Glass transition, crystallization and thermoreversible gelation in ternary PPO solutions; relationship to asymmetric membrane formation

The phase equilibrium behaviour of PPO (poly 2,6-dimethyl-1,4-phenylene oxide) in mixtures of trichloroethylene with non-solvents ethanol, methanol or octanol has been studied. Attention has been given to the phenomena of glass transition, crystallization and thermoreversible gelation. The effect of solvent-non-solvent mixtures on the glass transition temperature depression was studied using differential scanning calorimetry (d.s.c.) and found to be well described by a ternary form of the Kelley-Bueche equation, using a modified best fit value of 3.67 for the solvent/polymer thermal expansivity ratio. Crystallization and gelation behaviour was studied using both d.s.c. and falling ball methods. D.s.c. scans showed a two-peaked behaviour when solvent trichloroethylene (TCE) or mixtures of TCE with octanol were used. Analysis showed that gel and crystal melting (associated with the higher temperature peak) are in a one-to-one correspondence and both exhibit a behaviour pattern with solvent concentration suggestive of simple melting point depression. However, addition of small amounts of non-solvent methanol or ethanol had a significant effect on the melting behaviour in that only a single endotherm resulted, whose depression in temperature could not be accurately described using the ternary form of the Flory melting point expression. A semiquantitative discussion is given emphasizing the role of these phenomena in controlling both the nature of the ternary phase diagram and the structure formation Processes associated with asymmetric membrane production.