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 ≅ 20 mol % in pure water and at f Dod ≅ 10 mol % in 0.1 M NaCl, showing a saturation tendency near f Dod ≅ 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.

Interactions of random copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonate and N-dodecylmethacrylamide (DodMAm) with n-dodecyl hexaethylene glycol monoether (C12E6) with and without added n-hexadecyltrimethylammonium chloride (CTAC) in 0.2 M NaCl aqueous solutions were investigated by fluorescence and light-scattering techniques. The polymers with the DodMAm contents (f Dod) ranging from 10 to 50 mol % were singly labeled with pyrene (1 mol %) or doubly labeled with pyrene (1 mol %) and naphthalene (4 mol %). These polymers form unimolecular micelles (unimer micelles) owing to intramolecular self-association of polymer-bound dodecyl groups, the compactness of the unimer micelle depending on f Dod. The unimer micelles of the polymers with f Dod ≥ 30 mol % have a particularly compact nanostructure where polymer chains are highly collapsed. In the presence of C12E6/CTAC mixed micelles, the polymer with f Dod = 10 mol % abruptly undergoes bulk phase separation as the mole fraction of CTAC (Y) in the mixed micelle is increased to a certain critical level (Y p) (i.e., Y p ≈ 0.15). With an increase in f Dod, the bulk phase separation becomes significantly less abrupt, accompanying a marked increase in Y p. For the polymers with f Dod ≥ 40 mol %, no such bulk phase separation was observed in a region of 0 ≤ Y ≤ 0.6. Intrapolymer nonradiative energy transfer indicates that the unimer micelles of the polymers with f Dod ≥ 30 mol % are unfolded by interactions with C12E6. The formation of soluble polymer−micelle complexes was manifested by an increase in the hydrodynamic radius (R h) upon addition of C12E6 to polymer solutions. The R h for the unimer micelle with f Dod = 50 mol % increased from 8.2 to 15.8 nm by the addition of 5.0 mM of C12E6 at a polymer concentration of 1.5 g/L. The R h at Y = 0 further increased to 21.5 nm as CTAC was added to Y = 0.07, indicating an interplay of hydrophobic and electrostatic interactions in the complexation between the unimer micelle and C12E6/CTAC mixed micelles.

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