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 (M w) 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, M w 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.

Reversible pH-induced changes in self-associative behavior in water of random copolymers of 50 mol % sodium 2-(acrylamido)-2-methylpropanesulfonate (NaAMPS) and 50 mol % 6-acrylamidohexanoic acid (AmH), 8-acrylamidooctanoic acid (AmO), or 11-acrylamidoundecanoic acid (AmU) were investigated by various fluorescence techniques with use of fluorescence-labeled polymers. Fluorescence intensities, lifetimes, depolarization, and quenching for the polymers labeled with naphthalene (4 mol %) indicated that all the polymers adopt an open-chain conformation at basic pHs whereas NaAMPS/AmO and NaAMPS/AmU polymers collapsed into a compact conformation upon a decrease in pH to an acidic region. No such pH-induced conformational change was found for NaAMPS/AmH polymers. Nonradiative energy transfer (NRET) for NaAMPS/AmO and NaAMPS/AmU polymers doubly labeled with naphthalene (4 mol %) and pyrene (1 mol %) indicated a strong tendency for intrapolymer hydrophobic self-association, the polymers with pendent octanoic acid and undecanoic acid groups forming unimolecular micelles (unimer micelles) at acidic pHs. The mean aggregation numbers (N agg) of these alkyl carboxylic acid residues in the unimer micelles were determined to be 86 and 74, respectively. These N agg values were fairly close to the numbers of the AmO and AmU units per polymer chain (85 and 69, respectively), indicative of the formation of unimer micelles. The unimer micelles were disrupted into an open-chain conformation when pH was increased to a basic pH, thus exhibiting reversible pH-induced formation and disruption behavior.

No abstract