Dynamics of PEO-PPO-PEO triblock copolymer in aqueous solutions investigated by internal friction

The dynamics of poly( ethylene oxide)-block-poly(propylene oxide)-block-poly( ethylene oxide) ( PEO-PPO-PEO) in aqueous solutions in the range of the polymer concentration phi = 10.0-45.0 wt% has been investigated by the tube inversion method and the internal friction technique. At phi <= 31.0 wt%, two internal friction peaks are observed with the characteristics of the first-order phase transition, corresponding to the opaque liquid-to-liquid transition and the phase separation. The opaque liquid-to-liquid transition is due to the sphere-to-rod transition in shape of the micelles, and the phase separation is caused by the destruction of core-shell structure. At phi >= 32.0 wt%, besides the internal friction peak associated with phase separation, two other peaks are observed, which correspond to the solution-to-gel ( liquid-to-crystal) transition and gel-to-solution ( crystal-to-liquid) transition, respectively. The former is due to the micelle crystallization, whereas the latter is the result of spherical micelles fusing into wormlike or rodlike micelles at elevated temperatures.