Interaction, critical, percolation and kinetic glass transitions in pluronic L-64 micellar solutions

We briefly discuss results of analyses of an extensive set of small angle n eutron scattering (SANS) intensity distributions from a class of Pluronic t ri-block copolymer micelles in aqueous solutions. This class of Pluronic ha s 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 substanti al hydration. We use a previously developed 'cap-and-gown' model for the mi crostructure of the micelle, taking into account the polymer segmental dist ribution and water penetration profile in the core and corona regions. We t reat the inter-micellar correlations using a sticky hard sphere model. With this combination, we are able to fit all SANS intensities satisfactorily f or micellar solutions within the range of disordered micellar phase in abso lute scale. The structure and interaction of micelle stay is essentially th e same as the concentration increases. But the aggregation number and surfa ce stickiness increases, and the micelle becomes less hydrated with increas ing temperature. Micellar core is not completely dry but contains up to 20% (volume fraction) of solvent molecules at lower temperatures. We then disc uss, in some details, Pluronic L64 (PEO13PPO30PEO13) micellar system. This system shows an inverted bi-nodal line with a lower critical consolute poin t and a percolation line. We investigated the structure and interaction bet ween these micelles as temperature approaches the bi-nodal line along iso-c oncentration lines. The model developed above is able to describe SANS data in critical region also satisfactorily. As one approaches the bi-nodal lin e 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 po lymer on the critical phenomenon. We conclude that in both the pure and imp ure systems, the micellar solution shows a critical demixing point where mi celles stay spherical but interact strongly with each other by a short rang e temperature dependent attraction. Furthermore, from this fitting procedur e we find relationships between the stickiness parameter and temperature, t he volume fraction of micelles and the polymer concentration. Using these t wo relations, we are able to map the phase diagram of the sticky sphere mod el 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 sca ling plot of SANS data below and above the transition. (C) 2001 Elsevier Sc ience B.V. All rights reserved.