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.

The structure of (deuterated ethylene oxide)23−(propylene oxide)34−(deuterated ethylene oxide)23 block copolymer micelles was studied in the unimer-to-micelle transition region using small angle neutron scattering (SANS). A three-parameter core−corona model fit to the set of scattering curves obtained with various mixtures of D2O and H2O as the solvent yielded the aggregation number, the core and corona radii, and the water contents of the core and the corona. The results showed that the micellar core cannot consist of PPO only but must contain significant quantities of water. The micelle aggregation number increased with temperature while the core and corona radii were constant over a 10 °C temperature region above the critical micellization temperature. The values of the corona radius obtained from the model fit (56−57 ± 3 Å) are in a good agreement with the hydrodynamic radius of the micelle (56−61 ± 2 Å) measured by dynamic light scattering.

The dynamics of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) micelle rearrangements were studied using iodine laser temperature jump experiments with light-scattering detection. Two relaxation processes were detected: the first, fast one was accompanied by an increase in scattered light intensity, while the second, slow process was accompanied by a reduction in light intensity. The Aniansson-Wall theory was used to interpret the experimental results. The equilibrium micelle structures at the start and end points of the temperature jump experiment were used as input for the Aniansson-Wall equations. The solution of these equations agrees qualitatively with experimental data and suggests the mechanisms associated with the two time constants. The first time scale, in the tens of microseconds to about 10 ms range, is attributed to unimer insertion into micelles. The other time scale, in the 1-100 ms range, is associated with the rearrangement of the micelle size distribution. It is shown that the second process is often not observed either because the unimer supply is insufficient or because the micelle number density is not temperature dependent.