Studies on gas solution and transport mechanisms in glassy polymers

The solubility of argon, carbon dioxide, methane, and ethane in poly(n-butyl methacrylate), poly(n-propyl methacrylate), and poly(ethyl methacrylate) was determined at temperatures below the T$\sb{\rm g}$ of the polymers, or encompassing T$\sb{\rm g}$, at pressures from 1 to 30 atm. Contrary to expectations, the solubility coefficients, S, for argon and methane were found to be independent of gas pressures (or concentration in polymers) even at temperatures 30 to 40$\sp\circ$C below T$\sb{\rm g}$. Moreover, van\u27t Hoff plots of 1n S versus 1/T, the reciprocal absolute temperature, for argon and methane were linear in a range of temperature encompassing T$\sb{\rm g}$. The absence of dual-mode sorption behavior of argon and methane in the above polymers is attributed to the fact that poly(alkyl methacrylates) have a smaller excess free volume in the glassy state than other polymers. It is suggested that the values of the product $\Delta\alpha\cdot$T$\sb{\rm g}$ offer a guide as to whether dual-mode sorption behavior will be observed with a given polymer as the temperature is lowered through T$\sb{\rm g}$ or at some lower temperature; $\Delta\alpha = \alpha\sb{\rm r}\ -\ \alpha\sb{\rm g}$, where $\alpha\sb{\rm r}$ and $\alpha\sb{\rm g}$ are the coefficients of thermal expansion of a polymer in the rubbery and glassy states, respectively. As another test of the above hypothesis, solubility for argon and methane in glassy poly(vinyl chloride) (PVC) were measured at five temperatures from 40 to 80$\sp\circ$C and at pressures from 1 to 20 atm by a gravimetric technique. The onset of dual-mode sorption behavior was observed at a temperature just below the T$\sb{\rm g}$ (=88$\sp\circ$C) of this polymer, as expected from the value of $\Delta\alpha\cdot$T$\sb{\rm g}$ (=0.126) for PVC. In addition, the solubility of hydrogen in glassy poly(vinyl acetate) (PVAc, T$\sb{\rm g}$ = 32$\sp\circ$C) was also found to be described by the dual-mode sorption model. The theoretical analysis of the effects of polymer plasticization assumes the existence of two penetrant populations with different mobilities in glassy polymers. Expressions for the effective diffusion and permeability coefficients were derived assuming that D$\sb{\rm D}$ and D$\sb{\rm H}$ are exponential functions of concentration