Chapter 2.pdf

C H A P T E R I I

DESCRIPTION OF THE EXPERIMENTAL SET UP

This c h a p t e r d e s c r i b e s t h e e x p e r i m e n t a l s e t up u s e d in t h e d i e l e c t r i c s t u d i e s r e p o r t e d in t h i s thesis.

2.1 The Cell

T h e s c h e m a t i c d i a g r a m of t h e cell 1 used is g i v e n in fig.2.la.

It c o n s i s t s of t w o aluminium c o a t e d glass p l a t e s which s e r v e a s t h e e l e c t r o d e s . T h e s e p a r a t i o n b e t w e e n t h e m w a s d e f i n e d by t w o n a r r o w s t r i p s of mylar (or K a p t o n f o r t e m p e r a t u r e s a b o v e 170°C).

T h e t h i c k n e s s of t h e s p a c e r s used ranged f r o m 12.5 Um t o 100 Urn, d e p e n d i n g on t h e m a t e r i a l i n v e s t i g a t e d a n d t h e g e o m e t r y of

t h e set up. C a r e w a s t a k e n t o e l i m i n a t e t h e c o n t r i b u t i o n of t h e m y l a r t o t h e t o t a l c a p a c i t a n c e of t h e cell by positioning t h e m y l a r s / K a p t o n s o u t s i d e t h e a c t i v e a r e a of t h e c e l l w h i c h w a s a b o u t 1 sq.cm. A b e v e l , m a d e o n t h e s h o r t e r s i d e of o n e of t h e p l a t e s , w a s used in filling t h e s a m p l e . The t w o e l e c t r o d e s w e r e o f f s e t along t h e l e n g t h and t h e e x t e n d i n g a r e a w a s used t o m a k e e l e c t r i c a l c o n t a c t s . T h e c e l l w a s held rigidly in a c o p p e r f r a m e ( s e e Fig.2.la) s h a p e d like a r e c t a n g u l a r ' G ' c l a m p . T o avoid u n e v e n p r e s s i n g of t h e c e l l by t h e brass s c r e w s , a f l a t c o p p e r p l a t e w a s u s e d t o c o v e r t h e glass plates. R e c t a n g u l a r s l o t s m a d e in t h e f r a m e a n d in t h e

F i g u r e 2 . 1 : S c h e m a t i c d i a g r a m o f ( a ) D i e l e c t r i c c e l l a n d ( b ) h e a t e r . 1 . E l e c t r o d e s , 2 . B e v e l , 3. C o p p e r c l a m p , 4 . C o p p e r p l a t e , 5 . B r a s s s c r e w s ,

6 . Windows, 7 . C o p p e r c a p , 8 . N e o p r e n e g a s k e t , g . N o z z l e s , 10. B r a s s n u t s w i t h p y r o p h i l i t e b u s h e s , 1 1 . C h r o m e l - c o n s t a n t a n t h e r m o c o u p l e .

H r e p r e s e n t s t h e d i r e c t i o n o f t h e m a g n e t i c f i e l d f o r E, m e a s u r e m e n t s

c o p p e r p l a t e s e r v e d t o c h e c k t h e a l i g n m e n t of t h e s a m p l e by visual o b s e r v a t i o n b e t w e e n c r o s s e d polarisers. This c e l l a l l o w e d us t o c a r r y o u t m e a s u r e m e n t s e v e n a t t e m p e r a t u r e s as high as 220°C.

Also, i t h a s t h e a d v a n t a g e of requiring only a s m a l l q u a n t i t y of s a m p l e .

2.2 Heater

A s c h e m a t i c d i a g r a m of t h e h e a t e r is shown in Fig.2.lb.

I t c o n s i s t e d of a long r e c t a n g u l a r c o p p e r tube. T h e l e n g t h of t h e h e a t e r w a s d e l i b e r a t e l y m a d e very l a r g e c o m p a r e d t o t h e s i z e of t h e d i e l e c t r i c c e l l t o e n s u r e t h a t t h e r e a r e n o t e m p e r a t u r e g r a d i e n t s in t h e sample. Its c r o s s s e c t i o n a l s i z e in t h e c e n t r a l position w a s minimised in o r d e r t o o b t a i n a s high a m a g n e t i c field as possible.

N i c h r o m e h e a t e r w i r e w a s wound a t both e n d s of t h e h e a t e r . T h e h y l a m s h e e t s c o v e r i n g t h e e n t i r e s u r f a c e of t h e h e a t e r body provided v e r y good t h e r m a l insulation. I t w a s m a d e s u r e t h a t t h e c e l l wks a l w a y s positioned a t t h e c e n t r e of t h e h e a t e r w h e n i t w a s i n t r o d u c e d i n t o i t f r o m t h e top. T h e h e a t e r could be r o t a t e d a b o u t a v e r t i c a l a x i s a n d i t s position could be r e a d t o a n a c c u r a c y of 0.1" o n a c i r c u l a r s c a l e a t t a c h e d t o i t s b o t t o m . With t h i s a r r a n g e m e n t t h e c e l l could be r o t a t e d e x a c t l y by 90°, t h e r e b y e n a b l i n g us t o m a k e b o t h E and E m e a s u r e m e n t s (in t h e n e m a t i c phase) using t h e

I I

^L

s a m e s a m p l e . T h e h e a t e r w a s s e a l e d by t i g h t e n i n g a c o p p e r c a p w i t h a n e o p r e n e '0' ring. Two brass n u t s w e r e f i x e d in t h e body

o f t h e h e a t e r on t h e s i d e s with windows. T h e s e w e r e i n s u l a t e d f r o m t h e h e a t e r by p y r o p h i l i t e bushes. Two s c r e w s c o u l d b e m a d e t o m o v e f o r w a r d or b a c k w a r d in t h e nuts in o r d e r t o m a k e o r b r e a k e l e c t r i c a l c o n t a c t with t h e e l e c t r o d e s w i t h o u t d i s t u r b i n g t h e s e a l e d e n v i r o n m e n t . T h e brass n u t s w e r e c o n n e c t e d t o t h e m e a s u r i n g i n s t r u - m e n t ( I m p e d a n c e Analyser) via coaxial cables. T h e h e a t e r a s s e m b l y w a s p l a c e d on a p l a t f o r m which w a s provided w i t h levelling s c r e w s . T h e p u r p o s e of t h i s a r r a n g e m e n t i s two-fold:

(i) t o e n s u r e t h a t t h e h e a t e r is p e r f e c t l y n o r m a l w i t h r e s p e c t t o t h e d i r e c t i o n of t h e m a g n e t i c field a n d

(ii) t o a d j u s t t h e h e i g h t of t h e h e a t e r r e l a t i v e t o t h e pole- p i e c e s of t h e m a g n e t such t h a t t h e c e l l is s i t u a t e d e x a c t l y a t t h e c e n t r e of t h e pole- pieces. B e f o r e t h e c o m m e n c e m e n t of a n y e x p e r i m e n t t h e h e a t e r w a s f l u s h e d w i t h dry n i t r o g e n f o r a b o u t t e n m i n u t e s t h r o u g h a p a i r of n o z z l e s provided a t t h e t o p of t h e h e a t e r cap. T h i s p r e v e n t e d o x i d a t i o n a n d d e t e r i o r a t i o n of t h e s a m p l e a t high t e m p e r a t u r e s .

2.3 TEMPERATURE CONTROL AND MEASUREMENT

T h e e l e c t r i c a l p o w e r t o t h e h e a t e r w a s supplied by a s t a b i l i s e d DC p o w e r supply. To p r o b e t h e t e m p e r a t u r e of t h e s a m p l e a c h r o m e l - c o n s t a n t a n t h e r m o c o u p l e w a s s e l e c t e d a s both t h e s e e l e m e n t s a r e

n o n - m a g n e t i c m a t e r i a l s . T h e t h e r m o c o u p l e w a s u s e d in c o n j u n c t i o n w i t h a digital m u l t i m e t e r (Keithley, Model No. 195A). Using s i m i l a r p r o b e s at various positions inside t h e h e a t e r , i t w a s initially a s c e r t a i - . n e d t h a t t h e s a m p l e did n o t e x p e r i e n c e a n y n o t i c e a b l e t e m p e r a t u r e

g r a d i e n t s . By varying t h e c u r r e n t through t h e h e a t e r a t a c o n t r o l l e d r a t e , t h e t e m p e r a t u r e of t h e s a m p l e could be c h a n g e d a t a n y d e s i r e d r a t e . Typically, during t h e m e a s u r e m e n t of t h e s t a t i c d i e l e c t r i c c o n s t a n t , t e m p e r a t u r e w a s varied a t a r a t e of a b o u t 4"C/hour, t h i s r a t e being m u c h s l o w e r (I-2"C/hour) n e a r a p h a s e t r a n s i t i o n . T h e t e m p e r a t u r e of t h e s a m p l e was m a i n t a i n e d t o b e t t e r t h a n + 2 5 mK during dispersion m e a s u r e m e n t s .

2.4 TMEPERATURE CALIBRATION OF THE CELL

S e v e r a l liquid c r y s t a l l i n e m a t e r i a l s t h a t w e r e of good c h e m i c a l p u r i t y w e r e used f o r t h e t e m p e r a t u r e c a l i b r a t i o n of t h e cell. T h e n a m e s of t h e c o m p o u n d s as well a s their n e m a t i c - i s o t r o p i c tra::sition t e m p e r a t u r e s as m e a s u r e d with t h e polarizing m i c r o s c o p e f i t t e d w i t h a p r o g r a m m a b l e h o t s t a g e a r e listed in T a b l e 2.1. T h e t h e r m a l v a r i a t i o n of s t a t i c d i e l e c t r i c c o n s t a n t

/I

( t h e m e a s u r i n g f i e l d i?

11

-f n, t h e d i r e c t o r ) n e a r t h e n e m a t i c - i s o t r o p i c t r a n s i t i o n is s t u d i e d f o r e a c h compound.

11

v e r s u s t h e r m o e m f p l o t s a r e given in F i g u r e s 2.2-2.5. A slow h e a t i n g r a t e (-4-5"C/hr.) w a s a d o p t e d f o r t h e m e a - s u r e m e n t . T h e t h e r m o e m f m e a s u r e d at t h e t r a n s i t i o n f o r e a c h c o m p o u n d is a l s o given in T a b l e 2.1. T h e plot of t h e t e m p e r a t u r e

TABLE 2.1

C o m p o u n d s u s e d in t h e t e m p e r a t u r e c a l i b r a t i o n o f t h e d i e l e c t r i c c e l l a n d t h e i r n e m a t i c - i s o t r o p i c t r a n s i t i o n t e m p e r a t u r e s

A c t u a l t e m p e r a t u r e DMM S.No. N a m e of t h e c o m p o u n d

("C) b y M e t t l e r r e a d i n g

I 4 ' - n - P e n t y l 4 - c y a n o b i p h e n y l ( ~ c B ) ~

2 4 - C y a n o c y c l o h e x y l - 4 ' - e t h y l c y c l o -

h e x a n e ( C C H * ) 3 48.5 1

3 4 - N i t r o p h e n y l 4'-n-octyloxy

b e n z o a t e ( 8 . 0 . ~ 0 ~ ) ~ 68.24

4 4'-n-Octyluxy 4 - c y a n o b i p h e n y l ( 8 0 ~ ~ ) ~ 80.35 5 4-n-Hexyloxy p h e n y l 4 ' - c y a n o -

b e n z o a t e (cN.o.~) 5 95.89

6 4-Biphenylyl-4"-n-propyl b e n z o a t e

( B ~ B ) ~ 108.58

7 4-Biphenylyl-4"-n-nonyloxy b e n z o a t e

( 8 9 0 ~ ) ~ 127.86

8 4 - U n d e c y l o x y phenyl-4'-(4"-cyano-

b e n z y l o x y ) b e n z o a t e ( I I O P C B O B ) 7 160.05

THERMO EMF ( r n V )

Figure 2.2

Temperature variation of dielectric constant E , near the nematic-isotropic transition for ( a ) 4'-n-octyloxy.4-cyanobiphenyl (80CB) and (b) 4-biphenylyl- 4"-n-propyl benzoate (B3B).

10.5 10.6 10.7 THERMO EMF (mV)

F i g u r e 2 . 3

T e m p e r a t u r e v a r i a t i o n o f d i e l e c t r i c c o n s t a n t E l , n e a r t h e n e m a t i c - i s o t r o p i c t r a n s i t i o n f o r ( a ) 4 - u n d e c y l o x y phenyl-4'-(4"-cyanobenzoyloxy) b e n z o a t e (11OPCBOB) a n d ( b ) 4-cyanocyclohexyl-4'-ethyl c y c l o h e x a n e (CCH2)

8.2 8.3 8 . 4 THERMO EMF (mV)

Figure 2.4

Temperature variation of dielectric constant El, near the nematic- isotropic transition for ( a ) 4 - b i p h e n y l y l - 4 ~ 1 - n - n o n y l o x y b e n z o a t e (B90B) and ( b ) 4'-n-penty1-4-cyano biphenyl ( 5 C B )

THERMO EMF (mV)

Figure 2.5

Temperature variation o f dielectric constant El, near the nematic-isotropic transition for ( a ) 4-n-hexyloxyphenyl 4'-cyanobenzoate (CN.0.6) and

of t r a n s i t i o n as m e a s u r e d using t h e M e t t l e r h o t s t a g e (in " C ) a g a i n s t t h e t h e r m o e m f (in mV) a t t h e t r a n s i t i o n as m e a s u r e d by t h e dielec- t r i c s t u d y is shown in fig.2.6. T h e d a t a w e r e f i t t e d t o a polynomial e x p r e s s i o n of t h e t y p e ,

( w h e r e T = t h e t e m p e r a t u r e in "C and X i s t h e t h e r m o e m f in mV.) Using a linear l e a s t s q u a r e f i t p r o g r a m m e , t h e v a l u e s o b t a i n e d f o r t h e c o n s t a n t s A A l a n d A2 a r e 1.2216, 16.2543 a n d -0.1246

0' r e s p e c t i v e l y .

2.5 Sample Alignment

In g e n e r a l , a s t r o n g m a g n e t i c field w a s u s e d t o o r i e n t t h e s a m p l e . A homogeneous o r p l a n a r a l i g n m e n t ( m e a s u r i n g fieid

+ + + ^-f

E l n ) g a v e E while a h o m e o t r o p i c a l i g n m e n t (E

11

n) g a v e E~~ ( s e e

i

fig.2.7). S i n c e i t is well known t h a t a l i g n m e n t s of liquid c r y s t a l s a r e s t r o n g l y i n f l u e n c e d by s u r f a c e conditions, t h e g l a s s p l a t e s c o n s t i - t u t i n g t h e e l e c t r o d e s w e r e s e l e c t e d a f t e r c a r e f u l s c r u t i n y . T h e y w e r e f i r s t c l e a n e d with a d e t e r g e n t (Teepoi, BDH ~ t d . ) and t h e n with f r e s h l y p r e p a r e d c h r o m i c acid. Both h o m e o t r o p i c a n d homo- g e n e o u s a l i g n m e n t s w e r e a c h i e v e d by applying a m a g n e t i c field of a b o u t 2.4 T e s l a which was m o r e t h a n t w i c e t h e s a t u r a t i o n f i e l d f o r t h e t h i c k n e s s of t h e s a m p l e used (50- 100 ~ r n ) . ~ T h e m a g n e t

Figure 2.7

Schematic representation of (a) homogeneous and ( b ) homeotropic structures

used w a s BRUKER (\lode1 No. B-E25) w i t h t a p e r e d c o b a l t - i r o n pole- pieces. T h e s a m p l e a l i g n m e n t in t h e s m e c t i c A p h a s e w a s o b t a i n e d by slowly cooling t h e well aligned n e m a t i c in t h e p r e s e n c e of t h e m a g n e t i c field. In c a s e of i s o t r o p i c - s m e c t i c A t r a n s i t i o n s , t h e a l i g n m e n t s w e r e o b t a i n e d by m e a n s of s u r f a c t a n t s , o c t a d e c y l t r i e t h o x y s i l a n e a n d polymide R e s i n (ZLI-2650) being u s e d f o r h o m e o - t r o p i c a n d homogeneous a l i g n m e n t s respectively.

2.6 MEASURING SET U P

T h e d i e l e c t r i c c o n s t a n t s w e r e m e a s u r e d using a H e w l e t t - P a c k a r d I m p e d a n c e Analyser (4192A). The f u n c t i o n i n g of t h e Impe- d a n c e Analyser is based on a n a u t o b a l a n c e b r i d g e m e a s u r e m e n t c i r c u i t . T h e c a p a c i t a n c e m e a s u r e m e n t f u n c t i o n of t h e a n a l y s e r is based on v e c t o r - v o l t a g e - c u r r e n t r a t i o m e a s u r e m e n t m e t h o d . E v e n a t low oscillator v o l t a g e levels, high p r e c i s i o n c a p a c i t a n c e m e a s u r e m e n t s could be c a r r i e d o u t o v e r a wide r a n g e of f r e q u e n c i e s , viz., 5 H z - 1 3 MHz. C a p a c i t a n c e measuring r a n g e w a s 0.1 p F - 100 m F with a basic a c c u r a c y of 0.1% o v e r t h e e n t i r e range. A t t h e l o w e r e n d of i t s f r e q u e n c y s p e c t r u m (up t o 10 KHz), t h e v a l u e of t h e s e t f r e q u e n c y could b e r e a d t o a n a c c u r a c y of 1 m H z while in t h e r a n g e of 10

MHz,

t o a n a c c u r a c y of 1 Hz. T h e o s c i l l a t o r l e v e l could b e s e t t o a s low a v a l u e a s 5 mV rms.

In o r d e r t o minimize t h e e f f e c t of p a r a s i t i c c a p a c i t a n c e

o n t h e m e a s u r e d values, a s u i t a b l e m e t h o d w a s a d o p t e d which basi- c a l l y involves r e p r e s e n t i n g all m e a s u r e m e n t e r r o r s a s f o u r r e s i d u a l p a r a m e t e r s R, X, G a n d B in t h e fashion ( 1 ) R + J X a n d (2) G+JB.

T h e f o r m e r w a s c o r r e c t e d by a " z e r o s h o r t " o p e r a t i o n while t h e l a t t e r with " z e r o open" o p e r a t i o n . T h e values t h u s o b t a i n e d in d i f f e - r e n t f r e q u e n c y r a n g e s w e r e s t o r e d in t h e m e m o r y of t h e i m p e d a n c e a n a l y s e r in d i f f e r e n t r e g i s t e r s and could be r e c a l l e d l a t e r . T h e recalling of t h e p a r a m e t e r s e n a b l e d a u t o m a t i c lead c a p a c i t a n c e c o r r e c t i o n during t h e m e a s u r e m e n t s .

Most of t h e work d e s c r i b e d in t h i s t h e s i s ( C h a p t e r s 111, IV, V I a n d VII) w e r e c a r r i e d o u t by c o l l e c t i n g t h e d a t a manually. How- e v e r in o r d e r t o i m p r o v e t h e quality of t h e d a t a and t o p e r f o r m s p e e d i e r and a u t o m a t i c m e a s u r e m e n t s in r e a l t i m e , a p e r s o n a l c o m p u t e r ( H e w l e t t - P a c k a r d 86B) w a s u s e d f o r t h e m e a s u r e m e n t s d e s c r i b e d in c h a p t e r V. S c h e m a t i c d i a g r a m of t h e e x p e r i m e n t a l s e t up used f o r t h e s e s t u d i e s is shown in Fig.2.8. T h e d i a g r a m is self - explanatory. T h e d a t a a c q u i r e d was s t o r e d on floppy d i s k e t t e s f o r analysis which could be c a r r i e d o u t l a t e r . A display m o n i t o r a n d a n on- line p l o t t e r provided t h e g r a p h i c s o u t p u t as a n d w h e n t h e i n f o r m a t i o n w a s being c o l l e c t e d .

2.7 MEASUREMENT OF DIELECTRIC CONSTANTS T h e t e m p e r a t u r e d e p e n d e n c e of

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I I -

F i g u r e 2 . 8 . S c h e m a t i c d i a g r a m o f t h e e x p e r i m e n t a l s e t u p u s e d f o r t h e d i e l e c t r i c s t u d i e s .

TEMP.

READ OUT DMM

l E E E 4 8 8 B U S

1 I

P OW E R SUPPLY

POWER SUPPLY READ OUT

DMM

.

A

HP 4 1 9 2 A

at 1 K H z , t h i s f r e q u e n c y being chosen s i n c e i t is m u c h below t h e f r e q u e n c i e s of a b s o r p t i o n d o m a i n s a n d h i g h e r t h a n t h e f r e q u e n - c i e s at which t h e c o n d u c t i v i t y m e c h a n i s m b e c o m e s d o m i n a n t . A s

m e n t i o n e d e a r l i e r , a slow a n d uniform r a t e of h e a t i n g / c o o l i n g (-4-5"C/hr.) w a s m a i n t a i n e d during t h e m e a s u r e m e n t . T h e o s c i l l a t o r l e v e l of t h e m e a s u r i n g v o l t a g e a c r o s s t h e s a m p l e w a s k e p t v e r y low ( 300 mV) s o t h a t t h e r e w a s no d i s t u r b i n g i n f l u e n c e o n t h e sample. T h e v a l u e of t h e d i e l e c t r i c c o n s t a n t in t h e i s o t r o p i c p h a s e o b t a i n e d independently f r o m t h e t w o g e o m e t r i e s m a t c h e d t o within 1%.

2.8 MEASUREMENT OF THE DIELECTRIC RELAXATION OF E

I I

It is well known t h a t t h e d i e l e c t r i c c o n s t a n t s of liquid c r y s t a l s a r e f r e q u e n c y dependent.9 While ^{€ 1} e x h i b i t s a r e l a x a t i o n in t h e m i c r o w a v e r e g i o n only,

11

^{shows a} r e l a x a t i o n in t h e low f r e q u e n c y region also. T h e low f r e q u e n c y r e l a x a t i o n m e c h a n i s m c a n b e a t t r i - b u t e d t o t h e r e o r i e n t a t i o n m o t i o n of t h e m o l e c u l e s a r o u n d a n a x i s p e r p e n d i c u l a r t o t h e i r longitudinal axis.

T h e low f r e q u e n c y dispersion of

11

w a s m e a s u r e d using t h e s e t up d e s c r i b e d e a r l i e r . At any t e m p e r a t u r e ( m a i n t a i n e d con-

I I 1 I

s t a n t t o within + 2 5 m K ) t h e v a l u e s ^Ell a n d ^E ( = E x D . D b e i n g d i e l e c t r i c loss f a c t o r ) w e r e d e t e r m i n e d a s f u n c t i o n of f r e q u e n c y .

I I

E v e r s u s f r e q u e n c y c u r v e (known a s loss c u r v e ) g a v e t h e f r e q u e n c y o f r e l a x a t i o n f R ( t h e f r e q u e n c y corresponding t o t h e ^E"

I I

^{m a x ):}

1 11

A

-

p l o t , known as t h e Cole- Cole p l o t a l s o g i v e s f R v a l u e a n d i n d i c a t e s t h e n a t u r e of t h e relaxation. A s e m i c i r c l e p l o t

I

w i t h c e n t r e lying on t h e

E

I 1

a x i s d e n o t e s a s i n g l e r e l a x a t i o n t i m e while a n y d e v i a t i o n i n d i c a t e s a distribution of r e l a x a t i o n t i m e s . In all t h e e x p e r i m e n t s d e s c r i b e d in this thesis, t h e v a l u e of f

R o b t a i n e d f r o m t h e Cole- Cole p l o t a g r e e d with t h a t d e t e r m i n e d f r o m t h e loss c u r v e t o b e t t e r t h a n 2%. In a n y case t h e a v e r a g e value of f R ( d e t e r m i n e d f r o m t h e s e t w o t y p e s of p l o t s ) w a s u s e d f o r t h e e v a l u a t i o n of W, t h e a c t i v a t i o n e n e r g y which c a n b e asso- c i a t e d with t h e r e o r i e n t a t i o n of t h e molecule a b o u t i t s s h o r t axis.

T h e s e t up d e s c r i b e d s o f a r has been used t o s t u d y t h e d i e l e c - t r i c p r o p e r t i e s of a v a r i e t y of liquid c r y s t a l l i n e m a t e r i a l s . T h e r e s u l t s of t h e s e i n v e s t i g a t i o n s will be discussed in t h e f o l l o w i n g c h a p t e r s .

REFERENCES

1 B.R.Ratna, " D i e l e c t r i c P r o p e r t i e s a n d S h o r t r a n g e O r d e r i n Liquid Crystals" , Ph.D.Thesis, U n i v e r s i t y of M y s o r e (1978).

2 G. W.Gray, K.J.Harrison, J.A.Nash, J . C o n s t a n t , D.S.Hulme, J.Kirton a n d E.P.Raynes, "Liquid C r y s t a l s a n d O r d e r e d Fluids", Vo1.2, Eds. J.F.Johnson a n d R.S.Porter ( P l e n u m Press: N e w Y o r k , London, 1973) p.617.

3 R.Eidenschink, D.Erdmann, J.Krause, L.Poh1, Angew. C h e m . 90,

1 3 3 (1978).

5 A. Gobl- Wunsch, G. H e p p k e and R. Hopf, 2. N a t u r f o r s c h . 3 6 a , 2 1 3 (1981).

6 B.K.Sadashiva, G.S.R. Subba Rao, C u r r e n t S c i e n c e , 4 4 , 222 (1 975).

7 Nguyen Huu Tinh, C. D e s t r a d e , Mol. C r y s t . Liq. C r y s t . L e t t . 92, 257 (1984).

8 P. G. d e Gennes, " T h e P h y s i c s of Liquid C r y s t a l s " , ( C l a r e n d o n P r e s s , Oxford, 1974).

9 Von W.Maier and G. Meier, Z.Naturforsch. 1 6 a , 1200 (1961).

10 N.E.Hil1, \V.E.Vaughan, A.H.Price, M.Davies, " D i e l e c t r i c P r o p e r t i e s and Molecular Behaviour" (Van N o s t r a n d Reinhold Company: London, N e w York, T o r o n t o , M e l b o u r n e , 1969).

11 C.J.F.Bottcher a n d P.Bordewijk, " Theory of E l e c t r i c P o l a r i z a - tion", Vol.11 ( E l s e v i e r S c i e n t i f i c Publishing C o m p a n y : A m s t e r - d a m , Oxford, New York, 1978).