Irreversible structural transitions in mixed micelles of oppositely charged diblock copolymers in aqueous solution

Using light scattering (titration) measurements, we have shown that micelles can be formed in aqueous solutions of a mixture of poly(4-(2-amino hydrochloride-ethylthio)butylene)-block-poly(ethylene oxide), PAETB(49)-b-PEO212, and poly(4-(2-sodium carboxylate-ethylthio)butylene)-block-poly(ethylene oxide), PCETB47-b-PEO212. The driving force is not only electrostatic attraction between the oppositely charged polyelectrolyte blocks, but also hydrophobic interaction contributes. For pH <5.3 or pH > 9.7 the single acid or alkaline diblock copolymer also forms micelles due to absence of electrostatic repulsion and the presence of only hydrophobic interaction. The mixed micelles formed under so-called optimal conditions (pH = 7.2, 10 mM NaNO3, T = 25.0 degrees C) irreversibly shrink upon an increase in pH, ionic strength, and temperature and upon a decrease in pH. Restoring pH or temperature to the critical value has no effect on the hydrodynamic radius. We propose to relate these changes to an irreversible transition of the micellar core from a metastable fluidlike state (complex coacervate like) to a more stable glasslike state, triggered by a shift in the balance between electrostatic and hydrophobic interactions.