(342b) The Long-Awaited Universality in Polymer Physics

Predicting and manipulating dynamics of polymeric liquids is of great importance in the design and processing of soft materials. While fundamentals of slow flow dynamics were resolved half a century ago, fundamentals of fast flow dynamics are still unsolved, especially due to the lack of experimental data. Thus the primary goal in polymer physics is to develop a universal framework that will predict the linear and nonlinear flow behavior of polymer melts from a small number of physical parameters. Universality of molecular dynamics is the underlying framework of the most successful theoretical model, known as the “tube model”. According to the tube model a polymer melt can be coarse-grained into four independent parameters: a tube diameter (a), the number of entanglements per chain (Z), the equilibration time of one entanglement (τ_c), and the the number of Kuhn segments, N. The model predicts that different polymers (i.e. polymers with different chemical structures) exhibit identical rheological responses (normalized) if they have the same values of Z and λ_max.  To date, no two molecular systems involving melts or polymer solutions have exhibited identical nonlinear behavior. In this work, we first demonstrate the similarities between the nonlinear extensional flow dynamics of two melts, poly(methyl ethacrylate) (PMMA) and polystyrene (PS),  with identical Z, relatively similar N,  but very different chemistry. Secondly, we manipulate a PMMA solution into exhibiting the same flow behavior as a PS melt, considering the latest concepts of monomeric friction reduction. Despite great differences in chemistry and fluid topology, strikingly similar responses of these fluids are observed for both slow flows and fast extensional flows. This unique data set confirms universality of nonlinear polymer dynamics and therefore clearly outlines the missing physics in our current models.