Aqueous solutions of some amphiphilic poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymers. A thermodynamic study over a wide concentration range at temperatures between 288.15 and 328.15 K

Abstract

Aiming to probe their aggregative properties in solution, an extensive thermodynamic study of aqueous solutions of some poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide)block copolymers (pluronics) was carried out for concentrations up to 20% w/w in the temperature interval 288.15–328.15 K. For dilute solutions of pluronics L31, L35, L64, F68, P123 and 10R5, density measurements were performed at 298.15 and 318.15 K, and heat capacities were determined at 298.15 K. From these data accurate partial molar volumes and heat capacities at infinite dilution are obtained. They are compared with calculated values using group-contribution additive schemes for monomeric repeating units. It is concluded that experimental values are sensitive to the structural state of the pluronic in solution. Depending on either the molar mass and the PO/EO ratio of the pluronic or the temperature, the following different structural states are present in solution: dispersed monomers, mixed monomers−micelles species and predominantly micelles. The aqueous solutions of L31, L64 and P123 were selected for a more detailed study of densities between 288.15 and 328.15 K, and of sound speeds between 288.15 and 308.15 K. Using these experimental data, apparent molar volumes, isobaric molar expansibilities and isentropic compressibilities were calculated and their concentration and temperature dependence were derived. From the analysis of changes in these properties, thermally induced transitions are shown and related to the hydrophobic character of the pluronic. Apparent molar expansibilities, heat capacities and the temperature dependence of isentropic compressibilities are shown to be very sensitive to the aggregation process. From their sharp maxima in transition regions a clear relationship between critical micelle concentrations and critical micelle temperatures is obtained.