1. Department of Materials Science and Technology of Polymers and MESA+ Research Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands, and Centre de Recherches sur les Macromolécules Végétales, Centre National de la Recherche Scientifique, B.P. 53, 38041 Grenoble Cedex 9, France