Physical ageing glassy state

The nonequilibrium glassy state, 5(t) = f(t) -f, is determined by solving the kinetic equations which describe the local motion of holes in response to molecular fluctuations during vitrification and physical aging. The solution is (11)... 

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). 

Although different models of diffusion rate constants have been proposed in the literature, one must bear in mind that it is very difficult to capture the evolution of the system in the glassy state with a simple mathematical model. The main problem is the physical aging that occurs in parallel with the advance in conversion. Physical aging produces a densifica-tion of the glass, which brings two consequences ... 

Physical aging resulting from water absorption is probably the most widely studied process. The structure-property relationships in this field are relatively well understood, except for diffu-sivity, which seems to depend sharply on the strength of polymer-water interactions. The resulting plasticization is also relatively well understood, but the research field on swelling in the glassy state still remains open. 

The experimental results described above show that the gas-permeability properties of thin glassy polymer films (submicrometer in thickness) are more time- or history-dependent than much thicker films (the bulk state for example, 50 pm or thicker) seem to be. This is manifested in terms of physical aging over a period of 1 year and more. The observed permeability values for the current thin films are all initially greater than the reported bulk values but approach or become less than these values after a few days or weeks, depending on the thickness. After a year, the thin films may be as much as four times less permeable than the reported bulk values. Selectivity increases with aging time, as might be expected from a densification process. 

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. 

The physical aging rates increase from zero above Tg to a constant below T and then decrease to zero at 200 K below Te. In contrast to all the published expressions, the new equations also predict increasingly smaller activation energies with decreasing temperature in the glassy state. 

The JG y9-relaxation in the glassy state, like the a -relaxation, is sensitive to thermal history, physical aging, and the particular thermod5mamic (TyP) path used to arrive at the glassy state. 

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