PEO−PPO−PEO Block Copolymer Micelles in Aqueous Electrolyte Solutions: Effect of Carbonate Anions and Temperature on the Micellar Structure and Interaction

Abstract

The structure and interaction of micelles formed by an amphiphilic triblock copolymer P(EO)103−P(PO)39−P(EO)103 (F88) in aqueous electrolyte solutions of potassium carbonate have been investigated using small-angle neutron scattering as functions of temperature and salt concentration. Modeling the SANS data from the copolymer solutions using analytical expressions for the scattering intensity revealed the remarkable effects of carbonate anion concentration and the temperature on the self-assembly of F88 into spherical and cylindrical aggregates and subsequent phase separation. The critical micellization temperature (cmt) of 5 wt % F88 decreases with an increase in the salt concentration, and the cmt approaches ambient temperatures in a solution with 0.1 M K2CO3. An increase in salt concentration also decreases the temperature at which the transition from spherical to cylindrical micelles occurs as well as the phase separation temperature of the copolymer from the solution. These observations clearly indicate that the evolution of the micellar structure and the phase separation are through gradual dehydration of the copolymer chains with an increase in the temperature and/or the salt concentration. The effects of carbonate ions are quite dramatic when compared to those of monovalent ions as the structural transitions occur at a concentration that is lower by an order of magnitude than the published results on monovalent ions.