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 less-than-or-equal-to 15-degrees-C) and low polymer concentrations the unimers are fully dissolved GaUssian chains with radius R(g) = 17 angstrom. Close to ambient temperature, the hydrophobic nature of ppo causes aggregation of the polymers into spherical micelles with core sizes of the order of 40-50 angstrom, 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-degrees-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-degrees-C, large aggregates of polymers ordered in lamellae structure are formed, leading to an opaque suspension.