1. Laboratory of Polymer Chemistry (SPC), Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, Rhodia Recherches, Centre de Recherches d'Aubervilliers, 52, rue de la Haie Coq, 93308 Aubervilliers Cedex, France, and Gradipore Ltd, P.O. Box 6126, Frenchs Forest NSW 2086, Sydney, Australia