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Mechanical properties of crosslinked polymethacrylate glass
Crosslinked polymethacrylate glass
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RINSKH, Vysokomol. soyed. A9: 2185, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 10, 2470 J. A. KREUZ, A. L. ANDREY, F. P. GAY and C. Ye. SR00G, J. Polymer Sei. 4, A-1 : 2607, 1966 I. Ye. KARDASH, A. Ya. ARDASHNIKOV, F. S. YAKUSHIN and A. N. PRAVEDNIKOV~ Vysokomol. soyed. A17: 598, 1975 (Translated in Polymer Sei. U.S.S.R. 17: 3, 689, 1975) A. G. CHERNOVA, L. S. BUBL]X, L. P. OKUNEVA, V. V. RODIONOV, A. V. IVANOVA and V. D. VOROB'YEV, Plast. massy, No. 3, 11, 1975 I. A. ARKHTPOVA, B. A. ZHUBANOV, S. R. RAFIKOV, S. B. SAIDENOVA and N. L BUKETOVA, Dok]. AN SSSR 209: 93, 1973 N. A. ADROVA, A. I. A R ~ 0 V , Yu. G. BAKI.AGINA, T. I. BORISOVA, M. M. KOTON, Ye. V. KUVSHINSKII, A. MIRZAYEV, N. V. MIKHAILOVA, V. N. NIKITIN, A. V. SIDOROVICH, Vysokomol. soyed. A14: 2166, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 10, 2539, 1972) G. S. KOLESNIKOV, O. Ya. FEDOTOVA and E. I. HOFBAUER, Vysokomol. soyed. Bl1: 617, 1969 (Not translated in Polymer Sci. U.S.S.R.) B. A. ZHUBANOV, G. I. BOIKO, S. A. MASH~EVICH and S. R. RAFIKOV, Izv. AI~ KazSSR, set. khim., 27, 1972
[MECHANICAL PROPERTIES OF CROSSLINKED POLYMETHACRYLATE GLASS* Z. S. BELOKON', A. YE. SKOROBOGATOYA, N. YA. GRIBKOVA, S. A. ~ Z H A K O V ,
N. i~. BAKEYEV, P. V. KOZLOV and V. A. KABANOV
(Received 19 April 1976) A study was made of the mechanical properties of methyl methacrylatedimethacrylate ethylene glycol copolymers. The effect of an adsorption active medium and a plasticizer on deformation and strength properties of crosslinked glass was examined. Results were interpreted using the irregular distribution of intermoleeular chemical bonds in the polymer.
THE customary description of mechanical properties of crosslinked polymer systems is based on the concepts of a statistically homogeneous molecular network, the units of which are evenly distributed in the sample volume [1]. Direct and indirect information concerning the structural heterogeneity and the presence of ranges of ordering in linear amorphous polymers (see e.g. [2]) which are, apparently, also retained during crosslinking by various methods inevitably result in a new way of presenting the problem concerning the distribution o f intermolecular chemical bonds in crosslinked polymer systems. These con* Vysokomol. soyed. A18: No. 12, 2772-2779, 1976.
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~iderations h a v e a l r e a d y b e e n d e a l t w i t h in several studies m a i n l y c o n c e r n e d w i t h s t r u c t u r a l a s p e c t s of v u l c a n i z a t i o n of r u b b e r s (see e.g. [3]). V e r y few inv e s t i g a t i o n s h a v e b e e n concerned w i t h s t r u c t u r a l a n d m e c h a n i c a l a s p e c t s o f t h e b e h a v i o u r o f erosslinked p o l y m e r glass [4]. These p r o b l e m s were d e a l t w i t h b y t h e a u t h o r s in a p a p e r p r e v i o u s l y p u b l i s h e d [5]. R e s u l t s a r e described in this s t u d y of t h e s y s t e m a t i c e x a m i n a t i o n of t h e effect o f i n t e r m o l e c u l a r chemical b o n d s on t h e d e f o r m a t i o n p r o p e r t i e s of crosslinked P M M A in t h e glassy s t a t e a n d t h e effect of a d s o r p t i o n a c t i v e m e d i a a n d plasticizers on crosslinked s y s t e m s . Methyl methacrylate (MMA) copolymers with differing proportions of ethylene glycol dimethacrylate (EGI)M) were used. Network formation occurred directly in the course of macromoleeular extension during bulk polymerization, i.e. transition from monomer to solid polymer. Polymer glass was obtained in the form of silicate glass, the degree of crosslinking being regulated by the amount of EGDM added to the reaction mixture (0.05-11 mole°/o): When studying the effect of plasticizer on properties of crosslinked polymers polymer glass was obtained from a monomer mixture, into which the requisite amount of dibutylphthalate (DBP) was added. Dieyclohexylperoxydicarbonate was used as initiator in proportions of 0.3% and 0.005 wt.% for glass 1 and 20 mm thick, respectively. The samples were prepared for testing by mechanical working: by compression in the form of cylinders using sheets 20 m m in thickness and elongation, in the form of two way blades with a cross section of 2 × 1 m m 2 using sheets 1 m m thick. Elongation in air and white spirit chosen as an adsorption-active medium was cartied out in a Polanyi device at a rate of 1 mm/min. Reduction kinetics of compressed samples were measured by the following method. Samples were compressed at 20 ° at a rate of 0.1 mm/min in a universal TsDM testing ma. chine. Deformation was measured using an indicator with a multiplying factor of 0.01 mm~ After attaining the requisite deformation, the load was relieved and the sample placed in a device to measure height with a continuous increase of temperature. An IZV-2 optical length measuring device with a thermocryo-compartment was used for this purpose. Temperature was raised at a rate of 0.8 deg/min, adjustment being made by a RUS-01M automatic electronic programming regulator. From results of measuring the height of the deformed sample while heating, the dependence of relative residual deformation (subtracting linear expansion) on temperature was plotted. Compression isotherms at 20-100 ° were obtained using an Instron machine, the rate ~)f movement of the mobile clamp being 0.5 ram/rain.
Effect of intermolecular chemical bonds on deformation properties of glass. F i g u r e 1 shows i s o t h e r m a l curves o f u n i a x i a l c o m p r e s s i o n o f s a m p l e s w i t h diff e r e n t crosslink c o n t e n t s (EGDM). T h e f a c t should b e n o t e d first o f all t h a t e v e n s a m p l e s c o n t a i n i n g E G D M in a p r o p o r t i o n r e a c h i n g a b o u t one u n i t p e r 10 u n i t s o f M M A are c a p a b l e o f u n d e r g o i n g forced elastic d e f o r m a t i o n . According t o r e s u l t s o b t a i n e d b y B e e v e r s a n d W h i t e [6], t h e m o l e c u l a r w e i g h t o f a m e c h a n i c a l s e g m e n t o f ' P M M A is 10,000. L a t e r m e a s u r e m e n t s carried o u t b y T h o m p s o n [7] g a v e v a l u e s r a n g i n g f r o m 30,000 t o 60,000, i.e. so t h a t t h e a v e r a g e d i s t a n c e b e t w e e n c h e m i c a l crosslinks m a y r e a c h t h e l e n g t h of t h e m e c h a n i c a l s e g m e n t ; for t h i s i t is sufficient t o i n t r o d u c e 1 crosslink p e r 60-120 p o l y m e r chain units. I n t h e ¢ o p o l y m e r s a m p l e s s t u d i e d w i t h a m a x i m u m c o n t e n t of EGD1K I R s p e c t r o s c o p y
Crosslinked polymothaerylate glass
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had not shown residual double bonds. Consequently, the average density of crosslinks is known to be higher than that of the mechanical segment. It m a y be assumed that this is also valid for samples with lower EGDM con~ents. This is
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FIG. 1. Compreslon isotherms at 20 (a) and 100° (b) of MMA-EGDM eopolyrners containing 0 and 0-05 (•); 0.5 (2); 2.5 (3); 5 (a4), ll (b4, 6) and 8 mole~/o EGDM (5). FIe. 2. Relation between the glass temperature according to Vicar and EGDM content in a MMA-EGDM eopolymer. confirmed b y a continuous increase in glass temperature determined b y Vica~ over the entire range of copolymcr composition subjected to mechanical tests (Fig. 2). I f we assume that the distribution of intermoleeular chemical bonds in copolymers is statistically homogeneous, with an average distance between the chemical network units which is less than the length of the kinetic segment, brittle rupture of loaded samples could be expected without forced elastic deformation. The absence of brittle rupture, apparently, is simple evidence of statisticM network heterogeneity in the polymers studied. I t is also typical that the limit of forced elasticity ar on compression isotherm at 20 ° is completely independent of cross]ink density (Fig. la), however, the dependence is apparent when increasing test temperature (Fig. lb and 3). Forced elastic deformation is also valid in uniaxial elongation of cross]inked samples (Fig. 4). I~ appears that the value of af is independent of crosslink density up to t~mperatures close to Tg of
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linear PMMA (Fig. 5). An increase in a r with an increase in the average number of intermolecular chemical bonds in unit volume in e]ongation is only observed at 120% Extensive information confirms that chemical network units formed during
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F r o . 3. R e l a t i o n b e t w e e n t h e l i m i t o f forced elasticity in compression a n d t h e t e s t t e m p e r a * ture of ~ A - E G D M copolymers. C o n t e n t of E G D M , m o l e % : 1 m 0 a n d 0.05; 2 - - 0.5; 3--5;4-11. FIG. 4. E l o n g a t i o n i s o t h e r m s of M M A - E G D M c o p o l y m e r s at 20 (I) a n d 110 ° (II). Content, of E G D M , m o l e % : 1 - - 0 a n d 0.5; "2 m 1.5; 3 - - 3; 4 - - 6.5; 5 - - 11.
radical copolymerization of MMA with EGDM during the formation of a polymer unit are basically localized inside structural elements, which, over the tempera. ture range in which af is independent of crosslink density, are not practically
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FIG. 5. R e l a t i o n b e t w e e n t h e limit of forced elasticity a n d t h e c o n t e n t of EGD1K i n t h e c o p o l y m e r d u r i n g elongation in air (1-4) a n d w h i t e spirit (1'-4') at 40 (1, 1'), 60 (2, 2'), 80 (3, 3') a n d 110 ° (4, 4').
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subject to internal regrouping at the initial stages of forced elastic deformation. Transition from a constant value of ar to an increased value with an increase in EGDM content in copolymers taking place in the range of increased temperature, proves that other structural elements are involved in deformation which, in contrast to the former, are secured to a large extent by intermolecular chemical bonds, whereby in the case of uniaxial compression this occurs at lower temperatures than in the case of uniaxial elongation. Beyond the limits of forced elasticity compression isotherms and elongation isotherms of various samples diverge. The higher the network density, the higher the rate at which stress increases with high deformation. As noted previously, experimental results cannot be explained within the framework of ideas concerning a homogeneous statistical network. Their interpretation should, apparently, be sought in close connection with general ideas concerning the existence of several levels of organization in amorphous polymers. Recently, by analysing numerous structural and physico-mechanieal results obtained by Arzhakov, Bakeyev and Kabanov [8] a concrete structural model of an amorphous polymer was proposed. In this model ideas were summarized previously expressed by Arzhakov and Kabanov [9] concerning the existence in an amorphous polymer of "supermoleeular domains connected by continuous chains", which were required to explain relaxation kinetics of deformation of PMMA and later appeared useful in interpreting similar kinetic relations of relaxation of glass [10] and independent results and ideas by Yeh [11] and Klement and Geil [12] concerning the existence of super-domain organization, as well as domain organization. Ideas were introduced in the model [8] concerning fibrils formed of domains, which could later be directly observed with an electron microscope [13] and the concept of super-domains as fibrillar packing was more accurately defined, these fibrils being similar to some extent to spherulites in crystalline polymers. To interpret the experimental results described in this paper, the problem concerning the type of macromolecular packing within the domain is insignificant. It is essential and sufficient, however, to assume that there are at least two levels of structural heterogeneities in an amorphous polymer. Within the framework of model [8] the first level of structural heterogeneity is formed of domains and inter-domain regions (within the range of fibrils). The second level is formed of superdomains and interfaces between them. According to an earlier study [10], deformation of a glassy polymer far from T~ up to achieving the limit of forced elasticity causes the formation of ndw interfaces along the physical boundaries of some super-domains. Owing to the deformation of the entire super-domain frame the domains are somewhat displaced in relation to each other, which is accompanied by a change in the conformation of continuous chains in ranges between domains. The fact that the value of af is independent of the average concentration of crosslinks, as observed in some temperature ranges means that, the boundaries along which at given
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temperatures micro-cracks are formed in the samples studied, are not secured practically with further chemical crosslinks. The deviation of isothermal curves of compression and elongation with a deformation at which according to a former s t u d y [10], internal regrouping of super-domains takes place, dependent on the rotation of domains inside the fibrils and their separation (fragments) in the direction of tangential stress indicates that chemical crosslinks hinder these processes. I t is therefore reasonable to assume t h a t at least part of t h e m is concentrated between the chains inside and on the surface of domains, thus preventing the fragmentation of domains and elongation of chain segments in the region between domains.
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FIG. 6. Polytherms of reduction of PMMA copolymer samples containing 0.5 (a) and 11 EGDM (b). Degree of compression: a - 5.6 (1); 10.3 (2); 14.5 (3); 20.8 (4); 80.2 (5) and 50% (6); b - - 6.7 (•); 10-8 (2); 16.6 (3); 21.7 (4) and 26% (5). On increasing temperature the strength of contacts between super-domains a n d fibrils 'generally decreases and converges. I n view of the high temperature coefficient of strength boundaries between fibrils m a y become even weaker t h a n those between super-domains. I n the temperature range indicated where the value of ar depends on the density of crosslinks the morphological pattern o f deformation evidently changes: in the first section of deformation new interfaces a r e formed along the interfibrillar boundaries (separation of super-domains). Chemical cross]inks prevent this process (increase of af with an increase in EGDM content in the samples). Consequently, it should be assumed within t h e framework of the model examined t h a t some network units connect adjacen~ fibrils.
The mechanism proposed is in agreement with results of studying relaxation iknetics of deformation of crossJin~ed samples previously subjec~d to un/axial compression. When studying polytherms of reduction of linear Pl~d_~ samples
Crosslinked polymethacrylate glass
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subjected to uniaxial compression it was shown [10] t h a t in the general case forced elastic deformation is formed of two components, one of which el is lower during relaxation, while the other, e2, is only lower in the temperature range of glass transition. During deformation at stresses not exceeding the limit of forced elasticity, subsequent reduction is completed below Tg. As noted, in this range the domains are not re-grouped and the distortion of super-domain frame is only accompanied by some change in conformations of continuous chains in regions between domains, which after relieving load are relaxed below Tg by the action of internal stress accumulated in the system, i.e. the entire deformation falls on cl. With deformations at stress exceeding af, the value of E2 is added to ~1. According to a previous study [10], this is due to a re-grouping of superdomains, mutual displacement, rotation and separation of domains. Figure 6 shows polytherms of reduction of weakly and strongly crosslinked PMMA-glass samples subjected to uniaxial compression to varying degrees. I t can be seen t h a t for a weakly erosslinked sample at fairly high degrees of compression ranges of low temperature and high temperature reduction are clearly observed (Fig. 6a). In this case, both mechanisms of deformation and relaxation operate. Reduction of a strongly crosslinked sample (Fig. 6b) with the same degrees of compression is practically complete below Tg. Chemical crosslinks localized in domains and between adjacent fibrils hinder mutual displacement, rotation and separation of domains. Therefore, the relative effect on the overaU deformation of the affme deformation of the entire system and domains as a whole, only accompanied by a conformation, change of continuous chain segments fixed between adjacent domains, appears to be dominant in the range of degrees of compression studied. Effect of the adsorption active medium. Considerations concerning the localization of crosslinks which are in agreement with the views described, m a y be expressed using mechanical test results of samples in an adsorption active medium, such as white spirit, which does not dissolve PMMA, but satisfactorily moistens it and therefore, can be absorbed at the interfaces between structural elements. In Fig. 5 a broken line shows the dependence of af on the concentration of crosslinks, obtained in uniaxial elongation of samples immersed into white spirit. I t can be seen t h a t in an adsorption active medium an increase in af with crosslink density occurs at temperatures much lower t h a n during deformation in air. As a result of views formulated it is natural to assume that the action of white spirit causes the strength of interfibrillar boundaries to be reduced by adsorption so t h a t t h e y become locations of formation of new interfaces at the initial stage of deformation. Hence the sensitivity of af to the density of crosslinks which, according to the model examined, are localized particularly on intcr-fibrillar boundaries. Effect of plasticizer. I n order to explain the effect of plasticizer on crosslinked of forced polymethacrylate glass, a comparison was made of the dependence of limits of elasticity in uniaxial elongation on the gravimetric content of dibutyl phthalato
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(DBP) in linear PMMA samples and copolymers with different EGDM contents a t different temperatures (Fig. 7). Let us first examine results obtained for linear PMMA. It should be noted t h a t in the range of very low contents of good plasticizers, which is not normally being dealt with by scientists, the value of af shows extremal dependence. At 40 °.
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the addition of a plasticizer in a proportion of 1 wt.%, contrary to expectation, does not reduce, but increases the value of af, compared with the unplasticized sample. With higher DBP contents the value of af shows a regular reduction. Thus, the dependence of af on plasticizer content passes through a maximum. At higher temperatures of testing the addition of a plasticizer first reduces af in the region of even lower concentrations of DBP, which is followed by an increase with a maximum of about the same 1%, i.e. a minimum precedes the previous maximum under these conditions. Small amounts of DBP added to crosslinked glass (5% EGDM) also increase el, which passes through a maximum in the 0.5-1% region. However, in the latter case no minima are observed at a n y of the temperatures studied which would precede the maxima. Effects of hardening of crosslinked glass with small amounts of plasticizer (anti-plasticization) are very significant; for a sample with 5% EGDM at 120° hardening reaches 300//o of the initial value of af. The anti-plasticization mechanism is of considerable interest and so far has not been explained satisfactorily in the literature. It may be assumed that small amounts of plasticizer contribute to the orienta~ o n hardening of the material in the top part of micro-cracks formed at the initial stages of deformation. I t is difficult, however, to explain from this point of view, the coexistence of minima and maxima on curves showing of--DBP concentration for unplasticized PMMA at test temperatures higher than 40 °. In fact, an increase in temperature itself should contribute to orientatrion hardening, i.e. act in the same direction a s the plasticizer. Then, a further amount
Crosslinked polymethacrylate glass
3179
o f plasticizer could not in any way cause the hardening of the sample, if with low D B P contents the limit of forced elasticity already appeared to be reduced. Within the framework of the model used the entire combination of effects has found a logical interpretation of mechanical properties which we examined in previous chapters. Considering concrete levels of structural heterogeneity of polymer glass it is natural to assume t h a t a good plasticizer introduced in small amounts is first of all localized both inside the fibrils, being absorbed by macromolecules in relatively damaged regions between the domains and on inter-fibrillar boundaries. Adsorption of DBP by continuous chains in regions between domains should result in a local reduction of free volume and therefore, an effective reduction of continuous chain flexibility at a given temperature, i.e. hardening of fibrils. Since deformation in a section preceding the limit of forced elasticity is due to the deformation of fibrils, in which conformation mobility of continuous chain segments is observed, this factor will contribute to an increase in the value of ~f. The adsorption of the plasticizer at inter-fibrillar boundaries should be regarded as a factor which can potentially act in the opposite direction, i.e. reduce ~f under test conditions, when the overall strength of contacts is reduced on these boundaries to such an extent that t h e y become locations of forming new interfaces while stressing the sample. During elongation of linear PMY[A samples at low temperatures in the range of low plasticizer contents, the strength of inter-fibrillar boundaries still remains fairly high and deformation beigns with the decomposition of physical bonds on less perfectly packed joints between super-domains. Only the first factor operates under these conditions, i.e. the fibrils become harder. Hence follows the increase in the value of af, i.e. antiplasticization with smM1 additions of DBP. At increased temperature deformation at early stages involves weakened inter-fibrillar boundaries (decomposition of super-domains). Under these conditions both factors are observed, and according to plasticizer concentration either an adsorption reduction of the strength of inter-fibrillar bonds, or hardening of fibrils as a result of the adsorption of the plasticizer b y continuous chains predominate, i.e. the density of regions between domains increases. Two extrema follow from here (minimum and maximum) on curves showing af and D B P concentration. In crosslinked glass inter-fibrillar boundaries are further reinforced by intermolecular chemical bonds. Therefore, the adsorption reduction of strength degenerates; the minimum on the temperature dependence of af on D B P concentration also degenerates in the range of low plasticizer contents. Anti-plasticization with small additions of D B P is only observed in these system. I t is natural that DBP, being a good solvent for PMMA, introduced into the samples in considerable quantities, disrupts the bonds between macromolecules inside the domains, i.e. results in a gradual "dispersioll" of domains, maeroscopicMly degenerating during swelling of the sample and resulting in an increase in flexibility according to classical rules of plasticization.
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Z.S. B~T.OKON'e~ ed.
Conclusion. Structural and mechanical properties of crosslinked polymer systems were interpreted using t he particular example of PMMA glass, in which the three dimensional network of intermoleeular chemical bonds was formed during bulk copolymerization. At the same time the main ideas are apparently, applicable to a v e r y wide circle of crosslinked polymers in the glassy and highelastic states. The applicability of this approach has recently been demonstrated when studying t he behaviour of homogeneous and heterogeneous vulcani_~tes of several rubbers [14]. According to t he m e t h o d of forming chemical networks, distribution according to supermolecular structural elements and therefore, effect on macroscopic properties of samples m a y be different with the same average crosslink density. I t is significant, however, t h a t intermolecular chemical bonds do n o t pr o b ab ly cause major changes in general macromo!ecular packing, which is also typical of amorphous polymers formed of linear chains. TranslaSed by E. SEMERR REFERENCES
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RINSKH, Vysokomol. soyed. A9: 2185, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 10, 2470 J. A. KREUZ, A. L. ANDREY, F. P. GAY and C. Ye. SR00G, J. Polymer Sei. 4, A-1 : 2607, 1966 I. Ye. KARDASH, A. Ya. ARDASHNIKOV, F. S. YAKUSHIN and A. N. PRAVEDNIKOV~ Vysokomol. soyed. A17: 598, 1975 (Translated in Polymer Sei. U.S.S.R. 17: 3, 689, 1975) A. G. CHERNOVA, L. S. BUBL]X, L. P. OKUNEVA, V. V. RODIONOV, A. V. IVANOVA and V. D. VOROB'YEV, Plast. massy, No. 3, 11, 1975 I. A. ARKHTPOVA, B. A. ZHUBANOV, S. R. RAFIKOV, S. B. SAIDENOVA and N. L BUKETOVA, Dok]. AN SSSR 209: 93, 1973 N. A. ADROVA, A. I. A R ~ 0 V , Yu. G. BAKI.AGINA, T. I. BORISOVA, M. M. KOTON, Ye. V. KUVSHINSKII, A. MIRZAYEV, N. V. MIKHAILOVA, V. N. NIKITIN, A. V. SIDOROVICH, Vysokomol. soyed. A14: 2166, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 10, 2539, 1972) G. S. KOLESNIKOV, O. Ya. FEDOTOVA and E. I. HOFBAUER, Vysokomol. soyed. Bl1: 617, 1969 (Not translated in Polymer Sci. U.S.S.R.) B. A. ZHUBANOV, G. I. BOIKO, S. A. MASH~EVICH and S. R. RAFIKOV, Izv. AI~ KazSSR, set. khim., 27, 1972
[MECHANICAL PROPERTIES OF CROSSLINKED POLYMETHACRYLATE GLASS* Z. S. BELOKON', A. YE. SKOROBOGATOYA, N. YA. GRIBKOVA, S. A. ~ Z H A K O V ,
N. i~. BAKEYEV, P. V. KOZLOV and V. A. KABANOV
(Received 19 April 1976) A study was made of the mechanical properties of methyl methacrylatedimethacrylate ethylene glycol copolymers. The effect of an adsorption active medium and a plasticizer on deformation and strength properties of crosslinked glass was examined. Results were interpreted using the irregular distribution of intermoleeular chemical bonds in the polymer.
THE customary description of mechanical properties of crosslinked polymer systems is based on the concepts of a statistically homogeneous molecular network, the units of which are evenly distributed in the sample volume [1]. Direct and indirect information concerning the structural heterogeneity and the presence of ranges of ordering in linear amorphous polymers (see e.g. [2]) which are, apparently, also retained during crosslinking by various methods inevitably result in a new way of presenting the problem concerning the distribution o f intermolecular chemical bonds in crosslinked polymer systems. These con* Vysokomol. soyed. A18: No. 12, 2772-2779, 1976.
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~iderations h a v e a l r e a d y b e e n d e a l t w i t h in several studies m a i n l y c o n c e r n e d w i t h s t r u c t u r a l a s p e c t s of v u l c a n i z a t i o n of r u b b e r s (see e.g. [3]). V e r y few inv e s t i g a t i o n s h a v e b e e n concerned w i t h s t r u c t u r a l a n d m e c h a n i c a l a s p e c t s o f t h e b e h a v i o u r o f erosslinked p o l y m e r glass [4]. These p r o b l e m s were d e a l t w i t h b y t h e a u t h o r s in a p a p e r p r e v i o u s l y p u b l i s h e d [5]. R e s u l t s a r e described in this s t u d y of t h e s y s t e m a t i c e x a m i n a t i o n of t h e effect o f i n t e r m o l e c u l a r chemical b o n d s on t h e d e f o r m a t i o n p r o p e r t i e s of crosslinked P M M A in t h e glassy s t a t e a n d t h e effect of a d s o r p t i o n a c t i v e m e d i a a n d plasticizers on crosslinked s y s t e m s . Methyl methacrylate (MMA) copolymers with differing proportions of ethylene glycol dimethacrylate (EGI)M) were used. Network formation occurred directly in the course of macromoleeular extension during bulk polymerization, i.e. transition from monomer to solid polymer. Polymer glass was obtained in the form of silicate glass, the degree of crosslinking being regulated by the amount of EGDM added to the reaction mixture (0.05-11 mole°/o): When studying the effect of plasticizer on properties of crosslinked polymers polymer glass was obtained from a monomer mixture, into which the requisite amount of dibutylphthalate (DBP) was added. Dieyclohexylperoxydicarbonate was used as initiator in proportions of 0.3% and 0.005 wt.% for glass 1 and 20 mm thick, respectively. The samples were prepared for testing by mechanical working: by compression in the form of cylinders using sheets 20 m m in thickness and elongation, in the form of two way blades with a cross section of 2 × 1 m m 2 using sheets 1 m m thick. Elongation in air and white spirit chosen as an adsorption-active medium was cartied out in a Polanyi device at a rate of 1 mm/min. Reduction kinetics of compressed samples were measured by the following method. Samples were compressed at 20 ° at a rate of 0.1 mm/min in a universal TsDM testing ma. chine. Deformation was measured using an indicator with a multiplying factor of 0.01 mm~ After attaining the requisite deformation, the load was relieved and the sample placed in a device to measure height with a continuous increase of temperature. An IZV-2 optical length measuring device with a thermocryo-compartment was used for this purpose. Temperature was raised at a rate of 0.8 deg/min, adjustment being made by a RUS-01M automatic electronic programming regulator. From results of measuring the height of the deformed sample while heating, the dependence of relative residual deformation (subtracting linear expansion) on temperature was plotted. Compression isotherms at 20-100 ° were obtained using an Instron machine, the rate ~)f movement of the mobile clamp being 0.5 ram/rain.
Effect of intermolecular chemical bonds on deformation properties of glass. F i g u r e 1 shows i s o t h e r m a l curves o f u n i a x i a l c o m p r e s s i o n o f s a m p l e s w i t h diff e r e n t crosslink c o n t e n t s (EGDM). T h e f a c t should b e n o t e d first o f all t h a t e v e n s a m p l e s c o n t a i n i n g E G D M in a p r o p o r t i o n r e a c h i n g a b o u t one u n i t p e r 10 u n i t s o f M M A are c a p a b l e o f u n d e r g o i n g forced elastic d e f o r m a t i o n . According t o r e s u l t s o b t a i n e d b y B e e v e r s a n d W h i t e [6], t h e m o l e c u l a r w e i g h t o f a m e c h a n i c a l s e g m e n t o f ' P M M A is 10,000. L a t e r m e a s u r e m e n t s carried o u t b y T h o m p s o n [7] g a v e v a l u e s r a n g i n g f r o m 30,000 t o 60,000, i.e. so t h a t t h e a v e r a g e d i s t a n c e b e t w e e n c h e m i c a l crosslinks m a y r e a c h t h e l e n g t h of t h e m e c h a n i c a l s e g m e n t ; for t h i s i t is sufficient t o i n t r o d u c e 1 crosslink p e r 60-120 p o l y m e r chain units. I n t h e ¢ o p o l y m e r s a m p l e s s t u d i e d w i t h a m a x i m u m c o n t e n t of EGD1K I R s p e c t r o s c o p y
Crosslinked polymothaerylate glass
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had not shown residual double bonds. Consequently, the average density of crosslinks is known to be higher than that of the mechanical segment. It m a y be assumed that this is also valid for samples with lower EGDM con~ents. This is
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FIG. 1. Compreslon isotherms at 20 (a) and 100° (b) of MMA-EGDM eopolyrners containing 0 and 0-05 (•); 0.5 (2); 2.5 (3); 5 (a4), ll (b4, 6) and 8 mole~/o EGDM (5). FIe. 2. Relation between the glass temperature according to Vicar and EGDM content in a MMA-EGDM eopolymer. confirmed b y a continuous increase in glass temperature determined b y Vica~ over the entire range of copolymcr composition subjected to mechanical tests (Fig. 2). I f we assume that the distribution of intermoleeular chemical bonds in copolymers is statistically homogeneous, with an average distance between the chemical network units which is less than the length of the kinetic segment, brittle rupture of loaded samples could be expected without forced elastic deformation. The absence of brittle rupture, apparently, is simple evidence of statisticM network heterogeneity in the polymers studied. I t is also typical that the limit of forced elasticity ar on compression isotherm at 20 ° is completely independent of cross]ink density (Fig. la), however, the dependence is apparent when increasing test temperature (Fig. lb and 3). Forced elastic deformation is also valid in uniaxial elongation of cross]inked samples (Fig. 4). I~ appears that the value of af is independent of crosslink density up to t~mperatures close to Tg of
Z . S . BELOKON' e$ al.
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linear PMMA (Fig. 5). An increase in a r with an increase in the average number of intermolecular chemical bonds in unit volume in e]ongation is only observed at 120% Extensive information confirms that chemical network units formed during
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F r o . 3. R e l a t i o n b e t w e e n t h e l i m i t o f forced elasticity in compression a n d t h e t e s t t e m p e r a * ture of ~ A - E G D M copolymers. C o n t e n t of E G D M , m o l e % : 1 m 0 a n d 0.05; 2 - - 0.5; 3--5;4-11. FIG. 4. E l o n g a t i o n i s o t h e r m s of M M A - E G D M c o p o l y m e r s at 20 (I) a n d 110 ° (II). Content, of E G D M , m o l e % : 1 - - 0 a n d 0.5; "2 m 1.5; 3 - - 3; 4 - - 6.5; 5 - - 11.
radical copolymerization of MMA with EGDM during the formation of a polymer unit are basically localized inside structural elements, which, over the tempera. ture range in which af is independent of crosslink density, are not practically
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FIG. 5. R e l a t i o n b e t w e e n t h e limit of forced elasticity a n d t h e c o n t e n t of EGD1K i n t h e c o p o l y m e r d u r i n g elongation in air (1-4) a n d w h i t e spirit (1'-4') at 40 (1, 1'), 60 (2, 2'), 80 (3, 3') a n d 110 ° (4, 4').
Crosslinked polymethacrylate glass
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subject to internal regrouping at the initial stages of forced elastic deformation. Transition from a constant value of ar to an increased value with an increase in EGDM content in copolymers taking place in the range of increased temperature, proves that other structural elements are involved in deformation which, in contrast to the former, are secured to a large extent by intermolecular chemical bonds, whereby in the case of uniaxial compression this occurs at lower temperatures than in the case of uniaxial elongation. Beyond the limits of forced elasticity compression isotherms and elongation isotherms of various samples diverge. The higher the network density, the higher the rate at which stress increases with high deformation. As noted previously, experimental results cannot be explained within the framework of ideas concerning a homogeneous statistical network. Their interpretation should, apparently, be sought in close connection with general ideas concerning the existence of several levels of organization in amorphous polymers. Recently, by analysing numerous structural and physico-mechanieal results obtained by Arzhakov, Bakeyev and Kabanov [8] a concrete structural model of an amorphous polymer was proposed. In this model ideas were summarized previously expressed by Arzhakov and Kabanov [9] concerning the existence in an amorphous polymer of "supermoleeular domains connected by continuous chains", which were required to explain relaxation kinetics of deformation of PMMA and later appeared useful in interpreting similar kinetic relations of relaxation of glass [10] and independent results and ideas by Yeh [11] and Klement and Geil [12] concerning the existence of super-domain organization, as well as domain organization. Ideas were introduced in the model [8] concerning fibrils formed of domains, which could later be directly observed with an electron microscope [13] and the concept of super-domains as fibrillar packing was more accurately defined, these fibrils being similar to some extent to spherulites in crystalline polymers. To interpret the experimental results described in this paper, the problem concerning the type of macromolecular packing within the domain is insignificant. It is essential and sufficient, however, to assume that there are at least two levels of structural heterogeneities in an amorphous polymer. Within the framework of model [8] the first level of structural heterogeneity is formed of domains and inter-domain regions (within the range of fibrils). The second level is formed of superdomains and interfaces between them. According to an earlier study [10], deformation of a glassy polymer far from T~ up to achieving the limit of forced elasticity causes the formation of ndw interfaces along the physical boundaries of some super-domains. Owing to the deformation of the entire super-domain frame the domains are somewhat displaced in relation to each other, which is accompanied by a change in the conformation of continuous chains in ranges between domains. The fact that the value of af is independent of the average concentration of crosslinks, as observed in some temperature ranges means that, the boundaries along which at given
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temperatures micro-cracks are formed in the samples studied, are not secured practically with further chemical crosslinks. The deviation of isothermal curves of compression and elongation with a deformation at which according to a former s t u d y [10], internal regrouping of super-domains takes place, dependent on the rotation of domains inside the fibrils and their separation (fragments) in the direction of tangential stress indicates that chemical crosslinks hinder these processes. I t is therefore reasonable to assume t h a t at least part of t h e m is concentrated between the chains inside and on the surface of domains, thus preventing the fragmentation of domains and elongation of chain segments in the region between domains.
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FIG. 6. Polytherms of reduction of PMMA copolymer samples containing 0.5 (a) and 11 EGDM (b). Degree of compression: a - 5.6 (1); 10.3 (2); 14.5 (3); 20.8 (4); 80.2 (5) and 50% (6); b - - 6.7 (•); 10-8 (2); 16.6 (3); 21.7 (4) and 26% (5). On increasing temperature the strength of contacts between super-domains a n d fibrils 'generally decreases and converges. I n view of the high temperature coefficient of strength boundaries between fibrils m a y become even weaker t h a n those between super-domains. I n the temperature range indicated where the value of ar depends on the density of crosslinks the morphological pattern o f deformation evidently changes: in the first section of deformation new interfaces a r e formed along the interfibrillar boundaries (separation of super-domains). Chemical cross]inks prevent this process (increase of af with an increase in EGDM content in the samples). Consequently, it should be assumed within t h e framework of the model examined t h a t some network units connect adjacen~ fibrils.
The mechanism proposed is in agreement with results of studying relaxation iknetics of deformation of crossJin~ed samples previously subjec~d to un/axial compression. When studying polytherms of reduction of linear Pl~d_~ samples
Crosslinked polymethacrylate glass
3177
subjected to uniaxial compression it was shown [10] t h a t in the general case forced elastic deformation is formed of two components, one of which el is lower during relaxation, while the other, e2, is only lower in the temperature range of glass transition. During deformation at stresses not exceeding the limit of forced elasticity, subsequent reduction is completed below Tg. As noted, in this range the domains are not re-grouped and the distortion of super-domain frame is only accompanied by some change in conformations of continuous chains in regions between domains, which after relieving load are relaxed below Tg by the action of internal stress accumulated in the system, i.e. the entire deformation falls on cl. With deformations at stress exceeding af, the value of E2 is added to ~1. According to a previous study [10], this is due to a re-grouping of superdomains, mutual displacement, rotation and separation of domains. Figure 6 shows polytherms of reduction of weakly and strongly crosslinked PMMA-glass samples subjected to uniaxial compression to varying degrees. I t can be seen t h a t for a weakly erosslinked sample at fairly high degrees of compression ranges of low temperature and high temperature reduction are clearly observed (Fig. 6a). In this case, both mechanisms of deformation and relaxation operate. Reduction of a strongly crosslinked sample (Fig. 6b) with the same degrees of compression is practically complete below Tg. Chemical crosslinks localized in domains and between adjacent fibrils hinder mutual displacement, rotation and separation of domains. Therefore, the relative effect on the overaU deformation of the affme deformation of the entire system and domains as a whole, only accompanied by a conformation, change of continuous chain segments fixed between adjacent domains, appears to be dominant in the range of degrees of compression studied. Effect of the adsorption active medium. Considerations concerning the localization of crosslinks which are in agreement with the views described, m a y be expressed using mechanical test results of samples in an adsorption active medium, such as white spirit, which does not dissolve PMMA, but satisfactorily moistens it and therefore, can be absorbed at the interfaces between structural elements. In Fig. 5 a broken line shows the dependence of af on the concentration of crosslinks, obtained in uniaxial elongation of samples immersed into white spirit. I t can be seen t h a t in an adsorption active medium an increase in af with crosslink density occurs at temperatures much lower t h a n during deformation in air. As a result of views formulated it is natural to assume that the action of white spirit causes the strength of interfibrillar boundaries to be reduced by adsorption so t h a t t h e y become locations of formation of new interfaces at the initial stage of deformation. Hence the sensitivity of af to the density of crosslinks which, according to the model examined, are localized particularly on intcr-fibrillar boundaries. Effect of plasticizer. I n order to explain the effect of plasticizer on crosslinked of forced polymethacrylate glass, a comparison was made of the dependence of limits of elasticity in uniaxial elongation on the gravimetric content of dibutyl phthalato
~178
Z . S . BELOKON' et al.
(DBP) in linear PMMA samples and copolymers with different EGDM contents a t different temperatures (Fig. 7). Let us first examine results obtained for linear PMMA. It should be noted t h a t in the range of very low contents of good plasticizers, which is not normally being dealt with by scientists, the value of af shows extremal dependence. At 40 °.
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/5
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1
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80 (1).
the addition of a plasticizer in a proportion of 1 wt.%, contrary to expectation, does not reduce, but increases the value of af, compared with the unplasticized sample. With higher DBP contents the value of af shows a regular reduction. Thus, the dependence of af on plasticizer content passes through a maximum. At higher temperatures of testing the addition of a plasticizer first reduces af in the region of even lower concentrations of DBP, which is followed by an increase with a maximum of about the same 1%, i.e. a minimum precedes the previous maximum under these conditions. Small amounts of DBP added to crosslinked glass (5% EGDM) also increase el, which passes through a maximum in the 0.5-1% region. However, in the latter case no minima are observed at a n y of the temperatures studied which would precede the maxima. Effects of hardening of crosslinked glass with small amounts of plasticizer (anti-plasticization) are very significant; for a sample with 5% EGDM at 120° hardening reaches 300//o of the initial value of af. The anti-plasticization mechanism is of considerable interest and so far has not been explained satisfactorily in the literature. It may be assumed that small amounts of plasticizer contribute to the orienta~ o n hardening of the material in the top part of micro-cracks formed at the initial stages of deformation. I t is difficult, however, to explain from this point of view, the coexistence of minima and maxima on curves showing of--DBP concentration for unplasticized PMMA at test temperatures higher than 40 °. In fact, an increase in temperature itself should contribute to orientatrion hardening, i.e. act in the same direction a s the plasticizer. Then, a further amount
Crosslinked polymethacrylate glass
3179
o f plasticizer could not in any way cause the hardening of the sample, if with low D B P contents the limit of forced elasticity already appeared to be reduced. Within the framework of the model used the entire combination of effects has found a logical interpretation of mechanical properties which we examined in previous chapters. Considering concrete levels of structural heterogeneity of polymer glass it is natural to assume t h a t a good plasticizer introduced in small amounts is first of all localized both inside the fibrils, being absorbed by macromolecules in relatively damaged regions between the domains and on inter-fibrillar boundaries. Adsorption of DBP by continuous chains in regions between domains should result in a local reduction of free volume and therefore, an effective reduction of continuous chain flexibility at a given temperature, i.e. hardening of fibrils. Since deformation in a section preceding the limit of forced elasticity is due to the deformation of fibrils, in which conformation mobility of continuous chain segments is observed, this factor will contribute to an increase in the value of ~f. The adsorption of the plasticizer at inter-fibrillar boundaries should be regarded as a factor which can potentially act in the opposite direction, i.e. reduce ~f under test conditions, when the overall strength of contacts is reduced on these boundaries to such an extent that t h e y become locations of forming new interfaces while stressing the sample. During elongation of linear PMY[A samples at low temperatures in the range of low plasticizer contents, the strength of inter-fibrillar boundaries still remains fairly high and deformation beigns with the decomposition of physical bonds on less perfectly packed joints between super-domains. Only the first factor operates under these conditions, i.e. the fibrils become harder. Hence follows the increase in the value of af, i.e. antiplasticization with smM1 additions of DBP. At increased temperature deformation at early stages involves weakened inter-fibrillar boundaries (decomposition of super-domains). Under these conditions both factors are observed, and according to plasticizer concentration either an adsorption reduction of the strength of inter-fibrillar bonds, or hardening of fibrils as a result of the adsorption of the plasticizer b y continuous chains predominate, i.e. the density of regions between domains increases. Two extrema follow from here (minimum and maximum) on curves showing af and D B P concentration. In crosslinked glass inter-fibrillar boundaries are further reinforced by intermolecular chemical bonds. Therefore, the adsorption reduction of strength degenerates; the minimum on the temperature dependence of af on D B P concentration also degenerates in the range of low plasticizer contents. Anti-plasticization with small additions of D B P is only observed in these system. I t is natural that DBP, being a good solvent for PMMA, introduced into the samples in considerable quantities, disrupts the bonds between macromolecules inside the domains, i.e. results in a gradual "dispersioll" of domains, maeroscopicMly degenerating during swelling of the sample and resulting in an increase in flexibility according to classical rules of plasticization.
3180
Z.S. B~T.OKON'e~ ed.
Conclusion. Structural and mechanical properties of crosslinked polymer systems were interpreted using t he particular example of PMMA glass, in which the three dimensional network of intermoleeular chemical bonds was formed during bulk copolymerization. At the same time the main ideas are apparently, applicable to a v e r y wide circle of crosslinked polymers in the glassy and highelastic states. The applicability of this approach has recently been demonstrated when studying t he behaviour of homogeneous and heterogeneous vulcani_~tes of several rubbers [14]. According to t he m e t h o d of forming chemical networks, distribution according to supermolecular structural elements and therefore, effect on macroscopic properties of samples m a y be different with the same average crosslink density. I t is significant, however, t h a t intermolecular chemical bonds do n o t pr o b ab ly cause major changes in general macromo!ecular packing, which is also typical of amorphous polymers formed of linear chains. TranslaSed by E. SEMERR REFERENCES
1. T. ALFREY, Mekhanichesldye svoistva vysokopolimerov (Mechanical Properties of High Polymers). Izcl. inostr, lit., 1952; G. FERRY, Yyazkouprugiye svoistva polimerov (Visco-Elstic Properties of Polymers). Izd. inostr, lit., 1963 2. V. A. KARGIN and G. L. SLONIMSKH, Kratldye ocherld po fiziko-khimii polimerov (Brief Outlines of the Physieo-Chemistry of Polymers). Izd. "Khimiya", 1967 3. V. A. KARGIN, T. I. SOGOLOVA and B. I. AIKHODZHAYEV, Izv. AN UzbSSR, set. khlm., 49; 1957, B. I. ArIKIIODZHAYEV, T. I. SOGOLOVA and V. A. KARGIN, Zh. fiz. khlmli 51: 2552, 1957; V. A. KARGIN, T. I. SOGOLOVAand B. I. AIKHODZHAYEV, ])old. AN SSSR 120: 1277, 1958 4. V. A. KARGIN, I. V. PIS'MENKO and Ye. P. CHERNEVA, Vysokomol. soyed. AI0: 846, 1968 (Translated in Polymer Sci. U.S.S.R. 1O: 4, 981, 1968) 5. Z. S. BELOKON', A. Ye. SKOROBOGAT0VA, N. Ya. GRIBKOVA, S. A. ARZHAKOV, N. F. BAKEYEV, P. V. KOZLOV and V. A. KABANOV, Dold. AN SSR 214: 1069, 1974~ 6. R. B. BEEVERS and E. F. T. WHITE, Trans. Faraday Soc. 56: 744, 1960 7. E. THOMPSON, J. Polymer Sci. 4, A-2; 199, 1966 8. S. A. ARZHAKOV, N. F. BAIKEYEV and V. A. KABANOV, Vysokomol. soyed. A15.* 1154, 1973 (Translated in Polymer Sei. U.S.S.R. 15: 5, 1296, 1973) 9. S. A. ARZHAKOV and V. A. KABANOV, Vysokomo]. soyed. B13: 318, 1971 (Not, translated in Polymer Sei. U.S.S.R.) 10. A. Ye. SKOROBOGATOVA, S. A. ARZHAKOV, N. F. BA]KEYEVand V. A. KABANOV, ]:)old. AN SSSR 211: 151, 1972 11. G. S. YEH, J. Maeromolee. Sei. B6: 465, 1972 12. J. J. KLEMENT and P. H. GEHJ, ft. Maeromolee. Sei. BS: 505, 1971 13. D. N. BORT, V. D. ROMANOV and S. A. ARZHAKOV, Vysokomol. soyed. B16: 323~ 1974 (Not translated in Polymer Sci. U.S.S.R.) 14. A. A. DONTSOV, Dissertation, 1974