We probed the gelation and its effect on the photophysical property of a conjugated semiconducting polymer, poly(9,9-dioctylfluorene-2,7-diyl) (PF8), in toluene. The conformational structure characterized by the radius of gyration and the persistence length of PF8 in the dilute toluene solution depended strongly on temperature, where the conjugated chains exhibited a more extended conformation as the temperature was raised. The interchain interaction above the overlap concentration induced aggregation of PF8 to form network clusters in the solution. Gelation took place when the semidilute PF8 solutions were cooled and subsequently aged at −20 °C. The gel composed of the dynamically arrested PF8-enriched phase and the isotropic phase due to the occurrence of a macrophase separation at the subambient temperature. The PF8-enriched phase was mesomorphic, consisting of sheetlike aggregates or membranes. A fraction of the PF8 chains or segments within these sheetlike aggregates formed the β-phase which dominated the photoluminescence of the gel. The gel structure could be disrupted by heating to ca. 45 °C, above which the corresponding PL spectra displayed a blue shift because of reduced amount of the β-phase.

In this work, the effect of cononsolvency on the phase transition and preferential adsorption phenomenon behaviors of poly(N-isopropylacrylamide)/methanol/ water ternary solutions was studied. In this cononsolvent system, the DV E , DG m , and v 12 values show a nonlinear behavior and the minimum values of DV E and DG m , while the maximum value of v 12 at / 2 is around 0.7. These facts indicated that one water molecule could directly bond with one methanol molecule to form the H 2 OÀ ÀMeOH complex. The H 2 OÀ ÀMeOH complex structure was found to remarkably affect the phase transition of poly(N-isopropylacrylamide) (PNIPAM) in ternary solution. However, at the composition of mixed cononsolvent, / 2 < 0.2, the PNIPAM molecules may preferentially adsorbed pure water molecules; therefore, the LCST decreases slightly with composition of mixed cosolvent and this may be because of the small amount of H 2 OÀ ÀMeOH complexes in the mixed cononsolvent. While, at / 2 > 0.7, the PNIPAM molecules may preferentially adsorbed pure methanol molecules. PNIPAM ternary solutions were transparent and no transition occurred in this region. This indicates that the PNIPAM coils exhibited a much-extended conformation in solutions. In contrast, at 0.2 < / 2 < 0.4 and 0.4 < / 2 < 0.7, PNIPAM molecules preferentially adsorbed water and methanol molecules, respectively, and also adsorbed large amount of H 2 OÀ ÀMeOH complexes. In these regions, the clathrate-like structure around the side chain of PNIPAM molecule became more defected with adsorbing H 2 OÀ ÀMeOH complex. Therefore, we considered that the various thermodynamic behaviors between PNIPAM and mixture solvents must be related different preferential adsorption phenomena, which were mainly related to different degrees of polymer-solvent interaction and structures of solvent used.