1. Laboratory of Stimuli-responsive Functional Materials and Devices, Eindhoven University of Technology, P.O. Box 513, Eindhoven 5600 MB, The Netherlands
2. DSM Material Science Center, Urmonderbaan 22, 6167RD Geleen, The Netherlands
3. School of Engineering and Materials Science, Queen Mary, University of London, London E1 4NS, U.K.