Mol. Crysr. Liq. Cryst. 1990, Vol. 192, pp. 75-78
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TBSTING METHODS OB MEASUREMENTS OF LIQUID CRYSTAL'20-SOLID SURFACE ANCHORING ENERGY
O.LAVRENTOVICH, ToWUSIY, YUeRESNIKOV, VaSERGU
Inst.of Phys.Acad.of ScL, 252650, Kiev, USSR
Abstract
Anchoring energy values were
using two testing methods in
LC-cell for the pair NLC 5CB - silicon
orienting surface coincided within the
experimental accuracy.
determined
the same
elastomer
limits of
Anisotropic intermolecular bteraction at a liquid
crystal-non-liquid crystal interface moetly determines
NLC uniform alignment. Anchoring induced by LC cell bounded surfaces is characterized by anchoring energy Wo
and interaction potential Ws. Theae quantitative characteristics of anchoring produce a decisive effect on
LC devices operating parameters: threshold of orientational' effects, relaxation times eta. The development
of reliable method8 of the anchoring energy determination is still urgent unresolved problem. A large body
of data concerning measurements of Wo has been amassed
over the past years, but the spread of YB, values meaau~
red by the available method^ is rather high. even for the
'
same NU-surface pair.'~uch a spread in values may be
caused by techn.ologica1 inaqulvalenoe of cells or by the
difference in techniques based on some external impact.
It Ls interesting to measure the ancholring energy
by two methods using the same LC cell. The development
of testing methods, when LC layer ia subjected only to
76/[5 121
0.LAVRENTOVICH ET AL.
W e a l i g n i n g a c t i o n of the c e l l s u b s t r a t e s , e a p e s i a l l y
suits t h e s i t u a t i o n . Two t e s t i n g methods were chosen:
measurement of the topological-def e c t
and
t h e r e c e n t l y proposed technique of the small-angle l a s e r l i g h t s c a t t e r i n g on t h e BtLC d i r e c t o r fluctuations3.
Using t h e s e two methods we determined t h e value of anchoring energy of 5CB on s i l i c o n elastomer, which a l i g n s
t h i s LC homeotropically i n 50pm thiok c e l l .
The s i l i c o n elastomer ( [CH3))3SiO( (CH3) 2 ~ i O ) h ~ i ( ~ H 3 ) J ,
n=2500)! l a y e r was p r e c i p i t a t e d out of 105%s o l u t i o n of
elastomer i n toluol. a f t e r t h e p r e c i p i t a t i o n the obtained
100 nm t h i c k l a y e r was d r y i n g a t ~ x > O C . Using both, techniques, the anchoring energy corresponding t o the di,rect o r deviation from i t s av-ge
p o a i t i o n on the plane
perpendicular t o the o e l l s u b s t r a t e s w e a determined. I n
the topological-defect method one needs t o measure the
steady-state width D of domain walls, where the d i r e c t o r
b i s t s about t h e a x i s p a r a l l e l t~ t h e substrate. Such 8
width %a determined by t h e e q u a l i t y of t h e e l a s t i c and
surface energy contrib&ions t o the energy balance. Assuming-a uniform t d s t i i n g and using the surface poitend i a l W ~ ~ ~ c o s where
~ l 3 , f3 i a the angle betaem. the direct o r and s u b e t r a t e surface, and minimizing the sum of
t h e e l a s t i c and surface energy with respect t o D, one
obtaltins t h e expression f o r Wo:
Efere %2 i s t h e txdst e l a s t i c constant, L is t h e c e l l
ehicknesa. The measured value of D was about )tlym,
K22=4-% ~ - ~ d f~o nr 5CB,
e
thus Po=( 2 2 ~) 1 ()-3erg/cm2,
It i s well known tha% thermal f l u c t u ~ a t i o n sof t h e
d i r e c t o r around its equilibrium p o s i t i o n give r i s e t o
an i n t e n s e s c a t t e r i n g of l i g h t . The second t e s t i n g method of the anchoring energy determination i s baaed
TESTING PbEASUREMENP
' S OF ANCHORING ENERGY
[s 131177
on the s u f f i c i e n t b f l u e n c e of a n c h o ~ l n gon the long
wavelength s p e c t m of nemath d i r e c t o r ozdentati3anal
+ -b
fluctwt&ona < I ~ ~ ( P , W ~
where
) I ~ >i s the s c a t t e r i n g
wave veotor. X a c r o s c o p ~ c ~ lthis
y
dependence shows i t s e l f in the NLC-cell l i g h t s c a t t e r i n g c h a r a c t ~ r i s ~ i c s ,
namely i n the angular d i s t r i b u t i o n of the scattered
l i g h t i n t e n s i t y i n the region of small $.3
Experimentally the influence of polar anchoring
could be noticed, f o r example, i n the s c a t t e r i n g of extraordinary wave without changing, of the s t a t e of po,
l a r i e a t i o n Ce-e scatterLng) i n the region of very s m a l l
<
s c a t t e r i n g angles 6 . Intense e-e s c a t t e r i n g takes place
i n the o p t i c a l geometlry when the wave vectoo of the incident l i g h t makes an angle tf'f 0 with the director.
Bor t h i s geomet~y, choaeing the d i r e c t o ~to be p a r a l l e l
t o the scatterimg plane, t h e expression f o r the soattering cmss-section is:3
where A€ i s the o p t i c a l aniso-py
&,-- &,, d is t h e
-b
angle between E-vector of t h e incident l i g h t and the
d i r e c t o r , d = 90'
cp f o r the homeotropic alignment,
nT, i s the amplitude of the splay-bend mode; i t s explic i t e form i s preeented i n Ref. 3,.
To determine the anchoring energy value f o r a LCs o l i d surface p a i r one needs t o reaord the angular d i s t r i b u t i o n of the scattered l i g h t r e l a t i v e i n t e n s i t y i n
the region of 8 s 3
115mrad with high s p a t i a l resolut i o n , and f i t the experimenta data t o a s e t of theoret i c a l curves, calculated f o r d i f f e r e n t values of Wo.
To achieve high s p a t i a l resolution, l a s e r source of
l i g h t has t;r, be used with divergence not higher than
1
1,5 mrad and low power (.liO
115 mW), not t o a f f e c t
-
-
-
-
0.LAVRENTOVICH ET AL.
78/[514]
t h e LC-layer.
Brperiments were performed employing '
z
A o r b-Ne
TENoo l a s e r s , whose emission wavelengthes a r e n o t absorbed by NLC 5CB. Experimental s e t up w a s described
i n prevf;oua papers .3* Measurements were performed
using t h e same c e l l , f i l l e d w i t h 5CB, o r i e n t e d by sil i c o n elastomer l a y e r . The obtained anchoring energy
value coincides w i t h i n t h e limit8 of experimental accuracy w i t h t h a t measured by t h e topological-defect
method. T h i s r e s u l t shows t h e r e l i a b i l i t y of t h e s e two
t e s t i n g methods, whiah o f f e r s an advantage t o measure
Wo not only i n s p e c i a l l y prepared a m p l e but i n commercial c e l l s .
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