Rubbery polymers penetrant clustering

In general, tte diffusivities of penetrants that swell glassy and rubbery polymers increase with concentratioiu The sorption i tterms are normally well-described by the Flory-Hug ns equaticm. Clustering of penetrant can also occur and cause deviations from this behavior In the case of as prdymers and strong swelling solvents, so-called Case II transport can occur . As drown in Fig. 6 an initial linear increase in samjde weight with time characterizes II uptake in film samples. 

The pressure at which plasticizing effects begin to overcome permeability depression depends primarily on the ratio of the partial pressure of the penetrant in the feed stream to the corresponding vapor pressure of the penetrant at the system temperature. As the pressure approaches the vapor pressure, solubility tends to increase rapidly, thereby inducing plasticization or clustering responses such as those already discussed for rubbery polymers. Even below the plasticization point, additional complexities not encountered with rubbery polymers are often observed in glassy materials. For example, low vapor pressure components such as water can cause depression of the permeability of other gases when water is... 

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