Physical properties and alignment of a polymer-monomer ferroelectric liquid crystal mixture

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Physical properties and alignment of a

polymer-monomer ferroelectric liquid crystal mixture

D.S. Parmar, N.A. Clark, P. Keller, D.M. Walba, M. D. Wand

To cite this version:

Physical properties

and

_alignment

of

a

_{polymer-monomer}

ferroelectric

_liquid

_crystal

mixture

D. S. Parmar

_(1),

N. A. Clark

_(1),

P. Keller

_(4),

D. M. Walba

₍₂₎

and M. D. Wand

₍₃₎

(1)

Department

of

_Physics

and Center for

_{Optoelectronic}

_Computing

_Systems,

Boulder,

Colorado 80309, U.S.A.

(2)

Department

of

_Chemistry

and Center for

_{Optoelectronic Computing Systems, University}

of

Colorado, Boulder, CO _80309, U.S.A.

(3)

Displaytech,

Inc., 2200 Central Ave., Boulder, CO _80301, U.S.A.

(4)

Laboratoire Leon Brillouin

_(CNRS),

CEN

Saclay,

91191

Gif-sur-Yvette,

France

(Reçu

le 29

juin

1989,

accepté

le 5 octobre

₁₉₈₉₎

Résumé. 2014

Nous décrivons un nouveau

_{polymère mésomorphe}

en

_{peigne ferroélectrique}

appartenant à

la famille des

_{polysiloxanes}

_qui

est miscible en toutes

_proportions

avec un

_composé

ferroélectrique

classique,

le

_{4-décyloxybenzoate}

de

_{4’-(s)(4-méthylhexyl)oxyphényl.}

Le

_mélange

présente

une transition

d’alignement :

pour les concentrations élevées en

_polymère,

les

_plans

smectiques

s’orientent

_{parallèlement}

à la _{direction de frottement alors que pour des} concentra-tions faibles en

_polymère,

les

_{plans smectiques}

s’orientent

_{perpendiculairement à}

cette direction

(cette

dernière situation est

_typique

des

_petites

molécules

_{ferroélectriques).}

Les échantillons orientés

_présentent

une commutation

_{électro-optique}

avec un contraste _élevé, ce

_qui

a

_permis

d’étudier le

_signe

et

_{l’amplitude}

de la

_{polarisation ferroélectrique}

et sa variation avec la

température,

ainsi que l’évolution de

l’angle

d’inclinaison des molécules avec la

_température

et la

réponse optique

en fonction du

_{champ appliqué.}

Abstract. 2014 We

present a new Ferroelectric

_{Liquid Crystal}

_(FLC)

siloxane-based

_polymer

which exhibits

_{complete miscibility}

with the low molecular

_weight

FLC

(4’[(S)-(4-methyl-hexyl)oxy]phenyl-4-(decyloxy)benzoate).

The mixture exhibits an

_alignment

_transition,

_orienting

with the smectic

_{layers along}

the

rubbing

direction at

_{high polymer}

concentrations and,

typical

of low molecular

_{weight liquid crystals,}

normal to the

_rubbing

direction at low

_polymer

concentra-tions. The

_aligned

mixture exhibits

_high

contrast

_{electro-optic switching}

and has been

_physically

characterized with respect to the

sign

and

magnitude

of the ferroelectric

_polarization

and its

temperature

dependence,

temperature variation of

_optical

tilt

_angle

and the

_optical

_response to an

applied

electric field. Classification

Physics

Abstracts 61.30 - 64.40

1. Introduction.

During

the

_past

_decade,

a number of

_{liquid crystalline polymers}

have been

_synthesized

and

characterized with

_respect

to their

_{thermodynamic}

and

_{physical properties.}

These

_polymer

liquid crystals

include

_mesogenic

_{groups in the main chain}

_[1]

or as

_part

_{of the side group}

_[2]

or

_polymers

with more

_complex

structures

_[3].

_Recently,

there has been

_increasing

interest in

polymer

ferroelectric

_{liquid crystals}

in which a _{permanent electric}

_{dipole density}

is obtained

by

incorporation

of _{chiral side groups} to form a tilted chiral smectic

_{phase analogous}

to those formed

_by

low molecular

_weight

_liquid

_crystals.

Of

_particular

interest is the

_potential

of the

polymer

ferroelectric

_liquid

_crystals

and their mixtures with low mass ferroelectric smectic C*

liquid crystals

for

_application

in the fast

_switching,

bistable surface stabilized ferroelectric

liquid crystal

(SSFLC) electro-optic

devices of Clark and

_Lagerwall

_{[4, 5].}

A

_variety

of

_polymer

ferroelectric

_liquid

_crystals

have been

_synthesized

to date

_[6-16],

however,

not much work has been done on mixtures with monomer

_homologs

or on the

physical

characterization of

_polymer

FLC materials and mixtures for SSFLC

_{applications.}

In the

_present

_paper, we

_report

our results on SSFLC

_{alignment, phase}

characterization,

electro-optic switching

responses,

optical

tilt

_angles

and ferroelectric

_polarization

measurements on a new

_polysiloxane

[17]-chiral

_epoxide

_[18]

based side chain

_polymer

ferroelectric

_{liquid crystal}

W219

_[19]

_synthesized

in our

_laboratory,

and its mixtures with the low mass FLC W82

_[20].

2. Phase

diagram.

The chemical structure and the

_phase

transition

_temperatures

for W219 and W82 are

_given

in

figure

1. The

_phase

identification has been done

_{using polarizing microscopy.}

On

_cooling

from

_{isotropic liquid phase,}

W219

_undergoes

a

_phase

transition to the C*

_phase

at 147 °C and continues in this

_phase

until 85

°C,

below which it

_undergoes

a

_glass

transition which is maintained down to room

_temperature.

We have not seen

_samples

form the

_crystal

phase

from the

_{glass phase}

even after

_keeping

them for several weeks at room

_temperature.

Fig.

1. -

Structure and transition _temperatures for W219 and W82.

Fig.

2. - Phase

Since the

_starting

material at room

_temperature

is a

_{crystalline phase,}

we believe the

_glassy

state is

_supercooled

from the C*

_phase.

The

_miscibility

of the

_polymer

ferroelectric

_{liquid crystal}

W219 with the low mass

ferroelectric

_liquid

_crystal

W82 has been tested. These

_components

_{mix very well in the entire} range of

polymer

FLC concentrations as shown in

_figure

_{2. It may be noted that} at - 80 % of

W219,

the smectic A

_(SmA)

_phase

_{appears and continues}

_through

_{the entire range towards} W82. The C* range is

quite

large

in all the W219-W82 mixtures.

3.

_Alignment.

In smectic

_polymer

side chain

_liquid

_crystals

the _{side groups} are

_organized

into

_layers,

much

like the monomeric

_smectics,

while the backbone chains are confined

_largely

to lie in the

layering planes.

Thus the

_phase

orientation

_{produced by}

an

_anisotropic

surface treatment will

depend

on which

_part

of the

_polymer

molecule is most

_{strongly coupled}

to the surface

anisotropy.

An

_anisotropic

surface

_(e.g.

rubbed

_nylon)

which orients the backbone

_parallel

to

a

_particular

direction

_{(z )}

in the surface will induce smectic

_{layers parallel}

to z.

_If,

on the other

hand,

the

_coupling

is

_primarily

to the _{side groups,}

_then,

as in low mass

_liquid

_crystals,

the side

chain molecules line up

along

z

_forming

the smectic

_layers

_{perpendicular}

to z. In our side chain

polymer-monomer homolog

liquid

crystal

mixture,

we observe a transition between these two

alignment modes, obtaining

the former

_(layers

//

_z)

at

_{high polymer}

concentration and the latter

_(layers

..L

_z)

at low

_polymer

concentration. We used

_nylon

brushed surfaces to

_align

our

polymer

FLC

_samples.

The ITO coated

_glass

_plates,

one of which

_{having polyimide}

_{spacers of}

~ 3

um, were

_nylon

surface coated and then hair brushed in one direction and assembled in a

Fig.

3

_(continued).

way such that the

rubbing

direction on the two surfaces

_sandwiching

the

_liquid

_crystal

was

parallel.

The

_polymer

FLC was heated to the

_isotropic

_liquid

_phase

to _{fill the cell. It may be}

Fig.

4. - A shear

_aligned

_{pure W219 cell} on

_nylon

coated and brushed ITO

glass.

(a)

Bright

state and

(b)

Black state with

switching

the field in

appropriate

direction. The cell is

_strongly

bistable and the two states are surface stabilized in the entire range of temperature. The arrows indicate the

_rubbing,

shear

As _{the pure W219}

_sample

is cooled from the

_isotropic

_phase,

smectic C focal conics start

appearing

below 146 °C as shown in

_figure

3a. The focal conics are lined up

_{perpendicular}

to

the direction of

_rubbing

_indicating

the

_layer

orientation

_along

the

_rubbing

direction,

indicated

by

the arrows. The focal conics grow in size as the

_sample

is further cooled

_{(Fig. 3b).}

A

_slight

shear

_along

the direction of

_rubbing

makes these focal conics _{grow normal} to the

_shearing

direction

_{(Fig. 3c).}

Simultaneous

_cooling

and

_shearing

leave the

_sample

in a

_{perfect planar}

orientation

_(layers

_parallel

to the

_rubbing

and

_shearing

_direction),

the surface stabilized

_bright

state of which is shown in

_figure

4a.

_Application

of an electric field in

_appropriate

direction

switches the

_sample

to the other surface stabilized state shown as black in

_figure

4b. The cell in

_figure

4 is

bistable,

either state

_remaining

even when the field is removed.

It may be noted here that in pure

W219,

the

_alignment

is maintained even below the

_glass

transition

_{(85 °C)}

_temperature.

_Upon

_cooling

the cell under a d.c. field

( -

10 7

V/m)

to room

temperature,

it was found to be

_poled,

a

_voltage

of - 0.6 V

_appearing

measured across it upon

removing

the external

_voltage

source at room

_temperature.

A further

_interesting

_aspect

of the

W219 cell is the absence of

_zig-zag

walls

_[21].

-W219 was mixed with W82 and a 20 %

_{by weight}

mixture of W219 in W82 was observed to

have a

_phase

_sequence I-A-C*. the actual

_phase

transition

temperatures

are shown in

_figure

2. Smectic A

_layers

in this mixture are formed

_{perpendicular}

to direction of

_rubbing

and shear as

is

_typical

for low molecular

_{weight liquid crystals}

on the

_nylon

coated

_{glass plates.}

The

typical

texture of the shear

_aligned

cell in the C*

_phase

has a texture as shown in

_figure

5a. The

commonly

observed

_zig-zag

wall defects

_[21]

are observed in this case as shown in

_figure

5b.

Fig.

5. - C* texture in _a shear

aligned

cell of 20 % W219 in W82.

_(a)

The smectic

_layers

are

Fig.

5

_(continued).

Thus as the concentration of W219 is

decreased,

the

_{layer alignment}

shifts from

_layers

_parallel

to the

_rubbing

_(polymer)

to

_{layers perpendicular}

to the

_rubbing

_(monomer).

The crossover occurs for an

_{approximately}

1:1 mixture.

Cooling

the

_sample

of 1:1

mixture,

in a

_nylon

coated and rubbed ITO

_glass

cell,

from

parallel

to the direction of

_rubbing

and the other

_{perpendicular}

to it. There are more focal

conics oriented

_{perpendicular}

to the

_rubbing

direction than

_along

it. Thus shear

_alignment

along

the

_rubbing

direction is easier and

_produces

a cell with fewer defects as shown in

figure

5c where the

_{general alignment}

is

_good

and

_planar

with

_only

a small number of defects

seen in the form of black lines. No

_zig-zag

wall defects are observed in this case. This

_sample

is surface stabilized and bistable as for _pure

W219,

it

_ending

_{up in} a

_glassy

state _upon

_cooling

to

room

_temperature

as does W219.

4. Polarization and tilt

_angle

measurements.

The

_{typical sample}

_geometry

for the measurements is shown in

_figure

6. The C*

_phase

is sandwiched in between two ITO coated

_{glass plates}

on which a _{very thin}

_nylon

film is

deposited

and the

_surfaces

are brushed

_parallel

to the

_plates.

The

_plates

are

separated by

a - 3

um thick

polyimide

spacer

deposited

on one of the

_plates.

The area of the ITO electrode is 1.17

cm2.

The SSFLC

_sample

cell thus behaves like a thin

_birefringent

slab in which the

optic

axis rotation can be induced

_by

the

_application

of a

_voltage

_[22].

Tilt

_{angle 0}

has been measured as a function of

_temperature

_{by measuring}

the

_angle

between the two stable states of the cell with an electric field

_applied.

For a

_given

direction of

the

field,

the C* director is

_{placed along}

one of the

_polarization

directions of the crossed

Fig.

6. 2013

polarizers

of the

_{optical microscope.}

In this situation the cell

_{extinguishes.}

The electric field is reversed and

_depending

on the

_sign

of

_{polarization,}

the

_sample

is rotated so that the director lines up

along

the same

_polarization

direction. The

_angle

between these two director orientations is a measure of twice the tilt

_angle

0.

The

_polarization

P has been measured

_{by integration}

of the

_polarization

current

_peak

observed

_during

the

_application

of a

_triangular

wave field. The

_typical

current

_peak

and the

driving triangular

wave field traces on a dual trace

_oscillogram

are shown in

figure

7. The

current

_peak

is

_integrated

with the

_help

of the

_storage

_oscilloscope

and a

_computer.

Fig.

7. -

Typical oscillograms

and the

_{resulting polarization}

current

_peak

with a 30 V

_peak

to

_peak

triangular

wave

_driving

field on W219

_{(200 ms/div).}

Typical

variation of tilt

_angle

0 and

_polarization

P with

_temperature

are shown in

figure

8a,

b for pure

W219,

W82 and their mixtures

_given

in table I. It is

_interesting

to note _{that for pure}

W219, 0

stays

almost constant art - 29.5° in a

_large

_temperature

_{range and}

_{drops only}

_slightly

near the first order I-C* transition. For W82 and its other mixtures with W219 which all show A

_phase

above the C*

_{phase, 0 drops continuously}

to zero near the A-C* transition

temperature

(Tc)

indicative of a second order A-C* transition. In

_figure

_8,

_Tc

for W219 is the I-C* transition

_temperature.

There is a

_jump

in 0 even for lower concentrations of W219

(Fig. 8a).

For

_example,

at

_{Tc -}

T = 21.0

°C,

0

jumps

from 16.5° for 100 % W82 to 26.2° for a

20 % mixture of W219 in W82. Further increase in W219 concentration does not

_{change 0}

to

Table I. 2013 Materials used in the

any considerable extent as is evident from the fact that the saturation value of 0 for pure W219 is 31.0°.

The

_temperature

variation of P for W219 and its 20 % and 50 % mixtures with W82 is shown in

_figure

8b.

Since,

W82 is very

weakly

ferroelectric,

independent

measurement of its

polarization

has not been

_possible

and the saturated value of its

_polarization

has been

determined

_{by extrapolation}

of

_polarization

with various mixtures. The

_polarization

of W82 is P = - 0.4

nC/cm 2.

It is further noted from

figure

8b that whereas P for W219-W82 mixtures

falls

_continuously

to zero at

_{Tc -}

T * = 0 °C for pure

W219,

it

drops only slightly

_near the

I-C* transition

_temperature.

Fig.

8. -

Temperature dependence

of

_(a)

0 and

_(b)

P for W219 concentrations :

0 - 0 % ; . - 20 % ;

o - 50 %; + - 100 %.

Fig.

9. - The variation of the saturation value of P with W219 concentration in W82 is linear.

From the

_extrapolated

value of P for W82 and

_using

the saturated values of P for its mixtures with

_W219,

a linear

_relationship

between the W219 concentration and P is obtained

5.

Response

time measurements.

Optical

response to an

_applied

_step

_voltage

has been measured

_{using polarizing microscopy}

and an associated

_{photodetector}

_arrangement.

A

_typical

dual trace

_{oscillogram respectively}

for a

_voltage

_step

from - 15 V to + 15 V and the

_{corresponding optical}

_response

_signal

for W219 is shown in

_figure

_{10. Similar responses} are obtained for all the materials under

investigation.

The

_symmetry

in the

_shape

of the

_optical

_response curve for both directions of

the electric field

_suggests

that the C*

_layers

are more or less normal to the

_plates

_[22]

in W219 and no

_{significant layer}

tilt can be

_interpreted

from these observations.

_X-ray

measurements are,

however,

_required

to confirm this

_preliminary

observation.

_By

_changing

the

_magnitude

of the

_{applied voltage,}

the time for 10 % to 90 % rise in

_intensity

of the transmitted

_light

_(defined

here as rise time

_Tr)

and rise time variation with

_applied

field at different

temperatures

for

Fig.

10. - Dual

trace

_oscillogram

of a

_{typical optical}

_response curve with a 30 V p-p square wave

applied

to W219

_{(100 ms/div).}

W219 has been measured as shown in

figure

11a. The

delay

time Td defined as the time for the first 10 % rise in transmitted

_{light intensity}

at the same

_temperatures

is shown in

_figure

11b. In a

_{log-log plot,}

the rise time vs. the

_applied

field

_{relationship,}

as shown in

_figure

12 is linear.

Accordingly

the _T

_{r = 11 1 P E}

_relationship

holds for

_{polymer liquid crystals}

too. For a fixed

applied

field of - 1.5 x

107 V/m,

we have measured the _Tr as a function of

_temperature

for W219 and other materials under

_{investigation.}

The results are shown in

_figure

13.

It is

_interesting

to note that :

1)

For

_W219,

_T-r r increases near the I-C* transition

temperature.

On the other

hand,

mixtures with W82 demonstrate a fall near the A-C*

transition ;

Fig.

11. - Variation of

(a)

rise and

_(b)

_delay

times with

_{applied voltage}

for a 3 um thick

sample

of

W219 ; 0 -

135* ;

n -

_{130° ; # -}

120 *C.

Fig.

12. 2013

Fig.

13. - Rise time

vs.

_{Tc -}

T for the W219 and W82 mixtures of table 1.. - 20

% ;

D - 50 % ;

+ -

100 %.

6. Conclusions.

1.

_By

_using

rubbed

_nylon

surfaces,

we have been able to

_{align polymer liquid crystals}

and their mixtures with low mass ferroelectric

_{liquid crystals.}

The

_alignment

so obtained is as

_good

as for any low mass FLC.

2. The

_growth

of C* focal conic domains relative to the direction of

_rubbing

on a

_nylon

surface indicates that the

_polysiloxane

backbone in these side chain

_{polymers prefers}

to line up

along

the

rubbing

direction

resulting

in smectic

_layer

orientation in the

_rubbing

direction. Since the extinction even in the

_{isotropic liquid phase}

_{of the pure}

_polymer

is

_incomplete,

we

believe that the

_polymer

backbone remains

_always

_{lined up} on the surface in the

_rubbing

direction.

3. As a result of the

_polymer

backbone

_lining

_up

_along

the

_{rubbing direction,}

for

_polymer

concentrations of

_greater

than 50 % the smectic

_layers

are formed

_along

the

_rubbing

direction

with the

_polymer

backbone in the

_plane

of the

_layers

and the

_sample

needs to be sheared in the

_rubbing

direction in order to

_get

almost

_{perfect planar alignment.}

_{This may be}

_compared

to low mass FLC’s where

_shearing

normal to the

_rubbing

direction leads to

_{good alignment.}

At low

_polymer

concentration,

the usual

_alignment

of smectic

_layers

normal to the

_rubbing

is obtained.

4. The absence of

_zig-zag

walls at

_{high polymer liquid crystal}

concentration indicates that the smectic

_layers

are more or less normal to the

_plates

and do not form chevron texture

_[21].

5. The tilt

_angle

and

_polarization

behaviour with

_temperature

of the

_polymer

_{liquid crystal}

is similar to that for low mass

_{liquid crystals. Although}

the

_optical

_{response is slow}

_owing

to

very

high

orientational

viscosity

of the

_polymer,

the

_switching

mechanism seems to be the

same as for the low mass FLC’s.

Also,

the mixtures with low mass FLC’s reveal the total

Acknowledgments.

This work was

_{supported by}

NSF Contract DMR

8611192, by

NSF Grant CDR 8622236

through

the National Science Foundation

_Engineering

Research Center for

_{Opto-electronic}

Computing Systems,

and

_by

the Office of Naval Research.

References

[1]

Eds. M. Gordon and N. A. Platé,

Liquid Crystal Polymers,

Adv.

_Polymer

Sci. I-III

₍₁₉₈₄₎

59.

[2]

Eds. A. Ciferri and W. R.

_{Krigbaum, Liquid Crystal Polymers}

_{(Academic Press)}

1982.

[3]

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₍₁₉₈₄₎

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SHIBAEV V. P., KOZLOVSKY M. V., BERESNEV L. A., BLINOV L. M. and PLATE N. A.,

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DECOBERT G., DUBOIS J. C., ESSELIN S. and NOEL C.,

Liq. Cryst.

1

₍₁₉₈₆₎

307.

[8]

ESSELIN S., BOSIO L., NOEL C., DECOBERT G. and DUBOIS J. C.,

Liq. Cryst.

2

₍₁₉₈₇₎

505.

[9]

DECOBERT _G., SOYER F., DUBOIS J. C. and DAVIDSON P.,

Polym.

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₍₁₉₈₆₎

549.

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₍₁₉₈₈₎

125.

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Phys.

Rev. A 37

₍₁₉₈₈₎

2251.

[13]

ZENTEL _R.,

_{Liq. Cryst.}

3

₍₁₉₈₈₎

531.

[14]

MÜLLER U. , SCHEROWSKY G., SPRINGER J., TRAPP W., UZMANN M., Presented at the Twelfth Int’1.

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ZENTEL R., RECKERT R., BECK B.,

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2

₍₁₉₈₇₎

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[16]

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_{(in press) ;}

KAPITZA H. and ZENTEL R., Makromol. Chem.

_{(in press)}

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₍₁₉₈₈₎

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[18]

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₍₁₉₈₉₎

8273; W219 is shown in

_figure

1.

[20]

W82 is

_{4’[(S)-(4-methylhexyl)oxy]phenyl-4-(decyloxy)benzoate}

described in

_(KELLER

P., Ferroelec-trics 58

_{(1984) 3)}

obtained from

Displaytech

Inc.,

Boulder,

Co. 80301.

-

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₍₁₉₈₇₎

2658 ;

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₍₁₉₈₈₎

79.