Measurement of transport parameters in polymer films: Flux overshoot produced by a step change in temperature

When a polymer film, initially at steady state with respect to a partial pressure difference of a gas, is subjected to a step increase in temperature, the gas flux out of the film increases beyond and then relaxes back to a new steady-state value. The flux overshoot occurs as follows: In polymer films, heat transfer is much more rapid than mass transfer. Therefore, following a step change in temperature at the outside surfaces of the film, the temperature throughout the film goes from the old to new value before there can be a significant change in the concentration profile. If the gas solubility in the polymer decreases with temperature (generally true), the concentration profile slowly relaxes to a new, lower, steady-state level at the higher temperature. The flux of gas out of the film goes through a transient maximum immediately after the step change because the diffusion coefficient is representative of the new higher temperature but the concentration gradient is still representative of the original lower temperature. A simple model employing a Fick's law diffusion equation with temperature-dependent solubility and diffusion coefficients was found to give excellent quantitative agreement with the experimental measurements. A method for separately determining the temperature dependence of the diffusion coefficient and the solubility arises from the analysis of this phenomenon.