Mol. C y s t . Liq. C y s t . 1990, Vol. 193, pp. 217-221
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ELECTROOPTICAL EFFECTS I N THE POLYMER DISPERSED
NEMATIC LIQUID CRYSTALS: RESPONSE TIMES
AoVoKOVAL'CHUK,
and V,V,SERGAN
MoVoKURIK, O,D,LA?ZtENTOVICH,
I n s t i t u t e of Physics Academy of Sciences of t h e
Ukrainian SSR, Kiev-38, pr.Nauki, 46, U.S.S,R.
Abstract
The response times f o r two t e s of
polymer d i s p e r s e d l i q u i d c r y s t a l s (PDLCywith
normally and t a n g e n t i a l l y anchored drops a r e
s t u d i e d experimentally. It is shown t h a t t h e
normal anchoring provides t h e s h o r t e r turn-on
and turn-off times,
PDLC f i l m s , c o n s i s t i n g of micron s i z e l i q u i d cryst a l l i n e d r o p l e t s , d i s p e r s e d i n a polymer matrix, have
r e c e n t l y been r e p o r t e d a s new e l e c t r i c a l l y switched
l i g h t shutters.' E l e c t r o o p t i c a l response s t r o n g l y
depends on t h e boundary c o n d i t i o n s , v i z , , anchoring
of t h e d i r e c t o r on t h e polymer s u r f a c e and t h e shape
of t h e drops.
For example, it was p r e d i c t e d q u a l i t a t i v e l y ,294v5 t h a t normally anchored drops might
provide s h o r t e r response times t h a n t a n g e n t i a l l y anchored ones, We r e p o r t t h e response times f o r two
corresponding t y p e s of PDLC f i l m s , with t a n g e n t i a l
boundary c o n d i t i o n s (T-films) and normal ones (N-films>
I n % f i l m s l i q u i d c r y s t a l l i n e drops (ZhK-807) were
d i s p e r s e d i n p o l y v i n y l a l c o h o l , I n o r d e r t o prepare
N - f i l m s with similar parameters, a s m a l l amount (0,01-
2181[9981
A.V.KOVN;'CHUX
e t al.
0.1 p.c.
of weight) of l e c i t h i n was added a t t h e f i n a l
s t a g e of T - f i l m formation. A s a r e s u l t , f i l m thickness, concentration and average r a d i u s of t h e drops
(~s2p.1) were t h e same i n both films. T- and N - f i l m s
d i f f e r e d only i n t h e drop s t r u c t u r e s : t h e former
possesses b i p o l a r s t r u c t u r e s and t h e l a t t e r possesses
r a d i a l ones.
Fig.? shows t h e responses of f i l m s t o an a p p l i e d
r e c t a n g u l a r AC (IOkHz) p u l s e f o r l i g h t of He-Ne l a s e r ,
normally i n c i d e n t on t h e f i l m . N-film possesses s h o r t e r
turn-on ( t o n ) and turn-off ( t o f f ) times t h a n T-film:
T
N
ton -2 m s , ton-8 m s , toff-5 m s , tEff=20 ms. It i s
convenient t o d i s c u s s t h e s e i n e q u a l i t i e s f o r toffonly,
because toni s determined mainly by t h e f i e l d .
T-f i l m
Due t o t h e b i p o l a r s t r u c t u r e of drops, t h e dominant
f a c t o r i n t h e e l e c t r o o p t i c a l response of T - f i l m s i s a
shape of t h e s e drops. 2-6 For s p h e r i c a l b i p o l a r drops
Hono r e s t o r i n g e l a s t i c torque e x i s t s and
wever, nonspherical drops possess such torque due t o
t h e d i f f e r e n c e i n t h e e l a s t i c energy F f o r d i f f e r e n t
o r i e n t a t i o n s of t h e s t r u c t u r e . 2 9 6 Relaxation time i s
determined by balance of t h e e l a s t i c and viscous
torque. A s it was shown by 3.-G.Wu e t a l . , 3
tLf- .
where 1 i s o r i e n t a t i o n a l v i s c o s i t y , K i s e f f e c t i v e
e l a s t i c c o n s t a n t , I-a/b i s t h e a s p e c t r a t i o of t h e
d r o p l e t , a and b a r e t h e l e n g t h s of t h e minor and major
~ m 151.5
axes, For
=7-10
- -2 P, K = ? O - ~ ~ ~N =, 2 * 1 0 - ~and
T
S h o r t e r times may be achieone o b t a i n s t O f f = 2 2 m s ,
ved with l a r g e r 1, Bowever, i n c r e a s i n g of 1 l e a d s t o
RESPONSE TIIZ'iES O F PDLC FILMS
[9991/219
i n c r e a s i n g of t h e t h r e s h o l d f i e l d . 2-6 m e "passive"
c h a r a c t e r of t h e r e s t o r i n g torque i s a deficiency of
T-films.
I n N - f i l m s t h i s torque i s a c t i v e due t o anchor i n g forces.
N-film
The t r a n s i t i o n between on- and o f f - s t a t e s may be considered a s a c o n t r a c t i o n of t h e f l a t d i s c l i n a t i o n ring2t4
with v a r i a b l e r a d i u s r i n t h e e q u a t o r i a l plane of t h e
s p h e r i c a l drop. A s a r e s u l t of t h e c o n t r a c t i o n i n offs t a t e we g e t a r a d i a l hedgehog. The l a t t e r i s s t a b l e
due t o t h e s u r f a c e anchoring. Equation f o r torques may
be w r i t t e n a s
25
where Bearcsin (r/R) , F = 8 5 ~ ( ~ - r ) + Kru,
2
u=Sa(l+ln(WR/2X))/2, c% is a geometrical parameter,
W is anchoring energy. Here we assume t h a t t h e d i r e c t o r
d i s t r i b u t i o n i s uniform i n s i d e t h e sphere with r a d i u s r
and r a d i a l o u t s i d e t h i s sphere. One e s t i m a t e s from ( 2 )
t h e time constant f o r t h e r e l a x a t i o n time
2
J / m , and e a r l i e r
For a -3 ( s e e xef. 2 ) , w=~---Io-~
N
values f o r EC, g , R model y i e l d s toff=
5 m s . 'Phe
1J
s h o r t e r toffmay be achieved i n N-films with s t r o n g e r
anchoring, r a t h e r than with deformations of t h e drops.
A s a r u l e , e l e c t r o o p t i c response of PDLC c o n t a i n s
two components: f a s t , t h a t was discussed above, and
slow. Slow component manifestes i n a h y s t e r e s i s of t h e
transmittance v s voltage curves, Fig. 2 . N - f i l m
possesses s m a l l e r h y s t e r e s i s than T-Silm. Analysis of
2201[1000]
A.V.KOVAL'CHUK
e t al.
PIGUIiE 1 L i g h t t r a n s m i t t a n c e of T ( a ) and N (b)
f i l m s d u r i n g ( 1 ) and a f t e r
( 2 ) an a p p l i e d 100 V
d r i v i n g ,AC pulse. Right s i d e : corresponding types
of s t r u c t u r a l t r a n s f o r m a t i o n s i n flron?.
T.%
FIGURE 2
.
Transmittance v s v o l t a g e curves f o r
T ( a ) and N ( b ) f i l m s . 0 i n c r e a s i n g v o l t a g e ,
decreasing voltage.
2
RESPONSE TINES OF PDLC FILMS
t h i s e f f e c t i s p o s s i b l e w i t h i n t h e scope of above
mentioned models.
Real shape of t h e drops may be more complicated
i n comparison with t h e simply elongated o r f l a t t e n e d
drops and corresponds t o a few minima of e l a s t i c energy
r a t h e r t h a n one. I n T - f i l m s when t h e f i e l d i s removed
r e l a x a t i o n of t h e s t r u c t u r e a x i s may be hindered by
l o c a l " t r a p s " of t h e drop shape. I n N-films t h e r e l a x a t i o n i s caused by a c t i v e anchoring f o r c e and h i s t e r e sis i s smaller. Remaining h y s t e r e s i s may be caused by
s u r f a c e pinning of d i s c l i n a t i o n r i n g s .
The a u t h o r s wish t o thank B.I.Lev and V.N.Pergamentshchik f o r h e l p f u l discussions.
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