General properties of polymer networks - Physical Gels from Biological and Synthetic Polymers - page 100

Of course natural rubber is now used to a much more limited extent than previously;
instead, synthetic rubbers based on poly(butadiene), styrene

butadiene or silicone chem-
istries are commonly employed. For these the cross-linking chemistry is different, but is
still commonly known as vulcanization. Such non-traditional vulcanizing agents include
peroxides and urethane cross-linkers. Vulcanization, whether by sulphur or other agents,
is only one possible means of cross-linking preformed chains. Conventional free-radical-
initiated polymerization of vinyl polymers is very commonly used for linear chain
systems, but incorporation of small amounts of difunctional monomers can also produce
a cross-linked system. For example, the addition of divinylbenzenes to styrene can soon
produce a rigid resin, but the process can be controlled by suspension polymerization, by
adding a non-solvent such as water. Physical methods for cross-linking, such as radiation,
can also be employed; the work by Charlesby and co-workers (Charlesby and Swallow,
1959 ) pioneered this approach in the 1950s. Finally, many commercial ion-exchange
resins are formed by reacting difunctional styrene with divinylbenzenes and then mod-
ifying the polymer by adding the appropriate ionic groups.
For control and

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flexibility of material properties, silicones have a number of advan-
tages over vinyl polymers, and polydimethylsiloxane (PDMS) elastomers have proven
invaluable in academic studies because they can be persuaded to form elastomers by end-
linking, a process which allows far greater control and quantitative conversion degrees
(Erman and Mark, 1997 ).
The critical degree of cross-linking is approximately equal to 1/n 0 , where n 0 is the
number of potential cross-linking sites, which is in turn related to chain length. The exact
proportionality actually depends upon the primary chain molecular mass distribution; for
a monodisperse system it will be as stated, but for a most probable or Flory distribution it
tends towards 2/n 0 . However, in any practical process, the majority of cross-links formed
are not intermolecular, but are wasted in forming various defects such as loops or cycles.
At the same time, other network defects such as entanglements can also contribute,
so predicting the exact properties of an elastomeric network is not nearly as simple as,
for example, the Flory gel point estimate ( 3.3 ) might suggest. This has a number of
implications when we come to consider swelling theories.

4.1.3 Poly(acrylamide) and poly(NIPAm) gels

Despite the history of polymer networks discussed above for bulk systems, one of
the most important classes of chemical gels are those derived from poly(acrylamide).
These are widely used in a number of applications, most particularly in the technique of
poly(acrylamide) gel electrophoresis (PAGE). Early-generation DNA sequencing tech-
niques used poly(acrylamide) gels to separate DNA fragments differing in length by a
single base-pair, so the sequence could be read. Although now largely replaced in this
application by agarose gels (see Chapter 7 ), PAGE is still used in some applications
(Stryer, 1981 ).
A poly(acrylamide) gel is prepared by mixing acrylamide and a small amount of an
analogous branching component, typically the bifunctional form bisacrylamide, with a
persulphate, usually ammonium persulphate, and tetramethylethylenediamine (TEMED)