MICELLIZATION OF POLY(ETHYLENE OXIDE)-POLY(PROPYLENE OXIDE) BLOCK-COPOLYMERS IN AQUEOUS-SOLUTION

The micellization in an aqueous solution of polymers containing ethylene oxide (EO) and propylene oxide (PO) was modeled on the basis of a mean-field lattice theory for multicomponent mixtures of copolymers with internal states occurring in heterogeneous systems. The critical micelle concentration, the aggregation number, and the hydrodynamic radius, as well as their temperature dependence of these, were examined for a number of Pluronic triblock copolymers and for two related block copolymers. A semiquantitative description of the strong temperature dependence of these quantities as observed experimentally for Pluronic F127 [(EO)99(PO)65(EO)99] was obtained. An increase in the molecular mass or a decrease in the EO/PO ratio was found to reduce the temperature at the onset of micellization at a given polymer concentration and accordingly to decrease the critical micellization concentration at a given temperature. A smaller critical micellar concentration was associated with a larger aggregation number. A similar trend was found when the molecular structure was changed from that of a triblock to a diblock copolymer. Segment density profiles indicate the Pluronic micelles to consist of a hydrophobic core composed mainly of PO and an outer layer composed of a mixture of EO and water, the EO density gradually diminishing with radial distance.