Applications of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 74

phase known from achiral mesogens: the director is oriented parallel to the layer
normal, perpendicular to the layer plane. However, if an electric field is applied
perpendicular to the layer normal of the s A * phase, the mesogens tilt perpendicular to
the field (see Fig. 14b ) . Hence the field induces a transition from an orthogonal
smectic phase (s A ) to a tilted smectic phase (s C *). This occurs because the tilting
reduces the symmetry and induces a macroscopic polarization, which can interact
with the external field. This effect is strongest at the (second-order) phase transition
temperature (Curie temperature) between the s A * and the s C * phase. In combination
with chiral smectic elastomers, this behavior is very effective at inducing shape
variation because the sample shrinks parallel to the smectic layer normal (each layer
gets thinner by tilting, see Fig. 14b ) while it expands in the perpendicular direction.
In addition to chiral rod-like smectics, ferroelectricity has also been observed in
LC phases from bent-core mesogens. In the case of these banana-shaped molecules,
the core shape, together with their dense packing, biases the rotation around the
long molecular axis [ 144 , 145 ]. This leads to a summation of the molecular dipoles
in the direction of the kink [ 146 - 148 ] (see Fig. 14c ). Because of their molecular
shape, not only smectic phases but also biaxial nematic phases show ferroelectricity
[ 149 , 150 ]. In this context, one has to bear in mind that alignment of the mesogens
is essential to obtain devices. Smectic materials with a low viscosity and a low
phase transition temperature (like the s C * phase discussed above) usually meet
these prerequisites better than the highly viscous polar phases of banana-shaped
mesogens that normally exist at temperatures above 100 C. A recent example of
a bent-core elastomer will be discussed at the end of this section.
We will concentrate here on systems in which the LC director is coupled to the
network without losing its softness. This means that a rotation of the polar axis is
still possible (see Fig. 14a ) and real FLCEs can be prepared [ 23 , 36 - 38 ] . Densely
crosslinked systems that possess a polar axis, but cannot be switched [ 151 - 153 ] ,
will be excluded. Although FLCEs can be switched, the state prior to crosslinking is
stored. This is manifested in the optical hysteresis curve (see Fig. 15 ) . The center of
the curve is shifted from zero voltage in the direction opposite to that of the polarity
of the field applied during crosslinking. This behavior can be explained by an
additional internal mechanical field [ 23 ] that exhibits a minimum at the switching
state in which the sample exists during crosslinking. This asymmetric switching is
also observed in the smectic-A* phase. However, in that case the hysteresis is gone.
The mesogens do not rotate between two ferroelectric states, but change their
induced tilt angle due to the electroclinic effect (see Fig. 14b ) . Here again, the
mechanical field shifts the switching curves to one side [ 23 ].
The FLCEs prepared in this way can be used either as sensor components,
transforming a mechanical deformation into an electric signal, or as actuators that
change their shape on application of an electric field. From the chemical point of
view, they can be made by covalently linking the mesogenic groups to form
a slightly crosslinked rubbery polymer network structure [ 3 , 5 , 8 , 10 , 154 - 156 ] or
by dispersing a low-molar-mass liquid crystal in a phase-separated network
structure [ 7 , 40 , 157 - 162 ]. These two systems possess very different structures
locally. Macroscopically, however, they show very similar properties.