INTERNAL FRICTION OF SOLID SAMPLES COVERED WITH SURFACE LAYER OF LIQUID CRYSTAL

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INTERNAL FRICTION OF SOLID SAMPLES COVERED WITH SURFACE LAYER OF LIQUID

CRYSTAL

D. Samatowicz

To cite this version:

D. Samatowicz. INTERNAL FRICTION OF SOLID SAMPLES COVERED WITH SURFACE LAYER OF LIQUID CRYSTAL. Journal de Physique Colloques, 1987, 48 (C8), pp.C8-525-C8-529.

�10.1051/jphyscol:1987882�. �jpa-00227186�

INTERNAL FRICTION OF SOLID SAMPLES COVERED WITH SURFACE LAYER OF LIQUID CRYSTAL

D. SAMATOWICZ

Gd&sk Technical University, Department of Technical Physics and Applied Mathematics, PL-80-952 Gdadsk, Poland

A b s t r a c t

-

I n t e r n a l f r i c t i o n measurements were made f o r g l a s s and m e t a l samples covered w i t h t h i n l a y e r o f l i q u i d c r y s t a l MBBA, NPOOB and CN, The measurements were p e r f o r m e d w i t h t h e

t o r s i o n pendulum i n t h e f r e q u e n c y range 1

+

40 H z and h e a t i n g r a t e < l O C/min. The t h i n l i q u i d c r y s t a l l a y e r on t h e s u r f a c e o f s o l i d sample causes t h e appear nce o f d i s t i n c t maxima i n t h e i n t e r n a l f r i c t i o n spectrum Q - ? ( T ) and l a r g e changes o f modulus G. The temperatures o f m e l t i n g and phase t r a n s i t i o n s f o r t h e i n v e s t i g a t e d l i q u i d c r y s t a l a r e $ l o s e t o t h e tempe- r a t u r e s o f t h e maxima i n t h e spectrum Q- ( T ) .

I n t r o d u c t i o n

I n r e c e n t y e a r s some e x p e r i m e n t a l r e s u l t ? d e t e r m i n i n g t h e e f f e c t o f s u r f a c e l a y e r on i n t e r n a l f r i c t i o n 'Q and e l a s t i c i t y modulus G o f m e t a l s [1,2,3,4, have been p r e s e n t e d m a i n l y b y P o l i s h a u t h o r s

a t European and P o l i s

2

conferences. The f i n a l c o n c l u s i o n from t h e p r e v i o u s i n v e s t i g a t i o n s o f i n t e r n a l f r i c t i o n c a r r i e d o u t on samp- l e s o f m e t a l covered w i t h a t h i n l a y e r o f hydrocarbon, o i l o r Wood's a l l o y i s t h e conne t i o n between t h e i n t e r n a l f r i c t i o n maxi-

mum a p p e a r i n g on c u r v e Q-'[T) and t h e m e l t i n g temperature o f t h e l a y e r m a t e r i a l .

I n a l l t h e p u b l i s h e d papers t h e problem o f t h e i n t e r p r e t a t i o n o f t h e r e m a i n i n g p a r t o f spectrum Q - ~ ( T ) a t h i g h e r temperatures i s s t i l l unsolved. S i n c e t h e r e a r e no t h e o r e t i c a l papers concer- n i n g t h e a d s o r p t i o n o f o r g a n i c compounds and s t r u c t u r e o f t h i n l a y e r s o f such substances t h e problem seems t o be v e r y d i f f i c u l t t o e x p l a i n even i n t h e e x p e r i m e n t a l way.

I n t h i s work i n t e r n a l f r i c t i o n measurements were made f o r sam- p l e s covered w i t h t h i n l a y e r o f known l i q u i d crystals-MBBA, NPOOB and CN,

E x p e r i m e n t a l

The measurements o f t h e i n t e r n a l f r i c t i o n were c a r r i e d o u t on a s i m p l e t o r s i o n a l pendulum o f K; t y p e i n f r e q u e n c y range l + 4 0

Hz.

The measurement p r i n c i p l e c o n s i s t e d i n t h e d e t e r m i n a t i o n o f l o g a - r y t h m i c decrement

d

o f f r e e v i b r a t i o n s o f t h e sample 6'' =

6 / ~

t h e f r e q u e n c y connected w i t h e l a s t i c i t y modulus f o r s h e a r i n g B o t h q u a n t i t i e s were c o n t i n u a l l y r e g i s t e r e d from n i t r o - gen t e m p e r a t u r e t o h i g h temperatures /below t h e d e s o r p t i o n tempe- r a t u r e o f t h e c o v e r i n g substance/ w i t h h e a t i n g r a t e <0.5 K/min.

The v i b r a t i o n s w i t h d e f o r m a t i o n a m p l i t u d e o f o r d e r

lom5

were ana- l y z e d . The samples used h e r e were rod-shaped o f 20

+

⁵⁰ mm l e n g t h

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987882

C8-526 JOURNAL DE PHYSIQUE

and d i a m e t e r 0.5

5

2 mm made o f t e c h n o l o g i c a l copper, p l a t i n u m , s t e e l and s i l i c a t e glass. The samples, a f t e r t h e i n i t i a l measure- ment o f t h e background were covered w i t h a l i q u i d l a y e r f i r m l y adsorbed on t h e sample. The l a y e r s o f l i q u i d c r y s t a l were o b t a i n e d by l u b r i c a t i n g . The l a y e r t h i c k n e s s was c a l c u l a t e d from t h e

w e i g h t s o f t h e sample w i t h o u t and w i t h l i q u i d c r y s t a l . The l a y e r t h i c k n e s s was o f o r d e r o f a s e v e r a l micrometers. The c o v e r i n g l a y e r s were t y p i c a l l i q u i d c r y s t a l s

-

MBBA, NPOOB and CN.

The I n v e s t i q a t i o n R e s u l t s and D i s c u s s i o n

A t h i n l i q u i d l a y e r s e t on t h e s u r f a c e o f a m e t a l o r g l a s s causes t h e appearance o f d i s t i n c t maxima i n t h e i n t e r n a l f r i c t i o n spectrum Q - ~ ( T ) , W t t h t h e maxima correspond l a r g e changes o f e l a - s t i c i t y modulu G The temperatures o f t h e appearance o f maxima i n spectrum Q-?(T/ a r e c l o s e t o t h e temperatures o f t h e phase t r a n s i t i o n s o f l i q u i d c r y s t a l s .

F i r s t , t h e h i g h e s t maximum f r e q u e n t l y appears a t t h e l o w e s t tempe- r a t u r e s and i s connected w i t h t h e m e l t i n g t e m p e r a t u r e o f l i q u i d c r y s t a l i n d e p e n d e n t l y o f t h e phase t o w h i c h t h e l i q u i d c r y s t a l t r a n s f o r m s .

The n e x t maxima appear a t temperatures c l o s e t o t h e temperatures o f t h e c o r r e s p o n d i n phase tr n s i t i o n s .

9

Fig. 1 shows t h e Q0Q( T) and f

(T)

c u r v e s f o r P t sample, o l e a n and covered w i t h NPOOB, F o r p u r e p l a t i n u m , i n t h e i n v e s t i g a t e d range o f t e m p e r a t u r e s t h e r e a r e no maxima

-

o n l y a s m a l l t e m p e r a t u r e i n c r e a s e o f t h e background i s observed.

F o r t h e sample w i t h NPOOB, i n a d d i t i o n t o t h e h i g h maximum c o r r e s - ponding t o t h e l i q u i d c r y s t a l m e l t i n g , two s m a l l maxima appear a t temperatures where t h e t r a n s i t i o n s SAc* N and N - I occur. The s i m i l a r s i t u a t i o n can be observed f o r Cu samples covered w i t h MBBA

-

Fig.2 and 3. Here, b e s i d e s t h e two maxima a t t h e temperatures of about 295 K and 315 K /i.e. t h e m e l t i n g t e m p e r a t u r e o f MBBA and phase t r a n s i t i o n N c r I / a d d i t i o n a l two s l i g h t maxima appear a t about 220 #. A t t h i s t e m p e r a t u r e t h e l i q u i d c r y s t a l i s t h e so- l i d phase. The c a l o r i m e t r i c measurements&5] s t a t e d t h e metasta- b l e phases o c c u r e i n g a t t h i s temperature, and t h e i r p r o p e r t i e s a r e c l o s e l y connected w i t h h e a t t r e a t m e n t

03

t h e sample / c o o l i n g r a t e and range/. On F i g . 4 t h e Q - ~ ( T ) and f ( T ) c u r v e s f o r Cu sample covered w i t h m o n o t r o p i c u n p o l a r l i q u i d c r y s t a l CN a r e pre- sented. I n a d d i t i o n t o t h e two maxima c o r r e s p o n d i n g t o t h e n o r m a l phase t r a n s f o r m a t i o n s K 353.5 Ch 365 I, t h e maximum below t h e m e l t i n g o f t h i s l i q u i d c r y s t a l appears d u r i n g h e a t i n g t h e sample.

The t e m p e r a t u r e o f t h e appearance o f maximum / l v 3 5 0 K/ i s c l o s e t o t h e Ch-S t r a n s i t i o n t e m p e r a t u r e w h i c h i s observed d u r i n g coo- l i n g .

CN i s t h e l i q u i d c r y s t a l v e r y s l o w l y c r y s t a l l i z i n g from t h e smec- t i c phase. P r o b a b l y , t h e i n t e r n a l f r i c t i o n maximum a p p e a r i n g below t h e m e l t i n g p o i n t d u r i n g h e a t i n g may be connected w i t h t h e s l i g h t s u p e r c o o l e d p a r t o f t h e substance i n t h e s m e c t i c phase, now t h a t t h e maximum i s h i g h e r when t h e sample was c o o l e d s h o r t l y a f t e r s e t t i n g CN from i s o t r o p i c phase on t h e Cu s u r f a c e / b e f o r e t h e p r o p e r measurement/.

The r e s u l t p r e s e n t e d h e r e corresponds t o one-day c o o l i n g a t room temperature. F o r t h e samples w h i c h were c o o l e d o n l y f o r 354 hours, t h e h e i g h t o f t h i s maximum i s n e a r l y t h e same as f o r t h e maximum a p p e a r i n g a t t h e m e l t i n g p o i n t .

T a b l e 1 shows t h e temperatures o f phase t r a n s i t i o n s f o r t h e l i q u i d c r y s t a l s p r e s e n t e d h e r e and t h e temperatures o f t h e c o r r e s - ponding maxima o f i n t e r n a l f r i c t i o n ,

L i q u i d

c r y s t a l T e m p e r a t u r e s o f T e m p e r a t u r e s o f p h a s e t r a n s i t i o n s i n t e r n a l f r i c t i o n

maxima

MBBA C r y s t a l / n e m a t i c 2 9 5

f 300 K

p h a s e

K

2 9 5

N

3 2 0

I

2 9 5

K

/ p - m e t h o x y b e n z y l i d e n e

p-bu t y l a n i l i n e n e m a t i c p h a s e / i s o t r o - p i c p h a s e

3 2 0

K 315 K

NPOOB C r y s t a l / s m e c t i c p h a s e

K

3 2 2 SA

334 N

341 1

3 2 2

K

3 2 5

K

/ 4 - n - p e n t y l p h e n y l - s m e c t i c / n e m a t i c - 4 ' - n - h e p t y l o x y t h i o l - p h a s e

b e n z o a t e /

334 K 336 I<

n e m a t i c / i s o t r o p i c p h a s e

341 K 340 K

CN c r y s t a l / c h o l e s t e r i c

p h a s e

K 350.5 353.5

Ch 3 5 3 . 5

K

3 5 2

K

365 I

/ p e l a r g o n a t e c h o l e -

s t e r o l / c h o l e r t e r i c / i s o t r o p i c p h a s e

3 6 5

^I< 365

/ h e a t i n g /

360

/ c o o l i n g / c h o l e s t e r i c / s m e c t i c

p h a s e

3 5 0 . 5

K 340

/ c o o l i n g /

W 5

s t a b l e / n e m a t i c p h a s e

n e m a t i c c r y s t a l 2 5 3

^I<

2 4 3 -

^{263 K}

I<

2 5 3

N 335 I

n e m a t i c / i s o t r o p i c p h a s e

335 I<

3 1 5 -

^{335 K}

From t h e e x p e r i m e n t a l r e s u l t s

i t

f o l l o w s t h a t t h e maxima o b t a i - n e d a t t h e f r e q u e n c y

1 f 5

Hz a r e t h e h i g h e s t , o f c o u r s e when t h e s a m e d y n a m i c s o f m e a s u r e m e n t w a s a p p l i e d .

O n l y t h e s l i g h t l o w e r i n g o f t h e t e m p e r a t u r e s o f maxima i s o b s e r v e d a t t h e

N-I

t r a n s i t i o n s f o r n e m a t i c s , b u t t h e s h a p e o f t h e s p e c t r u m /a n u m b e r o f maxima/ i s n o c o n n e c t e d w i t h t h e k i n d o f s u b s t r a t e .

I t

i s c o n f i r m e d b y t h e 4 - I ( T ) c u r v e s p r e s e n t e d o n F i g .

5

d e t e r m i n e d f o r d i f f e r e n t s a m p l e s c o v e r e d w i t h t h e

W 5

com- p o s i t i o n w h i c h e x h i b i t s t h e n e m a t i c p h a s e i n t h e w i d e t e m p e r a t u - re r a n g e f r o m a b o u t

260 K

t o

335 K.

T h e p r e v i o u s m e a s u r e m e n t s p e r f o r m e d w i t h t h e

W 5

c o m p o s i t i o n i n d i c a t e d t h a t t h e h e i g h t o f t h e f i r s t maximum a t t h e t e m p e r a t u r e a b o u t

220 K

i s s t r i c t l y p r o - p o r t i o n a l t o t h e a m o u n t o f t h e l i q u i d c r y s t a l p u t o n a s a m p l e

63,

A l l t h e o b s e r v e d i n t e r n a l f r i c t i o n maxima a r e a c c o m p a n i e d

\ y

t h e l a r g e c h a n g e s o f f ( T ) , T h e g r e a t e s t c h a n g e s c a n b e f o u n d when

t h e l a y e r m a t e r i a l t r a n s f o r m s f r o m s o l i d t o l i q u i d p h a s e . T h e

f 2 ( ~ ) c h a n g e s c o n n e c t e d w i t h t h e o t h e r maxima s e e m t o b e r e l a t e d

t o t h e t e m p e r a t u r e v a r i a t i o n o f v i s c o s i t y o f a l a y e r material.

C8-528 JOURNAL DE PHYSIQUE

The p r i m a r y r e s u l t s o b t a i n e d f o r t h e l i q u i d c r y s t a l s a l l o w t o connect t h e observed phenomena w i t h t h e phase t r a n s f o r m a t i o n s o c c u r e i n g i n t h e l i q u i d c r y s t a l l a y e r . These processes a r e n o t t y - p i c a l o f t h e r e l a x a t i o n p r o c e s s s t u d i e d b y t h e i n t e r n a l f r i c t i o n method b u t some o f t h e phase t r a n s f o r m a t i o n t h e o r i e s a p p l i e d t o t h e d e s c r i p t i o n o f phase t r a n s f o r m a t i o n s i n s o l i d s may b r i n g a con- s i d e r a b l e c o n t r i b u t i o n t o t h e p r e s e n t i n v e s t i g a t i o n s .

I t s h o u l d be p o i n t e d o u t t h a t t h e i n t e r n a l f r i c t i o n method i s v e r y s e n s i t i v e and t h a t i t i s p o s s i b l e t o a p p l y t h i s method f o r d e t e r m i n i n g t h e p r o p e r t i e s o f t h e l a y e r m a t e r i a l . An a t t e m p t o f t h e m a t h e m a t i c a l d e s c r i p t i o n o f processes r e s p o n s i b l e f o r t h e ob- served maxima w i l l be p r e s e n t e d i n t h e n e x t paper.

Fig. 1. Q*'(T) and f 2 ( T ) s p e c t r a F i g . 2. Q-'(T) and f 2 ( ~ ) s p e c t r a f o r c l e a n P t

/---/

and P t Sam- f o r c l e a n Cu

/- - ^-/

^{and C u}

p l e covered w i t h NPOOB /-/at sample covered w i t h MBBA

/-/

f293K 1 7 Hz. a t f293K = 2.5 HZ.

Fig.3. Q-'(T) and f ( T ) s p e c t r a f o r c l e a n 2 Cu

/- - ^-/

and Cu sample

/

a t f293K = 33 Hz.

covered w i t h MBBA

/-

Fig.4. Q -1

and

f 2 ( ~ ) s p e c t r a f o r c l e a n Cu

/- - ^-/

and Cu sam- p l e covered w i t h CN

/- /

a t f293K = 3.6 Hz. Measure- ments on h e a t i n g ^---)--.

on c o o l i n g -.

.

s u b t r a c t i o n t h e back- grounds/ f o r d i f f e r e n t samples /Cu, s t e e l , Au, s i l i c a t e g l a s s / cove- r e d w i t h W5 nematic, o b t a i n e d a t t h e f r e - quencies 20

+

^{30 Hz.}

T h i s work i s p a r t l y s u p p o r t e d i n t h e frame o f CPBR 8.12 Problem.

References

1. W.Chomka, E-Denga, J.Phys.,

44,

09-505 /1983/

2. B.Augustyniak, G.Fantozzi, J.Phys.,

44,

09-493 /1983/

3, I'J,Chomka, €.Dengal P r o c e e d i n g o f RENIOM-84, Katowice, S i l e s i y n U n i v e r s i t y 1984, p.191

4. G.Haneczok

,

T.Poloczek

,

J.W.Moroh, P r o c e e d i n g o f RENIOM-84, Katowice, S i l e s i a n U n i v e r s i t y 1984, p.200

5. M.Rachwalska, "Advances i n L i q u i d Research and A p p l i c a t i o n s " . ed. L a j o s Bata, Budapest 1980, p.189

6. D.Samatowicz, P r o c e e d i n g o f J u r a t a - 8 5 , Warsaw, Academy o f M i l i t a r y , t o be p u b l i s h e d .