Small-angle neutron scattering investigation of the temperature-dependent aggregation behavior of the block copolymer Pluronic L64 in aqueous solution

Small-angle neutron scattering (SANS) was used to investigate the temperature dependence of the structure of micelles formed by the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) Pluronic L64 amphiphilic block copolymer in aqueous (D₂O) solution. Different models for the form factor and the structure factor have been considered for fitting the SANS scattering patterns. By comparing the various models, we found that in the intermediate block copolymer concentration range, e.g., 2.5 wt % Pluronic L64, the micelles are well separated while the intermicellar interaction remains strong. Thus the more appropriate model was that of core-corona form factor and hard sphere structure factor, which allowed the determination of the core, micelle, and hard sphere interaction radii, the micelle association number, and the solvent volume fraction in the micelle core and corona. Upon an increase of temperature (in the range 35-55 °C) the micelle radius increased by ≈10%, accompanied by the loss of water in the micelle core. These trends coincide with the second relaxation process (attributed to the micelle core dehydration) observed in recent kinetics studies. The SANS data also indicated a stronger increase of the micelle association number occurring above 47 °C, reflecting a strong increase of the intermicellar interactions. These results support the attribution of the third relaxation process observed in the aforementioned kinetic studies to micelle clustering.