Effects of salts on the micellization and gelation of a triblock copolymer studied by rheology and light scattering

The phase behavior and aggregation properties of a triblock copolymer of ethylene oxide (EO) and propylene oxide (PO), with a measured composition (EO)(29)(PO)(40)(EO)(29), in aqueous solutions containing salt, have been examined using dynamic light scattering, rheological techniques, and sedimentation and viscosity measurements. The copolymer is dissolved as a unimer at low temperatures and forms spherical micelles with increasing temperature. At higher temperatures, a sphere-to-rod transition is seen for the micelles. Two types of gel are formed at higher concentrations of the copolymer. With different inorganic salts, the micellization and gelation properties of the copolymer follow the same type of transitions as the salt-free system, but all transition temperatures are shifted. The spherical micelles thus transform into rod-like micelles at around 38 degrees C in 1 M KF, which is approximately 36 deg below the transition temperature in the salt-free system. Rod lengths in 1 M KF are between 1000 and 1800 Angstrom, at 40 degrees C. The higher-temperature gel phase is seen at all concentrations down to 0.5 - 1 wt %. The elasticity of this gel is due to hindered rotation of rods. Its relaxation time decreases with increasing concentration, indicating that the gel relaxes due to a partial breakdown or dissolution of the rods at the cross points. The strain dependence of this gel suggests that ordered structures of rods are formed at concentrations above 27 wt %.