Evidence of micelle growth in aqueous solutions of the amphiphilic poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymers from differential scanning microcalorimetry

The aqueous solutions of two poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymers, the Pluronic P84 (E19P43E19) and P85 (E26P40E26) where E=ethylene oxide and P=propylene oxide, have been investigated by DSC (differential scanning microcalorimetry). The DSC scans of aqueous solutions of both copolymers show three endothermal peaks, a large one at low temperature and two others of smaller amplitude at higher temperature. As in previous studies the first and third peaks have been unambiguously attributed to micelle formation and to clouding (phase separation), respectively. Reported studies of P85 permitted us to assign the second DSC peak to micelle growth, and not to the sol–gel transition which occurs at a slightly higher temperature. For P84, complementary investigations by means of turbidimetry, light scattering, and rheological methods permitted us to show that the middle DSC peak is also due to micelle growth. The middle peak had not been observed in previous DSC studies of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymers.