Fluid ingress into polymers

In soft material segmental motion is fast, and solvent is taken up easily due to only weak resistance which the solvent molecules encounter on their way into the host polymer matrix. The advancing solvent front has to be supplied with a sufficient amount of 

If the penetrant enters the glassy matrix faster than the polymer can adapt itself by volume relaxation, the solvent front advances linearly with time. This behaviour is called case-II diffusion or relaxation-controlled diffusion. It is a special case of anomalous diffusion, where the mean square particle displacement is proportional to t. It commonly applies to polymers in the glassy state [Wei2]. Here the system 1,4-dioxane/PVC is an example (Fig. 10.1.8(b)). Due to the softening of the material behind the diffusion front, the polymer relaxation in the already swollen matrix is fast enough to adapt to a new situation created by further solvent uptake. Therefore, solvent ingress as well as swelling behind the diffusion front is Fickian. 

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Useful insight into the mechanism of fluid ingress into polymers and polymeric composites can be gained by noting and interpreting the so-called reverse thermal effect. The earliest discovery of this phenomenon was made independently and interpreted differently (Apicella et al. 1979 Adamson 1980). 

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