Phase separation phenomena in solutions of poly(2,6-dimethyl-1,4-phenylene oxide). IV. Thermodynamic parameters for solutions in a series of homologous solvents: Toluene to hexylbenzene

Melting-point curves for solutions of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) in a series of homologous solvents (toluene to n-hexylbenzene) have been obtained from visual and differential scanning calorimetry measurements. The measured melting points were used to calculate thermodynamic interaction parameters. It was found that consistent values were obtained with the Flory-Hoffman melting-point depression equation, if the assumption was made that solvent molecules are incorporated in the crystal lattice. To this end, an adapted dependence of the enthalpy of melting per polymer unit on the cocrystallizing solvent was used. The values of the thermodynamic interaction parameters in the series of solvents and their dependence on polymer weight fraction are explained qualitatively with simplified versions of equation-of-state theory and solubility parameter theory.