1. Chevron Phillips Chemical Company, 1862 Kingwood Drive, Kingwood, Texas 77339, Department of Chemical Engineering, Yale University, P.O. Box 208286, New Haven, Connecticut 06520-8286, CEA Saclay, DSM/DRECAM/SPAM, Laboratoire Francis Perrin, Bât. 522, P. 249A, 91191 Gif sur Yvette Cedex, France, and Department of Biomedical Engineering, Yale University, P.O. Box 208284, New Haven, Connecticut 06520-8284