Multiple phase equilibria in polar polymer solutions containing poly (acrylic acid)

Abstract

The cloud-point curves for solutions of poly(acrylic acid) in 1,4-dloxane, 1,4-dloxane/water mixtures, tetrahydrofuran and tetrahydrofuran/water mixtures as a function of molecular weight and solvent composition were determined. These systems are unusual in that they exhibit three phase separation boundaries on raising the temperature from room temperature to 500K designated as pseudo-LCST (p-LCST), UCST and LCST behaviour respectively in order of Increasing temperature. Systems consisting of poly(acrylic acid) together with small chain poly(ethylene glycol)s and poly(propylene glycol)s In 1,4-dioxane were also studied and, in general, they exhibited simple LCST behaviour. Critical points for the pseudo-LCST phase separation boundary were determined for the higher molecular weight poly(acrylic acid)s in 1,4-dloxane, using the phase volume ratio method. The variation of the intrinsic viscosity of solutions below the pseudo-LCST was measured as a function of temperature increasing towards the phase separation boundary. The structure and possible complex formation in the system at temperatures below the pseudo-LCST were determined using laser raman spectroscopy. Preferential solvation studies, using light scattering techniques, were also made on the mixed solvent system 1,4-dioxane/water, from which data were obtained which could be compared with those derived from solution viscosity. The variation of the enthalpic interaction parameter, Xh, was determined from heat of dilution data derived from microcalorimetry experiments. The cloud-point curves for the poly(acrylic acid) In 1,4-dloxane system were compared to poly(N-isopropylacrylamide) in water, which exhibited simple LCST behaviour and aqueous solutions of poly(vinyl alcohol) which formed a closed-loop of immiscibility.