Chapter 16 Synthesis, Structure, and Properties of Functionalized Liquid-Crystalline Polymers 1
1
2
Rudolf Zentel , Heinrich Kapitza , Friedrich Kremer , and Sven Uwe Vallerien 2
1
Institut für Organische Chemie, Universität Mainz, J.-J.-Becher-Weg 18-20, D-5600 Mainz, Federal Republic of Germany Max-Plank-Institut für Polymerforschung, Postfach 3148, D-6500 Mainz, Federal Republic of Germany
2
C h i r a l lc-polymers can be prepared by a proper function a l i z a t i o n of lc-polymers with c h i r a l and reactive groups. These elastomers are i n t e r e s t i n g , because they combine the mechanical o r i e n t a b i l i t y of a c h i r a l lc -elastomers with the properties of c h i r a l lc-phases, e . g . the f e r r o e l e c t r i c properties of the c h i r a l smectic C* phase. The synthesis of these elastomers was very complicated so f a r , but the use of lc-polymers, which are functionalized with hydroxyl-groups, has opened an easy access to these systems. Also photocrosslinkable c h i r a l lc-polymers can be prepared v i a t h i s route. During the l a s t years our i n t e r e s t was focused on structure-property r e l a t i o n s i n various types of functionalized l i q u i d c r y s t a l l i n e (lc) polymers. At the beginning these were lc-polymers (main chain, side group and combined polymers) functionalized with reactive groups, which allow a network formation v i a a c r o s s l i n k i n g r e a c t i o n . The schematic representation of the l c elastomers prepared i n t h i s way i s given i n Figure 1. For a discussion of the synthesis and properties of these materials see Refs. (1-2). The most obvious property of the l c elastomers i s t h e i r good mechanical o r i e n t a b i l i t y that means s t r a i n s as small as 20% are enough to obtain a perfect o r i e n t a t i o n of a c h i r a l l i q u i d c r y s t a l l i n e phases. The mechanical o r i e n t a b i l i t y made i t i n t e r e s t i n g to look for a combination of t h i s elastomer property with the properties of c h i r a l l i q u i d c r y s t a l l i n e phases (cholesteric and c h i r a l smetic C*, see Figure 2), which r e s u l t from a proper f u n c t i o n a l i z a t i o n of low molar mass l i q u i d c r y s t a l s with c h i r a l groups. These phases show very s p e c i a l properties, which are: s e l e c t i v e r e f l e c t i o n of l i g h t (chole s t e r i c phase) and f e r r o e l e c t r i c properties ( c h i r a l smectic C* phase). The goal of t h i s work was not to prepare densely crosslinked thermo sets, i n which the structure of these phases i s locked i n unchanged up to the decomposition temperatures (3-4), but to prepare s l i g h t l y 0097-6156/90/0435-0207$06.00/0 © 1990 American Chemical Society
LIQUID-CRYSTALLINE POLYMERS
208
Crosslinked Side-Group Polymers
Crosslinked Main-Chain Polymers
Crosslinked Combined Main-Chain/ Side-Group Polymers
Figure 1. Schematic representation of different types of l c elastomers.
16. ZENTELETAL.
Functionalized Liquid-Crystalline Polymers
Cholesteric Phase
209
Chiral Smectic C* Phase
Figure 2. Schematic representation of the c h o l e s t e r i c and c h i r a l smectic C* phase. The repeating distance along the h e l i c a l axis (pitch) i s between 200 nm to some um.
210
LIQUID-CRYSTALLINE POLYMERS
crosslinked elastomers and to study the influence of mechanical stretching on the h e l i c a l superstructure of the c h o l e s t e r i c and the c h i r a l smectic C* phase (see Figure 2 ) . In t h i s case a change of the s e l e c t i v e r e f l e c t i o n (cholesteric) or a p i e z o - e l e c t r i c response ( c h i r a l smectic C* ) should r e s u l t from a mechanically induced change of the h e l i c a l superstructure (see Ref. (5) for a d e t a i l e d discussion of t h i s t o p i c ) . Combined l c polymers (6) were selected as s t a r t i n g materials for t h i s approach, because they form broad l c phases and they often show l i q u i d c r y s t a l l i n e polymorphism. Hence the chance to observe the desired c h i r a l phases i n the newly prepared polymers was great. In addition half of the mesogens i n these polymers are incorporated i n t o the polymer chain, while the other half (the mesogenic side groups) orients p a r a l l e l to the polymer chains (7). Therefore a strong influence of the stretching of the sample, which leads p r i m a r i l y to an o r i e n t a t i o n of the polymer chains, on the o r i e n t a t i o n of the mesogenic groups can be expected. The f i r s t c h i r a l combined l c polymers prepared for t h i s purpose showed the desired c h o l e s t e r i c and c h i r a l smectic C* phases only at high temperatures (8) (the melting point was always above 100°C). By using l a t e r a l substituents (see Figure 3) i t i s possible however to suppress the melting temperature and to obtain polymers with a glass t r a n s i t i o n temperature of about room temperature, without l o s i n g the c h o l e s t e r i c and c h i r a l smectic C* phases (9). The synthesis of c h i r a l elastomers of the schematic structure shown i n Figure 4 can be accomplished by c r o s s l i n k i n g c h i r a l copolymers (5) according to Scheme I (for one c h i r a l elastomer based on l c side group polymers see Ref. (10)). The phase behavior of some of these l c elastomers i s summarized i n Table I . Cholesteric and c h i r a l smectic C* phases are found depending on the temperature. The assignment of these phases was p r i m a r i l y done on the basis of X-ray measurements and p o l a r i z i n g microscopy (5). The f e r r o e l e c t r i c pro perties of some of these polymers i n the c h i r a l smectic C* phase were l a t e r confirmed by d i e l e c t r i c spectroscopy (frequency range: 10" 10 Hz) (11) (see Figure 5 ) . The observation of both the Goldstone and the Soft mode i s a d i r e c t proof of the f e r r o e l e c t r i c properties. F i r s t X-ray measurements show that the h e l i c a l superstructure of the c h o l e s t e r i c and c h i r a l smectic C* phase can be untwisted by stretching the elastomer (5). High s t r a i n s of 300% are necessary for t h i s purpose (compared to 20% for the a c h i r a l elastomers). Nevertheless these r e s u l t s show that the c h i r a l l c elastomers have the p o t e n t i a l to act as mechano-optical couplers (cholesteric phase) or as piezo-elements ( c h i r a l smectic C* phase) (5), because the mechanically induced change of the h e l i c a l superstructure has to change the o p t i c a l transmission or r e f l e c t i o n properties or the spontaneous p o l a r i z a t i o n . Both effects however have not yet been measured d i r e c t l y . The uncrosslinked and the crosslinked polymers described i n Table I s t i l l have some drawbacks. To begin with, the synthesis of polymers with strong l a t e r a l dipole moments (see polymer 3a, b i n Table I (5)) i s rather complicated, because the c h i r a l groups have to be introduced p r i o r to the polycondensation reaction (9), which they must survive unchanged. This l i m i t s the number of useful c h i r a l groups and excludes e.g. c h i r a l esters, which are w e l l known from low molar mass l i q u i d c r y s t a l s (12). In addition the c r o s s l i n k i n g has to 1
9
16. ZENTELETAL. B
Functionalized Liquid-Crystalline Polymers 1
—[-0-(CH ) -0-^^-X*-^^-0-(CH ) -OOC-CH-CO-] — 2
6
2
6
x
(CH ) 2
A,B,B X,X
R:
%
6
: Br , H
: — , - N= N - , "N^N-
-CH -CH-CH -CH CH. 2
2
-CH -CH-CH CI 2
=R
3
= R
3
1
2
-CH -CH-CH-CH -CH CI CH - CH- (CH ) -CH =R CH 2
2
3
3
2
5
3
4
3
Figure 3. Combined l c polymers with l a t e r a l substituents
[9],
Figure 4. Schematic representation of a network prepared from c h i r a l combined polymers [5].
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LIQUID-CRYSTALLINE POLYMERS
Table I:
No.
la lb
Phase t r a n s i t i o n s of uncrosslinked (a) and crosslinked (b) c h i r a l combined polymers (5)(see Scheme I)
Y
X2
R
molecular Crossweight(GPC) l i n k e r (mol%)
— 10
-N=N-N=N-
-N=N-N=N-
Ri Ri
60 000
2a 2b
-N(0)=N- -N=N-N(0)=N- -N=N-
-N=N-N=N-
Ri Ri
47 000
—
3a 3b
-N(0)=N-N(0)=N-
— —
R R
17 000
—
a )
-N=N-N=N-
Xi
— —
2
2
— — —
10 20
Phase t r a n s i t i o n s / °C > a
cl09sc*124n*149i c 99sc*114n*141i g24sc*115n*149i g25sc*110n*147i g20sc*124n*130i gl9sc*110n*123i
c : c r y s t a l l i n e or highly ordered smectic phase, g:glassy frozen phase, sc*: c h i r a l smectic C*, n*: cholesteric phase, i : i s o t r o p i c melt
F i g u r e 5. D i e l e c t r i c l o s s e" v s temperature and v s l o g a r i t h m 10 o f f r e q u e n c y f o r a t h i n (10 gm) and a l i g n e d sample o f polymer 3a (see Tab. 1 ) .
16. ZENTELETAL.
Functionalized Liquid-Crystalline Polymers
213
-f-fO-A—00C—CH-CO^O—A-OOC-CH-COJQ^ S
S
1
2
1a-3a
Rv —CH —CH—C H 2
2
CH CHo
3
5
R : —CH —CH—CH—C H 2
2
2
CI
CH
5
3
CHo
i
i
msi-o^si-H CH
3
CH3
•
Crosslinked chiral elastomers
1b-3b
Scheme I be done i n i s o t r o p i c s o l u t i o n , because the l c polymer and the siloxane (see Scheme I) are not m i s c i b l e . It would, however, be most i n t e r e s t i n g to c r o s s l i n k i n substance i n the d i f f e r e n t liquid c r y s t a l l i n e phases. This would make i t possible to c r o s s l i n k oriented l c samples and to investigate the influence of d i f f e r e n t l i q u i d c r y s t a l l i n e phases on the c r o s s l i n k i n g reaction and on the properties of the r e s u l t i n g network. A photo-crosslinking of groups covalently bound to the polymer chain would make t h i s p o s s i b l e . In order to achieve t h i s goal a synthetic route to f u n c t i o n a l i z e d l c polymers was developed (13) (see Scheme II and Table I I ) . Due to the much higher r e a c t i v i t y of hydroxyl groups compared to phenolic groups, i t i s possible to prepare l i n e a r polymers of the structure presented i n Scheme I I . S t a r t i n g from these polymers i t i s possible to introduce the c h i r a l acids known from low molar mass l i q u i d c r y s t a l s (12) and to obtain the c h i r a l homopolymers presented i n Scheme III and Table I I I . These polymers show a high spontaneous p o l a r i z a t i o n i n the c h i r a l smectic C* phase (14) (see polymer 7, Table III) and s e l e c t i v e r e f l e c t i o n of v i s i b l e l i g h t i n the c h o l e s t e r i c phase (see polymer 9, Table III) (13).
214
LIQUID-CRYSTALLINE POLYMERS
COO—C H 2
H — O - ^ " ^ — Y — ^
5
^ — O H C H ) Q C H 2
C O O - C
2
H
5
4-6 Scheme II
Table I I :
Phase t r a n s i t i o n s of the f u n c t i o n a l i z e d polymers 4-6 (13) (see Scheme II)
No
X
Y
4 5 6
-N(0)=N-
-N(0)=N-
Phase t r a n s i t i o n s > i n °C a
c 141 i c 37 i g 24 l c 139 i
«>see footnote to Tab. l c: c r y s t a l l i n e , l c : l i q u i d c r y s t a l l i n e phase, not further s p e c i f i e d f
16. ZENTEL ET AL.
215
Functionalized Liquid-Crystalline Polymers
-{0-(CH2)gO
O-fCHa^OC-CH-CO^
(CH ). 2
R—COOH
£T7
N=C=N
-[0-(CH >gO
DCCI
0-(CH >gOOC—CH—CC%
2
2
(CH ) 2
R*-CO-0—0
v—Y—v
6
V—O
7-9
Scheme III Table III:
Phasetransitions of the chiral homopolymers 7-9 ( 1 3 U ) (see Scheme M|) Z
Y
No 7
~
8
-
C H -*CH-*CH2
5
i
CH
c 114 s
I
3
-N(0)=N-
5
CH
3
a
in ° C
121 s * 133 i
x
c
CI
C H - *CH*CHI i 2
Phasetransitions
s
y
84 s
A
112 i
CI
0 N -CHO-^2
9
CjH
1 3
g 18 s 1 0 3 n*125 i
-N(0)=N-
A
CH, a)
see footnote to Tab. 1; s or s : highly ordered smectic phase, not further identified, s : smectic A x
A
y
Figure 6 . Photochemical c r o s s l i n k i n g of an l c copolymer functionalized with acrylate groups.
The Ic-phases are retained during crosslinking
2
20% CH =CH—CO'
0\
16. ZENTEL ETAL.
Functionalized Liquid-Crystalline Polymers
217
In order to prepare photochemically crosslinkable l c polymers, copolymers were prepared, i n which 70-80% of the mesogenic side groups were functionalized with c h i r a l groups, while the remaining 20-30% were modified with acrylate groups (13) (see Figure 6 ) . In order to prevent a thermally induced c r o s s l i n k i n g , these polymers have to be s t a b i l i z e d with 1-2% of bis (3 t e r t . butyl-4-hydroxy-5methylphenyl) s u l f i d e . They can be photochemically crosslinked by adding a Kodak i n i t i a t o r system (15) and by i l l u m i n a t i n g the sample i n the l i q u i d c r y s t a l l i n e phase (13) with a mercury lamp (see Figure 6). Further investigations of these photochemically crosslinked elastomers are i n progress. Literature Cited 1. Gleim, W.; Finkelmann, H.; In Side Chain Liquid Crystal Polymers, McArdle, C.B., Ed.; Blackie and Son, Glasgow, 1989, p.287 2. Zentel, R.; Adv. Mater. 1989, 321; Angew. Chem. Int. Ed. Engl., Adv. Mater. 1989, 28, 1407 3. Strzelecki,L.; Liebert, L.; B u l l . Soc. Chim. Fr. 1973, 597 4. Bhadani, S.N.; Gray, D.G.; Mol. Cryst. L i q . Cryst. (Lett.) 1984, 120, 255 5. Zentel, R.; L i q . Cryst. 1988, 3, 531; Zentel, R.; Reckert, G.; Bualek, S.; Kapitza, H.; Makromol. Chem. 1989, 190, 2869 6. Beck, B.; Ringsdorf, H.; Makromol. Chem. Rapid Commun. 1985, 6, 291 7. Zentel, R.; Schmidt, G.F.; Meyer, J . ; Benalia, M.; L i q . Cryst. 1987, 2, 651 8. Zentel, R.; Reckert, G.; Reck, B.; L i q . Cryst. 1987, 2, 83 9. Kapitza, H.; Zentel, R.; Makromol. Chem. 1988, 189, 1793 10.Finkelmann, H.; Kock, H.-J.; Rehage, G.;Makromol. Chem. Rapid Commun. 1981, 2, 317 11.Vallerien, S.U.; Zentel, R.; Kremer, F.; Kapitza, H.; Fischer, E.W.; Makromol. Chem. Rapid Commun. 1989, 10, 333 12.Terashima, K.; Ichihashi, M.; Kikuchi, M.; Furukawa, K.; Inukai, T.; Mol. Cryst. L i q . Cryst. 1986, 141, 237 (1986); Bahr, C.; Heppke, G.; Mol. Cryst. L i q . Cryst. Lett. 1986, 4, 31 13.Kapitza, H.; Ph.D. Thesis, Universität Mainz, FR-Germany, 1990 14.Vallerien, S.U.; Zentel, R.; Kremer, F.; Kapitza, H.; Fischer, E.W.; Proceedings of the Second Ferroelectric Liquid Crystal Conference at Göteborg, Sweden (June 1989), Ferroelectrics i n press; Vallerien, S.U.; Kremer, F.; Kapitza, H.; Zentel, R.; Fischer, E.W.; Proceedings of the Seventh International Meeting on F e r r o e l e c t r i c i t y (IMF-7) at Saarbrücken, Germany (August 1989), Ferroelectrics i n press. 15.Williams, J.R.; Specht, D.P.; Farid, S.;Polym. Eng. and S c i . 1983, 23, 1022 RECEIVED April 10, 1990
