Plasticizing penetrants, transport glassy polymers

The dissolution of a polymer in a penetrant involves two transport processes, namely penetration of the solvent into the polymer, followed by disentanglement of the macromolecular chains. When an uncrosslinked, amorphous, glassy polymer is in contact with a thermodynamically compatible liquid (solvent), the latter diffuses into the polymer. A gel-like layer is formed adjacent to the solvent-polymer interface due to plasticization of the polymer by the solvent. After an induction time, the polymer is dissolved. A schematic diagram of solvent diffusion and polymer dissolution is shown in Fig. 1. However, there also exist cases where a polymer cracks when placed in a solvent. 

It is generally accepted that mass transport in dense polymer membranes takes place according to the well-known solution-diffusion mechanism [18,19]. For non-swelling and non-plasticizing species the amount of penetrant that can dissolve in the glassy polymer matrix depends on the available sorption sites and often a typical dual mode sorption behaviour is observed [20]. The number of Langmuir sorption sites is strongly related to the free volume distribution of the sample. 

The kinetics of transport depends on the nature and concentration of the penetrant and on whether the plastic is in the glassy or rubbery state. The simplest situation is found when the penetrant is a gas and the polymer is above its glass transition. Under these conditions Fick s law, with a concentration independent diffusion coefficient, D, and Henry s law are obeyed. Differences in concentration, C, are related to the flux of matter passing through the unit area in unit time, Jx, and to the concentration gradient by,... 

At such extraordinarily low penetrant concentrations, plasticization of the overall matrix is certainly not anticipated. Motions involving relatively few repeat units are believed to give rise to most short term glassy state properties. In rubbery polymers, on the other hand, longer chain concerted motions occur over relatively short time scales, and one expects plasticization to be easier to induce in these materials. Interestingly, no known transport studies in rubbers have indicated plasticization at the low sorption levels noted above for PVC and PET. 

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