General properties of polymer networks - Physical Gels from Biological and Synthetic Polymers - page 98

address these cases in detail, we must spell out that the major difference between polymer
networks and polymer gels is simply the presence of a

or swelling agent. As we
will see below and elsewhere in this volume, the elastic constraints which follow from the
three-dimensional network structure ensure that a polymer network can swell to a limited
extent, but cannot dissolve. All viable theories of swelling are, in essence, concerned with
the balance between dissolution (i.e. in

' solvent '

nite swelling) and the abovementioned elastic
constraints, which follow from macroscopic network connectivity.

4.1 Chemically cross-linked networks and gels

The earliest true science of polymer networks goes back to studies on the nature of rubber
elasticity associated with such names as Gough, Joule and Kelvin in the nineteenth
century, well before there was any appreciation of the underlying network structure
(Treloar, 1975 ). Staudinger s demonstration that there was a class of materials which

consisted of very high molecular mass species, both linear and non-linearly linked, as
opposed to the original view that such species were simply colloidal aggregates, estab-
lished the existence of polymer science. Subsequently it was appreciated that rubber
elasticity is predominantly attributable to the reduction in entropy induced by the
deformation of linked polymer chains, and many papers on the classical theory of rubber
elasticity and swelling followed in the period 1936

'

1945, due to such seminal names as
Kuhn, Flory, Treloar and their respective co-workers.
As mentioned in Chapter 3 , the treatment of non-linear systems was a natural exten-
sion of work on linear polymerizations, which had been established by, among others,
Carothers and co-workers in the development of the synthetic polyamides, now known as
nylons, around 1934 at the DuPont Company.

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4.1.1 Non-linear materials formed from the reaction of functional groups

The chemistry of these systems has already been mentioned, and studies have tended to
follow the principles established in early work with reactant species a multi-functional

m-ol is reacted with, say, a multi-functional n-acid, e.g. pentaerythritol (functionality,
f m = 4) and the dibasic adipic acid (f n = 2), as in a series of experiments by Stockmayer
and Weil (Flory, 1953 ). Other systems include those from benzene triacetic acid (f n =3)
and decamethylene glycol (f m = 2) (BTA

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DMG), described in Chapter 3 .
Such materials, including such diverse systems as the phenol-formaldehyde, urea-
formaldehyde, epoxy and alkyd resins, are of commercial interest. Epoxy resins are
produced by reacting (typically) a diepoxide with a polyfunctional amine such as
triethylenetetramine (TETA) (f = 4). The well-known Araldite ® resin is an epoxy system
which uses epichlorhydrin and bisphenol-A to produce bisphenol-A diglycidyl ether
(f = 2), which is then

-

' '

with TETA.
Such chemical networks are used as adhesives and/or rigid moulding resins by
carrying out the reaction to high conversion, but such materials appear to be a long
way away from what are normally considered to be polymer gels. Indeed many, such as

cured