ANISOTROPY EFFECTS ON CHAIN CHAIN INTERACTIONS IN STRETCHED RUBBER

When a polymer network is stretched, the distribution of chain end-to-end vectors becomes anisotropic. The effect of this anisotropy upon excluded-volume interactions between the chains of this system is studied in this paper by the molecular dynamics simulation of an idealized model of a dense system that we term an oriented melt. This is a polymer melt in which the chain vectors are maintained constant in length and direction but otherwise they and the rest of the chains are free to move. We find that in a stretched four-chain model the chain vectors cease to be a principal axis of the chain stress due to the anistropic chain-chain interaction; this is contrary to an assumption made by us in earlier calculations on this model. We also study an oriented melt with chain vectors randomly oriented in the reference state. We find that the anisotropy in the stretched state has important effects on the chain stress and produces significant segment orientation enhancement for chains nearly perpendicular to the stretch direction. The simulations show that the stress at fixed extension increases with the reduced density rho = n sigma-3/v where n/v is the atom number density and sigma is an effective hard-sphere diameter; this is in qualitative agreement with experimental observations on the effect of large hydrostatic pressure in rubber elasticity. Finally, the simulations show clearly that on the atomic level the deviatoric or anisotropic portion of the stress in a stretched rubberlike solid is transmitted by the excluded-volume interactions, which are screened by the covalently bonded structure.