GAS PERMEATION THROUGH A SIDE-CHAIN LIQUID-CRYSTALLINE POLYSILOXANE-BASED MEMBRANE

A membrane was prepared by sandwiching the side-chain liquid-crystalline polysiloxane (LCP) 3 between two porous poly(propylene) (Celgard) films. Permeation properties for H-2, O2, N2, CH4 and CO2 through the side-chain LCP-based membrane was measured using a gas-chromatographic method at several different temperatures corresponding to the different states of of the side-chain LCP. The gas permeation behaviour depends strongly on the different states of the side-chain LCP. The plots of permeability versus reciprocal temperature show an abrupt change of the slope in the vicinity of the glass transition temperature (T(g)) of the side-chain LCP for all five gases. The activation energy for permeation obtained at these different states exhibits different values. The value obtained in the liquid-crystalline state is the highest. Separation factors (P(O2)/P(N2)) and (P(CO2)/P(CH4)) exhibit an unusual behaviour when the temperature changes across T(g). They increase abruptly when the temperature increases and the phase changes from the glassy state to the liquid-crystalline state of the side-chain LCP. The fact that both permeability and selectivity increases with increasing temperature above the glass transition is attributed to the supermolecular arrangement of the liquid-crystalline state of the side-chain LCP.