The effect of microstructural evolution during deformation on the post-yielding behavior of self-associated polyamide blends

Abstract

The deformation-induced microstructure evolution of self-associated polymer blends, i.e., long chain polyamide alloys (LCPAs), was investigated and correlated with their mechanical performance. A PA1012 (soft phase)/PA612 (hard phase) blend was selected for this study. It is interesting that both Young's modulus and yield stress exhibit a nearly linear dependence with composition, which follows a simple additive mixing law. With strong intra-associated hydrogen bonds, the LCPAs studied here were found to be immiscible, but mechanically compatible because of the strong interfacial adhesion of the two constituents. Moreover, when the blend was deformed close to or above its glass transition temperature (Tg), the simultaneous occurrence of fracture reinforcement and toughness was achieved, which was defined as “soft phase-reinforcing-hard phase” (SRH). On the other hand, below Tg, a “hard phase-reinforcing-soft phase” (HRS) mechanical behavior was identified at the expense of elongation at break. Upon uniaxial deformation above Tg, the PA1012 component imparted to the blends a superior fracture stress due to strong strain-induced crystallization effect and the subsequent formation of a microfibrillar structure. However, the PA612 phase contributed little to the improvement of the mechanical properties of the blends. The large discrepancy in the contributions of the blend components to the fracture stress, primarily originates from the flexible nature of the PA1012 chains and the thin crystals nucleated by the already crystallized PA612 phase. However, below Tg, tilt, slippage and fragmentation of the lamellae occur in both phases, which are accompanied by apparent void formation and directly lead to catastrophic fracture. Considering the higher density of hydrogen bonds in PA612, the PA612-rich blends displayed higher fracture stresses as compared to the rest of the compositions. Overall, the distinctive microstructure evolution of the constituent phases and their roles in stress enhancement at large strains have been successfully established. The results presented here shed light on the deformation mechanism of self-associated polymers and offer a new pathway for the development of mechanically reinforcing materials.