Physical aging enthalpy recovery

Simon, S. L., Plazek, D. J., Sobieski, J. W., and McGregor, E. T., Physical aging of a polyether-imide volume recovery and its comparison to creep and enthalpy measurements, J. Polym. 

Cowie, J. M. G., Elliott, S., Ferguson, R Simha, R Physical aging smdies on poly(vinyl acetate) - enthalpy relaxation and its relation to volume recovery. Polymer Communications, 28(11), pp. 298-300 (1987). 

While at a temperature below Tg, some degree of molecular motion does occur. With time the molecular system does approach the true equilibrium state, i.e., the equilibrium values of volume, enthalpy or other state function variables. Physical aging represents a thermodynamic drive towards the equilibrium glassy state, and the recovery process involves a decrease in free volume as the material ages. 

In this article on physical aging, the phenomena associated with structural recovery and physical aging are described, starting with discussion of volume recovery, enthalpy recovery, viscoelastic properties, and failure. There are more in-depth reviews of the phenomena and models associated with the glass transition... 

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