Thermodynamic studies on the gel-sol transition of some pluronic polyols

Abstract

Studies have been carried out on a number of pluronic polyols with the aim of determining factors which influence the transition temperature of the hydrogels. The gel-sol transition temperatures, T m, were measured for aqueous solutions of the polyols with and without additives such as sodium chloride, potassium chloride, urea, ethanol, sodium sulfate and sodium dodecylsulfate. A linear relationship was found between the logarithm of the pluronic polyol concentration and the reciprocal of the gel-sol transition temperature. Although no linear relationship was observed between log M w (molecular weight) and the reciprocal of the gel-sol transition temperature for all polymers, such a relationship does seem to exist among the polymers having the same ratio of poly(oxypropylene) to poly(oxyethylene) units per mole of polymer, (P/E ratio). All of the pluronic polyols studied showed endothermic enthalpy change for the sol-gel process. These results were substantiated with data from calorimetric studies. The driving force for thermal gelation is a result of the large entropic change. Neither Δ H gel o (for sol-gel) nor ΔS gel o seem to correlate with the molecular weight of the polymer, per se; however, the number of poly(oxypropylene) units per mole of polymer plays a major role in the gelation process. The presence of sodium chloride, potassium chloride and sodium sulfate decreased the transition temperature whereas the opposite effect was observed with urea, alcohol and sodium dodecylsulfate. The enthalpy of gel formation was not