Applications of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 55

Route (d) has the advantage of a low viscosity of the monomer mixture. However, it
is not trivial to obtain a system in which phase type and temperature range of the LC
phase of polymer and monomer overlap well. From the synthetic side, either
acrylate groups (photoinitiated radical polymerization) can be incorporated into
an LC polymer by a polymer-analogous reaction [ 23 , 36 , 39 , 41 ] or groups leading
to direct photocrosslinking can be used, like benzophenone units [ 42 , 43 ] . As a
result of these efforts, neat (i.e., solvent-free) LC materials have become available,
in which a complex director pattern can be created by the methods known from low-
molar-mass LC systems. The resulting director patterns can afterwards be stabilized
by photoinitiated crosslinking. As photocrosslinking can be done in a spatially
resolved manner, this route allows the creation of different director patterns in
different parts of the sample.
An interesting alternative to covalent crosslinking is the use of secondary
interactions like ionic interactions [ 34 ] or complex formation [ 44 - 46 ] . Redox
reactions that change the valence of the center ion can be used for reversible
crosslinking. Also, block copolymers have been used for the formation of supra-
molecular networks [ 33 , 47 ]. In this case, the crosslinks are formed by hard polymer
segments that are in the glassy state surrounded by LC polymer segments. If such
a block copolymer is heated above the T g of the hard segments, it becomes melt-
processable. As with ionic systems, the crosslinking of the block copolymers is
reversible and reuse becomes possible. In the case of the covalent networks mostly
investigated, recycling is not an option, as there is no way to selectively break the
chemical bonds formed during preparation.
2 Actuators Powered by a Phase Transition
2.1 Designing Actuators with Defined Specifications
In this section, we will discuss LCEs in which shape variation is induced by
a change of the order parameter S . This is most pronounced in the vicinity of an
NI phase transition (see Fig. 1 and Sect. 1 ). In addition, we will discuss how the size
and shape of aligned (and therefore actuating) LCE samples can be controlled in
order to meet certain specifications dictated by an application. We will address the
question of how the mechanical and actuation properties of the materials are
influenced by the chemical nature of the LCE system.
From an application-based point of view, the most important characteristics of
an actuator are the stimulus under which actuation occurs, the direction of
actuation, the maximum displacement that can be produced, and the maximal
force that can be created. The maximum displacement and force that an LCE
actuator can produce depend on the chemical structure of the material. This aspect
will be discussed in Sect. 2.1.1 . Concerning the stimulus leading to the shape
change, temperature-driven and UV-driven systems can be distinguished. In both