Physical Gelation of Synthetic and Biological Macromolecules

Abstract

Historically, polymer networks can be divided into two main classes, chemically cross-linked materials (including bulk elastomers), and ‘entanglement networks’. The latter are formed by the topological interaction of polymer chains, either in the melt or in solution when the product of concentration and molecular weight becomes greater than some critical molecular weight M_e.^1,2 In this case they behave as ‘pseudo gels’ at frequencies higher (timescales shorter) than the lifetime of the topological entanglements. This depends for linear chains on M_r ³, where M_r is molecular weight. The covalently cross-linked materials, on the other hand, are formed by a variety of routes including cross-linking high molecular weight linear chains, either chemically or by radiation, by end-linking reactant chains with a branching unit, or by step-addition polymerisation of oligomeric multifunctional precursors. They are true macromolecules, where the molecular weight is nominally infinite, and they therefore possess an infinite relaxation time and an equilibrium modulus.¹