Mesomorphic Order in Polymers - American Chemical Society

7 A New Series of Mesomorphic Monomers: Bulk

Downloaded by UNIV ILLINOIS URBANA-CHAMPAIGN on October 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch007

Polymerization of 4-Cyano-4'-biphenyl (n + 2)-alkenoates J. C. DUBOIS, J. C. LAVENU, and A. ZANN Thomson-CSF, Domaine de Corbeville, 91401 Orsay, France

Polymers which exhibit mesomorphic order may have interesting anisotropic properties such as high o p t i c a l , e l e c t r i c a l , or mechanical anisotropies. Such polymers may be obtained either by polymerization of a non-mesomorphic monomer i n a l i q u i d c r y s t a l l i n e solvent (1,2) or by direct polymerization of a mesomorphic monomer i n its l i q u i d c r y s t a l l i n e state ( 3 - 9 ) . The organization i n the resulting polymers may be improved if the polymerization is done under orienting conditions i n e l e c t r i c or magnetic f i e l d s or within suitable boundary conditions (10,11,12,13). In this chapter a new series of mesomorphic monomers i s presented -- the 4-cyano-4'-biphenyl (n+2)-alkenoates. Their bulk polymerization i n the nematic state has been investigated, and t h i n layers obtained by thermal polymerization with or with out an orienting e l e c t r i c field have been studied. D i e l e c t r i c properties are discussed also. Monomers The monomers which have been synthesized have the general formula:

with n equal to 0, 1, or 2. Synthesis. The preparation of these compounds involved three steps. The first step was the synthesis of 4-cyano-4'hydroxybiphenyl (14):

0-8412-0419-5/78/47-074-086$05.00/0 © 1978 American Chemical Society

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

7. D U B O I S E T A L .

New Series of Mesomorphic Monomers

87

The complex was broken down by u s i n g concentrated h y d r o c h l o r i c a c i d and f e r r i c c h l o r i d e i n aqueous s o l u t i o n a t 60°C. The second step was the p r e p a r a t i o n of the a c i d c h l o r i d e from the s e l e c t e d v i n y l i c a c i d (15): C H = C H - ( C H ) - C 0 0 H +/ 0 YcOCl * CH =CH-(CH ) -C0Cl ~" \—/ heating Downloaded by UNIV ILLINOIS URBANA-CHAMPAIGN on October 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch007

2

2

2

n

2

n

The a c i d c h l o r i d e was s imultane ou s l y d i s t i l l e d and c o l l e c t e d on c o o l hydroquinone. The l a s t step, e s t e r i f i c a t i o n , was conducted a t room temperature i n Ν,Ν -dimethylaniline medium (l6): 1

C H = C H - ( C H ) - C 0 C l + H0-( 2

2

kS h r

0 )-( 0 ) - C N

n

CH =CH( 2

CH

2

) coo n

-^T^X^^)-

CN

The r e s u l t i n g compound was p u r i f i e d by chromatography on a s i l i c a g e l column w i t h a benzene/hexane mixture as e l u a n t and then was r e c r y s t a l l i z e d two or three times i n e t h a n o l . Mesomorphic P r o p e r t i e s . The s y n t h e s i z e d compounds have been i n v e s t i g a t e d by means of a p o l a r i z i n g microscope equipped w i t h a h e a t i n g and c o o l i n g stage ( M e t t l e r FP5). The temperature and e n t h a l p i e s of t r a n s i t i o n were determined by d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC 1 B-Perkin Elmer). The r e s u l t s are l i s t e d i n Table I where Κ, Ν, I a r e , r e s p e c t i v e l y , the c r y s t a l l i n e , nematic, and i s o t r o p i c phases, and AE^ i s the m e l t i n g enthalpy. The meso morphic p r o p e r t i e s d i f f e r g r e a t l y a c c o r d i n g t o the l e n g t h of the c h a i n . For comparison, the compounds o f the homologous s a t u r a t e d s e r i e s (17) are mesomorphic a t lower temperatures. The presence of the v i n y l l i n k a g e i n the unsaturated compounds s t a b i l i z e s the mesophase. Table I .

T r a n s i t i o n Temperatures and E n t h a l p i e s f o r the U-Cyano-^ -biphenyl(n+2)-alkenoates 1

η

Κ

0

.

100

.

I30

1

. .

136 73

. .

166 [73]

2

Ν

I

ΔΗ^

(Kcal/mol)

k,6

·

3,7

I n order t o widen the nematic range of the monomers and t o decrease t h e i r m e l t i n g temperatures, we have made b i n a r y mixtures of compounds η = 0 and η = 1 (Table i ) . We observed t h a t t h e experimental diagram d i d not e x h i b i t an e u t e c t i c , as i s g e n e r a l l y observed w i t h other l i q u i d c r y s t a l s . The two compounds are probably m i s c i b l e i n a l l p r o p o r t i o n s i n t h e i r c r y s t a l l i n e s t a t e ( s o l i d s o l u t i o n ) . The phase diagrams, experimental and c a l c u l a t e d , are p l o t t e d on F i g u r e 1.


M E S O M O R P H I C ORDER I N P O L Y M E R S

88

The d i e l e c t r i c a n i s o t r o p y € o f the f i r s t monomer, the i f - c y a n o - V - a c r y l o y l o x y b i p h e n y l , was measured. The £ o f a 0.10/0.90 m o l a r - f r a c t i o n mixture o f t h i s compound, w i t h a nematic compound having an £ equal t o 0.1, i s 2.1. Since the a n i s o t r o p i c s a r e a d d i t i v e , the a n i s o t r o p y of the considered monomer i s approximately equal t o 2.0. fl

a


Bulk P o l y m e r i z a t i o n Bulk p o l y m e r i z a t i o n of each monomer has been s t u d i e d by means o f d i f f e r e n t i a l scanning c a l o r i m e t r y . The procedure f o r f o l l o w i n g t h e k i n e t i c s was s i m i l a r t o t h a t d e s c r i b e d by Blumstein and Hsu ( l 8 ) . A s m a l l q u a n t i t y o f t h e compound was sealed i n a DSC sample pan. The temperature was i n c r e a s e d l i n e a r l y t o the m e l t i n g p o i n t and was then s e t q u i c k l y t o the d e s i r e d v a l u e . The i s o t h e r m a l p o l y m e r i z a t i o n was performed d u r i n g a given time t . A t t h e end o f t h i s time, the sample pan was quenched w i t h a dry ice-methanol mixture t o stop the p o l y m e r i z a t i o n and t o a l l o w the r e c r y s t a l l i z a t i o n t o take p l a c e . A thermogram was then taken again. F o r each p o l y m e r i z a t i o n time, a new sample was used. The r a t e o f conversion i n fo i s equal t o ( l -ΔΗ/ΔΗ ) Χ 100 where H i s the m e l t i n g enthalpy o f the pure monomer, and Δ Η i s the t r a n s i t i o n enthalpy o f the remaining monomer a f t e r p a r t i a l p o l y m e r i z a t i o n . Thermal p o l y m e r i z a t i o n o f the f i r s t monomer o f the s e r i e s i n i t s nematic s t a t e i s very f a s t . The e v o l u t i o n o f i t s m e l t i n g peak i s shown i n F i g u r e 2. This f i g u r e a l s o shows t h a t the presence o f oxygen reduces the r a t e of p o l y m e r i z a t i o n . Since the two other monomers need a d d i t i o n o f i n i t i a t o r t o polymerize, only the k i n e t i c s t u d i e s o f thermal p o l y m e r i z a t i o n o f the i f - a c r y l o y l o x y - V -cyanobiphenyl a r e presented. The r a t e s of conversion i n wt. °jo vs. time f o r three temperatures are p l o t t e d on F i g u r e 3· The curves are p e r f e c t l y l i n e a r up t o 8θ/ο conversion, monomer and polymer are m i s c i b l e , and no phase segregation occurs. From these curves, the l o g a r i t h m o f the p o l y m e r i z a t i o n r a t e vs. ΐ/τ has been p l o t t e d (Figure k) and the energy o f activation c a l c u l a t e d . E i n t h e nematic phase i s equal t o 119 k J/mol. The b u l k polymer obtained a f t e r 80 min a t 1 1 0 ° C i n Np atmosphere i s white, very hard,and b r i t t l e . The t e x t u r e o f a t h i n f i l m o f t h i s polymer looks l i k e a d i s o r d e r e d mesophase t e x t u r e . The polymer i s i n s o l u b l e i n a l l the c l a s s i c a l s o l v e n t s except i n the dimethyIformamide. I t s w e l l s i n chloroform. The thermogram o f t h i s polymer does not e x h i b i t any t r a n s i t i o n from room temperature u n t i l 260 C. 0

0

}

a

1

Thin Layer P o l y m e r i z a t i o n of ^ - A c r y l o y l o x y - ^ - c y a n o b i p h e n y l I n v e s t i g a t i o n o f o p t i c a l and e l e c t r i c a l p r o p e r t i e s o f t h i s polymer may be f a c i l i t a t e d by making t h i n l a y e r s . Thus t h e p o l y m e r i z a t i o n has been done i n a ^Ο-μια t h i c k c e l l . Such a c e l l




DUBOIS E T A L .

1. t-=o 2. aïbtLr to ι** ait 1 Ι ο ( 1 Z. « f t ^ r ^ O H i v i ait H o ° C 6

Figure 2. Polymerization of 4-cyano4 -acryïoyloxyhiphenyl f


MESOMORPHIC

ORDER I N


!2û°d

Figure 3. Conversion of 4-cyano-4'-acryloyloxybiphenyl to polymer as a function of time at various temperatures

E

A

= iw

kJ/Mol.

o.i

o.oi

Figure 4. Arrhenius plot for the polymerization of 4-cyano-4 -acryloyloxyhiphenyl in the nematic phase f


POLYMERS


DUBOIS E T

AL.


Figure 5. Cell for thermal polymerization of a 50-pm film of a liquid crystal monomer orientated by an electric field

Vacuum, o r

\w4st

Mesomorphic Order in Polymers - American Chemical Society

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