Applications of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 71

(s C *). This transition can be induced by an electric field (electroclinic effect) and
leads to a decrease in the thickness of the smectic layers, resulting in a shape
variation.
3.1 Ferroelectric Liquid Crystals and Their Networks
We will first describe LC phases with ferroelectric properties and subsequently
outline the general properties of the ferroelectric LCEs (FLCEs). The most
intensely studied phase is the chiral smectic-C* phase (s C *) (see Fig. 14 ) . It is the
chiral modification of the smectic-C phase, a tilted smectic phase formed from
chiral (pure enantiomers) rod-like mesogens. Chirality is essential because it
eliminates the mirror plane present in the classical smectic-C phase. This reduces
the symmetry of the phase and allows a macroscopic polarization perpendicular to
the plane of the layer normal and the director, which now follows the average tilt
direction of the mesogens (for an overview see [ 113 - 119 ] ). This macroscopic
dipole moment (spontaneous polarization) is a consequence of the reduced symme-
try and the fact that the lateral dipole moments of individual mesogens no longer
cancel each other due to a slightly biased rotation around their long axis. This
symmetry argument applies to all tilted smectic phases formed by chiral rod-like
mesogens. However, for higher-ordered smectic phases than s C *, ferroelectric
switching (see Fig. 14a ), which is the final proof of ferroelectricity, is difficult to
perform due to the high viscosity. This problem is even more severe for FLCEs,
in which the switching times can be very long [ 37 , 38 ] .
In summary, chiral smectic-C* phases lack a center of symmetry. Hence they can
be used as materials for second-order nonlinear optics [ 120 - 124 ] , and possess piezo-
electric and pyroelectric properties. Pyroelectric measurements have been performed
on LC polymers [ 125 ] as well as on LCEs [ 126 - 128 ]. Irradiation of an FLCE sample
with light usually leads to a temperature increase resulting in a pyroelectric signal
[ 129 ]. More interesting are systems in which dye molecules like azobenzenes lead
to a shift of the phase transition temperature upon isomerization [ 19 ] .
Furthermore, in FLCEs the macroscopic electric dipole moment provides
a handle to apply a strong torque onto the director (see Fig. 14a ). The resulting
switching occurs on the cone of the so-called c-director, the projection of the
director on the smectic layer plane (see Fig. 14a ). Soon after the discovery of the
potential of chiral smectic-C* phases, the search for LC polymers with these phases
started [ 130 - 132 ] . However, as ferroelectric switching is the final proof for the
assignment of the phase, the more closely studied ferroelectric LC polymers were
limited to several LC polysiloxanes, which have a low T g and a relatively high
switching speed [ 25 , 66 , 133 - 136 ] (see Scheme 1 ) . These polymers form the basis
for most of the FLCEs discussed here.
Closely related to the chiral smectic-C* phase is the so-called chiral smectic-A*
phase (s A *, see Fig. 14b ) (for an overview see [ 24 , 25 , 119 , 142 , 143 ] ). Without the
presence of external electric fields, the s A * phase is identical to the smectic-A (s A )