General properties of polymer networks - Physical Gels from Biological and Synthetic Polymers - page 99

moulding resins like Araldite, are deliberately prepared (or driven by appropriate pro-
cessing) into the glassy state (Williams et al., 1998 ). However, at degrees of conversion
just greater than the corresponding gel point, i.e. when p

p c , they can be swollen, and
then their properties are not dissimilar to those of any other swollen network, whether
chemically or physically cross-linked, and so can be described, albeit with caution, as
polymer gels.
Perhaps the most important materials in this category are the polyesters and polyur-
ethanes (Stanford, 1998 ; Stepto, 1998 ). These form by the reaction of the constituents
with (usually) the elimination of a simple product. For example, the production of
polyesters involves the reaction of a polyacid with a polyol (often just a diol), with the
elimination of water. Commercial polyurethanes involve the reaction of a diol such as
ethylene glycol, a diisocyanate such as MDI (4,4 0 -diphenyl-methane diisocyanate) which
gives a urethane (structurally

≳

) linkage, and a polyfunctional additive such
as a hydroxyl-functionalized polyether (Stanford, 1998 ). In all cases a catalyst and high-
temperature/pressure processing is required to produce an acceptable commercial prod-
uct. For example, polyurethanes are commonly produced by the reactive injection
moulding (RIM) process, while polyurethane foams are producing by exposing the
reactive urethane groups to water or steam.

- O(CO)NH -

4.1.2 Non-linear materials from preformed polymer chains

These systems include the traditional elastomers, both natural and synthetic rubbers
(Erman and Mark, 1997 ). The vulcanization of natural rubber, a process some two
centuries old, involves heating the natural rubber latex extracted from the tree

-

essentially cis-poly(isoprene) with elemental sulphur. The sulphur forms cross-

links, normally assumed to be of the form

-

, between adjacent polymer chains,
a process commonly referred to as curing; see Figure 4.1 . Provided it has more than the
necessary number of intermolecular cross-links, the newly cross-linked rubber
becomes a solid (albeit a viscoelastic solid), whereas prior to this it was a viscoelastic

- S - S - S -

fluid, so curing a rubber ( ' gelling ' it) converts the material from liquid into solid.

Figure 4.1

Sulphur vulcanization of natural rubber, cis-poly(isoprene).