1. Contribution from the Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands, Institute Laue Langevin, BP156, Grenoble 38042 Cedex 09, France, and Polymer Materials and Engineering (PME), Department of Materials Science and Technology, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands