Mesomorphic Order in Polymers - ACS Publications - American

8 Polymerization in the Liquid Crystalline State: Monomer-Polymer Interactions F. CSER, K. NYITRAI, and G. HARDY

Downloaded by MONASH UNIV on November 26, 2014 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch008

Research Institute for Plastics, H-1950 Budapest, Hungary

Sadron e t al. (1) prepared l y o t r o p i c liquid crystalline systems u s i n g polymers and a polymerizable s o l v e n t and attempted t o fix t h e mesomorphic s t r u c t u r e by p o l y m e r i z i n g t h e monomeric s o l v e n t . Bouligand e t al. (2) attempted t o prepare liquid crystalline substances w i t h f i x e d s t r u c t u r e by copolymerizing mono- and bifunctional mesomorphic monomers. However, n e i t h e r group i n v e s t i g a t e d the phase c o n d i t i o n s o f the monomers o r o f the monomer and polymer. I n both cases identical homogeneous phases were assumed before and after p o l y m e r i z a t i o n . I n s o l i d - s t a t e p o l y m e r i z a t i o n , however, the polymer formed in the r e a c t i o n i n t e r a c t e d w i t h the initial monomer phase, forming a new thermodynamic system (3,4,5,6,7). The r e a c t i o n s were e i t h e r heterogeneous o r homogeneous phase topochemical r e a c t i o n s . The polymer remained isomorphous w i t h its monomer forming a one-phase system in homogeneous r e a c t i o n s only ( 3 , 6 ) . This type o f r e a c t i o n is, however, very r a r e (7,8,9,10). Heterogeneous r e a c t i o n s , in which the polymer and the monomer a r e not isomorphous, a r e much more frequent. I n many cases, polymerizat i o n s s t a r t i n g as homogeneous change i n t o a r e a c t i o n which is predominantly heterogeneous. The determining c o n d i t i o n f o r a homogeneous r e a c t i o n is the isomorphism o f the tactic polymer w i t h the monomer crystals. This isomorphism can o n l y be realized under s p e c i a l c o n d i t i o n s r e q u i r e d by the thermodynamics of the systern. These c o n d i t i o n s a r e as f o l l o w s : the chain p e r i o d o f the polymer must c o i n c i d e w i t h a translational period of the monomer crystal lattice, the o v e r l a p p i n g volumes o f monomeric u n i t s in the polymer chain should not differ g r e a t l y from t h a t o f t h e monomer molecule (11), and the volume c o n t r a c t i o n d u r i n g the c h a i n formation should not beint h e direction of the chain growth. These c o n d i t i o n s a r e fulfilled in c r y s t a l s o f monomers w i t h a long paraffinic chain s u b s t i t u e n t (12). F i g u r e 1 shows an example o f the isomorphous solid s o l u t i o n of p o l y ( c e t y l v i n y l e t h e r ) in its s i n g l e l a y e r o f pgg l a y e r symmetry. The isomorphism o f monomer c r y s t a l s and o f the

0-8412-0419-5/78/47-074-095$05.00/0 © 1978 American Chemical Society

In Mesomorphic Order in Polymers; Blumstein, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


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polymer formed has been reported by s e v e r a l authors (8,2,10,13). The topochemical aspect of the r e a c t i o n v a r i e s depending on the type o f mesomorphic s t a t e . Topochemical p o l y m e r i z a t i o n can be expected i n smectic mesophases (l2) where the f u n c t i o n a l groups o f the l a y e r formed by the p a r a l l e l chains are l o c a t e d i n one plane (lA,15). Homogeneous topochemical r e a c t i o n s may proceed i n a l a y e r w i t h e i t h e r u n i - or b i m o l e c u l a r s t r u c t u r e . I n the p o l y m e r i z a t i o n of c e t y l v i n y l e t h e r (l6) the polymer formed remained isomorphous w i t h the monomer, and the m e l t i n g p o i n t o f the system i n c r e a s e d . This isomorphism was destroyed, however, when the system reached i t s m e l t i n g p o i n t . A f t e r c o o l i n g i t formed a two-phase system. I n the p o l y m e r i z a t i o n o f c h o i e s t e r y l a c r y l a t e the i s o morphism o f the polymer w i t h the monomer could be preserved up t o the conversion l i m i t o f TOfo a t 0° C (lî), b u t a t 30° C the system became r a p i d l y heterogeneous because o f the increased m o b i l i t y of the molecules. A t both temperatures the monomer was i n the smectic G s t a t e . The experimental evidence i s shown i n F i g u r e 2. The decrease i n the m e l t i n g and/or c l e a r i n g p o i n t s o f the monomers caused by the presence o f the polymer was detected i n both the smectic s t a t e p o l y m e r i z a t i o n of v i n y l o l e a t e (l8,19) and i n the p o l y m e r i z a t i o n o f c h o i e s t e r y l a c r y l a t e i n the c h o i e s t e r i c s t a t e (lî)· The p o l y v i n y l o l e a t e formed i n t h i s r e a c t i o n was c r y s t a l l i n e w h i l e the p o l y c h o l e s t e r y l a c r y l a t e was amorphous. The temperature o f the phase t r a n s i t i o n was reduced i n both cases. The s i t u a t i o n i s completely d i f f e r e n t i n the c h o l e s t e r i c and the nematic s t a t e s . -The r a t e s o f p o l y m e r i z a t i o n s a r e u s u a l l y lower, and the a c t i v a t i o n energies o f r e a c t i o n s a r e higher than those i n the l i q u i d i s o t r o p i c s t a t e measured o r e x t r a p o l a t e d t o the same temperature (20,21,22). The c h o l e s t e r i c and nematic s t a t e s do not seem t o favor topochemic a l processes, and, t h e r e f o r e , a s t r u c t u r a l isomorphism cannot be expected e i t h e r . Because i n t h i s s t a t e the molecules are not packed densely, the f o r m a t i o n o f a homogeneous s o l u t i o n a l s o can be expected w i t h g r e a t e r d i f f e r e n c e s i n o v e r l a p p i n g volumes. The polymer/monomer s t a t e diagram o f ρ-methyl, JD - a c ryloyloxyazoxybenzene (23) i n F i g u r e 3 shows t h a t near the m e l t i n g p o i n t of the monomer a heterogeneous two-phase system i s formed i n which both phases have a nematic s t r u c t u r e . Above the m e l t i n g p o i n t o f the monomer the s o l u t i o n i s a homogeneous nematic one; the polymer i s d i s s o l v e d i n the i s o t r o p i c l i q u i d phase monomer. Therefore, i n the nematic phase of the monomer a heterogeneous phase p o l y m e r i z a t i o n can be expected. When the r e a c t i o n i s s t a r t e d i n the i s o t r o p i c l i q u i d phase, i t i s homo geneous up t o a g i v e n conversion, where a phase s e p a r a t i o n takes p l a c e . T h e r e a f t e r , as the r e a c t i o n proceeds, the nematic polymer/monomer s o l u t i o n d i s s o l v e s the remaining i s o t r o p i c 1


E T A L .

Polymerization in the Liquid Crystalline State


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Figure 2. X-ray powder pattern of chotesterylacryhte monomer (a), of its precipitated polymer (d), of polym erizing systerns polymerized at 0°C up to 7Οψο conversion (b), and at 30°C up to 28% conversion (c).



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monomer, and the r e a c t i o n becomes homogeneous. The Tg o f the p l a s t i c i z e d polymer system i s reached, and the r e a c t i o n terminates. The polymer/monomer s t a t e diagram of c h o l e s t e r y l v i n y l s u c c i n a t e shows another type of i n t e r a c t i o n (2^4-, 25 ) as can be seen on F i g u r e k. I n one of the three p o s s i b l e c r y s t a l l i n e phases a homogeneous t o p o t a c t i c s o l i d s t a t e p o l y m e r i z a t i o n takes p l a c e . The polymorph i s formed i n the presence o f even a t r a c e of polymer, and there i s a r e a l isomorphism o f the polymer and o f the monomer. The h i g h e s t polymer content i n t h i s s o l i d s o l u t i o n i s c a . 25$. A t h i g h e r tempera t u r e s the isomorphous polymer/monomer system has a smectic s t r u c t u r e . T h e r e a f t e r , i n the c h o l e s t e r i c s t a t e the system becomes heterogeneous. I n samples w i t h a h i g h polymer content two smectic s t a t e s were detected. A low v i s c o s i t y smectic s t r u c t u r e could be detected a t higher temperatures. The p o l y m e r i z a t i o n i n the c h o l e s t e r i c s t a t e o f t h i s monomer i s a heterogeneous phase r e a c t i o n . The polymer formed i s pre c i p i t a t e d as a smectic phase, and as the conversion i n c r e a s e s , the c h o l e s t e r i c monomer i s d i s s o l v e d i n the i s o t r o p i c system, and a homogeneous phase i s formed. As the r e a c t i o n proceeds f u r t h e r , the system becomes smectic (25). For a l o n g time we searched without success f o r a homo geneous r e a c t i o n i n the c h o l e s t e r i c s t a t e u n t i l we began t o study the c h o l e s t e r y l v i n y l f u m a r a t e monomer. I n the f o l l o w i n g we present the polymer/monomer s t a t e diagram of t h i s substance. F i g u r e 5 shows the phase t r a n s i t i o n p o i n t s found i n t h i s system w i t h a 10$ polymer content. F i g u r e 6 d i s p l a y s the DSC t r a n s i t i o n heats as a f u n c t i o n o f the composition. The next two f i g u r e s show the polymer/monomer s t a t e diagrams determined by DSC ( F i g u r e 7), by p o l a r i z i n g microscopy, and by thermomechanical methods ( F i g u r e 8). The f i g u r e s show a good agree ment between d i f f e r e n t methods. F i g u r e 9 d i s p l a y s wide-angle x-ray d i f f r a c t o g r a m s o f some c h a r a c t e r i s t i c compositions o f the polymer/monomer systems. The monomer i s i n the smectic G s t a t e (26). Systems w i t h polymer content o f l e s s than kO°jo c o n s i s t o f two phases - - a n amorphous phase o f the p l a s t i c i z e d monomer and a smectic Β phase o f the monomer c o n t a i n i n g the polymer. When the polymer content i s g r e a t e r than kO°jo, o n l y the amorphous phase i s detected. The upper p a r t o f F i g u r e 9 shows the x-ray d i f f r a c t o g r a m s o f systems p o l y m e r i z i n g a t 1^5 C i n the presence of 0.2$ benzoylperoxide as a f u n c t i o n o f r e a c t i o n time. I n these p o l y m e r i z i n g systems s i m i l a r i n t e r a c t i o n s occur as i n the melted and cooled systems. Samples w i t h polymer contents l e s s than 50$ d i s p l a y the green-blue r e f l e c t i o n c h a r a c t e r i s t i c o f the c h o l e s t e r i c s t a t e . The c h a r a c t e r i s t i c s o f the s t a t e s are as f o l l o w s : When the polymer content i s h i g h e r than 70$, the systems c o n s i s t o f a homogeneous phase which contains the p l a s t i c i z e d 0


Polymerization in the Liquid Crystalline State

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Figure 3. Polymer/monomer state diagram of p-methyl, p'acryloyloxyazoxybenzene ob tained by polarizing microscopy M composition (weight fraction of polymer). Characteristic points: (Αλ decreasing birefrin gence; (yf), mesomorphic transi tion; mesomorphic melting; (Φ), solidus; (O), liquidas. The meaning of the areas: (A), iso tropic liquid; (B), isotropic liquid + mesomorphic plasticized poly mer; (C), liquid; (D), glassy states of plasticized polymer in mesomorphic phase; (E), crystal line monomer -f- mesomorphic plasticized polymer in the glassy state; (F), crystalline monomer -f mesomorphic plasticized poly mer in the liquid state; (G), mesomorphic monomer -f- meso morphic plasticized polymer in the liquid state.

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Figure 4. Polymer/monomer state diagram of cholesterylvinyhuccinate. M , composition (weight fraction of monomer). ([J), T ; (O), change in the thermal exponent of deformation; (S7), T/, all measured by thermomechanics. (V), shift in the base line, if M < 0.5 or second peak if M > 0.5; (Φ), first peak on the DTA trace. (-{-), clearing point; (V), melting begins; (X), phase transformation, all measured by polarizing microscopy. The mean ing of the areas: (A), isotropic liquid; (B, C, D, and E), homo geneous mesomorphic plasticized polymer in liquid (B), in high elas tic (C and D), and in glassy state (E), respectively. Solid solutions are in solid (G) and in plasticized states (11). Mesomorphic solutions are in smectic (I) and in cholesteric (J) states. G, II, I and J are biphasic in the concentration range of0.5

Mesomorphic Order in Polymers - ACS Publications - American

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