Surface electroclinic effect in ferroelectric liquid crystal

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Surface electroclinic effect in ferroelectric liquid crystal

A. Biradar, S. Bawa, Kanchan Saxena, Subhas Chandra

To cite this version:

A. Biradar, S. Bawa, Kanchan Saxena, Subhas Chandra. Surface electroclinic effect in ferroelectric liq- uid crystal. Journal de Physique II, EDP Sciences, 1993, 3 (12), pp.1787-1793. �10.1051/jp2:1993103�.

�jpa-00247938�

Classification Phj,.<ic.s Ah.<tract.I

61.30 77.20 77.40

Surface electroclinic effect in ferroelectric liquid crystal

A. M.

Biradar,

S. S. Bawa, Kanchan Saxena and Subhas Chandra

National

Physical

Laboratory, ^K. S. Krishnan Road, New Delhi, l10012, India

(Received ²³ April J993, iei,ised 27 July J993, accepted 3J A~lgu.<t J993)

Abstract. Surface electroclinic effect in chiral nematic IN")

phase

of

high-tilt-angle

ferroelectric

liquid crystal

has been studied

by

dielectric relaxation and

electro-optical

method in

a

homogeneously aligned, thin samples in the frequency range of 400 Hz to loo kHz at different temperatures. ^The electroclinic effect in the N* phase ^{observed in the surface} layers is due to the effect of surface

anchoring

and smectic-like ordering of the molecules at the rubbed

boundary

surface of the sample ^{which has been confirmed}

by

dielectric and optical observations. The strong

anchoring

in the surface layers ^is also presented

by applying

_a dc bias field.

1. Introduction.

The electroclinic effect _near Smectic A-Smectic C*

(Sm

^A-Sm

C*) phase

transition is well- known

ii

in the ferroelectric

liquid crystals (FLCS).

A

large

number of data has been

reported

in SmA and in SmC*

phases

in

homogeneously aligned

thin cells from the

optical

observations

[2]

and dielectric measurements

[3, 4].

The electroclinic effect in the chiral nematic

IN

^*

phase

of FLCS have been

scarcely

studied

[5, 6].

The electroclinic effect _occurs when _an electric field

applied parallel

to the smectic

layers

induces _a molecular tilt relative to the

layer

normal.

Alignment

of ferroelectric

liquid crystal

^molecules is

generally

achieved

by properly treating

the surfaces of the _two substrates in the fabrication of FLC cells _or devices. Berreman

[7]

first

studied this

problem by rubbing

the

glass

substrates with diamond paste ^and

creating

microgrooves.

He calculated the elastic distortion due to the grooves and found that the lowest energy

configuration

_was for all the molecules to lie

along

the grooves,

creating

an

uniformly aligned

cell

along

the

rubbing

direction.

Geary, Goodby,

Kmetz and Patel

[8]

have

proposed

_a

different mechanism of the

alignment

of

liquid crystal

molecules _on the

polymer

coated

substrates rubbed with cloth.

Rubbing

of the films orients the

polymer

chains

along

_a

preferred

direction and the molecular interaction between the

liquid crystal (LC)

molecules and the stretched

polymer

induces the

alignment.

It has been found

[9]

that

alignment

is either due _to grooves formation _or interaction between the LC molecules and the stretched

polymer.

The surface

anchoring

_energy in thin cells is

strongly dependent

_on the

rubbing strength.

Several surface _treatments have been

successfully employed,

but the

phj~sical

mechanisms

affecting

1788 JOURNAL DE PHYSIQUE II N° 12

the surface induced bulk

alignment

are still not well understood. For

homogeneous

bulk

alignment

the

specially

treated surfaces often used _are rubbed

polymer

coated substrates. As _a surface induced

effect,

how the first

monolayer

of LC molecules _at the interface is oriented

by

the surface molecular interaction is fundamental to our

understanding

^of

homogeneous

alignment.

Different

techniques [9-11]

have been

employed

to

study

the surface induced

alignment

due to the

rubbing

of the

polymer

coated substrates in ferroelectric

liquid cry~tals.

^{In this} article,

we report ^the electrodinic effect due _to the interaction of LC molecules in chole~teric

phase (N *)

with the

polyamide

treated rubbed substrates in thin cells of

high

tilt

angle

ferroelectric

liquid crystals by

dielectric and

electro-optical

observations.

2.

Experimental.

Highly conducting

indium tin oxide

(ITO) glass plates

were used for electrodes. The distance

between the

plates

was

kept

around lo _~m and this distance _can be considered _as thin

compared

^{with the} helical

pitch

value

(15

_~m in Sm C* and 23 _~m in N*

phase)

of the material. The

high

tilt

angle (44°)

ferroelectric

liquid crystal

^material CS-2004

(Chisso Corporation, Japan)

used in this

study

has the

following phase

_sequence

Cryst

-- Sm-C * -- N * -- Iso

9 °C 62 °C 71°C

The material _was introduced in the cell

by

_means of

capillary

action at elevated temperatures to

ensure that the

filling

took

place

in its

isotropic (Jso) phase. Homogeneous alignment

of the

FLC

sample

is obtained with _one surface buffed, which has been

pretreated

with adhesion promoter ^and

nylon

and _a well defined monodomain

sample

_was obtained

by applying

an ac

field. A detailed

procedure

of the

alignment

and the dielectric studies in the Sm C*

phase

of

this material has been

reported

elsewhere

[12].

The smectic

layers

are assumed _to be

perpendicular

to the

plane

of the electrodes for this geometry. ^{Electric field is}

applied parallel

to the smectic

layers.

The dielectric measurements have been carried out

using

_a HP 4192A

Impedance Analyzer

in the

frequency

_range 400 Hz _to 100 kHz. The dielectric relaxation in N *

phase

at different temperatures ^{has been studied}

by applying

a dc electric field

(±

0 _to 30 V/10 _~m. The electroclinic effect in N*

phase

has been

further, analyzed by observing

under

polarizing microscope (Olympus BH-2).

3. Results and discussion.

The electroclinic effect in N*

phase

has been

analysed by

dielectric relaxation method. This mode is most

commonly

observed in Sm A

phase

as a soft mode

(which corresponds

to an

electroclinic

effect). Figure

I shows the tan 6 (loss

factor)

»ersiis

logjo

_v

plot

at different temperature~ ^{in N *}

phase.

^{The critical}

frequencies

of this dielectric mode have been

computed

from the

position

of the maxima _on the

plot

which is almost _constant with temperature. ^As described earlier _one of the

glass plates

used in the cell is coated with

polymer

and rubbed whereas other

plate

is without any surface _treatment for

aligning

the FLC molecules. This

means that the surface which is rubbed with

polymer

is

highly

anchored

[13]

and other surface

is

weakly

anchored. A

high

dc electric field is

applied

to unwind the twisted _{structure to see} whether the dielectric mode in N*

phase

comes due to surface _or bulk effect of the cell.

Figure

² shows tan 6 _{as a} function of

frequency

at different

applied

dc bias field in N*

phase.

The dielectric relaxation in N*

phase

in this

study

is not due to flexoelectric effect which appears on weak

anchoring

surfaces _or due _to the

breaking

of the cell symmetry (more than one

D .63.OC

°"~

osc. vw _~ o.5 v

i]I']]

Bias van ~o v

x .s9.o.c

~

[

_o-lo

O.05

2.O 3.O 4.O 5.O

log j~~ ^(Hz)

Fig, I. Frequency

dependence

of _tan 3 (loss factor) measured _at different temperature~ ^{in N±} phase.

o.<5

O ~OV

a ~5V

x ~<OV

. ~45V

o-lo

tan

6

o.05

2.O 3.O 4.O 5.O

log jo~ (Hz)

Fig. ^2. Frequency

dependence

of _tan 3 measured at different biasing voltages at 69 °C in N± phase.

domain) [14]

in N*

phase. Recently

Lee and Patel

[15]

have observed the flexoelectric effect in nematic

phase

where the difference between

anchoring strength

_on the two surfaces is very small. In the present

investigations

the

possibility

^of flexoelectric effect is ruled _out because I) the

anchoring

_energy in

polymer

^{rubbed surface is} _very strong

[13], it)

the

alignment

is uniform and _a

single

domain is checked under

polarizing microscope.

The dielectric relaxation

1790 JOURNAL DE

PHYSIQUE

II N° 12

in N*

phase (Fig, I)

_appears due _to strong

anchoring

effect _on the

polymer

rubbed surface which preserves the smectic-like

ordering

_near the rubbed surface

layers.

As it is known

[16, 17]

that the FLC

material,

which has _a Sm C*

phase

after the Sm A

phase

is

likely

_to form _a firm

layer

structure in the Sm C*

phase. Contrary

to this situation, the FLC material that ha~

Sm-C*

phase

without

passing through

the Sm A

phase

^would not form _a firm

layer

structure and

consequently

the direction of the molecules _near the surface is anchored rather

tightly

due

to the _nature of rubbed films

(grooved formation)

and this

anchoring

effect would be _more

pronounced

in thin

samples.

The electroclinic effect arises from interaction of the

liquid crystal

molecules with the local electric field _at the rubbed

boundary

surfaces in N*

phase.

This local field

originates

from the

polar anchoring

of the few

liquid crystal monolayers

_on the rubbed surface

[10].

The

anchoring

effect _at the rubbed surface is _so strong ^that even at the dc bias field of 10 VII 0 _~m the electroclinic effect in N*

phase

was observed

(Fi

_g.

2).

^This means that

a

high

bias field is needed to

nullify

^the

anchoring

effect _on the rubbed surface _so that the

surface and bulk molecules behave

identically.

However, above this

biasing

field

(above

10 V/10 _~m in

Fig.

2) the dielectric losses _or conductance due to electroclinic effect _{was not} consistent in N*

phase

which is shown

by

the _error bars in the

figure

and hence at

higher biasing

field the dielectric relaxation

frequency

could _not be

computed.

The smectic-like

ordering

in N*

phase

is confirmed due to the fact that the electroclinic

effect _was observed in N*

phase during

the

heating cycle

(Sm

C*-N*-Iso).

Whereas in the

cooling cycle (Iso-N*-Sm C*)

this effect _{was not} observed. This _means that when the

phase

transition is from Iso-N*-Sm C* then the LC molecules in N*

phase

are not

aligned along

the

rubbing

direction

resulting

into the

randomly lying (isotropically)

the molecules _on the surface and

therefore,

there would _not be any surface

anchoring

of the LC molecules _on rubbed

surface. When it is further cooled in SmC*

phase

the

layer

structure also

disappears

(multidomain cell)

which has been confirmed

by optical microscopic

observations.

Recently

Lee, Patel and

Goodby [18]

have also

reported

_a surface electroclinic

phenomenon

in the

isotropic phase

of _an FLC _{on a} rubbed

polymer

^{surface and}

interpreted

_as an electric field induced molecular tilt in _a chiral

liquid crystal

which has _a direct Iso-Sm A transition.

They

have found that the molecular tilt exists in about °C above the transition _to the bulk

isotropic phase

^and

predicted

smectic-like

ordering

within the surface

layer

which contributes to the

electroclinic effect.

Further, the electroclinic effect in the present

investigation

has been confirmed

by

electrical

~witching

method

by inducing

_a tilt

angle

in molecules

by applying

_a

high

electric field in N*

phase. Figure

3 shows the

change

in the _texture of the

sample

at different

applied

^static dc

voltages.

At _zero

voltage

the surface molecules and bulk molecules _are in the _same

phase.

Therefore, uniform _texture is _~een under the

microscope (Fig.

3A). The increase in the dc

voltage

which

gives

rise _{to an} induced tilt of the surface molecules with respect to the bulk

molecules,

re~ults into the

change

in the texture of the

sample (Figs.

38, C, D and

El.

However, if _a square

pulse

is

applied

at low

frequency

a

complete switching

in the

sample

is observed which is due to the fact that surface molecules and bulk molecules _are in unison. Thi~

means that the

switching

starts at the rubbed surface molecules and

percolate

in the bulk

resulting

into the

complete switching

in the N*

phase. Figure

4 shows

optical micrograph by applying

a

positive

or

negative

bias

voltage

to the rubbed surface. When 0 _{to +} 10 V bias is

applied

to rubbed surface _a smectic-like

ordering

is

clearly

_seen

(Fig. 4A)

which is due to the fact that the

polyamide

surface acts like

positively charged surface,

_as

suggested by

^{Patel and}

Goodby [19].

^When

negative

^bias is

applied

cholesteric type texture is _seen

(Fig. 48).

The

switching

_response and

polarization

rever~al current due _to electroclinic effect in N*

phase

is also shown in

figure

5

by applying

a square and

triangular pulses

at 45 Hz

frequency.

It should be stressed here that in the whole temperature range the surface electroclinic effect _was

A B c _o E

Fig. 3.

Microphotographs

of lo _~m cell in N*

phase

at 69 °C at different applied dc

voltages

(Al

r = 0 V, (B) v 8 V, (Cl _c 12 V, (D) v 17 V and (E) v 24 V.

A _B

Fig.

4.

Microphotograph

in N* phase at 69 °C by

applying

either positive or

negative voltage

_to the rubbed plate. (A) 0 _{to +} lo V and (B) 0 _to lo V.

1792 JOURNAL DE PHYSIQUE II N° 12

,_ ; ~

j[. _.~ j

/~ ~ i

&44~W~W,~

'f ^°'

~

Fig.

5. Polarization reversal _current and

switching

_response to triangular and square wave

pulses

for V~~ 15 V in N* phase (ii applied triangular pulse (2)

polarization

reversal _current due _to electroclinic

effect (3)

applied

_square

pulse

and (4)

switching

response due _to electroclinic effect in _a thin (10 ~m) sample.

observed in N*

phase

for the first time, even in the blue

phase

which is observed

just

below the

isotropic phase,

in the FLC material

possessing

Sm C*-N*-Iso

phase

transition.

The electrodinic effect appears due to the strong

polar anchoring [10]

and smectic-like

ordering [18]

of the molecules within the surface

layers

^of

polymer

rubbed

samples

in N*

phase.

One should

expect

a

spontaneous

two-dimensional electric

polarization

^{which resides}

very close to the surface and is oriented

perpendicular

to the director. In order _to

probe

the electroclinic effect in _terms of

polarization,

_{an ac} electric field

(E)

of

frequency

_v is

applied perpendicular

to the

director, thereby inducing

_a small rotation of the surface molecules. This surface-driven motion then propagates

elastically

into the

samples

interior

by

_a combination of bend and twist

elasticity resulting

into _an overall motion of the bulk

samples

which is observed

in _terms of

polarization

and response ^current in the electroclinic geometry.

As is _seen in the

figures

I and 2 the electrodinic effect observed

by

dielectric relaxation in N*

phase

of

high

tilt

angle

FLC material is weak due _to the fact that the

pitch

value in N*

phase

is

quite large

and also the tilt

angle

and

polarization

^value in the Sm C*

phase

of this material is almost

independent

of temperature. ^These temperature

independent

values of tilt

angle

and

polarization

are retained within the surface

layers

of the rubbed surface _even in N*

phase.

This has been confirmed

by optical

observations

(Figs.

3 and

4).

It is worth

mentioning

here that in _a

thick

sample (25 ~m)

a very weak electroclinic effect was observed very close to the transition temperature ^{in N.~}

phase

^which indicates that the electroclinic effect due _to surface

layers

is dominant in this

samples.

Similar results have been observed in

[5, 16, 17] by optical

method

using

thin

planar

unwound

samples. Large

electroclinic effect has

recently

been observed

[6]

in the FLC material where the helical

pitch

value of N*

phase

is

quite

small

(0.5

~m).

4. Conclusions.

In summary, surface electroclinic effect induced in _a

polyamide

coated rubbed surfaces _was observed

by

dielectric relaxation and electrical

switching

method in N*

phase

of

high

tilt

angle

ferroelectric

liquid crystal

which has _a

phase

_sequence of Sm-C*-N*-Iso in _a thin

sample.

It is concluded that the surface electroclinic effect in N*

phase

is due to the strong

anchoring

and

smectic-like

ordering

of the molecules within the surface

layers

^{of rubbed}

samples.

Acknowledgments.

The authors

sincerely

thank Professor E. S. R.

Gopal,

Director, National

Physical Laboratory

for continuous encouragement ^{and interest in this work.}

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