Small-Angle Neutron Scattering Study of PEO−PPO−PEO Micelle Structure in the Unimer-to-Micelle Transition Region

Abstract

The structure of (ethylene oxide) 26 (propylene oxide) 40 (ethylene oxide) 26 Pluronic P85 block copolymer micelles in the unimer-to-micelle transition region was studied using small-angle neutron scattering. At low concentration (1%), where no intermicellar interactions were evident, aggregation numbers were obtained using the contrast variation method. At higher concentrations a three parameter model accounting for the presence of water in the core and corona of the micelle, and for micelle−micelle interactions, was fitted. The micelle core water content decreases (from 60 to 10%) with increasing temperature in the transition region, with a corresponding increase in the micelle aggregation number (35−62). The micelle core radius remains constant (40 Å). Thus, the broad nature of the unimer-to-micelle transition region may be ascribed to the structural changes which accompany the replacement of micellar core water by polymer.