Reversible pH-Induced Formation and Disruption of Unimolecular Micelles of an Amphiphilic Polyelectrolyte

Abstract

Reversible pH-induced formation and disruption of a unimolecular micelle (unimer micelle) of a random copolymer of sodium 2-(acrylamido)-2-methylpropanesulfonate and 11-acrylamidoundecanoic acid (50 mol %) were investigated by static light scattering (SLS), quasi-elastic light scattering, viscometric, 1H NMR spin−spin relaxation, and fluorescence probe techniques. The weight-average molecular weight (Mw) value of the copolymer determined by SLS in a 0.1 M NaCl aqueous solution at pH 7 was virtually the same as that determined in a methanol solution, Mw in 0.1 M NaCl being practically independent of pH in the range 3 < pH < 9. These findings indicate that the copolymer exists as a single molecular state (unimer) in 0.1 M NaCl over the range 3 < pH < 9. At pH < 5, the copolymer exhibited very small values of the radius of gyration, hydrodynamic radius, reduced viscosity, and spin−spin relaxation time, indicative of the formation of a unimer micelle under acidic conditions. All these values increased significantly with increasing pH in the range 5 < pH < 8, reaching saturated values near pH 8. These observations indicate that the unimer micelle is disrupted into an open chain conformation at basic pHs. This is the first example of pH-responsive unimer micelles whose formation and disruption are controlled by the selective protonation and deprotonation of carboxyl groups attached at the terminal of polyelectrolyte-bound hydrophobes.