Chemistry Reference
In-Depth Information
PDMS has been combined with a number of other polymers to form
IPNs. Examples involve cellulose acetate butyrate,
176,
177
fluorinated acry-
lates,
178
polycarbonates,
179
epoxies,
180,
181
poly(vinyl alcohol),
182
and
poly(
N
-vinyl-2-pyrrolidone).
183
7.2.7 Sorption and Extraction of Diluents
7.2.7.1 General Approach
Swelling is an elastomeric deformation that specifically addresses the rate
at which diluent absorbs into a network of known “pore size,” and how
rapidly it can subsequently be extracted.
184-187
The rate of absorption or
extraction can be used to estimate diffusion coefficients. Extraction effi-
ciencies are related to the extent of reaction in the end-linking procedure
used to form the network and the degree to which the extractable chains
are entangled with the network chains. Of interest is the dependence of
these quantities on the molecular weight,
M
c
, of the network chains (as a
measure of pore size), the molecular weight,
M
d
, of the diluent, the struc-
ture of the diluent (linear, branched, or cyclic), and whether or not the
diluent is present during the end-linking process.
7.2.7.2 Linear Diluents
One way of obtaining a network swollen with diluent is to form the net-
work in a first step and then absorb an unreactive diluent into it. Alterna-
tively, the same diluent can be mixed into the reactive chains prior to end
linking. In either case, oligomeric and polymeric diluents are of greatest
interest. The diluent must be functionally inactive for it to “reptate”
through the network rather than being bonded to it. Both types of net-
works can then be extracted to determine the ease with which various
diluents can be removed, as a function of
M
d
and
M
c
.
The ease with which a diluent could be removed from a network was
found to decrease with increase in
M
d
and with decrease in
M
c
, as expected.
High molecular weight diluents are extremely hard to remove at values of
M
c
of interest in the preparation of model networks, complicating the
analysis of soluble polymer fractions in terms of degrees of perfection of
the network structure. The diluents added after the end linking are more
easily removed, possibly because they were less entangled with the net-
work structure.
