SANS study of the structure and interaction of L64 triblock copolymer micellar solution in the critical region

Pluronic L64 triblock copolymer is soluble in water at room temperature up to more than 30 wt%, forming disordered spherical micellar phase above the cmc-cmt line. We investigate the structure and interaction between these mi celles at temperature approaching the cloud point curve at approximate 57 d egrees C along isoconcentration lines. An extensive light scattering intens ity measurement indicates that the critical concentration is about 5wt%. We use a previously developed cap-and-gown model for calculating the micellar structure factor and a sticky hard sphere model for the intermicellar stru cture factor. SANS intensity distribution is fitted with four parameters: t he aggregation number of micelle N, the hydrophobic core radius a, the over all hard sphere volume fraction phi, and the stickiness between micelles 1/ tau. The model is found to describe all SANS data satisfactorily in absolut e scale. As one approaches the cloud point curve at constant weight fractio n, the aggregation number increases, the micellar core becomes dryer, and t he stickiness parameter increases. In particular, at weight fraction of 5%, the stickiness parameter approaches the critical value 10.2 at T=330.9 K. We thus conclude that there is a critical demixing point in this micellar s olution where micelles interact strongly with each other by a short range t emperature dependent attraction.