PHASE-BEHAVIOR OF POLY(ETHYLENE OXIDE)-POLY(PROPYLENE OXIDE)-POLY(ETHYLENE OXIDE) TRIBLOCK-COPOLYMER DISSOLVED IN WATER

The amphiphilic character of triblock-copolymers made of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) changes substantially as a function of temperature. This leads to interesting phase behaviour when the block-copolymers are dissolved in water: micelle formation above a critical temperature and critical concentration, and an apparent <<inverse crystallization>> characteristic as the micellar liquid <<freezes>> into a cubic crystal at elevated temperatures, characterized by true long-range correlation in the bond-angle correlation, but absence of long-range order in the bond-length correlation. At higher temperatures a phase of hexagonally packed rods is observed, but between the two crystalline states a liquid channel appears. It is shown that the micelle concentration depends linearly on both polymer concentration and temperature, and the cubic crystal is formed whenever the phase line corresponding to a micellar volume fraction-phi = 0.53 is crossed, in agreement with crystallization of hard-sphere colloidal suspensions. If the polycrystalline phases are exposed to shear, they coherently transform into single crystals. Thus by performing the scattering experiment under shear, unique insight is obtained in the phase diagram including melting and crystallization into single crystalline states.