Diffusion of organic vapors at low concentrations in glassy PVC, polystyrene, and PMMA

Abstract

Diffusion coefficients of various C 1 to C 6 organic vapors, at concentrations 0.5 wt. percent, have been determined by gravimetric sorption rate measurements on emulsion and suspension-polymerized powder samples of PVC, polystyrene, and PMMA. Fickian diffusion kinetics were observed at the lowest concentrations, with a second-stage, relaxation-controlled sorption appearing at higher concentrations. In conjunction with published data for diffusivities of fixed gases in these polymers, the results indicate that diffusivity decreases exponentially, and that diffusion activation energy ( E D) increases linearly, with increasing diameter of “spherical” penetrant molecules (e.g., the noble gases, CH 4, SF 6, CCl 4, and neopentane). Much of the observed scatter in these correlations is attributable to uncertainty in the molecular diameters. For C 4 and larger n-alkanes and other elongated or flattened molecules, diffusivities are higher, and E D lower, than for spherical molecules of similar molar volume. This finding suggests that anisometric molecules are oriented and move along their long axes during diffusion through the glassy polymer matrix. Correlations of diffusivities with molecular dimensions suggests that transport of anisometric molecules is governed by a diameter smaller than the mean (equivalent sphere) diameter but larger than the minimum dimension of their extended-chain conformation. Among the three polymers studied, diffusivity of each penetrant, at a given temperature, decreases in the order polystyrene > PVC ≥ PMMA.