ULTRASONIC PROPAGATION IN THE LIQUID CRYSTAL PHASES OF P-ALKOXYBENZOIC ACIDS

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Submitted on 1 Jan 1981

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ULTRASONIC PROPAGATION IN THE LIQUID

CRYSTAL PHASES OF P-ALKOXYBENZOIC ACIDS

L. Chou, S. Letcher

To cite this version:

JOURNAL

CoZZoque C S , suppZ6ment au nO1O, Tome 42, octobre 1981 page c5-587

ULTRASONIC PROPAGATION IN THE

LIQUID

CRYSTAL PHASES OF P-ALKOXYBENZOIC

A C I D S "

L.S.

S.V.

etcher*

I n s t i t u t e o f Electro-opticaZ Engineering, NationaZ Chiao-Tung, University Hsinchu, Taiwan, China

*i?epartment of Physics, University of Rhode Island, Kingston, Rhode IsZand 02881, U.S.A.

Abstract.- Ultrasonic absorption has been measured in p-alkoxy- benzoic acids in the isotropic, nematic and smectic phases in the the frequency range 9-85 M H z . The frequency and temperature dependence of the absorption indicates the presence of relaxation processes, which are attributed to rearrangements of the

equilibrium distributions of open dimers and cyclic dimers. The relaxation frequency is about 15 Mtiz in the nematic phase.

1. Introduction.- Recent investigations1-4 have shown that the p-alkoxy -benzoic acids have interesting structural and physical properties. They exhibit a nematic liquid crystal phase and the higher homologs also have a smectic phase. Depending on the temperature and thermal history, they can apparently exist as mixtures of various compositions of monomer, open dimer, closed dimer and (perhaps) polymer.

We have used the variable-path, pulse-transition method to measure the ultrasonic attenuation in p-heptyloxybenzoic acid, which exhibits both nematic ( 9 8 - 1 4 6 ~ ~ ) and smectic-C (92-98°~) phases. Molecular alignement was achieved in the nematic phase with a magnetic field of 4 kOe. The samples were obtained from Frinton Laboratories and were purified by recrystallization and zone refining. During the experiment.

(about four weeks), the sample was covered with high purity argon gas. All measurements were made after the sample had been heated to the isotropic liquid phase (usually about 1 6 0 ~ ~ ) and then cooled to the temperature of interest. In order to reduce spurious effects caused by dissolved acetone that may have survived the zonemelting process. Acetone is known to disrupt hydrogen bonds in p-nonyloxybenzoic acids. The theory of the nematic phase of Forster et al.' results in an expression for the sound absorption given by

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This work is supported by the National Science ~aundation.

wave a n d t h e b r e a k i n g o f h y d r o g e n b o n d s .

R e f e r e n c e s :

1. A.P. K a p u s t i n a n d N.T. B y k o v a , S o v . P h y s . C r y S t . 1 3 , 2 8 1 ( 1 9 6 8 ) . 2. B. D e l o c h e a n d B. C a b a n e , Mol. C r y s t . L i q . C r y s t . 1 9 , 2 5 ( 1 9 7 2 ) . 3 . L.S. Chou a n d E . F . C a r r , p h y s . Rev. A7, 1 6 3 9 ( 1 9 7 3 ) .

4 . L.S. Chou a n d E . F . C a r r , O r d e r e d F l u i d s a n d L i q u i d C r y s t a l s , 2 , 39 ( 1 9 7 4 ) . 5 . D . F o r s t e r , T.C. L u b e n s k y , P.C. M a r t i n , J . S w i f t a n d P.S. P e r s h a n , P h y s . Rev. L e t t e r s 2 6 , 1 0 1 6 ( 1 9 7 1 ) . 6 . G.C. W e t s e l , R.S. S p e e r , B.A. Lowry a n d M.R. W o o d a r d , J . A p p l . P h y s . 4 3 , 1 4 9 5 ( 1 9 7 2 ) . 7 . K . A . Kemp a n d S.V. L e t c h e r , O r d e r e d F l u i d s a n d L i q u i d C r y s t a l s , 2 , 3 5 1 ( 1 9 7 4 ) .

8. B.A. L o w r y , M.R. W o o d a r d , S.E. M o n r o e , a n d G . C . W e t s e l , B u l l . Amer. P h y s . S o c . 1 7 , 3 5 0 ( 1 9 7 2 ) .

9. A . A z i m a , C.W. B r o w n , a n d S.S. M i t r a ( P r i v a t e c o m m u n i c a t i o n s ) . 1 0 . S. N a g a i , P. ~ a r t i n o t y , S . C a n d a u a n d R.Zana, m o l . C r y s t . a n d L i q u i d