Effect of Chain Flexibility on Non-Linear Extensional Response of Linear Polymer Melts

Our work has recently shown the existence of universality in nonlinear flow behavior of polymer melts and polymer solutions [1]. The key conclusion from Wingstrand et al. is that in order for two systems to have the same non-linear flow dynamics, they must have three properties in common, (1) the same number of entanglements, (2) the same number of Kuhn segments between entanglements, and (3) the same monomeric friction between components. By extension, a comprehensive model capable of predicting both linear and nonlinear flow behavior must incorporate these fundamental properties. Since differences in polymer chemistry lead to fundamentally different values of (1)-(2) (and possibly (3)), these results show that the likelihood of two melts having the same nonlinear behavior is improbable. This work determines the dependence of the nonlinear flow behavior on (2) and (3) using a new commercial filament extensional rheometer, VADER 1000 (Rheo Filament ApS). More specifically, we study polystyrene, poly(methyl methacrylate), and poly(tert-butylstyrene) melts with tuned MW to give the same number of entanglements. The samples only differ in the number of Kuhn segments between entanglements (flexibility) and possibly monomeric friction. Small amplitude oscillatory shear is used to confirm the identical linear shear rheology between the three polymer melt samples. Constant strain rate extensional rheology is used to probe the fast flow dynamics of the polymers. The scaled stress growth coefficient as a function of scaled time shows the unique response of each polymer and the trend of increasing number of Kuhn segments per entanglements on strain hardening for various Weissenberg numbers.
[1] Wingstrand, S. L., Alvarez, N. J., Huang, Q. & Hassager, O. Phys. Rev. Lett. 115, 078302 (2015).