"Slip-stick" fracture and toughness enhancement in thermoset/thermoplastic polymer alloys under shear

Using large-scale molecular-dynamics simulation of a generic model, we study the mechanical behavior of thermoset/thermoplastic polymer alloys under shear. We investigate the effect of thermoplastic mass fraction Γ_l and the thermoplastic chain length N_l. Our results show two different fracture peaks in the stress-strain behavior. The first peak occurs at around 60% strain followed by a stress plateau, and the system fails at around 90% strain. This "slip-stick" fracture is independent of shear rate and only occurs when Γ_l is less than the threshold concentration Γ^* at which thermoplastic chains start to overlap. A micro-structural analysis suggests that the escape of thermoplastic chains from cavities near the fractured interfaces gives rise to slip-stick behavior. Slip-stick behavior has a strong chain length dependence and is only observed when N_l is greater than critical chain length N_l^c=40. Slip-stick fracture makes an alloy of Γ_l=4.7% more than 40% tougher than a neat thermoset.