Strain effects in the mass flux of methanol in poly(methyl methacrylate)^†

Diffusion of organic solvents into glassy polymers often results in a phase transformation of the hard, solid polymer into a swollen, rubbery material. During the sorption, internal stresses exist in the swollen and glassy parts of the polymer and are thought to contribute significantly to the “anomalous” diffusion observed in many penetrant–polymer systems. In this investigation, isothermal sorption data for the methanol–poly(methyl methacrylate) system have been obtained on plates ranging in thickness from 1/32 to ¼ in. The results show features characteristic of both a strain-dependent diffusion coefficient and of a stress gradient contribution to the mass flux. An attempt to reproduce these results by combining a strain-dependent diffusion coefficient model with a stress-induced contribution to the flux is presented.