Polymer Association Structures - American Chemical Society

Chapter 15

Dielectric and Electrooptical Properties of a Chiral Liquid Crystalline Polymer 1,4

1,5

1,6

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G. S. Attard , K. Araki , J. J. Moura-Ramos , G. Williams , A. C. Griffin , A. M. Bhatti, and R. S. L. Hung

Downloaded by CORNELL UNIV on May 22, 2017 | http://pubs.acs.org Publication Date: December 30, 1989 | doi: 10.1021/bk-1989-0384.ch015

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1Edward Davies Chemical Laboratories, University College of Wales, Aberystwyth, SY23 1NE, United Kingdom 2Department of Chemistry, University of Swansea, Singleton Park, Swansea SA2 8PP, United Kingdom Department of Chemistry and Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406 3

The dielectric properties of a chiral-nematic liquid crystalline polymer having applications in non-linear optics have been studied over wide ranges of frequency and temperature. Samples of different degrees of macroscopic alignment (homeotropic, planar) are shown to exhibit very different relaxation behaviour, and this is interpreted in terms of the anisotropic motions of the dipolar mesogenic groups. It is shown that fully homeotropic alignment is not achieved and that on removal of the directing electric field a relaxation of alignment occurs, which may be due to the reformation of chiral structures. There i s considerable current i n t e r e s t (1,2) i n the synthesis and properties of l i q u i d c r y s t a l l i n e (LC) polymers whose side groups cont a i n groups which possess large l i n e a r and higher order o p t i c a l p o l a r i z a b i l i t i e s . This i n t e r e s t stems from the f a c t that f i l m s (5 100 pm) made from these materials show promise as media f o r o p t i c a l information storage and processing, i n c l u d i n g second harmonic generation (SHG) of l a s e r r a d i a t i o n . Polymeric f i l m s may have some advantages over monomeric organic c r y s t a l s i n view of t h e i r ease of processing and t h e i r resistance t o damage by l a s e r beams. In a d d i t i o n , LC polymers may be aligned macroscopically using e l e c t r i c or magnetic f i e l d s , g i v i n g an enhanced s u s c e p t i b i l i t y and also allowing the macroscopic o p t i c a l properties to be varied continuously over a wide range. Most of the a p p l i c a t i o n s of LC polymers require a f i l m to be aligned macroscopically. In previous papers (3-11) i t was shown that acrylate or siloxane polymers could be aligned homeotropically, p l a n a r l y (or 4

Current address: Department of Chemistry, The University of Southampton, S09 5NH, United Kingdom On leave from Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo, Japan On leave from Departamento de Quimica Engenharia, Technical University of Lisbon, Portugal

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0097-6156/89/0384-0255$06.00/0 « 1989 American Chemical Society

El-Nokaly; Polymer Association Structures ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

POLYMER ASSOCIATION STRUCTURES


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homeogeneously) or to intermediate extents of alignment by moderate magnetic f i e l d s (3-6) or a.c. e l e c t r i c f i e l d s (7-11)· In the e l e c t r i c a l case the alignment process involves the d i e l e c t r i c prop e r t i e s of the polymer, which a r i s e due to the anisotropic motions of d i p o l a r mesogenic groups pendant to the chain (11). I t follows that a d i r e c t l i n k can be made between the macroscopic alignment behaviour and the molecular properties of the chain (7,10,11). In our e a r l i e r studies of siloxane polymers we found (7-13) that the homopolymers could not be aligned by a p p l i c a t i o n of the a.c. e l e c t r i c f i e l d to the m a t e r i a l i n i t s LC s t a t e , but copolymers would do so. A l l materials could be aligned by cooling the melt i n t o the LC state i n the presence of the e l e c t r i c f i e l d . Homeotropic alignment was achieved r e a d i l y but p l a n a r l y aligned material only formed i f the cross-over frequency ν was below normally accessible power frequencies (y 10^ Hz) at the cîearing temperature T (12,13). In t h i s paper we describe the behaviour of an LC polymer whose backbone structure i s d i f f e r e n t from that of the conventional a c r y l a t e , methacrylate or siloxane polymers (14,15). The polymer Î described below was designed f o r NLO a p p l i c a t i o n s i n that each repeat u n i t contained a mesogenic group which has a large second and t h i r d order o p t i c a l p o l a r i z a b i l i t y . By incorporating the e l e c t r o a c t i v e group i n the side chain of the polymer the problems of low s o l u b i l i t y and phase separation which occur i n guest/polymer host systems are overcome. Also polymer I has a c h i r a l group i n the main chain repeat u n i t . Using d i e l e c t r i c r e l a x a t i o n spectroscopy we have been able to study the dynamics of the mesogenic groups and the e l e c t r i c - f i e l d induced alignment behaviour. I t i s shown that t h i s polymer may be aligned d i r e c t l y i n the LC state. Q

Experimental The polymer had the following structure -f 00C-CH-C00. CH. CH CH h I I 0

2

R

CH

0

I

2

3

where R i s C H

0

N

C

H

N

0

-< 2>6 -O * O 2 The material was chiral-nematic and had an apparent glass transi t i o n at 276 Κ and a c l e a r i n g point at 325.2 K, as judged by DSC measurements. The c l e a r i n g point was very sharp, the biphasic range being ^ 0.5 Κ as determined by o p t i c a l microscopy. D i e l e c t r i c spectra were obtained i n the frequency range 15 to 10 Hz using a computer-controlled GenRad 1689 P r e c i s i o n RLC D i g i bridge. The samples i n d i s c form (1 cm diameter, 120 pm t h i c k ) were contained i n a three terminal d i e l e c t r i c c e l l whose electrodes were made i n s t a i n l e s s s t e e l . Sample temperatures were c o n t r o l l e d to ± 0.01 Κ by immersing the c e l l i n a thermostatted water bath. E l e c t r o o p t i c a l measurements were made as f o l l o w s . A sample was prepared between conducting glass p l a t e s (ITO 1 cm ) at a separation of 24 pm, 5

2


Chiral Liquid Crystalline Polymer


15. ATTARD ET AL.

257

using a mylar spacer. P o l a r i z e r s were attached to both glass p l a t e s and were crossed. The sample-assembly was mounted i n a sealed metal block and was illuminated using l i g h t (from a tungsten filament lamp) which was propagated to the sample v i a an o p t i c - f i b r e bundle. The l i g h t transmitted through the sample was further transmitted along an equivalent f i b r e - o p t i c bundle to a CdS photodiode, whose output was compared with that from a second photodiode activated by the same bulb. This arrangement ensured that the o p t i c a l measurements would not be affected by f l u c t u a t i o n s i n the i n t e n s i t y of the l i g h t source. Samples were aligned by applying an a.c. e l e c t r i c f i e l d of 200 V rms ( f o r d i e l e c t r i c measurements) and 50 V rms ( f o r e l e c t r o - o p t i c a l measurements), and of v a r i a b l e frequency. I t was found that polymer I was two-frequency-addressable such that a p p l i c a t i o n of the e l e c t r i c f i e l d at 400 Hz to the melt followed by slow cooling gave a homeot r o p i c a l l y - a l i g n e d sample, while r a i s i n g the frequency to 10 kHz at t h i s voltage and repeating the cooling from the melt gave a p l a n a r l y aligned sample. Results and Discussion Figure 1 shows the d i e l e c t r i c loss spectra obtained at 306.2 Κ f o r the homeotropic (Η), unaligned (U) and planarly-aligned (P) sample, where the Η and Ρ materials were prepared by cooling from the melt i n the presence of saturating low and high frequency d i r e c t i n g e l e c t r i c f i e l d s . The Η sample has a well-defined low frequency l o s s peak accompanied by a broad high frequency shoulder. The Ρ sample gives a very broad, featureless absorption whose frequency of maximum loss occurs s i g n i f i c a n t l y higher than that f o r the Η sample. The U sample e x h i b i t s intermediate behaviour. The i s o b e s t i c point (at which the loss f a c t o r i s independent of macroscopic alignment of sample) i s seen c l e a r l y at 2 kHz. Figure 2 shows the p l o t s of sample capacitance against frequency at 306.2 Κ f o r the Η and Ρ samples. The crossover frequency v (which i s an i s o b e s t i c point) i s seen to occur at 410 Hz at t h i s sample temperature. v determines the nature of alignment i f an a.c. d i r e c t i n g e l e c t r i c f i e l d i s able to a l i g n a material i n i t s LC s t a t e . Η or Ρ alignment i s obtained f o r a d i r e c t i n g f i e l d having ν < v or ν > v r e s p e c t i v e l y at the f i x e d temperature (10,16,17). We hcive determined v from such data (Figure 2) f o r a wide range of sample temperatures i n the LC state of the polymer. Figure 3 shows the p l o t of log v - v s - ( T / K ) " l and the rapid v a r i a t i o n observed i s simply due to the c r i t i c a l slowing-down of the d i e l e c t r i c relaxations i n Η and Ρ material as the apparent T of the polymer i s approached. As indicated i n the f i g u r e , a d i r e c t i n g f i e l d of ν > v w i l l produce planarly-aligned material while that f o r ν < v w i l l produce a homeot r o p i c a l l y - a l i g n e d material f o r a given sample temperature. In contrast with our e a r l i e r findings f o r siloxane homopolymers, we found that polymer I could be aligned d i r e c t l y by a p p l i c a t i o n of a strong a.c. e l e c t r i c f i e l d to the polymer i n i t s LC state. The k i n e t i c s of the alignment behaviour w i l l be described i n a future paper, but i t should be said that we found that the rate of macro scopic alignment decreased r a p i d l y as the sample temperature was lowered below T . As one example of the alignment behaviour, Figure 4 shows data we obtained at 319.2 K. A sample was prepared homeotropically-aligned by cooling from the melt i n a saturating low c

c

c

c

c

c

g

c

c

c


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POLYMER ASSOCIATION STRUCTURES

306.2 Κ homeotropic

20

Ρ 10


2

3 log (v/Hz )

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/y

Figure 1. e C against log^ (v/Hz) f o r homeotropic, unaligned and planarly-aligned samples at 306.2 K. a

0

2

3 log (v/Hz )

c

a

a

n s t

Figure 2. C (= ε'0 + j ) g i log (v/Hz) f o r homeotropic and planarly-aligned samples at 306.2 K. &

e

Polymer Association Structures - American Chemical Society

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