Chemistry Reference
In-Depth Information
7.2.7.3 Branched Diluents
The extraction of branched diluents is quite important. For example, in
the preparation of networks by radiation cross linking there are presuma-
bly large numbers of soluble molecules formed that are highly branched
(because of the essentially uncontrolled formation and coupling of free-
radical species). 188 Another example occurs in the area of controlled re-
lease of drugs from cross-linked reservoirs. 189 Some of the drugs diffusing
out of such delivery systems are assuredly “nonlinear,” so understanding
such systems is likely to become more important as interest shifts to
larger and larger molecules (such as polypeptides). Finally, molecules with
long branches can greatly affect the flow characteristics of a polymer, 190 to
the extent that they can be used as processing aids in polymeric systems
that will subsequently be cross linked. The ultimate in branched systems,
“ dendr imers,” 191-194 should also find applications in network structures.
7.2.7.4 Polar Diluents
There is a complication that can occur in the case of networks of polar
polymers at relatively high degrees of swelling. 170, 195 The observation is
that different solvents, at the same degree of swelling, can have signifi-
cantly different effects on the elastic force, apparently due to a “specific
solvent effect” on the unperturbed dimensions that appears in the various
molecular forms of the elastic equations of state. Although frequently ob-
served in studies of the solution properties of un-cross-linked polymers, 196,
197 the effect is not well understood. The effect is apparently partly due to
the influence of the solvent's dielectric constant on the coulombic interac-
tions between parts of a chain, but probably also to solvent-polymer seg-
ment interactions, which change the conformational preferences of the
chain backbone. 195
7.3 MULTIMODAL NETWORKS
7.3.1 Introduction
Multimodal networks represent a method to determine the effect of net-
work chain-length distribution 124, 126 on rubberlike elasticity. Chain-
length distribution has not received much attention even though
manipulation of the chain-length distribution can give large improve-
ments in mechanical properties. There are two primary reasons for this
 
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