Investigation of the low temperature ageing kinetics of glassy polycarbonate by mechanical damping spectroscopy

Abstract

Polycarbonate samples were submitted to ageing at room temperature or at 8°C after quenching from above Tg or de-ageing by pre-yield tensile deformation. Loss curves were measured in the temperature range located between the β and the α transitions. These curves exhibit an α′ peak on the low temperature side of the α peak. The position of the α′ peak is clearly dependent on the history of the sample: the α′ peak located in the low temperature range after quenching or mechanical de-ageing is shifted to higher temperatures during further ageing. The time and temperature dependence of the α′ peak location has shown that the kinetics of the α′ relaxation near and below room temperature is controlled by a lower activation energy process than the annealing process prevailing near Tg and already identified. A classical formalism modified by taking into account two α′ relaxation processes prevailing either at low or higher temperatures yields a good description of the α′ loss curves of most samples aged for a long term at room temperature, but fails to describe the α′ peaks of specimens submitted to another ageing history. To model the loss curves of such specimens the relaxation time spectrum involved in the description of the α′ peak is assumed to be dependent on the history of the sample and on the current test temperature.