1. Department of Chemical Engineering, University of WisconsinMadison, 1415 Engineering Drive, Madison, Wisconsin 53706; DuPont de Nemours International, S.A., European Technical Center, 2, chemin du Pavillon, CH-1218 Geneva, Switzerland; and University of Liège, Center for Education and Research on Macromolecules (CERM), Sart-Tilman, B6, 4000 Liège, Belgium