Molecular modelling of the elastic behaviour of polymer chains in networks. Comparison of polymethylene and poly(dimethylsi1oxane)

Abstract

It has been shown recently that Monte Carlo (MC) simulations of the elastic behaviour of polymethylene (PM) and poly(dimethylsiloxane)(PDMS) networks, using a realistic rotational-isomeric-state (RIS) network-chain model, are able to reproduce experimentally observed deviations from Gaussian network behaviour in uniaxial extension. An increase in the proportion of fully extended chains with increasing macroscopic strain gives rise to a steady decrease in the rate of network Helmholtz energy change, causing a reduction in the network modulus at moderate macroscopic strains. There is no need to invoke a transition from affine to phantom chain behaviour as deformation increases. In the present paper, the MC simulation method is used to compare the room-temperature behaviour of PM and PDMS network chains. Differences are interpreted in terms of the end-to-end distributions and chain-flexibilities. The stiffer PM chains generally show larger deviations from Gaussian behaviour.