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-meth ylpropane-sulfonate (AMPS) with N-hexylmethacrylamide (C6MAm) and with N-oc tadecylmethacrylamide (C18MAm) in aqueous solutions were investigated compa ring with those of AMPS copolymers with N-dodecyl-methacrylamide (C12MAm) r eported previously. For characterization by fluorescence, all the polymers were labeled with 1 mol % of naphthalene by terpolymerization using N-(1-na phthylmethyl)methacrylamide. The polymers possessing C-6 or C-12 alkyl chai ns exhibit a strong tendency for intramolecular hydrophobic association to form unimolecular micelles when the CnMAm (n = 6, 12) contents in the polym ers 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 C18MAm contents are in the range of 5-20 mol %, but po lymer micelles formed were not completely unimolecular. Both the lower and upper limits of the CnMAm contents for all the three polymers decrease sign ificantly with increasing the length of the alkyl chain. When the CnMAm con tents are either lower or higher than these limits, the polymer-bound alkyl chains undergo interpolymer associations. Fluorescence depolarization and H-1 NMR relaxation times indicate that local motions of naphthalene labels and alkyl chains are more pronouncedly restricted in hydrophobic domains fo rmed by longer alkyl chains. An inclination to the self-association of thes e polymers was discussed in terms of the ratio of the total number of carbo n atoms in hydrophobes to the number of SO3- in a polymer chain. A conclusi on 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 hydro phobe associations. In other words, the CH2 and CH3 residues in the C-6 cha in are much less effective than those in the C-12 and C-18 chains for the s elf-association when compared at the same hydrophobe/charge ratio in a poly mer 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 p er carbon atom to associate is in the order C-18 greater than or equal to C -12 much greater than C-6.