The Very Long-Term Volume Recovery of Polycarbonate: Is Self-Retardation Finite?

Abstract

The specific volume change of Bisphenol A polycarbonate was measured at room temperature for several years. The effects of formation conditions like cooling rate and pressure, the addition of low molecular additives, and injection molding and quenching were investigated. The volume shrinkage at long times was found to be much higher than extrapolated from short term measurements. A general pattern is observed in the volume recovery curves. They start with a small constant slope on the logarithmic time scale. After about 107 s a transition is observed to a much steeper slope. Whereas in the first section the characteristic increase in τeff is observed, the volume recovery during the transition can be fitted quite well with a single exponential. The sharp transition suggests that there is a long retardation time, which may represent the genuine α-relaxation. It also means that the self-retardation during physical aging is finite. The slope at long times (>107 s) scales with the distance from equilibrium, as extrapolated from measurements close to Tg. The cooling rate during vitrification affects the slope in the first zone only, while the formation pressure leads to a vertical shift of the curves. Addition of low molecular weight additives leads to a dramatic densification and accelerates the volume recovery.