Micellization of miktoarm star S_nI_n copolymers in block copolymer/homopolymer blends

Aseries of miktoarm (polystyrene)_n(polyisoprene)_n, S_nI_n, star-block copolymers, comprising n equal arms of polystyrene and n equal arms of polyisoprene, is utilized to investigate the effect of macromolecular architecture on the characteristics of the micelles formed by the copolymers in a polyisoprene homopolymer matrix at various concentrations 0.05-2 wt%. The characteristics of the micelles are studied with small-angle X-ray scattering (SAXS) as a function of the functionality of the star copolymer n. The radius of the micellar core is found to be independent of n, the aggregation number decreases with increasing n exhibiting a n ^-1 power law dependence, while the volume fraction of copolymer chains participating in micelles is constant; these imply that the junction point of the copolymer does not affect the micellization process in any significant way. A simple thermodynamic model is developed following the methodology of Leibler in order to describe theoretically the micellization of A_nB_n copolymers in a B homopolymer matrix. The predictions of the model are in quantitative agreement with the experimental results, thus signifying that the assumptions of the simple model are valid.