Physical aging of polycarbonate : Enthalpy relaxation, creep response, and yielding behavior

The effect of annealing polycarbonate at 125 °C (≈Tg − 20 K) for aging times up to almost 2000 h has been investigated by differential scanning calorimetry, and the kinetics of the enthalpy relaxation process are compared with the effects of aging at the same temperature on the creep response and on the yield behavior. The enthalpy relaxation is analyzed by the peak shift method, and the following kinetic parameters are obtained:  nonlinearity parameter x = 0.46 ± 0.02; apparent activation energy Δh* = 1160 kJ mol-1; nonexponentiality parameter β is in the range 0.456 < β < 0.6. The similarities and/or differences between these results and others quoted in the literature are discussed. The creep response is analyzed by the commonly accepted procedure of horizontal and vertical shifting of deflection vs log(creep time) curves, and a shift rate of μ = 0.87 is obtained, with an excellent master curve. It is shown that a similar shift rate for enthalpy relaxation can be defined, and a value of μH = 0.49 is fo...