VIBRATIONAL DENSITY OF STATES OF AMORPHOUS AND CRYSTALLINE LIQUID CRYSTAL SUBSTANCES

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VIBRATIONAL DENSITY OF STATES OF

AMORPHOUS AND CRYSTALLINE LIQUID

CRYSTAL SUBSTANCES

A. Bielushkin, I. Natkaniec, V. Dolganov, E. Sheka

To cite this version:

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JOURNAL DE PHYSIQUE

CoZZoque C6, supp2dment au n012, Tome 42, ddcembre 1981

V I B R A T I O N A L D E N S I T Y OF STATES OF AMORPHOUS AND CRYSTALLINE L I Q U I D CRYSTAL SUBSTANCES

A.V. B i e l u s h k i n , I. ~ a t k a n i e c * , V.K. ~ o l ~ a n o v * a n d E.F. ~ h e k a *

Joint I n s t i t u t ~ ; for NLlcZear Research, 141980, Eubna, USSR

* ~ n s t i t u t e of Solid State Physics, Acad. of Sciences of the USSR, 142432 Chernogo Zovka, USSR

A b s t r a c t .

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We h a v e measured by t h e TOF n e u t r o n s p e c t r o s c o p y method t h e am- p l i t u d e - w e i g h t e d f r e q u e n c y d i s t r i b u t i o n f u n c t i o n o f t h e g l a s s y a n d c r y s t a l l i n e p h a s e s o f t h e EBBA, MBBA a n d PAA compounds. E v i d e n t c h a n g e s i n t h e v i b r a t i o - n a l d e n s i t y o f s t a t e s i n t h e f r e q u e n c y r a n g e up t o 300 cm-l a r e o b s e r v e d when p a s s i n g from t h e amorphous t o o r d e r e d c r y s t a l l i n e p h a s e s . The p a r t l y d e u t e r a - t e d d6-PAA s p e c t r u m h a v e shown t h a t t h e s e v i b r a t i o n s a r e due t o t r a n s l a t i o n a l and l i b r a t i o n a l m o t i o n s o f t h e whole m o l e c u l e a n d d u e t o low f r e q u e n c y o s c i l - l a t i o n s o f t h e end g r o u p s o f t h e m o l e c u l e .

L i q u i d c r y s t a l (LC) s u b s t a n c e s d e p e n d e n t o n t h e i r t h e r m a l h i s t o r y may form d i f f e r e n t s o l i d m o d i f i c a t i o n s . The PAA, MBBA a n d EBBA compounds a r e t h e most f r e q u e n t l y s t u d i e d LC s u b s t a n c e s and t h e y are known t o h a v e d i f f e r e n t p o l i m o r p h i c c r y s t a l l i n e p h a s e s ,

s e e r e f . ( l - 3 ) a n d r e f s t h e r e i n

.

By t h e f a s t c o o l i n g o f t h e mesophase o f t h e s e s u b - s t a n c e s a g l a s s y s t a t e (GLC) i s formed. The X-ray and n e u t r o n d i f f r a c t i o n methods a s well a s o p t i c a l s p e c t r o s c o p y o n e s were u s e d t o i n v e s t i g a t e t h e s e GLC I t h a s been shown t h a t t h e m o l e c u l a r s t r u c t u r e a n d o r d e r p a r a m e t e r o f t h e GLC a r e s i m i - l a r a s i n t h e n e m a t i c p h a s e o f LC. The GLC s t a t e is m e t a s t a b l e a t low t e m p e r a t u r e s a n d may b e t r a n s f o r m e d i n t o t h e o r d e r e d c r y s t a l l i n e p h a s e s w i t h o u t t h e m e l t i n g o f t h e s u b s t a n c e . T h i s t r a n s f o r m a t i o n c a n b e s t o p p e d by t h e l o w e r i n g o f t h e t e m p e r a t u r e . So, a n y i n t e r m e d i a t e p a r t l y o r d e r e d s t a t e may b e s t u d i e d . S i n c e t h e GLC s y s t e m s e x i s t a t low t e m p e r a t u r e s , where t h e a n h a r m o n i c e f f e c t s a n d m u l t i p h o n o n s c a t t e r i n g p r o c e s - s e s a r e s t r o n g l y r e d u c e d , t h e y a r e more s u i t a b l e f o r d y n a m i c a l s t u d i e s , , t h a n LC sys- tems. Here we p r e s e n t t h e a m p l i t u d e - w e i g h t e d f r e q u e n c y d i s t r i b u t i o n f u n c t i o n G _H( & ) ( 7 ) o f t h e GLC a n d c r y s t a l l i n e states o f t h e most p o p u l a r LC s u b s t a n c e s .

The i n e l a s t i c i n c o h e r e n t n e u t r o n s c a t t e r i n g ( I I N S ) s p e c t r a h a v e b e e n measured u s i n g t h e KDSOG i n v e r t e d geometry t i m e - o f - f l i g h t s p e c t r o m e t e r a t t h e IBK p u l s e d re- a c t o r o f t h e JINR ~ u b n a ' ~ ) . The s a m p l e s of EBBA, MBBA and PAA were p o u r e d i n t h e aluminum c o n t a i n e r s o f a b o u t 0 . 8

mm

t h i c k and w i t h t h e 1 8 0 x 1 6 0 mm r e c t a n g u l a r c r o s s s e c t i o n . The GLC p h a s e s were p r o d u c e d by t h e d i p i n l i q u i d n i t r o g e n o f t h e c o n t a i n e r p l a c e d a t t h e bottom o f t h e h e l i u m c r y o s t a t . The IINS s p e c t r a have been measured a t 5 K a t s e v e n s c a t t e r i n q a n g l e s from 30' t o 150' e v e r y 20' s i m u l t a n e o u s l y . R e c e n t l y

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some o f t h e IINS s p e c t r a from t h e MBBA s a m p l e were p u b l i s h e d

.

The measured IINS s p e c t r a h a v e b e e n t r a n s f o r m e d i n t o t h e GH(E) f u n c t i o n a c c o r - d i n g t o t h e one-phonon i n c o h e r e n t s c a t t e r i n g c r o s s s e c t i o n f o r m u l a . Such approxima- t i o n i s j u s t i f i e d by t h e f a c t t h a t IINS s p e c t r a o f m o l e c u l a r c r y s t a l s below 300 cm-1 X

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measured a t 5 K agree w e l l w i t h c a l c u l a t e d ones (10711). The r e s o l u t i o n f u n c t i o n o f t h e KDSOG spectrometer i n t h e mentioned frequency range does n o t d i s t u r b s i g n i f i c a n t - l y t h e GH(&) f u n c t i o n . The s i n g u l a r i t i e s o f t h i s f u n c t i o n correspond t o those o f t h e f u n c t i o n o f phonon d e n s i t y o f s t a t e s (10-12). So, we b e l i e v e t h a t t h e s p e c t r a p r e - sented h e r e r e f l e c t t h e behaviour o f t h e v i b r a t i o n a l d e n s i t y o f s t a t e s i n d i f f e r e n t phases o f t h e LC substances.

Fig.1: The hydrogen-amplitude weighted d e n s i t y o f s t a t e s

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_{GH(E) f u n c t i o n s o b t a i n e d}

.

from t h e I I N S s p e c t r a o f d i f - f e r e n t s o l i d m o d i f i c a t i o n s o f t h e MBBA (p-methoxybenzy- l i d e n e p, n

-

b u t y l a n i l i n e ) A i s t h e GLC sample o b t a i n e d

.

from t h e i s o t r o p i c l i q u i d by t h e f a s t c o o l i n g . ' B i s t h e GLC sample o b t a i n e d from t h e nematic LC phase by t h e f a s t c o o l i n g . C and D a r e t h e s t a b l e and . metastable c r y s t a l l i n e modi- f i c a t i o n s , r e s p e c t i v e l y (see r e f s . 1 and 3).

E

i s t h e n e u t r o n energy - _{t r a n s f e r i n cm-l}

.

H o r i z o n t a l b a r s correspond t o t h e FWHM o f t h e r e s o l u t i o n f u n c t i o n o f t h e KDSOG spec-

The G,,(E) f u n c t i o n s o f t h e MB6A and EBBA compounds a r e shown i n Figs.1 and 2, r e s p e c t i v e l y . One can see t h a t t h e t r a n s i t i o n from t h e GLC s t r u c t u r e t o c r y s t a l l i n e o r d e r does n o t change s i g n i f i c a n t l y t h e GH(E) f u n c t i o n s . Thus t h e l a r g e changes i n v i b r a t i o n a l s p e c t r a o f t h e d i f f e r e n t s o l i d m o d i f i c a t i o n s measured by t h e o p t i c a l spectroscopy methods ( 3 - 6 ) a r e due t o t h e l o s s o f s p a t i a l coherency o f v i b r a t i o n s and n o t t o s i g n i f i c a n t changes o f t h e v i b r a t i o n a l d e n s i t y o f s t a t e s .

Fig.2: The GH(E) f u n c t i o n s o f t h e EBBA (p-ethoxybenzylidene p,n

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b u t y l a n i l i n e ) compound:

A i s t h e GLC sample o b t a i n e d from t h e nematic LC phase by t h e f a s t c o o l i n g .

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JOURNAL DE PHYSIQUE

20

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PAA

15

0 ~ ' ~ ~ " " ' ~ " ' 100 200 300 400 500 EICKII

Fiq. 3.: The

G

(E)

functions of H

the PAA (p-azoxyanizo le) compound:

A is the sample obtained from the nematic LC phase of the normal d -PAA (CH3-PAA) by the fast cooliZg.

B is the stable crystalline sample of the d -PAA and partly deuterated dz-PAA (CD3-PAA). The PAA spectra will be discus- sed in detail in the separate publication by A.V.Bielushkin, E.L.Bokhenkov, A.I.Kolesnikov, I.Natkaniec, E.F.Sheka and S.Urban.

The essential changes of GH(&) after the transition to the crystalline state occur in low frequency region ( E

<

300 em-'). Vibrational density of states of the

GLC

substances seems to be a linear function of frequency at

E - r

0 . In Fig.1 the slope of this function fitted to the spectrum of the sample cooled from isotropic liquid is compared with the spectra of other solid modifications. The distinct maxi- ma in the spectra of crystalline samples at the &

>

100 cm-' should correspond to the internal molecular vibrations. These low frequency bands undergo a considerable broa- dening when passing to the GLC state.

The comparison of the spectra of normal and partly deuterated samples of crystalline PAA (see Fig.3) show us that the lattice vibrations of this substance cut-

1

off at about 140 cm-

.

The internal vibrations of this molecule in the frequency range below 300 cm-l belong mainly to the oscillations of the end-CH groups. We ho-

3

pe that the study of the IINS spectra of partly deuterated LC substances will help in the assignment of these oscillations and will serve to the better understandiny of molecular conformations in different modifications of these substances.

References

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Zf?,

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V.K.Dolganov, Fiz.Tverd.Tela, Z l , 2629 (1979).

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