Study and characterization of the hysteresis behavior of polyimide membranes in the thermal cycle process of pervaporation separation

The thermal hysteresis behavior of Matrimid^® 5218 polyimide membranes in the pervaporation of a tert-butanol and water mixture containing 85 wt.% tert-butanol was investigated. The membrane flux decreases while separation factor increases after the thermal cycle process. The percentage changes of flux and separation factor after cycle are strongly path-dependent. Membrane starting from a high temperature has a more stable performance than that from a low temperature. Three factors play important roles in the hysteresis behavior; namely, interactions between feed molecules and membrane, non-equilibrium nature of dense-selective skin, and asymmetric membrane swelling. As a result, the collapse of micro-porous structure underneath the dense-selective skin and the thickening of the dense-selective skin were observed by SEM pictures after pervaporation. The d-space value measured by WXRD also indicates a reduced interstitial chain space after pervaporation, suggesting a thermal and solvent induced densification. Similar to the asymmetric membranes, a higher flux and separation factor was obtained at 100°C if a dense membrane was tested starting from a low temperature. The interesting behavior may arise from a greater relaxation of polymer chains and an easier cooperative packing of benzene rings induced by the penetrant molecules. As a result, the membrane treated at 100°C has a narrower interstitial chain space and possesses a higher selectivity towards larger molecules transportation. The narrower interstitial chain space and a higher selectivity observed at high temperature may be due to the loss of local chain rigidity and straightness resulting from the nature of Matrimid.