Double yielding in PA11: discriminating the mechanical contributions of the amorphous and crystalline fractions

The stress-strain curve of PA11 exhibits two well-distinct successive yield points during tensile deformation below T_g. This phenomenon, denoted double yielding (DY), has been observed as well in other semicrystalline polymers such as PA6 or PE. An investigation of the origin of such phenomenon and its relationship to the deformation mechanisms occurring in the amorphous and crystalline phases of PA11 has been conducted. PA11 samples were purposely modified prior to or during tensile testing and the resulting effects on the two yield points were systematically monitored. A strong link between ageing-, temperature/rate- or plasticity-induced molecular mobility in the amorphous phase and the first yield point has been established. On the other hand, the second yield point is clearly sensitive to crystalline restructuring due to annealing or slow cooling from the melt. However, it is also sensitive to changes in mobility in the amorphous phase. A thermomechanical activation model describing the DY phenomenon that predicts the non-trivial evolution of σ_y2 and suggests that plastic flow in the amorphous phase must first be activated to initiate crystalline mechanisms has been proposed. More generally new insights into the complex microscopic deformation mechanisms in semicrystalline thermoplastics have been provided.