Modelling relative humidity and temperature effects on CO2 gas transport in polyetherimide

The aim of this work is to study the effect of relative humidity (RH) on CO2 gas transport in polyetherimide (PEI) membranes. The Non-Random Hydrogen Bonding model, extended to the case of non-equilibrium glassy polymers (NETGP-NRHB model), is used to interpret the sorption thermodynamics in glassy polymer/penetrant mixtures. Furthermore, a new diffusion model (namely NETGP-NRHB-DM), in the spirit of the Free Volume Theory, is used in combination with the NETGP-NRHB model to interpret the gas permeation. The parameters of the two models have been obtained partly from the literature and partly from a dedicated experimental campaign. The full set of parameters is used in a predictive manner to calculate the permeability coefficient of CO2 in PEI at different temperatures and relative humidity conditions. The model results are validated against a complete set of experimental data specifically carried out for the present investigation. The CO2 permeability predictions are satisfactory, but a slight deviation between the calculated results and the experimental data is observed when the average amount of water in the membrane is high, probably due to the onset of water clustering. In fact, the phenomenological expression of the mobility coefficient of NETGP-NRHB-DM is not properly suited to describe the complex picture involving different kind of water mers starting from the water clustering concentration onset.