Synergy of Supercritical C[O.sub.2] and Superheated [H.sub.2]O for Enhanced Processability of Polyethersulfone Towards Open Cell Foams

Processing of a high glass transition ([T.sub.g]) polymer such as polyethersulfone (PES; [T.sub.g] = 225[degrees]C) poses a challenge as it requires high processing temperatures or sometimes toxic solvents. In this work, we report of a facile process using superheated water (s[h.sub.[H.sub.2]]O) and supercritical carbon dioxide (scC[O.sub.2]) co-media. PES solids were foamed in the scC[O.sub.2]/sh[H.sub.2]O co-media or scC[O.sub.2] alone in a batch process at different temperatures. The scC[O.sub.2]/sh[H.sub.2]O produced a synergistic effect and achieved PES foams even at processing temperatures as low as 85[degrees]C below the nominal [T.sub.g]; whereas, scC[O.sub.2] alone required higher processing temperatures. Moreover, the scC[O.sub.2]/sh[H.sub.2]O comedia produced highly porous PES foams that were at least 23% higher in porosity than what was obtained using scC[O.sub.2] alone. In addition, the scC[O.sub.2]/sh[H.sub.2]O produced open cell foams at some processing conditions; whereas, scC[O.sub.2] produced closed cell morphologies. Since both C[O.sub.2] and [H.sub.2]O are innocuous, this approach has potential for use in the preparation of ultrafiltration membranes, which currently require the use of toxic solvents for their fabrication by way of the phase inversion process. Moreover, the use of scC[O.sub.2]/sh[H.sub.2]O is a cost-effective approach for the processing of high [T.sub.g] polymers at significantly lower temperatures.