Hydrophobically modified poly(sodium 2-acrylamido-2-methylpropanesulfonate)s bearing octadecyl groups: A fluorescence study of their solution properties in water

Fluorescently labeled amphiphilic polyelectrolytes have been prepared by free-radical copolymerization in dimethylformamide using azobis(isobutyronitrile) as the initiator of sodium 2-acrylamido-2-methylpropanesulfonate (AMPS) and (1) N-[4-(1-pyrenyl)butyl]-N-n-octadecylacrylamide (PyODA) in 98:2 and 95:5 molar ratios, (2) N-[2-(1-naphthyl)ethyl]-N-n-octadecylacrylamide (NpODA) in 98:2 and 95:5 molar ratios, and (3) a mixture of the two labeled monomers in a molar ratio of 95:4:1 (AMPS: NpODA:PyODA). The solution properties of the copolymers in water and in salt solutions have been studied by viscometry, H-1 NMR spectroscopy, and fluorescence spectroscopy. Evidence from nonradiative energy transfer between excited naphthalene (Np*) and pyrene (Py) in aqueous solutions of the doubly labeled polymers points to the formation of polymeric micelles. The inefficiency of nonradiative energy transfer between Np* and Py in mixed solutions of the singly labeled polymers indicates that the micelles are mostly unimolecular. The quenching of fluorescence of polymer-linked pyrene by nitromethane and thallium nitrate has been used to assess the accessibility of the chromophore to neutral molecules and to cationic species, respectively, in water and in solutions of increasing ionic strength. The results are discussed in light of previous studies of the properties in solution of neutral polymers carrying the same hydrophobic substituents as PAMPS-Py(Np)ODA and of amphiphilic PAMPS substituted with various other hydrophobic groups.