Membrane preparation
PDM/PSF composite membrane was prepared by interfacial crosslinking of a PDM coating layer on the PSF substrate [30]. A solution of PDM in ethanol (10 g/L) was coated onto the surface of the PSF substrate for 15 min. The excess coating solution was removed and the membrane was air dried. Then the membrane surface was allowed to contact with p-xylylene dichloride (XD) dissolved in heptane (10 g/L) for 4 h, during which period PDM was crosslinked with XD at the solid–liquid interface. The crosslinking led to a quaternary ammonium network through alkylation reaction between the tertiary amino groups in the side chains of PDM and the chloromethyl groups in XD. Then the excess crosslinking solution was removed and the membrane surface was washed with heptane to remove the unreacted residual XD on the membrane surface. After the membrane was air dried, the above coating-crosslinking procedure was repeated one more time to form a double-coated composite membrane, which was shown to be defect-free on the membrane surface.