Self-Association in Water of Copolymers of Acrylic Acid and N-Dodecylmethacrylamide As Studied by Fluorescence, Dynamic Light Scattering, and Rheological Techniques

The self-association of random copolymers of acrylic acid (AA) and N -dodecylmethacrylamide (DodMAm) with 5.1−28.4 mol % DodMAm contents ( f Dod ) in water was investigated by fluorescence, quasi-elastic light scattering (QELS), and rheological methods as a function of f Dod and pH. The copolymers exhibited a strong tendency for interpolymer association, forming micelle-like multipolymer aggregates. This is a striking contrast to the previous finding that random copolymers of 2-(acrylamido)-2-methylpropanesulfonate (AMPS) and DodMAm exhibited a strong tendency for intrapolymer association, forming unimolecular micelles. At pH 10, the AA/DodMAm copolymers with a smaller f Dod formed multipolymer aggregates with a larger size, the size decreasing with increasing f Dod . At pH 4, this trend was opposite; the size of the multipolymer aggregate increased with increasing f Dod . Independent of f Dod and pH, the size of the multipolymer aggregates increased with increasing polymer concentration ( C p ). The association behavior and the nature of multipolymer aggregates of the AA/DodMAm copolymers with small f Dod values (≤9.1 mol %) were markedly different from those with larger f Dod (≥12.8 mol %). As the pH was decreased, the size of the multipolymer aggregate decreased when f Dod ≤ 9.1 mol % whereas it increased when f Dod ≥ 12.8 mol %. Independent of f Dod, the mean aggregation number ( N agg ) of dodecyl groups in one hydrophobic microdomain decreased with decreasing pH. Multipolymer aggregates formed from the AA/DodMAm copolymer of a small f Dod (9.1 mol %) at high C p were more dynamic in nature than those from the copolymer of larger f Dod (22.4 mol %), the former showing shear rate dependent viscosity behavior while the latter did not. On the basis of the characterization data, multipolymer association models for the AA/DodMAm copolymers were proposed.