Dynamics of polymer-dispersed cholesteric liquid crystals

Dynamics of polymer-dispersed cholesteric liquid crystals H.-S. Kitzerow (*), J. Rand and P. Crooker Department of Physics and Astronomy, University H...

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Dynamics of polymer-dispersed cholesteric liquid crystals H.-S. Kitzerow, J. Rand, P. Crooker

To cite this version: H.-S. Kitzerow, J. Rand, P. Crooker. Dynamics of polymer-dispersed cholesteric liquid crystals. Journal de Physique II, EDP Sciences, 1992, 2 (2), pp.227-234. <10.1051/jp2:1992126>.

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Phys.

J.

France

II

(1992)

2

227-234

1992,

FEBRUARY

PAGE

227

Classification

Physics

Abstracts

61.30

61.40K

78.20J

Dynamics H.-S.

(*), J.

Kitzerow

Department 96822,

HI

polymer-dispersed

of

Physics

of

Rand

and

liquid

crystals

Crooker

P. P.

University

Astronomy,

and

cholesteric

of

Hawaii,

2505

Correa

Road,

Honolulu,

U.S.A.

(Received

accepted

1991,

August

13

October

22

1991)

les propri£t4s £lectro-optiques des cristaux liquides cholest4riques polymbre. L'application d'un champ 41ectrique fait [email protected] une un l'amplitude du d'apparition de cette r4flection tone r£flection de Bragg. La vitesse augmente avec diminution diambtre gouttelettes. du des champ £lectrique, la temp6rature et avec la avec L'interruption du champ dlectrique cause deux rdactions trbs diffdrentes, ddpendant du voltage et

R4sum4.

Nous

dispers6s

du

diambtre

£tud14

avons

dans

matdriau

des

gouttes.

liquid crystal cholesteric studied the electrooptic properties of droplets of dispersed in a polymer. Application of AC electric fields to the droplets leads to a conversion of When the field is suddenly the droplet from a nonreflecting to a selectively reflecting appearance. strongly with increasing field strengtli, witli switched times found to decrease are on, the switching both witli decreasing drop size. For tile switching off process, increasing and temperature different tliat relaxation switching times and microscopic indicate two textures processes can depending on the applied voltage and on tile drop size, occur, We

Abstract.

have

Introduction.

1.

for suitable which have proved to be liquid crystals (PDLC), Polymer-dispersed nematic electrooptic display applications, have been extensively investigated during the last decade have recently invented a Crooker extended by and Yang [2], who ill. This idea has been device (PDCLC). This cholesteric liquid crystals display using polymer-dispersed new unlike reflectivity. nematic of different However, provides switching between two states

displays,

PDLC the

In

PDCLC

anisotropy

and

known

exhibit

the

to

there

is

also

a

a

strong

color

effect,

dielectric (I.e., cholesteric) materials with negative sufficiently substances is small, these used. pitch p are selective reflection in the visible wavelength range [3] according to

display, chiral high chirality are

nematic

If the

equation A

(*) StraBe

Permanent

des

17.

address:

Juni135,

1000

=

Iwan-N.-Stranski-Institut, Berlin 12, Gennany.

(1)

np

Technische

Universitit

Berlin,

Sekr.

ERll,

228

JOURNAL

refractive index), provided a mean sample is observed along the helix droplets in a polymer matrix, anchoring This sudace droplet sudace. orientation

(n is

that

the

axis.

causes

Thus,

field.

director

However,

for

in

reorientation

a

absence

the

with

materials of

of

a

negative

the

PHYSIQUE

DE

helix

the

If

causes

a

II

N° 2

is

stntcture

cholesteric

planar

uniformly

materials

director

are

orientation

leads, in tum, to a complicated field, such droplets do not show

anisotropy,

dielectric

cholesteric

structure

to

a

application

uniform

oriented

and

dispersed

everywhere configuration selective of

at

the

of

the

reflection. electric

an

of

orientation

that small

as

the

field

helix

axis

direction. Consequently, the display shows a bright selective reflection if along the field sufficiently high voltages are applied. configurations occurring in cholesteric director droplets have been studied earlier by The microscopic investigations of dispersed chiral liquid crystals exhibiting large pitch [4-10]. The characteristic indicates configuration known observation of fingerprint the textures a as Frank-Pryce model [4, 5]. Application of both magnetic [7, 8] and electric fields [9, 10] has influence reorientation of the field inside the of the revealed director the droplets under a field. focuses determination This experimental of the dynamic behaviour of the on paper electrooptic switching in PDCLC samples. Using the same material reported in reference as [2], we investigate the dependence of the switching times on applied voltage, temperature and droplet size. In sections 3.1-3, we describe of the intensity of reflected light measurements time if step voltages are applied to the sample. Section 3.4 contains complemenversus some observations. tary microscopic

2.

Experiment.

The

U.S.A.),

28.0 fb

of

investigation

under

material

Aldrich,

the

22.0 fb wide

consists

chiral

temperature

of

50.0 9b

monomer

CE2

range

nematic

(by weight) poly-(vinylbutyral) (PVB Great Britain) and Dntg House, 2806 mixture ZLI (Merck, Germany)

(British

exhibiting negative dielectric anisotropy. Samples were prepared by first mixing the together with components diameter spheres with a of 31.2 ~Lm either 9.7 ~Lm or added to were thickness. A glass slide with an electrically coated conducting ITO layer

chloroform.

Glass

sample covered by the was chloroform mixture and the allowed Then, the sample was hours. evaporate for several to was heated to 130 °C in order to complete the evaporation of the solvent and the sample was ITO-coated covered by the second glass slide. In order to obtain drops of uniform size, the cooled sample down from 130°C 90°C with cooling ranging from to rate was a 3 °C/mn to 0.01°C/mn. of cholesteric observed The droplets was about at appearance 105 °C. Depending on the cooling rate, drop diameters between about 4 ~Lm (for fast cooling) obtained. and about 32 ~Lm (for slow cooling) were The controlled PID thermostat temperature of the sample was regulated using a pulsewidth (Instec/Apple He) with an than better ±0.01K. sample was The observed in accuracy reflection using a Zeiss Universal microscope with polarizers. A Jarrel crossed Ash monochromator and photomultiplier connected the microscope tube in order to to were reflected the intensity spectntm or the intensity at constant wavelength either measure versus field strength or time. I kHz altemating voltages were used throughout reported voltages are

3.

define

the

values.

rms

Results.

Under

the

influence

of

ac

fields,

electric

given by equation (I). Typically,

we

our

observed

a

samples selectively spectral halfwidth

reflect between

at

40

the and

wavelength 50

nnJ

for

DYNAMICS

N° 2

229

PDCLC

OF

reflection peak. The spectra have been previously published [2, III. The dependence of reflectivity on the field strength shows a threshold behaviour. Above the threshold Eo, the reflectivity increases reaching 3Eo. The continuously until saturation E=2 at threshold voltage Eo was found to decrease with increasing temperature from 28 V/9.7 ~Lm 67.3 °C to 20 V/9.7 ~Lm at 95.0 °C for c~m~~j 22.0 fb CE2. The cholesteric pitch of our at and thus the materialselective reflection wavelengthwith increasing also decreases Reported intensities respective wavelength of maximum measured temperature. at the were reflectivity. the

the

=

3.I

EFFECT

measured

after

t

I(t)

FIELD

fit

well

In order to investigate behaviour, the dynamic we intensity I at the selective reflection wavelength versus voltage is suddenly applied to the sample (Fig. I, inset). We find of two exponentials : sum STRENGTH.

reflected

constant

a

is

oF

.the

ac

by

a

(t)

I The

two

switching

times

rj

=

and

figure I. Both time constants strengths between 20 V/9.7 ~Lm by the relation

lo r~

+

AI [x e~

are

decrease and 60

shown

observed

the

nematic

x) e~

(l

that

(2)

~'~~]

r

(3)

E,

cc

relaxation times has also been straight line in figure I. The appearance of two of the field in reorientation director droplets [12] where they are due to disclinations sudace, respectively. of the Chiral the bulk of the droplet and drop movement at identified have unique droplets undergo a texture change, however, and two not yet we as

in

+

time

plotted versus the rms value of the applied voltage in with increasing voltage. For field exponentially V/9.7 ~Lm, this dependence is approximately described log

by

~'~~

have

processes.

io3

io2 n

~

~s

v~

~

~Ql

Q

$

D

Tz~~

~

'

my

)

1~0

aov

~l,on -zo

10~l

o

zo,o

0

constants

for

each

eo

20 E

Switch-on Fig. I. functions exponential

«

~~"

60

40

80

/9.7/tm)

(V

field strength for T 95 °C, A 452 nm. I (t) is fit by versus (inset, solid lines), so that the behaviour of the sample is described voltage. Solid lines are straight line fits to equation (4). time

=

=

a

sum

by

of two

two

time

230

JOURNAL

The

results

for

time

versus

We

in

switch-off times

in

value,

switching

the

initial

however, inset). For

note,

lines

for

time.

E

50

~

order

figure 2 irrespective

of

in

the

lower

voltage

V/9.7

~Lm,

I(t)

to



2.

The

~Lm,

35

V/9.7

range is reasonably found exhibit to

was

switching for

II

figure

25 V/9.7 =

the compare therefore the times

are

shown

process are of E

voltages

only

that

In

off

tum-off

PHYSIQUE

DE

times

shows

~Lm,

and

the

50

intensity V/9.7

~Lm.

by exponentials (solid negative the curvature at different voltages, the switch-off 10 fb of its original to retum to

for

reflectivity

the

inset

2

fit

shape.

curve

io3 50

I

v

(

~

j 4

I

l

~

z5

V

0~

I

~ -20

0

20

~

t

40

60

80

(s)

101 20

0

60

40

(v)

v intensity Fig. 2. Switch-off times v~~~ (where the 95 °C, A 452nm. strengtli for T In contrast to exponentials only for E 50 V/9.7 ~m (inset). For E

retums

to

the

10 fb

switch-on

=

~

~

switch-off

3.2 E

50

V/9.7

~m,

of

its

value) versus (Fig. I), I(t) is negative curvature

initial

process has I (t

field fit

by

at

the

time.

TEMPERATURE

Temperature

EFFECTS.-

increases

the

switching

(measured

times

at

within the V/9.7 ~Lm) by about two orders of magnitude as the temperature is reduced cholesteric (Fig. 3). The dependence of the switch-on time temperature temperature range 50

=

constants

r~,

and

r~

~~

(Eq. (2))

is

well

described

by

an

Arrhenius

equation

according

to

the

~~

relation In where

activation

the

obtain

we

In rj, ~~ When

energy activation

the

and the

temperature.

In r~, ~~, field is For

exhibits r~~~ similar that to

r~~~,

T~

'

w

2.72

10~

~

(r~ ~~)

cc

e~/kT,

(4)

e~ depends on the applied e~/k=27.8K energies and

electric

field

strength.

e~/k=25.9K

from

From the

figure 3a, slopes of

respectively.

off, we find different shapes for I(t), depending on the curve temperatures, I (t) is fit by a single exponential, with a time constant Arrhenius (Fig. 3b, solid line) with dependence slope temperature an a higher (Tm95°C~ obtained for and For temperatures rj,~~ r~,~~. switched

lower

K~ ~), I

(t) is

not

exponential

and

the

relaxation

times

can

be

very

slow.

From



DYNAMICS

2

OF

231

PDCLC

10~

(a)

Tz,on u

~~3

a

Ti,an

~

i~~ z j /ioi loo

D

u

(b)Q I I I I

I

10~ l I

~s

W

I

~'

z~

l I

lo i

I I

2.7

Z-B

2.9

3

1/T (1/1000 K) Fig. v2

3.

plot

Awhenius

corresponding

for

these

defined

as

text.

experiments

relaxation may

in

not

and

from

mechanisms be

an

such

parameters,

are

oft fits

(t) to

to

double

equation (4)

in

effect,

field

Eo,

the

but

(a)

Switch-on

3. I,

slow rather

we

conclude

mechanism due

to

the

DROP

Different

EFFECTS.

SIZE

cooling the sample for field (measured

130°C

from

to

90°C

and

process : v~ Switch-off

that

~~

times

eye.

different

two

high temperatures dependence of other

at

temperature.

on

for the sample same cooling Time rates. 150V/31.2 ~m) tend increase with increasing drop to D for both the switch-on and the switch-off (Fig. 4). These dependencies processes ( as might be expected from follow obvious mathematical rifle, such as r~, any cc D diffusive For the switch-off I (t) again shows time dependences, two process. process, slow mechanism occurring for 32 droplets. very ~Lm

3.3

drop

;

~m).

exponential (Eq. (2)). (b) dashed line is guide to the

section

of

occurrence

thermal

V/9.7

50 =

a

reported

results The

threihold

the

(E

constants

fits

the

exist.

intrinsically as

time

square Solid lines

on

v~j,

tile

least

to

sizes

were

with

obtained

cell

different

by

constants

diameter do

not

purely

a

~~

3.4

MICROSCOPIC

reflection axis

originates

oriented

along

OBSERVATIONS.

from the

a

uniformly

field

direction

-Microscopic oriented region (Fig. 5a).

The

observations in the

reveal

of the

center

diameter

of

these

that

droplet regions

the

with

the

selective

with

its

grows

helix with

JOURNAL

232

PHYSIQUE

DE



II

,,~

io4 ,'

2

Toff

,

,'

io3

,"

R" , , ,

io z

~

w

, , '

-

~

,,

10~

,

~

,' ,' ,

~

iQ0

10

0

Fig.

Influence

4.-

V/31.2

150

E

of

the

drop

diameter

30

20

D

40

~~lm)

D

the

on

switching

times

T~,~~,

v~,~~

and

v~~~

for

~Lm.

=

investigations on with microscopic observation is in agreement increasing field strength. This droplets with a pitch of several ~Lm [9, 10]. cholesteric large droplets, the dynamics of the reflectivity change corresponds to a reversible For small growth and shrinkage of the oriented region in the drop center. The material at the periphery observed that high oriented by the field. For large droplets, we however, of the drop is not, droplets the region. For these surface entire drop, including the field strengths can orient the minutes several quite few seconds is stable from selectively reflecting up to a appearance (depending on the drop size and the field strength) after the voltage is switched off. After this of the drop while it is still selectively in the time, a non-reflecting region center appears after metastable configuration 5b). This (Fig. droplet periphery reflecting at the some : seems reorganization and complete undergoes determined time, the droplet randomly texture a I

Our

size

demonstrate on

texture.

This

slow

curvature

just

after

relaxation the

switch-off

process time.

field

corresponds

to

those

cases

conclusions.

and

results

droplet

negative

(t) has

Summary

4.

sphentlitic

the

to

retums

where

the

the

dynamic

influence

of

the

parameters

behavior

of

the

electric

field

strength,

effect

in

temperature cholesteric

and

droplets

spherical cavities in a polymer matrix. Previous investigations [9] on the configuration within larger such droplets have shown that the director pitch is cholesteric described by the sphentlite model [4] which is droplets with large characterized by planar anchoring at the droplet sudace. In these previous studies, the same compositions) were used as in the system reported here and the ratio materials (but different similar (D/p 50 ~Lm/5 ~Lm) to that of our between the drop diameter D and the pitch p was 5 ~Lm/300 nnJ). Therefore, the director configuration in our droplets is most samples (D/p described model. In with previous investigations [9], the likely by the agreement same reflection occurring in our samples under the influence of electric fields is due to the selective of each droplet, of a helicoidal with the helix axis parallel in the center structure appearance, diameter field. On increasing voltage, the of this reoriented region grows continuously to the with increasing field strength. confined

static

in

small

behaviour

of

=

=



DYNAMICS

2

OF

233

PDCLC

a)

$#

«

W

b) (a) Textures of relatively small droplets at moderate field strength : the central regions exhibit off. (b) which become continuously smaller when switched the voltage is planar texture loo V/31.2 ~m). Some of of large drops about 10 min after switching off a high voltage (E Texture relaxation non-planar oriented region appears in the center, inverse the larger drops show process an a direction. axis oriented parallel to the former field while at the outside of the drop the helix remains

Fig. 5.

uniform

a

=

We

have

shown

high voltages, can

the

be

well

time JOURNAL

that

high

DE

by

described

is

constants PHYSIQIJE

II

switch-on

the

double

a

described T

2, N' 2,

by

FEBRUARY

polymer-dispersed droplet diameters. The exponential and the function, times

and

temperature,

an 1992

for

small

Arrhenius

relation.

However,

droplets is reduced by time intensity curve versus dependence of temperature for the switch-off process,

chiral

ii

234

JOURNAL

intensity

both

relaxation

spherulitic high voltages the

is

removed.

that

in

induce

can

this

first

by

affected

can

the

from

configuration

reorientation

field

and

can

initiate

which from

starts

persisting boundary layer

a

sudace

the

at

the

the

relaxation,

the

the

of

is

2



microscopic investigations small droplets and small

inward

director

a

case

case

occur.

the

II

For

proceeds

texture

In

the

and

measurements

mechanisms

PHYSIQUE

DE

that

indicate

voltages droplet.

metastable

large

For

when

while

in

the

second

We

field

conclude

interface case

to

droplets,

extemal

the

droplet. center polymerfliquid-crystal of the

different

two

reorientation

the

high

is

not

field

volume of the drop. This leads to an director field in the entire strengths. field high electric indicate results that small droplet sizes and high With respect to display applications, our Conceming the applied field strength, compromises have favour fast switching. temperatures but too high voltages found : high voltages hasten the switching to be process, can on cause PDCLC displays interesting On hand, be long switch-off times. the other storage may as very

strengths

devices

change

cause

extremely

due

of the

for

relaxation

slow

their

to

slow

very

relaxation

at

low

temperatures.

Acknowledgments. H.-S.

K.

Hawaii

like

would

for

their

thank

to

very

kind

the Department hospitality. This

of

Physics

research

and was

Astronomy at the University of partially supported by Research

Corporation.

References

ill

DOANE

J. W.,

Cryst.

165

GOLEMME

(1988)

A.,

WEST

J.

L.,

WHITEHEAD

J. B.

and

WU

B.-G.,

Mol.

Cryst. Liq.

511-532.

2529-31. [2] CROOKER P. P. and YANG D. K., Appl. Phys. Lett. 57 (1990) 219-226. [3] DE VRIES H. L., Acta Crystallog. 4 (1951) Faraday Soc. 25 (1958) 29-42. [4] ROBINSON C., WARD J. C. and BEEVERS R. B., Disc. 467-494. [5] RoBiNsoN C., Mol. Cryst. 1 (1988) [6] CANDAU S., LE ROY P. and DEBEAUVAIS F., Mol. Cryst. Liq. Cryst. 23 (1973) 283-297. 1599-1607. [7] PATEL D. L. and DU PRE D. B., J. Polymer. Sci. 18 (1980) 1899-1923. [8] BOULIGAND Y. and LIVOLANT F., J. Phys. 45 (1984) 245-51. [9] YANG D. K. and CROOKER P. P., Liq. Cryst. 9 (1991), [10] KITzEROW H.-S. and CROOKER P. P., Liq. Cryst., in press. Ferroelectrics 122 (1991) 183-196. H.-S. [ll] KITzEROW and CROOKER P. P., [12] DRzAIC P. S., Liq. Cryst. 3 (1988) 1543-59.