Preparation of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 7

replaced by chiral dopands, e.g., cholesteryl derivatives that exhibit high helical
twisting power. In such co-elastomers of chiral and non-chiral side chains, the
phase behavior and the helical pitch which define the optical properties of the
material can easily be tuned. Smectic-A phase behavior can be induced reliably by
using perfluorinated tails, which are much stiffer compared to alkyl chains and may
further promote the segregation into a lamellar phase structure. The incorporation
of polar ethylene oxide (EO) side chains or of mesogenic units carrying EO tails can
cause lyotropic phase behavior when the elastomer is swollen with water.
In the following we will outline two basic methods to synthesize LC side chain
elastomers. As a starting point for the synthesis of LC elastomers a mixture of
mesogenic monomers and bi- or multi-functional crosslinker molecules may serve.
This will be discussed in the first part of the section. Alternatively, polymer
analogous reactions, where the mesogenic moieties are attached to a polymer
backbone, can be employed, which will be discussed in the second part of the
section.
If LC monomers are used as starting materials, it is very important to consider
that monomer and polymer networks can differ in their phase behavior as previ-
ously mentioned. This is a particular issue for nematic elastomers. Only very few
examples are known in which the temperature regime of a nematic phase of a
monomer overlaps with the nematic temperature regime of the polymer. The
systematic that was hereby obtained for the LC phase behavior of linear polymers
also holds for LC polymer networks because for elastomers the concentration of the
mesogens is much higher than that of the crosslink. The chemistry that can be used
for the crosslink is determined by the chemistry of the polymerization technique.
The only practicable chemical reaction to prepare LC elastomers from functional
monomers is radical polymerization . Acrylates or methacrylates are mainly used as
starting materials. It has to be ensured that the mesogenic group and the crosslinker
have the same reactivity, so that a statistical arrangement along the chains can be
achieved and a change in concentration ratio with advancing reaction is avoided.
In principal, radical polymerization can be carried out in bulk. However, only if the
polymerization temperature is within the overlapping temperature regime where
monomer and polymer exhibit the same LC phase a homogeneous reaction can take
place. Otherwise de-mixing occurs, which causes an uncontrolled network forma-
tion. Furthermore, the large reaction enthalpy of the polymerization restricts this
method to the preparation of thin films only, in which a suitable heat transfer and
control is guaranteed. To avoid these problems, the reaction can be carried out in
solution. When the reaction is completed, the solvent has to be carefully removed in
a de-swelling process. In some cases, especially if macroscopically ordered LCE
are to be synthesized (see Sect. 4 ), low molar mass LCs of appropriate phase
behavior can also serve as a solvent and have to be removed in an extraction
process after synthesis [ 13 ] .
An example for the synthesis of a side chain elastomer using radical polymeri-
zation of acrylates is presented in Scheme 1 and was demonstrated by Thomsen and
co-workers [ 14 ] . Here, the mesogenic groups are attached side-on to the polymer-
backbone. This attachment geometry is useful for a number of applications, because