Physical aging free-volume concept

Nearly all aspects of physical ageing can be explained by means of the "free volume" concept, i.e. the hypothesis that the mobility of particles (atoms, molecules) is mainly determined by their packing density. 

Kluin, J. E., Moaddel, H., Ruan, M. Y, Yu, Z., Jamieson, A. M., Simha, R., McGervey, J. D., Probe spectroscopy, free-volume concepts, and physical aging of polymer glasses, in Structure-property relations in polymers, Adv. Chem. Ser, 236, Chapter 20, pp. 535 (1993). 

Due to the universality in all glasses, physical aging can be theoretically explained in a straightforward way based on the free-volume concept. As proposed by Struik, This is the basic and rather obvious idea that the transport mobility of particles in a closely packed system is primarily determined by the degree of packing of the system or by its inverse measure, viz. the free volume [2]. The idea could date back to 1943 when Alfrey et al. proposed that the isothermal aging below Tg can be attributed to the diffusion of free volume holes from the interior of polymers into the surface [34]. This free volume diffusion model (FVDM) was developed by Curro et al. [35] to quantitatively analyze the volume relaxation experiments of poly(vinyl acetate) [36, 37]. The motion of free volume holes can be described by a diffusion equation ... 

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