Small-angle neutron scattering analysis of the structure and interaction of triblock copolymer micelles in aqueous solution

We report analysis of an extensive set of small angle neutron scattering intensity distributions from triblock copolymer micelles in aqueous solutions. We investigated two Pluronics, P84 (PEO19 PPO43 PEO19) and P104 (PEO27 PPO61 PEO27), in an entire range of disordered spherical micellar phases both in temperature and in concentration. At room temperature, both poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) are hydrophilic but at elevated temperature, PPO becomes significantly less hydrophilic, thus creating a thermodynamic driving force for micellization. It has been known that the resultant spherical micelle consists of a hydrophobic core and a hydrophilic corona region. In this paper, we propose a "cap-and-gown" model for the microstructure of the micelle, taking into consideration the polymer segmental distribution and water penetration profile in the core and corona regions. We take into account the intermicellar correlation using an adhesive hard sphere model of Baxter. With this combined model, we are able to fit satisfactorily all SANS intensities for the entire micellar range in an absolute scale. We obtain consistent trends for important parameters such as the aggregation number, hydration number per polymer in both the core and corona, and surface stickiness. The structure of micelle stays essentially constant as a function of concentration, but changes rapidly with temperature. Both the aggregation number and surface stickiness increase, and the micelle becomes drier with increasing temperature. The micellar core is not completely dry but contains up to 20% (volume fraction) of solvent molecules at lower temperatures.