PEO-PPO-PEO triblock copolymer in aqueous solution. Micelle formation and crystallization

The phase behavior of poly(ethylene oxide) - poly(propylene oxide) - poly(ethylene oxide), PEO-PPO-PEO, tri-block copolymers dissolved in water has been studied using small-angle neutron scattering, using a Couette device for shear alignments. The structural properties has been studied as a function of polymer concentration and temperature. At low-temperature and low polymer concentrations the unimers are dissolved as individual Gaussian chains. At temperatures close to ambient, the hydrophobic nature of PPO causes aggregation of the polymers into spherical micelles. The aggregation number is relative small close to the critical micellation temperature, and increases markedly with temperature. The micellar volume fraction increases roughly linearly with temperature, until either a saturation is reached, where all polymers are part of a micelle, or the volume fraction reaches the critical value for hard-sphere crystallization. At higher temperatures the aggregates transform from spherical form to rod-like micelles, causing first melting of the cubic structure, and subsequently crystallization into a phase of hexagonally ordered rods