Fluorescence Studies of Dansyl-Labeled Poly(N-isopropylacrylamide) Gels and Polymers in Mixed Water/Methanol Solutions

Abstract

The photophysical properties of solutions of dansyl [5-(dimethylamino)naphthalenyl-1-sulfonyl] labeled poly(N-isopropylacrylamide) (PNIPAM-Da ; M v 220 000 ; label content, 0.06 molar %) were examined in mixtures of water and methanol in order to investigate the phenomenon of cononsolvency exhibited by the PNIPAM/water/methanol ternary system. They were compared to the photophysical properties of dansyl-labeled poly(N-isopropylacrylamide) gels (cross-linker, N,N'-methylenebis(acrylamide) ; cross-linker content, 1.2 molar % ; probe content, 0.06 molar %) equilibrated in mixtures of methanol and water. The swelling of these gels decreases abruptly in aqueous solutions containing 7-25 mol % methanol and increases gradually in systems with a higher methanol concentration. Steady-state emission and excitation spectra, fluorescence decays, and fluorescence polarization were recorded at 20 °C from polymer solutions and from equilibrated gels. Shifts in the wavelength of maximum emission and changes in the fluorescence lifetimes and in the rotational diffusion coefficients of the probe were correlated to the macroscopic changes in swelling volume.