Evidence for swelling-induced pore structure in dense PDMS nanofiltration membranes

A dense polydimethylsiloxane (PDMS) membrane was used to assess the flux and separation performance of a range of solutes (e.g. poly-nuclear aromatics and organometallics) and organic solvents (e.g. heptane and xylene). Solvent flux was modelled with the Hagen-Poiseuille equation and found to fit the model well, with the degree of swelling influencing the effective pore size and porosity of the membrane. The rejection mechanism for low-polarity solutes was found to be predominantly size exclusion. The rejection varied with solvent type and rejections were higher in poorer-swelling solvents. For instance, the rejection of 9,10 Diphenylanthracene was 2% in a pure heptane solvent compared with 15% in xylene. It is postulated that dense PDMS membranes exhibit the characteristics of a porous structure when swollen with solvent, and that the degree of swelling impacts on the separation performance of the membrane. A comparison between the Hildebrand solubility parameters for the PDMS membrane and the challenge solvent was found to be a good indicator of flux/rejection performance.