No abstract

Micellar solutions of a highly hydrophilic ethylene oxide−propylene oxide triblock copolymer, pluronic F88 (EO10 3PO39EO10 3), in aqueous solution are examined by small-angle neutron scattering (SANS) at different concentrations and temperatures and in the presence of different salts. At temperature less than 30 °C, F88 solution in water (5 wt %) showed unimers which are fully dissolved Gaussian chains. The unimer-to-micelle transition takes place when the temperature or concentration is increased. Added neutral salt favors micellization of the copolymer at lower concentration/temperature. At temperature close to ambient, block copolymer forms micelles that consist of a central core, presumably dominated by the propylene oxide blocks, surrounded by a corona of highly hydrated ethylene oxide subchains. SANS analysis shows that the size of hydrophobic core increases as a part of PEO adjacent to the PPO core loses water with increasing temperature or salt (KCl) concentration. The micellar volume fraction increases with increasing concentration of block copolymer. The salting out effect of different electrolytes on structure of micelles is in the order KCI > KBr > KI. The effect of temperature on micellar solutions in the presence of KCl is also examined. It has been observed that the effect of added salt on the structural phase behavior of block copolymer is analogous to that of temperature. In all the above study, micelles are found to be spherical.

We have examined the polymer/surfactant interaction in mixed aqueous solutions of cationic surfactants and anionic polyelectrolytes combining various techniques: tensiometry, potentiometry with surfactant-selective electrodes, and viscosimetry. We have investigated the role of varying polymer charge density, polymer concentration, surfactant chain length, polymer backbone rigidity, and molecular weight on the critical aggregation concentration (Cac) of mixed polymer/surfactant systems. The Cac of these systems, estimated from tensiometry and potentiometry, is found to be in close agreement. Different Cac variations with polymer charge density and surfactant chain length were observed with polymers having persistence lengths either smaller or larger than surfactant micelle size, which might reflect a different type of molecular organization in the polymer/surfactant complexes. The surfactant concentration at which the viscosity starts to decrease sharply is different from the Cac and probably reflects the polymer chain shrinkage due to surfactant binding.