Formation of low molecular weight compounds during mechanical degradation of macromolecules

l~olYmer Science U.S.S.R. Vol. 24, lfo. 9, pp. 2202-2210, 1982 Printed in Poland

0032-3950]82 $7.50+.00 © 1983 Pergamon Presa Ltd.

FORMATION OF LOW MOLECULAR WEIGHT COMPOUNDS DURING MECHANICAL DEGRADATION OF MACROMOLECULE$* A. M. D~nvsKAYA and A. N. STRELETSKII Chemical Physics Institute, U.S.S.R. Academy of Sciences

(Received 20 May 1981) kinetic study has been made of the formation of free-radlcal and molecular (volatile) products during the low-temperature mechanical dispersion of polysilsesquioxanes. I t was found that evolution of hydrogen, methane and ethane takes place in polymethylsilsesquioxane, and the evolution of benzene in polyphenylsilsesquioxane. This results are attributed to decay of Si-- C and C-- C bonds of vibration. excited macroradicals appearing in the degradation of Si--O bonds. A kinetic soheme of formation of molecular products is proposed.

Mv.o~rA~c~ treatment of polymers (stretching of films, mechanical dispersion) leads in many cases [1-4] to the formation of low molecular weight products. It was found [1, 2] that mechanical degradation of macromoloculos of various structural types takes place by a free-radical mechanism, and that the formation of volatile products is generally duo to decay of polymeric free radicals [2]. At the same time some experimental results viz. the formation of monomers during low temperature mechanical degradation of polyisobutylene and polyformaldehyde [3], when rates of radical decay reactions are very inconsiderable, and the composition of gaseous products of mochanodegradation of epoxy resins (atomic hydrogon, methane, etc.) [4] are not in agreement with explanations based on free radical decay. To explain this a hypothesis was introduced in [3] regarding formation of vibration-excited radicals during mechanical degradation of macromoleculos. However there are as yet insufficient experimental data available on which to base any final conclusion in regard to the mechanism of formation of low molecular weight compounds during the mechanical degradation of polymers. To do so painstaking experiments are necessary to compare amounts of radical and moleeular products obtained under conditions precluding decay of thermolyzed free radicals. As suitable study objects we took polycyclic polysiloxanes, namely polymethylsilsesquioxane (PMS) and polyphenylsilsesquioxane (PPS), whose mechanical degradation results in the formation of large amounts of free radicals and low molecular weight compounds of relatively simple composition. In addition, volatile products may be collected at low temperatures, whereby macroradicals observed by ESR retain their stability. * Vysokomol. soyed. A24: No. 9, 1924-1930p 1982. 2202

F~mabion of low .mc~mtdav ,weight compotmds

2203

This pa~er relates to our investigation of the compositio~ and the kinetics of f o r m a t i o n o f v o l a t i l e p r o d u c t s o f t h e low t e m p e r a t u r e ( N 8 0 K ) m e c h a n i c a l degTachttion o f P M S a n d P P S a l o n g w i t h a k i n e t i c s t u d y o f t h e a c c u m u l a t i o n o f free radicals. PMS and PPS (polysiloxanes having simply Si-O bonds in the main chain) were synthesized by the method described in [5]. Mechanical degradation was carried out in a laboratory vibromill in glass ampoules containing glass or steel balls [5]. Before experiments were performed specimens were painstakingly evacuated at 3 70-420 K, and were eomminuted i n v a c u o or in an appropriate gas atmosphere at ~ 8 0 1~. When dispersion had been carried out ampoules were connected, without warm-up, to a vacuum apparatus, and the quantitative composition of the volatile products given off was determined. To separate the mixture of volatiles the temperature of the specimens and of the traps was varied. A membrane type pressure gauge (sensitivity 7 X 10TM molecules per division of the recording device) was used to measure the amount of the gases; an analysis of the gas mixture was carried out, using an APDM-1 partial pressure analyser. To verify mass spectra assi~ments the results were in most eases compared with the mass spectra of standard compound~ prepared under like conditions. The structure and concentration of free radicals were determined using an EPR-20 spectrometer. The ESR spectra were recorded at 77 K immediately after dispersion w i t h o u t an i n t e r m e ~ warm~u~ .or oontaet w/th air. A,mmm~ of votat,i~ predators a~l. fl~e zmiioaJs we~9 ~)rnmlly comtm~ed in a sim~le experiment, but sometimee in two :l~l~llel o~es, oenducted under strictly identical eondit~ns. MASS 8PY$C'JlA OF VOT.A_~qLE PROD~YOT8 ~ G R M I D IN ' x ~ DIGI~g&DATION

Ole POLYSILSE~TYIOXANES

Poly. IProd. I met uct

m/e

I/tel

i PMSI

H,

cK, Cjte PPS

CJ~e

2

1

lO0 5 16 15 14 13 12 100 82 16 30 29 28

8 2 27 26 15

15 27 100 35 22 9 79 78 77 76 74 63 ~2 51 50 39 38 37 28 27 26 12 100 21

3

3

5 23 27 24 17 12

9

8

7

9

T h e m e c h a n i c a l d e g r a d a t i o n o f P M S a n d P P S r e s u l t s in t h e f o r m a t i o n o f free r a d i c a l s , w h o s e s t r u c t u r e a n d p r o p e r t i e s h a v e b e e n a n a l y s e d in [5]. T h e gaseous products of mechanodegradation of PMS are a mixture of hydrogen, methane and e t h a n e ( ~ T a b i e ) . A t 80 K o n e c a n collect o n l y h y d l m g e n ( m e t h a n e a n d e t h a n e in a s p e c i m e n a m o u n t i n g t o o n l y s e v e r a l p e r c e n t a g e s ) , I f t h e s p e c i m e n is d e f r o s t e d t o 1 7 0 K m e t h a n e is g i v e n off a l o n g w i t h a n insignificant a d m i x t u r e o f h y d r o g e n a n d e t h a n e . A t t h i s t e m p e r a t u r e free r a d i c a l s in t h e P M S a r e stable, w h i c h m e a n s t h a t m e t h a n e e v o l u t i o n is n o t d u e to r e a c t i o n s o f r a d i c a l s o b s e r v e d b y E S R .

A. M. Du~I~S~YA and A. N. S ~ ~

2204

Ethane* is given off on heating the specimen to room temperature. Apparently, methane and ethane as well as hydrogen are evolved during mechanical degradation of PMS at ~ 80 K, though molecules of these gases are in the adsorbed state on the polymer surface at low temperatures. The reference experiment showed t h a t methane is completely and reversibly adsorbed on the comminuted PI~IS surface in the amounts detected during dispersion.

c,lo-ffkj8

a

2

h /

o

7×

Z 0

20

4O

2o

40

?'/me ~hp

FIG. 1. Kinetics of accumulation of molecular and radical products during mechanical degradation of PMS (a) and PPS (b). a" 1--[H~]; 2--[GHa]; 3--[R']; b: 1--[CeHe]; $--[R']. c--Concentration of products. Figure l a shows the kinetics of formation of H~ and OH4 and the kinetics of the accumulation of free radicals during mechanical degradation of PMS. The rate of liberation of volatile products is constant at the outset, and is then gradually reduced. T h e linear portion of t h e curve of free radical accumulation is significantly shorter, though the rate of rad~ca! generation is constant over a fairly long period of time, and cessation of growth in the number of free radicals is attributable to radical decay reactions [6]. The rate of H~ evolution was corn" the formation " ~ " " pared with of methane, ethane ~and free radmals (Fig. 2). The F i gure shows the results of a large number of experiments in which dispersion conditions were varied within wide limits (time of degradation, design of the operating vessel, glass or ste~l balls). The degradation intensity characterized b y the rate of free radical formation varied b y a factor of up to 50 in different experiments. Outside the dependence on the degradation intensity and hall material the number of free radicals formed over a wide range of concentration (up to 2 X 10~2 kg -1) is equal to the number of hydrogen molecules evolved, while the number of methane * Defrosting of a specimen and traps to room temperatttre is invariably also accompanied by the evolution of a small amount of water. This is because the initial PMS is the product of three-dimensional polycondensation [5], and has in its structure trapped molecules of water wl~ich cannot be altogether eliminated by prior evacuation of the specimen. Mechanical dispersion leads to a loosening of the polymer structure, and to liberation of H i e molecules.

Formation of low moleoular w i g h t oompounds

2205

molecules is approximately twice the number of hydrogen molecules. The a m o u n t of ethane evolved is 10-15 times less t h a n t h e a m o u n t o£ methane (the determiuation accuracy is lowest for ethane owing to difficulty in separating a m i x t u r e of methane and ethane). Thus the concentration ratio of radical and molecular volatile products of mechanical degradation of PMS will be [R] : [H a : [CH4] :

H

log c

+¢

__P o/

.z

÷/

/I

2 I

VJ

i

1

J]~

116 2!

ZI 2J log [H~][~-']

Fro. 2

EIJ iI

/!

Rc

FIG. 3

FIG. 2. Relationship between coneentrations of product~: I--JR'J; II--[CH~]; III--[CsH,]. Dispersion with glass (1-3) and steel (4, 5) balls, l ~ t e of hydrogen evolution: 3.3 (1); 12 (9); 13 (8); 50 (4) and 150×10 ~7 kg-~seo-1 (5). FIG. 3. ESR spectra of radical produets of raeehanieal degradation of PPS° Dispersion an& spectrum recording at $0 (a) and 295 K (b)."

2206

A. M. DUBINSKAYA a n d A. N~ S~R~E~i~KII

: [C~H~]=I : 1 : 3-2 : 0.2-0.3. I t can be seen t h a t the total amount o f volatile products formed in the PMS exceeds the observed number of free radicals b y a factor of ~4.5. I n PPS the rates of formation of volatile products and free radicals are commensurate (Fig. lb). I n this case benzene makes up the main part of the molecular products ( > 9 0 ~ ) , and m a y be collected only on warming the specimen to room temperature. In addition, the gaseous products contain an insignificant ( ~ 1%) amount of hydrogen. Neither benzene nor free radicals were found in reference experiments on mechanical dispersion of octaphenylsilsesquioxane, which is a low-molecular weight analogue of PPS. Thus valency bond rupture and liberation volatile molecular products during comminution of the material are characteristics of the ma cromolecules only. • The composition of volatile products of mechanodegradation of polysilsesquioxanes shows t h a t low-molecular weight free radicals of types H', CH~ and ~C6H~ are formed. The formation of H atoms is substantiated b y the ESR spectra o f products of comminution of PPS (Fig. 3) in which lines of cyclohexadienyl t y p e radicals (all (4)----48-{-1 gauss, and a H (3,5} --'--10:~:0"3 gauss) are clearly seen 5

6

H

t 3

Si//

2

These radicals appear as a result of H atom addition to phenyl rings of She polymer. These radicals amount to N 20o/o on the total number of radicals formed after dispersion of PPS at ~ 80 K, irrespective of the time of comminutiorg the other 800/0 are also cyclohexadienyl type radicals, but ~)f a structurally different variety

I! R:c

Here R denotes polymeric radicals generated at the site of Si--O bond scission, T h e value of a ~ ( 4 ) = 4 1 ~ 2 gauss; at low ¢~mperatures subsplitting on other H atoms of the ring is not pecmit¢~l (Fig. 3a). I f PPS is dispersed at room temperature the amount of radicals R~yenc increases to 50~/0, and at room temperature their spectrum is much better resolved (Fig. 3b)--splitting on H atoms in rectapositions of the phenyl ring (a~<2,e)~3 gauss) can be seen.

F0rmation o f low molecular weight compounds

2207

Thus mechanical degradation of polysilsesquioxanes is accompanied by the emergence of low-molecular weight free radicals. L e t us consider possible ways of formation of these radicals. It was found by experiments and supported by theoretical considerations in [1] that direct action of mechanical stress causes weakening and rupture of valency bonds in polymer backbones; bonds of side substituents may merely be reinforced. Mechanical degradation of polymers is accompanied by electron emission; in some cases the emitted electron energy may be as much as hundreds of electron volts [7]. Theoretically, the formation of low-molecular weight products could result from the influence of emitted electron radiation. However, on comparing the amounts of products formed with radiation yields in polysiloxanes [8] we obtain unreasonably large doses (~50 Mrad in 30 min comminution) of "internal" irradiation doses. On a previous occasion it was found [5] that mechanical degradation of polysilsesquioxanes takes place through main Si--O bonds with the formation of two types of primary macroradicals: --~SiO"and ;Si'. The formation of low-molecular / weight compounds during mechanical degradation of these polymers could be attributed to bimolecular reactions of macroradicals which take place very rapidly during mechanical dispersion of po|ymarie materials [2]. For instance, for the formation of CH~ and (likewise (Jstt~) one may propose the following scheme \ -s /

0

\ o'+-o-si-cn /

0

~O~Si--O~S~ -i /

\

(i)

where the energy gain is 130-170 kJ/mole. To verify this possible scheme we ra n additional experiments in mechanical degradation of polymers in carbon mono xide or hydrogen, whose molecules react at 80 K with macroradicals generated through Si---O bond scission [5, 9]. Campetition of these reactions with reaction (I) would necessarily lead to a lower yield of methane in PMS, and of benzene in PPS. It is seen from Fig. lb that in the presence of CO the benzene yield is practically the same as under vacuum, although a sigaificant proportion ( ~ 35~) of the ~Si--O" radicals added CO molecules. Similarly, there is no change in the methane yield when PMS is comminuted in an H.z atmosphere. Thus the formation of low-molecular weight products is not due to bimolecular reactions of macroradicals. In our view the sole mechanism that could account for the experimental results would be the appearance, during degradation of the polymers, of vibration-excited macroradicals whose decay leads to the expulsion of low-molecular weight radicals. In PMS the latter are H" and CH~, i.e. very light particles capable of rapid movement along the polymer surface. Their relocation leads to free valency migration and to decay, so that the experimentally observed number of free radicals in PMS is invariably less than the amount of volatiles. The following

~2os

A. ~t. D~ms~YA and A. Z¢.

scheme is proposed for the reactions involved:

M o,...~ R~*+R~* ., I-~R~ Rt - - -~MI+H"

(1) (2a) (a) (2b) (4)

R'* I-+R~

(5) CH~+M~CH4+R~

(O)

H'+H'-,H~

(7)

H"+ CH; ~ CHt

(8)

CH~+Ctt~-+C~H.

(9)

H"+ R~(R~, R~) -, HRI(HR~, HRs)

(10)

OH; + R~(R~, R~) -, CHaR~(CHaR~, CHAR8)

(11) (12)

R~+R~ ~

In this scheme M, M1 and M2 are macromolecules, and R1, R~ and R a are macroradicals. At the start of the dispersion process, when the macroradical concentration is low, their interaction with one another may be neglected. Prolonged maintenance of a constant rate of radical accumulation (Fig. 1) shows that the rate of reactions (10) and (11) is quite low. These data accord with the results of direct measurement of rates of interaction of atomic hydrogen with macroradicals of various structural types [10]. In the light of the experimental results and in view of the published data* we conclude that volatile radicals disappear mainly in reactions with one another (reactions (7)-(9)), not with polymer, as the rate ratios are wT/w6~10S and wS/w~ 106. This being so, we may determine the ratio of rates of formation of H" atoms and of methyl radicals d[~+3

~.=2---~ we.;=

+

d[CHo]

a---~

d[CHd -+-2 d[C~H~]

~

The experimental results show that w~'[wcrl;~ 1.4, and that the total rate of generation of volatile radicals (H" and CH~) exceeds the measured rate of * We used published data on recombination rates of H atoms on polymer surfaces [11] a n d c/n a quartz surface [12, 13] a n d on the kinetics of reactions of H" atoms with po. lymers [I0] a n d of methyl radicals with polydime~hylsiloxane [14].

Formation of low molemdar weight oompounds

2209

formation of polymeric radieahs by a factor of ~ . ~ o s e d s ~ s ~ tha~ the decay probability of "hot" radieale is very high (~0'0~. Moreover, tl~e results show that the true rate of chemical i~ond scissloh in PM~ i n a c r o m o l e c ~ exceeds

o served rate of

emergence o f

maeroradiea]s, The expulsion in'thiS ease of

very light particles migrating over the polymer surface leads to free valency migration and decay. In view of this the rate of free radical accumulation measured by ESR does not in this instance reflect the true pattern of bond scission. In the case of PPS likewise cleavage of H atoms and of phenyl radicals from vibration-excited macroradicals takes place. However the rate at which H" atoms add to phenyl rings of the polymer exceeds that of reaction (7) (see kinetic data for the interaction of H" atoms with phonyl rings of PS [10]), and so only insignificant amounts of hydrogen are detected in PPS. An analysis of the kinetic data for PPS shows that the total rate of generation of low molecular radicals (H" and CeH~) is twice the rate of emergence of radicals Rb'yene,and, accordingly, the decay probability for hot radicals in PPS will be not below 0.6-0.7. Thus the idea that hot macroradicals are formed during mechanical degradation and that decay of these macroradicals leads to the emergence of low molecular weight free radicals is currently the best explanation of the results obtained and accounts for the composition and ratio of molecular and radical products. Within the framework of the above scheme an analysis of the kinetic data points to a very high probability of decay of vibration-excited polymeric radicals, which evidences high excess energy of the r ~ c a l s during mechanodegradation. The authors thank A. Yu. Rabkin for kindly supplying polysilsesquioxane

samples. ~'ransazted by R. J. A. HEx-nxY REFERENCES

1. V. R. REGEL, A. L SLUT~ER and E. Ye. TOMASHEVSKII, gin~etiehesk~y~priroda prochnosti tverdykh tel (Kinetic Nature of the Strength of Solid Bodies). Nal~]ea, Moscow, 1971 2. P. Yu. BUTYAGIN,A. M. DUBINSKAYAand V. A. RADTSIG, Uspekhi khimii 88: 4, 593, 1968

3. P. Yu. BuTIrAGIN, Vysokomol. soyed. Ag: 1, 136, 1967 (Translated in Polymer Sol. U.S.S.R. 9: ~ 149, 1967) 4. L. S. ZARKHI~, Pat. of discussion at Cand. Chem. Degree Competition, Chem. Physios Institute, U.S.S.R. Academy of Sci., Moscow, 1979 5. A. M. D I T B ~ Y A , S. F. NIKI~'SIiffN, A. Yu. ~ A and B. G, ZAVIN, Vysokernel soyed. A22: 9, 2019, 1980 6. V. A. RADTSIG and A. M. DUBINSKAYA, In: Mekhanoemissiya i mokhanokhizm'ya

tverdykh tel (Mechanoemission and Meohanochemistry of Solid Bodies), p. 218, Ilim, Frunze, 1974 7. N. K. BARAMBOIM, Mekhanckhimiya vysokomolekulyarnykh soyedinenii (MechanocherniAtry Of High Molecular Compounds). p. 57, Khimiys, Moscow, 1978 8. A. A. iWn.T.ER, J. Amer. Chem. Soc. 85: 14, 3519, 1960 9. V. A. RADTSIG, Kinetika i kataliz 20: 2, 466, 1979

2210

10. 11. 12. 13. 14.

YE. P. STOGOVA and G. M. BAJt~'J~v

A. M. DUBINSKAYA, Uspekhi khimii 47: 7, 1169, 1978 A. M. D ~ Y A , Kinetika i kataliz 19: 6, 1171, 1978 B. J. WOOD and H. WISE, J. Phys. Chem. 66: 6, 1049, 1962 A. GELB and Sh. K. KIM, J. Chem. Phys. 55: 8, 4035, 1971 E. L. ZHUZHGOV, U.S.S.R. P a t at Cand. Chem. Degree Competition, United Academio Council for Chem. Sciences, U.S,S.R. Academy of Sciences, Novosibirsk, 1973

Polymer Science U.S.S.R. Vol. 24, Ifo. 9, pp. 2210-2214, 1982 Printed in Poland

0082-8950/82 $ 7 . 5 0 ÷ . 0 0 © 1983 Pergamon Press Ltd.

INFLUENCE OF THE PRESSURE OF GASEOUS MEDIA ON THE RELAXATION PROPERTIES OF ELASTOMERS* YE. P. SToGovA and G. M. BART~S~,V Research Institute of the Rubber Industry Physical Chemistry Institute, U.S.S.R. Academy of Sciences

(Received 29 May 1981) A study has been made of the influence of air pressure (up to 30 MPa) at 30-130 ° on the relaxation properties of a butadiene.methylstyreno elastomer. A pressure increase results in retardation of physical relaxation and leads to some augmentation of the activation energy owing to increased intermolecular interaction and to a free volume reduction. Chemical relaxation is accelerated in a high pressure air medium, and the activation energy is reduced. This reduction is the result of an intensification of thermooxidative processes on account of an increase in the concentration of dissolved oxygen in the elastomer. RELA-XXTIO~< processes in p o l y m e r s a s s o c i a t e d w i t h t h e r m a l m o t i o n of s t r u c t u r a l e l e m e n t s h a v e b e e n c o n v e n t i o n a l l y s u b d i v i d e d into t w o g r o u p s w h i c h d e t e r m i n e r a p i d a n d slow stages o f r e l a x a t i o n r e s p e c t i v e l y [1]. T h e r a p i d s t a g e is m a i n l y a s s o c i a t e d w i t h t h e ~-process, i.e. w i t h s e g m e n t a l m o b i l i t y a n d , w h e n t h e t e m p e r a t u r e falls, is r e s p o n s i b l e for glass t r a n s i t i o n of t h e p o l y m e r . T h e slow s t a g e following t h e r a p i d one is t h e result o f w h a t are t e r m e d )~-processes a n d is a s s o c i a t e d w i t h m o l e c u l a r m o b i l i t y o f p h y s i c a l j u n c t i o n s in t h e e l a s t o m e r n e t w o r k , w h i c h are ordered micro-blocks of supermolecular structure. Moreover chemical relaxation is classed w i t h slow r e l a x a t i o n processes. I n general, t h e slow s t a g e of stress r e l a x a t i o n m a y , w i t h i n t h e limits of linear visco-elasticity, b e described as t h e s u m o f t h e e x p o n e n t s r e l a t i n g to t h e discrete r e l a x a t i o n t i m e s p e c t r u m ~1, ~2. . . . , zn tb

a=sE(t)=8 F. Ele -tl~, lnl

* Vysokomol. soyed. A24: No. 9, 1931-1941, 1982.