Noble gases as diffusion probes for molecular-selective gas transfer through polymeric membranes

摘要

Traditionally the membrane gas separation technologies (air separation, hydrogen recovery from dehydrogenation processes, etc.) use the glassy polymer membranes with dominating permeability of "small" gas molecules based on the lower fraction free volume (FFV) glassy polymers (e.g., polysulfone, tetrabromopolycarbonate and some polyamides). On the other hand, an application of membrane methods for VOCs and some toxic gas recovery from air, separation of the lower hydrocarbons containing mixtures (in petrochemistry and oil refining) needs the membranes with preferable penetration of components with relatively larger molecular sizes. In general, it is characterized for rubbers and for the high free volume glassy polymers. Data files accumulated (more than 1500 polymeric materials) represent the region of parameters "inside" of these "boundaries." The estimation of FFV of polymers by annihilation of ortho-positronium can't explain the variation of selectivity for whole set of gases. One of the reasons is that FFV obtained from the positronium mobility in polymers does not reflect FFV available for heavy noble gases or C3-C4 hydrocarbons. Contrariwise the estimation of the diffusion mobility in polymeric matrix by EPR methods includes the using of the large-dimension radicals which cannot correctly correlate with mobility of small molecules (helium, hydrogen, etc.). To understand the evolution from "standard" selectivity to "inverse" one this paper presents an attempt to demonstrate the using of noble gases as diffusing probes in polymers of different classes.