ANALYSIS OF THE RUBBER ELASTICITY OF POLYETHYLENE NETWORKS BASED ON THE SLIP LINK MODEL OF EDWARDS,S.F.

Force-extension data, obtained at 140°C on polyethylene monofilaments, electron-irradiated with doses from 0.7 to 6.0 Mrad, are interpreted according to a recent model of rubber elasticity proposed by Edwards and Vilgis. In this model, the free energy of single chains between the crosslinks, Nc, is described by the usual statistical theory, whereas the reduction of entropy due to entanglements with neighboring chains is described by a number, Ns, of slip links, which are free to slide along the arc length of the chain between crosslinks. The degree of slip link freedom is measured by a parameter η. Finally, the extra constraints introduced by the entanglements limit the extensibility of the chains long before the single chain limit is reached. The effect of strain rate and orientation history on the samples is also described. Our data suggest that slip links are dynamic, and can behave as crosslinks, at higher strain rate. The effect of solid state drawing on an irradiated polymer is to alter permanently the structure of the network; increasing the number of effective crosslinks, at higher dose, and removing a fraction of non-permanent entanglements, at lower dose.