Physical aging of polycarbonate: Elastic modulus, hardness, creep, endothermic peak, molecular weight distribution, and infrared data

For the first time, load and depth sensing indentation (DSI) was used in order to monitor physical aging of bisphenol A polycarbonate for 30 months at room temperature and for 1 month at an elevated temperature. The DSI experiments were combined with differential scanning calorimetry, gel permeation chromatography, and infrared spectroscopy. The endothermic peak of polycarbonate shifted toward higher temperatures upon aging at an elevated temperature and did not change its location upon aging at room temperature. The elastic modulus and hardness of polycarbonate increased in a stepwise fashion during aging at room temperature. The molecular weight distribution broadened slightly, and the trans-trans conformational population increased during annealing. No simple correlation between changes in the mechanical properties and the shift of the endothermic peak during annealing was found. These changes seem to be caused by phenomena of different nature; namely, the changes in the mechanical properties appeared to have a reasonable correlation with free volume relaxation of the polymer, whereas the changes in the endothermic peak may be associated with internal energy changes. The similarities and differences between our results and the results of others are discussed.