Why is Understanding Glassy Polymer Mechanics So Difficult?

In this Perspective, I describe recent work on systems in which the traditional distinctions between (i) unentangled versus well-entangled systems and (ii) melts versus glasses seem least useful, and argue for the broader use in glassy polymer mechanics of two more dichotomies: systems which possess (iii) unary versus binary and (iv) cooperative versus noncooperative relaxation dynamics. I discuss the applicability of (iii-iv) to understanding the functional form of strain hardening. Results from molecular dynamics simulations show that the dramatic hardening observed in densely entangled systems is associated with a crossover from unary, noncooperative to binary, cooperative relaxation as strain increases; chains stretch between entanglement points, altering the character of local plasticity. Promising approaches for future research along these lines are discussed. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 979-984, 2011