Deeper issues in strain hardening Shi-Qing Wang, University of Akron, DMR The aim of the project is to delineate the difference in polymer chain dynamics between linear melts and melts with long chain branching (LCB). LCB has been shown to suppress elastic yielding after large step extension. More recently, polystyrene of centipede-like architecture has shown surprising shear strain hardening in solutions, as reported in Phys. Rev. Lett. 111, (2013). A dumbbell-like PS, as shown in Fig. 1, was then studied with the knowledge that the side chains might be too short for this molecule to be a pom-pom model system. Surprisingly, rheometric measurements revealed strain hardening as shown in Fig. 2, i.e., the stress builds up more quickly as the strain increases, contrary to the familiar stress overshoot. The synthetic difficulty has made it difficult to obtain a true pom-pom polymer. The available PS depicted in Fig. 1 and 2 should be rheologically equivalent to the linear PS. On the other hand, Fig. 2 resembles the strain hardening behavior reported of the LCB PS. Such similarity inspired the further research to probe the possibility that linear PS solutions could also display strain hardening. By resort to an ultra high molecular weight linear PS, it is possible to access this regime of strain hardening. Fig. 3 shows for the first time that linear PS chains form an entanglement network that tightens under strong shear. Two more steps were taken to put this finding in proper perspective. First, similar, entangled, linear chain solution of 1,4- polybutadiene was also examined under comparable shearing conditions. It did not show strain hardening. Then, PMMA solution is investigated and strain hardening was observed, indicating a polymer with high T g is necessary to observe this unusual feature. Fig. 1 Fig. 2 Fig. 3

Deeper issues in strain hardening Shi-Qing Wang, University of Akron, DMR On the broader issue of advancing the field of polymer dynamics beyond the reptation/tube theory, a letter has been published [J. RHeol. 58, 1059 (2014)] to point that a recent counter-study of shear banding in entangled polymer solutions is flawed on numerous accounts. It is shown that the authors (Li et al.) of J. Rheo. 58, 1071 (2014) did not know that their samples had either the wrong molecular weight or much higher polydispersity, making it improbable to observe shear banding under the conditions of weak shear, which they deliberately chose to avoid strong shear. In other words, as shown in the diagram on the left hand side, the new study of Li et al. did not reach into the "phase space" explored by previous studies, shown by the purple box. As another aspect of broader impacts, the PI was invited to present a keynote speech at the Polymer Processing Society in June, PI continues to reach the community with our seminar at that has received nearly 3500 visits since March Finally, PI has made a decision to spend a major part of his time writing a monograph “Physics of Nonlinear Polymer Rheology: macroscopic phenomenology and conceptual foundation”, aiming to finish by the end of summer 2015.