MICELLE FORMATION AND SOLUBILIZATION OF FLUORESCENT-PROBES IN POLY(OXYETHYLENE-B-OXYPROPYLENE-B-OXYETHYLENE) SOLUTIONS

Micellization of poly(oxyethylene-b-oxypropylene-b-oxyethylene) triblo ck copolymers (Pluronic polymers F68, P85, and F108) in aqueous soluti ons was studied, and critical micellization concentrations (cmc) were determined using surface tension measurements and fluorescent probes ( pyrene, 1,6-diphenyl-1,3,5-hexatriene), The dependence of cmc on tempe rature was observed, and critical micellization temperatures character izing temperature-dependent transitions of Pluronic unimers to multimo lecular micelles were measured. The molecular characteristics of P85 a nd F108 micelles including their dimensions, molecular masses and surf actant aggregation numbers were determined using lightscattering and u ltracentrifugation techniques. Depending on the type of Pluronic, the micelles had an average hydrodynamic diameter ranging from about 15 to about 35 nm, a molecular mass of about 200 kDa and aggregation number s ranging from one to several dozens. The partitioning of fluorescent probes between aqueous and micellar phases was analyzed within the fra me of a pseudophase model, and the partitioning coefficients were dete rmined using the fluorescence data. The results are compared with prev ious reports and are discussed in relationship to the application of b lock copolymer micelles as microcontainers for drug delivery.