Structure of densely crosslinked polymers of dimethacrylates and oligo-p-methacrylates

Polymers of dimethacrylates and oligo-p-met.haerylato~

2111

REFERENCES 1. G. S. SHAPOVAL and T. E. LIPATOVA, Elektrokhimieheskoye initsirovaniye polimerizatsii (ElectrocheInical Initiation of Polymerization). p. 235, N a u k o v a dumka, Kiev, 1977 2. Ya. D. ZYTNER mid K. A. MAKAROV, Elekgrokhimiya 16: 28, 1980 3. J. OUDIAN, Osnovy khimii polimerov (Bases of Polymer Chemistry). p. 614, Mir, Moscow, 1974 4. A. M. TOROPTSEVA, K. V. BELOGORODSKAYA and V. M. BONDARENKO, Laboratornyi p r a k t i k u m po khimii i tekhnologii vysokomolekulyarnykh soyedinenii (Laboratory Prae, tieum on the Chemistry and Technology of High Molecular Weight C,)mpo,mds). p. 416, Khimiya, Leningrad, 1972 5. A. P. KARPINETS and V. D. BEZUGLYI, Elektrokhinfiya 14: 1552, 1978 6. Ya. D. ZYTNER mad K. A. MAKAROV, Vysokomol. soyed. A22: 2612, 1980 (Translated in Polymer Set. U.S.S.R. A22:11, 2866, 1980)

Polymer ,qeienceU .S.g.R. Vol. 25, No. 9, pp. 2111-2120, 1983 Printed in Poland

0032-q950/83 $10.00 t-.00 © 1984 Pergamon Pres.~Ltd.

STRUCTURE OF DENSELY CROSSLINKED POLYMERS OF DIMETHACRYLATES AND OLIGO-p-METHACRYLATES * I{. V, OZERKOVSKII, V. D. I~LOTNIKOV a n d V. P . ROSHCHVeKIN Division of the I n s t i t u t e of Chemical Physics, U.S.S.R. Academy of Sciences

(Received 4 December ] 981 ) 1R spectroscopy has been used to stn(ly the microtaetieity of densely crosslinked polymers obtained b y radical polymerization of oligomerie dimethaerylates and oligop-nlethaerylates and to evaluate the size of the vibratory segment of the polymethacrylate chain ill the reticular polymer. The molecular probe method has shown that in polydimethaeryl'~tes microregions exist, differing in the density of molecular packing. The influence of high pressure on the process of polymerization of dimethacrylates and the structure of the polymer formed has been studied.

AN IMPORTANTfeature of the formation of densely crosslinked glasses through polymerization of oligomeric dimethacrylates is that the chemical reaction of chain growth is accompanied by rigid fixation of the spatial structure of the polymer formed. The dense m~sh of chemical bonds m~y fundamentally influence the process of polymerization and determine the specific features of the microstructure of the polymer. The present work studies these features by comparing the *Vysokomol. soyod. A25: 1go. 9, 1816-1822, 1983.

2112

B. ~¢'. OZERKOVSKII et al.

microstructure of p o l y m e t h a e r y l a t e chains in linear PMMA a n d in densely crosslinked p o l y d i m e t h y l a e r y l a t e s (PDMA). I t is k n o w n t h a t the s t r u c t u r e of the PMMA chains (microtacticity of the a t t a c h m e n t of the m o n o m e r units, a t t a c h m e n t type) depends on the conditions of polymerization: m e t h o d of initiation, t e m p e r a t u r e , pressure, solvent [ 1, 2]. B y polymerizing d i m e t h a c r y l a t e s in different conditions and comparing the microstructure of the p o l y m e t h a c r y l a t e chains in reticular and linear polymers it is possible to obtain the most direct and clear information on the influence of the crosslinks on the processes of radical polymerization and f o r m a t i o n of the physical s t r u c t u r e of densely crosslinked p o l y m e r glasses. Microtacticity of densely crosslinked PDMA. T h e microtacticity of a t t a c h m e n t of methaerylie groups on p o l y m e r i z a t i o n of the oligomers of the t y p e H 2 - - - - C C H a C - O - - X - - O - - C ( C H a ) C ~ C H 2 (referred to h e r e a f t e r as oligomers o f t y p e A) was studied b y I R spectroscopy. As the characteristic of stereoregularity ef the polymethacrylate chains we used the parameter I which for PMMA is determined from the optical densities of the absorption bands 968, 1060, 1388 and 1483 cm-* using the relations [3] I~(I,--~I~)/2 where nl4sa I,~179D*°6°-~27 and I2~- 81.4 - - - -- 43. According to [3] for syndiotactic PMMA D~68

D138~

the parameter I~--100-115 and for the isotactic I--25-35. The values I = 40--80, in the view of the authors of [3], indicate a stereoblock structure of the polymer chains.* It is known that syndiotactie PMMA is formed on radical and isotactic on anionic polymerization of methylmethacrylatc initiated by n-butyl lithium in toluene solution at --70°C [1]. Densely crosslinked polymem were obtained on radical (in bulk or in benzene solution) and anionic polymerization of the triethylene glycol dimethacrylate (hereafter this oligomer will be denoted as I) and the dimethacrylates of n-diols distinguished by the chemical nature and length of the group X. Analysis of the spectra of densely crosslinked P D M A in the region sensitive to stereoregularity of the p o l y m e t h a c r y l a t e chains gives for all the polymers studied values of I in the interval 45-75°C (Table 1). I t is v e r y i m p o r t a n t to n o t e t h a t w h a t e v e r the conditions a reticular p o l y m e r c a n n o t be o b t a i n e d with a high degree of iso- or syndiotaetic a t t a c h m e n t of the methacrylic groups. Consequently, the t e n d e n c y towards equalization of the probabilities of iso- a n d syndiotaetic a t t a c h m e n t is a characteristic feature of three dimensional polymerization of dimethacrylates. The results suggest t h a t the t h e r m o d y n a m i c features of the f o r m a t i o n of the spatially-meshed s t r u c t u r e of P D M A d o m i n a t e over the factors determining the stereospecifieity of the chain growth reaction of linear PMMA [2]. I n fact, in * Such a classification is arbitrary since tile chains of syndio- and isotaetie PMMA are also characterized by the mean length of tlle stereosequences of the units ("blocks") I. According to reference [4] in the syndiotactic polymer i 1 - 2, la = 5 and ilx the isotactic [~-~9,

Polymers of dimethacrylates and oligo-p-methacrylates

2113

T A B L E l . S t ' E C T R A L CHARACTERISTICS OF T H E S T E R E O R E G U L A R I T Y OF T H E POLY~IETHACRYLATE CHAINS I N R E T I C U L A R POLYI~ERS OF D1METHACRYLATES

H ~C = CCH3C--

X in oligomer --(--CH~--CH,--0--),--- (-- CH2CH,--O -- )~--- ( -- CH:-- CH, -- O -- )~ ---(--CH~--h--- (CH2--

h--

-- (CH2--

),, - -

--(--CH~-ho--

O-- X--

O -- CCH3C

II

II

o

o

= CH:

Conditions of polymerization Radical Anionic In 10% solution in benzene Radical

1 71 44 75 62 63 69 64

reference [5] it is shown that PDMAs with a different type of stereoregularity of the polymethaerylate chain differ in the size of the cavities and topology of the network. As a result the polymers containing commensurate numbers of units of different type must have a very wide size distribution of the cavities in the network. Therefore, because of the entropic factor the formation of such polymers is more preferential than that o~ polymers with stereoregularity of one particular type. A fundamental influence on mierotactieity must be exerted b y steric hindrances and shrinkage stresses [6] appearing in the network in the vicinity of the growing macroradical. In fact, after reaching the critical stress level the syndictactic attachment of the units may become thermodynamically adverse and the chain will continue to grow through isotactic attachment of the methaerylic groups. The resulting sharp change in the conditions of packing reduces the stresses to a level at which the reaction returns to the path of thermodynamically more advantageous syndiotactic chain growth. Thus, the presence of an inverse link between the stresses in the polymer matrix and the thermodynamics of the chemical reaction may cause periodic "switching" of the chain growth reaction from one type of attachment of methacrylic groups to another. Inhomogeneity of the packing density of densely erosslinked polymers. The packing of the nets plays an important role determining the contribution of the intermolecular-interactions to the properties of the polymer bodies [6]. However, because of the microinhomogeneity of the dense nets to characterize their packing b y the averaged coefficient of molecular packing used to characterize linear polymers is meaningless. To detect and evaluate the inhomogeneity of the packing density of PDMA the method of the molecular probe has been used [7] based on the dependence of the frequencies of the electron-vibratory transitions in the absorption spectra of the aromatic molecules on the density of the surrounding polymer matrix. The probe used was anthracene molecules which were dissolved in the initial

2114

]3. V.

OZEI~KOVSKII

eta{.

oligomer in an amount of 10 -4 moleL After radical polymerization the anthracene molecules were found to be incorporated into the reticular polymer matrix. Figure 1 presents the electron absorption spectra of anthracene in the polymer of oligomer I obtained in the usual variant (spectrum 1) and in the form of the derivative absorption curve for a difference in wavelengths of 1 nm (spectrum 2). As is clear, the bands of spectrum I have weakly marked low frequency shoulders which in spectrum 2 are manifest in the form of three well resolved components. The spectrum of anthracene in the 1)MMA matrix consists of single components of electron-vibratory structure. "

1

J

li

;

I

I

I

,,

_•

I

D

1

i(

;

?

1 1.

7

Z1

~:

7

,

/ ~ / r 1

.;

-

_

7

f

I

I-2 1

I

/"

,I

I

I

1.

1

/

J

$

,.t:l

/ 4

2

I -~7-~ ~

I / ! _./

I k

I0~-,

-/ 1 I

I

I

1

t

I -~

x,to-f,4 lQcl. 1

i

/

i

" /12

/ •

,~,/

-

I

.¢

,~,.~4. ~

b FiG. 2

lezc. I. Initial (I) and deriw~tive (2) eleet,ron -~bsorption spectra of anthracene in polymer matrix of oligomer I. FIG. 2. Schematic representation of the formation of the structure of network polymers from oligomers of type A (a) and oligo-p-methaerylates (b); 1--polymethacrylate chains; 2--oligomer chains; 3--n-dimensional methacrylato chains; 4--residual structure-isolated double bonds. The appearance of new absorption bands in the spectrum of anthracene introduced into the polymer obtained from oligomer I indicates t h a t this polymer has three types of microregions differing in packing density in which the probe molecules are localized. Ill the case of tile polymer of butanediol diinethacrylate the method reveals two regions with different densities.

Polymers of dimethacrylatcs and oligo-p-m('lllacrylates

2113

From the results (Table 2) we evaluated the relative differences in the densities p of the individual microregions using the formula [7] Av=Ap ~ where Av is the value of the spectral shift equal to the difference between the frequencies of the purely electron transitions of the free molecules of anthracene vapour (v0=27,590 cm -1) and the molecules penetrating into the polymer matrix. From Table 2 it may be seen that the differences in density in the individual regions of the densely crosslinked polymers may reach several tens of percentages. The presence in dimethacrylate polymers of microregions of different density is evidently due to the structural inhomogeneity of the polymer body produced by the heterophasic character of polymerization [8]. Vibratory segment in P D M A . Fundamental to an understanding of the dynamic structure and properties of densely crosslinked dimethacrylate polymers is the establishment of the minimal size of the portion of the chain ("vibratory segment") the spectrum of which is identical; to that of the polymethacrylate chain in linear t?MMA. In linear polymers the vibratory segment is usuMly determined by investigating the spectra of the oligomers with a different degree of polymerization. In reference [9] a number of MMA oligomers with a degree of polymerization increasing from n = 2 - 3 to n = 4 0 - 5 0 was studied. It turns out that in oligomers with short chains the parameter of the stereoregularity depends on n. n I

4 44

5-6 52

7-8 68

9 81

15 93

40 96

10()0 105

The oligomers with ~ < 9 contMn a considerable quantity of units with isotaetic attachment and from the vMues of the parameter I may be assigned to tile stereoblock type. Oligomers with n > 9 have vMues of I a.pproaching the magnitude characteristic of high molecular weight syndiotactic. 3'he polymer obtained from oligomer I has the parameter 1 - 7 1 which corresponds to the linear methylmethact)qate oligomer with a degree of polymerization n ~ 7. However, it should be borne in mind that since the stereocomposition of the short oligomers depends on their length, then it somewhat differs from the composition of the portions of the chain isolated in the central part of the macromolecule. Therefore the evaluation made of the size of the vibratory segment of the polymethacrylate chain in the reticular polymer gives its upper limit. The experimental results show that the vibratory segment of the linear PMMA polymer includes more monomer units than polymethacrylate chains in a densely erosslinked polymer based on oligomer I. Fall in the number of the monomer units in the vibratory segment, of the polymethacrylate chain in PDMA is apparently due to the alveolar structure of reticular polymers which leads to a different redistribution of the vibratory interactions along the chains and between adjacent chains as compared with linear PMMA. In connexion with this tbr densely erosslinke<] polymers it is desirable to speck not of the "vibratory segment of the chain" but, the "vibratory frag-

B.V. OZErU~OVSKIIet al.

2116

'.['ABLE 2. ] ~ R E Q U E N C I E S OF T H E E L E C T R O N T R A N S I T I O N S Y0 A N D S P E C T R A L S H I F T S ~

IN THE

A B S O R P T I O N S P E C T R A OF T H E P R O B E M O L E C U L E S OF A N T H R A C E N E AND R E L A T I V E D I F F E R E N C E S I N D E N S I T Y OF T H E M I C R O R E G I O N S I N R E T I C U L A R P O L Y M E R S

X in oligomer -- (--CH2--CH2--0--)3--

--(-CH~--h--

Vo, cm -1

Jr, cm -x

26,240 25,790 25,440 26,300 26,060

1350 1800 2150 1290 1350

p, rel. units 1

1-155 1.262 1 1.089

ment of the network". The concept of the mutual link of the vibrations of all chemical bonds in the cyclic alveoli of the network may prove useful in analysing the special features of the thermodegradation of densely crosslinked PDMAs. According to reference [10], on thermodegradation of the PDMA the intact molecules of oligomer I split off. This indicates the possibility of synchronous depolymerization of crosslinked polymethacrylate chains. The reason for such synchronization may be the strong vibratory bond of the monomer units of the adjacent chains through the oligomer blocks X.

Formation of densely crosslinked polymer glasses from oligo-p-methacrylates. The radical polymerization of the oligomers of type I is characterized by heavy self-acceleration of the reaction, the heterogeneity of the polymerisate [8] and the appearance of shrinkage stresses in the polymer body [5]. The scale of these negative phenomena may be greatly reduced by passing to the formation of reticular PDMAs in two stages: at first obtain oligo-p-methacrylates (-- CH2--CCH3-- ),~

l

O:C-- O--X--CCH~C~CH,,

with a low degree of polymerization n and then carry out polymerization at the C----C bonds in the side chains. With such a method chemical shrinkage falls by 50~o, which must reduce the polymerization stresses. According to the ideas outlined in the first part of the

paper fall in the stresses must increase the probability of syndiotactie attachment of the methacrylic groups. The values of the parameter of stereoregularity I presented in Table 3 show that this is actually so and the polymers obtained by polymerizing oligo-p-methacrylates have a predominantly syndiotactic structure of the polymethacrylate chains. An important feature of the two-stage method of obtaining PDMAs is that it gives a bimodal MWD of the polymethacrylate chains. As is schematically shown in Fig. 2, this leads to the formation of a mesh of distinctive "segmented" structure consisting of short and long polymethacrylate chains crosslinked between themselves. Such a structure must promote the better packing of the atomic

Polymers of dimet hacrylafes and oligo-p-methaerylates

2117

groups, the relaxation of the internal stresses and improvement of the physicomechanical properties of the material. In fact, the elasticity modulus of the polymer based on oligo-p-methaerylate with n--8 is 25< 105 kg/cm 2 while t h a t of tho triethylene glycol dimethacrylate polymer obtained in the traditional way is only 1-5 × 105 kg/cm 2. Formation of densely crosslinked polymer glasses at high pressures. Study of the influence of pressure on the microstructure of polydimethacrylates is of interest for two reasons. Firstly it helps to clarify the special nature of the effect of pressure on the stereoregulalqty of the chains, the type of attachment of the monomer units and the residual double bonds on three dimensional radical polymerization of the oligomers of type A. Secondly, the establishment of the experimental dependence of the microstructural parameters on pressure makes it possible to pass from elements of structure little amenable to a generalized description to thermodynamic parameters and to the equation of state of densely crosslinked polymers. The influence of pressure on the formation of densely crosslinked polymers was studied with reference to the radical polymerization of oligomer I. Analysis of the spectra of the polymers obtained in the region of pendulmn vibrations the methylene groups (Fig. 3), showed that pressure influence the type of attachment of the methacrylie groups. In the network polymers obtained at normal pressure in this region a single band 751 cm- ~ is observed relating to the vibrations of the CH2-groups of the polymethacrylate chain. In the polymer obtained under pressure 6--20 kbar together with this band a new intense band 732 cm -1 appears indicating the formation of a considerable number of - - C H 2 - C H 2 units. The formatiori of these units is apparently due to increase in the probability of the attachment of the methacrvlic groups of the "tail to tail" type under the unfluence of high pressures. According to reference [1 I] carrying out radical polymerization of methylmethacrylate at a pressure of 8 kbar increases the contribution of isotactic attachment of methaerylie groups from 25 to 38%. Rise in pressure within these limits TABLE

3 . CHAIC~ACTERISTICS O F

OLIGO-2)-METHACr4YLATES A N D

I ~ E T I C U L A R ~POLY~EI~8 B A S E D

O N THI~] ~I

8

ll 15

Conversion of C=C bonds of oligomer I, °jo

Parameter I for oligomer

Limiting conversion of C=C bonds in oligomer, ~o

Parameter I for polymer

53 65 7l

78 85 .I)-2

54 q0 4

84 96 98

* T h e ~,~ g o - p - m e ,h~ err M e ~ w4,vi~old,ain~,c b y l'adieM polynh~riz~Hh)n ol~ frh,|hy]~lle g[yq~o| diltlelJtacLvilde ill pre~ellco

o f eata ysts of chMn fransfi~r (e(,pper p|lth.'tlocyalJin, viPlmlt l.~rphvritD The degz'o¢: or" p(dyn~e]'ization was reglllatod b y the coil '(~!ntl';ll i,~[i D[" th0 tt'~),nsfi:r agent.

~118

B, V. OZE~I~OVSKnet al.

on polymerization of oligomer I also increases the contribution of the isotactie attachment of the methacrylic groups. However further increase in the pressure judging from the values of the parameter I (Fig. 4) practically does not influence the stereocomposition of the polymethacrylate chains. ~T arb. un.

O'09~a

o

Q

I

l

I

C

~

4,,

t 16

t

20 O.O7|t-

i

I

7O

Y I 7

0

i

I

8 V ,' I0- i cm -1

Fro. 3

_

50F"

I

o

8 1

i

p, kbar

FIo. 4

Fie. 3. IR spectra of polymers of oligomer I obtained in the usual conditions (1) and under pressure of 12 kbar (2). FIe. 4. Content ~ of residual C=C bonds (a) and parameter I(b) of polymers of oligomer I as a function of pressure.

The effect of pressure on the depth of polymerization of dimethacrylate (concentration of crosslinks) is of a complex character (Fig. 4). At first rise in pressure deepens polymerization. Such a dependence is natural since polymerization is accompanied b y fall in volume Av and the pressure shifts the equilibrium toward polymerization by reducing the free energy AG according to the relation d(AG)/ / d p = A v . However, further rise in pressure practically does not influence the concentration of crosslinks in the densely crosslinked polymer. Let us consider the dependence of the concentration of the crosslinks on pressure. It was earlier shown [6] that the presence in polymers of a high concentration of crosslinks produces a loosening effect the essence of which is that the chemical crosslinks starting from a certain critical concentration impede the packing of the carcass of the mesh optimal for the intermolecular interaction. Therefore it may be assumed that in the extremely crosslinked reticular polymer equilibrium between the crosslinks is set up reducing the volume of the system and the loosening crosslinks. I f the existence of such equilibrium is accepted then the chemical potentials of the crosslinks for a given pressure are equal between themselves: llo,l=tlo. Differentiating this cquatio~, and dividing the equation

Polymers of dimcthacrylatcs and ollgo-p-mcthacrylates

2119

obtained by dp we get

where a is the molar fraction of crosslinks. According to the equation for the free energy

--~/~,T=Ve,1 and t~/T-re' where Vc,l is the partial molar volume of the loosening erosslinks; ve is the partial molar volume of the crosslinks reducing the volume of the system. Regarding the loosening crosslinks as a component distributed in the densely crosslinked polymer one may write

pc, l=/~c, sv+/~T In ac

-

'~;-Iv,T=

L- ~g-/~,'~

Substituting these values in equation (]) we find for the limiting value (O~/~p)

\~P/~

~(t '? ~'~

. . . .

"~) ll

- -

(2)

--

Equation (2) on the assumption of an ideal system (ln ac = I n ~) assumes the form

-~-:p--)m=

RT

and the limiting concentration ~lim is expressed as

Avl.p

In ~ = - - ~ -

(a)

From equation (3) one may expect fall in the limiting concentration of erosslinks in the densely crosslinked polymer with rise in pressure owing to the presenco of an additional "loosening" increment of the free volume Avl. Tran,~lated by A. CROZY

2120

B. YA, TEITEL;I~A'USI e t a l . REFER.CNCE5

1. A. A. KOROTKOV, S. P. MITSENGENDLER, A. L K R A S U L I N A and L. A. VOLKOVA, Vysokomol. soycd. A I : 1319, 1959 (Translated in Polymer Sci. U.S.S.R. 1: 3, 506, 1960) 2. It. $AVADA, Tcrmodimunik~ polilnerizo~tsii (Thermodynamics of Polymerization) Khimiya, Moscow, 1979 3. W. GOOD, ¥. OWENS, It. FELLMAN, W. SNYDER and J. MOORE, J. Polymer Sei. 46: 317, 1960 4. I. Ya. 8LONIM and A. NI LYUBIMOV, Yadernyi magnityi rezonans v polimerakh (Nuclear Magnetic Resonance in Polymers). p. 268, Khimiya, Moscow, 1966 5. V. P. ItOSItCHUPKIN and B. V. OZERKOVSKII, Karbotsepnye polimery (Carbon Chain Polymers). p. 132, Nauka, Moscow, 1977 6. B. V. OZEItKOVSKII and V. P. ROSHCHUPKIN, Dokl. Akad. :Nauk SSSR, 248: 657, 1979 7. Ye. G. MOISYA, Metod molekulyarnogo zonda v issledovanii polimerov (Method of the Molecular Probe in Study of Polymers). In: Novye metody issledovaniya polimerov (:New Methods for Studying Polymers). p. 32, Naukova dumka, Kiev, 1975 8. V. P. ROSHCHUPKIN, B. V. OZERKOVSKII, Yu. B. KAL'MYKOV and G. V. KOROLEV, Vysokomol. soyed. A19: 699, 1977 (Translated in Polymer Sci. U.S.S.R. 19: 4, 809, 1977) 9. B. V. OZERKOVSKII and V. P. ROSHCHUPKIN, Dokl. Akad. N a u k SSSR 254: 137, 1980 10. R. M. ASEVA, T. V. ZELENETSKAYA, O. G. SEL'SKAYA and A. A. BERLIN, Vysokotool. soyed. A14: 1573, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 7, 1766, 1972) 11. D. HAM, Polimerizatsiya vinilovykh nlOl).onlerov (Polymerization of Vinyl Monomers). Khimiya, Moscow, 1973

Polylner ScienceU.S.~.R. Vol. 25, .No.9. pp. 2120-2127. 1953 Priu~d in Poland

0032-3950/83 $10.00+ .00 ,~ 19~t PergamonPress Lt(t,

INFLUENCE OF TEMPERATURE ON DOMAIN STRUCTURING IN SEGMENTED URETHANE POLYMERS * ]3. YA. TEITEL'BAUM, YE. T. MAGDALEV, T. A. YAG•AICOVA, M. G. ZIMI•A a n d N . 1~. APUKHTINA Arbuzov I n s t i t u t e of Ot'ganie and Physical Chemistry, U.S.S.R. Academy of Sciences Lebedev All-Union Synthetic P~ubber lCesearch Instituto

(Received 29 January 1982) X - r a y and thermomechanieal methods have been used to s t u d y the formation of the domain structure and thermnmechanical properties of polyether urethane urea obtained by hardc-ning the prepolymer at 120°C. According to the results, the per* Vysokomol. soyed. A~.~" ~¢,,. 9, 1823-1829, 1983.