General properties of polymer networks - Physical Gels from Biological and Synthetic Polymers - page 97

4 General properties of polymer

networks

As noted in Chapter 1 , there are essentially three types of polymer network structures:
covalently cross-linked networks, in which the junctions are formed by chemical bonds;
entangled networks (melts, concentrated solutions) in which the junctions are simply
localized topological constraints; and physical networks where the junctions have com-
plex structures, stabilized by secondary forces (such as hydrogen bonds, hydrophobic
interactions or labile ionic complexes). The present volume is concerned predominantly
with physicals gels and gelation. However, both from the historical viewpoint and from
the overlaps in structure

property relationships which follow from the commonality of
topology, it would be quite inappropriate to attempt to address the nature and science of
physical gels without considering the many features of interest and theories that have
been carried over, either modi

-

ed or unmodi ed, from the older and scienti cally more

mature field of chemical gels and networks.

Interestingly, physical gels and networks have been known, at least through the
application of their properties, for millennia. The early Egyptians were familiar with
both starch and animal-based glues some 4000 years ago, while the technology of
papermaking dates back to the Chinese Han dynasty, c.100 AD. By contrast, chemical
networks have been known for only around two hundred years, starting with
Mackintosh

s early work (1823) on producing a waterproof fabric using natural rubber,
and the subsequent progress by Goodyear in extending the applicability of natural rubber
latex by the process of sulphur vulcanization, invented in 1839. The

'

first synthetic resin
was the phenol-formaldehyde system Bakelite, invented by Leo Baekeland in the rst

decade of the twentieth century. Subsequent work on urea-formaldehyde and polyester
resins followed in the next 20 years.
Much of the above work is described in Stepto

s 1998 monograph (Stepto, 1998 ). This
volume describes howmuch of the simple chemistry was known well before its extension
to useful cross-linked resins. What is of real signi

'

cance is that much of this work

predates Staudinger s macromolecular hypothesis of the early 1920s, and a real under-

standing of the nature of polymer networks did not become clear until Flory

'

s work on
non-linear polymerization, published in a series of classic papers in the late 1930s and
early 1940s.
What this work also made clear for the

'

first time was the similarities and differences

between now more correctly referred to as step-growth or end-

linked polymerization, and involving low molecular mass species

' polycondensation '-

,
the cross-linking of pre-existing polymeric (i.e. high molecular mass) chains. Before we

- and ' vulcanization '