1. Polymer Institute of the Slovak Academy of Sciences, SK-842 36 Bratislava, Slovak Republic, Institute of Physics of the Slovak Academy of Sciences, SK-842 28 Bratislava, Slovak Republic, Institute for Chemical Research, Kyoto University, Uji 611, Japan, and Deutsches Kunststoff-Institut, Schlossgartenstrasse 6, D-642 89 Darmstadt, Germany