5.2 Viscoelasticity of polymers
The simplest models for the deformation behaviour of an ideal material are those of Hookean linear elasticity in the solid state, and Newtonian linear viscosity in the liquid state. The end point of elastic deformation is either fracture or plastic flow, with the latter taking place at a constant yield stress in the ideal case. Whilst the behaviour of many real materials does approximate to these idealised models, that of polymers deviates markedly from them. In particular, their solid state deformation is time-dependent and nonlinear and so resembles some combination of elastic and viscous responses, whilst their melt rheology is also significantly nonlinear. To start with, let's consider polymer behaviour in the context of the idealised models of elastic deformation and of viscous flow.