Applications of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 82

the layers. This effect is independent of the polarity (tilting to the left or the right).
The shrinking rate perpendicular to the smectic layers is comparable to the shrink-
ing of the homeotropic film mentioned before. As described for the thermal transi-
tion, the thickness change that is caused by the electroclinic switching is always
accompanied by shear parallel to the smectic layers.
So far we have discussed networks containing calamitic rod-like mesogens. Due
to their kinked shape, bent-core LCs (BCLCs) also have ferroelectric properties.
Though they have been known for about 15 years, because of their high phase
transition temperatures it is hard to obtain oriented samples. As a consequence, low-
molar-mass BCLC systems have mainly been investigated so far. In a first experi-
ment, direct measurement of the flexoelectric effect was performed [ 112 ] . The
mechanical deformation resulted in induced currents of up to 8 nA (at 5 Hz
and a 4.8 cm 2 electrode area). This corresponds to a bend flexoelectric coefficient
| e 3 | of 60 nC/m. In nonaligned samples, smaller values are observed that increase
upon mechanical treatment. This is attributed to improvement of the degree of
orientational order. In the case of smectic BCLCs, the behavior is even more
complicated. These phases are primarily composed of alternating layers that pre-
vent a macroscopic polar orientation. Such samples have to be poled to align the
polar axis and to obtain a piezoelectric signal. After poling, a piezoelectric current
of up to 20 nA (2 cm 2 sample area) has been measured [ 181 ] . This corresponds to
a piezoelectric constant of 100 pC/N, a value comparable to PVDF and ceramic
materials. However, due to a thermal back-relaxation to the macroscopically apolar
state, this value cannot be maintained. At 60 C the half-life time of the measured
current is only 50 s.
Crosslinking can be used to overcome the aforementioned obstacles, as in
the case of the calamitic networks in which the orientation and polarity are
preserved. First experiments were performed by swelling a calamitic LCE with a
low-molar-mass BCLC [ 182 , 183 ]. Up to 35 mol% of this material could be
incorporated into the network. The resulting gel had a flexoelectric constant | e 3 |
of 20 nC/m. This is one-third of the value of the low-molar-mass BCLC, which
corresponds to the volume fraction of the BCLC. Only recently has a pure bent-core
elastomer (BCFLCE) been made [ 139 , 169 ]. Following the synthetic route of
Finkelmann, an oriented, transparent nematic elastomer sample could be obtained.
As a polysiloxane backbone was used, the T g was close to room temperature. The
flexoelectric constant of 40 nC/m is somewhat smaller than for the pure bend-core
monomer, but larger than the value obtained for the swollen elastomer.

4 Conclusions and Outlook

LCEs have been under investigation for several years. These systems are unique
because of the combination of order and softness. During recent years, several
developments have evolved that bring them closer to application. One important
achievement

is the introduction of various stimuli like electric current and