Interaction, Critical, Percolation and Kinetic Glass Transition in Pluronic L-64 Micellar Solutions

We briefly discuss results of analyses of an extensive set of small angle neutron scattering (SANS) intensity distributions from a class of Pluronic tri-block copolymer micelles in aqueous solutions. This class of Pluronic has a symmetric structure (PEOMPPONPEOM), with PPO/PEO molecular weight ratio of 60/40. It is shown that the micelles are spherical, each consisting of a hydrophobic core and a diffuse hydrophilic corona region having substantial hydration. We use a previously developed 'cap-and-gown' model for the microstructure of the micelle, taking into account the polymer segmental distribution and water penetration profile in the core and corona regions. We treat the inter-micellar correlations using a sticky hard sphere model. With this combination, we are able to fit all SANS intensities satisfactorily for micellar solutions within the range of disordered micellar phase in absolute scale. The structure and interaction of micelle stay is essentially the same as the concentration increases. But the aggregation number and surface stickiness increases, and the micelle becomes less hydrated with increasing temperature. Micellar core is not completely dry but contains up to 20% (volume fraction) of solvent molecules at lower temperatures. We then discuss, in some details, Pluronic L64 (PEO13PPO30PEO13) micellar system. This system shows an inverted bi-nodal line with a lower critical consolute point and a percolation line. We investigated the structure and interaction between these micelles as temperature approaches the bi-nodal line along iso-concentration lines. The model developed above is able to describe SANS data in critical region also satisfactorily. As one approaches the bi-nodal line at constant weight fraction of the copolymer, the aggregation number and the stickiness parameter increase. In particular, at weight fraction of 5%, the stickiness parameter approaches the critical value 10.2 at T = 330.9 K. We investigated the effect of hydrophobic impurities in the commercial polymer on the critical phenomenon. We conclude that in both the pure and impure systems, the micellar solution shows a critical demixing point where micelles stay spherical but interact strongly with each other by a short range temperature dependent attraction. Furthermore, from this fitting procedure we find relationships between the stickiness parameter and temperature, the volume fraction of micelles and the polymer concentration. Using these two relations, we are able to map the phase diagram of the sticky sphere model onto that of the micellar solution. The agreement between the theory and experimental phase behavior is satisfactory. Finally, we briefly describe the recently found kinetic glass transition line in this system using a scaling plot of SANS data below and above the transition.