On the mechanism of mass transport of low molecular weight compounds in polyimides: Modeling and 2D-FTIR analysis

Mass transport mechanism of low molecular weight compounds (water and carbon dioxide) into polyimide films has been analysed and modelled on the basis of the relevant findings of an in situ FTIR spectroscopic analysis. In particular, in the case of water, it has been assumed that concurrent diffusion occurs of single water molecules and water dimers, which display different mobilities. A non-linear instantaneous equilibrium between the local concentrations of these two species has been imposed, based on a two layers BET theory. The proposed approach is able to give a good qualitative and quantitative interpretation of both sorption equilibrium and of sorption/desorption kinetics data.