Novel open-cellular polysulfone morphologies produced with trace concentrations of solvents as pore opener

As a new method of membrane formation, we have investigated microcellular f oaming of thin (similar to 100 mum) polysulfone films containing varying tr ace concentrations of tetrahydrofuran using carbon dioxide as a physical bl owing agent. Membrane morphologies were obtained by first saturating the po lymer with carbon dioxide at 5 MPa, and subsequently heating the sample abo ve the glass transition temperature (T-g) Of the polymer/gas mixture at atm ospheric pressure. The presence of tetrahydrofuran in the polymer at concen trations above 0.04 wt.% led to a transition from a closed cellular structu re into novel open-cellular morphologies. The open structure manifests itse lf by small spot-like openings (diameters between 10 and 100 nm) in the cel l walls. The mass transport resistances of the porous films were quantified using gas permeation measurements, and a Knudsen-type separation mechanism was observed. Detailed investigation showed that the transport resistance can mainly be controlled by two variables: (1) the concentration of the res idual solvent in the polymer film, and (2) the foaming temperature. At opti mal foaming temperatures, thin cell walls are obtained, which break up when fluctuations in the wall thickness are amplified by plasticizing solvent m olecules. (C) 2001 Elsevier Science B.V. All rights reserved.