Micellar structure of an ethylene oxide propylene oxide block copolymer: A small-angle neutron scattering study

Micellar solutions of a highly hydrophilic ethylene oxide-propylene oxide triblock copolymer, pluronic F88 EO103PO39EO103), in aqueous solution are examined by small-angle neutron scattering,(SANS) at different concentrations and temperatures and in the presence of different salts. At temperature less than 30 degrees C, F88 solution in water (5 wt %) showed unimers which are fully dissolved Gaussian chains. The unimer-to-micelle transition takes place when the temperature or concentration is increased. Added neutral salt favors micellization of the copolymer at lowerconcentration/temperature. At temperature close to ambient, block copolymer forms micelles that consist of a central core, presumably dominated by the propylene oxide blocks, surrounded by a corona of highly hydrated ethylene oxide subchains. SANS analysis shows that the size of hydrophobic core increases as a part of PEO adjacent to the PPO core loses water with increasing temperature or salt (KCl) concentration. The micellar volume fraction increases with increasing concentration of block copolymer. The salting out effect of different electrolytes on structure of micelles is in the order KCI > KBr > KI. The effect of temperature on micellar solutions in the presence-of KCl is also examined. It has been observed that the effect of added salt on the structural phase behavior of block copolymer is analogous to that of temperature. In all the above study, micelles are found to be spherical.