Location of three critical phase boundaries in poly(acrylic acid)-dioxane solutions

The phase behaviour of poly(acrylic acid)-dioxane solutions has been studied over a wide range of solution concentration and temperature. Three distinct phase boundaries have been observed on increasing the temperature from ambient where homogeneous solutions are obtained. Phase separation occurs, below the boiling point of dioxane, when the solutions are heated and this first cloud-point curve has been called a quasi-lower critical solution temperature (LCST) boundary. Further heating of the two-phase system in sealed tubes leads to redissolution of the polymer and the location of an upper critical solution temperature (UCST) for the solution. If the temperature is allowed to rise further, phase separation again takes place at an LCST that is due to free-volume differences in the solution. The heats of dilution and laser Raman spectra of the poly(acrylic acid) solutions and those of a model compound, propionic acid, dissolved in dioxane have been examined below the quasi-LCST. Evidence for hydrogen bonding between the solvent and the carboxylic groups in the polymer has been found. It is suggested that the ability of dioxane to dissolve the polymer at ambient temperatures but not at higher temperatures below the solvent boiling point is a result of a changing balance between specific, polar, polymer-solvent interactions and normal, non-polar, dispersion forces, the ratio of which alters with changing temperature.