Gelation behavior of poly(9,9-dioctylfluorene)/poly[9,9-di(2-ethylhexyl)- fluorenyl-2,7-diyl] blend in methylcyclohexane solutions

We investigated the gelation behavior of the blend of two light-emitting polyfluorenes, i.e., poly(9,9-dioctylfluorene-2,7-diyl) (PF₈) and poly[9,9-di(2-ethylhexyl)-fluorenyl-2,7-diyl] (PF_2/6), in solutions with methylcyclohexane (MCH). Upon prolonged aging at room temperature, dynamic light scattering revealed that phase separation in the initially well-mixed solutions occurred via spinodal decomposition, yielding a PF₈/PF _2/6-enriched phase and an isotropic solvent-rich phase. Sheetlike aggregates in which a portion of the PF₈ chains adopted the β conformation were formed in the polymer-enriched phase, as revealed by small-angle X-ray scattering and optical spectroscopy. Macroscopically, the phase separation transformed the original viscous liquid solution into a gel. Measurements of the gelation temperature (T_gel), gel-to-sol transition temperature (T_g-s), and spinodal decomposition temperature (T_SD) indicated that gelation (and hence phase separation) became more difficult with increasing content of PF_2/6. The presence of PF_2/6 also hindered formation of the β-phase of PF_8. Therefore, the microstructure and photophysical properties of PF ₈/PF_2/6 in MCH solution can be tuned by the composition of the two polymers.