Chiral Liquid-Crystalline Copolymers for Electrooptical Applications

Chapter 22

Chiral Liquid-Crystalline Copolymers for Electrooptical Applications 1

1,2

3

3

Downloaded by UNIV MASSACHUSETTS AMHERST on October 13, 2012 | http://pubs.acs.org Publication Date: June 10, 1992 | doi: 10.1021/bk-1992-0493.ch022

E. Chiellini , G. Galli , A. S. Angeloni , M . Laus , and D. Caretti

3

1

Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Risorgimento 35, Pisa 56126, Italy Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853 Dipartimento di Chimica Industriale e dei Materials, Università di Bologna, Viale Risorgimento 4, Bologna 40136, Italy 2

3

The synthesis and liquid crystalline properties of four new series of copolymers consisting of variously substituted azobenzene mesogenic groups are presented. While three series contain chiral, optically active counits, the fourth series contains achiral dipolar chromophore counits. The incidence and stability of different chiral (nematic and smectic) mesophases are discussed with relevance to their potential ferroelectric and nonlinear optical properties. These samples may serve as models of liquid crystalline polymers to be used in electrooptical applications.

L i q u i d c r y s t a l l i n e ( L C ) polymers present a unique c o m b i n a t i o n of the characteristics peculiar to liquid crystals w i t h those typical of polymers. W h i l e the f o r m e r i n c l u d e m o l e c u l a r p o l a r i z a b i l i t y , s e l f - a s s e m b l y tendency, diversity of structures, and fast response to external electric or m a g n e t i c f i e l d s , the l a t t e r can f e a t u r e v a r i e t y of m o l e c u l a r architectures, dimensional stability, mechanical orientability, and ease of p r o c e s s a b i l i t y (1). The i n t r o d u c t i o n of c h i r a l i t y i n the m o l e c u l a r s t r u c t u r e of L C m a t e r i a l s i n d u c e s the f o r m a t i o n of c h i r a l n e m a t i c (cholesteric) or c h i r a l smectic supermolecular assemblies endowed w i t h a macroscopic twist superposed on t h e m (2).This consistently offers a n a d d i t i o n a l v a l u a b l e means of t u n i n g the l i q u i d c r y s t a l b e h a v i o r a n d a d d r e s s i n g specific responses of c h i r a l L C p o l y m e r s i n o p t i c s a n d electrooptics. I n t h i s context, the ferroelectric and n o n l i n e a r o p t i c a l properties of side chain L C polymers are currently the focus of intense research (3). The ferroelectric properties of the c h i r a l smectic C * mesophase are r e c o g n i z e d i n a n u m b e r of t h e r m o t r o p i c p o l y m e r s (4-9). I n t h i s m e s o p h a s e the o p t i c a l l y a c t i v e mesogens w i t h h i g h s p o n t a n e o u s p o l a r i z a t i o n are assembled i n a layered, h e l i c a l superstructure w h i c h must then be untwisted into another superstructure w i t h a resulting 0097-6156/92/0493-0280$06.00/0 © 1992 American Chemical Society

In Macromolecular Assemblies in Polymeric Systems; Stroeve, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


22. CHIELLINI ET A L

Chiral Liquid-Crystalline Copolymers

281

macroscopic electric p o l a r i z a t i o n a l l o w i n g s w i t c h i n g b i s t a b i l i t y . While the switching times measured for ferroelctric L C polymers are i n the range of milliseconds, it may be argued that the associated electroclinic effect (10) should provide faster response rates i n an orthogonal smectic phase, such as the smectic A phase. The second-order nonlinear optical properties of organic polymers are also w e l l documented (9,11-15). For a material to exhibit s i g n i f i c a n t second-order nonlinear optical responses it must have a n o n c e n t r o s y m m e t r i c structure. E l e c t r i c field p o l i n g of p o l y m e r s c o n t a i n i n g a d i p o l a r chromophore is a common method to introduce a p o l a r a x i s i n t o the L C m e d i u m , and the c o n s e q u e n t l y r e d u c e d o r i e n t a t i o n a l a v e r a g i n g of molecular dipoles allows for a net secondorder susceptibility to result. A n a l t e r n a t i v e a p p r o a c h to o r i g i n a t i n g n o n c e n t r o s y m m e t r y m i g h t be to i n t r o d u c e c h i r a l i t y a n d t h e o t h e r s t r u c t u r a l r e q u i r e m e n t s of the f e r r o e l e c t r i c smectic C * phase into a p o l y m e r i c s t r u c t u r e c o n t a i n i n g chromophores capable of p r o d u c i n g a nonlinear response without poling i n an external electric field (9,16). H o w e v e r , t h e d e t a i l s of t h e c r u c i a l i n t e r p l a y o f c h e m i c a l , stereochemical and electronic factors i n effecting the s t r u c t u r e s a n d properties of c h i r a l supermolecular assemblies are s t i l l far from being elucidated (9,17,18). F o l l o w i n g our interest i n c h i r a l L C polymers (18), we have recently developed the synthesis and c h e m i c a l m o d i f i c a t i o n of c h i r a l L C side c h a i n p o l y m e r s based on different mesogenic u n i t s (19,20, A n g e l o n i , A . S . et a l . Chirality, i n press). A m o n g these, the azobenzene mesogenic core is characterized by sufficient chemical and t h e r m a l stability and

Comonomer Β

Comonomer A R C H CH*(CH3)CH20C H CH*(CH3)CH20C H CH*(CH3)CH20NO2 2

5

2

5

2

5

No.

R'

1

71-C6H13O-

1 1 5

Ti-CeHtfOn-Ci H iOn-CioH iO0

2

2

R" Η CH3 Η Η

Copolymer No. 2 3 4 4

Series 1/2 1/3 1/4 4/5



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MACROMOLECULAR ASSEMBLIES IN POLYMERIC SYSTEMS

π - e l e c t r o n c o n j u g a t i o n a n d c a n be f u n c t i o n a l i z e d w i t h v a r i o u s s u b s t i t u e n t groups i n order to t u n e the s t r u c t u r a l a n d p h y s i c a l properties of the resulting L C polymers. Therefore, major objectives of t h i s w o r k w e r e t h e d e s i g n a n d s y n t h e s i s of c h i r a l L C s i d e c h a i n polymers (see general formula above) exhibiting a variety of mesophases according to the distinct tendencies of the azobenzene comonomers to i m p a r t either cholesteric or smectic properties. One further objective w a s to i n c o r p o r a t e d i p o l a r c h r o m o p h o r e c o u n i t s w i t h p o t e n t i a l n o n l i n e a r optical characteristics i n L C polymers. We believe that these p o l y m e r s m a y s e r v e as m o d e l s to a d d r e s s the s t r u c t u r e - p r o p e r t y correlation i n polymeric materials for electrooptical applications. Experimental Synthesis of monomers. The synthesis of monomer 1 is d e s c r i b e d i n detail as a typical example. 4-((S)-2-methylbutoxy)-4'-hydroxyazobenzene: A solution of 12.0 g (0.174 mol) of N a N 0 i n 30 m L of water was added dropwise to a solution of 25.8 g (0.144 mol) of 4-((S)-2-methylbutoxy)aniline (6, X = ( S ) - C 2 H C H * ( C H ) C H 0 ) i n 150 m L of 3 M HC1 at 0-5°C w i t h vigorous s t i r r i n g . A f t e r 1 h , the excess N a N 0 was decomposed by addition of urea a n d the solution was slowly poured into a solution of 13.6 g (0.144 mol) of phenol (7, R " = H) i n 150 m L of 2 M N a O H . After 10 m i n , the solution was acidified w i t h HC1 a n d the precipitated 8 was washed w i t h water, d r i e d a n d finally 2

5

3

2

2

crystallized i n cyclohexane (yield 47%): m.p. 85°C., [ c c ] ^ +10.4° (CHCI3). 6-chlorohexyl methacrylate: A s o l u t i o n of 15.3 g (0.146 m o l ) o f m e t h a c r y l o y l c h l o r i d e i n 50 m L of a n h y d r o u s 1,2-dichloroethane was added dropwise w i t h vigorous s t i r r i n g to a mixture of 20.0 g (0.146 mol) of 6-chlorohexanol, 29.6 g (0.293 mol) of triethylamine, and 0.5 g of 2,6-dii e r i - b u t y l - 4 - m e t h y l p h e n o l at a m b i e n t t e m p e r a t u r e u n d e r n i t r o g e n atmosphere. The reaction m i x t u r e was t h e n r e f l u x e d for 1 h , cooled down, washed w i t h 1 M HC1, water and dried over Na SC>4. The product (9) was finally purified by distillation (yield 75%): b.p. 107°C/2mm. 4'((S)-2-methylbutoxy)-4'-(6-methacryloyloxyte (1): A m i x t u r e of 5.4 g (0.019 mol) of 4 - ( ( S ) - 2 - m e t h y l b u t o x y ) - 4 ' - h y d r o x y azobenzene, 3.9 g (0.019 mol) of 6-chlorohexyl methacrylate, a n d 4.0 g (0.029 mol) of anhydrous K C Û 3 i n 60 m L of dry dimethyl sulfoxide was stirred at 80°C for 2 h , poured into 300 m L of cold water and washed w i t h 1 M N a O H a n d w a t e r . The s o l i d product ( l ) w a s c r y s t a l l i z e d t w i c e i n methanol (yield 82%): m.p. 82°C., [ C C F D +6.9° (chloroform). Copolymerization. I n a t y p i c a l c o p o l y m e r i z a t i o n r e a c t i o n (cf. (1/2)a c o p o l y m e r ) 0.542 g (1.20 m m o l ) of 4 - ( ( S ) - 2 - m e t h y l b u t o x y ) - 4 ' - ( 6 methacryloyloxyhexyloxy)azobenzene (1) and 0.361 g (0.75 mmol) of 4hexyloxy-4'-(6-methacryloyloxyhexyloxy)azobenzene (2) were dissolved i n 5 m L of dry benzene i n the presence of 5 mg of A I B N . The solution w a s i n t r o d u c e d u n d e r n i t r o g e n i n t o a g l a s s v i a l t h a t w a s s e a l e d after repeated freeze-thaw p u m p cycles. A f t e r r e a c t i n g 48 h at 60°C., the m i x t u r e was poured into 100 m L of m e t h a n o l . The p o l y m e r i c p r o d u c t was p u r i f i e d by s e v e r a l p r e c i p i t a t i o n s from c h l o r o f o r m s o l u t i o n i n t o methanol (yield 90%). 2

2

2



22. CfflELLINl ET AL.


283

Characterization. Optical rotatory power measurements were c a r r i e d out w i t h a P e r k i n E l m e r 141 spectropolarimeter on chloroform solutions. A v e r a g e m o l e c u l a r weights of the polymers were d e t e r m i n e d by size e x c l u s i o n chromatography ( S E C ) of chloroform s o l u t i o n s u s i n g a 590 W a t e r s c h r o m a t o g r a p h w i t h a Shodex K F 8 0 4 c o l u m n . P o l y s t y r e n e standards were employed for the universal calibration method. Differential scanning calorimetry (DSC) was performed w i t h a P e r k i n - E l m e r D S C 7 apparatus. The transition temperatures were taken as corresponding to the m a x i m u m of the enthalpic peaks obtained at a heating/cooling rate of 10 K / m i n . The mesophase textures were observed between crossed polarizers on a R e i c h e r t microscope equipped w i t h a M e t t l e r F P 5 2 heating stage. X-ray diffraction data were recorded at the C H E S S f a c i l i t i e s at C o r n e l l U n i v e r s i t y . O r i e n t e d f i b e r s , f i l m s a n d powders were examined using a M e t t l e r F P 8 2 hot stage mounted i n the b e a m p a t h to c o n t r o l s a m p l e t e m p e r a t u r e s . F i b e r s were f o r m e d by t a k i n g the polymer to the isotropic melt on a F i s h e r - J o h n s a p p a r a t u s a n d d r a w i n g out the m o l t e n p o l y m e r w i t h tweezers. M o n o b r o m a t e d radiation of λ = 1.565 À was used. The experimental setup and a n a l y t i c a l procedures have been previously described (21). Results and Discussion M e t h a c r y l a t e monomers 1-5 were prepared following the synthetic route o u t l i n e d i n S c h e m e 1. A m o n g t h e m , o n l y 2 p r e s e n t s a m o n o t r o p i c nematic phase (isotropic-nematic t r a n s i t i o n at 354 K ) and 4 exhibits a smectic phase (smectic-isotropic transition at 363 K ) . The polymers were obtained by free r a d i c a l i n i t i a t i o n ( A I B N ) at 60°C w i t h polymerization yields greater t h a n 85-90%. I n the Η - Ν Μ Η and C - N M R spectra the relative intensities of the signals o r i g i n a t i n g from the R, R', and R" substituents do not depend on the polymerization yield and correspond to those of the initial comonomer m i x t u r e s . T a k i n g into account that the methacrylate moiety is w e l l spaced apart from the m e s o g e n i c c o r e , we m a y r e a s o n a b l y a s s u m e t h a t t h e d i f f e r e n t monomers are characterized by a comparable reactivity and hence the c o r r e s p o n d i n g copolymers have a r a n d o m d i s t r i b u t i o n of m o n o m e r c o u n i t s . The copolymers i n c o r p o r a t i n g the c h i r a l c o m o n o m e r l a r e o p t i c a l l y active. There exists a l i n e a r r e l a t i o n s h i p b e t w e e n o p t i c a l rotatory power and weight fraction of 1 (Xi) of the different c o p o l y m e r s y s t e m s , the h i g h e r m o l a r optical rotation being detected for p o l y ( l ) ( [ Φ ] ^ = +31° ( C H C I 3 ) ) . It was, however, difficult to measure the optical rotatory power of the copolymers due to t h e i r strong absorption of the U V - v i s i b l e l i g h t , p a r t i c u l a r l y i n copolymers w i t h l o w e r c o n t e n t s of c o u n i t s f r o m 1. T h e c h i r o p t i c a l p r o p e r t i e s a n d t h e trans-cis p h o t o i n d u c e d i s o m e r i z a t i o n i n s o l u t i o n o f t h e s e p o l y m e r s w i l l be described i n a forthcoming paper. The m o l e c u l a r w e i g h t s of the polymers were determined by S E C and their number average molecular w e i g h t ( M ) r a n g e s f r o m 1 9 0 , 0 0 0 to 7 6 0 , 0 0 0 g / m o l w i t h f i r s t polydispersity index ( M / M ) between 1.9 and 2.6. Χ

1 3

2

n

w

n


284


Scheme L Reaction pathway for the synthesis of methacrylates 1-5. l)NaN0 /HCl 2

/

-NH

2

OH


V τ

:

N N

~O - -Q-

8

OH

R' CH =C-COO(CH2)6Cl 2

9

CH,

CH

3

0(CH ) OOC-C=CH 2

6

1 R

- 5

M

X = (S)-C2H CH*(CH3)CH 0, η - ϋ β Η ^ Ο , / i - C i H i O , N 0 5

2

2

R" = Η , C H

0

2

2

3

A l l of the polymers exhibit L C properties. There is no c r y s t a l l i n i t y detectable by D S C or X-ray scattering i n the samples at t e m p e r a t u r e s below the onset of the thermotropic mesophase, w h i c h therefore extends f r o m t h e g l a s s t r a n s i t i o n t e m p e r a t u r e ( T ) to t h e i s o t r o p i z a t i o n temperature (TO. In most cases we could not detect the glass transition by D S C a n d the l o w e r t e m p e r a t u r e l i m i t for the L C r a n g e o f t h e s e copolymers r e m a i n s to be identified better. Poly(l) a n d poly(3) have Tg values of 384 Κ and 364 K , respectively, and this suggests t h a t i n these systems the mesophase can easily be locked-in at room temperature. The L C properties of 1/2 copolymers are s u m m a r i z e d i n T a b l e I. F o r these samples there exists one smectic mesophase t h r o u g h o u t a l l the c o m p o s i t i o n r a n g e a n d the s m e c t i c - i s o t r o p i c t e m p e r a t u r e i n c r e a s e s l i n e a r l y w i t h i n c r e a s i n g 2 content from that of poly(l) to that of poly(2). T h i s reflects the tendency of counits from 1 to give rise to a less stable smectic mesophase, consistent w i t h the presence of the branched c h i r a l group, a n d suggests that the nature of the mesophase may be the same for a l l 1/2 copolymers. I n agreement w i t h this interpretation, the values of the smectic-isotropic entropy are very s i m i l a r along the series. g


22. CHIELLINI ET AL.

Table L L C properties of 1/2 copolymers containing azobenzene mesogens with R • (S)-2-methyIbutoxy (1) and R' • n-hexyloxy (2) substituents M„W M /M W ASi Sample Xl*> Ti (K) (J/mol-K) 18.7 1 388 poly(l) 760,000 1.9 16.7 (l/2)a 395 0.8 490,000 2.3 (l/2)b 402 18.8 0.6 390,000 2.6 0.4 19.4 (l/2)c 2.4 410 500,000 20.4 (l/2)d 0.2 2.2 417 540,000 poly(2) 18.8 0 440,000 2.4 423 w


285


a

n

b

> Weight fraction of monomer 1. > By S E C .

A s i m i l a r trend is observable i n the 1/4 copolymer series (Table II). A l l of these members e x h i b i t smectic behavior and the i s o t r o p i z a t i o n temperature decreases w i t h increasing X i (Figure 1). However, there is a p a r a l l e l m a r k e d decrease i n the smectic-isotropic entropy i n g o i n g from poly(4) to poly(l) and the smectic mesophases i n the copolymers m a y be d i f f e r e n t i n c h a r a c t e r . N o t e also t h a t for poly(4) a n d 1/4 copolymers the smectic-isotropic t e m p e r a t u r e is c o n s i s t e n t l y h i g h e r than for poly(2) and respective 1/2 copolymers, this difference b e c o m i n g less m a r k e d w i t h i n c r e a s i n g Χχ. These results are i n accordance w i t h the general observation that longer alkyloxy substituents on mesogenic cores tend to favor the incidence of more stable and persistent smectic mesophases.

Table Π. L C properties of 1/4 copolymers containing azobenzene mesogens with R • (S)-2-methylbutoxy (1) and R • w-decyloxy (4) substituents f

Sample poly(l) (l/4)a (l/4)b (l/4)c poly(4) a

b

Mn > 1 0.8 0.6 0.4 0

M /M„W W

760,000 580,000 540,000 530,000 190,000

) Weight fraction of comonomer 1.

1.9 2.2 2.2 2.1 2.3 b

Ti (K) 388 398 410 435 445

ASi (J/mol-K) 18.7 16.4 19.4 22.4 24.7

) By S E C .

A more complex mesomorphic behavior is shown by 1/3 copolymers (Table III). Poly(3) presents one nematic mesophase of limited extension (4 K ) . Incorporation of counits from 3 i n c h i r a l 1/3 copolymers results i n t h e o c c u r r e n c e of a c h o l e s t e r i c a n d a n e w s m e c t i c m e s o p h a s e at i n t e r m e d i a t e compositions, a n d e v e n t u a l l y the c o p o l y m e r s become purely smectic at 1 weight fractions greater than approximately 0.80.


286


Table HL L C properties of 1/3 copolymers containing azobenzene mesogens with R • (S)-2-methyIbutoxy (1) and R' • n-hexyloxy, R" • methyl (3) substitaents Δ Sj S a m p l e Xl») M /M„W Ti AS Ch TsCh (K) (J/mol-K) (J/mol-K) (K) — — 18.7 388 1 polyd) 760,000 1.9 — — 380 13.3 (l/3)a 0.8 360,000 2.5 3.2 8.2 375 (l/3)b 0.6 480,000 368 2.3 3.4 370 0.4 5.6 (l/3)c 470,000 2.3 367 369 3.7 0.2 364 3.5 (l/3)d 320,000 2.6 — — 4.0 368 poly(3) 0 2.4 390,000 c )

c)


W

a

> Weight fraction of comonomer 1. transition.

S

M By S E C .

c )

Smectic-cholesteric

The introduction of the lateral methyl substituent i n 3 reduces the aspect ratio of the azobenzene mesogen relative to 2, which greatly depresses its s m e c t o g e n i c t e n d e n c y t h u s p e r m i t t i n g the e x i s t e n c e o f two c h i r a l mesophases i n 1/3 copolymers (0 < X i < 0.80). I n agreement w i t h t h i s mesophase behavior, the isotropization entropy (Figure 2) is l o w for the n e m a t i c p h a s e (Χχ = 0) a n d lowest for the c h o l e s t e r i c m e s o p h a s e whereas i t is h i g h for the smectic mesophase. Conversely, the smecticcholesteric entropy increases r e g u l a r l y w i t h i n c r e a s i n g 1 content and, as a net result, the total entropy change from the glass to the isotropic l i q u i d increases almost linearly w i t h Χχ. 4/5 c o p o l y m e r s p r e s e n t o n e s m e c t i c m e s o p h a s e , w h o s e isotropization temperature and entropy are not substantially altered by the incorporation of significant amounts (higher t h a n 10 wt.%) of the n i t r o s u b s t i t u t e d c o m o n o m e r 5 (Table I V ) . T h i s p o l y m e r s m e c t i c structure can, therefore, easily accommodate different mesogenic u n i t s with strongly mismatching terminal substituents, including chromophores w i t h pronounced dipolar character. The s t r u c t u r e of the mesophases was i n v e s t i g a t e d u s i n g a h i g h energy s y n c h r o t r o n source w h i c h a l l o w e d to perform r e a l t i m e X - r a y s c a t t e r i n g e x p e r i m e n t s . The phase t r a n s i t i o n s c o u l d , therefore, be followed d y n a m i c a l l y as they were t a k i n g place i n the same conditions as w i t h D S C and p o l a r i z i n g microscopy. The X - r a y observations are i n agreement w i t h the above results. G e n e r a l l y , the recorded diffraction patterns can be divided into two spectral regions (Figures 3 and 4). T h e s m a l l angle region signals (inner rings) are associated w i t h the layered stacking of the molecules i n the smectic phase, whereas the wide angle region diffractions (outer rings) occur from the l a t e r a l p a c k i n g of the molecules w i t h i n the smectic l a y e r s . In a l l cases a very diffuse outer signal is observed indicative of a liquid-like arrangement of the polymer side chains i n a disordered smectic phase w i t h an a v e r a g e i n t e r m o l e c u l a r d i s t a n c e D ~ 4.4-4.5 À. T h e cholesteric phase of the polymers displays a broad wide angle ring (D ~ 4.5-4.6 À) only. A typical diffraction pattern of the smectic phase of an unoriented



287



F i g u r e 2. P h a s e t r a n s i t i o n e n t r o p i e s ( A S ) for 1/3 c o p o l y m e r s as function of 1 weight fraction ( X i ) : cholesteric (or smectic)-to-isotropic ( · ) and smectic-to-cholesteric (A).



288

MACROMOLECULAR ASSEMBLIES IN POLYMERIC

SYSTEMS

F i g u r e 3. X - r a y diagram of the smectic phase of poly(4) at 160°C.

Figure 4. Fiber X - r a y diagram of the smectic phase of poly(l) at room temperature (vertical fiber axis).


22.

CHIELLINI ET AL.


Table IV. L C properties of 4/5 copolymers containing azobenzene mesogens with R • n-decyloxy (4) and R' • nitro (5) substituents M /M„W AS{ Sample X a) M„» Ti (J/molK) (K) 24.7 poly(4) 1 445 190,000 2.3 (4/5)a 445 23.5 0.98 2.4 450,000 (4/5)b 449 24.7 0.95 480,000 2.3 22.9 (4/5)c 449 0.90 240,000 2.6 W

4


289

a

) Weight fraction of comonomer 4.

b

) By S E C .

polymer sample is given i n Figure 3 for poly(4). There are three orders of reflection i n the inner region w i t h corresponding d spacings i n the ratio 1:2:3 associated w i t h the regular layer periodicity of the mesophase. T h i s seems to be a common feature i n thermotropic side c h a i n p o l y m e r s , i n complete contrast to s m a l l molecule l i q u i d crystals for w h i c h one s m a l l angle signal is observed and it is generally accepted that the projection of the electron density profile along the director can be described by the i d e a l m o d e l of a s i n g l e s i n u s o i d a l m o d u l a t i o n (22). T h e m e a s u r e d d spacing (d = 54 À) is much longer than the calculated length of the side c h a i n s i n t h e i r m o s t e x t e n d e d c o n f o r m a t i o n ( L ~ 37 Â ) a n d t h e mesophase must have a bilayer structure w i t h p a r t i a l l y interdigitated mesogenic units ( S A 2 or S c 2 ) (23). A t y p i c a l fiber diffraction p a t t e r n is reported i n Figure 4 for the smectic phase of poly(l). The six s m a l l angle B r a g g arcs correspond to the three first orders of reflection on the layer plane. They are located on the equator, w h i c h shows t h a t the s m e c t i c layers, and consequently the polymer backbones, are oriented p a r a l l e l to the fiber axis. The detected d spacing of 44 À is m u c h longer t h a n the calculated side chain length (L - 29 À). The two broad crescents at wide angle are located on the m e r i d i a n a n d suggest t h a t the p o l y m e r side chains are orthogonal to the fiber axis and to the smectic layers of a S A 2 p h a s e . H o w e v e r , the existence of a s m a l l t i l t a n g l e i n t h e p r e s e n t samples cannot be completely ruled out and investigations on annealed fibers and m a g n e t i c a l l y a l i g n e d samples are u n d e r w a y . A c o m p l e t e report w i l l be given elsewhere. Conclusions A variety of chiral thermotropic copolymers can be prepared exhibiting a c h i r a l smectic mesophase and, i n some cases, a cholesteric mesophase. W h i l e the o v e r a l l mesophase behavior is e s s e n t i a l l y d i c t a t e d by the structure of the two parent comonomers, it can be somewhat accurately tuned by the proper adjustment of the relative amounts of the counits. We also anticipate that, by the synthetic scheme proposed, several c h i r a l mesophases can be developed i n copolymers c o n t a i n i n g strong d i p o l a r c h r o m o p h o r e s to be t e s t e d i n f e r r o e l e c t r i c a n d n o n l i n e a r o p t i c a l applications.


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Acknowledgments T h i s w o r k w a s c a r r i e d out w i t h p a r t i a l f i n a n c i a l s u p p o r t from t h e M i n i s t e r o d e l l ' U n i v e r s i t à e Ricerca Scientifica of Italy. G . G . also t h a n k s I t a l i a n C N R for a N A T O F e l l o w s h i p a n d t h e C o r n e l l H i g h Energy Synchrotron Source ( C H E S S ) for use of the facility, as w e l l as Prof. C . K . Ober, Cornell University, for helpful discussions.


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Chiral Liquid-Crystalline Copolymers for Electrooptical Applications

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