Preparation of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 30

cause the elastomer to break when later a mechanical load is applied during
the orientation process.
In order to remove the cast elastomer film from the Teflon support, a strip
of Kapton based adhesive tape is fixed at one end of the swollen gel. Kapton is
used as it can withstand high temperatures and is stable towards organic
solvents. The film is now removed from the support by slowly pulling the
tape. In some cases it is useful to rinse the space between the elastomer film
and the Teflon foil with toluene. The elastomer film is fixed at a metal holder
and hung vertically. At the lower end of the film a small load, e.g., a paper
clip, is attached and held steady with Kapton tape from both sides (Fig. 10 ).
Some elastomers cannot be prepared as described above because they are too
sticky or mechanically not stable enough. This often holds for side chain side-on
elastomers, for elastomers with mesogens carrying ethylene oxide or fluorinated
chains, or for very low crosslinking densities. In that case the gels can be prepared
over a liquid nitrogen bath. The gel is frozen to the glassy state while the Teflon
still remains flexible at this temperature, allowing for an easier removal. For this
procedure, a small Dewar is filled with liquid nitrogen and a cork ring is placed
inside. On the cork ring a PVC Petri dish is placed with the rim facing down-
wards. The elastomer film is put on the Petri dish with the Teflon foil facing the
Petri dish. After the gel is frozen (test with tweezers) the sample ist turned over
and the Teflon foil is removed using a pair of tweezers. In the case where the
electrostatic interaction is too strong, one can earth oneself during preparation.
Then two strips of Kapton are attached to the ends of the film and thereby warmed
with one's thumbs, because they are not sticky in the cold. One of the Kapton
strips should already have a paper clip attached because the Kapton tends to curl
in the cold and it is difficult to attach the clip afterwards. The film is then attached
to the holder and fixed with additional Kapton strips as described above.
For the orientation process, the load is increased successively. For nematic
elastomers one typically waits until the gel becomes slightly turbid before more
load is applied. For nematic side chain elastomers the load needed to orient the
film ranges between 300 mg and about 4 g. For nematic main chain elastomers,
loads of about 100 g are often used. It is useful to leave nematic elastomers under
load overnight so that they become stable enough for further crosslinking in the
oven. Before putting the elastomers in an oven to perform the second
crosslinking step, the load should be reduced to the minimum necessary to
induce a uniform orientation. The elastomers are typically crosslinked in an
oven at T ¼ 60 C, the optimum temperature for the catalyst. However, one

should be careful not to crosslink at temperatures too close to T ni because this
leads to disorder in the elastomer. The elastomers are usually left in the oven for
1 week to ensure complete reaction and good reproducibility.
After complete crosslinking, the elastomer is carefully extracted with a good
solvent as described in Sect. 2.3 .