ABSTRACT
When an amorphous polymer such as bisphenol A polycarbonate (BPA-PC) (I) is rapidly cooled from the liquid state into the glassy state, it is not at thermodynamic equilibrium but rather in a metastable condition. Such glasses contain excess volume as well as residual thermal and orientational strains and stresses (24). Annealing is one method whereby the residual strains, stresses, and orientation can be reduced or eliminated and has been defined as the tempering of materials by heat in the case of inorganic glasses and metals. In the case of thermoplastics, it is the process used to relax residual thermal stresses and to recover residual orientational strains (5-16). It is normally performed just below the glass transition temperature (Tg) of the material to reduce residual thermal strains and stresses and slightly above the glass transition temperature to remove orientation. Annealing is a secondary operation. Changes may also be observed in the volume and the thermal, mechanical, optical, and dielectric properties when samples are allowed to age at temperatures between the low-temperature p transition and the glass transition over long periods of time (15-24). These changes occur as a result of internal processes in the absence of external forces. This process is known as physical aging because of the changes in the physical properties (6). It is clear that the terms annealing and physical aging are being used to describe some of the same phenomena.
