Physical model, glass polymers conditioning

In approaching the problem of modelling diffusion in polymers, regardless if in a classical or computational manner, an important feature must be highlighted, namely that markedly different diffusion mechanisms operate at temperatures above and below the glass transition temperature, Tg, of the polymer. This is due in principal to the fact that polymers at temperatures T > Tg, so-called rubbery polymers, respond rapidly to changes in their physical condition. Therefore, the penetrant polymer system adjusts immediately to a new equilibrium when a penetrant species is... 

Some of the most important early experimental observations were of transitions from the quasi-brittle crazing deformation mode to the ductile shear deformation mechanisms with changes in the experimental conditions, such as temperature and strain rate, as well as in polymer variables, such as polymer backbone architecture, blend composition, crosslinking and physical aging state of the polymer glass. One of the strengths of the model of craze growth outlined above is that it allows one to make sense out of some experimentally observed craze-to-shear transitions that had previously defied explanation . The idea behind this explanation is quite simple One writes an expression for the shear yield stress, viz ... 

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