Preparation of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 15

esterification chemistry to obtain polyesters. They did not use a flexible chain-
extender like the short tetrasiloxane in the examples discussed above, but started
from a flexible dialcohol, an aromatic di-ester as a mesogenic group, and a trifunc-
tional aromatic ester as a crosslinker.
A new and elegant solvent-free approach to main chain LCEs was found by
Yang et al. [ 39 ] who, based on the work on linear LC polymers by Lub et al.
[ 40 - 42 ], made use of the photo-induced addition of thiols and olefins (click-
chemistry) to synthesize nematic polymer networks. Starting from a mixture of
the mesogen, a tetrafunctional crosslinker and a photo-initiator networks with a T ni
around 170 C were obtained by UV crosslinking (Scheme 8 ) .

2.3 Basic Characterization of LC Networks

The first question that has to be addressed after the synthesis of a polymer network
is whether the crosslinking reaction was successful and is reproducible. Therefore,
it is advisable to measure the soluble content of the elastomer, that is, the weight
loss in percent after the extraction of unreacted monomers, oligomers, or polymers.
For the extraction the elastomer is placed in a poor solvent of the linear polymer
(e.g., isohexane). Subsequently the solvent quality is increased by slowly adding a
good solvent (e.g., toluene). The swelling of the elastomers has to be carried out
very carefully as inhomogeneous or too fast swelling can produce local mechanical
stress which might cause the sample to break. This is especially important for
macroscopically oriented samples (LSCEs) for which the LC-isotropic phase trans-
formation that occurs upon swelling is accompanied with large length-changes of
the sample. The extraction is usually done over about 1 day for nematic main chain
elastomers; for the less sensitive side chain elastomers or for smectic elastomers
this can be done faster. After the extraction of unreacted material is completed the
elastomers have to be deswollen again. This is done by the reverse procedure by
adding poor solvent. Afterwards the elastomer is dried at elevated temperatures to
evaporate any remaining solvent. This extraction procedure is crucial to obtain
samples with reproducible properties.
Information on the crosslinking density of the synthesized elastomer can be
obtained using various methods. Mechanical characterization, especially stress-strain
measurements, thermoelastic measurements in the isotropic phase, and swelling
experiments yield information about the molecular weight of the network chains
M c . In a first approach, the affine network model, introduced by Kuhn, may be used
to correlate the experimental results with the chemical constitution of the networks.
This network model assumes that the average squared distance of the chain ends of a
network chain is the same as for an uncrosslinked polymer of the same length.
Furthermore, a deformation of the network does not change the sample's volume so
that two polymer segments are separated about the same factor

that is determined by
the macroscopic deformation. Finally, the enthalpy of network chains does not change
under deformation. Network defects like loops or dangling chain ends are not

l