Relaxed enthalpy and volume during physical aging: Their interrelation

Several papers have recently presented results of measurements of physical aging by studying the behavior of glassy materials quenched from temperatures above their glass transition temperature $T_g$. The evolution of the aging process is usually followed by plotting the relaxed enthalpy versus the accompanying decrease in volume. Here, we focus on the slope of such plots, which are found to be similar to the inverse value of the isothermal compressibility close to $T_g$. An explanation of this empirical result is attempted in the frame of a model that interconnects the defect Gibbs energy with properties of the bulk material.