Micellization of PEO-PPO-PEO triblock copolymers in aqueous electrolyte solutions

Small angle neutron scattering was used to study the effects of several potassium salts (K_2CO_3, K_3PO _4, K_2P_4O_7, KNO_3, KF and KCl) on the structural evolution and inter aggregate interactions of the Pluronic F88 (P(EO)_103-P(PO)_39-P(EO)_103) in aqueous (D_2O) solution. Addition of the salts lowered the micellization temperature, implying that they favored self-aggregation. The progression of the micellar structures (from spherical to cylindrical) with a variation in the salt concentration is similar in nature to that observed with a change in temperature in F88/pure D_2O system with the differences found only in the temperature value at which the structural transformation takes place. Dimensions of the micelles were obtained by modeling the scattering data using analytical expressions for the scattered intensity. The obtained values for the characteristic sizes of the micelles indicated that the salts were effective in the order K_2CO₃ > K_3PO₄ > K_2P_4O₇ > KF ~ KNO₃ > KCl in promoting micellization. The calculated dimensions of the micelles, coupled with certain assumptions, were used to obtain several measures of interest, like the amount of water in the core. The quantities thus obtained were then used to interpret the mechanism of the structural behavior in terms of the effects of the different salts studied, primarily within the context of an approach developed earlier, that the process of progressive dehydration of the copolymer is the cause of all the structural transformations observed.