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 H2, O2, N2, CH4 and CO2 through the side‐chain LCP‐based membrane was measured using a gaschromatographic method at several different temperatures corresponding to the different states 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 (Tg) 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 (PO2/PN2) and (PCO2/PCH4) exhibit an unusual behaviour when the temperature changes across Tg. 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.