Supramolecular Structure of Poly[2-methoxy-5( 2 -ethylhexyloxy) -1,4-phenylenevinylene] (MEH-PPV) Probed Using Wide-angle X-ray Diffraction and Photoluminescence

The aggregation of hairy-rod segments in solid films was observed using wide angle X-ray diffraction (WAXD). The films cast by evaporating the THF solvent from MEH-PPVITHF were extremely rigid due to the strong aggregation among the main chains of MEH-PPV and could not be disaggregated by heat. In contrast, aggregations in the films cast by evaporating the toluene solvent from MEHPPV/ toluene could be disaggregated due to the weak interactions among the side chains of MEH-PPV. Accordingly, the MEH-PPV/toluene system exhibits good thermal reversibility between aggregation while cooling and disaggregation when heated. The aggregation of the semi-rigid polymers occurs even at a concentration of well below the threshold lyotropic concentration. This process takes place in semi-rigid polymers with disk-like domains which serve as nuclei for the development of a macroscopic nematic phase as the polymer concentration increases during solvent evaporation. A film with a dominant lyotropic liquid crystalline (LC) phase was formed. As the temperature of the cast polymer film was increased, the thermotropic LC phase, rather than the lyotropic LC phase, was dominant between ca. 100°C and 230°C. However, both thermotropic and lyotropic LCs coexisted and associated with aggregation. The photoluminescence (PL) emission spectra and UV-vis absorption spectra of MEH-PPV in solution were obtained to elucidate the self-organized supramolecular aggregated conformation in various solvents, concentrations, and temperatures. The peaks exhibited a red shift in the PL emission spectra accompanied by a right shoulder when aggregation occurred. The shoulder grew as the concentration was increased.