Associative Properties in Water of Copolymers of Sodium 2-(Acrylamido)-2-methylpropanesulfonate and Methacrylamides Substituted with Alkyl Groups of Varying Lengths

Associative properties of random copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonate (AMPS) with N -hexylmethacrylamide (C 6 MAm) and with N -octadecylmethacrylamide (C 18 MAm) in aqueous solutions were investigated comparing with those of AMPS copolymers with N -dodecylmethacrylamide (C 12 MAm) reported previously. For characterization by fluorescence, all the polymers were labeled with 1 mol % of naphthalene by terpolymerization using N -(1-naphthylmethyl)methacrylamide. The polymers possessing C 6 or C 12 alkyl chains exhibit a strong tendency for intramolecular hydrophobic association to form unimolecular micelles when the C n MAm ( n = 6, 12) contents in the polymers are in the ranges of 30−70 and 10−50 mol % for the C 6 and C 12 chains, respectively. A similar tendency was observed for the polymers possessing C 18 chains when the C 18 MAm contents are in the range of 5−20 mol %, but polymer micelles formed were not completely unimolecular. Both the lower and upper limits of the C n MAm contents for all the three polymers decrease significantly with increasing the length of the alkyl chain. When the C n MAm contents are either lower or higher than these limits, the polymer-bound alkyl chains undergo interpolymer associations. Fluorescence depolarization and 1 H NMR relaxation times indicate that local motions of naphthalene labels and alkyl chains are more pronouncedly restricted in hydrophobic domains formed by longer alkyl chains. An inclination to the self-association of these polymers was discussed in terms of the ratio of the total number of carbon atoms in hydrophobes to the number of SO 3 - in a polymer chain. A conclusion is that with C 6 chains one needs to incorporate more carbon atoms into a polymer than with C 12 and C 18 chains to attain the same extent of hydrophobe associations. In other words, the CH 2 and CH 3 residues in the C 6 chain are much less effective than those in the C 12 and C 18 chains for the self-association when compared at the same hydrophobe/charge ratio in a polymer chain, whereas there is no significant difference between those in the C 12 and C 18 chains. Thus, the ability of the C 6, C 12, and C 18 chains per carbon atom to associate is in the order C 18 ≥ C 12 ≫ C 6 .