Introduction - Physical Gels from Biological and Synthetic Polymers - page 2

solution), entirely solid (jelly), or a mixture of the two; or whether the physical change is or is not
accompanied by chemical change.

He recalls also that:

Graham ' is nomenclature is as follows: The fluid state, colloidal solution, is the ' sol ' , the solid state

the ' gel ' . The fluid constituent is indicated by a pre x. Thus an aqueous solution of gelatin is a

' hydrosol ' , and on setting it becomes a ' hydrogel ' .

This nomenclature is still largely valid nowadays, other extensions having been intro-
duced. Gelatin, in some ways the paradigm hydrogel, and which derives its name directly
from , is produced by hydrolysis of collagen, so gelatin gels were probably recog-

nized by early man during cooking, much before their properties were understood.
Gelation was initially regarded in the same light for supersaturated solutions of
inorganic compounds or natural polymers, although now the two are regarded very
differently. Natural rubber gave the

' gel '

first estimate of molecular mass (referred to infor-

mally, and in the early literature, as ) in the thousands of daltons, but

these were then largely dismissed and so posed the question of the nature of such
molecules. Paul J. Flory wrote:

' molecular weight '

'

The gap between molecules of ordinary size and those
of hundreds or thousands times as large was too great to be bridged in a single leap '

(Flory, 1953 ). Indeed, recognition of polymer molecules (macromolecules) came much
later, when Hermann Staudinger, against the views of many of his contemporaries,
asserted the existence of covalently bound long chains (Mülhaupt, 2004 ). Rubber, gelatin
and cellulose were all considered to belong to this category. The gels described in this
volume mostly involve such polymeric chains. Covalently bound polymer networks
became very important in polymer science, since, depending upon conditions, they can
form

rigid resins or foams.
According to Treloar ( 1975 ) such elastomer systems, for example, have been studied for
many years, going back to the work of Gough and Joule in the nineteenth century. In
excess of solvent such a polymer network no longer dissolves; instead it simply swells

' rubber-like ' elastomers - they often show high elasticity -

-

often, but not exclusively, since this depends on the polymer - solvent system - to ll the

volume of its container. The solvent can be replaced by another fluid inside the network,

giving rise to macroscopic changes of gel volume either by swelling or by shrinking
Covalently linked networks are permanent, since the junctions can be assumed to be
formed irreversibly; in physical networks the junctions are sometimes irreversible even
though they do not involve covalent linkages (Ca alginates or, heat-set protein gels),
but they can also sometimes associate or dissociate reversibly under thermodynamic
(temperature, pH, ionic strength) or mechanical action (shear, elongation), depending
on their species and mode of formation. In fact any de nition of physical gels which

simply implies shear or temperature reversibility is far too narrow. As we will see
below, some of the systems of interest to us are reversible when they are subject to, for
example, stirring (

), others are temperature reversible, but some are neither of
these. Many papers, particularly some theoretical papers, seem not to have grasped this
essential facet.
That said, the widely cited de

' shear '

nition by Dorothy Jordan Lloyd that ' the colloidal

condition, the gel, is one which is easier to recognise than to de ne ' (Jordan Lloyd, 1926 )