UV-cured cholesteric polymer-dispersed liquid crystal display

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UV-cured cholesteric polymer-dispersed liquid crystal display

H.-S. Kitzerow, P. Crooker

To cite this version:

H.-S. Kitzerow, P. Crooker. UV-cured cholesteric polymer-dispersed liquid crystal display. Journal de

Physique II, EDP Sciences, 1993, 3 (5), pp.719-726. �10.1051/jp2:1993162�. �jpa-00247866�

Classification Physics Abstracts

61.30G 61.40K 78.20J

UV-cured cholesteric polymer-dispersed liquid crystal display

H.-S. Kitzerow

(I)

and P. P. Crooker

(2)

(1) Iwan-N.-Stranski-Institut, Technische Universitfit Berlin, Sekr. ERll, Str. des 17. Juni 135, 1000 Berlin 12, Germany

(2)

Department

of Physics and

Astronomy,

University ^of Hawaii, 2505 Correa Road, Honolulu, HI 96822, U-S-A-

(Received15 October 1992,

accepted

in

final form

19 January 1993)

Abstract. We describe the

preparation

and

electro-optic

characteristics of _a color

display

which makes _use of the selective reflection of cholesteric

droplets

embedded in _a

polymer

film. The

polymer

film is formed by a UV-curable adhesive, _as

opposed

to the

thermoplastic polymers

which have been used

previously.

The _new systems are easy to prepare and suitable for

operation

at room

temperature. They show fair contrast, reasonable threshold

voltages

and enhanced

switching dynamics.

1. Introduction.

Polymer-dispersed liquid crystals (PDLC)

and their

electro-optic properties

have been

extensively

studied

during

the last decade

[1, 2]. By using

the

principle

of refractive index

matching,

the field-induced reorientation of nematic

droplets

embedded in _a

polymer

film _can be

applied

to build flexible

displays, large

_area

light

shutters which _are used for

privacy windows,

_or

spatial light

modulators with

high

transmission which have

proved

to be suitable for TV

projection displays.

In the usual

mode,

the nematic PDLC

display

_{appears opaque} in

the OFF _state due _to

mismatching

of the refractive index of the

polymer

and the effective

refractive index of the

liquid crystal.

Due _to reorientation of the

liquid crystal,

its effective refractive index becomes

equal

to the

ordinary

refractive

index,

and thus the

display

becomes transparent ^{if the refractive index of the}

polymer

and the

ordinary

refractive index of the

liquid crystal

_are

properly

matched.

Switching

times less than _{a ms} and _contrast ratios well above 100

have been achieved for such devices

[1, 2].

Several

techniques

have been

developed

in order to

produce

^{PDLC films. The PDLC} can be obtained either

by

an

encapsulation

_process from _an emulsion of

liquid crystal

in _a

liquid (nematic

curvilinear

aligned phase, NCAP)

or

by phase separation

of the

liquid crystal

from _a

polymer.

In the latter _case, the

phase separation

_may be realized

by cooling

a solution of

liquid crystal

and

thermoplastic polymer (thermally

induced

phase separation, TIPS), by evaporation

of _a solvent in which both the

liquid crystal

and the

polymer

are dissolved

(solvent

induced

720 JOURNAL DE PHYSIQUE ^{II N° 5}

phase separation, SIPS),

_or

by polymerizing

the monomeric components ^of a

liquid crystal/monomer

solution

(polymerization

induced

phase separation, PIPS) [1, 2].

Recently,

^it has been demonstrated that cholesteric

[3-5]

and chiral smectic

liquid crystals [6]

_are also suitable for PDLC

applications.

The cholesteric PDLC color

display

invented

by

Crooker and

Yang [3]

makes _use of the selective reflection of

highly

chiral oriented cholesteric

droplets. By using liquid crystals

with

negative

dielectric

anisotropy

_{e~, an}

applied

^electric

field

(field-on state)

_causes a

planar alignment

of the cholesteric

pitch

_axes within the

droplets, thereby inducing

_a colored

reflecting

state of the

display.

When the electric field is zero

(field-

off

state),

the

display

_appears colorless due _to the

non-planar

orientation of the

pitch

axes.

To date, TIPS has been used _to obtain cholesteric PDLC

samples.

^The miscibilities of

liquid crystals

with

negative

dielectric

anisotropies

in various

polymers

have, however, been found to be

quite

different from those with

positive

dielectric

anisotropy.

For this _reason, many of the

polymers,

such as

poly(methyl methacrylate),

^{used for nematic PDLC}

displays (where

F~ ~

0),

do not work for cholesteric PDLCS. While _success has been achieved with

poly(vinyl butyral) [3],

the lack of suitable

polymers

for the TIPS process ^{has motivated} a search for other

techniques.

In this paper, we present ^{the first results} on cholesteric PDLC

displays prepared using

PIPS.

For the

polymer

matrix _we used _a commercial adhesive which is cured

by

tJV radiation. The

new

displays

_{are easy} to prepare and have

properties

which _{are an}

improvement

_over systems studied

previously.

In _{contrast to}

TIPS,

_no

complicated cooling

schedules _are necessary. The

droplet size,

which affects the

electrooptic properties,

can

easily

be controlled

by

the UV

intensity. However,

the most remarkable

properties

of the _new systems ^with

respect

to

applications

_are their

operation

at room temperature ^{and their fast}

switching

times. We found

switching

times of a few _ms, which is two orders of

magnitude

below the fastest time _constants

reported

for the cholesteric PDLC

samples

studied

previously.

2.

Experiment.

The

polymer

used in _our

investigations

was the UV-curable

polymer

adhesive NOA-65

(Norland).

We tried several commercial nematic

liquid crystal mixtures, namely

EN18

(Chisso, Japan),

EN38

(Chisso, Japan),

ZLI-2585

(Merck, Germany)

and ZLI-4788-000

(Merck, Germany).

These _were chiralized

by

the addition of

S-(-)-4-[(1-methyl-heptyl)-

oxybenzoyl)]-4-hexyloxy-benzoate

C~HI~O ~ ~ COO~COO-~H-C~HI~

CH~

(S811, Merck).

For EN38 and ZLI-2585 _we

got

poor

phase separation. Thus,

_{we report}

only

the results of the

following

two mixtures _:

System

A _: 10.60 ffi

5811,

32.27 ffi ENI

8,

57,13 ffi NOA-65

System

B _: 9,15 ffi S811, 27.98 ffi

ZLI-4788-000,

62.87 ffi NOA-65.

We

speculate

that the different results of the

phase separation

are related _to the chemical

structure of the

liquid crystals.

While ZLI-2585 consists of

cyano-substituted bicyclohexyl

derivatives,

the mixture ZLI-4788-000 contains aromatic

compounds

with

laterally

attached fluor atoms.

The components were stirred _at 80 °C for _ten minutes _to

give

_a

homogeneous isotropic

mixture.

Cylindrical glass

_spacers

(diameter

it

~m)

were added to the mixture while

stirring

and then the mixture _was sandwiched between _two ITO coated

glass

slides. For the

curing

process, the

samples

_were illuminated

by

UV radiation

(type A,

320-400

nm)

from _a 400 W

metal-halogen

_source.

Using filters,

the irradiance _was varied between 12

mW/cm~ (no filter,

exposure time 30

s)

and 0.12

mW/cm~ _(filter

with absorbance

2,

_exposure time 50

min).

Preliminary

studies indicated poor

phase separation

for

long

_exposure times due to

heating

of the

sample.

The

sample

was therefore held at 20 °C

during

the

curing

_process

by

_means of _a

Peltier element.

For

electrooptic studies,

AC fields up ^to 120

V~Jll

_~m at a

frequency

of I kHz _were

applied

to the

sample.

All observations _were carried _{out at room} temperature,

by reflection,

between crossed

polars.

The

intensity

of the reflected

light

_versus field

strength

was measured

using

_a

photomultiplier

connected to the

microscope tube,

and the

intensity readings

stored and

averaged

with _a

digital

storage

oscilloscope (Gould

model

4072). Switching

times _were measured

by

step ^{modulation of the}

voltage amplitude (on/off~

and

recording

of both the

applied voltage

and the

photomultiplier signal.

To _measure the reflection

spectrum,

a multichannel

spectral analyzer (Photo

Research

model

PR702A)

was used. The

sample

was illuminated

by white, linearly polarized light

incident _{at an}

angle

of 5° with respect to the surface normal. The reflected

spectra

were

recorded _{at a} reflection

angle

of 5°

using

_a crossed

analyzer.

The

angular dependence

of the reflected

light

_was studied for normal

light

incidence

using

the _same

experimental equipment.

3. Results.

In _{contrast to}

thermoplastic

PDLC

samples,

which have

only

been made to work at

high

temperatures,

our new

systems

^{A and B showed}

electrooptic

color effects _{at room} temperature.

The maximum

reflectivity

of system A _versus

voltage

is shown in

figure

la. In the field-off

state the

sample

shows _no selective reflection due _to the

non-planar

texture of the

droplets.

With

increasing field,

the

liquid crystal

becomes

gradually

reoriented with the

pitch

_axes

along

the field direction and the

reflectivity

increases. Detailed studies _on the mechanism of this reorientation have shown that the reorientation starts in the

droplet

center

[7]

and that this transformation is connected _to the appearance of _a disclination

ring

which grows with

increasing

field

strength [8].

35

30 20

25

15

$$

20

W

~~ ~ ^is

ill _lo

5

5

O O

O 20 40 60 80 lOO120 O 20 40 60 80 loo

E (V

/

11 pm) E (V

/

11 pm)

a) b)

Fig.

I. a)

Reflectivity

_versus

voltage

for

samples

of 58 II /ENl 8/NOA-65 (system A). b) Reflectivity

versus

voltage

for

samples

of S811/zLI-4788-coo/NOA 65

(system

B) cured with different UV

intensities _: (*)

12mW/cm2,

o.5 min

(Q) 1.2mW/cm2,

5 min (Z£)

o.12mW/cm2,

50 min.

JOURNAL DE PHYSIQUE II T 3. N'5. MAY I991 28

722 JOURNAL DE PHYSIQUE II N° 5

The effect of the

curing

_process is shown in

figure

16. Here the

composition

of the mixture remains constant but the

curing

time and UV

intensity

is varied _so that the total _amount of

light (that is,

^the total number of

photons) deposited

_on the

sample

is

kept

constant. The

sample

cured at

high

UV

intensity (12 mW/cm~)

_shows

a low saturation value of the

reflectivity

and

high switching voltage,

while the two

samples

cured at lower

intensity

show _a

high reflectivity

and low

switching voltages.

This result _can be understood if _we compare ^{it to} our

microscope

observations. For the

sample

cured _at low

intensity

and

long curing times,

_we observed

droplet

diameters of _a few

~m ; for the

sample

cured _at

high intensity

and short

curing

times _{we were} not able to

distinguish

individual

droplets

in the

microscope,

which indicates the

droplets

are

sub-~m

size. In order to estimate how the

scattering intensity

varies with

droplet size,

let the

sample

have N

droplets

of diameter D _so that the total volume of all the

drops

is V

~ND~.

_The

backscattered

intensity

is

just

_I~~~

=

NI~~~~,

^{and since the}

peak intensity

from _a

single drop [9]

is I~~~~

D~,

then _I~~~

~

VD~.

_Thus,

assuming

the total volume of

phase-separated liquid crystal

is

independent

of the

curing

_rate, the

larger drops

and

higher intensity

of the slow-cure process

are consistent.

The

dynamic

behavior is shown in

figure

2.

Figure

2a shows the response of the

peak

^{of the}

reflection spectrum to the

switching

_on and off of the

sample voltage.

We have found

switching

times down to _a few milliseconds

(Fig. 2a), depending

_on the

curing

rate.

Qualitatively,

_we find that the

intensity

versus time for the

switching-on

_{process can} be fit

by

a

single exponential

rise, while the

switching-off

_process

requires

_{a sum} of _two

exponential decays.

Rather than

perform

_a

precise

data

fit, however,

_we

present only

the _tum-on time vgo

(the

time

required

for _an increase of the

intensity

_to 90 ffi of its maximum

value)

and the tum-off time vj

(the

time

required

for the

intensity

to

drop

to 10 fb of its initial

value)

in

figure

2b. A

comparison

of

samples

cured at different intensities shows that the

switching

times increase with

decreasing curing

rate.

Again,

this result is consistent with _our

loo

§

o ^~'

-loo ~

i 5

fl

_jo

t~

5

o

O loo 200 300 400

t

(ms)

a)

Fig.

2. a)

Applied voltage (top)

^and

reflected light intensity

(bottom) versus time for _a

sample

of S811/EN18/NOA-65,

curing intensity 1.2mW/cm2.

b) Turn-on time r~ (*) and tum~off time

r~~ (O) versus

curing

UV

intensity

for

samples

of S811/ZLI-4788-coo/NOA~65

E~~

=

52 VIII _~m.

loco

~i

^loo

E

~

lo

i

i lo loo

(w/m2)

b)

Fig.

² (continued).

observation that _a slower _{cure causes}

larger droplets which,

in tum, cause

longer switching

times.

Although

the actual processes are

doubtlessly

_more

complicated,

a

simple

tum~off mechanism which _assumes the director is driven

by

Frank

elasticity

and slowed

by

viscous forces results in the diffusion relation _{: v}

=

y/Kq~ yD~/K,

where _y is _a

viscosity

^and

K is a Frank elastic coefficient.

Thus,

in

constructing

_a

practical cell,

a trade-off has to be made between the fast

switching

times which _occur for fast cures

(small droplets),

and the

higher reflectivity

and lower

switching voltages

favored

by

slow cures

(large droplets).

Finally, by affecting

the

droplet

radius the

curing

rate also affects the way the

reflectivity depends

_on

wavelength

and

viewing angle. Figure

3a shows the

reflectivity

_versus

wavelength

for _two

samples

of the _same material

subjected

to UV

curing

intensities of 1.2 and

30 30

~ ~

~ 20 ^~ 20

Ct £r

lo Jo

o o

400 soo soo 700 o lo 20 30 40

1 (nm) _e (°)

a) b)

Fig.

3. a)

Reflectivity

_spectra for two

samples

of 5811/ZLI-4788-000/NOA-65, cured _at different UV intensities, b)

Angular dependence

of the

reflectivity

at normal

light

incidence for the _same

samples.

(O) 1.2

mW/cm2 (Q)

12

mW/cm2.

724 JOURNAL DE PHYSIQUE II N° 5

12

mW/cm~.

As before, the

curing

times _were

adjusted

_so that each received the _same overall UV exposure.

Since there is _no

theory

yet ^for

light scattering

from

selectively reflecting droplets,

_we have tried _to understand these

lineshapes by starting

with the

assumption

that the

droplets

are

Bragg- scattering spheres.

Since the dielectric _tensor field for _a cholesteric structure with

pitch

p exhibits the

periodicity p/2,

the

peak wavelength

for the scattered

light

is

given by

the

Bragg

condition

i~

= p cos

I ₍₁₎

where

I

_{is the}

_angle

_between the

light

beam and the helical axis. The

quantities

lo

_and

^I

in

equation (I)

_are defined within the

sample.

Due to refraction _at the

sample

^surface

these

quantities

are related to the

actually

measured values

Ao

^{and outside the}

sample by

lo

=

Ao/n

^and

I

=

sin~

(sin 9/n), (2)

where _n is _an average refractive index. The line

shape

for

light

scattered from _a

Bragg- scattering sphere

is

given by [9]

f(a

=

[3 (sin

_a a cos a

) la

~]~

(3)

where _{a =} D

w/2,

« is the _vector which connects the actual wavevector

I

_{of the scattered}

light

^{with that}

^scattering

wavevector

lo

_which satisfies the

Bragg

condition

exactly.

In the _case of

backscattering,

_{a m}

Dj ()io) )I)

_~/2

=

aTnDj (A Ao)/Aj,

where _{we use}

Dj

to indicate that D is measured

parallel

to the

light

direction. The half width at half the maximum

intensity

(HWHM)

is then

given by

AA

~w~~ ⁼ 1.81

A(/(aTnDjj ).

The fits _to

equation (3)

are

given by

the solid lines in

figure

3a. The

shapes

of the

experimental

_spectra are in

good

_agreement with the

theory

_except that the

theory

has

wings

which

approach

_zero

intensity

_more

rapidly

than the data. This

rapid approach

to _zero is caused

by

the

Bragg planes

^of a

perfect Bragg-scattering sphere being

^{well defined} out to the

edge

of the

sphere.

Since _our data does not show the _same

behavior,

we conclude that the

Bragg (actually

^selective

reflection)

condition for

planes

in _our

droplets

is not satisfied to the

edge

^of the

droplet.

Note also the

slight

decrease of the half-width for the

droplets

^cured under low

curing

rates.

Qualitatively

this _means that these

drops

have _a

larger

diameter.

In

figure 3b,

_we

plot,

for the _same two

samples,

^{the reflected}

intensity

_versus

viewing angle

for

normally

incident

light.

The solid lines _are

again

fits _to

equation (3),

where _now

K =4 _aTn sin

(R/2)/Ao. Again

9, the

angle

^within the

sample,

is related to the

angle

9 outside the

sample

as described

by equation (2).

From

equation (3),

it _can be shown that the half width of this

plot

is

given by Afi~w~~

=

2 sin~

[l.81 Ao/(2 aTnD~ )],

where

D~

is the diameter of the

drop

measured

roughly perpendicular

to the

light

direction. As in

figure 3a,

the fits _are reasonable except ^{in the}

wings,

for the _{same reasons as} before.

Again,

note that

droplets

^cured under low

intensity

^{UV have} a narrower half width and hence

larger

diameter.

This feature is

qualitatively

in

agreement

with

figure

3a.

A

quantitative comparison

of the halfwidths of

figures

3a and 3b is

given

in

figure

4. Here the

droplet

diameter D is calculated from the

respective

halfwidths AA

~w~~

(giving Djj)

^and

AiHWHM (giving Di

_{as a} function of UV

curing intensity.

The decrease of the

droplet

diameter

with

increasing curing

rate is in

qualitative

agreement ^with our observations _on

scattering

6

s

(4

~3

_

a

²

1

o

i lo loo

(W/m2)

Fig.

4.

Dependence

of the effective

droplet

diameter _on the UV curing

intensity. (Q)

Dj, calculated from the

spectral

halfwidth AA~W~~ (O) D~ calculated from the

angular

width AiHWHM.

intensity.

From the

lineshape

measurements,

however,

and the ratio of Djj

/Di

=

2-3,

it _can be

concluded

that,

while the

drops

themselves _may be

spherical,

the

planar selectively reflecting

cholesteric texture is not

preserved

out to the walls of the

sphere.

This result is consistent with earlier observations made _on

large droplets

with

larger pitch

size

[7, 8],

in which the

region

of

planar

texture

properly

oriented for selective reflection _was observed to be

longer

in the direction of the field than in the direction

perpendicular

to the field. In fact, the whole

question

of textures and defects in cholesteric

liquid crystals

confined _to small volumes and

subjected

to

electric fields remains

unresolved,

and is overdue for both theoretical and

experimental

attention

[10].

4. Conclusions.

We have demonstrated that

polymer-dispersed

cholesteric

liquid crystal displays

_can be

obtained

by

_means of _a UV-curable adhesive. In

comparison

with the systems ^described

previously [3-5],

_{our new}

samples

have the

advantage

that

they

are easier to prepare, are

operable

at room

temperature,

^and are

capable

of faster

switching

times. Time _constants down to a few ms are

found,

which is _an

improvement

_over the minimum values of about 0, I s which have been found for cholesteric

droplets

in

TIPS-prepared thermoplastic polymer

films. In the latter systems, ^the

droplet

size and thus the

electrooptic properties

are very

dependent

_on the

polymer/liquid crystal

miscibilities _{as a} function of

temperature

^{and the}

cooling

rate

during

the

phase separation. However,

in the _new

PIPS-prepared samples presented here,

the

droplet

size is determined

by

the

polymerization

rate which in _tum

depends

_on the UV

intensity

used for the

curing

_process.

As usual,

compromises

have to be made in order _to

optimize

the

preparation. Large droplet

sizes

(obtained by

low UV intensities and

long curing times)

lead to

high

contrast and low threshold

voltages,

whereas small

droplet

sizes _are

required

for fast

switching. Using

medium UV

intensity (1.2 mW/cm~)

_we have succeeded in

getting displays

which _are very

promising

with

respect

to both low threshold

voltages

and fast

switching

times. We expect,

however,

that

726 JOURNAL DE

PHYSIQUE

II N° 5

the

optical

contrast can be further enhanced

by increasing

the ratio between the amounts of the

liquid crystal

and the

polymer.

Further

experiments

to

optimize

this process are in progress.

Acknowledgements.

The authors would like _to thank the Deutsche

Forschungsgemeinschaft (Sfb 335)

and the

Office of

Technology

Transfer and Economic

Development

of the

University

of Hawaii for

support

^{of this} work.

References

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16S (1988) 5 II -532.

[2] DOANE J. W.,

Liquid Crystals. Applications

and Uses, Vol. I B. Balladur Ed. (World Scientific Publishing,

Singapore,

1990) _pp. 361-395.

[3] CROOKER P. P. and YANG D. K.,

Appl. Phys.

Lett. 57 (1990) 2529-2531.

[4] KiTzERow H.-S., RAND J. and CROOKER P. P., J. Phys. ^{ii France 2} (1992) 227-234.

[5] KiTzERow H.-S., CROOKER P. P. and HEPPKE G., Liq.

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[6] KiTzERow H.-S., MOLSEN H. and HEPPKE G., Appl. Phys. Lett. 60 (1992) 3093-3095.

[7] KiTzERow H.-S. and CROOKER P. P., Liq.

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[8] KiTzERow H.-S. and CROOKER P. P., Liq.

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[9] See LANDAU L. D., LifsHiTz E. M. and PITAEVSKII L. P.,

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[10] BEzit J. and 2uMER S., Liq. Cryst, ll (1992) 593-619.