THE NONLINEAR STRAIN MEASURE OF POLYMER MELTS AND RUBBERS - A UNIFYING APPROACH

Uniaxial elongation and compression data of linear and branched polyme r melts as well as of rubbers are analyzed with respect to their nonli near strain measure. This is found to be the result of two contributio ns: (1) affine orientation of network strands, and (2) isotropic stran d extension. Network strand extension is caused by an increasing restr iction of lateral movement of polymer chains due to deformation, and i s modelled by a molecular stress function f, which in the tube concept of Doi and Edwards is the inverse of the relative tube diameter. At s mall strains, f2 is found to be linear in the average stretch for melt s as well as for rubbers, which corresponds to an affine deformation o f the tube. At large strains, melts show maximum molecular tension dep ending on the degree of long-chain branching, while rubbers show maxim um extensibility.