ABSTRACT
Polymer networks can be divided formally into two classes, chemically cross-linked materials (including bulk elastomers), and “entanglement networks”. Between these two categories fall a number of systems, which consist of chains “physically” cross-linked into networks. Physical gels can be formed from both synthetic and naturally occurring polymers. Amongst the latter are gelatin, the seaweed and plant polysaccharides such as agarose, the carrageenans and pectin, starches and cellulose derivatives, globular protein gels, formed either by heating or by change of chemical conditions (pH etc.) and fibrillar gels of actin and myosin. This article discusses the structure and mechanical properties of these gels, and the nature of the cross-links in the above increasing order of structural complexity. A difficulty in their characterisation is that such systems are often under kinetic rather than thermodynamic control.
