PERVAPORATION OF ETHANOL WATER MIXTURES USING NOVEL HYDROPHOBIC MEMBRANES CONTAINING POLYDIMETHYLSILOXANE

Novel hydrophobic membranes made of crosslinked polydimethylsiloxane (PDMS), segmented polyurethaneurea containing PDMS as the soft segment and PDMS/polystyrene graft copolymer were prepared and pervaporation experiments were carried out with ethanol/water mixtures. It was found that ethanol permselectivity (separation factor of ethanol) increased with an increase in the crosslink distance, with soft segment chain length and with the side chain length of PDMS. Constant selectivity values were obtained with the number of dimethylsiloxane units > 50. Viscoelastic properties and oxygen permeability of these membranes were measured to clarify the ethanol permselective mechanism. These studies indicate that relationships exist between the glass transition temperature or the oxygen permeability and the permselectivity. That is, ethanol permselectivity increases with a decrease in the glass transition temperature and with an increase in oxygen permeability. These results suggest that the free volume theory for gas can be applied to the permeability of ethanol/water by pervaporation through PDMS membranes. This may be explained by assuring that ethanol and water permeate in the state of vapor since the surface energy of PDMS membranes is very low and the surface is water-repellent.