Fickian diffusion of alkanes through glassy polymers: Effects of temperature, diffusant size, and polymer structure

Diffusivities D ranging over six orders of magnitude with values as low as 2 × 10 −13 cm 2 /s have been obtained by a recently developed permeation apparatus, employing a gas‐flow method and a flame ionization detector; Log D for hydrocarbons in bisphenol‐A polycarbonate (PC) at 120°C is proportional to the square of the molecular diameter ( d 2 ) as given by the Lennard‐Jones 6–12 potential. This correlation holds even for the nonspherical n‐hexane molecule. The activation energy for diffusion is also linearly related to d 2 , with values of 9.5 and 23 kcal/mol for methane and neopentane in PC, respectively. Comparison of PC with two similar polymers of higher glass‐transition temperatures ( T g ) indicates that our diffusion data do not correlate with the T g of these polymers. The presence of subsidiary transitions, however, appears to enhance segmental mobilities, increasing the rate of diffusion of the hydrocarbons. The thermodynamic solubility of alkanes in glassy PC can be directly related to their boiling points, and in addition, their enthalpy of solution is linearly related to the heat of condensation of these permeants.