Ordering in Liquid Crystals Owing to Electric and Magnetic Fields

8

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 16, 2014 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0063.ch008

Ordering i n L i q u i d Crystals Owing to Electric and Magnetic Fields EDWARD F. CARR Physics Department, University of Maine, Orono, Maine

A great deal of work has been carried out using electric fields to align molecules in the anisotropic liquid phase, and the results have often been conflicting. Results are shown for anisal-p -aminoazobenzene, which exhibits positive dielectric anisotropy, and for p-azoxyanisole, which exhibits negative dielectric anisot ropy. The molecular alignment was investigated in the presence of external electric and magnetic fields acting individually and also simultaneously. Measurements of the dielectric loss at a microwave frequency were used to indicate the extent of the order ing. The results indicate the existence of a process causing molecular alignment which depends on the electric or magnetic field but is not associated with either the anisotropy in the di electric constant or the permeability.

H P h e effect of a n e x t e r n a l l y a p p l i e d electric field o n t h e a l i g n m e n t of molecules i n l i q u i d c r y s t a l s has been s t u d i e d b y m a n y i n v e s t i g a t o r s , a n d t h e results h a v e f r e q u e n t l y been c o n f l i c t i n g .

T h e literature regarding

t h e effect o w i n g t o electric fields has been r e v i e w e d r e c e n t l y b y G r a y (8). T h i s p a p e r is concerned w i t h t h e l i q u i d c r y s t a l s , p - a z o x y a n i s o l e a n d anisal-p-aminoazobenzene. T h e s t r u c t u r a l f o r m u l a s a r e :

COMPOUND Anisol - ρ ominoazobenzene

ρ - Azoxyanisole

STRUCTURE

C

H

3~ ~O~Ç

= Ν - φ - Ν

0

=

Ν - φ

Η

C

H

3 ~

0

" O "

N

s

N

O " ° "

C

H

3

δ

I n t h e presence of e x t e r n a l m a g n e t i c fields b o t h substances show a m o l e c u l a r a l i g n m e n t , w i t h t h e l o n g axes of t h e molecules p r e f e r r i n g a d i r e c t i o n 76 In Ordered Fluids and Liquid Crystals; Porter, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

8.

Ordering

CARR

parallel to the

field.

in

77

Crystals

M e a s u r e m e n t s b y Z w e t k o f f a n d M a r i n i n (26) i n v o l v

i n g t h e K e r r effect i n d i c a t e d t h a t i n anisal-p-aminoazobenzene a n a l i g n m e n t s h o u l d exist, w i t h t h e l o n g axes of t h e molecules p r e f e r r i n g a d i r e c t i o n p a r a l l e l to a n e x t e r n a l l y a p p l i e d electric

field.

M e a s u r e m e n t s (S) w h i c h i n

v o l v e m i c r o w a v e d i e l e c t r i c techniques agree w i t h t h e i r w o r k as t o t h e d i r e c t i o n of t h e m o l e c u l a r axes r e l a t i v e to a n e x t e r n a l electric

field.

for p - a z o x y a n i s o l e i n a s t a t i c electric field h a v e been c o n f l i c t i n g .

T h e results Since the

l o w frequency dielectric constant (15) is greatest p e r p e n d i c u l a r t o t h e l o n g Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 16, 2014 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0063.ch008

axes of t h e molecules, one w o u l d expect a n a l i g n m e n t p e r p e n d i c u l a r t o t h e electric

field.

S o m e observers agreed w i t h t h i s p r e d i c t i o n , b u t K a s t

(14)

a n d l a t e r other investigators r e p o r t e d a l i g n m e n t s w i t h t h e l o n g m o l e c u l a r axes p r e f e r r i n g a d i r e c t i o n p a r a l l e l to t h e

field.

I t is generally agreed t h a t

for frequencies of a p p r o x i m a t e l y 0.5 M c . t h e o r i e n t a t i o n of t h e l o n g m o l e c u l a r axes w i l l be p e r p e n d i c u l a r t o t h e field. Exp erim

ental

E x c e p t for a few d e t a i l s , t h e e x p e r i m e n t a l techniques used for t h e w o r k discussed h a v e been described (1, 4) δ), so o n l y a few c o m m e n t s are m a d e here. T h e cell is a section of i i - b a n d w a v e g u i d e as i l l u s t r a t e d i n F i g u r e 1 A. F i g u r e 1, Β a n d C., shows e n d v i e w s of t h e cell w i t h a center p l a t e e l e c t r i c a l l y i n s u l a t e d f r o m t h e guide. T h e cell is d i v i d e d i n t o t w o sections, each c o n t a i n i n g t h e same a m o u n t of s a m p l e . A s t h e m i c r o w a v e b e a m enters t h e

Figure 1. A. B. C.

Diagram of cell

Wave guide for sample cell Magnetic field parallel to polarized microwave electric field Magnetic field perpendicular to polarized microwave electric field

cell, i t d i v i d e s i n s u c h a w a y t h a t we h a v e a p o l a r i z e d w a v e w i t h t h e m i c r o w a v e electric field p e r p e n d i c u l a r t o t h e w i d e side of t h e guide i n each sec t i o n . A n e x t e r n a l a.c. electric field, a p p l i e d b e t w e e n t h e center p l a t e a n d t h e w a v e guide, produces a n e x t e r n a l electric field p a r a l l e l t o t h e m i c r o w a v e

In Ordered Fluids and Liquid Crystals; Porter, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.


78

ORDERED FLUIDS A N D LIQUID CRYSTALS

electric field. T h e a r r a n g e m e n t s h o w n i n F i g u r e 1, B, p r o v i d e s for a n ex t e r n a l m a g n e t i c field p a r a l l e l t o t h e e x t e r n a l electric field w h i l e F i g u r e 1, C., shows t h e a r r a n g e m e n t for a m a g n e t i c field p e r p e n d i c u l a r t o t h e electric field. T h e center p l a t e a n d T e f l o n i n s u l a t i n g s t r i p s w i l l p r o d u c e some m i s m a t c h i n t h e guide, b u t t h i s is n o t serious since o n l y r e l a t i v e measurements of t h e t r a n s m i t t e d p o w e r at a f r e q u e n c y of 24 k M c . were recorded w i t h t h i s arrangement. F o r measurements i n a n i s a l - p - a m i n o a z o b e n z e n e t h e m a g n e t i c field w a s v a r i e d f r o m 500 t o 3000 gauss, a n d for p - a z o x y a n i s o l e o n l y v a l u e s of 500 a n d 1000 gauss were used. A l t h o u g h m a g n e t i c fields a b o v e 500 gauss m a y p r o d u c e a m o l e c u l a r a l i g n m e n t w h i c h is s l i g h t l y m o r e complete t h a n a field of 500 gauss, t h e change d i d n o t appear to affect t h e results discussed i n this article appreciably. T h e samples of a n i s a l - p - a m i n o a z o b e n z e n e were o b t a i n e d c o m m e r c i a l l y a n d were p u r i f i e d b y d i s s o l v i n g i n h o t m e t h a n o l , followed b y filtration a n d r e c r y s t a l l i z a t i o n . T h e c l e a r i n g p o i n t was 185°C. T h e samples of p - a z o x y a n i s o l e were also o b t a i n e d c o m m e r c i a l l y a n d p u r i f i e d b y r e c r y s t a l l i z a tion. Anisal-p-aminoazobenzene Dielectric Loss in the Presence of an External Magnetic Field. F i g u r e 2 shows t h e t e m p e r a t u r e dependence of t h e dielectric loss at a m i c r o w a v e frequency of 24 k M c . (3). T h e u p p e r c u r v e represents t h e loss i n a field of 2800 gauss p e r p e n d i c u l a r to the m i c r o w a v e electric field, hence a m o l e c u l a r

.,1 150

ι

I 160

ι

I

I

170

180

TEMPERATURE

ι

I 190

ι

I 200

- ° C

Figure 2. Temperature dependence of dielectric loss for anisal-p-aminoazobenzene at a frequency of 24 kMc. External magneticfieldof 2800 gauss applied parallel and perpendicular to microwave electric field

a l i g n m e n t w i t h t h e l o n g axes of t h e molecules p r e f e r r i n g a d i r e c t i o n p e r p e n d i c u l a r to t h e electric field. T h e lower c u r v e shows t h e loss w i t h t h e ex t e r n a l field p a r a l l e l to the m i c r o w a v e electric field, hence a m o l e c u l a r a l i g n m e n t p a r a l l e l to t h e electric field. I n c r e a s i n g t h e m a g n e t i c field to v a l u e s greater t h a n 2800 gauss d i d n o t produce a noticeable change i n t h e d i e l e c -


8.

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Ordering

79

in Crystals

Molecular Alignment for Electric and Magnetic Fields Parallel to Each Other. I n F i g u r e 1, B, b o t h t h e electric a n d m a g n e t i c fields p r o d u c e a l i g n m e n t s of the molecules w i t h t h e l o n g axes p a r a l l e l t o t h e m i c r o w a v e electric field. W h e n a 370-kc. electric field of 1100 v o l t s per c m . was replaced b y a d.c. m a g n e t i c field of 3000 gauss, n o change i n t h e dielectric loss was d e tectable. A change i n t h e dielectric loss of a p p r o x i m a t e l y 0 . 5 % of t h e d i f ference for t h e p a r a l l e l a n d p e r p e n d i c u l a r o r i e n t a t i o n s r e l a t i v e t o t h e m i c r o w a v e electric field s h o u l d be detectable w i t h t h i s arrangement. F o r electric fields of frequencies 60 c.p.s. a n d 1.7 M c . t h e m o l e c u l a r a l i g n m e n t was not as complete as i n t h e presence of a m a g n e t i c field. T h i s w o r k indicates t h a t t h e degree of m o l e c u l a r a l i g n m e n t w h i c h c a n be o b t a i n e d w i t h m a g n e t i c fields is t h e same as w i t h electric fields if t h e p r o p e r frequency is chosen. I f one assumes t h a t t h e a l i g n m e n t is p r i m a r i l y caused b y t h e a n i s o t r o p y i n t h e l o w - f r e q u e n c y dielectric constant a n d t h e p e r m e a b i l i t y , t h i s w o r k also shows t h a t t h e d i r e c t i o n f o r t h e m a x i m u m v a l u e o f t h e l o w frequency d i e l e c t r i c constant is p a r a l l e l t o t h e d i r e c t i o n for w h i c h t h e absolute v a l u e o f t h e d i a m a g n e t i c s u s e p t a b i l i t y is a m i n i m u m i n a n ordered sample. • .

1

1

r- M O L E C U L A R ALIGNMENT \ E L E C T R I C FIEl.D

'ERPENDIC ULAR

TO »MICROWAVE

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80


Molecular Alignment in Magnetic and Electric Fields at Right Angles to Each Other. F i g u r e 3 (4) is a p l o t of t h e dielectric loss, e", as a f u n c t i o n of t h e e x t e r n a l l y a p p l i e d electric fields f o r v a r i o u s c o n s t a n t v a l u e s of t h e steady m a g n e t i c field. T h e frequency of t h e e x t e r n a l electric field w a s 370 kC., a n d 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 155°C. T h e m a x i m u m v a l u e of e" = 0.40 (upper d a s h e d line) for each of t h e m a g n e t i c fields a t l o w electric fields represents a n a l i g n m e n t of t h e molecules p e r p e n d i c u l a r t o the m i c r o w a v e electric field a n d hence p a r a l l e l t o t h e m a g n e t i c field. F o r sufficiently l a r g e e x t e r n a l l y a p p l i e d e l e c t r i c fields, t h e m i n i m u m v a l u e of e" = 0.2 (lowest d a s h e d line) i s a p p r o a c h e d , i n d i c a t i n g a n a l i g n m e n t p a r a l l e l t o t h e m i c r o w a v e electric field a n d therefore p a r a l l e l t o t h e e x t e r n a l l y a p p l i e d electric field. F i g u r e 3 (4) shows t h a t f o r c e r t a i n v a l u e s of t h e e x t e r n a l electric field s m a l l changes i n t h e electric field w i l l p r o d u c e m a r k e d changes i n t h e a l i g n m e n t of t h e molecules. E l e c t r i c a n d m a g n e t i c fields were c o m p a r e d , f o r p r o d u c i n g m o l e c u l a r a l i g n m e n t , a t a dielectric loss of e" = 0.30. T h i s c o r responds t o a r a n d o m o r i e n t a t i o n of t h e molecules i n t h e p l a n e of Ε a n d H. T a b l e I shows t h e r a t i o s of t h e electric field t o t h e m a g n e t i c field f o r d i f ferent values of t h e magnetic field. T h i s r a t i o does n o t a l l o w for a n y effect Table I. Ratio of Electric Field to Magnetic Field for Random Orientation of Molecules in Plane of Ε and Η with a 370-Kc. Electric F i e l d Τ = 175°C.

Τ = 155°C. Ε Η, gauss 500 1000 2000 3000

Η,

Ε

V cm. gauss 0.374 0.377 0.374 0.377

Η, gauss 500 1000 2000 3000

H t

V cm. gauss 0.374 0.380 0.382 0.382

o w i n g t o w a l l s , t e m p e r a t u r e gradients, o r o t h e r a l i g n i n g processes t h a t m i g h t b e present w h i c h d o n o t i n v o l v e t h e electric o r m a g n e t i c field. E/H for Τ = 155°C. i s constant w i t h i n t h e l i m i t s of t h e e x p e r i m e n t a l error. E/H f o r Τ = 175°C. appears t o be s l i g h t l y lower t h a n f o r 155°C., b u t t h i s difference c o u l d be caused b y e x p e r i m e n t a l error. M e a s u r e m e n t s s i m i l a r t o those i l l u s t r a t e d i n F i g u r e 3 were also m a d e w i t h a 60-c.p.s. e x t e r n a l electric field. T h e s e results were s i m i l a r t o those i n F i g u r e 3, a n d t h e ratios of E/H agreed w i t h those g i v e n i n T a b l e I w i t h i n t h e l i m i t s of e x p e r i m e n t a l error. A f t e r t h e sample h a d been used a p p r o x i m a t e l y 10 hours a t 155°C. a n d 2 h o u r s a t 175°C., i t was h e a t e d t o a p p r o x i m a t e l y 180°C. a n d left f o r a b o u t 15 hours. F i g u r e 4 (4) illustrates t h e m o l e c u l a r a l i g n m e n t i n t h e presence of a 370-kc./second a n d a 60-c.p.s. electric field. T a b l e II shows t h e ratios of E/H f o r e" = 0.30. T h e s e ratios for t h e 370 k c . / s e c o n d field are consistent w i t h those s h o w n i n T a b l e I, b u t those



CARR

Ordering

in

Crystals

Figure 4- Dielectric loss in a decomposed sample of anisal-p-amino azobenzene at a microwave frequency of 24 kMc. as a function of an externally applied a.c. electric field Upper. Frequency of electricfield370 kc. Lower. Frequency 60 c.p.s. Individual curves are for different values of magneticfieldapplied per pendicular to external electric field Temperature 156°C.

Table II. Ratio of Electric F i e l d to Magnetic F i e l d for Random Orientation of Molecules i n Plane of Ε and Η at 155°C. 60 C.p.s.

370 Kc. H, gauss 1000 2000 3000

E_

V

Η,

cm. gauss 0.373 0.373 0.375

Ε Η, gauss 1000 2000 3000

Η ,

V cm. gauss 0.349 0.338 0.338



82


for t h e 60-c.p.s. field are n o t i c e a b l y lower. Since the s a m p l e is a p p r e c i a b l y decomposed, t h e results do n o t h a v e m u c h significance except to i l l u s t r a t e t h e effect of d e c o m p o s i t i o n . T h e l o w e r v a l u e s for the 60-c.p.s. field i m p l y t h a t fields of t h i s frequency are m o r e effective i n p r o d u c i n g a m o l e c u l a r a l i g n m e n t t h a n fields of m u c h h i g h e r frequencies. T h i s seems to be c o n sistent w i t h some of t h e results o n l i q u i d c r y s t a l s w i t h a n e g a t i v e d i e l e c t r i c a n i s o t r o p y , i n t h a t l o w frequencies seem to be more effective i n p r o d u c i n g a n a l i g n m e n t w i t h t h e l o n g axes of the molecules p r e f e r r i n g a d i r e c t i o n p a r a l l e l to t h e field. T h e change i n t h e m o l e c u l a r a l i g n m e n t is n o t as s h a r p for t h e 60-c.p.s. field as for t h e 370 k c . / s e c o n d field, w h i c h indicates t h e presence of other disturbances i n t h e sample. I f i t is assumed t h a t t h e a l i g n m e n t of t h e molecules is caused o n l y b y t h e anisotropies i n t h e low-frequency dielectric constant a n d t h e p e r m e a bility, the following relation should hold : (€„' -

e ')E ±

2

= (μ„ -

(1)

μ )Η ±

2

where en' a n d μ H are t h e dielectric constant a n d p e r m e a b i l i t y , r e s p e c t i v e l y , p a r a l l e l to the axis of s y m m e t r y w h e n t h e m a x i m u m m o l e c u l a r a l i g n m e n t has been a t t a i n e d . e± a n d μ are the dielectric constant a n d p e r m e a b i l i t y p e r p e n d i c u l a r to t h e axis of s y m m e t r y . U s i n g the v a l u e of (μ — μ±) at 155°C. g i v e n b y Z w e t k o f f a n d S o s n o v s k y (27) a n d d e n s i t y measurements b y P o r t e r a n d J o h n s o n (19), one o b t a i n s t h e v a l u e of 1.09 for (en' — e±). ( « ι ι ' — e±) at 6 k M c . is 0.48 a n d decreasing w i t h frequency, w h i c h i m p l i e s t h a t t h e a n i s o t r o p y i n the dielectric constant is not sufficient to e x p l a i n t h e ratios of E/H o b t a i n e d i n t h i s w o r k . H o w e v e r , a crude check o n t h e l o w frequency dielectric constant has i n d i c a t e d t h a t (en' — e±') is greater t h a n 0.48. S i n c e there appear to be no accurate measurements of t h e l o w - f r e q u e n c y dielectric constant for t h i s m a t e r i a l , a n d the r a t i o of E/H appears to be constant, one c a n w r i t e as a first a p p r o x i m a t i o n ±

Μ

[C + ( « „ ' -

ei')]E = 2

[μ

η

-μ±]Η

2

(2)

where c is a constant t h a t p o s s i b l y m i g h t be associated w i t h the a n i s o t r o p y i n t h e c o n d u c t i v i t y . T h e v a l u e of c m a y be a v e r y s m a l l n u m b e r o r even zero for a v e r y pure c o m p o u n d , b u t w o u l d be s o m e t h i n g o t h e r t h a n zero for a decomposed sample. R e c e n t measurements w i t h techniques i d e n t i c a l t o those m e n t i o n e d i n t h i s article h a v e been m a d e b y T w i t c h e l l a n d C a r r (23) i n p-azoxyanisole w i t h a 500-c.p.s. e x t e r n a l electric field. E q u a t i o n 2 d e scribes t h e b e h a v i o r w e l l for h i g h fields, a n d t h e v a l u e of c was f o u n d to be m u c h larger t h a n t h e difference i n t h e l o w - f r e q u e n c y d i e l e c t r i c constants. p-Azoxyanisole Molecular Alignment in A . C . Electric and Static Magnetic Fields Parallel to Each Other. F r e e d e r i c k s z a n d Z w e t k o f f (7) h a v e i n v e s t i g a t e d the o r d e r i n g i n the anisotropic l i q u i d phase of p-azoxyanisole u s i n g o p t i c a l


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techniques a n d c o m p a r e d t h e effectiveness of electric a n d m a g n e t i c fields. T h e i r i n v e s t i g a t i o n was c a r r i e d o u t w i t h static m a g n e t i c fields a n d electric fields w i t h frequencies greater t h a n 300 k c . / s e c o n d . F o r g i v e n values of the m a g n e t i c field t h e y o b t a i n e d v a l u e s of t h e electric field i n t e n s i t y w h i c h w o u l d be e q u i v a l e n t t o t h e m a g n e t i c fields f o r p r o d u c i n g m o l e c u l a r a l i g n m e n t . T h e s e results i n d i c a t e d t h a t t h e r a t i o of t h e electric field t o t h e m a g netic field w a s independent of t h e m a g n e t i c field b u t d i d v a r y w i t h t e m perature. F i g u r e 5 is a p l o t of t h e dielectric loss,e", as a f u n c t i o n of the e x t e r n a l l y a p p l i e d electric field f o r v a r i o u s constant v a l u e s of t h e steady m a g n e t i c field. T h e frequency of t h e e x t e r n a l field was 370 k c . / s e c o n d , a n d t h e t e m perature of t h e sample was 132°C. T h e e x p e r i m e n t a l arrangement for these .68

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Ordering in Liquid Crystals Owing to Electric and Magnetic Fields

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