Characterization of surface compositions of phase-separated structures in conjugated poly(phenylene vinylene) blends by scanning near-field optical microscopy

The phase-separated structures in a thin-film conjugated polymer blend, poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene)/poly(methyl methacrylate) (MDMO-PPV/PMMA) was studied by scanning near-field optical microscopy (SNOM), which can probe both of the surface topography and the chemical composition at high spatial resolution. In thin films of MDMO-PPV/PMMA, the PMMA-rich phase with less fluorescent emission appeared as a raised structure while the MDMO-PPV-rich phase with highly fluorescent emission was present as a collapsed phase with a low height. Such the vertical structure of the phase separation was dependent on the solvent quality for each component. On the other hand, the lateral structure was determined by the surface property of the substrate. As the degree of hydrophobic character of the substrate increased, the PMMA domains tended to form a continuous form to a separated spherical form. This evolution arise from a change in affinity between the substrate surface and PMMA, which the hydrophilic substrate has higher affinity to PMMA for maintaining the continuous structures and the hydrophobic substrate with lower affinity to PMMA can not hold the structures continuously.