Phase-Separation-Induced Gelation of Poly(9,9-dioctylfluorene) Methylcyclohexane Solution

The gelation behavior of the solution of a conjugated polymer, poly(9,9-dioctylfluorene) (PF8), with a poor solvent, methylcyclohexane (MCH), induced by aging at room temperature has been investigated. Light scattering and optical microscopy revealed that the gelation was driven by a macrophase separation occurred through a spinodal decomposition mechanism. Although the spinodal decomposition could proceed to the late stage, the interconnected morphology was arrested to give rise to the gel property of the system. The phase-separated gel was composed fan isotropic phase and a PF8-enriched liquid crystalline phase. The liquid crystalline phase was further found to consist of sheet like aggregates (as revealed by small-angle X-ray scattering) in which a fraction of the PF8 chains formed the beta-phase (as disclosed by the optical spectra) which dominated the photoluminescence spectrum of the gel. The PF8/MCH gel could be disintegrated by moderate heating to ca. 70 degrees C. This gel-to-sot transition was accompanied by the disruptions of the sheet like aggregates and the beta-phase which led to homogenization of the solution.