Simultaneous reptation and constraint release in polymer melts

A numerical solution to the parallel processes model for combined reptation and constraint release in polymer melts is presented. The discrete chain solution to the problem exhibits an exponent of 3 in the molecular weight dependence of the viscosity, just as in the strict reptation case; the magnitude of the viscosity is overestimated and no crossover to reptation is observed, as in Graessley's independent processes model. It is shown that linear combinations of reptation and constraint release, as modeled here, cannot describe the viscosity results observed experimentally.