Fluorescence studies of hydrophobic association of random copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonate and N-dodecylmethacrylamide in water

Intrapolymer self-association of random copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonate (AMPS) and N-dodecylmethacrylamide (DodMAm) with varying compositions in aqueous solution was investigated by various fluorescence techniques. The polymers were labeled with 1 mol % of naphthalene (Np) or pyrene (Py) or doubly labeled with Np (4 mol %) and Py (1 mol %). Vibronic fine structures of Py fluorescence, lifetimes of Np and Py fluorescence, intrapolymer nonradiative energy transfer (NRET) from singlet excited Np to ground-state Py labels, and fluorescence quenching by thallium cations were investigated as a function of the DodMAm content (f(Dod)) in the copolymers with or without added salt. Results from all these fluorescence studies indicate that with increasing f(Dod), hydrophobic association commences at f(Dod) congruent to 20 mol % in pure water and at f(Dod) congruent to 10 mol % in 0.1 M NaCl, showing a saturation tendency near f(Dod) congruent to 40 mol % in the salt solution. In contrast, viscosity data show that the polymer size markedly decreases in the regime 5 < f(Dod) < 20 mol % owing to intrapolymer hydrophobic association of dodecyl groups. This decrease in the macroscopic size of the polymer in the low f(Dod) regime could not be detected by any fluorescence techniques employed. Although the viscosity data do not show any further contraction of the polymer chains at f(Dod) > 20 mol %, the NRET results indicate a considerable increase in the compactness of polymer chains at f(Dod) > 20 mol %. The combination of these fluorescence techniques proved to provide a sensitive tool to detect hydrophobic associations and conformational changes in the hydrophobically modified polymers, while viscosity reflects only global changes in the polymer size.