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 ox ide triblock copolymer, pluronic F88 EO103PO39EO103), in aqueous solut ion are examined by small-angle neutron scattering,(SANS) at different concentrations and temperatures and in the presence of different salt s. At temperature less than 30 degrees C, F88 solution in water (5 wt %) showed unimers which are fully dissolved Gaussian chains. The unime r-to-micelle transition takes place when the temperature or concentrat ion is increased. Added neutral salt favors micellization of the copol ymer at lowerconcentration/temperature. At temperature close to ambien t, block copolymer forms micelles that consist of a central core, pres umably 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 t he PPO core loses water with increasing temperature or salt (KCl) conc entration. The micellar volume fraction increases with increasing conc entration of block copolymer. The salting out effect of different elec trolytes 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 t he structural phase behavior of block copolymer is analogous to that o f temperature. In all the above study, micelles are found to be spheri cal.