Preparation and characterization of new poly(dimethylsiloxane) membrane series via a ‘cross-linking’ reaction using monomolecular trichloro(alkyl)silane of different alkyl chain and type

Abstract

Poly(dimethylsiloxane) (PDMS) membrane, which is generally prepared by cross-linking a hydroxyl-terminated-poly(dimethylsiloxane) liquid with a polymethylhydrosiloxane cross-linker through a condensation reaction between the hydroxyl end groups and hydride groups liberating hydrogen, is one of the most studied polymeric membranes for the separation of gases and liquids. Herein, a new PDMS membrane series prepared by direct cross-linking hydroxyl terminated polydimethylsiloxane pre-polymer liquid with RSiCl₃ molecules of different types in n-heptane solvent under a nitrogen environment were studied, wherein the alkyl chain, R, was varied as methyl CH₃, octyl C₈H₁₇, perfluorooctyl C₈H₄F₁₃, and octadecyl C₁₈H₃₇. For each membrane series, the amount of cross-linker to pre-polymer was varied as 13 : 87, 33 : 67 and 50 : 50 (w/w) to compare the membranes at different cross-linking densities. The cross-linked network structure of the membrane comprised dimethylsiloxane network structures of two cross-links and alkylsiloxane network structure of two or three cross-links. The changes in the structure and properties of the membranes depending on the concentration and type of trichloro(alkyl)silane used were observed by XRD, SEM, TEM, SANS, TGA, DSC, ²⁹Si NMR, IR, cross-linking density, contact angle (water) and ethanol–water separation performance measurements. The membranes with an optimal amount of trichloro(alkyl)silane cross-linker of approximately 10–30% (w/w) showed better separation performance than the other reported conventional PDMS membranes in terms of the ethanol selectivity and flux from an aqueous feed containing 5% (w/w) ethanol. Among the membranes prepared, the membranes cross-linked with perfluorooctylsiloxane/octadecylsiloxane showed better separation performance than those membranes cross-linked with methylsiloxane/octylsiloxane.