The thermodynamics of the alpha (a) phase to beta (P) phase transition was investigated in solution of poly(9,9'-dioctylfluorene) in a variety of solvents with Uv-vis absorption spectroscopy, differential scanning calorimetry, fluorescence spectroscopy, atomic force microscopy, and near-field scanning optical microscopy. The results show that the solvent quality has a strong affect on the (alpha- to beta-conformational change. The trend in enthalpies and transition temperatures indicates that the transition results from an increase in intramolecular interactions upon chain collapse at lower temperatures. This transition leads to subsequent gelation and/or aggregation that stabilizes the P-phase at higher temperatures and leads to a large hysteresis in the transition temperature. The enthalpy for the transition from an aggregated beta-phase to a fully solvated alpha-phase is found to be 21.04 kJ mol(-1) of monomer for toluene solutions. Differences between the measured heat and those previously reported are discussed. (c) 2007 Elsevier Ltd. All rights reserved.