A UNIVERSAL STRESS-STRAIN RELATION FOR POLYMER MELTS AND RUBBERS

Uniaxial elongation and compression data of linear and branched polyme r melts as well as of crosslinked rubbers are analyzed with respect to their nonlinear strain rate. This is found to be the result of two co ntributions: (1) affine orientation of network strands, and (2) isotro pic strand extension. Network strand extension is caused by an increas ing restriction of lateral movement of polymer chains due to deformati on, and is modelled by a molecular stress function f which in the tube concept of Doi and Edwards is the inverse of the relative tube diamet er. Up to moderate strains. f2 is found to be linear in the average st retch for melts as well as for rubbers, which corresponds to a constan t volume of the tube. At large strains, melts show maximum molecular t ension depending on the degree of long-chain branching, while rubbers show maximum extensibility. The influence of crosslink density on the strain measure is considered.