Chemistry Reference
In-Depth Information
be kept in order to obtain high molecular weight products. Therefore, non-
volatile reactants are weighed first before adding more volatile starting
materials. For the synthesis of main chain elastomers based on a hydrosilylation
reaction, 1,2-dihydrotetramethyldisiloxane has often been used as co-monomer,
as it acts as flexible chain extender. Owing to its very low vapor pressure it
should be added quickly to the glass vessel after the mesogen and crosslinker. It
is used in slight excess and then left to evaporate until the excess to stoichiomet-
ric conditions is about 2%. Then the solvent is added and after all components
are dissolved the mixture is quickly transferred to the centrifuge cell (it is often
better to do that without prior filtration). The slight excess of the chain extender
is needed to compensate for the loss due to evaporation during the transfer of the
solution. If there are problems in dissolving the mesogen quickly in the solvent
used, it is advisable to recrystallize it in an appropriate solvent and cooling
rapidly using an ice-bath so that small, fluffy crystals are obtained.
After the reaction mixture has been transferred into the centrifuge cell, it is
spun at 5,000 rpm and an interior cell temperature of 60
C. For the reaction
time of the first crosslinking step, the minimum time to obtain a stable gel film
at the cell's wall is chosen. This minimizes the number of crosslinks formed
in the isotropic state. The film has to be mechanically stable enough to be
removed from the Teflon support and to be loaded with a small weight, e.g., a
paper clip. It should not be too sticky and it should not be possible to draw
fibers from the elastomer film. The reaction time usually has to be optimized
for each system and can range from 45 min to 24 h. At a given temperature it
basically depends on the purity of the starting materials and the crosslinker
concentration used. Especially after longer storage times it is advisable to
recrystallize mesogens and crosslinkers followed by drying in vacuo in order
to remove impurities and humidity.
In order to interrupt the crosslinking/casting reaction the cell is removed from
the centrifuge and cooled for about 30 s in liquid nitrogen to room temperature.
This is preferable to water cooling since traces of water can prevent the proceed-
ing of the reaction when they happen to get into the gel. If no stable gel is obtained
the crosslinking reaction under spinning is continued. One has to be very careful
to add more catalyst to the reaction mixture, as close to the gel state this can cause
heterogeneous reactions.
Once the reaction is successful, the elastomer strip is loosened from the
centrifuge wall with the non-cutting side of a thin lancet. The elastomer film
is carefully removed from the cell together with the Teflon support and cut
into smaller (usually two or three) pieces. For cutting the swollen polymer gel
the best method is to use a carpet knife and a hammer. That way, a clean cut of
both the gel and the Teflon band is achieved. It is useful, especially if working
with sticky side chain elastomers, to keep some toluene at the preparation
table to clean the tools before reuse. If possible, both long rims of the film
should also be cut with a carpet knife, because small defects at the rim can
(continued)
Search WWH ::
Custom Search