| The effects of adding low molecular weight diluents or additives on gas permeation properties of polysulfone, PSF, and substituted, high free volume polysulfones and polyarylates were analyzed. Such diluents to glassy polymers lead to the phenomenon called antiplasticization, i.e. they increase modulus and retard certain segmental motions.; Additives based on naphthalene, bisphenol A and fluorene structures were incorporated into PSF. The incorporation of low concentrations of such additives increases the selectivity and reduces permeability of PSF. The largest increases in selectivity are observed for the additives that cause the largest reductions in the glass transition temperature and fractional free volume; typically, these additives have low glass transition temperature and are planar molecules containing polar or small asymmetric protuberances. The productivity-selectivity balance response of the modified PSF membranes reveals that the naphthalene-based additives containing asymmetric groups of atoms leads to membranes with higher selectivity and with only small losses in permeability relative to the unmodified PSF membranes.; The permselectivity properties of tetramethylhexafluoro polysulfone, TMHFPSF, and tetramethylhexafluoro bisphenol A t-butyl isophthalate, TMHFBPA-tBIA, modified with a low molecular weight glassy additive Kenflex A, KXA, were compared to the permselectivity properties shown by the base, unsubstituted polysulfone, PSF, and bisphenol A t-butyl isophthalate, BPA-tBIA. The addition of modest amounts of KXA (ca 20 wt%) into TMHFPSF or TMHFBPA-tBIA, leads to materials whose permeability/selectivity combination is better than that of the PSF or BPA-tBIA materials. The polymer TMHFPSF responds more beneficially to the incorporation of KXA than TMHFBPA-tBIA.; A mathematical model to describe the effect of incorporating diluents on the specific volume of polymers is presented. Comparisons of the predicted to the experimental specific volume reveals that the negative departure from volume additivity, observed in antiplasticized polymers, is the result of the relaxation of the excess volume of the glassy mixture relative to the equilibrium state caused by mixing two components with different glass transition temperatures. Model predictions show that the specific volume of a glassy mixture contracts more relative to the pure polymer the lower the glass transition temperature of the diluent, in agreement with experimental observations. |
