Molecular weight dependence of the physical aging of polycarbonate

In our previous paper, we reported that the impact value of polycarbonate (PC) increases with increasing molecular weight and decreases upon annealing [Polymer 2012, 53, 895-896]. To clarify these phenomena, the effects of molecular weight and heat treatment on the fracture of PC are investigated in this study. The results show that the stress for craze nucleation increased as the molecular weight increased but was not affected by annealing. The shear modulus and bulk modulus do not exhibit molecular weight dependence, but annealing resulted in a slightly increased shear modulus and a decreased bulk modulus. Furthermore, the amplitude and correlation length of density fluctuations of the Debye-Buche mode in small-angle X-ray scattering profiles were shown to increase upon annealing. On the basis of these results, it is suggested that, as the molecular weight increases, the toughness of PC is enhanced because the increase in molecular weight increases the stress of slippage for entangled molecular chains. In addition, it is proposed that the change in the fraction of fine voids resulting from density fluctuations may affect the toughness, resulting in the reduced toughness observed for annealed PC.