The effect of sorbed penetrants on the aging of previously dilated glassy polymer powders. I. Lower alcohol and water sorption in poly(methyl methacrylate)

Abstract

AbstractSorption kinetics and equilibria for methanol, ethanol, and n‐propanol in 0.544 μm diameter poly(methyl methacrylate) microspheres were determined at 35°C over a wide range of relative pressures. Sorption isotherms were concave to the pressure axis at low relative pressures and convex to the pressure axis at higher relative pressures. These results, considered in the context of recently reported data for high pressure sorption of gases in polymeric glasses, suggest that the S‐shaped isotherms reported here are examples of a generalized isotherm which describes sorption behavior of all penetrants in glassy polymers if an appropriate range of concentration is traversed by the experimental protocol. The effects of dialating the microspheres by preswelling with methanol were studied by subsequent low pressure sorption of water, methanol, ethanol, and n‐propanol at 35°C. The preswollen microspheres exhibited initially higher sorption capacities than the as‐Received samples, but tended to consolidate with time following the preswelling treatment. The aging process, monitored by periodic short‐term sorption with the various penetrant probes, was arrested by contacting the microspheres with an activity of n‐propanol sufficient to maintain a sorbed concentration of approximately 1 wt %. The aging was significantly retarded by the presence of low concentrations of water and ethanol. Conversely, the aging process appeared to be essentially unaffected by the presence of correspondingly low concentrations of methanol. The complex kinetics describing the sorption of the various penetrants ranged from Fickian diffusion to polymer relaxation‐controlled absorption, depending upon penetrant, relative pressure, and prior exposure history. The low temperature preswelling of the microspheres markedly increased the rate of sorption as well as the respective apparent equilibrium sorption.