THE ROLE OF THE ORIENTATION TENSOR IN THE RHEOLOGY OF FLEXIBLE POLYMERS

The stress-strain relations for transversely isotropic deformations of linear and branched polymer melts as well as of (crosslinked) rubbers are discussed in terms of the orientation tenser. It is shown that or ientation and network strand extension are decoupled, and that the rel ative tube diameter and its inverse, the molecular stress function f, can be extracted directly from experimental data, if the effect of net work orientation is accounted for by the order parameter. The tension of the average network strand increases with increasing deformation. T his is caused by an increasing restriction of lateral movement of poly mer chains due to deformation. At small strains, f(2) is found to be l inear in the average stretch for melts as well as for rubbers, which c orresponds to a constant volume of the tube. At large strains, melts s how maximum molecular tension, depending on the degree of long-chain b ranching, while rubbers show maximum extensibility.