Introduction - Physical Gels from Biological and Synthetic Polymers - page 3

still re cation of the gel state

was noted by Hermans ( 1949 ), but still stresses the traditional colloidal viewpoint.
According to Hermans

ects the unclear description. An early, more formal classi

' de nition:

* Gels should be coherent two-component systems formed by solid substances

finely

dispersed or dissolved in a solvent.
* They should exhibit solid-like behaviour.
* The dispersed component and the solvent should extend continuously throughout the
whole system (bicontinuous systems).

In an earlier review (Djabourov, 1988 ), four different structures were illustrated: aggre-
gates of colloidal particles, with crystalline or amorphous structures; frameworks of rod-
like particles as in supersaturated solutions; polymer gels with long linear chains held
together by physical bonds such as

' crystalline junction zones ' ; and covalent bonds.

1.2 Structural characteristics and their study

At a Royal Society of Chemistry Faraday Division Meeting, Flory ( 1974 ) proposed a
structural classi

'

cation of gels which has some features in common with Hermans

de nition, but with more emphasis on speci cally polymeric systems:

* Well-ordered lamellar structure, including gel mesophases
* Covalent polymeric networks, completely disordered
* Polymer networks formed through physical aggregation, predominantly disordered,
but with regions of local order
* Particular, disordered structures.

In the present volume we are concerned with aspects of all of these classes, although, as
we have already stated, the second of these, the so-called chemical gels, although very
important, falls naturally outside the de

nition of a physical gel. Instead we will be
concerned with the other three types. Incidentally, the term

' physical gel ' itself has not

been in existence for that long: although it is dif cult to establish quite who first coined

the term, it seems to go back to the early 1970s, to Pierre-Gilles de Gennes.
At a later Faraday Division Meeting, Andrew Keller, another distinguished polymer
scientist, noted (Keller, 1995 ) that

there is no simple and unique definition of the gel state. Its main constituent is a fluid, yet it retains
its shape, which is a characteristic feature of the solid state matter. It can support large strains to a
high elastic limit, which, however, a real solid cannot do. Everyday one can experience that a jelly
wobbles! The retention of shape implies a connectedness of the system which, in view of the fact
that the major component is a fluid, means that there is a connective pathway along the non-fluid
component which is the existence of a ' network ' .

'

Consequently he coined a further de nition in which gels were 'fluid containing self

supporting disperse systems where the non- uid connecting elements are not con ned to

individual chain molecules but can also be larger assemblies of molecules, finely divided

struts or even membranes of the appropriate solid phase ' .