Understanding the elastomeric properties of polymer networks

It is shown that Monte‐Carlo (MC) simulations of the elastic behaviour of chains in networks using realistic rotational‐isomeric‐state (RIS) chain models are able to reproduce experimentally observed deviations from Gaussian network behaviour in uniaxial extension and also to interpret, quantitatively, stress‐optical properties. In stress‐strain behaviour, an increase in the proportion of fully extended chains with increasing macroscopic strain gives rise to a steady decrease in the rate of change of the Helmholtz energy of a network, causing a reduction in network modulus at moderate macroscopic strains. There is no need to invoke a transition from affine to phantom chain behaviour as deformation increases. To evaluate stress‐optical properties, the average orientation of segments with respect to the deformation axis is calculated, taking into account the interdependence of segment orientation and chain orientation as chains become more extended and aligned under uniaxial stress. The MC method gives, in agreement with experiment, values 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 of the simulation methods to the quantitative modelling of the stress‐strain behaviour of poly(dimethyl siloxane) networks and the stress‐optical behaviour of polyethylene networks are described.