Effect of free volume and sorption on membrane gas transport

Polysulfone (PSF) was cast from three different solvents to form three membranes of different free volume. The effect of free volume on the sorption and permeation of carbon dioxide was analyzed by dual mobility model. It was found that both the solubility coefficient and permeability increased with the increase in free volume. However, the increase in solubility was largely due to the increase in Langmuir sorption but not in Henry’s mode. According to dual mobility model we found that the diffusivity in Henry’s sorption area dominated the gas transport. The effect of free volume on gas permeation was mainly contributed by the increase in diffusivity rather than in solubility. To further examine the effect of gas solubility, membranes of similar free volume but different chemical structure were made from three bisphenol-A polymers: polycarbonate (PC), polysulfone, and polyetherimide (PEI). It was found that the PEI membrane owned the highest gas solubility but the lowest permeability, and the PC membrane had the lowest gas sorption but the highest permeability. The high CO2 solubility was found strongly affected by the sorption in Langmuir’s mode. But it seemed that the high Langmuir sorption led to a lower diffusivity in the same mode. Although the gas permeation was still dominated by the diffusion in Henry’s mode, the Langmuir diffusion also had its contribution. The Henry’s diffusivity in PC membrane was slightly lower than that in PSF, but the Langmuir’s diffusivity was much higher. As a result the overall CO2 permeability through PC membrane was higher than that through PSF