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251

FERROELECTRIC LIQUID
CRYSTALLINE ELASTOMERS

Introduction

Ferroelectric materials are a subclass of pyro- and piezoelectric materials
(Fig. 1) (see P

IEZOELECTRIC

P

OLYMERS

). They are very rarely found in crystalline

organic or polymeric materials because ferroelectric hysteresis requires enough
molecular mobility to reorient molecular dipoles in space. So semicrystalline
poly(vinylidene fluoride) (PVDF) is nearly the only known compound (1). On the
contrary, ferroelectric behavior is very often observed in chiral liquid crystalline
materials, both low molar mass and polymeric. For an overview of ferroelectric
liquid crystals, see Reference 2. Tilted smectic liquid crystals that are made from
chiral molecules lack the symmetry plane perpendicular to the smectic layer struc-
ture (Fig. 2). Therefore, they develop a spontaneous electric polarization, which is
oriented perpendicular to the layer normal and perpendicular to the tilt direction.
Because of the liquid-like structure inside the smectic layers, the direction of the
tilt and thus the polar axis can be easily switched in external electric fields (see
Figs. 2 and 3).

Here, we discuss materials (LC-elastomers) that combine a liquid crystalline

phase and ferroelectric properties (preferably the chiral smectic C

∗ phase) in a

polymer network structure (see Fig. 4). The coupling of the liquid crystalline di-
rector to the network or the softness of the network is chosen so that reorientation
of the polar axis is still possible. Thus, densely cross-linked systems that possess
a polar axis but cannot be switched (11) will be excluded. It is the role of the
network (1) to form a rubbery matrix for the liquid crystalline phase and (2) to
stabilize a director configuration. LC-materials that have these properties can be
made either (see Fig. 4) by covalently linking the mesogenic groups to a slightly

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

252

FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS

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P

s

E

Ferroelectric

Pyroelectric

Piezoelectric

Ferroelectric

Pyroelectric

Piezoelectric

Fig. 1.

Ferroelectric hysteresis that shows the spontaneous polarization P

S

of a ferroelec-

tric material reversed by an applied electric field E.

P

E

Electrodes

+

θ

z

n

−

θ

z

n

E

P

Fig. 2.

Schematic drawing of the bistable switching of a ferroelectric liquid crystal in the

“surface stabilized FLC” configuration.

cross-linked rubbery polymer network structure (see Fig. 4a) (4–6) or by dispers-
ing a phase-separated polymer network structure within a low molar mass liquid
crystal (see Fig. 4b) (9,12). Both systems possess locally a very different structure.
They may show, however, macroscopically similar properties.

LC-elastomers (see Fig. 4a) have been investigated in detail (4–8). Although

the liquid crystalline phase transitions are nearly unaffected by the network, the
network retains the memory of the phase and director pattern during cross-linking

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FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS

253

(a)

(b)

Fig. 3.

Network: soft, can be transformed like rubber band, but retains its shape and

couples to director orientation because (a) director is preferably parallel (or perpendicular)
to polymer chains (LC-elastomer) (4–9); (b) director aligns (parallel) to chains in oriented
phase-separated polymer network structure (low molar mass LC in LC-thermoset) (9,10).

(7,8). In addition, it freezes fluctuations of the smectic layers and leads to a real
long range order in one dimension (13). An attempt to change the director pat-
tern by electric or magnetic fields in LC-elastomers leads to a deformation of the
network and to an elastic response (see Fig. 3). As a consequence of this, nematic
LC-elastomers could never be switched in electric fields, if the shape of the elas-
tomer was kept fixed. For freely suspended pieces of nematic LC-elastomers, shape
variations in electric fields have been observed sometimes (14,15). In ferroelectric
liquid crystals, the interaction with the electric field is, however, much larger.
Thus, it has been possible to prepare real ferroelectric LC-elastomers (see Fig. 3)
(3,16). In these systems, the polymer network stabilizes one switching state like
a soft spring. It is, however, soft enough to allow ferroelectric switching. There-
fore the ferroelectric hysteresis can therefore be measured in these systems. It is,
however, shifted away from zero voltage (see Fig. 3).

Synthesis of Ferroelectric LC-Elastomers

The ferroelectric LC-elastomers described so far (3,16–21) are mostly prepared
from cross-linkable ferroelectric polysiloxanes (see Fig. 5), which are prepared by
hydrosilylation of precursor polysiloxanes (22) (see S

ILICONES

). The cross-linking

is finally initiated by irradiating a photoradical generator, which leads to oligomer-
ization of acrylamide or acrylate groups (see Fig. 5). The functionality of the net
points is thus high (equal to the degree of polymerization) and varies with the
cross-linking conditions.

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FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS

Vol. 6

(a)

(c)

Uncross-linked

−80 −40

0

40

80

Voltage, V

Optical response

Cross-linked

−200 −100

0

100

200

Voltage, V

(b)

E

Fig. 4.

Schematic drawing of the ferroelectric LC-elastomer and its two switching states

(3): (a) A polymer chain acts as cross-linking point by connecting different mesogenic groups
attached to the main polymer chains. A ferroelectric switching in this elastomer extends
polymer chains. (b) The entropy elasticity arising from this acts like a spring that stabilizes
one state. (c) For the uncross-linked system (left), the hysteresis is symmetrical to zero
voltage and both states are equal. After cross-linking in one polar state (right), only that
state is stable with no electric field, and the hysteresis is no longer symmetrical to zero
voltage.

The advantage of this photochemical-initiated cross-linking is that the cross-

linking can be started—at will after the liquid crystalline polymer is oriented and
sufficiently characterized in the uncross-linked state (see Fig. 6). The advantage
of using polymerizable groups (acrylates) for cross-linking is that small amounts
of these groups are sufficient to transform a soluble polymer into a polymer gel
and that the chemical reaction happens far away from the mesogen. Cinnamoyl
moieties, on the other hand (23), require a high concentration of these groups for
cross-linking. The dimers thus formed are, in addition, nonmesogenic. Figure 7
summarizes the ferroelectric LC-elastomers discussed in this article. Two different
positions of cross-linkable groups are used. In polymer P1, the cross-linking group
is close to the siloxane chains, which are known to microphase-separate from
the mesogenic groups (22,23). Therefore, the cross-linking should proceed mostly
within the siloxane sublayers. In polymers P2 and P3, the cross-linking group is
located at the end of mesogens. Therefore, the cross-linking should proceed mostly

Vol. 6

FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS

255

C

Si

O

H

3

C

Si

O

H

H

3

H

3

C

Si

O

CH

3

+

(CH

2

)

9

O

O C CH

3

(CH

2

)

11

O

O

C CH

3

H

3

C

Si

O

CH

3

H

2

N

NH

2

H

3

C

Si

O

(CH

2

)

11

O

1

n

OH

H

3

C

Si

O

CH

3

2.7

n

+ HO

C

CH

CH

C

2

H

5

Cl

CH

3

O

O

+ HO

C

(CH

2

)

5

O

N

H

C

O

DCC, DMAP, THF, CH

2

Cl

2

H

3

C

Si

O

CH

2

0.9

n

(CH

2

)

10

O

O

C

CH

CH

C

2

H

5

Cl

CH

3

O

H

3

C

Si

O

CH

2

0.1

n

(CH

2

)

10

O

O

O

C

(CH

2

)

5

N

H

C

O

H

3

C

Si

O

CH

3

2.7

n

= 365 nm

in

smectic C*

O

OCH

3

OCH

3

1

n

2.7

n

O

2.7

n

1

n

λ

LC network

THE

C

10

H

12

PtCl

2

toluene

ⴢ

ⴢ

ⴢ

ⴢ

Fig. 5.

Synthetic route to the cross-linkable polysiloxane P2 and the following preparation

of the oriented smectic C

∗ network using uv light in the presence of a photoinitiator (1,1-

dimethoxy-1-phenyl-acetophenone) (3).

256

FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS

Vol. 6

ITO

Polydomain

Electrical
field

Initiator
hv
electrical field

Monodomain

Oriented s

c

-network

Fig. 6.

Preparation of polar smectic C

∗ monodomains (3,16) (ITO: indium tin oxide).

between different siloxane layers (see Fig. 7). A comparison of these elastomers
allows evaluating structure–property relationships (18,24,25).

Properties and Characterization

Ferroelectric Characterization (Uncross-linked Systems).

Before

cross-linking, polarization, tilt angle, and switching times can be determined in
the usual way (18,22,26). Figure 8 shows the temperature dependence of the spon-
taneous polarization for polymers P1–P3. For the homopolymer related to poly-
mer P2, all relevant parameters were determined in a careful study (27). It seems
that the electroclinic effect is especially strong in these polysiloxanes (16). This
has implications for the freezing of a memory of the tilt angle present during

Vol. 6

FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS

257

cross-linking. Therefore, ferroelectric elastomers, which have been crosslinked in
the smectic A phase while applying an electric field, produce a stable macroscopic
polarization (tilt) after cooling into the smectic C

∗ phase (18).

Mechanical Properties of Ferroelectric LC-Elastomers.

The cross-

linking reactions of a series of copolymers analogous to polymer P2, but differing
in the amount of cross-linkable groups, were studied by ftir spectroscopy (17).
These measurements show a decrease of the acrylamide double bond on irradi-
ation. Conversions between 60 and 84% were observed. The uncertainty of the
conversion, however, is high because only very few double bonds are present in
polymer P2 and they are visible in the infrared spectrum at rather low intensity.

Mechanical measurements which show how this photo-chemical cross-

linking (conversion of double bonds) leads to an elastic response of the network
are, however, still at the beginning because photocross-linking can be performed
only in thin layers of a few micrometers. It is best performed between two glass
slides to exclude oxygen.

AFM measurements of photocross-linked free standing films show changes

in topology during stretching (24). They, however, do not allow measuring elastic
moduli.

The most promising approach to obtaining elastic data for these ferroelectric

elastomers is investigation of LC-elastomer balloons (25,28). For this purpose,
an experimental setup was developed on the basis of an apparatus designed to
study smectic bubbles (28). Freely suspended films of the uncross-linked mate-
rial behave like ordinary smectic films. They can be inflated to spherical bubbles
several millimeters in diameter (the thickness of a smectic-layer skin is about
50 nm). These bubbles are stabilized by the smectic-layer structure and their in-
ner pressure p is related to the surface tension and the bubble radius R by the
Laplace–Young equation, p

∝ 1/R. After exposure to uv light, the material is cross-

linked, and an anisotropic elastomer is formed. When the cross-linked bubbles are
inflated/deflated, the radius–pressure curve reverses its slope and gives direct ac-
cess to the elastic moduli of the material (25). Because the deformation during
inflation of the balloon is isotropic in the layer plane, the material should contract
in the direction of the layer normal.

Mechanical measurements of chemically crosslinked LC-elastomers have

been made extensively (4,5,29–34). For these systems, it can be shown that stretch-
ing allows orientation of the liquid crystalline phase. In ideal situations, it is thus
possible to prepare a ferroelectric monodomain by stretching (30,31,35). This re-
sult can be rationalized as a two-stage deformation process (see Fig. 9) (31). This
possibility of orienting or reorienting the polar axis mechanically is the basis for
the piezoelectric properties to be discussed later. Ferroelectric switching could
not be observed for any of the chemically crosslinked systems. This may occur
because chemically cross-linked films are too thick (several 100

µm compared to

about 10

µm for photochemically cross-linked systems) and the electric field ap-

plied is therefore too small. In addition, the cross-linking density in chemically
cross-linked systems is presumably higher.

Ferroelectric Properties (Cross-linked Systems).

The ferroelectric

properties of the photochemically crosslinked elastomers E1–E3 differ signif-
icantly and depend on the topology of the network formed. For the systems
that have interlayer cross-linking (see Fig. 7, E2 and E3), the switching time

H

3

C

Si

CH

2

O

(CH

2

)

10

O

O

Cl

O

O

Si

O

Si

H

3

C

H

3

C

CH

2

CH

3

CH

2

CH

2

O

O P1

0.1

n

0.9

n

Phase transitions [

°C]: s

X

32 s

C

∗

2.7

n

H

3

C

Si

CH

2

O

(CH

2

)

10

O

O

Cl

O

O

Si

O

Si

H

3

C

H

3

C

CH

2

CH

3

(CH

2

)

10

O

0.1

n

0.9

n

2.7

n

n

= 30

n

= 30

O

N

O

O

H

H

3

C

Si

CH

2

O

(CH

2

)

10

O

O

O

Si

O

Si

H

3

C

H

3

C

CH

2

CH

3

(CH

2

)

10

O

0.15

n

0.85

n

1.5

n

n

= 15

O

P2

P3

C

C

*

NO

2

CH

3

H

C

6

H

13

C

O

O

O

(CH

2

)

6

O

E2, E3

P2, P3

Photoinitiator

E

h

E1

P1

Photoinitiator

E

h

60 s

A

92 i

Phase transitions [

°C]: s

X

29 s

C

∗

53 s

A

89 i

Phase transitions [

°C]: s

X

32 s

C∗

117 s

A

152 i

ν

ν

Fig. 7.

Chemical structure and phase-transition temperatures of polymers P1–P3 (18). (a) P1 is designed to favor intralayer cross-linking;

(b) P2 and P3 form an interlayer network.

258

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FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS

259

5

−50 −45 −40 −35 −30 −25 −20 −15 −10 −5

0

0

20

40

60

80

100

120

140

160

P

s

,

nC/cm

2

T

−T

c

,

°C

Fig. 8.

Temperature dependence of the spontaneous polarization P

S

for the polymers

P1–P3 measured by the triangular wave method (18). p1 : , p2 : , p3 :

.

First deformation

Second deformation

Fig. 9.

Two-step deformation process of a chiral smectic C

∗ elastomer that displays macro-

scopic polarization at the end (31).

is increased greatly. Therefore, spontaneous polarization can no longer be deter-
mined by the triangular wave method. Slow switching is, however, still possi-
ble and therefore ferroelectric hysteresis can be measured optically (see Figs. 3c
and 10) (3,16). After photochemical cross-linking in a ferroelectric monodomain,
the ferroelectric hysteresis shows stabilization of the orientation present during
cross-linking. At zero external voltage, only this state is stable. The second switch-
ing state can, however, be reached. Therefore, the network acts like a spring that
stabilized one state because switching to the other state leads to a deviation from
the most probable conformation of the polymer chain (36) (see Fig. 10). Then, the
shift of the center of the hysteretic loop away from zero voltage gives the magni-
tude of the electric field necessary to balance the mechanical field of the network.
The asymmetry of the hysteresis increases with the cross-linking density (18). For

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FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS

Vol. 6

Temperature,

°C

42 44 46 48

Optical response

−200

200

−100

100

0

Voltage, V

Temperature,

°C

56

50

54

Optical response

−200

200

−100

100

0

Voltage, V

Fig. 10.

Temperature dependence of the optical hysteresis of elastomer E2 (S

C

∗ 49

◦

C S

A

)

(15). (a) Ferroelectric behavior of the S

C

∗ phase (42, 44, 46, and 48

◦

C, respectively). (b)

Electroclinic behavior of the S

A

phase (50, 54, and 56

◦

C, respectively).

high cross-linking densities, switching remains possible only if the spontaneous
polarization is rather high (18). Otherwise, the network prohibits switching.
The asymmetry of ferroelectric switching could also be proven by polarized ftir
spectroscopy (37). Increasing the temperature of this ferroelectric elastomer leads
to narrowing of the hysteretic loop, which is lost at the transition to the smectic
A phase (see Fig. 10).

This behavior is best interpreted by plotting the liquid crystalline potential,

the elastic potential of the network, and their superposition in one graph (16) (see
Fig. 11). As the network is formed in the smectic C

∗ phase, an internal elastic

field is created, which has its minimum value for the tilt angle and tilt direction
during cross-linking. Other tilt angles are destabilized.

The elastomer that has preferable intralayer cross-linking (E1, see Fig. 7)

shows completely different behavior (see Fig. 12) (18,38). In this case, the switch-
ing time increases by less than a factor of 2, the polarization can still be deter-
mined, and measurement of the ferroelectric hysteresis shows no stabilization
of the switching state present during cross-linking. Then, the coupling between
the orientation of the mesogens and the network conformation is obviously very

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FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS

261

Tilt angle

θ, deg

−30

−20

−10

0

10

20

30

8000

6000

4000

2000

0

−2000

10000

∆

g

J/m

Fig. 11.

Effect of network force on the free energy density (16) 2 K below the phase

transition, S

C

∗ phase: (◦) calculated potential of the S

C

∗phase, ( ) force due to the network,

and ( ) superposition of both.

T

−T

c

°C

, ms

τ

50

40

30

20

10

0

25

20

15

10

5

0

Fig. 12.

Temperature dependence of the switching time

τ (defined as 0–100% change in

transmission) for P1 and E1 [see Ref. 18 for comparison].

weak. The network stabilizes the smectic layer structure, but it does not stabi-
lize the tilt direction. Therefore, the polar axis can be switched easily. This is
the result of the network topology (see Fig. 7) in which interlayer cross-linking is
rare.

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FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS

Vol. 6

AFM Imaging of Thin Films.

Freestanding films can be prepared from

uncross-linked polymers (see also the smectic balloons in this context). They can
be photocross-linked and transferred to a solid substrate. Thereafter, the topology
of the films can be imaged by afm, which gives direct visualization of the smectic
layer structure at low temperatures. The uncross-linked polymers can only be
imaged at low temperatures, deep inside the smectic phase and in the tapping
mode, which does not induce strong lateral forces. At higher temperatures, the
sample is too soft and mobile to allow imaging. Cross-linked elastomers, on the
other hand, are mechanically stable, and films sustain the tapping mode and also
the contact mode of the atomic force microscope (39). This holds both for intra-
and interlayer cross-linked systems. Because measurements can be done in all
phases, it is also possible to determine the change of the smectic layer thickness
at the phase transitions directly. For elastomer E1, for example, the smectic layer
thickness is 4.2 nm in the smectic C

∗ phase (36

◦

C, tilt angle about 30

◦

). It increases

to 4.4 nm at 50

◦

C in the smectic A phase (39). This corresponds to x-ray measure-

ments.

To analyze the impact of the molecular structure on network properties, elas-

tomers are compared, which are identical except for the molecular position of the
cross-linkable group: (1) elastomer E1 that has cross-linkable groups attached to
the backbone via a short spacer (intralayer cross-linking) and (2) elastomer E2
where the cross-linkable group is in the terminal position of a mesogenic side group
(interlayer cross-linking) (24,40). When mechanical stress (stretching) is imposed
on thin films in homeotropic orientation, the two elastomers react differently to the
deformation (24,40), as seen by afm imaging of the surface topology (see Fig. 13).
For elastomer E1, “intralayer” cross-linking results in two-dimensional networks
in the backbone layers, separated by liquid-like FLC side-group layers. Because
there are practically no vertical connections in this intralayer network, no verti-
cal distortions occur. Therefore, this elastomer can be stretched up to 100%, the
surface remains smooth, and the layers deform affinely. In elastomer E2, a three-
dimensional “interlayer” network is formed; the system reacts by distorting the
smectic layering. Therefore, only smaller stretching ratios are accessible and the
surface roughens and buckles during stretching. The distortion strength increases
with a higher cross-linking density.

Piezoelectric Properties of Ferroelectric LC-Elastomers.

Because a

ferroelectric material has to be piezoelectric (see Fig. 1), observation of a piezore-
sponse is natural. It has been observed for ferroelectric LC-elastomers (3,16,19–
21) and also for more densely cross-linked systems (10,31,41,42) for which no
ferroelectric switching could be observed. For the elastomers described here it is,
however, possible to change the piezoresponse (3) by reorienting the polar axis
in an external field (see E2 in Fig. 14). For this experiment, the polar axis was
kept in one orientation during cross-linking. This resulted in a positive piezore-
sponse (see Fig. 14). Thereafter, the direction of the polar axis was inverted by
applying an external field of opposite direction. Then, the external field was re-
moved and the piezocoefficient was measured. At first, a piezoresponse of op-
posite sign (negative) but identical value is determined (see Fig. 14). In the
field-free state, this piezoresponse continuously decreases, it goes through zero,
increases again, and finally reaches the original positive value. This experiment
is comparable to the hysteresis measurements of Figs. 3 and 10 because it shows

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FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS

263

(a)

(b)

(c)

(d)

(e)

( f )

Fig. 13.

Surface topography of prepared and stretched transferred films of elastomer

E1

x

= 0.1

(a:

λ = 0, b: λ = 10%), E2

x

= 0.07

(c:

λ = 0, d: λ = 12%), and E2

x

= 0.25

(e:

λ = 0, f: λ =

2%,). Scale bars 1

µm, height scale 25 nm valid for all images. The surface of all polymers

show plateau patterns. In the E2 samples, the lateral strain leads to surface deformation
(40).

that two polar states are accessible, but the one present during cross-linking is
stabilized.

The shape variation under application of an external electric field was most

intensively studied for microtomized pieces of ferroelectric elastomers, which had
been oriented by drawing (19–21). These experiments show only a small shape
variation if the field is applied parallel to the polar axis of the monodomain.
The effects become, on the other hand, rather large if the smectic layer struc-
ture (chevron texture) rearranges (19,21). To get a large electrostrictive response,
which can be understood on a molecular level, the geometry presented in Fig. 15
was chosen (43). The application of an electric field parallel to a smectic layer
leads to a tilt of the mesogens (electroclinic effect). Thereby, the thickness of
the layer decreases. For a stack of layers, the effect sums up over all layers.
As a result, the thickness perpendicular to the smectic layers decreases if a
field is applied parallel to the layers (see Fig. 15). Because the electroclinic ef-
fect is relatively large in these polymers (16,37), a large variation in thickness is
expected.

X-ray diffraction measurements prove the electrically induced shrinkage of

single smectic layers. A freestanding ultrathin (75 nm) film of a ferroelectric liquid

264

FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS

Vol. 6

0.8

0.6

0.4

0.2

−0.2
−0.4
−0.6
−0.8

0

0

20

40

60

80

100

Time, s

d

33

, pC/N

Charge
det.

Fast cooling
to room
temperature

d

33

80 min.

−d

33

d

33

Poling

200 V. 65

°

Fig. 14.

Relaxation of the piezocoefficient d

33

of elastomer E2 at room temperature after

reversal poling at 65

◦

C (S

A

phase) (3).

Single smectic layer

∆h/z

h

0

−∆h

z

for

E < 0

for

E > 0

for

E = 0

θ

θ

−

h

0

−∆h

E

≠ 0

z smectic layers

Electric
field

E

E = 0

z smectic layers

h

0

Fig. 15.

In the S

A

∗ phase, the mesogenic parts (depicted as ellipsoids) of the elastomeric

macromolecule stand upright (

θ = 0

◦

) inside the single smectic layers. By applying a lateral

electric field (perpendicular to the plane of the paper), a tilt angle

θ that is proportional

to the electric field E can be induced (electroclinic effect). The sign of the tilt depends
on the sign of the electric field E. Hence, each smectic layer shrinks by

h/z twice dur-

ing one period of the electric field. The shrinkage

h of the whole film is measured by

the interferometer as an optical phase shift between the sample beam and the reference
beam.

Vol. 6

FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS

265

2

nd

harmonic

1

st

harmonic

4

3

2

1

0

0

250

500

750

1000

1250

1500

U

ac

, V

h

, nm

∆

S

A

∗

S

C

∗

7

6

5

4

3

2

1

0

55

60

70

75

a

(10,000 nm

2

/V

2

)

Temperature,

°C

65

Fig. 16.

Electrostriction of a ferroelectric LC-elastomer (43). Big diagram: Thickness vari-

ation

h as a function of the applied ac voltage U

ac

. Interferometric data were obtained

at the fundamental frequency of the electric field (piezoelectricity, first harmonic:

+) and

at twice the frequency (electrostriction, second harmonic:

◦). Sample temperature: 60

◦

C.

Inset: Electrostrictive coefficient a (

+) versus temperature. At the temperature where the

non-cross-linked polymer would have its phase transition S

C

∗–S

A

∗ (about 62.5

◦

C), the tilt

angle of 0

◦

is unstable. That is why the electroclinic effect is most effective at this temper-

ature. An electric field of only 1.5 MV/m is sufficient to induce lateral strains of more than
4%.

crystalline elastomer (similiar to E2) was used to measure the shape variation
(electrostrictive response) associated with this (43). It was measured by a high
precision (

±3 pm at 133 Hz) Michelson interferometer. The measurements (see

Fig. 16) exhibit extremely high electrostrictive strains of 4% in an electric field
of only 1.5 MV/m, which is, to our knowledge, a new world record for the cor-
responding electrostrictive coefficient a. The effect exhibits typical electroclinic
behavior, which means that it is caused by an electrically induced tilt of the chiral
LC molecules. As a consequence of chirality, the primary strain is perpendicular
to the applied field. Hence, a new material that has a giant electrostriction effect
is introduced, where the effect can be fully understood on a molecular level.

BIBLIOGRAPHY

1. M. G. Broadhurst and G. T. Davis, in P. M. Galetti, D. E. De Rossi, and A. S. De Reggi,

eds., Medical Applications of Piezoelectric Polymers, Gordon and Breach, Amsterdam,
1988, pp. 3–14.

266

FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS

Vol. 6

2. J. W. Goodby and co-workers, Ferroelectric Liquid Crystals, Principles, Properties and

Applications, Ferroelectricity and Related Phenomena, Gordon and Breach, Philadel-
phia, 1991.

3. M. Brehmer and co-workers, Macromol. Chem. Phys. 195, 1891–1904 (1994).
4. R. Zentel, Angew. Chem. 101, 1437–1445 (1989).
5. M. Brehmer and R. Zentel, Mol. Cryst. Liq. Cryst. 243, 353–376 (1994).
6. F. J. Davis, J. Mater. Chem. 3, 551–562 (1993).
7. W. Gleim and H. Finkelmann, in C. B. McArdle, ed., Side Chain Liquid Crystalline

Polymers, Blackie and Son, Glasgow, 1989.

8. H. Finkelmann, in A. Ciferri, ed., Liquid Crystallinity in Polymers, VCH, Weinheim,

1991.

9. R. A. M. Hikmet, H. M. Boots, and M. Michielsen, Liq. Cryst. 19, 65–76 (1995).

10. R. A. M. Hikmet and J. Lub, Prog. Polym. Sci. 21, 1165–1209 (1996).
11. A. Hult and co-workers, Liq. Cryst. 20, 23–28 (1996).
12. R. A. M. Hikmet and H. Kempermann, Nature 392, 476–478 (1998).
13. G. C. L. Wong and co-workers, Nature 389, 576–579 (1997).
14. R. Zentel, Liq. Cryst. 1, 589 (1986).
15. N. R. Barnes, F. J. Davis, and G. R. Mitchell, Mol. Cryst. Liq. Cryst. 13, 168

(1989).

16. M. Brehmer and co-workers, Liq. Cryst. 21, 589–596 (1996).
17. E. Gebhard and co-workers, in K. te Nijenhuis and W. J. Mijs, eds., The Wiley Poly-

mer Networks Group Review Series, Vol. 1, Chemical and Physical Networks, Wiley,
Chichester, 1998, pp. 387–397.

18. E. Gebhard and R. Zentel, Macromol. Chem. Phys. 201, 902–922 (2000).
19. F. Kremer and co-workers, Polym. Adv. Technol. 9, 672–676 (1998).
20. W. Lehmann and co-workers, Ferroelectrics 208, 373–383 (1998).
21. W. Lehmann and co-workers, J. Appl. Phys. 86, 1647–1652 (1999).
22. H. Poths and R. Zentel, Liq. Cryst. 16, 749–767 (1994).
23. L.-C. Chien and L. G. Cada, Macromolecules 27, 3721 (1994).
24. H. M. Brodowsky and co-workers, Langmuir 15, 274–278 (1999).
25. H. Sch ¨

uring and co-workers, Macromolecules 34, 3962–3972 (2001).

26. H. Poths and co-workers, Liq. Cryst. 18, 811–818 (1995).
27. A. Kocot and co-workers, Phys. Rev. B 50, 16346–16357 (1994).
28. R. Stannarius and C. Cramer, Europhys. Lett. 42, 43 (1998).
29. J.

K ¨

upfer

and

H.

Finkelmann,

Macromol.

Chem.

Phys.

195,

1353–1367

(1994).

30. K.-H. Hanus and co-workers, Polym. Prepr. 30, 493 (1989).
31. I. Benn´e, K. Semmler, and H. Finkelmann, Macromolecules 28, 1854–1858

(1995).

32. E. Nishikawa, H. Finkelmann, and H.-R. Brand, Macromol. Rapid Commun. 18, 65

(1997).

33. J.

Kundler

and

H.

Finkelmann,

Macromol.

Chem.

Phys.

199,

677–686

(1998).

34. E.

Nishikawa

and

H.

Finkelmann,

Macromol.

Chem.

Phys.

200,

312–322

(1999).

35. R. Zentel and co-workers, Makromol. Chem. 190, 2869 (1989).
36. L. R. G. Treloar, The Physics of Rubber Elasticity, Clarendon, Oxford, 1975.
37. S. Shilov and co-workers, Macromolecules 32, 1570–1575 (1999).
38. M. Brehmer and R. Zentel, Macromol. Chem., Rapid Commun. 16, 659–662

(1995).

39. H. M. Brodowsky and co-workers, Langmuir 13, 5378–5382 (1997).
40. H. M. Brodowsky and co-workers, Ferroelectrics 243, 115–123 (2000).

Vol. 6

FIBERS, ELASTOMERIC

267

41. M. Mauzac and co-workers, Chem. Phys. Lett. 240, 461 (1995).
42. S. U. Vallerien and co-workers, Makromol. Chem., Rapid Commun. 11, 593 (1990).
43. W. Lehmann and co-workers, Nature 410, 447–450 (2001).
44. J. Zhao and co-workers, Jpn. J. Appl. Phys. 34, 5658 (1995).
45. T. Jaworek and co-workers, Science 279, 57 (1998).

R

UDOLF

Z

ENTEL

University of Mainz