No abstract

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.

The interaction between pyrene-labeled poly(acrylamido)-2-methylpropane sulfonate) (PyPAMPS) and mixed micelles of n-dodecylhexaoxyethylene glycol monoether/n-hexadecyltrimethylammonium chloride (C12E6/CTAC) was studied by turbidimetry, quasielastic light scattering (QELS), fluorescence quenching, and UV spectroscopy. The present report focuses on the effect of the pyrene label on the polymer-micelle interaction. With nonlabeled PAMPS, we observe by turbidity and QELS a critical mole fraction of ionic surfactant (Yc) corresponding to the onset of polyelectrolyte-micelle interaction. The same Yc is observed by turbidity and QELS for PyPAMPS. However, PyPAMPS shows a lower additional transition by the same methods, which we refer to as "Yc1" to distinguish it from "Yc2" which is seen for both PAMPS and PyPAMPS. Steady-state fluorescence, in the presence of a hydrophobic quencher (N,N-dimethylaniline) solubilized in the micelles, also shows a discontinuity at Yc1. Therefore, we conclude that Yc1 and Yc2 correspond to the binding of micelles to polymeric pyrene sites and AMPS sites, respectively. Analysis of UV spectra at varying Y demonstrates that the polymer-bound pyrene penetrates inside micelles and resides at or near the hydrophobic core. These results indicate preferential binding of micelles to pyrene binding sites; nevertheless, both Yc1 and Yc2 show a linear dependence on the square root of the ionic strength. This dependence suggests the dominant role of electrostatic forces, consistent with the observation that nonionic micelles will not bind to PyPAMPS. We conclude that conjoint hydrophobic and electrostatic effects determine the interaction between PyPAMPS and C12E6/ CTAC mixed micelles.

The interaction between poly(sodium 2-(acrylamido)-2-methylpropanesulfonate) labeled with 1 mol % pyrene (PyPAMPS) and mixed micelles of hexaethylene glycol n-dodecyl monoether (C12E6) and n-hexadecyltrimethylammonium chloride (CTAC), in which N,N-dimethylaniline (DMA) was solubilized, was studied by steady-state and time-dependent fluorescence quenching by varying the mole fraction of CTAC (Y) in the mixed micelle. At Y < 0.05, fluorescence quenching is essentially dynamic, arising from collisional encounters of the pyrene sites and DMA-carrying mixed micelles. Strong quenching begins to occur at Y ≅ 0.05, corresponding to the onset of polyelectrolyte−micelle interactions. In the region 0.05 < Y < 0.09, the quenching was shown to occur via transient complex formation between PyPAMPS and the mixed micelle. Changes in the vibrational structures of fluorescence spectra of pyrene in PyPAMPS indicate that pyrene groups are inserted in the hexa(oxyethylene) phase of the mixed micelle when the polymer−micelle complex is formed. The extent of quenching reaches a limiting value at Y > 0.11 in excess micelle solution, corresponding to the state in which all pyrene groups are micelle-bound.