Physical ageing concentration

Chemical aging resulting from water absorption (i.e., hydrolysis) has not been as widely studied as physical aging. It is relatively well understood at the molecular scale (chemical mechanisms). But macromolecular (kinetics of decrease of the elastically active chain concentration) and mechanical aspects (effect of chain scissions on mechanical properties) are far from being elucidated. 

Zheng, Y, Priestley, R. D., and MeKenna, G. B Physical aging of an epoxy subsequent to relative humidity jumps through the glass concentration, J. Polym. Sci. B, 42, 2107-2121 (2004). 

The decrease in volume that accompanies the physical aging process can be related to a change of the distribution of free volume holes. Since the size and concentration of free volume holes in amorphous polymers is closely linked to the thermal expansion, direct measurements of the free volume can be used to monitor the aging processes. 

Stress evolution in polymers stems fi om a number of factors in addition to mismatched coefficients of thermal expansion. Curing-induced shrinkage, contraction due to the evaporation of solvents and volatile by-products, and relaxation accompanying physical aging are three examples. Stress concentration near topographically sharp features, viscoelasticity, competition between curing kinetics and thermal processing history, as well as ambient environment further complicate the issue. 

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