Use of solubility parameters for predicting the separation characteristics of poly(dimethylsiloxane) and siloxane-containing membranes

Abstract

The separation characteristics of poly(dimethylsiloxane) (PDMS) and siloxane-containing membranes can be predicted by the use of the solubility parameter (δ) differences between the membranes and solvents and/or the solutes. The membranes considered were PDMS and siloxane derivatives such as polysiloxaneimide (PSI) and poly[ 1-(trimethylsilyl)-1-propyne] (PTMSP), respectively. PSI is a copolymer made of PDMS and polyimide (in this case using 3,3,4,4-benzophenonetetracarboxylic dianhydride (BTDA) while silicone is a substituent in PTMSP. The system considered was the separation of PDMS and PSI using different aqueous alcohol feed solutions through pervaporation. The separation characteristics of PSI membranes having varying PDMS content were also investigated using an aqueous ethanol feed solution. PDMS was investigated for the different alcohol separations from water while PSI was investigated for die recovery of ethanol or water from different aqueous ethanol solutions. It was observed that an increase in the δ-difference between the membrane and solvent resulted in a general decrease in the separation factor (SF). In all the cases investigated, the closeness in the δ-difference between the membrane and solvent compared to that of the membrane and solute indicated good membrane performance. However, an increasing concentration of ethanol in an aqueous ethanol feed resulted in membrane swelling that caused an increased flux and decreased SF of ethanol relative to water. It could be concluded from the systems considered here; that a small difference between the δ of the membrane and solvent compared with that of the membrane and solute corresponds to an improved SF. However if this difference is too small, it might cause excessive membrane swelling. Thus, this approach provides a quick way of selecting membranes including PDMS and siloxane-containing polymers for specific membrane applications. © 2007 American Chemical Society.