1. Department of Engineering Physics and Mathematics, Helsinki University of Technology, Rakentajanaukio 2, FIN-02150 Espoo, Finland, Department of Physics, University of Helsinki, P.O. Box 9, FIN-00014 Helsinki, Finland, SERC Daresbury Laboratory, Warrington WA4 4AD, United Kingdom, and Department of Polymer Science and Materials Science Center, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands