Mechanochemistry of polymers

Prog. Polym. Sci., Vol, 14, 451-596, 1989

007%6700/89 $0.00 + .50 Copyright © 1989 Pergamon Press plc

Printed in Great Britain. All rights reserved.

MECHANOCHEMISTRY

OF POLYMERS

JUNKICIrlI SOHMA

Faculty of Engineering, Hokkaido University, Sapporo, 060, Japan* CONTENTS 1. Introduction 2. Electron Spin Resonance (ESR) as a tool for mechanochemical research 2.1. Information obtained by ESR 2.2. Fundamentals of ESR 2.3. Analysis of a simple ESR spectrum 3 Mechanoradicals in the solid phase 3.1. Methods to produce mechanoradicals for ESR observation 3.1.1. Ball-milling 3.1.2. Drilling 3.1.3. Slicing 3.1.4. Large deformation 3.1.5. Sawing in liquid nitrogen 3,2. Identification of mechanoradicals based on ESR analysis 3.2.1. Polyethylene (PE) 3.2.2. Polytetrafluoroethylene (PTFE) 3.2.3. Poly(methyl methacrylate) (PMMA) 3.2.4. Other polymers 3.3. Verification of main-chain scissions 3.4. Heterolytic scission induced by mechanical force 4. Mechanoradicals in liquid solutions 4.1. Spin-trapping method 4.2. Main-chain scissions induced by ultrasonic waves 4.3. Main-chain scissions induced by high-speed stirring 5. Molecular approaches to fracture of polymers 5.1. Grilt]th's theory 5.2. Andrew's generalized theory 5.3. Rate process approaches to fracture 5.4. Z h u r k o v - Bueche model 5.5. Peterlin model 5.6. Microcrack formation initiated by mechanoradicals 5.7. Critical degree of polymerization for mechanically-induced chain scission 5.8. Mechanically-induced main-chain scission in the liquid phase 6. Structural changes induced by fracture of polymers 6.1. Experimental methods used for studies on structure changes 6.1.1. X-ray diffraction

453 455 455 456 458 463 463 463 464 464 465 465 465 465 469 470 473 473 4'77 477 478 480 482 488 490 491 491 493 495 497 499 504 51 ! 51 I 511

*Present address: Faculty of Science, Kanagawa University, 2946 Tsuchiya Hiratsuka, Kanagawa Prefecture, 259-12, Japan. 451

452

J. SOHMA

6.1.2. Determination of crystallite size 6.1.3. Differential radial distribution function (DRDF) 6.1.4. Nuclear Magnetic Resonance (NMR) for determination of crystallinity 6.1.5. Crystallinity determined by density measurements 6.2. Structure changes in polyethylene after mechanical fracture 6.2.1. Crystalline transformation 6.2.2. Decrease in crystallinity 6.2.3. Decrease in the crystallite size 6.3. Structural changes in polypropylene after fracture 6.3.1. Crystalline transformation 6.3.2. Decrease in crystallinity 6.3.3. Decrease in crystallite size 6.3.4. Decrease in crystallinity 6.3.5. ESR confirmation of crystallite size decrease in fractured polypropylene Trapping sites of mechanoradicals 7.1. Radical migration in polymer matrices 7.2. Trapping of mechanoradicals on fresh surfaces Effect of triboelectricity on radical reactions 8.1. Anomalous behavior of mechanoradicals in decay processes 8.2. Mechanism of anomalous decay of mechanoradicals 8.2.1. Role of excess electric charges produced by triboelectricity Mechanochemical polymerization initiated by mechanoradicals formed from polymers 9.1. General 9.2. Mechanochemical polymerization initiated by PTFE mechanoradicals 9.2.1. Post-polymerizations initiated by PTFE mechanoradicals 9.2.2. Simultaneous polymerizations initiated by PTFE mechanoradicals 9.2.3. Experimental evidence for copolymerization 9.3. Mechanochemical polymerization initiated by mechanoradicals of other polymers 10. Mechanochemical polymerization initiated by mechanoradicals of inorganic compounds. 10.1. General features of mechanoradicals of inorganic compounds 10.2. Mechanochemical polymerizations initiated by mechanoradicals derived from alumina 10.2.1. Polymerization of ethylene 10.2.2. Polymerization of propylene 10.2.3. Polymerization of acetylene 10.3. Mechanochemical polymerization initiated by mechanoradicals formed from other inorganic compounds 10.4. Characterization of polyethylene polymerized by the mechanochemical method 11. Mechanochemically-induced reactions of radicals 11.1. Mechanochemically-induced reaction of poly(methyl methacrylate) mechanoradicals 11.2. Mechanochemically-induced reactions of polyethylene and polypropylene radicals 11.2.1. Polyethylene I 1.2.2. Polypropylene 11.3. Mechanisms of radical conversion by milling 11.3.1. The direct effect

511 513 513 516 517 517 518 519 521 521 522 523 524 526 528 529 533 538 538 545 545 549 549 552 552 554 557 558 560 560 560 560 564 565 567 569 571 572 574 574 578 578 579

MECHANOCHEMISTRY OF POLYMERS 11.3.2. Local heating by friction 11.3.3. Effect of macroscopic electric charge separation 11.3.4. Effect of fresh surfaces 11.3.5. Combined effect 11.3.6. Sceptical claims against mechanochemical reactions 12. Potential applications of mechanochemistry to industry 12.l. Radical formation by mechanical processing and its after effects 12.2. Combined effect of mechanochemical and photochemical degradation 12.3. Surface modification by mechanochemical polymerization Acknowledgement References

453 581 582

582 583 583 584 584 585 587 588 588

1. I N T R O D U C T I O N

Mechanochemistry is defined as a branch of chemistry in which chemical phenomena, such as chemical reactions and changes in crystalline structures, induced by mechanical actions like fracture and large deformations, are studied. This discipline parallels other well-established branches of chemistry, such as photochemistry and radiation chemistry. Radiation chemistry is the chemistry of chemical phenomena initiated by ionizing radiation, Ionizing radiation, such as ),-rays, initially ionizes molecules in a system. These ionized molecules then initiate secondary, tertiary, etc. reactions, resulting in permanent changes in either species and/or structures of the system. In other words, radiation chemistry is concerned with chemical phenomena caused by energy introduced into a system in the form of ionizing radiation. Similarly, photochemistry is the branch of chemistry dealing with chemical phenomena caused by photons, which are another form of energy. Mechanical energy is another type of energy, analogous to electrical currents and photons. Thus, it should not be a surprise that mechanical energy introduced into a system can produce any chemical changes. Researchers in rubber technology were the first investigators to notice chemical changes in systems as a result of the introduction of mechanical energy. They fi~und quite long ago that molecular weights were decreased considerably after either rolling or calendering of rubbers and even copolymerization occurred sometimes in the case of mixtures. It is also well known that strong ultrasonic agitatien of a polymer solution decreases the molecular weight of the solute polymers. These are good examples of chemical effects of mechanical actions. However, mechanochemistry is a field which is not as well established as photochemistry and radiation chemistry. There are several reasons for its relative immaturity. One of the reasons is probably a lack of positive evidence that such chemical reactions originate purely from mechanical origins, even though the energy source is doubtlessly mechanical. For instance in the rolling of rubber, the strong mechanical action is always accompanied by friction which produces large amounts of heat in the system. Therefore, the observed chemical phenomena might be postulated as thermally caused. Thus, new methods of

454

J. SOHMA

investigation have been required to establish mechanochemistry as a separate valid discipline in chemistry. In the two previous decades, the formation of free radicals in polymers at low temperature (e.g. 77 K) under strong mechanical action has been reported by several researchers on the basis of Electron Spin Resonance (ESR) experiments. The temperatures at which free radicals were generated were so low that any possibility of thermal production of radicals was ruled out. These findings have convinced researchers working in this field that purely mechanical action can induce a chemical phenomenon, the scission of chemical bonds. Since free radicals are unstable and reactive regardless of the method used for generation, the mechanically produced free radicals then initiate a series of reactions, just as do unstable ionic species produced by ionizing radiation. Radiation chemistry is a discipline in which the fate of an unstable ion produced by ionizing radiation is studied from its formation to its end. Similarly, one of the main themes in mechanochemistry is the study of the fate of a mechanically produced free radical from its formation through the disappearance of its descendants. One may imagine that the mechanical production of free radicals in a polymer is closely related to the molecular mechanism of fracture of the polymer. Also, changes in radical species can be followed by ESR, and information on other behaviours of radicals is also available from ESR studies. Mechanochemistry, as an established discipline in chemistry, is based on studies on mechanoradicals, which are defined as free radicals produced by mechanical action. ESR, which is a sensitive tool for detecting free radicals, provides a breakthrough in the studies on chemical phenomena originating from mechanical actions. There is a branch of modern polymer science called "chemorheology".~ Since rheology is the science which deals with viscoelastic properties, one class of mechanical properties of materials, chemorheology seems to be similar to mechanochemistry. Chemorheology is defined as the discipline which studies the changes in viscoelastic behaviour of polymeric materials induced by chemical reactions. This is the inverse of mechanochemistry, which deals with chemical phenomena caused by mechanical action. Thus, these two disciplines were originally different from each other. However, chemical reactions induced by a mechanical action, such as a large deformation, may result in changes in the rheological properties of polymers, as will be mentioned later. In such a situation, the two disciplines overlap in scope and approaches from both sides are needed for studies of these phenomena. In recent years, several excellent books 2-4 and review papers 5 ~l relating to mechanochemistry have been published, although most of them have not explictly mentioned the term "mechanochemistry" in the title. However, their contents are surely associated with this discipline. The appearance of many reviews covering this field indicates strongly that mechanochemistry has developed extensively in this decade.

MECHANOCHEMISTRY OF POLYMERS

455

2. E L E C T R O N S P I N R E S O N A N C E (ESR) AS A T O O L F O R MECHANOCHEMICAL RESEARCH

2.1. Information obtained by ESR Many of the recent developments in mechanochemistry have been related to studies on mechanoradicals, which can be detected with high sensitivity by ESR. It is very important in a study of mechanochemistry to learn the fundamentals of ESR. This chapter is devoted to an elementary explanation of ESR, which is needed to understand the following chapters. Readers, who have no interest in ESR and merely wish to know conclusions derived from ESR studies, may skip over this chapter. Information obtained by ESR spectroscopy is summarized as the following: (i) Positive evidence for the presence of upaired electrons. An ESR spectrum observed from any system provides us with positive evidence for the existence of unpaired electrons, either in free radicals or at broken bonds, or as trapped electrons or in paramagnetic ions, in the system. (ii) Concentration of free radicals. The concentration of free radicals in a system is estimated from spectral intensity. Relative concentration of free radicals can be easily determined simply by comparing the intensities of two spectra. The determination of an absolute concentration of radicals is not difficult if a standard sample is available. Sometimes, a peak height in a spectrum is adequate for an approximate estimation of the concentration of a radical responsible for an observed ESR signal. (iii) Identification of radical species. A radical species can be identified by an appropriate analysis of its ESR spectrum. (iv) Reactions of free radicals. By observing changes in ESR spectra, one can follow changes in radical species. These result from reactions of radicals. If the total intensity of an ESR spectrum stays constant through a spectral change, this means that a radical is being converted into other radical species without any decay. If the total intensity decreases, radicals are decaying (being converted into nonradical species). (v) Molecular motion of radicals. A very rapid mode of molecular motion of free radicals, the time scale of which is measured in nanoseconds or picoseconds, is detected by an analysis of changes in an observed ESR spectrum. ESR detects free radicals selectively. This is a big advantage of ESR. A free radical may show a characteristic optical spectrum. But in the optical spectrum (UV, visible, IR, etc.) from a system containing a small concentration of free radicals, the effect of the radicals is easily masked because nonradicals also absorb photons. The ESR spectrum of free radicals is not disturbed at all by molecules in the trapping matrix because nonradicals show no ESR spectrum. Thus, one can detect free radicals without any difficulty if a sample contains more than 10 ~5 free radicals. This should be called high sensitivity for the detection of free radicals.

J. SOHMA

456

(" H=0

3E=giH rn~=-..L 2 H~=O

F1G. l. Zeeman splitting.

2.2. Fundamentals of E S R Since an electron has a spin, S = 1/2, and a magnetic moment, the degenerated energy of the electron is split into two energy states corresponding to the magnetic quantum number ms = + 1/2 or - 1/2, under a strong magnetic field. This energy splitting due to the magnetic interaction is called the "Zeeman splitting". The Zeeman levels are described by E(ms)

= gflHms

(1)

where fl is the Bohr magneton, H is an applied magentic field and g is a constant named the spectroscopic splitting factor, or simply the g factor. An energy gap AE between the two states, as shown in Fig. 1 is described by the following expression: AE

= gflH.

(2)

The value of the factor for a free electron, go, is known to be 2.0023. If an unpaired electron occupies an orbital, either atomic or molecular, the electron may interact with the orbital angular momentum and the g factor of the electron may shift from the free electron value go. Consider a photon whose energy hv is equal to gflHo, namely, a Zeeman splitting at a fixed magnetic field H0. If a system having unpaired electrons in a magnetic field is irradiated by these photons, a resonance occurs between the magnetic energies and the photons. That is, hv = gflHo

(3)

is a resonance condition between a photon and the Zeeman splitting. Unpaired electrons are distributed between the two magnetic states, ms = + 1/2 or - 1/2, and relative populations of the electrons at these two states are governed by the Boltzmann factor under either a thermal equilibrium or a weakly perturbed condition, that is N _ ( m s = + 1/2) N+(m s = --1/2) oc exp ( - g f l H o / k T ) .

(4)

MECHANOCHEMISTRY OF POLYMERS

A

457

B

FIG. 2. An ESR signal is observed at the value of magnetic field strength, which satisfies the resonance condition, when the frequency is fixed and the field is swept.

Since the population of the higher level, ms = + 1/2, is smaller than that of the lower level, ms = - 1 / 2 , the energy absorption N hv exceeds the induced emission, N+ hv. Hence the net energy balance P is an absorption expressed by the following equation: P =

N hv - N+hv

=

(N

- N+)hv =

nhv

(5)

where n is the population difference ( N - N+). The absorption power is proportional to the population difference between the Zeeman levels. Radiation energy is absorbed by the system containing unpaired electrons under a fixed magnetic field, and the absorption is easily detected. This is the Electron Spin Resonance (ESR) absorption, sometimes called Electron Paramagnetic Resonance (EPR). Most ESR spectrometers are designed to be operated in the microwave region (v ~ 9.5 GHz) and at a magnetic field near 3,200 Gauss. An ESR spectrometer detects an ESR absorption at a field satisfying the resonance condition of eq. (3) when the magnetic field is swept while the frequency of the microwave radiation is held fixed. An absorption curve is shown on a recorder as a function of the swept field, as shown in Fig. 2. An ESR observation with most commercial spectrometers is shown not in an absorption line shape but in its first derivative form in order to increase sensitivity, as shown in Fig. 2(B). It should be strongly emphasized that most chemical compounds have no unpaired electron and show no ESR signal. Compounds containing only ,covalent bonds have no unpaired electrons due to the Pauli principle. An ordinary ionic compound consists of ions having closed shell electronic configurations in which all electrons are paired. A trapped electron produced by ionizing radiation in a system is a typical unpaired electron. A broken bond, which lacks a partner in an electron pair, has an unpaired electron at the broken site. Some polycyclic aromatic compounds, such as anthracene, can attract a single electron (due to their strong electron affinity) to form an anion, in which the additional electron stays unpaired.

458

J. SOHMA

According to Ingram, t2 a compound having an unpaired electron is defined as a free radical, or simply a radical. A molecule having a broken bond is called a neutral free radical and a compound having an excess electron, like the anthracene ion, is called an ionic free radical. If there exists a sufficient number ( > 1015) of either free radicals or trapped electrons in a system, an ESR signal originating from these unpaired electrons is observed from the system. This is the reason why ESR signals are selectively observed from free radicals and an observation of an ESR signal is positive evidence for the presence of free radicals in a system. 2.3. Analysis of a simple ESR spectrum An unpaired electron occupying a molecular orbital of a free radical is not isolated but is involved in various interactions, such as the s p i n - o r b i t and s p i n - s p i n interactions. The most important interaction in an organic radical is the interaction between electron spin and nuclear spin, the so-called hyperfine interaction. A majority of the carbon atoms forming the backbone of an organic compound are ~2C, with no nuclear spin. Hence the hyperfine interaction in an organic radical originate mainly from couplings of the unpaired electron with protons or nitrogen atoms in the radical. The hyperfine interaction is expressed as A • S ' / , where A is the hyperfine constant, or simply the coupling constant, and S and I are the spin quantum numbers of an electron and a nucleus, respectively. Then, each level of the Zeeman splitting is further split into two more energy levels corresponding to mr = __+1/2 in the case of I = 1/2. Each energy level is described by the following equation

E(ms, ml) = gflHoms + Ares'mr,

ms = ___1/2, mr = +1/2.

(6)

Splittings are illustrated in Fig. 3. The selection rule among the levels is given by

Ams =

___1, Am1 =

0.

(7)

Therefore, two transitions are allowed as shown in Fig. 3, and two absorption lines at H~ =

H0-

1/2A0,

and

/-/2 =

Ho + I/2A0

(8)

as shown in Fig. 3a. In other words, a single absorption line is now split into a doublet by hyperfine interactions with a single proton having nuclear moment 1/2. The coupling constant A is estimated in units of m T from the separation of the two lines. Similarly, the hyperfine interaction with a nitrogen nucleus ~4N, I = 1, divides each o f the Zeeman levels into three sublevels corresponding to ml = + 1, 0, - 1, and a singlet is observed as a triplet with equal intensity as shown in Fig. 4.

MECHANOCHEMISTRY OF POLYMERS (a)

rnl

.½

"

i

(b)

N,

H2

(c)

k

A.--~

N-N FIG. 3. Splitting o f a n ESR signal into a doublet by electron interaction with a single proton.

r~ / - -

rr~

""

g),H.n

_i2

.

i

.1

-1

~


- 1

o *I

,,

I't FIG.

,"

: :

,,

,,

H2 Hs

4. Splitting o f an ESR signal into a triplet of equal intensities by electron interaction with a single nitrogen atom.

459

460

J. SOHMA

If an unpaired electron couples with two protons, each sub-level due to the coupling of the first proton is split into another two sub-levels by the interaction with the second proton. In the case of unequal coupling constants At and A z, the observed spectrum appears as a quartet with equal intensity. If the coupling constants are equal, the two levels are merged into one level and a spectrum is observed as a triplet with relative intensity 1 : 2 : I. The central peak, being twice as strong, is due to a statistical weight of 2 of the doubly degenerated state of the middle level, as shown in Fig. 5. One can extend the above described results to a general case. If an unpaired electron in a radical interacts with n protons unequally, a spectrum of 2" lines with equal intensities is observed. In the case of equal couplings with n protons, a spectrum of(n + 1) lines with relative intensities being equal to the expansion coefficients of (a + b)" is observed. In a radical, a proton bonded to a carbon atom having an unpaired electron is called an s-proton and a proton bonded to an adjacent carbon to the unpaired electron site is called a fl-proton. Interaction between an unpaired electron and or-protons is mainly composed of configuration interaction, ~3 and the coupling with fl-proton is mainly through the hyperconfiguration interaction.~4 These two interactions are roughly equal in most organic radicals, and interactions with protons separated farther from the unpaired electron are negligible as a first approximation. Suppose an alkyl radical R2 H

I

I

I

I

~ C-C', R3 Rl as an example, where Rl, R2 and R3 are either groups like methyl or atoms other than hydrogen. The expected ESR spectrum from this alkyl radical is a doublet due to the coupling with the single proton. If one of the R's is a proton, either RI

H

H

H

I

I

I

I

I

I

. ~ C - C"

or

,-~ C - C ' ,

I

R2 H

I

R] R 2

the expected spectrum is a triplet, with a relative intensity 1:2: 1, due to the equal coupling with the two protons. In the case of H

H

I

I

~ C-C',

I

I

R~ H

MECHANOCHEMISTRY

OF P O L Y M E R S

461

Proton Proton ml 1 m I .-- +~-

2",,,

gaHo

ml

/ __

+½

_.]__ /

/ - g1

_1

2',

(

'--'

',___+l," 2~

_l 2

+½ __

_!

2

I_

Ho /

] ,

1 I

H1 H2 H3 FK;. 5. S p l i t t i n g o f a n E S R s i g n a l into a t r i p l e t o f 1 : 2 : 1 i n t e n s i t i e s by e q u a l inter-

actions with two protons.

a 1 : 3 : 3 : 1 quartet is expected. G o i n g back over this process, one m a y identify a radical basis for the observed ESR spectrum, if the m o t h e r molecule is known. F o r example, if a 1 : 2 : 1 triplet is observed from a system containing a m o t h e r molecule o f the type

I R 1 -- C - C - R 4

I

'

I

R3 H tile radical species responsible for the observed triplet is R:_, H

I Ri-

I

C-C'.

I

I

R~ H If the m o t h e r molecule is k n o w n to be o f the type H

H

I

I

Ri -- C - C - R 4 ,

I

I

R2 R3

J. SOHMA

462

A

B

t t

t

t

o~

g~

g3

FIG. 6. First derivative curve of spectra observed from a radical having (A) an anisotropic g factor, g~, g2, g3, and (B) axial symmetry, gl = gll, g-' = g3 = g.t.

the radical species is identified as H

H

I

I

Ri- C-C'.

I

I

R2

R3

Strictly speaking, one can deduce only the number of protons coupling a given unpaired electron from the number of lines and their relative intensities in an observed spectrum. Thus, other additional information, such as a known mother molecule, is needed in order to identify a radical species unambiguously. However, it is not particularly difficult to obtain the additional information needed to enable an identification of radical species in many cases. The ESR parameters, the g factor, and the hyperfine splitting constant are actually not scalars but tensors. Thus, there exist three principal values, g~, g2 and g3 for the g factor and A~, A2 and A3 for the hyperfine splitting constant. Ira radical undergoes a very rapid rotational motion, the principal values of one of the ESR parameters is averaged out to be a scalar quantity. Then, a radical trapped in a frozen amorphous state shows a characteristic spectrum in the case of an anisotropic g factor, as shown in Fig. 6 as evidence for the presence of peroxy radicals in a system. The anisotropy of radical coupling with a fluorine nucleus is very large in comparison with that of protons, and a characteristic spectrum of characteristic line-shape is observed, not from coupling with protons, but from the radical species Rt

F

J

I

I

I

~C-C"

Ri R2 trapped in a rigid state. One can use this large anisotropy of the coupling constants to identify a radical containing fluorine atoms. A change from these characteristic line-shapes in Fig. 6 to a simple line shape as shown in Fig. 2 is

463

MECHANOCHEM!STRY OF POLYMERS

analyzed in order to obtain interesting information on the molecular motion of the radical responsibility for the observed spectrum. Readers who wish to study ESR further are urged to read one of the reference books,2,~s L9 on ESR cited in the list of references. 3. MECHANORAD1CALS IN THE SOLID PHASE 3. I. Methods to produce mechanoradicals f o r E S R observation

Any method causing mechanical breakdown of polymers may produce mechanoradicals. Several types of apparatus, which were convenient for ESR observation, were designed by various researchers. 3.1.1. Ball-milling - Use of a ball-milling method for this purpose was at first reported by Butyagin and Abagyan. 2° ,at ball-milling apparatus designed by Sohma and Sakaguchi 2~ is displayed in Fig. 7. A glass ampoule A, which contains glass balls of ca. 5-mm diameter as well as flakes of a polymer sample, is joined to an ESR sample tube B. Through the connector C the ampoule can be connected to a vacuum system and then evacuated to 10 5Torr, for example. After evacuation the connector is sealed off and the ampoule is removed from the vacuum system to a vibrator, S, which moves vertically at ca. 4 cycles per sec. This vertical cycling can be carried out in a Dewar flask containing a coolant, such as liquid nitrogen, for an isothermal experiment. After being subjected to this vibration for the desired time period, the crushed flakes of the sample are transferred to the ESR sample tube without raising the temperature

....-..U>g] ii m

FIG. 7. Schematicdiagram of ball-mill apparatus• A: glass ampoule, B: ESR sample tube, C: connector to a vacuum system, D: glass balls for milling, S: holder for the ampoule, M: motor, N: belt, P: pulley, R: crank, V: Dewar flask containing coolant.

464

J. SOHMA

FIG. 8. Schematic diagram of drilling apparatus by Lazar and Sz6cs. (1) Oil, (2) rubber packing, (3) vacuum pump, (4) stopcock, (5) thermal insulator, (6) drill (95 rev/min), (7) polymer sample, (8) liquid nitrogen, (9) ampoule, (I0) Dewar flask. of the sample. Then, the sample tube holding the comminuted sample is placed in an ESR cavity at a controlled temperature. The advantages of this ball-mill apparatus are (1) it allows milling in vacuum or in any gas, and (2) it allows milling at a fixed low temperature. Another type of bali-milling apparatus used for ESR work was described by Pilar and Ulbert. 22 3.1.2. D r i l l i n g An apparatus designed by Lazar and Sz6cs 23 is shown in Fig. 8. Mechanoradicals are produced by drilling a polymer i n v a c u o (10 3Torr) and at low temperatures. The powder of the sample polymer (7) made by the drill rotating at 95 rpm fell into a cooled ampoule (9), which was sealed off before ESR measurement. 3.1.3. S l i c i n g - Backman and DeVries 24 designed an apparatus for slicing a polymer sample. Dimensions of the sliced piece were approximately 1.1 x 0.5 x 0.07cm. About 50 such slices could be placed in the ESR cavity for a measurement. The slicing device was driven by a small dc electric m o t o r and a jet of nitrogen gas carried the newly formed slice through a tube cooled by liquid

MECHANOCHEMISTRY OF POLYMERS

465

nitrogen immediately after the slicing. The temperature and atmosphere under which the slicing was carried out were controlled by placing the entire slicing device in a double-walled glass vessel. The temperature was varied from - 30°C to + 110°C. A slicing technique was also reported by Zhurkov et al. ~-5 3.1.4. Large deJormation - Becht and Fisher 26 constructed an apparatus by which a polymer was stretched in an ESR cavity. A similar method was tried by Verma and Peterlin. 27 Nagamura and Takayanagi 2~ built up an apparatus, which is fitted in a gap in the magnet of an ESR spectrometer, for the purpose of in-situ measurement of the ESR spectrum of a sample during stretching. A diagram of the apparatus is shown in Fig. 9. Load is applied to a sample (by the lever connected to a loading machine) under a condition of either constant strain, or constant stress, or constant stress-strain or cyclic strain. The temperature is also variable from - 8 0 ° C to 160°C in nitrogen gas but at room temperature in vacuo. William and DeVries 29 constructed a more sophisticated apparatus, by which almost any kind of deformation mode is applicable and with which in-situ observation of an ESR spectrum can be carried out in the temperature range between - 160°C and 200°C. A simpler method to detect tile ESR spectrum of a loaded polymer involves a low temperature quenching of the sample immediately after stretching. This method was successfully used by several researchers. 3°'3~ 3.1.5. Sawing in liquid nitrogen 32"33 - A stick of polymer is sawed by a metal saw in a vessel filled with liquid nitrogen. Sawdust from the sample polymer is collected with liquid nitrogen in an ESR sample tube cooled at 77 K. Liquid nitrogen is then gradually pumped out from the sample tube while the tube is kept at 77 K. After this procedure the sample tube is sealed offand placed in an ESR cavity at a controlled temperature for observation. By this simple method E S R spectra of polymer sawdusts could be observed without heavy contamination by oxygen.

3.2. Identification o f mechanoradicals based on E S R analysis 3.2.1. Polyethylene ( P E ) 34 - An ESR spectrum observed from polyethylene fractured at 77 K and 10 -5 Torr by the ball-mill apparatus is shown in Fig. 10a. This sextet is derived from a radical species H

H

I

I

~C-C,

I

I

H

H

in which the coupling constant of the two ~ protons with one of the fl protons

466

J. S O H M A

apparatus

--,

tcer

FIG. 9. Schematic diagram of stretching apparatus for in-situ ESR observation.

is 23 Gauss and their coupling constant with the other fl proton is 45 Gauss. The theoretical spectrum derived from the assumptions of both the above coupling constants and a line-width of 17 Gauss is also shown as (b) in the same figure. A good agreement between the observed spectrum and the theoretical one indicates that the radical responsible for the observed spectrum is H

H

I

I

~C-C',

I

H

I

H

which is a mechanoradical produced by a main-chain scission of polyethylene.

MECHANOCHEM1STRY OF POLYMERS I

No

467

t

a A AA

cb Vvv AA FIG. 10. (a) Observed ESR spectrum of PE fractured by the ball-mill apparatus shown in Fig. 7. (b) Simulation spectrum based on the assumed radical, H

H

I I

~C-C'.

I I

H

H

An ESR spectrum obtained from polyethylene sawdust is shown as (a) in Fig. 11. The line-shape is almost the same as for the spectrum (a) in Fig. 10 except for an asymmetry in the centre of the spectrum. ESR spectra were observed from the same sawdust after heat treatments at 132K, 152K and 233 K. The shape was gradually changed after each heat treatment. The asymmetric spectrum shown as (d) is known to be from a peroxy radical ROO' 35(see Fig. 6(b)). This suggests that the slight asymmetry in the central part of the spectrum (a) is due to the presence of some peroxy radicals, H

H

I

I

-C -C-O-O,

I

I

H

H

among the primary radicals of polyethylene H

H

I

I

~C-C

I

I

H

H

J. SOHMA

468 H

H

.c-c'H

Mixtur~

(a)

~

A

13zK

(b) C- C

(c)

152 K

H H C- C-CH H 0 H 0

L

->,

(d)

H H ~C-C. H H

233K

26-8G H H H ~" ~ C - C - C H Thermal H H 02

02 H H H ,,-~C- C-CH Thermal H 0 H 0

FiG. I 1. ESR spectra observed from the sawdust of polyethylene. Observation temperatures were 77 K. (a) Immediately after the sawing and after heat treatment, (b) at 132K for 5min, (c) at 152K for 5min, (d) at 233K for 5min.

due to oxygen dissolved in liquid nitrogen. Thus, one may conclude that two kinds of mechanoradicals are generated by sawing of polyethylene in liquid nitrogen, that is, H

H

H

H

I

I

I

I

~C-C'

and

~C-C-O-O'.

I

I

I

I

H

H

H

H

The spectrum (b) is a superposition of the two spectra, (a) and (c), and therefore the state which gives the spectrum (b) is an intermediate state between (a) and (c). The spectrum (c) is mainly an octet superposed with the spectrum of the peroxy radical. The octet spectrum was identified to be due to a radical species H

H

H

f

I

J

~C-C-C

I

I

H

H

-H.

36

MECHANOCHEMISTRY OF POLYMERS

469

Based on the above identifications one may conclude that the radical H

H

[

I

~C-C

I

I

H

H

is the major radical produced by the mechanical action. Some of these primary radicals are converted inlo the peroxy radical tt

H

I C -

I (? -

I

I

H

H

OO

at 77 K in the presence of oxygen. The remaining primary radicals are gradually converted into more stable radicals like H

H

C -- C - CH~

I

H by

the mild heat treatments. Other species of radicals, such as CH2 - CIH - CH: ~ 37,35and ~ C H 2 - CH = CH - CH - CH 2,3~ were reported by Russian researchers. 3.2.2. P o l y t e t r a f l u o r o e t h y l e n e ( P T F E ) 2~ - ESR spectra of P T F E fractured by ball-milling at 77 K in v a c u o are shown in Fig. 12. The spectra (a) and (b) were observed at 77 K and 243 K, respectively. It is known that a spectrum having a strong central band with weak satellites like (a) in the figure is characteristic of a radical having two ~ fluorines due to strong anisotropic coupling with the fluorine nucleus. 39 The 1 : 2 : 1 triplet appearing in the spectrum (b) originates from the coupling of two fl-fluorines (the anisotropy of the coupling with c~-fluorines is averaged out by rapid molecular motion at the elevated temperature). The argument presented above leads us to the conclusion that the radical species showing the spectra in Fig. 12 is F

F

I

[

~C-C

I

I

F

F

produced by a main-chain scission of a P T F E molecule. In this case, again, mechanical fracture of P T F E causes scission of the main-chains.

470

J. S O H M A

(a)

,,50G (b) Fio. 12.Observed ESR spectraof milledPTFE. Observationtemperatureswcrc (a) 77 K and (b) room temperature. 3.2.3. Poly(methyl methacrylate) ( P M M A ) 4°'4' - The E S R spectrum observed from P M M A after 18 hr fracturc at 77 K in vacuo by the ball-millis shown as (a) in Fig. 13. Thc spcctrum obtained after milling for 0.I hr under the samc conditions is shown as (b) in Fig. 13. This lattcrspectrum is different from thc onc obscrved after thc long milling. The spcctrum observed after the long milling can bc decomposed into two components, a doublet (bold linc) and a nonct (dotted line),as shown in Fig. 14. The nonct is the well-known spcctrum of the P M M A radical H

I

CH 3

I

~ C-C"

I H

42

I CO2CH 3

The doublet is assigned to a radical CH3

I ~C

I CO 2CH 3

H

I

CH~

I

C-C~.

I CO 2CH3

MECHANOCHEMISTRY OF POLYMERS

471

Obs. at 77K

( Ohr

. ~ A~

0h 1 ,r

FIG. 13. Observed ESR spectra of PMMA after (a) 18hr milling and (b) 30min milling.

i,,

/i

j

'./

I

!

'

i I

I

FIG. 14. Decomposition of the ESR spectrum of PMMA after 18 hr milling at 77 K. The bold line is the spectrum obtained by the graphical subtraction of the nonet (dotted line) from the observed spectrum (thin line).

J. SOHMA

472

I 25.5 G 24 G i

/,

! /

!,,

1

#

i

,

!

s

20G.,

u

J FIG. 15. Decomposition of the ESR spectrum of PMMA after 0. I hr milling at 77 K. The thin line is the observed spectrum, the dotted line is the nonet. The bold line is the pattern obtained by the subtraction of the nonet from the observed spectrum. The bold line spectrum consists of the doublet and triplet shown in the stick diagrams.

The observed spectum (b) in Fig. 13 can be decomposed into three components as shown in Fig. 15, the nonet, the doublet and a triplet. The radical species responsible for the nonet and the doublet are the same as those mentioned above, because each component is identical to the corresponding components in Fig. 14. There are three radical species derived from the mother molecule P M M A which would be expected to show a triplet, that is, CH 3

I

H

CH:~ H

H

I

t

I

~C--C,

I

~C--C~,

and

CH 3

I

~C-C,-~

I

I

I

i

I

I

R

H

R

H

C

CO2CH~

In experiments on deuterated methyl P M M A samples the observed spectra were unchanged, ruling out the second and third radicals as sources of the observed triplet. Therefore, the triplet radical species consistent with the experimental data is CH 3

H

I

I

I

I

~C--C,

CO2CH 3 H

which is produced by main-chain scission.

MECHANOCHEMISTRY

OF POLYMERS

473

3.2.4. Other polymers - Trials to identify species of mechanoradicals were reported by various researchers on the following polymers; polypropylene, 21.34.43,44-45 polyisobutylene,45,46 polybutadiene] ~,33,34.47 polystyrene, ~-5.4~,4~,5° poly-~-methylstyrene,46'~15~"poly(vinyl alcohol), 48`s°'s3"54poly(vinyl acetate), -~5'4~'5~ poly(methyl methacrylate), 46'485°polyoxymethylene,3)'34'35poly(ethylene oxide), 5) poly(propylene oxide), s3 poly(ethylene terephthalate), 55 poly(2,6-dimethyl-pphenylene oxide), 56 polycaprolactam, 52'57 poly -methylcaprolactam, ~7 poly(hexamethylene adipamide), 24 polyurethanes, 58 polydimethylsilmethylene,s~ • qq polyMoxanes,- and celluloses. 6°'<~-' Observations of ESR spectra, which indicate rupture of chemical bonds, were reported on a number of polymers following large deformations. These included polyethylene, 26x'3 polypropylene, 63 polyisoprene, 3° polycaprolactam, 6364'~5 poly(ethylene terephthalate)] 6~'~ and others.2~.~,~,.~7 All these results demonstrate that the macroscopic fracture of solid polymers induces microscopic ruptures, namely scissions of chemical bonds in polymers. It is worth noting that no mechanoradicals were generated in the fracture of solids consisting of low molecular-weight compounds, such as n-paraffins, benzene, styrene and others, under the same conditions of milling which we used for the polymers. 34 One exception was found recently to be sucrose, which showed an ESR spectrum after mechanical fracture. 6~ Therefore, it is safe to believe that the generation of mechanoradicals is a characteristic of macromolecules.

3.3. Verili'cation o f main-chain scissions

The assigned structures for the mechanoradicals formed from PE and PTFE are chain-end radicals, as described in the last subsection. Most of the identified structures of mechanoradicals formed from other polymers are also chain-end radicals. Based on the generation of primarily chain end mechanoradicals, one might consider that main-chain scissions are the primary scissions caused by mechanical actions. Strictly speaking, observation of one species of a chain-end radical is not sufficient evidence for one to conclude that main-chain scissions are the primary scissions caused directly by mechanical action. It is known, for example, that main-chain scissions can be chemically caused sometimes by /~-scission o f a radical

R3

R3 R~

I I I ~c--t-c-c~ I I I I R4 R2 R4 R2

474

J. SOHMA

to generate a chain-end radical

Rl R3 I I 'C-C~

I

I

R2 R4

and double bond R3

R3

I

I

~C-C--~-C

I I I R, R2 R4 as secondary productsfl '69 If main-chains are directly broken by mechanical action, a pair of radicals, R3 RI

R3 Ri

I /

~C-C',

I

and

I

R4 R2

I

I

I

I

"C-C, R4 R2

will be generated from a mother molecule R3 RI

I

I

,-,C-C~.

I

I

R4 R2 Therefore, experimental evidence for the formation of this radical pair is required to verify experimentally that main-chain scission is the primary process by which radicals are mechanically generated. Such verification is impossible for both PE and PTFE because such a main-chain scission would produce two radicals indistinguishable from each other. However, formation of these pairs of mechanoradicals is directly observable for polypropylene (pp)21.43 and indirectly observed for PMMA41 . An ESR spectrum observed from PP ball-milled at 77 K in vacuum is displayed as (a) in Fig. 16. Spectrum (b) in the same figure is a theoretical spectrum derived from an assumption that two kinds of radicals, H

CH3

I

I

,-- C - C "

I

H

I

H

CH3 H and

I

-C--C,

I

H

I I

H

exist in a one-to-one ratio in the system. 21'43The good agreement between the

MECHANOCHEMISTRY OF POLYMERS

40G

(a)

(b) FIG. 16. (a) Observed ESR spectrum of PP ball-milled at 77 K in vacuum. (b) Simulation spectrum obtained from a one-to-one mixture of the octet from the radical H

CH~

I I

~C-C

I I

H

H

and the quartet from the radical CH3

H

I I ~C--C. I I H

Iq

475

476

J. SOHMA

observed spectrum and the theoretical one indicates that PP main-chains are directly ruptured by milling at 77 K to produce one-to-one pairs of the two species of radicals mentioned above. This is positive evidence that primary scission of the main-chains is the main mode of mechanoradical generation. The spectrum observed from P M M A milled at 77 K in vacuum for a short time (0.1 hr) can be decomposed into the three components shown in Fig. 14. The relative intensity of the nonet is approximately equal to the sum of those of the two other components, the triplet and the doublet. The radical identified from the triplet is the species CH 3 H

I

I

,,~ C - - C "

(R3).

I

I

R

H

H

CH3

I

I

This is the partner of the radical

--C--C'

I

I

H

R

(gg)

responsible for the nonet if both are considered as being formed by main-chain scission of PMMA. The other radical, CH 3

CH 3

I I ~c--C-c~ I I I R

H

(R,)

R

responsible for the observed doublet is a secondary product, generated by hydrogen abstraction from a PMMA molecule by the radical R3. The observed equality of the sum of intensities of the doublet and the triplet with the intensity of the nonet means that the sum of the concentrations of the radicals R 3 and R~ is equal to the concentration of the radical R9. These results lead us to conclude that pairs of radicals R3 and R 9 a r e primarily formed by mechanical action. However, some fraction of R 3 is converted to the secondary radical R2 during the milling. This conclusion is supported by the fact that the relative intensity of the doublet is approximately equal to that of the nonet in the spectrum (a) in Fig. 13, which was observed after the long milling of PMMA. The partner radical R 3 for the radical R9 in main-chain scissions is completely converted into the secondary radical R2 during long milling, while no conversion occurs of the radical Rg, which is known as a more stable species. 7°'7t These observed spectra of P M M A milled at 77 K indicate indirectly the for-

MECHANOCHEMISTRY OF POLYMERS

477

mation of pairs of mechanoradicals. Therefore, one may conclude that main chain scission of P M M A is the primary process caused by mechanical action. Such pairs of mechanoradicals are found only for the two cases mentioned above. However, one may have a good reason based on these facts to believe that main-chain scission is the primary chemical process caused by mechanical fracture at low temperatures such as 77 K, when the chain-end radicals can be identified.

3.4. Heterolytic scission induced by mechanical Jbrce It was shown in the last Section that homolytic scission of the main chains in ball-milled PP has been experimentally verified. This verification provides a ground to support the viewpoint that homolytic scission is the main mechanodegradation mode for many other polymers, for which the formation of neutral mechanoradicals can be experimentally shown. However, one cannot rule out a priori the possibility that a polymer chain is heterolytically cleaved by mechanical force to produce a pair of ions P+ and P . Recently Sakaguchi and his collaborators claimed that heterolytic scissions could be mechanically induced for PE, ppTe,73 and PTFE. 74 They observed tetracyanoethylene (TCNE) anion radicals by ESR after the ball milling of mixtures of these polymers with T C N E, which is a strong electron (anion) scavenger. They took the formation of the T N C E anion radicals as evidence for the presence of anionic precursors which had been mechanically produced. They therefore concluded that heterolytic scissions, P+ and P - , were mechanically induced. There is no doubt that the T C N E anion radical was actually produced in their experiments. However, it seems to this author that their conclusion stands on 'weak ground. Heterolytic scission is not the exclusive mechanism by which ;anionic species may be formed in the mechanical fracture of polymers. Anions can be secondarily formed through a process by which an excess electron is stabilized by attachment to a neutral radical with high electron affinity. A metastable anion is thus formed by this attachment. The anion has no unpaired ,electron and no ESR signal is observable from the anion. The presence of the strong electron scavenger like T C N E can induce a separation of the electron from the anion to form a T C N E anion radical and a neutral radical. This mechanism has been shown for polypropylene and will be discussed in detail in Section 8. In order to conclude that a heterolytic scission occurs, this author believes that one has to prove pair formation of a cation and an anion by direct main-chain scission. At least one must prove the presence of both an anion and a cation which are primarily formed by direct mechanical action. 4. MECHANORADICALS IN LIQUID SOLUTIONS It is well known that molecular weight of polymers in solution decreases after ultrasonic irradiation of these solutions. 7s'76The observed decreases in molecular

478

J. SOHMA

weight were theoretically analysed in terms of a rate equation. These analyses lead us to the conclusion that main-chain scissions are mechanically caused by the action of ultrasonic waves on the polymer solutions. 75-79However, a question as to whether the primary ultrasonic process is main-chain scission is still open. It is possible that main-chain scission is a secondary process involving a chemical reaction such as the fl-scission of an unstable intermediate, 4t'8° for example, a free radical produced by the primary ultrasonic process. Phenomenological approaches, like measurements of molecular weights, do not give us any direct information on the nature of the free radicals produced by the ultrasonic energy. Indirect detection of free radicals formed by ultrasonic waves was performed by Henglein 8t by adding stable free radicals to scavenge the formed radicals. Direct detection and identification of free radicals are also very important in the case of mechanical degradation of polymers in a solution. ESR is a useful technique to detect free radicals, as described before, and proves its utility for detection of mechanoradicals in solid polymers. However, lifetimes of free radicals in a liquid phase are too short for the radicals to be detected by a conventional ESR spectrometer. Hence no ESR studies on polymer solutions irradiated by ultrasonic waves have been reported until publication of the works by Tabata and Sohma. 82-84 In order to detect unstable free radicals in solution by ESR, we have to rely on a special technique of spin trapping, which was invented and reported by Janzen 85 in 1971.

4.1. Spin-trapping method Nitroso compounds, R - N = O , scavenge an unstable radical R~ to convert it into a stable nitroxide radical. These nitroxide radicals are called spin-adducts. R - N = O + R~ ~ R u - - N - - O ' . Since an unpaired electron in an unstable radical, R~, is trapped and stabilized in a stable nitroxide R u - N - O ' , this method is called spin-trapping. Several nitroso compounds 86are reported as good agents for spin-trapping. For example, pentamethylnitrosobenzene (PMNB) 84 is known as a spin-trapping agent with a high rate of trapping. It is useful for trapping unstable radicals which decay in several microseconds. Suppose PMNB traps an unstable tertiary alkyl radical, as shown in Fig. 17a. The resultant spin adduct has an ESR spectrum which is a simple triplet (Fig. 17) due to the coupling of the unpaired electron with nitrogen, because no hydrogen is present at a fl-site in this spin-adduct. The separation of the triplet is determined by the coupling constant aN of the nitrogen. If PMNB traps a secondary alkyl radical, the spin-adduct produced shows a double triplet as shown in Fig. 17(b). The separations of the triplet and the doublets are due to the couplings with the nitrogen and the hydrogen, respectively. Similarly if the

MECHANOCHEMISTRY OF POLYMERS

479

PMNB (ol

_ ~

N=O +

C

•~:~ ~:

~7~ 0 C t-carbon

.--~ ~ _ j ~ - N - C - Spin r 7

~

a~u~t

triplet

GN

_~

H

6H

"~--'~ '

'

s-carbon

C double triplet

OH

(C)

ON

H

~-~ 9 ~

O-carbon

N=O + "C'~ ~ - ~ - ' N-C--spin H / "~ H adduct triple triplet

QN

OH

FIe. 17. The expected spectra of spin adducts formed by PMNB. (a) Spectrum of a spin adduct of a tertiary carbon radical, (b) spectrum of a secondary carbon radical, (c) spectrum of a primary carbon radical, aN = Coupling constant with a nitrogen nucleus, a H = coupling constant with a proton.

original radical is a primary alkyl radical, the spin-adduct formed shows an ESR spectrum with a triple triplet as shown in Fig. 17(c). If one observes an ESR spectrum like the triplet shown in Fig. 17(a) from a system containing P M N B after mechanical agitation, one may conclude that tertiary free radicals are produced by the agitation. When either the double triplet or the triple triplet is observed, presence of either secondary or primary radicals in a system is concluded. This is a way to identify unstable radicals in a liquid phase by ESR combined with the spin-trapping method. This spintrapping method has both advantages and disadvantages. The largest advantage is its simplicity in comparison with the alternative, highly sophisticated technique of time-resolved ESR spectroscopy to detect short lived radicals. The main disadvantages of the spin-trapping techniqu e are two-fold. Firstly the spin trap is a foreign molecule which may perturb the system. Secondly the identification of a radical species is not unique but equivocal• Radical identification by this method is limited to determining whether a carbon radical is tertiary, secondary or primary. In order to make unique identifications, more information is required.

480

J. SOHMA 30 cm

T o

L

D1

Medium: Water ULtrasonic wave

tube

t ...t..'.t t t t ~tttt/ ULtrasonic generator

FIG. 18. Schematic representation of the apparatus used for ultrasonic irradiation.

4.2. Main-chain scissions induced by ultrasonic waves In this type of experiment, a commercial ultrasonic cleaner (frequency 45 KHz, power 100 W) was used as the ultrasonic source. An ESR sample tube containing both sample polymer and a spin-trapping agent in solution is immersed in the water bath of the ultrasonic cleaner for sonification (See Fig. 18). 87 The temperature of the sample during ultrasonic irradiation is controlled by thermostarting the water bath. A benzene solution of P M M A containing 10-1-10 -3 % PMNB was irradiated by the ultrasonic cleaner at room temperature. An example of an ESR spectrum observed from the P M N A - P M N B - b e n z e n e system after 2hr irradiation is shown in Fig. 19. 84 In a control experiment, a PMNB-benzene solution without polymer was irradiated with ultrasonic energy. No ESR spectrum was observed from this control sample. The observed spectrum is decomposed into two components, a triple-triplet and a triplet, as shown by stick diagrams in Fig. 19(a). The central peaks of the 1 : 2 : 1 triplets (formed by the coupling with two protons) are completely smeared out by a superposition of the triplet. This is because the nitrogen in the nitroxide radicals couples with the unpaired electron with the same coupling constant aN as the hydrogens. A decomposition into a triplet and a doubletriplet instead of a triple-triplet might be possible because there is no explicit observation of the central peaks of the triple-triplet. However, the separation of the doublet in the assumed double-triplet would be about 16 Gauss in this case. This value for the coupling constant with protons is much larger than the reported value 8s for the coupling constant with fl-protons in PMNB spinadducts. Therefore, the possibility of this doublet-triplet was ruled out. Spectrum simulation was carried out by superposing the triplet and the triple-triplet, assuming equal intensity. The separation of the triplets due to

MECItANOCHEM1STRY OF POLYMERS

481

(a) obs.r.t. M n 2+

T T-T

•-- aN--'1

l i

~aH~

l

I

!

I

i

I ] I.~- aN - • i

I

i

¢b) catc+

FIG. 19, (a) Observed ESR spectrum of P M M A in benzene solution after 2 hr ultrasonic irradiation. (b) A simulated spectrum on the assumption of superposition of a triplet and a triple-triplet with equal intensities.

nitrogen was taken as 13.8 Gauss, while the separation of the 1 : 2 : 1 triplet due to the protons was assumed as 8.28 Gauss. The simulated spectrum is shown in Fig. 19(b). The satisfactory agreement between the observed spectrum and the simulated one reconfirms the analysis of the spectrum and also demonstrates the presence of equal amounts of tertiary radicals and primary radicals. One could conceivably generate three different kinds of tertiary radicals from the mother molecule PMMA. These radicals are H

CH3 H

H

H

H

I

I

I

I

I

I

-C-

C

I

-C-C--C-

(I),

-C-C-C

(II),

CH 3

I

I

I

I

I

I

I

H

H

H

R

H

H

R

(hi)

where R means -CO2CH 3. No ESR spectra from spin-adducts of either the methyl radical or the ester radical R', which are partner radicals of the radicals (I) and (I|), respectively, were detected. Absence of the partner radicals in the system strongly suggests that neither one of the radicals (I) and (1I) exists in the system. Thus, the most plausible species for the observed triplet is the radical llI, which is produced by a main-chain scission of PMMA. Three different primary radicals could also conceivably be generated from the

482

J. SOHMA

mother molecule PMMA, namely H

CH2

H

CH3

I

I

I

I

~C-C~

(IV),

,-~C - C,-,

I

I

I

I

H

R

H

CO2CH2

CH 3 H

I (V)

and

I

~ C - - C'

I

I

R

H

(VI).

In order to determine which was the species responsible for the observed triple-triplet, experiments in which either a-methyl deuterated or ester methyl deuterated PMMAs were used, were carried out. In these experiments, identical spectra were observed. These experimental results eliminated the possibility that either one of the radical species (IV) and (V) was the origin of the triple-triplet. Therefore, the observed triple-triplet was attributed to the spin-adduct of the radical (VI), which is a partner radical of the radical (lid in a main-chain scission. This agreement with the simulated spectrum indicates that these two radicals are generated in equal concentration. All these results consistently indicate pair formation of the radicals (III) and (VI) which are the primary products of main-chain scission of PMMA. Thus, the spin trap experiments provide direct evidence that main-chain scissions is the primary process caused by ultrasonic irradiation. Mechanoradical formation caused by ultrasonic irradiation, probably due to main-chain scission, was reported for polystyrene,82 polyisoprene,82 poly(vinyl acetate), 83 poly(~-methyl styrene)83 and polypropylene,8z although identifications of the radical species were less unambiguous than for PMMA. 4.3. Main-chain scissions induced by high-speed stirring Degradation of polymers during turbulent flow is known. It is believed to have mechanical origins as in the case of ultrasonic action. 77 Although radical formation, originating from main-chain scission in turbulence, is anticipated, no positive evidence for this was reported until 1981. Tabata et al. designed an apparatus for the ESR study of polymer chains ruptured in a turbulent liquid under high-speed stirring. 89A schematic diagram of this is displayed in Fig. 20. A high-speed rotor (5), the rotation speed of which could be varied from 10,000 to 35,000 rev/min, was used for the experiments. Either a rotating blade or a rotating ball was attached to the end of the rotor. The blade and ball were made of stainless steel. Special caution was needed to keep the polymer concentration in the sample solution constant. This was accomplished by attaching a condenser (6) to prevent evaporation of the solvent during the stirring. Otherwise, the solvent gradually evaporated from the sample solution in the sample vessel (2), resulting in a gradual increase in concentration of the sample during an experiment. A water stream (indicated with bold arrows) was used to cool the condenser. The thin arrow shows the flow of nitrogen gas which was used to fill

MECHANOCHEMISTRY OF POLYMERS

483

H,o

FIG. 20. Schematic representation of the apparatus used for high speed stirring. (1) ESR sample tube, (2) glass container for liquid sample, (3) heatbath controlling temperature of the sample, (4) magnetic stirrer, (5) high speed rotating blade, (6) condenser, (7) rotor.

the entire vessel. All stirring experiments were performed in a purified nitrogen atmosphere to hinder the formation of peroxy radicals. PMMA-benzene solution was studied with this apparatus in order to find out whether or not mechanoradicals were generated by high speed stirring at 14,000rev/min. 9~ The spin-trap PMNB was also used in this experiment. P M M A (100mg) was dissolved in 2ml benzene containing 0.5 mg of the spin trap agent PMNB. The spectrum observed after stirring the solution at 20°C for 20 min is identical 9~ to that in Fig. 19(a) which is the spectrum of the P M M A benzene solution containing PMNB after ultrasonic irradiation. As discussed in the last subsection, this spectrum shows the formation of the radical pairs H

CH3

I

I

-C-C'

CH 3 H

i and

I

~C--C"

I

I

i

I

H

R

R

H

by main-chain scission of PMMA. Thus, one can conclude that main-chains of P M M A in the benzene solution are ruptured by high-speed stirring. This

484

J. SOHMA PSt

(a)

n

obs.r.t.

,

I,

~

,

~-

',

'

k ON-~

~-~-PSl

obs.r.t.

M n 2+

I V (bl

I" I]'

:

~

I

,

,

FIG. 21. Observed ESR spectrum of polystyrene (PS) with added PMNB at room temperature after high speed stirring. (a) Normal PS, (b) ~-deuterated PS. I and II are the stick diagram representations of the components.

conclusion was reconfirmed by the experiments in which samples of partially deuterated P M M A were used. 9~ The same results were obtained by using either the rotating blade or the rotating ball. This result suggests that the chain scissions are caused not by direct scission of polymer chains by the blade but by a mechanical effect of the turbulent motion of the sample liquid. Actually, cavitation due to the high-speed stirring was observed during the experiments. A similar experiment was carried out for polystyrene (PS)-benzene solution with the P M N B spin trap. 9°'9~ The observed spectra are shown in Fig. 21. The spectrum (a) of the normal PS can be decomposed into a double-triplet (I) and a triple-triplet (II), as shown with the stick diagrams in the figure. The tripletriplet is assigned to a primary carbon radical, which is the species H

H

I

I

~C-C"

I

J n

generated from the mother molecule PS. The double-triplet is attributed to a

M E C H A N O C H E M I S T R Y O F POLYMERS

PVAc

obs.r.t.

,

I

485

,

~

,

k-a;~

I,i,

n

Ill FIG. 22. Observed ESR spectrum of poly(vinylacetate) at room temperature after high speed stirring. I, II and Ill are the stick diagram representations of the components.

secondary carbon radical, which is presumably the H

H

I

I

~C -C"

I

I

H

4,

species generated from the mother molecule. The spectrum observed from an ~-deuterated PS is shown as (b) in Fig. 21. The spectrum consists of two components, a triplet (I') and a triple-triplet (II') identical to component II in Fig. 21(a). Since the coupling constant for a deuteron with an unpaired electron is smaller (ca. 1/6) than that for a proton the width of the doublet within the double-triplet (I) is smaller than the width of the lines themselves. Hence a triplet is expected from the ~-deuterated PS. This was the case as shown in Fig. 21 (b) and therefore the assignments of the radical species are reconfirmed. It is worth noting that the relative intensities of the triple-triplet and the triplet are approximately equal. This equality indicates formation of pairs of the two kinds of radicals resulting from the scission of PS main-chains. Similar experiments were also carried out with poly-~-methylstyrene. The experimental results also indicated main-chain scission in this case. 91 Spin trapping experiments were also carried out for poly(vinyl acetate) (PVAc) and polyethylene glycol (PEG). The results are shown in Fig. 22 (PVAc) and Fig. 23 (PEG), respectively. The observed spectrum shows in Fig. 22 is decomposed into three components, a triple-triplet (I), a double-triplet (II) and a triplet (III). The components I and II are reasonably attributed to the two

486

J. SOHMA obs. r.t

PEG

I

,

111 lV

I

I,

I

I

,,

I

I

I Q

FIG. 23. Observed ESR spectrum of polyethylene glycol at room temperature after high speed stirring. I, If, III and IV are the stick diagram representations of the components.

species of radical H

H

H

H

I

I

I

I

~C-C"

and

,-~C-C"

I

I

I

I

R

H

H

R

(R: - O C O C H 3 ) ,

respectively. 9' These are the pair of radicals produced by a main-chain scission. The third component, the triplet, is assigned to a radical species H

H

I I ~c-¢-c~. I l i H

R

H

This is the radical produced by hydrogen abstraction from the or-site of the mother molecule - a secondary reaction of the primary mechanoradicals. In the case of PEG a more complicated spectrum was observed9' as shown in Fig. 23. The spectrum consists of the four components I, If, ]II and IV as shown with stick diagrams. The decomposition of the spectrum was based on temperature variation experiments.9' Detailed analysis of the components l and II leads us to a conclusion that these components are attributed to the radicals "CH2-O-CH2 ~ and 'CH2-CH2-O ~ respectively.These are formed by mainchain scission of PEG. The third component is the spectrum of the oxy radical, "O-CH2-CH2 ~. The fourth component is ascribed to the radical ~OCH2(~H-O,,~, which is generated by a secondary reaction (hydrogen abstraction by the primary mechanoradicals). Remembering the high trapping rate 86 of PMNB, one can imagine that the secondary reactions caused by the primary mechanoradicals occur in times of the order of 1#sec in the liquid phase.

MECHANOCHEMISTRY OF POLYMERS

~'-MeSt-St Copolymer

obs.r.t

I

Mn 2+

II

'

487

I

,

Mn2+

, ~a.-~ aH----~

FIG. 24. Observed ESR spectrum of copolymer of ~-methylstyrene and styrene after high speed stirring at room temperature. I and II are the stick diagram representations of the components.

It seemed interesting to study the scission of a copolymer under high-speed stirring, because it was completely unknown as to whether a copolymer chain would be preferentially ruptured in a particular site or not. The trial was carried out for a copolymer of c~-methylstyrene (c~-MeSt) and styrene (St) by using the same experimental method. 9~ The result is shown in Fig. 24. The observed spectrum can be decomposed into two components, a triplet (I) and a tripletriplet (II). In analogy to the identification of the mechanoradicals of PMMA, the triplet, the spectrum from the tertiary carbon radical, can be ascribed to the radical CH3

i 'C --

C H 2 ,',-.

I The other component, the triple-triplet, can be assigned to the radical CH2

I CH 2-C-CH~

~.

f In the case of a copolymer of c~-MeSt and St there are four possibilities of different scission sites as shown in Fig. 25. In this diagram, head-to-tail repeating units are assumed. The radical species which would be produced by a scission at site 4, that is, between c~-MeSt units, agree with the experimentally identified ones,

488

J. SOHMA 1

2

3

r,.., C-CH2-C-CHc--C-~CH2-c ,---,

4

1) St scission

'~-C-CH;t" i Ph

2) St-MSt s¢ission

"C-CHz "~ I Ph

~

C,H3 .CH -C

Ph

3) St-MSt scission

Ph

I~ ~-" C-CH 2" Ph

4) MSt scission

CH3 'C --CH2 N

I~h

CH.1

CH3

,--CH2-,C- "CH2--C

"--"

Ph 5) Other

Ph

head to head

structure FIG. 25. Structure of copolymer of styrene and u-MeSt with four different possible sites for main-chain scission.

CH3

CH3

I

I

"C--CH2",~

and

I

"CH2-C--CH2"~.

I

No radicals can be detected which can be ascribed to scissions at the three other different sites. Thus, it is concluded that main-chain scissions of this copolymer occur not randomly but preferentially in one homopolymer sequence. However, we are not able to say whether such preferential scissions happen in other copolymers also. A summary of the mechanoradicals detected in solutions of different polymers stirred at high speed is given in Table I. 5. M O L E C U L A R

APPROACHES

TO F R A C T U R E

OF POLYMERS

Many studies of mechanical fracture of polymers were carried out mainly for practical purposes, for example, determination of the tensile strength of a commercial polymer under certain conditions. Of course, it is very important to find out how much load leads to failure of a solid polymer as an industrial material. However, theoretical studies, even from a phenomenological view point are far fewer in number than the experimental studies. One of the dif-

MECHANOCHEMISTRY OF POLYMERS

489

TABLE I. R a d i c a l species, hyperfine splitting c o n s t a n t , g-value and relative c o n c e n t r a t i o n ratio at r o o m t e m p e r a t u r e for p o l y m e r spin a d d u c t s Polymer

T r a p p e d radical

g-value,

a~'/G

a"/G

2.0062

13.3

2.0063

13.3

8.3

1.0

2.0063 2.0064

13.8 13.4

4.3 8.5

3.0 1.0

2.0063

13.4

2.0063

13.0

9.2

2.0

2.058

13.7

6.4

1.0

2.059

13.7

8.1

4.0

2.0059

13.7

2.0060 2.0060 2.0060 2.0061

13.7 13.7 17.6 13.0

8.7 11.0

12.0 3.0 1.0

2.0063

13.6

2.0063

13.2

C o n c e n t r a t i o n ratio

CH3

I PMMA

C -CH 2~

1.5

I COOCH 3 CH~

I I

"CH 2

C~ COOCH3

PS

" C H P h CH~ ~ "CH 2 - C H P h ~ CH 3

I

Poly-c~-MeSt

C-

I

CH 2~

1.0

Ph CH3

I

"CH 2 - C ~

I

COOCH 3 PVAc

CH - CH 2 ~

I OCOCH 3 "CH2-CH ~

I

OCOCH 3 CH 2 -C-CH

2~

|

OCOCH 3 PEG

~-MeSt-St copolymer

"CH 2 - O ~ "CH2CH 2 - O ~ "O-CH2CH 2 ~ ~OCH 2 -CH-O~ CH 3 ] " C - CH2 ~

5.4 1.0

I

Ph CH 3

I "CH 2 - C ~ I Ph

9.4

1.0

490

J. SOHMA

ficulties which hinder the theoretical treatment of fracture is probably the statistical nature of fracture in general. 92 Most of the experimental quantities relating to the phenomenon of fracture in polymers are not obtained as unique values but rather scatter in a wide range. This characteristic of fracture strongly suggests that fracture in polymers is a structure-sensitive phenomenon and that fluctuations in either the molecular structure, the crystalline structure or an amorphous phase may play an important role in the fracture, especially in the initiation of fracture. The statistical nature of fracture is discussed in detail in Kausch's book. 92 A theory, which is widely accepted to explain fracture on a phenomenological basis, is the Griffith theory. 93 In recent decades several researchers have proposed molecular models to explain the fracture of polymers. In particular, the physical aspects of the fracture of solid polymers were extensively discussed by Kausch in his book Polymer Fracture. In this survey the author should like to confine himself to the introduction of molecular models to explain chain breaking or radical formation. 5.1. Griffith's theory 93

One argument to explain the statistical nature of fracture is that a fracture starts from a microcrack produced by stress concentration in the vicinity of a flaw, pre-existing with the distribution of flaw sizes being very wide. Microcrack formation creates a new surface area, which increases the energy content of the material because of its surface energy. The increased surface energy is provided by a decrease in the elastic energy concentrated in the vicinity of the flaw. If the increased surface energy caused by crack formation exceeds the build-up of elastic energy, the creation of new surfaces will not occur in the system and the micro-crack will not propagate into a macroscopic fracture. In this case no break of the material results from the formation of the microcrack. However, the energy required for the formation of new surface in the microcrack is continuously supplied by the elastic energy stored in the vicinity of the flaw, the microcrack will propagate from the flaw and finally become a crack of macroscopic size. This ends in fracture of the material. Based on the argument mentioned above, Griflith equated the increase of the surface energy YcdA resulting from the creation of infinitessimal fresh surface dA to the decreased d U of the elastic energy stored. Then, dU/dA

=

~,,(dA/2a)

(5.1)

where 7, is the surface energy density and 2a is the length of the crack as shown in Fig. 26. The critical stress is derived from eq. (5.1) under the condition that an axial stress is applied to a plate having an elliptic flaw of size 2a. 6"

=

(27,E/rca) '/2

(5.2)

MECHANOCHEMISTRY OF POLYMERS

491

~ress

L stress

FI(;. 26. Grilfith model (pre-existing microcr;lck in a conlinut~us m~lterial)

where E is the elastic modulus of the material. This result indicates how the flaw size determines the critical stress of material, above which cracks propagate and result in a failure of the materials. This explains also the fact that the experimentally determined strength is much smaller in most materials than the theoretical strength of the flawless materials. 5.2. An(h'cw's generalized theory 94:'5 Andrew extended the Griffith theory to a generalized form, which links the phenomenological theory to the molecular approach. This is our primary concern in this survey. Andrew, at first, proposed a simple modification of the Griffith theory as follows. 9495In the limit of infinitesimally small void size, the quantity "'a'" in eq. (5.2) should be replaced with a = d, where d is the interatomic spacing. Moreover, the surface energy density 7, should be taken as half the energy needed to break a unit area of interatomic bonds across the fracture plane. Then, Griffith's theory is extended to the atomic scale and the quantity 7, should be an explicit function of the strength Of the atomic bonds in a material. Based on this idea Andrew proposed the nomenclature "'surface work" instead of "'surface energy". He defined the surface work as a function of surface energy density ;'~, crack speed, temperature, and strain in his generalized theory. ~'4 However, it is difficult to relate the surface work in a discernable manner to the physical properties of the solid and this must be treated as an empirical quantity. 5.3. Rate process approaches to fracture According to Eyring's rate process theory, % the flow rate between two states, 1 and 2, shown in Fig. 27 is determined by an activation free energy e,0. The number of molecules which pass over the barrier per second, K, is described by Ko =

J,(kT/h)(F+'/F) e ,:,~kT

where t,- is the transmission coefficient, F and F ~ are the partition functions at

492

J. SOHMA

u n ~

LIJ (1)

(21 d

k

)1

dithe stance alio direct onng of appliedstress FIG. 27. Two-site model for rate process approach for a fracture site: (I) bond intact, (2) bond broken. Thin line represents the shift in the relative energy levels of the two states under a strong mechanical stress. The degree of shift of the energy levels is proportional to the applied stress.

the bottom and at the top of barrier, respectively, h is the Planck constant, k is the Boltzmann constant, and T is the temperature. If the energies are equal at the two sites, I and 2, as shown by the bold curve in Fig. 27, the rate in either direction is identical and there is no net flow in this case. One of the fundamental assumptions of this rate process theory is that any movement of molecules is caused by thermal motion and that the entire rate is governed by a stochastic process. If a mechanical stress t7 is applied to a system along the direction shown in the figure, the energy of state l is increased by a • q • d/2, where q is the cross section normal to the direction of stress and d is the separation between the two states. The energy of state 2 is decreased by the same amount. Thus, the rate of flow from state 1 to state 2 is increased from K to K~ K,

=

Ko exp \ 2kt J

(5.3)

and the flow rate in the opposite direction is decreased to K 2. 1£2 =

K,, exp ( - c ~ q d / 2 k T ) .

(5.4)

Then the net flow rate is Kn

=

K~ -

K2 =

2k,, sinh ( a q d / 2 k T ) .

(5.5)

MECHANOCHEMISTRY OF POLYMERS

493

Since aqd/2 is half the difference in elastic energy w between the two states I and 2 and the net flow rate is described in terms of the elastic energy w, K,, =

2K, sinh (w/2kT).

(5.5)

Tobolsky and Eyring q7 were the first to apply this rate process theory to the fracture of polymeric materials. They treated the breaking as a slipping of secondary " b o n d s " (such as the Van der Waals force). This slippage was discussed in terms of the rate process theory. The decrease in number of bonds per unit time was assumed to be proportional to total number, N, of bonds per unit area perpendicular to an applied stress cr dN dt

-- k , N

(5.6)

where k, is the rate constant described by eq. (5.6). Since the elastic energy stored per bond is ~d/N in this case, the rate constant is k, =

2k~ sinh (6d/2NkT).

(5.7)

In the limit of large stress, with extremely high energy difference between states, the flow rate from state 2 to state I becomes negligible and the rate expressed by eq. (5.7) is approximated by a simple exponential function like eq. (5.4). -dN/dt

=

K,, exp ( a d / 2 k r ) N

-dN/N

=

exp (--o~d/2kT -

(5.8) 1/N) dt.

(5.9)

By integrating the equation from the initial value ao)./2N~kT to infinity one can obtain a relation which express the time to break t~ as a function of molecular parameters such as the average stress per bond. The integral of the left hand side of the equation is approximated by an exponential function, and therefore the logarithm of the lifetime is proportional to the elastic energy w. This linearity in the semilog plot is widely observed in the fracture of polymers, q2 5.4. Zhurkov-Bueche model Zhurkov 9s and Bueche q~ ~03 independently applied the rate process theory to studies of mechanical fracture. They assumed a two-state model in which the initial and the final states are taken as unbroken and broken bond states, respectively, and the rate from the initial state to the final is estimated on the basis of the rate process theory. Naturally, the unbroken bonds comprise the normal and lower energy state and the broken bond state is higher in energy approximately by the bond energy as shown by the thin line in Fig. 27. The energy barrier ~:o separating the two states is so much higher than k T that the probability of breaking a chemical bond by thermal agitation, that is the rate from

494

J. SOHMA

state 1 to state 2, is actually negligible because of its exponential dependence on e o / k T . If the stress applied to a solid polymer is concentrated on a particular

chemical bond, the energy of this bond is increased by the elastic energy, which is proportional to the concentrated stress a. The energy contour is then qualitatively displayed by the thin line in Fig. 27. Then, the rate constant Kb for bond fracture can be described as follows: Kb ---- K f e x p [--(~0 -- C a ) / k T ]

(5.10)

where Kf is the molecular oscillation frequency and C is a proportionality constant.The stress acting on the chemical bond is different from the bulk stress a 0 applied macroscopically to the polymer studied. Bueche ~°t'1°2estimated the microscopic stress acting on the bond based on the assumption that the applied stress is supported by the entanglements of polymer chains. He found the local stress a to be proportional to the bulk stress a0 with the proportionality constant C equal to the bond length. Zhurkov 98 used the bulk stress G0 in eq. (5.1 0) instead of the local stress, but he took the proportionality constant C to be a molecular parameter sensitive to structure. Thus, Zhurkov and Bueche gave different meanings to the parameters in eq. (5.10), but the rate of fracture has an exponential dependence on the applied stress in either case. Experimentally, the time t b to fracture of a material is easily measured and this t b must be inversely proportional to Kb, tb

-----

to exp [~0 -

C' • ao]kT

(5.11)

where to is the inverse of the molecular oscillation frequency and C' is a constant depending on molecular parameters. This equation indicates that the logarithm of the lifetime to is proportional to the applied stress and the gradient of this linear dependence is inversely proportional to the temperature. This expectation is supported by the experimental results. 99'~°°'~°4The parameters to, e0, and C' in eq. (5.1 l) were discussed in detail in Kausch's b o o k J °5 The relationship between the Griffith theory and the rate process approaches was discussed by Andrew and Reeds. 95 The energy difference between the two states A~ = ~2 - e~ is positive in the absence of stress, because the energy of the broken bond state is higher than that of the unbroken state. Then, no net flow from state i to state 2 may occur. When stress is applied, energy of the unbroken bond state is increased by the stored elastic energy C a under the concentrated stress and the energy difference Ae then decreases. I f Ae becomes zero, then the net reaction ! ~ 2 may occur. This means that there must be a minimum stress to break a chemical bond. This is what the Griffith theory predicted. Moreover, the condition Ae < 0 is more favourable for breaking bonds. Actually, bond breaking proceeds when the energy level of state 1 is elevated by the stored elastic energy much higher than the level of state 2. In such a situation the excess energy is released by the forward process 1 --* 2 and this excess energy is spent to generate more surface and cause propagation of the crack in the sample. This

495

MECHANOCHEMISTRY OF POLYMERS

is also what the Griffith theory assumed. It is interesting to find a good correspondence between the Griffith theory, which is a phenomenological approach in essence, and the rate process approaches, which are based on the molecular model. Hsiao-Kausch, ~°6~°7combined the rate process theory with a theory of deformation based on a model of rod-like elements. 5.:5. Peterlin model~°8 It has been shown that stress concentration plays a crucial role in the fracture of solids. The simplest molecular model assumed for the stress concentration in a solid polymer involves tie molecules which bear the load in the polymer. This model was proposed by Peterlin ~°4'~°8"~°9and discussed more quantitatively by Kausch. v2 The model is schematically illustrated in Fig. 28. The force F,, per molecule in an ideal crystal lattice oriented with the c axis, along which a stress is applied, is described by Aa, where A is the average cross section of one molecule expressed by the inverse of the number of polymer chains per unit cross-sectional area, n,.. The force F~ acting on a tie molecule is Aa multiplied by a stress concentration factor m: F~ = mAG. The stress concentration factor m is the consequence of two facts: (1) there are fewer tie molecules spanning a crack (or a purely amorphous region) than chains piercing a typical crosssectional plane of the lattice and the surface density of the tie molecule should hence be written ~n,. (fl ~ 1) instead of n,,, and (2) the length, the number and their distribution of tie molecules are subject to large fluctuations, as shown in Fig. 28. In this model the crystalline blocks are assumed to be sufficiently strong that they remain unaffected by the stress field. At a given strain the shortest tie molecule (A in Fig. 28(a)), which is stretched to its m a x i m u m length, bears most of the load and the stress is highly con-

(a)

t

(b)

t

(c/

t

I',

f,I FIG. 28. Peterlin model.

496

J. SOHMA

A\

l

t

\ \ S \

L

FiG. 29. Tie molecules between microfibrils. A and B are fully stretched and carry almost all the load.

centrated on this particular tie molecule. Then, at the moment of further elongation the tie molecule A is broken and the other tie molecule B being fully stretched will be broken at the next moment (Fig. 28(b)). This model is easily modified for the microfibrillar structure of a highly oriented polymer, as shown in Fig. 29. There exist inhomogeneities in the strain field in the microfibrillar structure and the stress is primarily born by the fibrillar tie molecules. The shortest tie molecules (A and B in Fig. 29) are stretched to breaking-strain first, while the rest remain well beyond this limit and intact. In the case of unoriented spherulitic samples, this Peterlin model is applicable to the tie molecules connecting spherulites. Brittle fracture occurs.along a path of minimum resistance. That is, cracks in a spherulite structure propagate along the boundary between adjacent spherulites, mainly by breaking the van der Waals interaction between the spherulites. Then, no chain breaking occurs and no mechanoradicals are generated as long as only van der Waals bonds are ruptured. However, it is known that there are generally very few tie molecules between spherulites. Thus, cracks can propagate between spherulites while breaking very few tie molecules. This argument leads to the conclusion that there is much less mechanoradical formation in an unoriented spherulitic sample than in a highly oriented sample following similar inputs of mechanical energy. Backman and DeVries j~° reported less production of mechanoradicals from unoriented polyethylene than from other polymers fractured at low temperature. The Peterlin model is also supported by other experimental results. 2j First, the number of broken chains depends exclusively on the maximum strain reached. Second, new radicals are formed as the strain is increased beyond the former maximum strain in the experiments involving unloading and restressing. Third,

497

MECHANOCHEMISTRY OF POLYMERS

the mechanoradicals showed less anisotropic ESR spectra than those expected R)r radicals formed within a crystal lattice, even in the oriented sample. These radicals are believed to be formed in the amorphous domain in which tie molecules are present. However, the Peterlin model fails to give quantitative agreement with experimental data for the radical concentration. 5.6. Microcrack i[brmation initiated hv meehanoradicals It is believed from the above rate process approaches that the molecular process of polymer fracture proceeds as follows. The stress concentrated on chemical bonds reduces the energy requirement for bond scissions from U,, to U,, - C • o. The bond scissions are then caused by thermal fluctuation. In other words, the activation energy of bond breakage (see Fig. 27) is supplied by the fluctuating thermal energy of molecular motion. Suppose a particular polymer has sutlScient thermal energy to supply the activation energy for the path 1 2 and then the chain is broken to produce a pair of mechanoradicals (RI and R~} (Fig. 30(a) and (b)). Naturally, the radicals (RI and R_;) generated by the main(a)

(b)

RI--R2--*

(c

~d)

R~H + R2H

RI H + R2 H -4-

R--CH=CH2 + 2 R'

2 (R--~H--CH2--R') {2) × •

end radical chain radical stable end

(e)

--',. R, H + R~H n ( R - C H = CH2) + 2 R'

R ~ : R--CH 2 -Rz : RLCHz - -

{Zhurkov et at I It-113) H H ® stands for the end radicaL, ~ C - - C °, H H H • H x for the chain radicaL, ~ C - - C - - C ~ a n d • for" the stable vinyl end of a polymer chain H H H Zhurkov mechanism,

FIG. 30. Zhurkov model. (a) Chain breaking, (b) primary formation of mechanoradicals (end radicals), (c) hydrogen abstraction by the primary radicals and formation of chain radicals tt

H

!

I

c-¢-c tf

H

,

H

(d)/J-scission of the chain radicals with formation of a double bond and another end radical, (e) void formation after many cycles of the reactions.

498

J. SOHMA TABLE 2. E x p e r i m e n t a l d a t a s u p p o r t i n g Z h u r k o v model Polymer

Submicrocrack concentration ( U / c m 3)

Radical concentration ( N / c m 3)

Broken b o n d concentration ( U / c m 3)

Polyethylene Polypropylene Polycaprolactam

6 X 1015 1 × 10 ~5 5 × 10 ~5

5 × 1015

9.9 x 1018 2.5 x 10 3

3 X 1016

chain scission are chain-end radicals. These end radicals abstract hydrogen atoms from neighbouring polymers to stabilize themselves (R~ H and R2H) and to produce two nonterminal radicals (R-CH-CH2-R") on the H-abstracted polymers (Fig. 30(c)). The chain radical undergoes//-scission (R-(~H-CH2 ~ R') to generate both a vinyl-terminated chain R-CH--CH2 and another end radical R' (Fig. 30(d)). By this hydrogen abstraction followed by/3-scission, the primary mechanoradicals convert into two shortened vinyl-terminated polymer chains and two more end radicals. Then, the cycle repeats the two end radicals reproduced again after forming two more chain ends with double bonds. This is a kind of cyclic reaction and many terminal double bonds can be formed from each pair of the primary mechanoradicals. Thus, a microcrack is generated after multiple cycles of this series of reactions, as shown in Fig. 29(e). This reaction scheme was proposed by Zhurkov and his collaborators.r~ ~t3 Since the microcracks in a solid polymer can be detected by small angle X-ray scattering, the detection of microcracks formed in polymers under mechanical load was successful and the concentration of microcracks was found to be as high as 10 ~2-1017c m - 3 . Formation of the terminal double bonds can be experimentally verified by IR spectroscopy. The concentration of double bonds could be determined from the observed differences in IR absorption in the C = C bands, for example 910-965 nm i for R C H = C H R ' , between undeformed and fractured specimens. The concentration of free radicals can be estimated from the intensity of the observed ESR spectrum. All these measurements were carried out by Zhurkov and his collaborators for polyethylene, polypropylene and polycaprolactam. ~j2 The results are tabulated in Table 2. The concentration of microcracks is nearly equal to that of mechanoradicals - both in the range of I 0 ~5per c m 3 . This equality corresponds to the hypothesis that one microcrack is generated from a pair of the primary mechanoradicals. The concentration of chain end double bonds is much higher (by three orders of magnitude) than that of mechanoradicals. This fact supports the mechanism, in which many double bonds are produced by the cyclic reactions started by a single mechanoradical, with each reaction repeated about 10 3 times to form thousands of double bonds in each microcrack. The set of experimental results shown in Table 2 is in accord with the Zhurkov mechanism, which assumes that

MECHANOCHEMISTRY OF POLYMERS

499

a chain reaction is initiated by a pair of the mechanoradicals produced by a chain scission and the microcrack is generated by the cyclic reactions. One may imagine various modifications to this mechanism. For example, if main-chain cleavage occurs in the presence of oxygen, the stable chain ends may not bc methylene groups as in Fig. 29 but rather carbonyl groups. In this Zhurkov mechanism two things are impicitly assumed; one is the formation of an initial pair of mechanoradicals and the other is that these reactions occur in the crystalline region. The former assumption was experimentally verified for polypropylene (Section 3.3). The latter is not explicitly mentioned by Zhurkov but the reactions should be restricted to the crystalline regions of the polymers. This is because only microcracks formed in the crystalline regions are detectable as cracks. F.ven if the cyclic reactions do proceed in an amorphous region of a polymer, the void formed in the amorphous region can hardly bc discerned from other voids originally present in the amorphous region. In addition to this, the series of rapid successive hydrogen abstractions assumed in the model can occur only in a region where adjacent molecules are tightly packed and easily accessible to a radical formed on an adjacent molecule. Such a condition is scarcely considered possible in an amorphous region but is found in a crystalline zone. Thus, the Zhurkov mechanism is applicable preferentially to a chain scission in a crystalline region. The Zhurkov model indicates that the mechanoradicals generated in a highly strained polymer result in void formation in a crystallite. The newly formed void in the crystallite may cause stress concentration nearby and a decrease m mechanical strength of the polymer. Then, mechanoradicals are more easily produced under the same load and more voids are formed in the sample. In such a way, breakdown of polymer chains may be accelerated by microcrack formation. Detailed and quantitative studies of this acceleration of cleavage of polymer chains seem to be interesting and should be explored from the viewpoints of both mechanochemistry and chemorheology.

5.7. Critical de,wee qf poD,merizalion Jbr mechanicalh'-induced chain scission It has been mentioned in Section 3.2 that formation of mechanoradicals is believed to be a characteristic feature of polymers. Many low molecular weight organic salts, such as monomers corresponding to most common polymers, were ball-milled or sawed at low temperatures. No ESR spectra could be observed from these fractured low molecular weight compounds so t:ar as was tried. This result shows that at least mechanoradicals stable enough to be observed were not produced from the frozen low molecular compounds. This result is not surprising at all because a crack propagating along the path of least resistance may proceed in the solid by breaking only van der Waals forces among the organic molecules. No scissions of covalent bonds are required in the destruction of such systems. We are convinced, therefore, that mechanoradical formation

J. SOHMA

500

i I

! I

I i

I I

A

B

n monomers

FIG. 31. C o o p e r a t i v e m o t i o n of the m o n o m e r units in two p o l y m e r s A and B in shearing motion. I n t e r a c t i o n between the m o n o m e r units is the van der W a a l s energy, E v .

occurs only for polymer materials. That is, only polymer chains having molecular weights higher than a certain critical size are broken by mechanical actions. A question to be answered is how long the critical size is. A systematic study was carried out by Sohma et al. to find out the critical molecular weight for chain breakage 2~ of polyethylene. A series of n-paraffins, having different molecular weights as well as polyethylene with different degrees of polymerization, were prepared and all these were ball-milled under the same conditions, and deaerated at 77 K. No ESR spectra were observed from the n-paraffins or from PE of degree of polymerization 71. ESR spectra were observable for polyethylenes with degree of polymerization larger than 100. No sample was available with degree of polymerization between 71 and 100. The critical degree of polymerization for mechanoradical generation was therefore experimentally determined as between 71 and 100 for polyethylene. Let us propose a simple model for estimating the critical degree of polymerization for chain cleavage. 34"7Ductile fracture of a solid results from a large shearing motion of constituents, such as grains or crystallites or molecules. In the case of a low molecular weight solid, the shearing motion of each molecule is independent and a crack may grow by ruptures of the van der Waals bonds among molecules present in regions of stress concentration. This may be the reason why no mechanoradicals are formed in solids consisting of low molecular weight compounds. In a polymer solid the situation is different from that in a solid composed of small molecules. A large shearing displacement of a single polymer chain, which leads to macroscopic fracture, requires the simultaneous and incorporated motion of each constituent monomer unit, which corresponds to each single molecule in the low molecular weight solid. This simultaneous shearing motion of polymer chains is schematically displayed in Fig. 31. Thus, such shearing movement in a polymer requires rr ~re energy than that needed for a molecule of low molecular weight. Let us take the activation energy, E,, of viscosity of a low molecular weight compound as the approximate measure of the energy, Ev, required for shearing

M E C H A N O C H E M I S T R Y OF POLYMERS

50l

motion of a single monomer. The energy needed for the shearing displacement of a polymer molecule is then expressed as nE,~, that is the energy E,~ multiplied by a number of monomers units, n, moving simultaneously. This energy nE,~ may exceed the bond energy Ec c of a c a r b o n - c a r b o n bond if the number of ,.simultaneously moving monomer units is large enough. Then, the critical degree of polymerization n~ is estimated by equating t'/~-E,~ =

Ecc

(5.12)

based on the postulate that a crack proceeds along a path of the least energy. The amount of energy needed for scission of a carbon-carbon bond is less than that for the simultaneous movement of either a molecule or a molecule segment longer than the degree of polymerization, n~. In this model, an assumption of equal interaction energy E~ over n monomer units is introduced for simplicity. This assumption is schematically illustrated by parallel movement of n monomer units in Fig. 31. In order to estimate the critical degree of polymerization nc for comparison with the experimental value obtained for polyethylene, the activation energy for viscosity must be known for ethane. which corresponds to a monomer unit of polyethylene. No experimental data on the activation energy is available for solid ethane at 77 K. It is an empiricai rule, ~14 however, that activation energy of viscosity is approximately !/2.45 of the molar energy of evaporation. Relying on this rule and the experimental value of molar energy of evaporation of ethane (2.51 kcal. mol L),~ E, 7 for a monomer of polyethylene is approximated as (1/2.45) × (2.51 kcal. mol ') = 1.01 kcal. mol ~. Inserting this value and Ec c, the bond energy of a c a r b o n carbon bond (83.1 kcal. mol ~) into eq. (5.12), one obtains roughly 83 for no. "l-hus, it is concluded from this model that polyethylene chains having a degree of polymerization larger than 83 can break their chains in preference to multi-breaking of many van der Waals bonds. In such a case the cracks may propagate by breaking C - C bonds, producing mechanoradicals. Because of the crudeness of the model and involved approximation, the value estimated above should be regarded as simply a rule of thumb. And then the critical degree of polymerization calculated from the model is approximately 80. This value agrees fairly well with the experimental value above which mechanoradicats are produced. The argument presented above helps to understand the reason why bond fractures were not observed for the paraffins, whose carbon numbers are less than 160. Therefore, one may be convinced of the validity of the model. The model was explained in terms of a ductile fracture, but this model is not restricted to this kind of fracture. In the case of brittle fracture, this model may apply if one considers that a bond rupture consumes less energy than that required for the simultaneous separation of all the van der Waals bonds between two chains each containing more than 160 carbon atoms. The fracture of polymers caused by milling at low temperature is presumably brittle and expert-

502

J. SOHMA stress

stress

t

Rn

crysta Ill te

crystallite

I

L

stress Stress

FIG. 32. Unfolding of folded chain pulled by fully stretched tie molecule. Fully stretched tie molecules may possibly unfold in this manner so as not to result in a chain break.

mental results obtained by the ESR studies at 77 K are concerned with brittle fractures. The comparison of the results derived from the model with the experimental results is still reasonable. In the Peterlin model (Fig. 28) the stress is concentrated on the particular tie molecule which will be next to rupture. This model is based on two assumptions; the first is the finite ability of a tie molecule to stretch before breaking and the second is strong anchoring of the tie molecule in the crystallites. The second assumption was not clearly mentioned by him. If the anchoring is not strong enough, the tie molecule pulls out the segment in the crystallite as the bulk strain of the sample increases (see Fig. 32). No chain breaking occurs in such a case because the longitudinal slip of single chains in the cystallite may release the strain on the tie molecules. The Peterlin model rests on the assumption (not previously justified) that no slippage of single chain occurs. The model presented in this subsection may provide a good justification for this assumption. The folding period of a crystallite of polyethylene is usually longer than 100A and the number of monomer units in one folding period exceeds the critical chain length in the model. Thus, it follows from the model that the longitudinal slip of a single chain is prevented in the crystallite. This implicit assumption in the Peterlin model is understandable in the frame of the model. The tie molecule, which is stretched to its full length and strongly anchored in the crystallite, is highly stressed. Peterlin assumed that a chain break would occur at a point on the tie molecule some distance between the two crystallites. However, it seems unlikely that the chain breaks exclusively in the

M E C H A N O C H E M I S T R Y OF POLYMERS

503

region between the two crystallites as Peterlin assumed. This is because the high stress is not limited to the portion of the chain which ties the crystallites together. Rather, the stress is along the chain, even through the portion within the crystallites, if the single tie molecule is approximated as an elastic string. If this is the case, it might logically follow that breakage of a fully stretched chain may occur either in the intercrystalline region or within the crystallite. Any carbon-carbon bond in the stressed chain may be thought of as having equal probability to break. If the breaking happens in the tieing part between the crystallite, this is the model proposed by Peterlin. If the chain breakage occurs in the part of the molecule within the crystallite, the mechanoradicals initiate chain reactions resulting in microcrack formation. This is the mechanism proposed by Zhurkov, as described in the last subsection. The critical size o1" molecule needed for chain cleavage plays an important role in both the Peterlin model and the Zhurkov mechanism. The Peterlin model is valid only for <'rystalline polymers, because no tie molecules are considered to exist in amorphous polymers. Experiments demonstrate that polymer chains are also. however, ruptured in amorphous polymers. Mechanoradical formation has been reported for many amorphous polymers, such as PMMA, polystyrene, polybutadiene and others mentioned in Section 3. It is not completely unreasonable to consider that two polymer chains in an amorphous domain can be parallel over a longerthan-critical distance because of the fluctuation in structure of the amorphous phase in polymers. If so, chain scission induced by the fracture of the polymer could be imagined from the model. The other possibility is presumably entanglement of polymer chains in an amorphous region. Importance of chain entanglements is emphasized by Bueche in his studies on mechnical breakdown of rubber. ~L~,..~7In fractures of rubbery materials in which polymer chains undergo micro-Brownian motions and are mobile, both crosslinks and entanglements must be very important. This is because these points cause molecules to resist deformation under shear and they increase the tension on molecules which connect to these points. Then the probability for a chain break at these sites, either cross-links or entanglements, is higher than at other sites because the stress ~ in eq. (5.11) is larger along the chains connected to either cross-links or entanglements than along the other chains. However, in the case of brittle fracture at low temperatures, where most ESR studies have been carried out, the entanglements probably are not so important because mobility of polymer molecules is very low and entanglements are not particularly discerned from other regions of the polymer chains from the viewpoint of molecular mobility. Stress concentrations in an amorphous polymer at low temperature are induced by density fluctuations in the amorphous phase, although the sample is macroscopically homogeneous. At present we are only in a position to speculate about the molecular mechanism of fracture in amorphous polymers at these low temperatures.

504

J. SOHMA

-

• Intact o After stirring

Mc=3xlO s

MoLecuLar

weight

F=G. 33. Change in molecular weight distribution of high density polyethylene after high speed stirring.

5.8. Mechanically induced main-chain scission in the liquid phase That main-chain scission can be caused in solution by either ultrasonic agitation or high-speed stirring can be concluded from experimental data, such as spin-trapping and observed decrease in molecular weight, as described in Section 4. Decreases in molecular weight after ultrasonic irradiation of polymer solutions were found for poly(methyl methacrylate), 78'~18polystyrene78 and poly(vinyl acetate). 78 Similar decreases in molecular weight were induced by highspeed stirring of polymer solutions.~lgJ2° An example ~20of changes in molecular weight distribution, which were detected by GPC after high-speed stirring, is shown for high density polyethylene in Fig. 33. Apparently there exists a critical molecular weight Me, above which molecules are ruptured. In this particular case, Mc is 3 x 105. Similar critical molecular weights were found for other polymers. 119 The molecular mechanism of chain scissions in liquid phase is quite different from that in the solid state, which was described in the former sections. Data on the ultrasonic degradation of polymers in the liquid phase have been accumulated and some theoretical approaches to the chain scission mechanism have been reported. 75'78'79'121 Two direct effects of ultrasonic irradiation on polymer molecules should be considered. A single chain molecule is assumed to be fixed at one end and lying in the direction of the plane of the ultrasonic waves. The frictional force acting on the molecule in the velocity field of the ultrasonic waves was calculated in the following way. 12°'~21 The velocity of the solvent molecules Vo is related to the ultrasonic intensity I by do CV02 1 = ~-

(5.13)

where do is the density of the solvent and C is the velocity of sound. The

M E C H A N O C H E M I S T R Y OF POLYMERS

505

frictional force f0 is given by the Stokes equation f~

=

P,6rctlrV o

where P,, is the degree of polymerization, r is the radius of a m o n o m e r unit and ~l is the viscosity coefficient. Taking the ultrasonic intensity I as 15 W cm -" and inserting reasonable values such as do = l, C = 1.2 × 105cmsec L, p,, = 3 × 103 , r = 3A_,and~/ = 6.2 x 10 3poise, then V0becomes 51.1cmsec and f~ = 5.37 x 10 4dyne. The other direct effect is the frictional force caused by the motion of polymer molecules relative to the solvent molecules. The relative velocity Vr is given by Schmid t= >; -

P, rno) P, 67tqr

V0

(5.14)

'where m is the mass of a m o n o m e r unit and to is the angular frequency of the ultrasonic waves. The frictional forcef~ resulting from this second effect is then given by .~

=

Po m " o)V~,

(5.15)

11~is estimated as 10 t~ dyne for the given values m = 104, o) = 10 ~'. Since the force necessary to rupture a C - C bond is 5.64 × 10 4dyne, it is clear that the second of these direct effects never causes chain cleavage. The force estimated from the first effect is similar in magnitude to the force needed for the bond scission. However, the assumption of the fixed end is too artificial to be believed for a polymer molecule in a liquid phase, Therefore some other mechanism is required to explain the observed chain scission in an ultrasonic field. Mostafa w has shown that the direct effect of a strong ultrasonic wave produces sufficient frictional force to break a C - C bond if entanglements are present in the system. However, it is believed among many researchers vS's° that chain scissions are caused by the cavitation which occurs in liquid medium under intense ultrasonic irradiation, because it was found that the polymer degradation rates are depressed in the absence of cavitation. When a liquid is irradiated at high ultrasonic intensity, with pressure amplitude on the order of 1 bar, small foggy bubbles appear in the liquid. The formation of these bubbles and their subsequent collapse is called cavitation. It is known that the collapse of a bubble exerts a large pressure on molecules in the immediate vicinity of the bubble and also generates a shock wave in the medium. The total energy produced by the collapse of the cavity is rather small but the energy density in the vicinity of the collapsing cavity is very high. The cavitation is considered to be a site where mechanical energy is concentrated just as stress is concentrated at sites in the strained solid.

506

J. SOHMA

The mechanism of ultrasonic degradation is still a controversial problem although cavitation is believed to be the cause of the degradation. Several mechanisms are well summarized in the review written by Basdow and Ebert. 8° One of the mechanisms proposed by Okuyama and Hirose ~23will be introduced. A frictional force, which is induced by the rapid flow occurring at the concentration step of a cavitation bubble, acts on a segment of a polymer molecule situated near the collapsing cavity. The segments stream along a flow line of the solvent directed toward the center of the cavity and are formed into a straight line by the tension. The maximum tension occurs at C0, the center of the straightened section, of the polymer chain. The longer the straightened chain, the larger is the tension. Only when the tension exceeds the ultimate strength of a C-C bond is the bond broken. This condition is described b y j > g, where g is a threshold chain length. During long irradiation each polymer chain may encounter a collapsing cavity and the polymer molecules having degree of polymerization larger than g are all cleaved. Then the final value of the degree of polymerization g is described by

(Z) where Fk is the interatomic force and Zmax is a kind of force constant for the frictional force, which is given by the equation Zmax =

1/2 l ~

(--R)m --R--

ax

(5.17)

in which le is the interatomic distance at zero slippage, ~ is the frictional constant, and (-l~/R)n,,x is the maximum of the shear velocity. Inserting the proper values for these quantities, one can obtain g = 100. This value is closer to the experimental value gcxp = 250, which was determined for polystyrene in toluene under ultrasonic irradiation at 500 KHz at a pressure of 4 bars. This comparison seems to suggest that this fracture mechanism is not far from the truth. Chain scissions induced by high-speed stirring have been little investigated in comparison with ultrasonic degradation of polymers. Systematic research on the degradation of P M M A in stirred benzene solution has been reported by Sohma and his collaborators. ~24 At first it should be stressed that primary main-chain scission has been experimentally verified by ESR combined with the spin trap method, as described in Section 4.3. The decrease in the molecular weight of P M M A after stirring at high speed (14,000 rev/min) was followed with time at room temperature and the results are shown in Fig. 34. Molecular weight decreases with time to a final plateau value which depends on the P M M A concentration in solution. This result demonstrates the presence of the limiting molecular weight above which polymer molecules are degraded and the result is consistent with the conclusion derived from the observed decrease in the

MECHANOCHEMISTRY OF POLYMERS o 20 mg/mL

IT /

0

507

• I0 mg/mL

I

2

I

4

t

I

6

8

10

Time (hr)

FIG. 34. Change of molecular weight of P M M A as a function of stirring time.

molecular weight shown in Fig. 32. The similar time dependence of the degradation rate was also reported for the case of ultrasonic degradation. ~22 Based on this experimental result, one may assume that the rate of degradation is proportional to the fraction of the total chains which exceed the limiting molecular weight, i.e. dJQ dt

k~(~/

-

~,~ )

(5. ~ 8)

where k, is an apparent rate of degradation, bar over M means an average and .M~ is the limiting molecular weight, which is determined as the final plateau value in an experiment. Since the rate constant of chain scission depends more or less on molecular weight and there is a distribution of molecular weights in the sample, the exact rate constant of degradation cannot be obtained by using such a sample with broad molecular weight distribution. In order to estimate the exact rate constant of degradation one has to prepare a monodisperse sample. A complete kinetic analysis, with simultaneous rate equations, should be solved taking account of the variation with time of molecular weight distribution during the stirring. This task is too difficult to be attacked. Hence one is obliged to be satisfied with changes in the averaged values, as shown in eq. (5.18), and the apparent rate constant, which is different from the real and exact rate constant. Although one should bear in mind this limitation, the apparent rate constant serves, at least, as a convenient parameter for a quantitative comparison. Equation (5.18) is integrated as follows: In (M, - M~,~) =

In (h~¢0 -

2Q~) - Kt

(5.19)

508

J. SOHMA

14~~,,ii~ I ~'~.~ IO --

o 20mg/ml • I0mg/ml " ~ I ~

8--

6

1

2

I

4

I

6

I

8

I

I0

Time (hr)

F~G. 35. Semi-log plot of the data shown in Fig. 34. Open circle = 2 0 m g m l 1, filled circle = 10mgm1-1.

where M0 is the initial molecular weight. A semi-log plot of molecular weight is displayed in Fig. 35. Experimental values are represented by eq. (5.19) and the rate constants Ka were experimentally obtained from the plots. The results are shown in Table 3 with their limiting molecular weights. The rate constants are equal within experimental error. This concentration dependence of the degradation rate in high-speed stirring is different from the concentration dependence of the rate of ultrasonic degradation, in which the limiting molecular weight rapidly decreased with increasing concentration, j25-~z9The concentration dependence of the rate was discussed in connection with entanglement of chains for the case of ultrasonic degradation, j26'128 However, the effect of concentration must be very complicated for both ultrasonic and stirring degradation, since entanglement and non-Newtonian flow are interdependent in complex ways. It is interesting to note the fact that the critical molecular weight for the concentrated solution is higher than that in the lower concentration, as shown in Fig. 34. The number of entanglements in a concentrated polymer solution is higher than for a dilute one. The result shown in Fig. 34 indicates that higher molecular weight is required for a chain scission at higher concentration. This means efficiency of chain rupture is lower in the concentrated solution, in which more entanglements of chains are formed than in the dilute one. It is naturally expected that mechanical force is concentrated at the entanglement points and TABLE 3. Concentration mg/ml

K

M~

20 10

0.52 0.48

5.01 x 105 3.80 x 105

MECHANOCHEMISTRY OF POLYMERS

509

O--,---,G

? 0m =

8--

7

--

I

2

I

_k

4

6

I

8

I I0

L

12

II 14

S p e e d x 10 - 3 ( r e v / m i n )

FIG. 36. Dependence of molecular weight decrease (after I hr stirring) upon the stirring speed.

this concentration of mechanical energy leads to chain cleavage. However, the entanglements of the chains also have another effect. That is, coupling between chains is stronger at the entanglement sites and mechanical energy is transferred more efficiently from one chain to another at an entanglement. Thus, it is considered that mechanical energy is more easily distributed all over a system if this system has more entanglements. In other words entanglements increase efficient energy dissipation, which reduces stress concentration on a particular site. Thus it seems to me that the effect of entanglements on chain rupture is not simple and one cannot dash to the conclusion that the entanglements are always the points of origin for chain scission. Dependence of the rate of chain scission on the rotating frequency was investigated and the result is shown in Fig, 36. The molecular weight stays unchanged after l-hr stirring period at rotation frequencies below 6,000 rev/min. Above this frequency decrease in the molecular weight starts gradually. Although the curve seems to level off near the frequency of 14,000 rev/min, which is the upper limit of the rotating frequency in the apparatus used, the levelling off has not been confirmed experimentally. This result demonstrates the existence of a threshold rotating frequency below which no chain scission is induced. Similarly a power threshold was found in the ultrasonic degradation of polymers. 13° No depolymerization occurred at low ultrasonic intensities which caused no cavitation. Depolymerization took place only in the presence of ultrasonic waves which are strong enough to produce cavitation. Thus, it is believed that ultrasonic depolymerization is caused by cavitation, s° The similarity between the power dependence in ultrasonic degradation and the frequency dependence in degradation by stirring seems to suggest that chain scission by high-speed stirring may also be caused by cavitation. Actually, bubbling was a very active process during the high-speed stirring. However, it is hard to discriminate between bubbles containing gases and true cavitation bubbles in this apparatus

510

J. SOHMA 8

6 E %4 n,.

0

I

5

I

I0

Time (hr)

FIG. 37. Comparison of number of chain scissions calculated from the observed

decreaseof molecularweightafter stirringand numberof radicals trapped as measured by observed ESR intensity. and bubbles were always found even at the lower frequency. Although cavitation caused by high-speed rotation seems to be a most plausible cause for polymer degradation, no definite and positive evidence to support this presumption has been obtained. The number of chain scissions per molecule is estimated by _

(s.2o)

where M, means number average molecular weight and the subscript zero indicates the initial valueJ 3. The mean scission number was easily estimated from the observed molecular weight after the stirring. The total number of new chain ends formed by scission was also evaluated by multiplying the mean scission number by mean molecular weight after a certain time of high-speed stirring. This scission is plotted as a function of the stirring time in Fig. 37. In the same diagram the number of chain-end radicals trapped by the spin trap method is also displayed. The number of radicals is smaller than the scission number, as shown in Fig. 37. The scission number should be half the radical number, because one chain rupture produces a pair of radicals. The discrepancy could be explained in two possible ways. One involves the trapping efficiency. If the trapping efficiency is not 100%, the number of chain scissions estimated from the concentration of the trapped radicals must be lower than the number of chain scissions which actually occurred. Another possibility to explain this discrepancy is self-degradation initiated by the primary radical. 4. The primary radical abstracts a hydrogen from a chain which then undergoes main-chain fl-scission, resulting in a degradation of a second chain. This mechanism is a cyclic reaction and therefore many chain breaks can be induced by each primary radical. These two mechanisms are not exclusive and either one or both could

MECHANOCHEMISTRY OF POLYMERS

511

be a reason for the observed discrepancy between the observed loss in molecular weight and the ESR intensity. 6. STRUCTURAL CHANGES INDUCED BY FRACTURE OF POLYMERS It is well known that mechanical fracture of inorganic material affects its crystalline structure. Changes in crystal structure induced by mechanical action are now among the major problems in the mechanochemistry of inorganic materials. ~32 A few examples will be cited as follows. Crystalline CaO changed its crystalline structure from the stable one, calcite, to the metastable one, aragonite, after crushing at room temperature. The structure returned back to the stable calcite form after annealing of the fractured sample at 450°C. k3~ Imperfections in crystalline TiO2 were increased by grinding and the material became amorphous after being ground for 9 6 h r ] 34 Changes in structure and increases in the number of imperfections were found in the surface layer of milled quartz] 35'~36Reduction in the size of crystallites was reported for milled nickel oxide. ~37 Dislocations were generated by fractures in silicon crystals. ~'~ Few studies have been carried out on changes in the crystalline structure of polymers after milling. Crystalline transformations induced by cold drawing of polyethylene L39and compression of polypropylene H° have been reported. 6.1. Experimental methods used f o r studies on structure changes

An X-ray diffraction pattern, which appears as discrete peaks, is observed when an X-ray beam is directed at a crystalline sample. The angles 0, at which the reflected X-ray peaks appear, depend on both the wavelength 2 of the X-rays and the separation d between adjacent planes in the crystal. The relation is known as the Bragg condition for diffraction, 6.1.1. X-ray diffraction -

n2 =

2dsin 0

(6.1)

where n is an integer, I, 2, 3 . . . . . The peak intensity in a diffraction pattern decreases with increased n, that is with increasing order. The crystalline structures of most commercial polymers have been determined by the analysis of observed X-ray diffraction p r o f i l e s . 141"142 X-rays are scattered also by noncrystalline materials, which lack periodic structure, and the profile of the diffraction pattern in this case appears as a very diffuse halo. A sample consisting both of crystallites and non-crystalline regions shows an X-ray diffraction profile which is the superposition of discrete peaks and a diffuse halo. The ratio of total peak intensity to total intensity of the diffraction pattern (peaks plus diffuse halo) is used to estimate the crystallinity of the sample] 43 6.1.2. Determination o f e r y s t a l l i t e size -- For polycrystalline samples the mean crystallite size can be determined from the line-width of observed peaks by using

512

J. S O H M A

the Scherrer equation ~44

thkl --

k2 /~ COS 0

(6.2)

where lhk ! is the mean dimension of the crystallites perpendicular to the plane (hkl), fl is the integral breadth, 2 is the peak position and k is a constant which is commonly taken as approximately unity. The mean dimension thkt is defined as

thkt =

52" dhkl

(6.3)

where 57 stands for the mean number of plane (hkl) corresponding to the dimension and dhk~ for the lattice constant of the plane (hkl). Although the Scherrer equation is convenient for estimation of a crystallite size in terms of thk~, one should remember that the equation is based on the assumption of a perfect crystal having no imperfections. This assumption is actually far from true for any crystallite in a polymeric material, because the imperfection density in polymer crystals is rather high and it is known that the imperfections also cause substantial broadening of the reflection peaks. Strictly speaking, therefore, numerical values determined by the Scherrer equation are not reliable for a polymer sample. Even so, thk~ is still an experimentally determined quantitative parameter describing crystallite size and one may use this value for comparison, bearing the restriction in mind. There is also another method to estimate crystal size, which was developed by HosemannJ 44 Hosemann introduced fluctuations in both lattice dimensions and electron distribution for crystalline polymers, and he proposed a paracrystalline model. ~45He classified distortions as being of the first and the second kinds. The first kind is considered as deviations of locations of monomer units from their lattice sites, in other words as thermal fluctuations in displacement. Distortions of the second kind are characterized by a loss of long-range order in a crystal. Each lattice point varies in position in reference not to the sites of an ideal lattice but rather to its nearest neighbours. A crystal having a distortion of the second kind is rather similar to the structure of liquids as an extreme case. Hosemann m derived an equation to estimate both the crystalline size and a parameter describing the second kind of distortion from the paracrystalline model, which describes the behavior much better of real crystallites in polymer samples than does the Scherrer equation. The Hosemann equation is expressed by

fie =

1/tZkl + (1~g)4n4/dZkl

(6.4)

where the second kind of distortion in crystallites is represented by a parameter g and n is the reflection order appearing in the Bragg equation (6.1). The parameter g is defined as Aa/fi, where Aa is a deviation of plane distance and a is the mean distance between the lattice planes. Both the size and degree of

MECHANOCHEMISTRY OF POLYMERS

513

distortion can be experimentally determined from the gradient and the intercept of the Hosemann plot, in which the square of the line-width 13is plotted against n4. The Hosemann equation is more reliable than the Scherrer equation for analysis of the X-ray diffraction profile of real polymer samples, because the model used in the derivation is much more appropriate to real polymer sample behavior than the assumption for the Scherrer equation. However, in a practical sense the Hosemann plot has the defect that accuracy is limited by poor signal/noise ratio of the higher order diffraction peaks as well as by the limited number of data points. Therefore, one should not rely solely on either equation but rather deduce a conclusion based on various experimental results. 6.1.3. Different&l radial distribution function (DRDF) - The radial distribution functionf(r) of a system of N particles is defined by the statement that number of molecules which are separated by a distance r is (N2/2 V)f(r)47zr 2dr, where V is the volume. In a crystalf(r) is periodic as shown in Fig. 38. The differential radial distribution function (DRDF) is the difference between the radial distribution function and the mean density. Spherical shells with lattice points on the surface, such as r~ and r3 (Fig. 38(a)) have a positive deviation from the mean density. A spherical shell with lattice points just outside its surface, such as r2, has a negative deviation from the mean density. The deviations become periodically positive and negative with increasing radius and the amplitude increases with larger radius because more particles are involved in a spherical shell with larger r. However, if the periodicity is lost at longer distances, the D R D F then tends to zero with increasing radius, because the distribution is averaged out to the mean density in the random distribution. The periodicity of the D R D F is schematically illustrated in Fig, 38(b). Therefore, the D R D F is very useful to find out how far the ordering of the space distribution persists. One may use the D R D F to distinguish a crystalline structure from an amorphous phase. The D R D F is experimentally obtained by a Fourier transform of the X-ray diffraction profile. For details of the analysis one is referred to the literature. ~4~ 6.1.4. Nuclear Magnetic Resonance ( N M R ) for determination of crystallinity The proton N M R spectrum observed from a solid sample is broadened by the dipolar coupling, which is expressed as a tensor. ~49 In the case of either polycrystalline or amorphous samples, the angular dependence of the dipolar interactions is so dispersed that a superposed line shape is observed. The superposed line appears very broad and the line-shape is expressed by a Gaussian function (Fig. 39(a)). If the molecular motions of carriers of protons, such as skeleton motion of polymers, are greatly speeded up, then the orientation dependence of the dipolar couplings will be averaged out due to random rotational motions. Then the N M R line width will be narrowed for mobile protons and the motionally narrowed line-shape can be represented by a Lorentzian curve. (Fig. 38(b)).

514

J. SOHMA

Differential Radial Distribution Function (DRDF)

(b) DRDF ÷

~r

FIG. 38. Schematic explanation of meaning of differential radial distribution function (DRDF). (a) Relation between crystal dimensions and radial distribution, (b) differential radial distribution function.

From a sample in which immobile protons and mobile protons coexist, the line-shape appears as a superposition of the broad component and the narrow one, shown as the solid line in Fig. 39(c). When such a superposed spectrum is observed from a sample, the observed spectrum can be decomposed into the two components, the broad (B) and the narrow (N). The ratio of the relative intensity of the narrow component to the total intensity indicates the fraction of mobile protons, because the N M R intensity of a component is proportional to the number of protons making up that component in the observed spectrum. This fraction is called the mobile fraction, fro.

fm

=

U/(U + B)

(6.5)

In an appropriate temperature range, protons in the amorphous regions will be mobile but the molecular motions of protons in crystallites will still be frozen. Then the mobile fraction, fro, can be determined by the decomposition of the observed spectrum. This fraction fm of mobile protons can be used to estimate a degree of crystallinity f~ .150 f~ =

1 - fm

(6.6)

The analysis of an N M R spectrum is much easier than the analysis of an X-ray profile and this is the easiest way to estimate the crystallinity. However, there are arguments on what is the best method to decompose a spectrum into broad

MECHANOCHEMISTRY OF POLYMERS

515

(a)

(b)

Fl(3. 39. First derivative line shapes in broad line NMR. (a) Gaussian line shape, (b) Lorentzian line shape, (c) superposed line shape of the Gaussian (broad component, B) and Lorentzian (narrow component, N), dotted line is a decomposition of the superposed line into the broad component and the narrow component.

and narrow components. '5~'152The decomposition is often simply performed by drawing a line passing through three points, the positive shoulder, the center and the negative shoulder in the derivative curve of a line-shape, as shown by the dotted line (c) in Fig. 39. The line-width is inversely proportional to the spin-spin relaxation time T2,~5~ which is not easily determined by continuous wave measurement of the N M R spectrum. Recently, pulse techniques have been developed for N M R measurements and commercially made pulse N M R spectrometers are now available. By the pulse technique the spin-spin relaxation time can be directly determined by a semi-log plot of an observed free induction decay (FID) curve against time. TM For a Lorentzian line shape the semi-log plot of FID against time appears linear. The semi-log plot of FID against the square of time becomes linear in the case of a Gaussian line shape, since the FID curve is a Fourier transform of a line

516

J. S O H M A

i

i

i

time FIG. 40. Schematicrepresentation of semilogplot of free inductiondecayof a pulse NMR spectrum. N = linear part corresponding to the Lorentzian line shape, B = deviation from linearity. Dotted lines are the extrapolations of the observed curve and the linear part. shape in the frequency domain into the time domain. If an N M R line shape is composed of both broad and narrow components, a plot of the logarithm of FID against time appears as a straight line for the narrow component and the part corresponding to the broad component deviates from the straight line (Fig. 40). The intercept of the straight line gives the mobile fraction, because the FID curve is usually normalized by taking the initial intensity, that is the total intensity, as unity (Fig. 40). If the logarithm of the deviation from the straight line is plotted against square of time and this plot forms another straight line, the broad line shape is then Gaussian. The semi-log plot in the time domain is more advantageous for the determination of crystallinity than the graphical decomposition of line shape shown in Fig. 39, because less ambiguity is involved in decomposition of a straight line than in that of a smooth curve of observed line shape. Thus, FID observation by the pulse technique is more recommended for the determination of crystallinity than is the line shape decomposition. It should be remembered that the crystallinity as determined by N M R measurement, by either of one of the two methods, is not a direct measurement of crystallinity but rather a measurement of nucleus mobility which is governed by both aggregation state and temperature. If the temperature of the observation is so high as to permit active motions of protons in crystallites, the crystallinity determined by N M R is lower than the true value. Actually the crystallinity estimated by N M R does not always agree with that determined by X-ray analysis, even for an identical sample. 6.1.5. Crystallinity determined by density measurements expressed by the following equation, f~ =

(0c/Q)(e - Qa/Q¢ -- 0a)

Crystallinity, f~, is (6.7)

where Q means density, suffixes c and a standing for crystalline and amorphous

MECHANOCHEMISTRYOF POLYMERS

517

respectively. The letter ~ without a suffix means the measured density of a sample. Since f~ is defined as .L =

w~/w

= ~c~;/~v

(6.8)

where W~ and W are the weight of the crystalline fraction and the total weigbt, respectively, V and V~mean the respective volumes of the crystalline fraction and the total polymer. Since the ratio V~/V is expressed as V~/V =

(0 - 0,)/(0 - 0~)

(6.9)

from the mass balance VO = 0c Vc + 04 V,, therefore eq. (6.7) can be derived from eqs (6.8) and (6.9). The density of a sample, 0, can be measured with good accuracy by either the density gradient tube method or the floating method. The crystalline density, O~, is estimated from the known unit cell parameters of the crystallite. The amorphous density 0, can be estimated from the density of the molten phase and the thermal expansion coefficient.

6.2. Structure changes in polyethylene q[ter mechanical fracture 155 6.2.1. Crystalline transformation - X-ray profiles of H D P E before and after ball-milling at 7 7 K for 20hr are shown in Fig. 41. The left peak in the profile from the non-fractured PE is due to the (110) plane of the orthorhombic crystal in PE and the second peak is attributed to the (200) plane) 56 After the milling one may note the appearance of a new peak at lower angle and an enhancement of the amorphous halo component. The new peak at 19.3 ° is assigned to the reflection from the (001) plane of the monoclinic crystal] 56 This change in the

a

¢C

y& I

I

20

20 20

FIG. 41. X-ray profiles of high density polyethylene (HDPE): (a) non-fractured sample, (b) sample milled at 77 K for 20 hr.

518

J. SOHMA

e-

_/4_ i

20

25

20 FIG. 42. X-ray profile of the HDPE milled at 77 K, after subsequent annealing at 353K for 15hr. profile demonstrates that a crystalline transformation from the stable orthorhombic structure to the monoclinic was induced by the milling at low temperature. Similar changes in the crystalline structure were observed even at room temperature, though the change in the profile was not so remarkable as in Fig. 41. The fractured PE was then annealed at 80°C for 15 hr in vacuum. The X-ray profile observed from the annealed sample is shown in Fig. 42. The profile was almost restored to that of non-fractured PE by the heat treatment. In this case, the monoclinic structure is a metastable structure caused by input of mechanical energy. The percentage of monoclinic structure in the milled PE was evaluated from the relative intensity of the (001) peak. This percentage was followed with change in the time o f milling and the result is shown in Fig. 43. At 77 K the percentage increased abruptly after 10 hr milling, while the percentage gradually increased with milling time at room temperature. 6.2.2. D e c r e a s e in c r y s t a l l i n i t y - Although the relative intensities of the (110) and (200) reflections in the X-ray diffraction profile were restored by heat treatment of the fractured sample, comparison of the two profiles in Fig. 41(a) and Fig. 42 indicates that the enhanced halo component still survived even after the heat treatment of the fractured sample. This fact means that the crystallinity in H D P E is decreased by the milling and the decrease in crystallinity is not completely restored by the heat treatment. That is, the decrease in the crystallinity was irreversible within the temperature range for the heat treatments in the experiments. The decrease in the crystallinity is plotted against the milling time

MECHANOCHEMISTRYOF POLYMERS

519

PE o at 77K o~ • at 293K 20 0

"a

n

I

I

10

20

Frocture time (hr) FIG. 43. Dependenceof the monoclinicfraction on the milling time (HDPE). in Fig. 44. No marked difference was observed when the milling temperature was changed from 7 7 K to room temperature. Apparently the crystallinity decreases more than 10% during ca. 3 hr of milling, and after that the rate of decrease is reduced considerably. The fractured samples do, however, recover some (though not all) of their original crystallinity after heat treatment, as indicated by comparison with Fig. 41(b) and Fig. 42. 6.2.3. D e c r e a s e in t h e c r y s t a l l i t e s & e - Comparing the profiles in Fig. 41 and Fig. 42 one may note that the reflection peaks were broadened by the fracture and the broadening was not completely eliminated by heat treatment. The broadening of the peaks was possibly caused either by decrease in crystallite size, by increase in the number of defects in the crystallite, or by both. Assuming decrease in crystallite size to be the main source of the broadening, we evaluated the size of the crystallite in terms of t110, the length perpendicular to the (110)

0.80~ "E

o.7oF%. "6 060

---4~

C~

0.5(; I

i

I

10 20 Fracture Time (hr) FIG. 44. Milling time dependenceof crystallinityof the milled HDPE. Filled circles: milling at 77 K. Half filled circles: milling at room temperature.

520

J. SOHMA PE o ot 77K

200

• ot 293K o~~ ~ o

I00

_

i 0

I

i

I

IO

20

Fracture time (hr) FIG. 45. Decrease of IH0 with increased milling time,

plane, by using the Scherrer equation. The results are plotted against the milling time (Fig. 45). Crystallite size decreased with increasing milling time in the same way at both temperatures of milling, 77 K and room temperature. The size was reduced by nearly one half within 5 hr and remained almost unchanged at longer milling times. The results seem to suggest existence of a limiting size for fracture, which had previously been commonly observed in the fracture of particles of inorganic materials. ~57'158Similar observation of a limiting size was reported for fractured quartz. ~58 The decrease of the crystallite size with increased milling time is also shown in the Hosemann plot in Fig. 46. Although the accuracy was not as high as was suggested in Section 6.1.2., usable crystallite sizes were obtained for the inverse of the intercepts of the two straight lines obtained for the fractured and the non-fractured samples, respectively. The reduction of the crystallite size from 200/~ to 100,~ agrees quantitatively with the result obtained from the of 15

% x

I01

t°

2. i O/o 2.0%

I

I

16

81 rl 4

FIG. 46. H o s e m a n n plots o f non-fractured and fractured HDPE. Both t and g appear in eq. (6.4).

MECHANOCHEMISTRY OF POLYMERS

52L

Scherrer equation, and the agreement provides us with a positive verification of the assumption that the line broadening of the X-ray profile originates from the decrease in crystallite size. 6.3. Structural changes in polypropylene after fracture 155 6.3.1. Crystalline transformation - X-ray diffraction profiles for polypropylene before and after milling for 20 hr are shown in Fig. 47. Change is more drastic (a)

S |

.e-. ul r"

i

m

h

~,

(b)

C:

~b

~

io

is

~o

20 (c)

u~ C

c

_J

~

2O

2b

fs

FIG. 47. X-ray profilesof polypropylene:(a) non-fractured (b) fractured, (c) fractured and annealed. Profile b was observed after 20 hr milling at 77 K. Annealing was performed at 381 K for 24hr.

522

J. SOHMA

TABLE4. Comparison of crystallinityfor unmilled, milled and annealed polypropyleneand polyethylene as determined from X-ray and density measurements Crystallinity

Density

From X-ray

From density

PP Non-fractured Fractured Fractured and heat-treated

71%

78%

53%

66%

PE Non-fractured Fractured

77% 59%

0.917 0.897 0.907

for PP than for PE. The diffraction profile before the fracture is identical to that of the monoclinic structure reported by Natta. 159 The other profile is known from the pseudo-hexagonal structure of PP which appears in cold-drawn samples of PP. The change in the profiles caused by fracture at 77 K demonstrates that a crystalline transformation from the stable monoclinic to the unstable pseudo-hexagonal is induced by the milling. In contrast with PE, complete transformation by fracture was observed for PP so far as the X-ray profile was concerned and no percentage of the stable structure was able to be estimated for the fractured PP. Again, an enhancement of the halo component was observed for the fractured PP. The positions of the reflection peaks were restored by heat treatment of the fractured sample at 343 K. Therefore, the crystalline transformation caused by the fracture is reversible. Densities were measured for the non-fractured, fractured, and fractured and heat-treated samples by using the flotation method and the results are listed in Table 4. The density of the fractured sample is a little higher than the crystalline density of the pseudo-hexagonal lattice, ds = 0.888. This difference is meaningful because the error of the measurements was +0.002. The higher density suggests that some monoclinic structure survives after the fracture as it does in PE, although no monoclinic component was observed in the X-ray profile. 6.3.2. D e c r e a s e in c r y s t a l l i n i t y Although the monoclinic structure was recovered by heat treatment of the fractured PP, the halo component appeared enhanced for the fractured sample even after the heat treatment. The crystallinity was estimated from the non-halo fraction and results are shown in Table 4. Crystallinity was also estimated from density measurements obtained by the flotation method. It is concluded from the results of the two methods that the crystallinity is decreased considerably by the fracture. The amount of the decrease is nearly the same in both PP and PE. In this experiment the crystallinity is not restored by the heat treatment and the crystallinity decrease is irreversible. It may be reasonably considered, however, that the temperature of

MECHANOCHEMISTRY OFPOLYMERS

523

the heat treatment, 343 K, may b e high enough to rearrange the polymer chains from the pseudo-hexagonal to the monoclinic form but might be too low for recrystallization. 6.3.3. D e c r e a s e in c r y s t a l l i t e size - By comparison of the X-ray profile after annealing with that of the unannealed fractured PP, one finds that the positions of peaks are restored but the widths of peaks are broadened. That is, the crystalline transformation caused by the fracture is clearly reversible but the increase in line width is irreversible. Although there are two factors causing the line broadening in the X-ray diffraction pattern as stated in Section 6.3, it is safe to assume that the line broadening originates primarily from decrease in the crystallite size as shown in a later section. Based on this assumption the crystallite size can be estimated from the observed line width [3 by using Scherrer's equation. "~ The crystallite size decreases rapidly from 220 A initially to 150 A after a 5 hr fracture and levels off after that, similar to Fig. 45. No marked difference was observed depending on the temperature of milling. However, one has to remember that the experimental results were obtained after room temperature annealing of the sample fractured at 77 K, because X-ray observation was not easy at 77 K, Thus, it is likely that the crystallite size was more or less restored by the annealing effect. The crystallite size can also be estimated by the Hosemann plot, as described in Section 6.1. The plot for PP is shown in Fig. 48 and the crystallite size is reduced from 129/k to 95A by ball-milling at 77K. No clear changes in distortion were evident from the plot. The differential radial distribution function (DRDF) of PP is shown in Fig. 49. The characteristic behavior of the D R D F of the fractured PP was not very much different from that of the non-fractured PP, in the range less than 12 A. Corresponding peaks in these two D R D F plots clearly agree with each other not only in peak position but also in line shape. This agreement indicates that the periodic structure within this distance range is not disturbed by the fracture. At larger distances, however,

43--~O

t'-~"

•

•

o

o 5 . g5 %

oNonfroctured

129A°

Froctured

1

95A

5.6%

1

i6

81 n4

FIG.48. Hosemannplotsof non-fracturedand fracturedPP,

524

J. SOHMA

5.01 a mr E}

R(/~I

501 b

I.L. 1:3 rr O

-5.0

R(~,)

FIG, 49, D R D F of non-fractured PP (a) and fractured PP (b).

the D R D F of the fractured PP becomes very different from that of the nonfractured material. The D R D F of the non-fractured PP oscillates with higher amplitude as the distance increases beyond 20A. This behavior indicates periodicity. On the other hand the D R D F of the fractured PP gradually decays to zero beyond 20A and almost disappears at 40A. This decay to the mean value means that the periodicity is gradually lost beyond 20 A and the density is nearly equal to the mean density at 40 A. That is, the crystal size is a maximum of 40 A. These different analyses of the observed X-ray profile consistently indicate that the crystallite size is decreased by fracture, although the estimated sizes do not always quantitatively agree. 6.3.4. D e c r e a s e in c r y s t a l l i n i t y - Comparison of the profiles a, b and c in Fig. 47 demonstrates that the halo fraction of total X-ray scattering was increased by the fracture but did not return to its initial value after annealing. This is an irreversible change caused by the fracture. By decomposing the observed spectra into the sharp peaks and the halo component, we evaluated the

MECHANOCHEMISTRYOF POLYMERS

525

,\ o

2_9 °m""-l L' 1

[3

'

'

'

i

o

--~-Ori gina[ Data ......

5b

....

t(,~sec)

FIG. 50. Semi-log plot of free inducing decay (F1D) of fractured PP. Observed at 333 K at a frequencyof 90MHz. crystaUinity for both fractured and non-fractured samples. The results are shown in Table 4. The crystallinity was decreased from 71% to 53% by the fracture. The crystallinity was similarly evaluated by density measurement in which reference densities of dc = 0.936 for the monoclinic phase, ds = 0.88 for the pseudo-hexagonal phase and da = 0.85 for amorphous PP were used. These results are also listed in Table 4. By this measurement, the fracture process reduced the crystallinity from 78% to 66%. The results obtained by the different measurements agree fairly well within experimental error. The mobile fraction, which corresponds to unity minus the degree of crystallinity, was estimated by N M R measurement. The logarithm of the observed pulse height is plotted against time in Fig. 50. The longer tail is well represented by a straight line, N, as shown in the figure. The differences of the observed points from the extrapolation of the line N were re-plotted. These re-plotted points are also well represented by another straight line, M. The differences of the observed points in the short time range from the line N are also plotted in the figure and these points (B) do not form a straight line, as shown in Fig. 50. However, when the same differences were plotted against square of time in the same figure, a good linearity was found (inset B). This means that the broad component of the N M R signal, which is expressed by a Gaussian curve, is involved in the observed FID curve. Thus, the mobile fraction was estimated from this curve. An increase in mobile fraction was found by the N M R pulse experiment for both fractured and subsequently annealed PP, as shown in Fig. 51. In other words, the irreversible decrease in the crystallinity of the fractured PP was reconfirmed by the N M R measurement.

526

J. SOHMA 60 /t

50

jt j,

40 ._o

~ 3C - 20, 0

3b0

.

.

.

.

Temperature (K)

3O

FIG. 51. Temperature dependence of mobile fraction: o = non-fractured PP, • = PP ball-milled at 77 K for 20hr, 0) = PP ball-milled at 77 K for 20hr, and then heat treated at 381 K for 24hr. 6.3.5. ESR confirmation of crystallite size decrease in fractured polypropylene Both non-fractured and fractured PP were irradiated by 7-rays under identical experimental conditions. ESR spectra were then observed on these two samples. The observed ESR spectra appear the same, as shown by the spectra (a) in Figs 52 and 53. These spectra are assigned to the radicals produced in pp.161 After observation of these spectra, both samples were brought into contact with methyl methacrylate ( M M A ) monomer. Then, the temperature of the two samples contacted with M M A was raised to 273 K and the samples were held at this temperature for 5 min. Following this procedure the ESR spectrum was observed again at 77 K for the samples which had been held at 0°C. No marked change was observed for the non-fractured sample, as shown in Fig. 52. However, a remarkable change in the line shape was found for the fractured PP, as shown in Fig. 53. The spectrum (b) in Fig. 53 is well known as that from the propagating radicals o f M M A . Therefore, it was proved by this experiment that the polymerization of M M A m o n o m e r was initiated by PP radicals only in the fractured PP. M M A polymerization is initiated by contact of M M A m o n o m e r with preexisting PP radicals. High accessibility of M M A m o n o m e r to the PP radicals is required for high efficiency of initiation of M M A polymerization by the PP radicals. The experiment indicated that the PP radicals are formed at sites highly accessible to the M M A m o n o m e r in the fractured PP. However, the radicals formed in the non-fractured PP are poorly accessible to the monomer. The M M A molecule is so bulky that its penetration into a PP crystallite is actually impossible. Therefore, only the PP radicals formed and trapped on surfaces of

MECHANOCHEMISTRY OF POLYMERS

527

After heat treotment A /f~ (b)

~

O°C o fort5 rain

j~ '"

~~J[

V

I 20 --

G

I

FIG. 52. (a) ESR spectrumafter ),-irradiationof non-fracturedPP. (b) ESR spectrum after heat treatment at 0°C for 5min of the ?'-irradiated PP containing methyl methacrylate (MMA) monomer.

FIG. 53. (a) ESR spectrum after 7-irradiation of fractured PP. (b) ESR spectrum after heat treatment at 0°C for 5min of fractured 7-irradiated PP containing M M A monomer.

528

J. SOHMA 0 0

Amorphous CrystoKine

0 0

0

0 ~ID • 0 •

F" o° I.

°/" o

0

Amorphous

o

••

.

° I.

y-irradiation i

,JO 0

..o •

I

. .I• ° o

Ioo

•

•

•1o

o o o Non-fractured 0

0

0

Vr~7~--~]

o

CrystalLine

o

o Fractured

o

FIG. 54. Schematic representation of accessibility of M M A monomer (o) to PP radicals (e). A sample consisting of smaller crystallites has higher accessibility of M M A monomer to PP radicals.

the crystailite or in the amorphous phase are accessible and reactive to M M A monomer. Thus, the high efficiency of the initiation reaction, which was observed only for the fractured PP, indicates the higher specific crystallite surface area per unit volume. This means a smaller crystallite size in the fractured PP. This is schematically represented by Fig. 54. Accordingly, the results in the ESR experiment on the radical conversion support strongly the assumption that the decrease in the crystallite size is the-main cause of the broadening of the peaks in the X-ray profile of the fractured PP. This ESR experiment is interpreted as positive evidence for decrease in the crystallite size due to milling. 7. T R A P P I N G

SITES

OF

MECHANORADICALS

Formation of mechanoradicals is always accompanied by creation of fresh surface by sample rupture, because any mechanical force resulting in the rupture of a solid polymer is a common cause for the formation of both mechanoradicals and cleavages. Therefore, it is reasonably assumed that the mechanoradicals are formed on the fresh surfaces which are formed simultaneously. This assumption, however, does not directly mean that the mechanoradicals detected by ESR are trapped on the fresh surfaces, because a radical in a polymer matrix can migrate from one site to another even at low temperatures. '64 There is a time interval between cleavage or milling of a sample and ESR observation. That is, the sites at which mechanoradicals are primarily generated are not always the trapping sites of the mechanoradicals from which ESR signals are observed long after the. initial generation of mechanoradicals. Accordingly, one cannot rule out the possibility that the mechanoradicals formed on the fresh surfaces migrate into

529

MECHANOCHEMISTRY OF POLYMERS

the polymer matrix before ESR observation. Experimental evidence is needed to determine whether or not mechanoradicals are trapped on the flesh surface. In this section, radical migration in polymer matrices will be first discussed. Both the experimental evidence and the discussion, which convinces us that trapping of mechanoradicals occurs on the fresh surface, will be presented.

7.1. Radical migration in polymer matrices As described in Section 5.6, a mechanoradical generated in a polymer matrix initiates a cyclic reaction. For example, a chain end radical can abstract a hydrogen atom to form a radical on an internal carbon atom of a polymer chain. This may then be followed by a /~-scission to generate a double bond and another chain end radical. That is, H

H

H

H

H

H

H

I

I

i

t

I

I

I

~C" + ~ C - C ~

--, ~ C H + ~ C - C - C - C Q

i

I

I

I

I

I

I

I

H

H

H

H

H

H

H

H

H

I --~ - C H 3 +

~CH~ + C=C~.

I

i

H H Since each radical occupies a position in a solid matrix, a free radical generated after several cycles of the above reaction is on a site far from the position of the primary radical. One possibility for radical migration is that a radical migrates from the original site to some other point due to such a chain reaction which reversibly produces the same radical species in each step. There is also another possibility for radical migration, proposed by S o h m a 164-166 and studied mostly by Japanese r e s e a r c h e r s . 36A64-tT° This migration is reversible and non-dissipative of energy, similar to an elastic collision of molecules in the gas phase. Consider an alkyl type radical of polyethylene, H

H

H

H

I

I

I

I

~ CI H

Crn I 1 - C m" - ~ H

H

I m ÷ l - C ~ I'

H

H

The unpaired electron localized at the carbon atom C,, may abstract a hydrogen atom from the adjacent carbon atoms, either C,,+~ or C,, ~, of the same molecule. By this hydrogen abstraction process the unpaired electron moves by one carbon atom along the chain. This chemical reaction can be called an intramolecular radical migration along the polymer chain. In this hydrogen abstraction

530

J. SOHMA

reaction the energy of the final state is identical to that of the initial state because of the homogeneous structure of polyethylene. Even in other polymers

C-C

,

with a structure less "homogeneous" than that of polyethylene, an identical energy state can be restored in a series of hydrogen abstraction reactions because of the periodic nature of the polymers. For simplicity, polyethylene will be taken as an example for the discussion. The energy state of the unpaired electron remains identical when the electron moves by one C - C bond. One thing we should bear in mind is the electronic configuration of the carbon atom, Cm, having the unpaired electron. The electronic configuration is sp 2 for the bond-forming orbitals and p~ for the unpaired electron. Thus, the configuration must be planar at the Cm site. The configuration of the other carbon atoms is, of course, a n sp 3 or tetragonal structure. Thus, a misfit of the steric configuration occurs at the site of the unpaired electron. This misfit may be relaxed by an internal rotation in a low molecular weight compound. However, such a relaxation can hardly occur in a polymer chain at low temperature, at which molecular motion of the chain is frozen and the chain segments on both sides of the "misfit" site are immobile. Thus, it is reasonable to assume that the chain is twisted at this particular site of the unpaired electron and that an amount of elastic energy is stored at this site. The energy of the carbon Cm with the unpaired electron must be higher by this elastic energy than that of the other carbon atoms (in addition to the chemical energy due to the unpaired electron). The situation is illustrated in Fig. 55 and the carbon atom Cm occupies an energy level raised by an amount Vr resulting from the twisted chain. The radical is known to be rather stable at low temperature and a potential barrier E is required for this stabilization. If a hydrogen atom at the adjacent carbon atom Cm+~ is abstracted by the unpaired electron, the elevated energy level moves from the site C~ to Cm+~. In this reaction the initial state is identical in energy to the final state. In other words, the energy is transferred from the carbon atom C,, to the carbon atom Cm+j without any loss. This reaction is, therefore, regarded as an elastic process, which is similar to an elementary process in diffusion of molecules in the gas phase. Based on the similarity between this hydrogen abstraction and an elastic collision of molecules in the gas phase, one may imagine that an unpaired electron can migrate back and forth randomly along the chain. Suppose a double bond exists between carbon atoms C2 and C3 (Fig. 55). The unpaired electron, which arrives at the carbon atom C4 by this random migration,

MECHANOCHEMISTRY OF POLYMERS

531

(a) H

H

H

--C,--Cz=C3--C4--C5 ..... H

H

H

H

H

H H H C._,--C,.--C~+~--C,.+2 H

H

H

H

.

......

H H C . _ , - - C o - - C.+,H

H

H

(b)

~T~>"

C3 C4

~E

C._,co co+,

FIG. 55. Schematic energy level for a polymer chain having unpaired electrons and a double bond.

is stabilized by delocalization as the alkyl radical is converted into an allylic radical. The stabilization energy is expressed by VAin Fig. 55. This alkyl-allyl conversion after storage of milled polymers was reported by Ohnishi. ~7~The energy depth VAof the trap is so shallow that the unpaired electron can be knocked out of this trap by UV light. This is a photo-induced allyl-alkyl conversion, which was also discovered by Ohnishi. If an unpaired electron migrates by a random process to the site C, ~, which is adjacent to another alkyl radical, a double bond is formed between the carbon atoms. In Fig. 55 the unpaired electron in such a situation is described as being stabilized in the deep trap liD. Although intramolecular migration, which is a hydrogen abstraction from a chain adjacent to the chain having the unpaired electron, is also conceivable in a polymer matrix. There are several items of experimental evidence, both indirect and direct, to support the radical migration mechanism defined above. ~64Free radicals, which had been homogeneously produced throughout a polymer matrix by y-radiation, decay in an amorphous region of the matrix. '72'~v3 This phenomenon can be rationalized only by assuming that the radicals generated within the crystallites of the matrix migrate to the amorphous regions. The radical decay was discussed on the basis of this migration mechanism and the experimental results were well explained by this model. '64 It was also reported that the concentration of free radicals (previously generated by y-irradiation of the polymer) increased in the surface region of the crystallites after mild heat treatment. ~v4'~75These results indicate radical migration from the crystalline zones to the disordered region, where the twisting of polymer chains may be relaxed and the unpaired electrons are stabilized by the resultant reduction in the elastic energy. Photoconversion of the allylic radical followed by the heat treatments is direct evidence to support the idea of radical migration. 36When polyethylene containing allylic radicals was irradiated by visible light, its ESR spectrum changed from the characteristic septet of the allylic radical 36 to an octet, as shown in

532

J. SOHMA

Storage at room temperature

H --C~C--C--C H H H

11.3G

H

H H visible -)(- C - - C - - ~ - - C : C ~ C : C H ~ Light

H H H + H C--C--C-H H

(septet)

(octet) H

CH3-- C - - C - - C " H H H

v

H

CH,--C--C--C-H

(octet) H . H CH3--C--C--C--C

(o)

H

(b)

H

(sextet)

....

C~C--

H H H (sextet)

storage at • ,.. room temperature

H H • H H C--C--C--C~-~-C-H

¢c)

H H (septet)

FIG. 56. Changes of ESR spectra caused by visible light irradiation at 77 K of a polymer containing allylic radicals and by subsequent heat treatments. Chemical reactions causing the spectral changes are shown. Fig. 56. The octet was identified as the spectrum from the end radical H 3C

-

(~ - CH2

"~

.36

J H The observed spectrum from the polymer matrix is broadened in line-width and has poor resolution. The two types of alkyl radicals, H3C-CH2-CH-CH2 (in which the unpaired electron is on the second methylene carbon atom from the end methyl group) and ~ C H 2 - C H - C H 2 ,~ (in which the unpaired electron is on a methylene carbon atom well inside the chain), show a similar broadened sextet. Therefore, the sextet appearing after warm up to - 20°C is the spectrum originating from the alkyl radical formed by the shift of the unpaired electron toward the middle of the polymer chain either by one carbon atom or a few carbon atoms. The sample stored at room temperature for a long period of time

MECHANOCHEMISTRY OF POLYMERS

533

showed the septet, which proves the presence of the allylic radical. During long storage, the unpaired electrons migrate along the chain until stabilized as allylic radicals. This is what we would have anticipated from the model shown in Fig. 55. Thus, the experimental results are believed to be positive evidence for radical migration. The rate constants of both the octet-sextet and the sextet-septet conversions have been quantitatively studied. 176The rate constant of the former is much higher than that of the latter. But the activation energies are equal within experimental error. This result demonstrates that many more steps are needed for conversion of the alkyl radical to an allyl radical in comparison with the conversion of the end radical to a chain radical. However, the energy barrier overcome by the unpaired electron is the same for these two processes. This conclusion means that the unpaired electron must migrate randomly for a long distance to meet a double bond, and this really fits the model of the radical migration. Similar spectral changes, which were interpreted as the conversion of the chain scission radical C H 2 - C H 2 - C H 2 ~ to CH3-CH2CH 2, were found for mechanoradicals generated from polyethylene. 34 It was also found that the end radical CH3-12H-CHz ~ was converted into the chain radical ~ CH~-I~HCH 2 ~ by heat treatment at 143 K for 5min. ~v7 Radicals produced at 77 K by mechanical fracture of polybutadiene containing an antioxidant, t-butyl-p-cresol, were identified not as mechanoradicals of polybutadiene but as cresol radicals. L7~ Since no low molecular weight compounds like cresol produce mechanoradicals, the primary radicals generated by fracture at 77 K must be the mechanoradicals of polybutadiene. But the primary mechanoradicals are actually trapped by the antioxidant, which was contained at a level of a few tenths of one percent in the system. This trapping by the antioxidant is understood only if one assumes radical migration in a soft and amorphous polymer like polybutadiene. The relation between the decay reaction and diffusion in a polyethylene matrix was discussed. ~6~'~v2 A mechanism of radical migration in a specific system, urea-polyethylene complex was investigated in detail by Kashiwabara eta/. 16g-17° Evidence so far accumulated indicates that radicals may migrate in a random way from the site of their primary generation to any site in a polymer matrix, depending on temperature as well as the properties of the matrix. Radical migration is very important as an elementary process for other radical reactions in a polymer matrix. 7.2. Trapping of mechanoradicals on fresh surfaces During fracture of a solid polymer, especially at a temperature as low as 77 K, many mechanical cleavages occur. A pair of new surfaces is produced by the separation of the solid material into two parts in a cleavage. This new surface is called a fresh surface produced by mechanical fracture. Mechano-

534

J. SOHMA ¢n

-6 -~

I00

o

/ jr°" ~ so g g 8

o

,tOO

150

I

I

200

250

Heat treatment temperature (K)

FIG. 57. Conversion of PP radicals into peroxy radicals after contact with oxygen. Conversion is plotted against annealing temperature. • = Mechanoradicals; O = radicals generated by 7-irradiation.

radicals are simultaneously produced by fractures of the solid polymer. Thus, the mechanoradicals are generated on the fresh surfaces during fracture of the solid sample. ESR observation is not carried out immediately after the fracture but after some time period, say several tens of minutes at least, after the stopping of the mechanical action. Then, the sites where the mechanoradicals are trapped are not necessarily the places where they were generated. That is, the sites o f radical generation are not always the trapping sites, because the radicals in a polymer matrix may migrate randomly from one site to another. The trapping sites of mechanoradicals will be discussed on the basis of experimental results t79 obtained on polypropylene (PP). PP radicals formed in an oxygen-free environment are converted into peroxy radicals after contact with oxygen at temperatures above 90 K. The conversion fraction is estimated by decomposition of the ESR spectrum observed at a given temperature into a component attributed to the PP radical and one attributed to the peroxy radical. The fraction o f PP radicals converted into peroxy radicals was plotted against annealing temperature. The results are shown in Fig. 57. In the case of mechanoradicals, complete conversion into the peroxy radical was found at 200 K, while 100% conversion was never obtained for the PP radicals generated by 7-irradiation. There is competition between the conversion (into peroxy radicals) and the decay of the PP radicals. The conversion is more efficient than the decay in the case o f mechanoradicals, but it is not so for the radicals produced by 7-irradiation. High reactivity of the PP mechanoradicals similar to that toward oxygen was found toward a more bulky molecule, methyl methacrylate (MMA). This molecule is too bulky to penetrate into the crystalline, or even the amorphous regions of solid PP. PP mechanoradicals were brought into contact with M M A at 77 K and heat-treated at 273 K for 5 min. The ESR spectra observed before

MECHANOCHEMISTRY OF POLYMERS

535

(b)

F~G. 58. Changes in the ESR spectrum of e e mechanoradicals after contact with MMA monomer followed by heat treatment at 0°C.

and after the heat treatment are shown in Fig. 58. Fig. 58(a) is the spectrum observed from the fractured PP immediately after the contact with M M A monomer at 77 K. The line-shape (a) is the spectrum of PP mechanoradicals. 43 Fig. 58(b) is the line-shape observed from the same sample after the heat treatment. This is known as the characteristic spectrum 7~ of the propagating PMMA radical. These facts demonstrate that PP mechanoradicals were almost completely scavanged by M M A monomers after treatment at 0°C. This complete scavenging of the mechanoradicals by the bulky M M A molecules indicates that the mechanoradicals are trapped at sites to which the bulky molecules are easily accessible. To make a comparison with the behavior of the mechanoradicals a similar experiment was carried out for PP radicals generated by y-irradiation. The spectra before and after the heat treatment are presented in Fig. 59. Apparently no drastic change was caused by heat treatment of the ),-irradiated PP after contact with M M A monomer. This result means that the radicals produced by y-irradiation are mostly trapped deep inside the solid, into which bulky MMA monomer cannot penetrate. That is, the comparison apparently demonstrates that the mechanoradicals are trapped in regions to which both oxygen molecules and bulky M M A molecules are easily accessible. That is, the surfaces formed by fracture. The isothermal decay at 313 K of the peroxy mechanoradicals was studied and the result is shown in Fig. 60. The same isothermal decay was followed for peroxy PP radicals produced by y-irradiation of solid PP in vacuum at 77 K followed by contact with air at an elevated temperature, for comparison. The result for the radicals generated by y-irradiation is also shown in the same figure. The isothermal decay of the peroxy radicals generated by y-irradiation was studied by Eda and Iwaskai.18° According to these researchers the decay behavior is divided into three stages; rapid decay up to 1 hr, slow decay from 1 to 2 hr and

536

J. SOHMA

FIG. 59. Comparison of ESR spectra of PP radicals generated by 7-irradiation. Before (a) and after (b) contact with M M A m o n o m e r followed by heat treatment at 0°C. Experimental conditions are identical with those for Fig. 58.

very slow decay after 2 hr. They ascribed the first stage of rapid decay to the decay of the radicals trapped on the surfaces of crystailites. (The decay behavior of the peroxy mechanoradicals can also be divided into three stages, as shown in Fig. 60). In analogy to the Eda--Iwasaki study, we interpret the first stage as the decay of the radicals trapped on the surfaces of crystallites for the case of the mechanoradicals also. The first stage accounts for the decay of more than 90%

10001 E O

O U

100 10~ 1 ~ ,

r~

r~ OZ

5 Time(hr) 10 Storage

15

FIG. 60. Isothermal decay of PP peroxy radicals at 313 K. O = Peroxy radicals produced by ),-irradiation; • = mechanochemically generated peroxy radicals.

MECHANOCHEMISTRY OF POLYMERS

(a)

537

~__

I

i

Fl6. 61. ESR spectra of peroxy mechanoradical observed at 199 K. (a) Peroxy radical before heat treatment, (b) peroxy radical after beat treatment at 317K (gain is increased by 10), (c) peroxy radical produced by ?,-irradiation.

of the total radicals, as shown in the figure. These results and interpretation indicate again that more than 90% of the mechanoradicals are trapped on the crystalline surfaces. A study of molecular motion may also provide information on trapping sites, because radicals trapped on a surface are more mobile than those trapped in a crystalline region. ESR spectra observed at 199K of PP peroxy radicals are shown in Fig. 61. The spectrum (a) is of peroxy mechanoradicals before heat treatment at 317K. The spectrum (c) is that of peroxy radicals formed by 7-irradiation. The spectrum (a) appears more symmetric than (c). The symmetric appearance of the ESR spectrum of a peroxy radical, which has an anisotropic g factor, is caused by averaging of the anisotropy due to rapid molecular motion of the radical. Therefore, the symmetric line-shape indicates rapid motion of the mechanoradicals at this temperature. The spectrum (b) is that observed after heat treatment of the sample containing the peroxy mechanoradicals. By comparison of the spectra (a) and (b) one may find that the deviation from symmetry in spectrum (a) is mainly due to the presence of some radicals with the spectrum (b). The (b) radicals are a minor component which represent the radicals which survive after the heat treatment. The results shown in Fig. 61 demonstrate that the majority of the mechanoradicals are trapped in the region where the molecular motions of the radicals become rapid at this temperature. By contrast, the molecular motions of the same species of the radicals originating from 7-irradiation are frozen at the same temperature. These facts indicate that the radicals generated by 7-irradiation are trapped in the crystallites, such that molecular motion is not activated at this

538

J. SOHMA

temperature. TM The heavily decreased intensity of the spectrum (b) in Fig. 61 can be consistently interpreted in terms of the decay behavior shown in Fig. 60. After the heat treatment at 317K most of the mechanoradicals had decayed. Only a small fraction of the mechanoradicals, which were trapped in crystallites and showed an anisotropic line-shape, survived. They survived because molecular motion in the crystalline region is not activated at temperatures as low as 40°C. 181 After the milling the sample flakes became finer and the total surface area of the finer flakes was increased. The fresh surface area is created by fracture which simultaneously produces mechanoradicals. The experimental results presented above provide good comparisons between the mechanoradicals and the radicals generated by 7-irradiation. Four different experiments, (1) the reactivity with oxygen, (2) reactivity with bulky MMA molecules, (3) the isothermal decay of the peroxy radicals, and (4) the differences in the molecular motion of the radicals, indicate covergently that the PP mechanoradicals are formed and trapped on the fresh surfaces produced by the fracture. The respective typical trapping sites of the mechanoradicals and the radicals formed by the y-irradiation are schematically represented in Fig. 54. In the case of y-irradiation the formed radicals distribute almost uniformly in the sample and the fraction of radicals trapped in the crystalline regions is proportional to the crystallinity of the sample. The radicals trapped on surfaces must be negligible in number, because the surface regions form a very small fraction of total sample mass. Non-bulky molecules like oxygen may penetrate rather easily into the amorphous regions and the radicals may react with oxygen in these areas to form peroxy radicals. In mechanical fracture, the sample is cleaved and separated into finer pieces, which results in increased surface area. The mechanoradicals are generated on the fresh surfaces by cleavages of the sample and are trapped on the surfaces, which are rich in defects and distorted in crystalline structure. Thus, it is safe to conclude that the mechanoradicals are trapped on fresh surfaces created simultaneously. This conclusion is experimentally supported only for polypropylene but it is probably applicable to mechanoradicals formed from other polymers under similar conditions. 8. E F F E C T

OF TRIBOELECTRICITY

ON RADICAL

REACTIONS

8.1. Anomalous behavior of mechanoradicals in decay processes Since free radicals are unstable intermediates, they are formed and trapped in polymer matrices at 77 K and decay at elevated temperatures. This was demonstrated in Fig. 60 in the last Section. This decay is a decrease of the radical concentration with time at a fixed temperature, that is, an isothermal decay. Suppose a polymer sample traps free radicals within itself at 77 K. Suppose we warm the sample to a definite temperature, say 100 K, and keep it there for a

MECHANOCHEMISTRY OF POLYMERS

~

0

0

-

539

~

8

7

,

5o--

~"

o

L I00

I 200

T 5 Q..,q~" ""-

L 300

Temperature of heoting (K)

FIG. 62. Examples of normal decay curve. O = Decay curve of radicals trapped in HDPE; • = decay curve of radicals trapped in LDPE.

definite time, say 10min, followed by cooling down to 77 K, at which temperature the ESR spectrum of the sample is observed. The relative intensity of the ESR spectrum after such a heat treatment to its original intensity is determined. If one changes the temperature of the heat treatment for a fixed time, the relative intensity of the ESR signal can then be plotted against the temperature of heat treatment as shown in Fig. 62. This plot reflects the decay behavior of radicals in a polymer matrix, and therefore it is often called a decay curve. It was found that radicals generated and trapped in the polymer matrices decayed stepwise. For example, polyethylene radicals decayed in three steps ~v3 and polypropylene radicals in two steps.172 In Fig. 63 the decay curve of polypropylene radicals produced by ,/-irradiation is represented with a dotted line. No radicals decay up to 160 K, beyond which temperature gradual decay starts. In the temperature range between 230 K and qSO

¢r" 03 UJ

.~ ~ ~o

I

I00

I

150

L

200

L

250

I

300

Temperoture of heating in K

FIG. 63. A n o m a l o u s behavior of decay curve of PP mechanoradicals (filled circles). Normal decay curve obtained for comparison (open circles).

540

J. SOHMA

o~

I00 0

50

--

o,

I IOO

150 Temperature

FIc. 64.

Decaycurve of

of heating

200 ( K )

PTFE mechanoradicals.

260 K the radical decay almost stops. However, the radicals decay more rapidly over temperature ranges corresponding to onsets of different types of molecular motions of the polymer matrices, in which the free radicals are trapped. ~7~'173'182 This stepwise decay is a normal behavior pattern for free radicals, because it is natural for a free radical, which is an unstable chemical species, to decay at higher temperatures through recombination or other processes. In Fig. 63 the decay curve of the polypropylene mechanoradicals formed by sawing in liquid nitrogen is shown with a solid line. This curve shows a nearly 50% increase in radical concentration in the temperature range from 120 K to 180 K. 34'~83This increase is quite an anomalous behavior for free radicals in the decay process. A similar increase was found for the mechanoradicals of polytetrafluoroethylene, as shown in Fig. 64, and for polyethylene mechanoradicals. So far as has been determined, this anomaly is characteristic of mechanoradicals. The increases in relative concentration of radicals observed over these temperature ranges mean that free radicals are being generated by heat treatment at temperatures much lower than room temperature. No excitation other than the heat treatment (that is, after the mechanical fracture) was applied to these systems. It is impossible to imagine that a chemical bond would be broken only by thermal energy at such low temperatures. Thus, the observed increase in radical concentration at some stages of heat treatment was a big puzzle. The anomalous behavior in the decay curve appeared most strongly for polypropylene mechanoradicals. Hence the factors affecting the decay of polypropylene mechanoradicals were investigated particularly closely, j83 The polypropylene sawdusts formed at 77 K were found to be electrically charged. The sawing is naturally accompanied by friction, which produces electric charges

MECHANOCHEMISTRY OF POLYMERS

541

L5-

.s I 0 - 55

.u 0 5

-

w

l IO0

150

200

250

500

Temperoture in K

Fl(;. 65. Discharge curve; decrease with increasing temperature of the excess negative charge produced by triboelectricity.

due to triboelectricity (charge separation caused by rubbing). Therefore, the samples, following mechanical action, are not electrically neutral but charged. This is the big difference between mechanical action and other radical-forming processes like ~,-irradiation. By 7-irradiation ionic species are produced in a system but the number of cations must be equal to the number of anions in the system, which stays in an electrically neutral state before and after the irradiation. Mechanical action, such as milling and sawing, always involves friction. This friction causes charge separation from particle to particle. The fractured particles are not electrically neutral but charged immediately after the fracture. These net electric charges are held on the individual particles for considerably longer times at low temperatures like 77 K than at higher temperatures. The polypropylene sawdust generated by the metal saw was negatively charged at 77 K and the negative electric charge was gradually discharged at temperatures above 160 K, as shown in Fig. 65. The reproducibility of the discharge curve was not so good as to permit a quantitative conclusion but was qualitatively reliable. It is important that the amount of excess electric charge begins to decrease near 150 K, at which temperature the anomalous increase of the radical concentration occurred in the decay curve of the mechanoradicals. This correspondence seems to suggest that the excess electric charges produced by the triboelectricity play an important role in this anomalous decay behavior of the mechanoradicals. Positive evidence for the effect of the excess charges was obtained in an experiment in which tetracyanoethylene (TCNE) was used as a strong electron scavenger. The sample containing T C N E was ball-milled at 77 K and the decay curve of the mechanoradicals from this sample was traced. The result is shown in Fig. 66, in which the decay curve of the mechanoradicals formed from polypropylene in the absence of T C N E is shown for comparison. No anomalous

542

J. SOHMA 150 la.i

>0

.--.

iO0

u

o\ I

I00

I

150

I

I

200

Temperaturein

250

I

300

K

FIG. 66. Comparison between the decay curves of PP mechanoradicals containing TCNE (open circles) and no TCNE (filled circles).

increase of radical concentration during heat treatment was observed for the sample containing TCNE. The mechanoradcials show no increase in concentration but the radical stays unvaried up to 250 K, beyond which the radicals decay monotonically. This is quite normal behavior in radical decay. The observed ESR spectra are shown in Fig. 67. The spectrum shown in Fig. 67(a) is that observed at 77 K from polypropylene with TCNE after milling at 77 K. The spectrum shown in Fig. 67(b) was observed at 263 K, a temperature at which most of the mechanoradicals decay as shown in Fig. 63. This broad singlet is the spectrum of the TCNE anion, (TCNE)-, which is line-broadened due to trapping in a frozen state. Based on this change the observed spectrum (a) can be decomposed into three components; the singlet from (TCNE)-, the octet from the polypropylene mechanoradical and a broad singlet from an unknown radical. This decomposition is shown in (c) in Fig. 67. Combining the normal decay behavior with the spectral analysis on the fractured polypropylene containing TCNE, one may conclude that the excess electrons are scavenged by TCNE and no anomalous increase of radical concentration during heat treatment is observed from the system in which both the polypropylene mechanoradicals and TCNE anions are present. In other words, the excess electrons, which are not somehow stabilized, are a cause of the anomalous behavior of the mechanoradicals in the decay process. The excess electrons are not trapped as in a glassy matrix similar to supercooled ethanol. 184'~85If this were the case, the trapped electrons would show a characteristic ESR spectrum but no such spectrum was observed from these systems. The ESR spectrum observed from the sawdust of polypropylene sawed in liquid nitrogen appeared as a spectrum superposed with the asymmetric spectrum of peroxy radicals. An example of a superposed spectrum is shown as (a) in Fig. 68. In the temperature range of the radical production the line shape gradually changed as shown at (b) and (c) in Fig. 68. These changes in the

543

MECHANOCHEMISTRYOF POLYMERS

(a)

~

_

~

77

(b)

(c)

FIG. 67. ESR spectra of mechanoradicals produced from PP containing TCNE. (a) Spectrum immediately after milling at 77 K, (b) a spectrum observed at 77 K after a heat treatment at 263 K, (c) decomposition of spectrum (a) into components.

line-shape indicate that the species of radical produced by the heat treatment in this temperature range is the peroxy radical. That is, the anomalous increase in the number of total radicals is caused by new peroxy radicals produced by the heat treatment. This conclusion is positively indicated by tracing the decay behavior of the mechanoradicals in a vacuum of 10-s Torr. Under this condition no anomalous increase was observed, as shown in Fig. 69. In the same figure the decay curve of the mechanoradicals in air is shown for the comparison. This comparison apparently demonstrates that the presence of oxygen is required for the anomaly in the decay curve of the mechanoradicals.

544

J. SOHMA

Iol __/x_f

(b)

(c)

77

_Y

K

143 K

Y

173 K

36.8 G

FIG. 68. Changes in ESR line shape with temperature of heat treatment. All spectra were observed at 77 K.

The experimental results so far accumulated lead us to the following conclusions. For the anomalous increase in the decay process of the polypropylene mechanoradicals to occur: (1) excess electric charges, negative in this case, are required. (2) the excess negative charge, that is, excess electrons, must not be in a trapped and isolated state like the so-called trapped electrons. (3) the presence of oxygen is required also.

150 --

.t/) ~:

f"x. ~ ' ~ "

;oo

~'g .E g

.~ e

so

o

I

I00

1

I

I

I

150

200

250

300

• Temperoture in

K

FIG. 69. Comparison between the decay curves ofPP mechanoradical produced in air

(filled circles) and at a deaerated condition (10 -5 Torr) (open circles).

MECHANOCHEMISTRY OF POLYMERS

545

8.2. Mechanism of anomalous decay of mechanoradicals 8.2.1. Role of excess electric charges produced by triboelectricity - The observed increase in ESR intensity shown in Fig. 63 might not be attributed to real increase in the radical concentration but to apparent enhancement of the spectrum. An ESR signal from a polymeric material is easily saturated. The ESR intensity depends on both microwave power and on relaxation mechanisms which cause the saturation. The relaxation mechanisms might be changed by the heat treatment. If so, the ESR signal intensity would be enhanced after the heat treatment without any increase in the radical concentration. In the experiments previously discussed, the ESR spectrum was observed at the fixed temperature of 77 K. Thus, it is reasonable to assume that relaxation mechanisms are the same at this fixed temperature. In order to check this possible effect of relaxation mechanisms on the ESR intensity, the decay curve was observed at different levels of microwave power. The decay curve appeared with enhancement even at the different microwave power levels. Therefore, the possibility that this enhancement of ESR spectrum is due not to increased radical concentration but to a change in the relaxation mechanism was ruled out. The enhanced intensity of the ESR spectrum after heat treatment was confirmed to be caused by increased radical concentration. The anomalous behavior of the mechanoradicals described in the last subsection is considered to be a characteristic specific to mechanoradicals, because no such anomalous increase in radical concentration during decay was found for the radicals produced by either photo- or ?,-irradiation of the same polymers. The excess electric charges, which were shown to play a crucial role in this anomalous increase of the radical concentration, are reasonably assumed to be created by triboelectricity, which is charge separation produced by rubbing, milling or other mechanical actions. Since any mechanical action, such as sawing and milling, is always accompanied by friction, that is rubbing, it is quite natural that the fractured pieces are not neutral but electrically charged. In the case of polypropylene sawed by the metal saw, the sawdust: was negatively charged. The presence of charged chemical species, either cations or anions, is quite common in chemical reactions occurring in radiation chemistry and other branches of chemistry. However, bulk samples containing cations and anions stay electrically neutral. In the case of triboelectricity, however, a sample becomes electrically charged as a whole and has a net excess charge, either positive or negative. These excess charges are quite different from the charged species (cation-anion pairs) formed in neutral samples. The excess charges in a sample are generally unstable and are easily discharged to return the sample to a neutral state. The discharge curve shown in Fig. 65 is an example of a return to the electrically neutral state during thermal treatment. The assumption that the excess charges due to the triboelectricity are a cause for the anomalous increase of the radical concentration

546

J. SOHMA

during the decay of the mechanoradicals is quite compatible with the fact that the anomaly was observed not for the radicals produced by non-mechanical methods but only for the mechanoradicals. This is because the triboelectricity occurs only in mechanically fractured materials. The fractured pieces of polypropylene were negatively charged, containing excess electrons. If the excess electrons are isolated and trapped in the polymer matrix at low temperatures, like the trapped electrons ~85formed by 7-irradiation of alkaline ice at 77 K, the ESR spectrum of the trapped electrons would then be expected to appear as a singlet, which is easily saturated. No such singlet was observed from the negatively charged polypropylene and therefore the excess electrons are not trapped in the isolated state but rather are associated with the molecules to form anions. It is really hard to imagine that radicals could be newly formed by the heat treatment at these low temperatures below 175 K, since the amount of thermal energy present at these temperatures is too small to break any covalent bond in an organic molecule. On the basis of the results summarized in the last subsection and the considerations presented above, the following mechanisms (described in Table 5) will be presented for the anomalous behavior of the mechanoradicals in the decay process. By fracture, the mechanoradicals of polypropylene, RI and R2, are produced by main-chain scission. (Process 1, Table 5). When the fracture is carried out in the presence of air, both radicals react with oxygen to form peroxy radicals (Process 2). Peroxidation of the radical R~ is known to be more rapid than peroxidation of the radical R1.43 Since friction accompanies fracture, excess electrons are simultaneously transferred to the polymer particles by triboelectricity. Some fraction of the excess electrons may leak to the earth during the fracture but the rest become associated with the mechanoradicals RI and R2 to form anions R ( and R• (Process 3). These anions are not paramagnetic because of the pairing of the unpaired electron in the radical by association with the excess electron. There are two reasons why we assume the association of the excess electrons with the radicals. The first is the high electron affinity of these particular radicals. Although no data for the electron affinity of these particular radicals, RI and R~, is specifically known, the electron affinity of the methyl radical is reported as 25 kcal mol -~ . 186.187There is no reason to believe that a big deviation from this value would occur for similar organic radicals like RI and R;. Thus, we have a good reason to believe that these radicals may have high electron affinity. The second reason is the small electron affinity of saturated organic molecules like n-paraffins and polypropylene. Therefore, it is most likely that the excess electrons are captured by the free radicals and stabilized at 77K, although no positive evidence for this capture is available at present. Process 3 in Table 5 represents the capture of the excess electrons by the radicals. No ESR signal is observed from the resultant anions.

547

MECHANOCHEMISTRY OF POLYMERS TABLE 5. (1) At the initial state of 77 K: H

CH~

I

~C+C--

H

I

H

CH,

H

CH~

I

L

II

C - C ~ ,,~ec~,nic.¢' ~ C

C'

+ "C - C

I I

H

H

CH~

I I

I I

I

L

H

H

H

H

H

H

(R;)

(I)

(R~_)

R I + 02--* R IO0"

(2)

R~ + 0 2 ~ R200" RI + ( - e ) --, RI-

anions nomradicals

R½ + (--e) ~ Rf

no ESR spectrum

(3)

(--e): an excess electron due to triboelectricity (ll) Heat treatment in the temperature range 100-175 K: N

R200

Rj 0 0

unvaried / no change in ESR intensity

(4)

R 2 --* R i + ( - e) "[ regeneration of the radicals R~' ~ R~ + ( - e ) J b Y the release of excess electrons

(5~

RI + O2-~ ROO;

)

Ri + 02 ~ R I O O ) formation of the stable R~ + 02 --* R2OO" l peroxy radicals

(6)

released electrons ( - e ) leak to the earth and the sample approaches electrical neutrality. ( l i d Heat treatment at temperatures above 175 K: Both R LOO' and R 2 0 thermally decay. All excess electrons leak to the earth and the sample becomes electrically neutral.

In the temperature range between 100K and 175K, no change in ESR intensity occurs. The peroxy radical R1 OO" is so stable in this temperature range that no peroxy radicals actually decay. The radical R; may react with oxygen in this temperature region but the ESR intensity is not changed by the transformation of one radical to another, though the line shape may change (Process 4). Thus, the presence of these radicals causes no increase in the ESR signal observed in this range, as shown for Process 4 under I! in Table 5. Suppose, however, that the electrons are released from the cations. The released electrons leak to earth and the charged sample is gradually discharged by this detachment of the electrons. This is observed as the discharge curve shown in Fig. 65, in which the discharge starts from nearly 150 K. Such release

548

J. SOHMA R. + 02 + (-e)

R-+O z

ROz" + (-e)

FIG. 70. Energy states o f a system consisting of a radical R', an 02 molecule and an electron ( - e). EA is the electron affinity and EB is the bonding energy of the radical with oxygen.

of electrons regenerates the radicals RI and R~, as shown in Process 5. The regeneration of these particular radicals probably does not directly cause the increase in the observed intensity of the ESR spectrum in this temperature region. The radicals RI and R~ are rather unstable at these temperatures and it is likely that the radicals decay before they can be observed by ESR. It is known that the presence of oxygen is needed for the increase in the ESR intensity. The radicals are easily converted into peroxy radicals in the presence of oxygen (Process 6) and the stable peroxy radicals, which are newly produced by the release of the electrons and the reaction with oxygen, contribute to the increase in the ESR intensity. This mechanism is consistent with the fact that the increased intensity is observed to be due to the increased contribution of peroxy radicals to the total spectrum, as mentioned in the last section. The presence of oxygen is required not only for the stabilization of the radical but also for another important reason. We have assumed very high electron affinity for the free radicals and one may therefore wonder why electrons are released from the stable attached state by the small thermal energy in this temperature region. Oxygen plays an important role in this release of electrons. There are three different states for a system consisting of a radical, an electron and an oxygen molecule. In Fig. 70, the state, in which the three components, the radical, the electron and the oxygen molecule are all separated from each other, is taken as the reference state. If the electron is attached to the radical to form an anion, the energy state of the anion and the separated oxygen molecule is decreased by the electron, affinity. For the other state corresponding to the ROO' radical and the isolated electron, the energy is lower than the reference value by the formation energy of the peroxy radical. Although no exact values for both the electron affinity and the bonding energy of the peroxy radicals are known, it is not unreasonable to assume that the bonding energy is a little lower than the electron affinity. If so, the reaction may proceed from the anion to the peroxy radical when the activation energy is overcome by the thermal energy. This is presumably why peroxy radicals are formed accompanied by release of electrons. It is safe to say, at least, that such release of electrons trapped by radicals with high electron affinity does not occur

MECHANOCHEMISTRY OF POLYMERS

549

in the absence of oxygen. At temperatures above 175K, the peroxy radicals begin to decay and the radical concentration decays monotonically at these temperatures. The arguments presented in the preceding paragraphs are quite compatible with the observed results and consistent in themselves. This compatibility and self-consistency lead one to believe that the excess charges produced by the triboelectricity affect the free radical reactions initiated by the mechanical fracture, although no positive evidence for the association of the excess electrons with, and their dissociation from the radicals has yet been obtained. Before closing this subsection, it should be mentioned that similar anomalous increases in the ESR signal during the decay process were reported by Pilar and Ulbert for mechanoradicals generated from poly(methyl methacrylate) L~ and poly(ethyleneglycol dimethacrylatef 2 in the presence of oxygen. However, they found that the intensity of the ESR spectrum of the mechanoradicals produced in vacuum was decreased by nearly 50% immediately after contact with oxygen and the ESR intensity was then enhanced by the heat treatment below ca. 210 K. They ascribed this anomalous increase in ESR intensity to the dissociation of diamagnetic tetraoxides, which had been formed immediately after the contact of oxygen with the tertiary carbon radicals. The equilibrium between the peroxy radicals and tetraoxide R O O + R O O --~ ROO - OOR was assumed to exist by Ingold and then investigated by Bartlett and Guaraldi) su Pilar and Ulbert pointed out the possibility that tetraoxide dissociation may explain the anomalous increase of the ESR signal of the mechanoradicals of polypropylene. Their view is compatible merely with the fact that oxygen is needed for this anomaly but fails to give a good reason why excess charges are needed for this anomaly. The presence of excess charges was well demonstrated by the discharge curve as well as by the formation of TCNE anions. Mead et alJ 9° also found that oxygen plays an important role in the anomalous decay. However, Andrew and Reed 9 seem to be a little sceptical about the role of the excess electrons in this anomalous decay.

9, M E C H A N O C H E M I C A L POLYMERIZATION INITIATED MECHANORADICALS FORMED FROM POLYMERS

BY

9.1. General

Mechanoradicals may initiate chemical reactions whenever they are brought into contact with any reactant capable of reacting with radicals. For example, it was found that polypropylene mechanoradicals are converted into peroxy radicals after contact with oxygen, as described in Section 7.2. This is a good example of a chemical reaction initiated by mechanoradicals. The reactivity of

550

J. SOHMA

Vecgum system

Dewar

MMA reactont R

.iq. N2 Fractured PTFE JJ

Apparatus

FIG. 71. Special ampoule for a study of post-polymerizationinitiatedby mechanoradicals. the mechanoradicals was much higher than that of the similar species of radicals produced by 7-irradiation. Thus, it would be reasonable to expect that reactive mechanoradicals may initiate polymerization reactions when brought into contact with monomers. Remembering that most of the primary mechanoradicals of linear polymers are generated by ruptures of their main chains, these mechanoradicals are then a kind of living polymer, which is able to initiate further polymerization starting from their chain ends. Thus, if a monomer, which is not the monomer of the matrix polymer, is contacted with a polymer containing mechanoradicals, a block copolymer is generated after the mechanoradical-initiated polymerization is finished. Moreover, such a block copolymer may be produced selectively on the surfaces of fractured polymers, because the mechanoradicals initiating copolymerization tend to be formed and trapped on fresh surfaces, as discussed in Section 7. Therefore, it is very interesting to investigate whether or not mechanoradicals can initiate polymerization. There are two methods to investigate mechanochemical polymerization. One is called a post-polymerization, in which monomers are brought into contact with mechanoradicals produced before the contact with monomers. In this method, polymerization may start after mechanical action has produced the mechanoradicals. No mechanical action need necessarily be applied during the polymerization process (although polymerization may be accelerated by heat treatment). A special ampoule, shown in Fig. 71, was designed by Sohma and his collaborators jgl'j92for studies of post-polymerization and other chemical reactions

MECHANOCHEMISTRY OF POLYMERS

551

initiated by mechanoradicals. This ampoule is a variation of the ampoule for ball-milling, described in Section 3.1. This ampoule for chemical reaction studies has two arms. One arm is for evacuation before production of mechanoradicals and this arm is sealed off after sufficient evacuation, as shown in Fig. 71. The second (connecting) arm, having a breakable seal, connects with a small tube containing a reactant. In Fig. 71 crushed polymer pieces are located at the bottom of the ESR sample tube, the temperature of which is controllable by the coolant used in the Dewar flask. The reactant in the evacuated vessel is transferred to the ESR sample tube by breaking the seal in the connecting arm. In this way the reactant comes into contact with mechanoradicals at a fixed temperature. The temperature of the sample after contact can be adjusted by varying the temperature of the coolant in the Dewar flask. After the sample is held at a fixed temperature for some duration, the ESR sample tube is then transferred to an ESR spectrometer for observation of the ESR spectrum. By such a procedure one can follow changes in the ESR spectrum after heat treatment of the sample in contact with the reactant. It is recommended that one carries on ESR observation at 77 K after each heat treatment, because the reactions are "frozen" at 77 K and an ESR spectrum containing information on the current reaction state is easily obtained at 77 K. The other method of mechanical polymerization is known as "simultaneous polymerization". In this method polymer flakes are fractured by ball-milling apparatus in the presence of monomers. For this purpose no special ampoule is needed but the ball-milling ampoule can be used for the experiment. The ball-milling ampoule, described in Section 3.1. is connected with a vacuum system for evacuation of the ampoule. After the evacuation, a monomer is introduced into the ampoule via the vacuum system. After estimation of the amount of monomer in the ampoule (by measuring the pressure), the arm is sealed off. At temperatures lower than the melting point of the monomer, such as 77 K, the polymer flakes in the ampoule are covered with frozen monomers. Ball-milling is carried out for this sample at a fixed temperature. The polymer flakes are milled in the presence of the monomers. One big difference between the "simultaneous polymerization" and the "post-polymerization" is that mechanical action, such as milling, is applied during the polymerization process. Thus, mechanical action may affect polymerization in this method. The postpolymerization is convenient for studies of the mechanisms of polymerization initiated by mechanoradicals because one can interrupt the polymerization by rapid cooling to 77 K. The simultaneous polymerization technique is suitable to carry on polymerization at high efficiency. The investigation of mechanochemical polymerization was carried out in more detail for polytetrafluoroethylene (PTFE) mechanoradicals, and the mechanochemical copolymerization of PTFE with methyl methacrylate, vinyl acetate and ethylene was experimentally proved.191'192Mechanochemical copolymerization was also successfully attempted for other polymers.

552

J. SOHMA

9.2. Mechanochemical polymerization &itiated by PTFE mechanoradicals 9.2.1. Post-polymerizations &itiated by PTFE mechanoradicals - F l a k e s o f P T F E in the a m p o u l e shown in Fig. 71 were ball-milled at 77 K for several hours a n d the f o r m a t i o n o f m e c h a n o r a d i c a l s was shown by o b s e r v a t i o n o f the E S R s p e c t r u m o f the P T F E m e c h a n o r a d i c a l s . T h e central p a r t o f this s p e c t r u m is shown in Fig. 72(a). In a d d i t i o n to this central b a n d , two wings c h a r a c t e r i s i n g the s p e c t r u m o f the radical F

I M E"

I F in a frozen state were clearly detected. A f t e r this c o n f i r m a t i o n o f the presence

(a)

~

(b)

//~

Ob$ 77 K

lOG

FIG. 72. ESR spectra (a) observed from PTFE mechanoradicals before MMA treatment, (b) observed at 77 K after heat and MMA treatment at 273 K for 1min, (c) observed at 77 K after heat and MMA treatment at 273 K for 5 min.

MECHANOCIfEMISTRY OF POLYMERS

553

of P T F E mechanoradicals, a methyl methacrylate (MMA) monomer was introduced into the sample tube at 77 K by breaking the breakable seal. After the contact of the m o n o m e r with the fractured P T F E flakes at 77 K, the ESR spectrum of the sample was observed and no change in the spectrum was found, even after the contact with the monomer. The sample was then warmed to 273 K and the ESR spectrum of the sample was observed after holding for I rain at this temperature. The spectrum observed after the heat treatment is shown in Fig. 72(b). This spectrum shows major changes from the original one. The ESR spectrum observed after longer heat (and M M A ) treatment at 273 K of the same sample is shown in Fig. 72(c). The main feature of this spectrum (c) is a nonet, which is the characteristic spectrum of the propagating radical of poly(methyl methacrylate). 1%~94'~9sThe spectrum (b) in Fig. 72 is a superposition of the two spectra (a) and (c). This means that the state of the system corresponding to the spectrum (b) is intermediate between the initial state and the final state. This experiment demonstrates clearly that polymerization of M M A was initiated by the PTFE mechanoradicals and propagation occurred under the heat treatment. The total intensity of the ESR spectra was approximately unvaried throughout the experiment. This means that almost all the PTFE mechanoradicals initiate M M A polymerization. Careful experiments proved to us that a temperature as high as 273 K is not necessary for the initiation of polymerization, but a temperature higher than the melting point of M M A is required for the initiation. Propagation occurred even at the temperature of initiation but the propagating rate was very small. It can be said, therefore, that the polymerization is initiated by the P T F E mechanoradicals at temperatures higher than the melting point of M M A , but no post-polymerization of solid M M A is initiated by PTFE mechanoradicals. The sample, in which a fi'action of the PTFE mechanoradicals had been converted into P M M A radicals by the short heat treatment, was then brought into contact with air at 77 K. The spectrum observed at 77 K after the air contact is shown in Fig. 73. The spectrum is apparently a superposition of the asymmetric component in the center, which is attributed to a peroxy radical, and the other component, marked with arrows, which is attributed to the P M M A propagating radicals. The result indicates that the PTFE mechanoradicals react with oxygen more efficiently than do the P M M A propagating radicals. After the formation of the P T F E peroxy radicals no conversion of them to P M M A propagating radicals was observed. This fact means that the initiation of the polymerization is inhibited by oxygen. Mechanochemical polymerization of ethylene by the P T F E mechanoradicals was also attempted by the post-polymerization method. Changes in the ESR spectrum were observed after heat treatment at 113 K, which is higher than the melting point (92 K) of ethylene. After the longer heat treatment at this temperature, nearly 100% conversion of the P T F E mechanoradicals into propagating polyethylene radicals was observedJ 91 The same conclusions were derived in

554

J. SOHMA

l

l

FIG. 73. ESR spectrum of PTFE mechanoradicals contacted with MMA monomer and then with air at 77 K observed at 77 K. Arrows indicate peaks corresponding to the strong quartet of the PMMA spectrum.

this case, that is (1) the need for a heat treatment at a temperature higher than the melting point of the monomer used, and (2) 100% conversion of the mechanoradicals to the propagating radicals. 9.2.2. Simultaneous polymer&ations &itiated by P T F E mechanoradicals - An ESR spectrum observed at 77 K after simultaneous polymerization of M M A at 77K is shown in Fig. 74. The spectrum is identical to that of the P M M A propagating radical and has no component from the PTFE mechanoradicals, as proved by a complete disappearance of the wing peaks. Since 77 K is lower than

- O l G~ 25.~G

v

FIG. 74. ESR spectrum observed at 77 K after simultaneous polymerizationof MMA by PTFE mechanoradicals.

555

MECHANOCHEMISTRY OF POLYMERS

peak 50G

L

,

x5

FIG. 75, ESR spectrum observed at 77 K after simultaneous polymerization of ethylene by PTFE mechanoradicals.

the melting point of MMA, the polymer flakes were covered with a coating of solid MMA. By the simultaneous polymerization method, both the PTFE polymer and the solid MMA are simultaneously fractured by the ball-mill. It is known, however, that no mechanoradicals are generated by fracture of a solid organic compound of low molecular weight. Thus, no mechanoradicals are produced by milling of the MMA solid and the propagating radicals in the polymerization of MMA do not originate from MMA mechanoradicals. Under such a milling condition the only radical species produced mechanically are the PTFE mechanoradicals, although no spectral component from the PTFE mechanoradicals was observed. The arguments presented in the preceding paragraphs lead one to conclude that the polymerization of MMA is initiated by the PTFE mechanoradicals with 100% efficiency and the polymerization of MMA propagates even at a temperature as low as 77 K in the simultaneous polymerization method. Since the PTFE flakes were covered with frozen MMA at 77 K, the PTFE mechanoradicals, as they are produced may easily react with nearby MMA monomer molecules. Similar simultaneous polymerization experiments were carried out for PTFE with ethylene and vinyl acetate. ~9~The observed spectrum for the simultaneous polymerization with ethylene is shown in Fig. 75. The spectrum appearing in the quintet is attributed to the propagating radicals of ethylene polymerization H

H

I

I

~C-C"

I

H

I

H

in which the four protons couple equally with the unpaired electron due to the high mobility. Such higher mobility was never observed for the mechanoradi-

556

J. SOHMA

X 4,6

No Wing peak 50 G

FIG. 76. ESR spectrum observed at 77 K after simultaneous polymerization of vinyl acetate by PTFE mechanoradicals.

cals, which were so strongly trapped on the surface as to hinder rapid motion. On the other hand, the propagating radicals of ethylene polymerization are presumably mobile if they are sufficiently separated from the sites trapped on the surface by being situated at the ends of sufficiently long polyethylene chains. If this is the case, the higher mobility suggests that the polymerization propagates with good efficiency even at 77 K. The observed spectrum for the simultaneous polymerization with vinyl acetate is shown in Fig. 76. The spectrum is characterized mainly as a triplet with satellites. The spectrum in no way resembles that of the PTFE mechanoradicals. The reported spectrum ~93of poly(vinyl acetate), which is attributed to a chain radical, H

H

H

I

I

I

"- C - - C - C ~

I

(R: - O C O C H 3 )

I

C R is not similar to the observed spectrum. The triplet is assigned to a radical resulting from chain scission H H

I

I

~ C - - C " 194

I

R

I

H

Although the assignment of the observed spectrum is not completely confirmed, it seems most likely that the observed spectrum originates from the propagating radical in the polymerization of vinyl acetate. For the two cases of ethylene and

MECHANOCHEMISTRY OF POLYMERS

557

vinyl acetate, one may conclude that polymerizations initiated by the PTFE mechanoradicals propagate in the simultaneous polymerization technique at 77 K. 9.2.3. Experimental evidence f o r copolymerization - Since mechanochemical polymerization is initiated by the mechanoradicals generated by fracture of the matrix polymer and polymerization propagates only via the added monomer. the polymerization must be a sharp block copolymerization with the blocks separated by the initation point. In other words, a new block made by this mechanochemical polymerization technique is chemically bonded to a block of the original polymer, P T F E in this case, in the matrix. However, this copolymerization needs to be experimentally proved. After the complete conversion of P T F E mechanoradicals to PMMA radicals had been verified by ESR, the radicals in the sample were thoroughly annihilated by heat treatment at higher temperatures. The annihilation of the propagating radicals meant that the polymerization reaction was terminated. The sample was then washed with a large amount of hot benzene in order to remove excess MMA monomer. Simultaneously, PMMA homopolymer, which was not chemically bonded to PTFE molecules, was dissolved by the hot benzene and removed from the sample. Thus, it is safe to assume that PMMA molecules unremoved by this treatment were chemically bonded to PTFE, which was not dissolved b~ the hot benzene. The washed sample was then dried under vacuum at room temperature. An IR spectrum from the dried sample, free from either excess monomer or P M M A homopolymer, was observed at room temperature. An example of the IR spectrum is shown in Fig. 77. The intense band in the range of ( l l 0 0 1400cm ~) is due to the C - F bond. Weak but clear peaks appear at both 1735 cm ~and 2906 cm L. The former is attributed to the characteristic peak of the C = O group and the latter to the methyl peak. The appearance of these peaks provides us with experimental evidence for the presence of P M M A in the system even after the washing. Thus, one is led to conclude that copolymerization of MMA with the matrix P T F E is initiated by the P T F E mechanoradicals. The IR spectrum was also observed from the mechanochemical polymerization of vinyl acetate by the P T F E mechanoradicals after the same treatment as described above. The spectrum is shown in Fig. 78. In addition to the main peak of the C - F band, the peak assigned to the C--O group appears clearly. Thus, the copolymerization of vinyl acetate with the P T F E matrix is experimentally evidenced. It is very interesting that one finds a drastic increase in the wettability of P T F E after mechanochemical copolymerization with vinyl acetate. Since P T F E is itself not wettable but poly(vinyl acetate) is very hydrophilic, the increased wettability of the sample indicates that the surface nature of the sample is determined not by the P T F E but by the poly(vinyl acetate) in spite of the fact that the bulk properties of the sample are still those of PTFE. This fact also

558

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-t)

F1G. 77. IR spectra: (a) PTFE, (b) MMA and PMMA, (c) spectrum from the washed mechanochemically copolymerized sample. Solid line is enhanced spectrum. means that the copolymerization of vinyl acetate is initiated by the radicals trapped on the surface. This experiment suggests the possibility that the surface nature can be selectively changed by mechanochemical polymerization without changing the bulk nature of a matrix polymer. 9.3. Mechanochemicalpolymerization initiated by mechanoradicals qf other

polymers Mechanochemical polymerizations initiated by mechanoradicals of polymers other than PTFE were also investigated. Polypropylene mechanoradicalinitiated mechanochemical polymerization of M M A proceeded with good efficiency by either the post-polymerization or simultaneous polymerization techniques. Although few combinations have been tried, there is no reason to

MECHANOCHEMISTRYOF POLYMERS 25 r

,

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FIG. 78. IR spectra: (a) PTFE, (b) vinyl acetate, (c) spectrum from the washed mechanochemically copolymerized sample.

imagine that mechanoradical-initiated polymerizations are exclusive to P F T E and polypropylene, for which mechanochemical polymerizations have been investigated. One is encouraged to try mechanochemical copolymerization for various combinations of matrix polymers and monomers. The mechanochemical polymerization of M M A with coal by the simultaneous polymerization method has also been experimentally verified. ~%Akabira coal was ball-milled with M M A monomers at 77 K and the ESR spectrum from the propagating radicals of P M M A was observed after the milling. The IR spectrum observed from the sample washed with hot benzene showed a peak corresponding to C = O band. Thus, one is convinced that M M A is copolymerized with the coal by the mechanochemical method.

J. SOHMA

560 I0. M E C H A N O C H E M I C A L MECHANORADICALS

POLYMERIZATION INITIATED OF INORGANIC COMPOUNDS

BY

10.1. General features of mechanoradicals of inorganic compounds Observation of an ESR signal from crushed silicon was reported by Walters and Estle 197 in 1961. This was almost at the same time when ESR began to be applied to the mechanochemistry of polymers. It was known among researchers on surface phenomena that the cleavage of a solid is one of the best methods to produce clean surfaces j98 and dangling bonds are produced by such cleavages. The jargon "dangling bond" is preferred by researchers in surface science to describe an orbital which can form a chemical bond but lacks a partner and has an unpaired electron. Thus, either an atom or a molecule with a dangling bond can be the same as a mechanoradical, defined earlier in this article. Several reports have been published of ESR studies on the dangling bonds (or mechanoradicals) produced by crushing certain inorganic materials such as silicon, 199-2°2 germanium, 2°2-2°4 SIO2, 2°5 several II-V semiconductors, 2°6'2°7 and graphitefl °8 ESR spectra observed from crushed ZnO were studied by Sadahiro. 2°9 Most of the ESR spectra were attributed to either defects on a fresh surface or to various surface states, but the chemical species responsible for these spectra observed from the crushed material are not well identified. More research from a chemical viewpoint will be required to identify the radical species produced by the crushing. Many researchers have reported chemical reactions induced by the fracture of either inorganic materials or metals. For example, silica crushed in the presence of styrene initiated a chemical reaction and some styrene became chemically bonded to the silica. 2j° Metal pieces undergoing friction while immersed in certain polymerizable organic liquids, such as styrene and acrylonitrile, became covered with solid materials in the vicinity of the frictional spots. Kargin and Plate 2~ reported that some monomers were polymerized by mechanically fractured metallic oxides such as alumina and by fractured silicates. Similar polymerizations have also been reported by other researchers. Examples of these mechanochemical reactions on both inorganic and metallic substances have been collected in a book by Kubo. 2 These reports encouraged us to investigate mechanochemical polymerizations of organic monomers initiated by mechanoradicals derived from inorganic compounds.

10.2. Mechanochemical polymerizations &it&ted by mechanoradicals derived

from alumina 212 10.2.1. Polymerization of ethylene - Powdered y-alumina (alumina oxide, ),A1203) was heat-treated under vacuum (10 4Torr) for several hours at 363 K, for the purpose of desorption of adsorbed water. After this pretreatment the

MECHANOCHEMISTRY OF POLYMERS

561

t

K Ob$.

FIG. 79. ESR spectrum observed from milled alumina

in v a c u o

at 77 K.

alumina sample was milled in v a c u o at 77 K by the ball-milling apparatus. An ESR spectrum observed from the milled alumina is shown in Fig. 79. The observation of an ESR spectrum is positive evidence for the formation of mechanoradicals from the alumina. However, it is prohibitively difficult for even ESR specialists to identify chemical species of mechanoradicals responsible for this observed broad spectrum. Mechanochemical polymerizations of ethylene initiated by the mechanoradicals from alumina were attempted by the methods of both "simultaneous polymerization" and "post-polymerization". The line shape (a) in Fig. 80 is a spectrum observed after the milling of the alumina with ethylene monomer, that is, from simultaneous polymerization. Apparently the spectrum is quite different from that of the alumina mechanoradical (see Fig. 79) but is identical to the spectrum of radicals derived by chain scission of polyethylene, shown in (b) in the same figure. In post-polymerization, no radical conversion was found at 77 K even after introduction of ethylene. However, changes in the line shape were observed after heat treatment at temperatures above the melting point of ethylene, and the ESR spectrum component produced by the heat treatment was ascribed to the propagating radicals. An example of these changes is shown in 2+

2+

(Q)

(b) ~._;._;.~ 2 0 G Obs. 77 K

FIG. 80. (a) ESR spectrum observed after milling of alumina with ethylene at 77 K. (b) ESR spectrum of the propagating radical of polyethylene.

562

J. SOIqMA

( b ) "--'~

"

20G 77 K Obs.

FIG. 81. ESR spectra observed from post-polymerization of ethylene by milled alumina. (a) Spectrum from the milled alumina after contact with ethylene at 77 K, (b) after subsequent heat treatment at 123 K for 5 min.

Fig. 81. The peaks of the multiplet in Fig. 81(b) correspond to (b) in Fig. 80 and the distortion of the spectrum is caused by superposition with the spectrum (a) from remaining alumina mechanoradicals. Little change in the spectral intensity was observed. This fact indicates that the conversion efficiency of the alumina mechanoradicals to the polyethylene propagating radicals was close to 100%. The absence of a spectral component from the alumina mechanoradical in the simultaneous polymerization (Fig. 80(a)) provides us with good evidence for 100% efficiency of radical conversion even at 77K for the simultaneous polymerization. Since the propagating radicals decay at higher temperatures (in the range of room temperature) only a weak ESR signal from the propagating radicals was observed at temperatures above 200 K. Thus ESR spectroscopy is not a suitable technique to obtain experimental evidence for mechanochemical polymerization at room temperature. A pressure drop of the monomer gas is expected if monomer molecules in a gas phase are polymerized to solid polymer. An ampoule having one more arm (for monitoring pressure in the ampoule) was designed to find out whether or not ethylene is polymerized at room temperature by milling of alumina in an atmosphere of ethylene. It was found that pressure dropped with increasing milling time. On the assumption that all the pressure drop is caused by the polymerization, one could estimate the amount of polymerized ethylene. This amount of polymerized ethylene per unit weight of alumina is plotted against the milling time in Fig. 82. The curve demonstrates that polymerization proceeds during milling. It is interesting that one finds no remarkable difference in polymerization efficiency between polymerizations at 77 K and at room temperature (at least during the early stages of polymerization).

MECHANOCHEM1STRY OF POLYMERS

563

0.6

05

_

0 a t R.T • at 77K

C~H4

04

0.3

E 02

OI

,

I

5O

I

IOO

F r a c t u r e time ( hr )

FIG. 82. Total monomer gas consumption as a function of milling time for the milling of alumina in the presence of gaseous monomers. Open circles show the results from the simultaneous polymerization of ethylene at room temperature. Filled circles show the results from the simultaneous polymerization of ethylene at 77K. Triangles show the results from the simultaneous polymerization of propylene at room temperature. IR spectra observed from the alumina and ethylene after the milling are shown in Fig. 83. Absorption peaks at the C - H stretching band (2900 cm ~) and lhe methylene deformation band (1470cm 1) appeared in the spectrum (a) as shown with the arrows. The milled sample was then washed thoroughly with hot toluene and the I R spectrum of the washed sample was observed (Fig. 83(b)). The absorption peaks at both 2900 and 1470cm ~ remained even after the extraction, although the peak intensity at 2900 cm ~was reduced by nearly half. Milled alumina, after the mechanochemical polymerization, is covered with polyethylene and the weight of the starting sample increased by the weight of the polymer formed. This weight can be evaluated by comparing the weights before and after calcination of the sample (after simultaneous polymerization). This is because only organic polymer is burned by the calcination. The weight loss by calcination, which corresponds to the weight percent of polyethylene produced by the mechanochemical method, is 7.8%. It is interesting to note that the percentage is reduced by nearly 50% after extraction with hot toluene. Both IR and the weight loss measurement demonstrate that 50% of the mechanochemically polymerized polyethylene is not extracted by the treatment. There are two possible mechanisms for the observed non-extraction of the mechanochemically polymerized ethylene. One is that non-extracted polymers are chemically bonded. This seems to be compatible with the concept

564

J. SOHMA

~xs

I 4000

I

I

I

I

I

r

I ~

3000

I

I

r t6oo

I

J

I

I 14oo

(cm-')

FIG. 83. IR spectra: (a) aftermillingof aluminawithethylene,(b) spectrumfromthe sample after washingby hot toluene.

of initiation of polymerization by the mechanoradicals of alumina. The polyethylene, chemically bonded with the alumina matrix, is assumed to have a bond whose type is either -O-AI-C-C- or -AI-O-C-C-. We do not have any experimental evidence showing a preference for either one of these species, because the species of the alumina mechanoradical is unknown. The other possibility which could account for the non-extraction is the high molecular weight of the formed polymers. It was recently found that polyethylene molecules, having extremely high molecular weights, c a . 106, were generated by mechanochemical polymerization.2~3 However, the fraction with extremely high molecular weight comprised much less than half the total product. Therefore, the former mechanism, that of chemical bonding of the polymer chains with the alumina matrix, seems to be more reasonable. If so, the mechanochemical method is presumably the easiest way to form a chemical bond between an organic molecule and an inorganic solid. 10.2.2. P o l y m e r i z a t i o n o f p r o p y l e n e 2~2 - Mechanochemical polymerization of propylene initiated by the alumina mechanoradicals was attempted by the simultaneous polymerization technique at 77 K. The observed ESR spectrum is identical to that of the propagating radicals observed in polymerization of propylene, and therefore propylene was found to be polymerized mechanochemically. By the measurement of the propylene gas consumption during the milling of alumina, which is shown in Fig. 82, the polymerization efficiency of propylene was found to be smaller than that of ethylene. The IR absorption at 2900 cm ~ was noted in the spectrum of alumina milled in the presence of

MECHANOCHEMISTRY OF POLYMERS

565

AH=I82G (a)

(b)

AH=i0.5G -

x3 Obs. R.T

FIG. 84. ESR spectra observed from alumina milled in an acetylene atmosphere at 298 K, (a) immediately after the milling, (b) after heat treatment at 408 K for 6hr.

propylene at room temperature. Thus, mechanochemical polymerization is verified by this IR observation. However, most of the polypropylene on the milled alumina could be extracted by ethyl ether and the extract was not solid, but very viscous and oily, after evaporation of the solvent. From the IR spectrum of this extract, the viscous liquid was identified as atactic polypropylene and its molecular weight was determined as 300-500 by means of the vapour pressure method. The failure to form polypropylene with a higher molecular weight is a remarkable difference from the behavior of ethylene, in spite of the fact that identical methods were taken for the mechanochemical polymerization of these two monomers. We are not in the position to consider reasons why such a difference occurs in the polymerizations of these similar monomers by the same method (although radical chain transfer to propylene monomer may, of course, be a factor). 10.2.3. P o l y m e r i z a t i o n o f acetylene 214 - v-Alumina was heat treated at 900 K for 20 min (after 5hr of evacuation at room temperature) before the milling experiments. The pretreated alumina was milled in an acetylene atmosphere at room temperature. The observed ESR spectrum after the milling is a broad singlet, as shown in Fig. 84(a). Although the line shape looks similar to that of the alumina milled in vacuo, the comparison of the spectra shown in Fig. 85 clearly demonstrates the difference in the line shape and the g factor. The g factor of the spectrum in Fig. 84 was determined as g = 2.0034. The singlet with this g factor was found m o r e stable than that of the alumina mechanoradical at higher temperatures, as shown in Fig. 84(b), and the line-width became narrower after

566

J. SOHMA /'~i~ Mr~+ - - : At,O:.+Cal'+I+,

]

",

,'

',,

20G,,

+ , K oOb+. o+. 77K

FIG. 85. ESR spectrum observed from alumina milled in vacuo at 77 K (dotted line). ESR spectrum observed from alumina milled with acetylene at 77 K (solid line).

heat treatment. The values determined for the g factor as well as the stability of the singlet spectrum strongly suggest that the radicals responsible for this spectrum are polyenyl radicals. 215 The line width of the singlet was found to decrease after heat treatment at 295 K for 5 min. The observed reduction of the line width after heat treatment can be interpreted as due to the decay of the smaller polyenyl radicals at the higher temperatures. This is because shorter polyenyl radicals show broader line-widths and are less stable than the longer ones. 215 It is also known that polyacetylene produced by Ziegler-type catalysts shows an ESR spectrum, 216-2j9 although polyacetylene is not a radical at all. The ESR signal arises from unpaired electrons existing due to the misfit of the chemical bonds caused by mixtures of cis and trans conformations. The presence of these unpaired electrons is inevitable in any polyacetylene. Also the g factor of the spectrum, shown in Fig. 84, is close to the reported values of the g factor for the ESR spectra of the various polyacetylenes. Therefore, it is quite reasonable to assume that the ESR spectrum observed after the milling of alumina with acetylene originates from polyenyl radicals in the polyacetylene. A pressure drop was observed when alumina was milled in an acetylene atmosphere at room temperature. 214 Since the pressure drop is caused when molecules in the gas phase become part of a solid phase, the polymerization of acetylene induced by the milling of alumina was experimentally evidenced by this observed pressure drop. The amount of polyacetylene formed per unit weight of alumina is plotted against milling time in Fig. 86. In the same figure the radical concentration estimated from the observed ESR intensity is also plotted vs the milling time. Both curves increase with milling time. This fact indicates that both polyacetylene and polyenyl radicals are produced simultaneously by the milling. It was also found that the colour of the milled sample (black) was different from that of the non-milled alumina. Both infrared and visible absorption spectroscopy were attempted on the milled product, but meaningful data were

MECHANOCHEMISTRY

OF POLYMERS

567

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+ E E 0.5

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30

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Fracture time (hr)

FIG. 86. Increase of acetylenegas consumption per gram of alumina (open circles) and radical concentration per gram of solids (filled circles) with milling time for alumina milled in an acetyleneatmosphere. not obtained. This is not surprising if one remembers that polymers formed by mechanochemical methods are located on the surfaces of milled particles and the ratio of the weight of the polymer to that of the bulk matrix is rather small. Then, the observed spectra come from mainly the bulk matrix and it is hard to detect a spectral component from the very small polymer fraction of the sample. ESR, however, detects selectively unpaired electrons and is not interferred with by the overwhelming number of paired electrons. The evidence so far accumulated (unfortunately not direct but indirect) indicates that polyacetylene is produced by the milling of alumina in the presence of acetylene monomer.

10.3. Mechanochemical polymerization inith~ted by mechanoradicals formed from other inorganic compounds As described in the preceding sections, it was established that mechanoradicals formed by milling of 7-alumina initiate mechanochemical polymerizations of ethylene, propylene and acetylene. The search was then extended to the milling of other substances, to look for the possibility of resultant mechanochemical polymerization by using ethylene as a probe. 22° In Fig. 87 the ESR spectra observed after simultaneous ethylene polymerization during milling at 77 K are shown for several compounds, (a) silica, (b) silica-alumina, (c) boron oxide and (d) titanium oxide. All the spectra contain the multiplet component from propagating radicals in ethylene polymerization. Hence, mechanoradicals derived from these compounds, which have not been identified chemically, are shown to be more or less able to initiate mechanochemical polymerization. Initiation efficiencies for mechanoradicals derived from silica and silica-alumina

568

J. SOHMA

ca,

SiO+zCzH4 _

JL~

(b) Si02'AL203~ ,20_~Gj+C2H4 VV~ .

TiO2+CsH4

FIG. 87. ESR spectra observed from several metal oxides milled in an ethylene atmosphere at 77 K.

were similar to those found in milling of alumina. However, titanium oxide was less effective in initiating mechanochemical polymerization. Other compounds, such as NaC1, MgO, Ag20, MnO2, CrO3, Cr203, NiSO4 and ZnO, were investigated by ESR as potential initiators of mechanochemical polymerization. However none of these showed positive evidence of the presence of propagating polyethylene radicals, though some of them showed an ESR spectrum of their own mechanoradicals.

MECHANOCHEMISTRY OF POLYMERS

569

;,,-Alumina, for which mechanochemical polymerization was studied in more detail, is known to have catalytic activity. Thus, there is a possibility that all the reactions described in the former section might be due to catalytic action. In order to check this possibility, two experiments were performed. 2~2'22°One was to find out whether or not the polymerization of ethylene is initiated and can proceed at room temperature without any mechanical action. The result was negative and the mechanical action was actually required for the polymerization. The second one is to use other alumina, which is much less porous and known as lacking in catalytic activity. Even when this type of alumina was used, it was found that the mechanochemical polymerization was initiated and proceeded with same efficiency under the milling. These results demonstrate that the real cause of the polymerization is not catalytic activity but mechanical action. 10.4. Characterization of polyethylene polymerized by the mechanochemical method 213 Polyethylene (PE) was produced by simultaneous polymerization initiated by milled alumina in an ethylene atmosphere at room temperature, as described in Section 10.2.1. The mechanochemically produced PE was characterized by solubility and GPC behavior. Nearly 50% of the product could be extracted by various organic solvents, such as petroleum ether, n-heptane, toluene, tetralin and o-dichlorobenzene. The other 50% of the product remained on the surfaces of the milled alumina. Characterization of the insoluble material was difficult. Characterization was attempted only for the extracted fractions of the mechanochemically produced PE. Molecular weight distributions were determined by GPC and examples are shown in Fig. 88. It was surprising to find the presence of PE with extending molecular weights as high a s 107 , because the longer chains could be more easily scissioned by the milling.

80--

Method ( I )

60_ o•°

5_0 "5

n -Heptane PetroLeum

~

ToLuene

4O

/ /\

%,.,et oL n

0 I02

iO3

104

iO5

iO6

107

iO8

MoLecuLar weight

FIG. 88. Molecular weight distributions determined from G P C measurements on polyethylene prepared by mechanochemical polymerization of ethylene on milled alumina. Distributions are of soluble fractions extracted by several hot organic solvents such as petroleum ether, n-heptane, toluene, tetralin and o-dichlorobenzene.

570

J. SOHMA Amyl-C 2 lon0 chain

'3C-NMR Long choin-C

Vloin chain

AmyI-C3~ Butyl.Amyl-Methine Long c

h a i n ~

40

Butyl.Amyl-Ci long chain

.__

III

L~]

I

30

. 11

-Ethyl-Cf

20 ppm

I0

FIG. 89. 13C-NMR spectrum of petroleum ether-extracted polyethylene from the mechanochemical polymerization of ethylene on milled alumina. TMS was used as an internal standard. Solvent used for NM R sample was deuterated chloroform. Branched chains are shown on the corresponding peaks.

13C-NMR spectra were obtained for both the hot petroleum ether-extracted PE and the hot tetralin-extracted PE, as shown in Fig. 89 and Fig. 90, respectively. The lower molecular weight PE, extracted by n-heptane, shows various peaks in addition to the main peak corresponding to the methylene carbons. This means that this PE has many branches. Examples of the identified branches are shown in Fig. 89. Therefore, the heptane-soluble fraction of mechanochemically generated PE is similar to branched low density polyethylene. In contrast, the 13C-spectrum of the tetralin-extracted PE is mainly a singlet, as shown in Fig. 90. This spectrum indicates that the tetralin-extracted PE is a non-branched one. In the conventional production of HDPE both the catalysts and pressures used are different from those used in the production of LDPE. It is interesting to note that the mechanochemical polymerization of ethylene by milled alumina produced simultaneously both LDPE and HDPE. Although the fraction of PE with higher molecular weight, which corresponds to HDPE, seems to be small in the observed GPC curve (Fig. 88), one should remember that nearly 50% of the products were unextracted and remained on the milled alumina. The CP-MAS '3C-NMR spectrum was observed for the mixture of insoluble polyethylene and remaining after extraction. The observed spectrum was a sharp singlet like the spectrum in Fig. 90. This means that the major component of the unextracted PE is a non-branched PE, that is HDPE, which corresponds to the PE with higher molecular weight in this method. Thus, it is safe to say that both LDPE with low molecular weight and HDPE with

MECHANOCHEMISTRY OF POLYMERS

571

m3C-NMR

,J L

50

I

40

l

I

~0

20

I

I0

I

0

ppm

FK;. 90. )~C-NMR spectrum of tetralin-extracted polyethylene from the mechanochemical polymerization of ethylene on milled alumina.

high molecular weight were simultaneously produced by this mechanochemical polymerization. 11. M E C H A N O C H E M I C A L L Y - I N D U C E D

R E A C T I O N S OF R A D I C A L S

In last two sections, mechanochemically-induced polymerizations were described. The post-polymerizations could be considered as a kind of thermal reaction, because the thermal energy supplied by the heat treatments was required for the propagation reactions in the polymerizations initiated by the mechanoradicals. In contrast to the post-polymerizations, the simultaneous polymerizations should be taken as mechanochemically induced reactions of radicals on account of two observed facts. The first is that the mechanical energy supplied by the milling is always needed for the polymerization reactions at 77 K. The second is that no polymerization propagation reactions occur when monomers are added to solids previously milled without monomers and the mixtures are then stored at 77K, a temperature at which, by contrast, the simultaneous polymerization proceeded. It was really a surprise when the simultaneous polymerization was first found to proceed at 77 K. Apparently the mechanical energy input is the original cause of these chemical reactions at 77 K. In other words the energy needed for the chemical reactions was supplied in the form of mechanical energy, which was then transferred into chemical energy. A mechanism, through which mechanical energy can be transferred into chemical energy, is a very interesting and important problem in mechanochemistry. This problem is still open to question and we are not yet at a stage to debate the mechanism. However, there are two possibilities for processes of energy transfer from the mechanical sources to chemical reactions.

572

J. SOHMA

One is the process, in which mechanical energy is converted to a form of thermal energy. For example, a sample could be locally heated by friction occurring due to mechanical action, even when the bulk temperature of the sample is kept constant. In this case the same reaction should be induced by heat treatment, because the reaction would occur when sufficient thermal energy is supplied, irrespective as to whether the origin is friction or not. By contrast, if the reaction is induced not by heat treatment but purely by a mechanical method, thermal energy is not involved in the process of energy transfer and the mechanical energy directly contributes to the chemical reaction. This is called a direct effect of mechnical energy on a chemical reaction, although details of the effect are completely unknown. The former suggested mechanism, a thermal conversion of mechanical energy, should be called an indirect effect. Of course it is possible that the two processes occur simultaneously. An example of the direct effect was observed for mechanochemical conversion of the mechanoradicals of poly(methyl methacrylate) (PMMA), as follows. Examples of the former process were found for polyethylene and polypropylene.

11.1. Mechanochemieally-induced reaction of poly(methyl methacrylate) mechanoradieals 41 Poly(methyl methacrylate) (PMMA) was ball-milled in vacuum at 77K. P M M A molecules are scissioned with very high efficiency by ball-milling at 77 K and only 0.1 hr of milling time was enough to produce mechanoradicals giving an ESR spectrum with a good signal-to-noise ratio. As the milling time increased from 0.1 hr to 18 hr, the changes in line shape of the ESR spectra were followed. Examples of the observed spectra are shown in Fig. 13. Apparently the line shapes were changed. Analyses of the observed spectra were performed as follows. The ESR spectrum of a P M M A radical is well known 221 as a nonet consisting of a quartet and a quintet and this spectrum is attributed to a main-chain scissioned radical o f PMMA, H CH3

J I ~ c - c (A). I J H COOCH3 Remembering this characteristic spectrum, the spectrum, Fig. 13(a), observed after long milling can be decoposed into the nonet and a doublet, as shown in Fig. 14. The doublet is assigned to a radical,

CH3

CH3

I I ~C CH-C~ (B). I I COOCH 3 COOCH 3

MECHANOCHEMISTRY OF POLYMERS

573

The spectrum (b) in Fig. 13, obtained after the short-milling, is apparently different from the spectrum (a) shown in Fig. 13. However, the outermost peaks in the spectrum, which correspond to the extreme outer peaks of the nonet, remain almost unvaried in separation as well as intensity during the milling. Moreover, there are peaks corresponding to the doublet component in the spectrum (b) and therefore the same doublet is involved in this spectrum. Then. the observed spectrum (b) can be decomposed into three components, the nonet, the doublet and a triplet, as shown in Fig. 15. Consequently, one may believe that the spectrum (b) is a superposed spectrum of the honer, the doublet and the triplet. The triplet spectrum can be reasonably assigned to the radical CH 3

I ~c COOCH 3

H I

c

(c).

H

It was found that the doublet is gradually enhanced with increasing milling time. 4~ As described in Section 3.3, main-chain scission is the primary reaction caused by the milling and the radical (B) is a secondary radical derived from the initially formed radical (C). The results obtained from the milling of PMMA at 77 K can be summarized as follows: (1) The main chains of PMMA are scissioned in the early stages of milling, (2) The conversion of the primary radical (C) into the radical (B) is induced by mechanical action at 77 K, and (3) the radical (A) remains stable and unchanged during the milling. In another experiment, milling was stopped after observation of the spectrum (b) in Fig. 13 and the sample was stored at 77 K for more than 18 hr. No change in the ESR spectrum was observed after storage. This means no radical conversion was caused by storage at 77 K without mechanical agitation. The sample was then heat-treated at various temperatures and the ESR spectrum was observed at 77K after each heat treatment. No change was observed after heat treatment at temperatures below 0°C. 222 The changes observed after heat treatment at temperatures above 0°C were quite different from those induced mechanochemically. These differences in the spectral changes observed after mechanical and thermal agitations indicate that the radical conversion caused by mechanical energy is not thermally induced. The conversion of the radical (C) to the radical (B), which is a hydrogen abstraction reaction by the radical (C), is caused not by the heat treatment but by the milling. Thus, the hydrogen abstraction reaction is an example of the direct effect defined in the last section. The comparison mentioned above encourages us to consider that mechanical action might have a particular effect on a chemical reaction, which is inherent in the mechanical excitation but not duplicated with other sources of agitation such as thermal.

574

J.

SOHMA

11.2. Mechanochemically-induced reactions o f polyethylene and polypropylene radicals 223,224 11.2.1. Polyethylene - ESR spectra observed after milling of polyethylene (PE) at 77 K in vacuum are shown for different milling times in Fig. 91. The intensity of the ESR spectrum increases with the milling time. The line-shape of the spectrum after 24 hr milling is different from that after 1 hr milling. The spectrum after 6 hr milling is a superposition of the spectra observed after short and long milling - this spectrum has peaks corresponding to each of the two other spectra. Changes in both intensity and line-shape indicate that the increase in radical concentration was accompanied by conversion to other radical species during long milling. The spectrum observed after 1 hr milling (Fig. 91(a)) is identical to that of the primary mechanoradical of PE (see Section 3.2). That is, the radical species responsible for the spectrum observed after short milling is the chain scission radical

H

H

I

I

~C-C

.,ehr

(A).

I

I

H

H

V'

(c)24

' /

25G

FIG. 91. ESR spectra observed from polyethylenemilled at 77 K. (a) I hr milling, (b) 6 hr milling,(c) 24 hr milling.

MECHANOCHEMISTRY OF POLYMERS

575

The spectrum observed after long milling is an octet, which is ascribed to the radical 36 FI H

I

H

I

I

(B).

~C-C-C-H

I

I

H

H

Since the spectrum observed at intermediate milling time (Fig, 91(b)) is a mixture of these two spectra, such changes in the line shape with increasing milling time indicate that the primary mechanoradical (A) is gradually converted into the secondary radical (B) during milling. Energy input into the system consists solely of mechanical energy supplied by ball-milling and therefore the radical conversion observed during the experiment is caused by the mechanical action. No radical conversion was found, after 24hr storage, for the sample showing the spectrum in Fig. 91(a). The results definitely indicate that the mechanical action is needed for the radical conversion and this conversion is another example of mechanochemically induced conversion of a radical. It is worth noting that no conversion was observed for the radicals produced by v-irradiation of polyethylene after the same milling of the v-irradiated sample at 77 K. Accordingly, the conversion is a characteristic behavior of the mechanoradicals. A thermal change of the radical (A) was investigated. After I hr of milling (followed by observation of a spectrum similar to Fig. 91(a)) the milled sample was heat-treated at several temperatures. Some of the results after the heat treatment are shown in Fig. 92. Although the signal/noise ratio of the spectrum decreases after the heat treatment, the line shape is also changed by the treatment. The decrease of the signal/noise ratio is due to decay of the radicals at the higher temperature and the Changes of the line shape demonstrate the conversion of the radicals. The heat treatment causes not only the decay but also the conversion of the radicals. The line shape observed after heat treatment at 123 K for 10 min (Fig. 92(b)) is an octet, which agrees with the spectrum shown in Fig. 91(c) in the main characteristic. Thus, one can assign the spectrum to the radical species (B). No conversion (A) ~ (B) was observed at any appreciable rate in the temperature range below 110°C. The line shape observed after heat treatment at the higher temperature of 143 K (Fig. 92(c)) is mainly a sextet, which is attributed to the following radical species:36

H

H

H

I

I

I

~~C-C-C~

l

I

H

H

(C).

576

J. SOHMA

(a)1hr

( b ) ~ (c)143K

FIG. 92. ESR spectra observed from milled polyethylene after heat treatment.

(a) After 1hr milling at 77 K before heat treatment, (b) after heat treatment at 123 K for 10min, (c) after heat treatment at 143 K for 5 rain. Therefore, the thermally induced conversion of the mechanoradical (A) can be summarized as follows: H

H

H

H

H

H

H

H

I

I

I

I

I

I

I

I

~C--C'

(A)

th. . . . I , - C - C - C - n

(n) ~

thermal

,~,C-C-C

~'

I

I

I

I

I

I

H

H

H

H

H

H

(c).

The radical (C) was not changed to other radicals but simply decayed upon further heating. Comparison of the results obtained by the two types of experiments, the mechanical and the thermal, indicates that the same conversion of the radical (A) to the radical (B) was induced by both the mechanical and thermal treatments. The observed similarity between the mechanical action and the heat treatment suggests strongly that local heating induced by friction during milling is at the origin of the radical conversion induced by milling at 77 K. However, no component of the sextet was seen in the spectrum observed after the longer

M E C H A N O C H E M I S T R Y OF POLYMERS

577

milling. This fact means that the local temperature at the sites trapping the radicals is not above 147 K even if local heating due to friction causes the radical conversion. This is because the radical (B) can be converted into the radical (C) giving the sextet, only if the local temperature is raised above 147 K. The results and discussion above leads us to conclude that the mechanochemically induced reaction of the mechanoradical (A) is attributed to local frictional heating during milling, in spite of the fact that the bulk temperature of the sample remained at 77 K during the experiment and also that the local heating did not exceed ca. 150 K. The absence of conversion of the radicals produced by ?-irradiation is compatible with this hypothesis of local heating, shown as follows. As mentioned in Section 7, the radicals produced by 7-irradiation are distributed almost homogeneously through a sample and most of the radicals are present not on the surfaces but buried inside the sample particle. On the other hand, the mechanoradicals are trapped on the surface. The friction occurs only on the surfaces and the local heating caused by the friction is limited to spots on the surfaces. Thus, the mechanoradicals, being present on the surfaces, are sensitive to local heating but the radicals inside the sample particles are never warmed up by the friction and stay at the constant bulk temperature of the sample. Thus, mechanochemical radical conversion is brought about for the mechanoradicals but not for the radicals formed by 7-irradiation. Local heating caused by friction is anticipated in any mechanical experiment, because there is no way to eliminate friction completely in any milling of a solid sample. Since the local heating is not a phenomenon of thermal equilibrium but rather a transient and unstable one, it is difficult to determine experimentally the maximum temperatures produced by the local heating. According to the results described above, radicals can be used as probes by which local temperature can be estimated, on the assumption that a particular type of radical conversion occurs in a definite temperature range. For example, the primary mechanoradical (A) of polyethylene is stable at 77 K but is converted into the secondary radical (B) at a significant rate at c a . 130 K. The radical (B) is converted into the radical (C) at c a . 150K and the radical (C) decays at higher temperatures. Therefore, if the main cause of the mechanochemical reaction is proved to be friction, one can evaluate an upper limit of local temperature by observing whether or not the radical conversion occurs. For example, if the conversion (A) --~ (B) is observed to occur, the upper limit of the local temperature at the sites trapping the radical (A) is roughly 110 K but lower than 150 K, above which the radical (B) is converted into the other species. In such a way one can use a radical as a probe for the estimation of local temperature. Relying on this radical-probe method, one can estimate the local temperature rise due to friction in the ball-milling of polyethylene at 77 K at more than c a . 30°C but less than c a . 70°C.

578

J. SOHMA

FIG. 93. ESR spectra observed from polypropylene milled at 77 K. (a) 1 hr milling, (b) 24hr milling.

11.2.2. Polypropylene - Spectra observed from polypropylene after milling at. 77K for 1 hr and 24hr are shown in Fig. 93. Although spectral intensity increased after the long milling, no change in line shape was observed, as shown in Fig. 93. The effect of heat treatment on line shape was then evaluated for the polypropylene mechanoradicals. No change in ESR spectrum was observed after heat treatments at temperatures below 183 K. Some spectral changes were seen at temperatures above 213 K. The fact that no change was induced mechanochemically at 77 K suggests that the temperature of local heating caused by friction on the polypropylene sample was lower than 213K, above which temperature the primary mechanoradicals derived from polypropylene are converted into other species. One is led to believe from these experiments that no conversion of the mechanoradicals of polypropylene was mechanochemically induced at 77 K even if local heating was indeed caused by friction. This is because the amount of thermal energy needed for the conversion of the polypropylene radical is larger than that required by the polyethylene radical, and the local heating caused by the friction is not sufficient to supply the amount of thermal energy needed for the conversion of the polypropylene mechanoradicals. 11.3. Mechanisms o f radical conversion by milling In the preceding sections the examples of radical conversion induced by ball-milling were described. The primary mechanoradical of PMMA was converted into the secondary radical by mechanical action and no such conversion was caused thermally. This mechanochemical conversion, in the case of PMMA, is regarded as a kind of direct chemical effect of mechanical energy. For PE, radical conversion was caused by the milling at 77 K, but the mechanical energy may be changed into thermal energy via friction and the radical conversion may

MECHANOCHEMISTRY OF POLYMERS

579

be accelerated by thermal energy at local spots heated by the friction. In either case the energy needed for the conversions, that is chemical reactions, is provided by the applied mechanical energy. In this light, one may refer to these chemical reactions as mechanochemical reactions. An important question to be answered is "what is the mechanism through which mechanical energy (such as from ball-milling) is transferred to the radicals to initiate a chemical reaction?" This is a central problem in the mechanochemistry but it is too difficult to be solved at present. However, four possible processes could be imagined for energy transfer from the form of mechanical energy to the form of chemical energy: (1) the direct effect, (2) local heating, (3) the effect of macroscopic electric charge separation, and (4) the effect of fresh surface. O f course, these effects, may also work in combination. 11.3.1. The direct effect - The first of these processes is a direct effect, which can be considered as a process by which mechanical energy can cause chemical phenomena without any thermal path. The best example of this direct effect is main-chain scission of a polymer caused by an applied mechanical stress. The scission of a main chain of polymer is a chemical phenomenon, because a chemical bond is ruptured to produce a pair of free radicals. Strictly speaking, no positive evidence, which proves the non-contribution of thermal energy to the bond breaking has yet been obtained. However, most researchers believe that the chain scission is caused by stress concentration on a single chain, as described in Section 5. The assumption that stress concentration leads to chain breakage is equivalent to a direct chemical effect of mechanical energy. Zhurkov 225 found a shift in the stretching band of C - C bonds in the IR spectrum of propylene under load as shown in Fig. 94. The major peak is shifted slightly and this means the majority of C - C bonds are stretched by an average stress, In addition to this main peak shift a band appears between the main peak and 950 cm ~. This large shift means that some fraction of the C - C bonds in the system suffers from a larger stress than the average. Thus, this large shift is considered to be experimental evidence for stress concentration on particular C - C bonds. This was observed from a sample in a stationary state under a stress. If the applied stress is increased by some amount, the stress concentrated on a particular C - C bond may result in bond breaking. This is a picture of the direct effect. The shift in I R spectrum for a sample under large stress can be interpreted as indicating that an extremely stress-concentrated C - C bond occupies an energy state higher by 50 c m ~than that of a C - C bond under average stress. The energy corresponding to 50 cm-1 is 0.06 eV. Since the C - C bond energy is about 3.5 eV, a greater shift in the I R peak is required to show enough energy input for a C - C bond scission. However, one should remember that the IR spectrum was observed in a stationary state. Local stress levels in a stressed sample may fluctuate either over time or over space. 226 Such a fluctuation can result occasionally in a high

580

J. SOHMA 9O ~-

I

I

70

2oLin10~ 940

,

,

9f:)O

j

..

98O9, cm-~

FIG. 94, Changes in profile of 975cm ~ absorption band in polypropylene under load: (I) no load, (II) load of 80 kg/mmL

concentration of stress on a particular bond (beyond 3.5 eV) and a bond can be directly scissioned by the fluctuating stress concentration. The formation of mechanoradicals is considered to be positive evidence for the occurrence of such a large stress concentration which results in chain breaking. In conversion of a free radical, such a large energy input as that required for bond rupture is not needed. This is because a free radical is an unstable intermediate and conversion of a free radical to a more stable species may proceed when a small amount of energy to overcome a potential barrier is supplied to a system. In our ball-mill experiment, weight of a glass ball is about 1 g and the distance which a ball falls is about I cm. If the ball hits the sample with gravity acceleration, the amount of mechanical energy supplied by the ball to a polymer flake is in the order of magnitude of 1 0 - 4 J . Consider a phonon of energy 0.06eV mentioned above. A collision of a ball with a polymer flake excites lattice vibration in the polymer and the energy input by the collision is about 10-4J, which creates 1016 phonons o f 0.03eV in the polymer. If these phonons are created completely homogeneously all over the sample, the density of phonons is very small. However, if the ball hits a very limited area of flake, the phonon density is enhanced very much depending on the hitting area. If so, the activation energy needed for a radical conversion, which is presumably several kcalmol -~, is easily provided by simultaneous absorption of two or three phonons. This is not a completely unreasonable process. Whether or not the radical conversion is caused via the direct process may depend on the phonon density, which is determined by the degree of concentration of the input of mechanical energy. Unfortunately no data is available on the phonon density in the impact region and we are not yet at the stage to claim this mechanism. The effect o f ultrasonic action in causing main-chain scissions of polymers in liquid solutions is believed not to be thermal but mechanical. For example, a bond rupture can be caused by the large pressure change occurring in the collapse of a cavitation bubble. 7s,8° In other words, main-chain scissions caused

MECHANOCHEMISTRY OF POLYMERS

581

by intense ultrasonic irradiation of a polymer solution are evidenced as pure mechanical actions. Thus, it is very unlikely that such pure mechanical action is exclusively limited to liquid phases and that no such mechanical action leading to bond rupture can occur in a solid phase. Thus, one cannot rule out the possibility of the direct effect, even though no definitive evidence for this direct effect is available at present. 1 1.3.2. L o c a l h e a t i n g b y . f r i c t i o n - It is hard to imagine a frictionless mechanical action. Mechanical actions such as milling, sawing, drilling and others, are always accompanied by friction. Friction produces local heating of the contact spots. The instantaneous temperature of these spots may be much higher than bulk temperature of the sample. The lheoretical estimation of the amount of heat created by friction between solids A and B is given by Bowden 227as follows: 0 -

0.236 # Wv / ( K A + Kl~)

where ~ stands for the friction coefficient, W for a weight, v for the moving velocity, 2l for the length of contacl area and K,, and KB for the thermal conductivities of solids A and B. Since no data are available for the parameters mentioned above for polymers, it is impossible to predict the degree of heating produced by friction of organic polymers theoretically. The temperature rise at the spots heated by friction between metal and glass, moving with respect to each other at the high speed of 380 cm sec ~ under a heavy load of 450 kg, has been measured to be as high as 7 0 0 ° C . 22g In the circumstances of our experiments, the milling of fragile polymer tlakes by small glass balls, both the load and the moving speed are much smaller than in the above example. Then, such a large temperature rise by several hundred degrees is very unlikely. Even so one cannot rule out the possibility of this local heating as an energy transfer pathway from mechanical to chemical energy. As described in Section 10.1, the post-polymerizations were never initiated by mechanoradicals at temperatures below the melting points of the monomers. However, mechanochemical polymerizations were initiated by mechanoradicals under the circumstances of simultaneous fracture at 77 K, a temperature at which the polymerizing monomers are in the solid state. Taking account of these post-polymerization results, it is hard to imagine that the polymerization is initiated and then propagates in the solid state. It is very likely that the solid monomers melt at particular spots heated locally by friction during the milling and that the polymerizations are then initiated by mechanoradicals and propagate at these locally heated spots. According to the radical probe method described in the last section, the temperature rise during local heating is something between 30°C and 70°C (that is, local temperatures may rise to between ca. 100K and 150K in a sample kept at 77K). Solid ethylene and propylene are

582

J. SOHMA

able to melt in this range of temperature rise caused by friction. The melting point of methyl methacrylate is beyond this range, but no positive evidence is available to determine whether or not methyl methacrylate melts partially due to the local heating. 11.3.3. Effect of macroscopic electric charge separation - Friction produces macroscopic electric charge separation due to triboelectricity; the sign of the excess charge on a particular type of material is negative in some cases but positive in other cases depending on the particular combination of materials between which friction occurs. As described in Section 8 the excess charges can change the course of radical reactions. It is not completely unlikely that excess electric charges on a material, either positive or negative, caused by triboelectricity may either initiate reactions or influence radical reactions. It is known that electrons released from tetramethyl-p-phenylene-diamine by visible light irradiation of polypropylene containing this material produced free radicals, such as methyl, n9 It is well established that both cations and anions produced by ionizing radiations can cause radical formation and other reactions. Thus, it is not unreasonable to imagine that ionic species, including isolated electrons, may initiate chemical reactions. 11.3.4. Effect of fresh surfaces - Fracture of a solid produces new surfaces by cleavage. These surfaces are called fresh surfaces. The production of mechanoradicals and fresh surfaces occur simultaneously because they have the same origin. Mechanoradicals are trapped on these fresh surfaces, as discussed in Section 8. Immediately after a fracture, the fresh surfaces produced are free from any adsorption of gas molecules. It is reasonable to assume that both defects and dislocations are much more common on a fresh surface than an ordinary surface because a fracture creates many dislocations, defects and misfits in a crystalline structure, as discussed in Section 6. Moreover, it is known that crystalline structures under large stress are often different from their unstressed counterparts, as described in Section 6. All these factors mentioned above indicate that more energy is stored on a fresh surface than on a similar area of normal surface. In other words, some of the mechanical energy used in cleaving a solid becomes stored on fresh surfaces in the forms of (1) the absence of adsorbed gas molecules, (2) defects, (3) dislocation and (4) unstable crystalline structures. The mechanoradicals are trapped on the fresh surface which is rich in excess energy. The above consideration leads us to believe that the environment surrounding mechanoradicals is quite different from that surrounding other radicals produced, for example by either 7 or photoirradiation. In the latter cases the environment around the radicals is in thermal equilibrium. This difference in the trapping sites characterizes the behavior of mechanoradicals in chemical reactions. Whenever a reaction caused by mechanoradicals is discussed, one should

MECHANOCHEMISTRY OF POLYMERS

583

bear in mind that the mechanoradicals are trapped in an energy-rich environment. Mechanoradicals may utilize excess energy stored on fresh surfaces in addition to thermal energy, while only thermal energy is available for radicals produced by radiation. This is probably a reason why mechanochemical polymerization proceeds at lower temperatures than otherwise attainable, such as 77 K in some simultaneous polymerizations. It is believed in the mechanochemistry of inorganic substances that the higher chemical reactivity of fractured material results from the high activity of fresh surfaces. 23° However, the excess energy stored on the fresh surfaces of either inorganic or metallic solids is larger than that on fresh surfaces of organic solids. This is because the bonding energy in either metals or inorganic substances is much higher than the van der Waals energy, which is the bonding energy in most organic solids, particularly organic polymer solids. The rupture of polymer solids is carried out not in ultra-high vacuum but in low vacuum such as 10 4Torr in most experiments. Under these conditions, one can not expect surfaces to be clean and free from any adsorbed molecules. Imperfections caused by fractures are presumably frozen at 77 K and the excess energy of the imperfection could be stored for a long time at 77 K. Again, there is no positive evidence supporting the anticipation that the excess energy stored on fresh surfaces may contribute to high reactivity of the mechanoradicals, but the discussion mentioned above makes it reasonable to assume that fresh surfaces are one of the main sources of the high reactivity of mechanoradicals. I 1.3.5. C o m b i n e d effect - The effects described above are not mutually exclusive and it is likely that two or more of these effects may contribute in a combined way to accelerate chemical reactions of mechanoradicals. Probably the simultaneous polymerization is a good example of the combined influence of the fresh surface effect and local heating. By local heating, solid monomers may melt even at a bulk-temperature of 77 K and m o n o m e r molecules in the resultant liquid phase may have sufficient mobility to move a fresh surface nearby. M o n o m e r molecules arriving at a fresh surface can be attacked by primary mechanoradicals on it and the reaction initiated by the mechanoradicals may be accelerated by excess energy stored on the surface. 11.3.6. Sceptical claims against m e c h a n o c h e m i c a l reactions - Although mechanochemical reactions have been demonstrated, as described in the last section, some researchers are still sceptical about these claimed mechanochemical reactions, even about the mechanochemistry, because of the following two allegations. The first is that the mechanical energy input may not be large enough to break a strong chemical bond. The second is that chemical reactions induced by mechanical agitation are not caused directly by mechanical action but rather by thermal energy deposited locally by mechanical friction. That is, they claim that a mechanochemical reaction is essentially a thermal reaction.

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The first claim rests on the assumption that mechanical energy is uniformly distributed throughout a system. Suppose 1 erg (107j) of mechanical energy is input into a system consisting of one mole of molecules, the mechanical energy per molecule is in the order of 10-24erg, which is really too small to break one C-C bond. However, this assumption is apparently not the case. It is well established that fracture of a material is initiated by stress concentration. One example is the Griffith theory. 93 According to this theory, a fracture starts from voids, at which applied stress is concentrated. That is, mechanical energy applied to a system distributes not uniformly but concentrates on specific sites. Such stress concentration occurs on a molecular scale. The Peterlin model ~8 is a good example, in which a mechanical load is supported by a single tie molecule. Therefore, one should always consider the existence of stress concentration at either the macroscopic or microscopic scale. Suppose elastic energy is concentrated at specific sites, at which a small number of molecules support the stress. Then the elastic energy per supporting molecule could be large enough to break a chemical bond. The second claim is based on the assumption that the local temperature rise caused by mechanical friction is so high as to thermally break a chemical bond, although the bulk temperature of the sample remains unvaried. One should consider that the local temperature may be quite a lot higher than the bulk temperature when mechanical friction becomes effective. It is rather difficult, however, to estimate the degree of local temperature rise either theoretically or experimentally as discussed in Section 1 1.3. The radical probe method, described in Section 1 1.2, provides us with an upper limit of the degree of local heating caused by mechanical friction. This is nearly 150 K for the polymers kept at 77K. Based on these experimental results one may say that the amount of thermal energy produced by the mechanical friction is not sufficient for breaking a chemical bond, at least in the experiment studied. Thus, we have good experimental evidence which leads us to believe that mechanochemical reactions are not caused by thermal energy. In addition to this, there is a finding that chemical reactions induced by mechanical excitation are different from thermal reactions, which become active at higher temperatures. This was discussed in Section 11.1 in detail. The experimental result shown indicates that the mechanochemical reaction does not originate from thermal excitation. Thus, neither of the two above objections is valid.

12. P O T E N T I A L

APPLICATIONS OF MECHANOCHEMISTRY TO INDUSTRY

12.1. Radical formation by mechanical processing and its after effects It was believed for a long time that a polymer suffers only mechanical effects during mechanical processing such as extrusion. Mechanochemical effects have

MECHANOCHEMISTRY OF POLYMERS

585

been well established in recent decades and mechanical processing may induce chemical effects. Fracture of solid polymers produces mechanoradicals as described in Section 3. The high-speed stirring of liquid solutions of polymers generates mechanoradicals in the liquid state. The solid (amorphous) and liquid states are two extremes of the molten state of polymers. Since mechanoradicals are produced in these two extreme states, it is reasonable to assume that generation of mechanoradicals occurs in the molten state under strong shearing force, as in an extruder. However, it was not easy to prove experimentally the generation of mechanoradicals in an extruder, because lifetimes of radicals are too short at the high temperature of extruder processing for the radicals to be detected. Spin trapping was also not applicable at these high temperatures because of the thermal instability of available spin trapping agents. Recently, hindered amines (HALS), which are used as effective stabilizers for photodegradation, were found to be good spin trapping agents at higher temperatures] 3~ HALS were successfully used to detect radical formation in an extruder. 232 Radical formation might initially be attributed to thermal degradation due to higher temperatures in the extruder. However, this possibility was ruled out, because few radicals were produced when the polymers were held at the same temperatures used in extension but in the absence of shear. Therefore, the formation of radicals during the extrusion process is due to the mechanochemical effect. Mechanoradicals generated during extrusion readily react with oxygen in the atmosphere to form peroxy radicals in molten polymers. Mechanoradicals may also initiate other reactors, such as cross-linking or disproportionation. Final products of these reactions may include ketones, double bonds and cross-links. ll should be stressed, therefore, that any polymer which has experienced mechanical processing contains such impurities, the final products of chemical reactions initiated by the mechanoradicals. 12.2. Combined effect of mechanochemical and photochemical degradation Polypropylene was milled in air at 77 K with a ball mill. The ESR spectrum observed from the milled polypropylene demonstrated the formation of peroxy radicals. All these peroxy radicals were eradicated by a long heat treatment at room temperature. No radicals survived as proved by the absence of an ESR signal from the sample after the treatment. This sample was then illuminated by UV light from a high pressure mercury lamp in vacuum and an ESR spectrum was observed from the sample after UV illumination. The observed ESR spectrum consisted of a broad multiplet superposed with a sharp quartet, with the quartet assigned to the spectrum of methyl radicals. 233 Although the origin of the broad multiplet, as well as how the methyl radicals are produced by the photoillumination, are unknown, the main features of the observed spectrum agree with those reported in the literature. 234 Photo-

586

J. SOHMA

12

_

/

Fractured PP

I0

"• 8 o

j

o

_..&-- Non-fractured PP

/"

6

&

r/

t

I0

i

I

20

30

I

40

I

50

UV irradiation time (rain)

FIG. 95. Comparison between fractured and non-fractured polypropylene in photodegradation.

degradation of polypropylene was well evidenced by the observed ESR signal. For comparison, UV illumination of non-fractured polypropylene was carried out under the same conditions. The observed ESR spectrum agrees with the previously reported spectrum. The efficiency of radical formation depends on the wavelengths of the light used for the illumination. Relative concentrations of free radicals estimated from the observed ESR spectra are plotted in Fig. 95. Apparently the radicals are formed more efficiently in the polypropylene fractured in air than in the non-fractured material. This is called a combined effect of mechanochemical and photochemical degradation. Primary mechanoradicals generated in air are rapidly transformed to peroxy radicals, as proved by ESR. The peroxy radicals decay at higher temperatures and ketones, O

In --C--

,

are produced as final products. These ketones present in the fractured polypropylene act as a chromophore, which induces photodegradation. Although the mechanism of this process is unknown, photodegradation is surely accelerated in polypropylene which has experienced mechanical degradation. This combined effect is very important in the photodegradation of polymeric materials in practical uses. The first point of concern is that polymeric materials in practical uses have inevitably experienced mechanical processing, such as extruder action, moulding by a moulding machine or inflation. As discussed in the last section, mechanoradicals are produced in these processes. Decay

M E C H A N O C H E M I S T R Y OF POLYMERS

587

products of these mechanoradicals are ketones. Thus, the combined mechanochemical and photochemical effects should be taken into account whenever photodegradation of polymers in practical use is discussed. It was recently fi3und by Carlsson et al. 23s that the rate of photodegradation of polymers depends on their processing history. Geuskens236also noticed the importance of processing history in photodegradation studies of polymers. The second point of concern is the effect of invisible damage caused by tiny mechanical actions, such as friction, as well as minute scratches. Polymer materials in practical uses inevitably experience minute mechanical damage, which should be considered as large scale from a microscopic view point. Mechanoradicals are produced even by slight scratching in air and the final products of this mechanical damage in air are ketones which, again, accelerate photodegradation. Thus, it is very reasonable to consider that photodegradation of polymers is enhanced by mechanical damage, even if this damage is invisibly small. This seems very important for estimation of photostability of polymer materials in practical uses. 12.3. Surface modification by mechanochemical polymerization Mechanochemical polymerizations, both post-polymerizations and simultaneous polymerizations, are initiated by mechanoradicals which have been formed and trapped on fresh surfaces and the reactions proceed on these surfaces. Most polymers mechanochemically produced on polymer surfaces are complete block copolymers, which consist of one block formed from added monomers and another block of matrix polymer, as described in Section 9. In other words, the polymer produced mechanochemically from added monomers is chemically bonded with the matrix polymer. By this mechanochemical polymerization one can selectively modify the nature of the surface of a solid polymer without changing its bulk properities. For example, a remarkable change in surface wettability of PTFE occurs without change in bulk properties after the simultaneous mechanochemical polymerization of vinyl acetate on a PTFE surface. ~92 The mechanochemical polymerization could be performed by scratching a polytetrafluoroethylene surface in the presence of monomers by a sapphire needle. By a mechanochemical method, methyl methacrylate was copolymerized on crushed coal 1% and the surface properties of the coal were modified by this copolymerization. There are several other methods to modify the surface properties of materials, such as coating and copolymerization by irradiation. Mechanochemical copolymerization is superior to coating, because the modified surface formed by mechanochemical polymerization is chemically bonded to the matrix. Chemical bonds can also be formed between coating polymers and inorganic matrices by mechanochemical polymerization, as demonstrated in Section 10. Copolymerization by ionizing radiation could be used, of course, for covering the

588

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surface o f a polymer matrix. The covering polymers then also become chemically b o n d e d to the matrix polymer. However, the irradiation damages the matrix polymer, which changes the bulk properties o f the matrix in most cases. Thus, one can expect that mechanochemical copolymerization m a y have the advantage o f longer service life o f the treated material, Moreover, the apparatus required for mechanochemical polymerization is much simpler than the radiation sources needed for radiation polymerization. It should be emphasized that mechanochemical methods are probably the simplest way to produce chemical bonds between organic molecules and inorganic solids. Taking advantage o f this fact, potential industrial applications m a y be presumably designed. ACKNOWLEDGEMENT The a u t h o r would like to express his cordial thanks to all his coworkers for their fruitful and kind collaborations. He is very grateful to the Materials Research L a b o r a t o r y o f the University o f Massachusetts for its kindness in giving him a chance to lecture on " M e c h a n o c h e m i s t r y o f polymers". This series o f lectures was the starting point for this review paper. He is also very thankful to Prof. G. Geuskens, Free University o f Brussels, and the Belgian Research F o u n d a t i o n for Sciences which made it possible for him to stay in Brussels where he could finish the final draft o f this review. He is also indebted to Miss Y. N a k a b a y a s h i for her typing o f the manuscript.

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