Micellar crystallization with a hysteresis in temperature.

We have investigated the phase diagram of the triblock copolymer P123 solved in water by viscosity measurements for different concentrations and temperatures. The structures of the different phases were identified by surface sensitive neutron diffraction. We find a pronounced hysteresis between heating and cooling. During heating, a highly viscous crystalline fcc phase is found before melting occurs at 44 °C with a simultaneous drop in viscosity. Upon cooling, first a hexagonal phase with low viscosity develops followed by a highly viscous fcc phase. Phase diagrams for the heating and cooling cycle for different concentrations are provided. The hysteric behavior is discussed in relation to the shape of the micelles.