Dual mechanism of equilibrium sorption in polymer systems : A generalized theory

A uniform system of models of isotherms for gas sorption by glassy and rubbery polymers was examined in a wide range of pressures and sorbate concentrations. For isoenergetic Langmuir sorption sites, a three-parameter equation, which is formally similar to the well known BET equation, was derived. In the limiting cases, this basic equation leads to the classical equation describing the basic dual mode sorption model and a new equation describing the sorption of gases by elastomers (this last equation is an alternative to that of the traditional Flory-Huggins model). The effect of energy distribution of the sorption sites on the shape of sorption isotherms was examined. For a uniform energy distribution of Langmuir sorption sites, a more general (three-parameter) equation was obtained, which, in the limiting case, also leads to the model that describes the behavior of the gas-polymer system above the glass transition temperature. The validity of the basic equation and of the equation for elastomers was illustrated by way of examples involving poly(vinyltrimethylsilane)-ethane, poly(dimethylsiloxane)-propane, and polybutadiene-ethane systems.