Melts of Linear Polymers in Fast Flows

We report on recent progress in our understanding of the rheological behavior of polymer melts, resulting from the work of several investigators, mostly on fast extensional flows. They contributed new data from cleverly conceived experiments, new results of Brownian and molecular dynamics simulations, and suitable theoretical models. The outcome of such collective effort is the plausible certainty that flow-induced coalignment of the Kuhn segments of the polymer modifies the friction coefficient, which may depart considerably from the equilibrium value. It is also found that the effect is sensitive to the chemistry, being different in different polymers. It so appears that the universality of the rheological behavior of polymer melts, well documented under equilibrium conditions and in relatively slow flows, is lost in fast ones. This feature, however, opens interesting perspectives of future research and application.