Interval Sorption of Alkyl Acetates and Benzenes in Poly(methyl acrylate)

Abstract

We describe a simple, low-cost, gas flow vapor sorption/desorption apparatus that enables “interval” or “differential” experiments on supported polymer films, with control of small step changes in diffusant weight fraction, ω1, to intervals as small as 0.1%. The apparatus permits accurate determination of the binary mutual diffusion coefficient D12(ω1) for ω1 up to at least 3% for “condensable” organic species that typically exhibit very sharp increases in D12 with the diffusant’s weight fraction. We report results for two homologous series of diffusants, n-alkyl acetates and alkyl benzenes, in monodisperse poly(methyl acrylate) (PMA). The Vrentas–Duda free volume theory adequately accounts for the strong concentration dependence of D12 seen in all cases. Nearly constant values of the jump-unit size ratio, ξ, for the homologous set of n-alkyl acetates in the range ξ = 0.61–0.67 closely match those reported previously. For the alkyl benzenes, ξ for benzene (ξ = 0.78 ± 0.04) was higher than those for the larger homologues toluene, ethylbenzene, and cumene (ξ = 0.58–0.60). The ξ values for both homologous series are well-predicted by a free volume based estimate for eccentric diffusants, which jump as a single unit in a mixture whose components contribute different amounts of free volume per jump unit, according to their geometries.