Abstract
Most users of polymeric materials have a good sense that the glass transition event is kinetic in nature, i.e., it depends on cooling or heating rate in conventional experiments, and that the glassy state is a non-equilibrium state. However, it is often not appreciated that the structural recovery which occurs as a glass attempts to reach equilibrium is non-linear (e.g., the rate of volume recovery depends on the instantaneous volume). The non-linear viscoelastic nature of structural recovery can lead to surprising behaviors in certain kinds of measurements. It results in, for example, the asymmetry of approach in up and down-temperature jumps. The features of enthalpy and volume recovery, including the sub-Tg peaks and excess enthalpy overshoots in differential scanning calorimetry, are well-described by models of structural recovery developed in the 1970's. The purpose of the present work is to describe structural recovery and physical aging and their impacts on material performance and measurement of material properties. In addition, we present new results from calculations using the structural recovery models in which we demonstrate that new analytical tools, such as Temperature Modulated Differential Scanning Calorimetry (TMDSC), need to be used with caution when glass-forming systems are studied because of the nonlinear viscoelastic nature of structural recovery.
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