Effect of chain stiffness on polymer glasses’ shear deformation micromechanisms

Using molecular dynamics simulations, we study how chain stiffness affects how glassy polymers deform under applied shear. Loosely entangled systems composed of flexible chains exhibit strong shear banding and subsequent strain softening whereas tightly entangled systems composed of semiflexible chains exhibit neither of these. For all systems, inflection points in their stress-strain curves correspond to the onset of chain scission, but nonlinear strain hardening continues well beyond this point. Tightly entangled systems build up considerable elastic energy before fracturing via chain scission. Loosely entanged systems’ deformation is far more dissipative and their fracture occurs via chain pullout. Their much larger fracture strain produces far greater prefracture chain alignment, which in turn reduces interchain friction and further favors pullout over scission. We further explain these differences in terms of microscale plasticity metrics. Tightly entangled systems’ much higher rate of chain scission causes their plastic flow to be far more heterogeneous, and they ultimately fail along significantly sharper fracture planes than their loosely entangled counterparts.