Molecular Weight Effect on the formation of β Phase Poly(9,9′-dioctylfluorene) in Dilute Solutions

The effect of molecular weight on the formation of β phase poly(9,9'-dioctylfluorene) (PF8) was studied in dilute solutions. Temperature-dependent fluorescence experiments of unique synthetic batches as well as size-excluded single batches of polyflourene were studied. Each batch had unique molecular weight, tetrahedral defect concentration, and polydispersity index (PDI). Polyflourene was found to exhibit a temperature-dependent transition between two phases with distinct electronic transition signatures: the α (primary) phase and the β (secondary) phase. In dilute solutions, the temperature at which the polymer exhibited a conversion between these phases showed a clear dependence on molecular weight. We model this transition temperature for β phase formation using the mean field theory for the coil-globule transition developed by Isaac Sanchez. Results show that temperature affects the average end-to-end distance corresponding to increases in secondary electronic absorption and that the dependence on temperature related to the coil-globule transition.