Sensitivity of damage to microstructure evolution occurring during long-term high-temperature annealing in a semi-crystalline polymer

This study aimed to highlight the role of microstructure evolutions induced by high temperature annealing on damage in a semi-crystalline polymer during long-term applications. It was based on a polar form of poly(vinylidene fluoride) (T g = −40 °C; T m = 170 °C) and the long term annealing context dealt with burst resistance tests performed for 1000 h in a temperature range from 95 °C up to 140 °C. A secondary crystallization was observed after annealing by Differential Scanning Calorimetry and a consistent phenomenology was evidenced in Dynamic Mechanical Analysis. A decrease of the amorphous phase mobility and a weak reorganization of primary crystals were observed at the same time. Tensile tests on annealed specimens pointed out modulus and yield stress reinforcement, partial disentanglement in the amorphous phase and a raise of the volume strain. Thermally-induced microstructure evolutions were shown to enhance cavitation and slow down crack opening displacement kinetics. This last effect would result from both a raise of the yield stress in primary crystals and secondary crystallization.