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

The effect of annealing polycarbonate at 125 degrees C (approximate to T-g - 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 Delta h* = 1160 kJ mol(-1); nonexponentiality parameter beta is in the range 0.456 < beta < 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 mu = 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 mu(H) = 0.49 is found. The difference between these two shift rates suggests that the time scales for the aging process are different when probed by the two techniques of creep and enthalpy relaxation. Similarly, it is found that the yield stress of annealed samples depends on log(aging time) in quite a different way from its dependence on log(strain rate), and it is argued that this provides further support for the contention that the time scales and rates of physical aging will be different when probed by different techniques.