Physical aging equilibrium

The kinetic character of the glass transition and the resulting non-equilibrium character of the glassy state are responsible for the phenomena of structural relaxation, glass transition hysteresis, and physical aging (Kovacs, 1963 Struik, 1978). 

The inplane shear stress-strain tests reported here have been well demonstrated to be a reliable test for matrix-dominated properties in composites 141). For the selected mechanical properties that were monitored, their sensitivity to the thermal history was well demonstrated. In particular, the embrittlement process during the sub-Tg annealing or physical aging has been clearly observed. This decrease in molecular mobility, which gives rise to an increase in relaxation time and hence a decrease in toughness, can be rationalized as a decrease in free volume in an approach towards the equilibrium glassy state. 

One problem encountered with polymeric membranes is their physical aging which can alter the permeation and the mechanical properties. Sinko et al. [144] noticed a decrease in the water permeability and in the mechanical rate of relaxation with time, which was proportional to the aging temperature. A relaxation coupling model could be used to predict equilibrium in the structural change profile. 

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