Applications of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 52

[ 23 - 25 ] can lead to a change of the tilt angle of the mesogens in chiral smectic-C
(smectic-C*) or chiral smectic A (smectic-A*) systems. This causes a change of the
smectic layer thickness, which in turn induces the actuation (see Sect. 3 ) . The
polymer network serves here mostly as a matrix, which transforms a “liquid-like”
ferroelectric phase into a “soft solid-like” ferroelectric, which can exert permanent
stress. The magnitude of the shape change achievable in this way is smaller than for
systems that rely on a nematic-isotropic (NI) phase transition. But, this actuation
principle has the advantage that shape variations happen in direct response to an
electric field (in fact it is a piezoelectric effect) and does not require heating.
LCEs can have different topologies depending on the attachment of the
mesogenic units (see Fig. 2 ) , which is important for the magnitude of the chain
anisotropy. If they are incorporated into the main chain, a large interaction between
LC phase and mechanical properties is expected. If the mesogens are attached as
side groups (LC side-chain polymers), their orientation is decoupled from that of
the polymer chains [ 26 ]. Thus orientational correlations will be smaller than in LC
main-chain polymers [ 10 , 27 ].
In this chapter, we will focus on the application of LCEs that can change their
shape in response to temperature, to irradiation with light, or to electric fields (either
by resistive heating or by a direct piezo-like response). Compared to inorganic
piezoelectric materials, LCEs show a much larger shape variation but smaller
forces. Compared to gels working by reversible swelling and deswelling, they
have the advantage that the shape variation does not require mass transport of
solvent.
The basic principles of preparation and operation of LCEs - especially with
regard to the route shown in Fig. 3a - have been described in [ 184 ]. Still, some

a b

Fig. 2 (a) Network containing side-chain mesogens; (b) network containing main-chain mesogens