Structural study on the micelle formation of Poly(ethylene oxide)-Poly(propylene oxide)-poly(ethylene oxide) triblock copolymer in aqueous solution

The phase behavior of poly (ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymers dissolved in water has been studied using small-angle neutron scattering. The structural properties have been studied as a function of polymer concentration and temperature. At low temperature (T ≤ 15 °C) and low polymer concentrations the unimers are fully dissolved Gaussian chains with radius R_g = 17 Å. Close to ambient temperature, the hydrophobic nature of PPO causes aggregation of the polymers into spherical micelles with core sizes of the order of40–50 Å, somewhat temperature dependent. The concentration of micelles increases roughly linearly with temperature, until either a saturation is reached, where all polymers are part of a micelle, or the volume density of micelles is so high that they lock into a crystalline structure of hard spheres. In the 60–70 °C temperature range, the micellar structure changes from spherical form to prolate ellipsoid, leading to a decreasing intermicelle interaction. At high concentration, this causes melting of the cubic lattice and leads successively to the formation of a rodlike structure with hexagonal symmetry. Close to 95 °C, large aggregates of polymers ordered in lamellae structure are formed, leading to an opaque suspension.