Understanding the elastomeric properties of polymer networks

It is shown that Monte-Carlo (MC) simulations of the elastic behaviour of c hains in networks using realistic rotational-isomeric-state (RIS) chain mod els are able to reproduce experimentally observed deviations from Gaussian network behaviour in uniaxial extension and also to interpret, quantitative ly, stress-optical properties. In stress-strain behaviour, an increase in t he proportion of fully extended chains with increasing macroscopic strain g ives rise to a steady decrease in the rate of change of the Helmholtz energ y of a network, causing a reduction in network modulus at moderate macrosco pic strains. There is no need to invoke a transition from affine to phantom chain behaviour as deformation increases. To evaluate stress-optical prope rties, the average orientation of segments with respect to the deformation axis is calculated, taking into account the interdependence of segment orie ntation and chain orientation as chains become more extended and aligned un der uniaxial stress. The MC method gives, in agreement with experiment, val ues of stress-optical coefficient that are dependent upon both deformation ratio and network-chain length. The method highlights serious shortcomings in the classical Gaussian model of stress-optical behaviour. Applications o f the simulation methods to the quantitative modelling of the stress-strain behaviour of poly(dimethyl siloxane) networks and the stress-optical behav iour of polyethylene networks are described.