Chemistry Reference
In-Depth Information
crystalline and the uniaxial mechanical stress induces a macroscopic orientation
of the nematic director. After complete alignment, the crosslinking reaction is
finished at elevated temperatures. This “second crosslinking step” has to be carried
out deeply enough in the nematic phase, as the crosslinking temperature has
significant impact on the crosslink topology and in the vicinity of
T
n,i
a more
disordered state is fixed.
Compared to approaches based on crosslinking of aligned LC prepolymers
is important to consider when discussing chain anisotropies, effects of random
disorder, or pretransformational effects.
In the following, a detailed description of the two-step crosslinking procedure,
including many practical details that proved to be useful in order to obtain high
quality samples will be presented. Readers not interested in experimental details
can proceed below.
The spin casting process can be carried out in a heatable centrifuge using a
spinning cell of the desired sample dimensions. It is advisable to cover the inner
wall of the cell with a strip of poly(tetrafluoroethylene) foil (Teflon) in order to
avoid adherence between the casted elastomer film and the wall and allow for
easy removal. The ends of the strip should not overlap and they should be
marked so that they can be recognized easily after synthesis.
For side chain elastomers, the reactants - mesogens, crosslinker, and
prepolymer - are dissolved in a minimum of solvent. Toluene is a good choice
as it evaporates slowly at room temperature and thus leaves enough time for the
orientation of the swollen polymer gel. For a hydrosilylation reaction the solvent
needs to be thiophene-free because otherwise the platinum catalyst is inhibited.
Before use, toluene has to be washed with concentrated sulfuric acid and distilled
several times. If one of the components is insoluble in toluene, dichloromethane,
or tetrahydrofuran also give good results. Both can be used at analytical purity
without prior distillation. However, the orientation process usually becomes more
difficult for solvents with low vapor pressure. If polymeric crosslinkers are used,
it is advisable to dissolve the mixtures overnight. To start the hydrosilylation
reaction, the catalyst is added to the clear solution of the starting materials. Next,
the mixture is transferred into the centrifuge cell through a millipore filter in order
to remove dust particles or other insoluble impurities. As a catalyst, a solution of
dichloro(1,5-cyclooctadiene)platinum-II in dichloromethane (5-15
m
L, 1.0 wt%)
is used. The catalyst has been developed for the insertion of terminal C
C double
bonds into the Si-H bond for vinylic spacers (except for allylic spacers). It
provides a fast conversion at moderate temperatures (
T ¼
¼
60
C) with a mini-
mum of side reactions. Higher quantities of catalyst or aged catalyst solutions
result in a brown color of the elastomers. If a catalyst with a higher reactivity is
required, solutions of hexachloroplatinum acid can be used.
For main chain elastomers, a proper preparation of the reaction mixture is
crucial as volatile reactants are often involved and the exact stoichiometry has to
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