Phase separation of poly(N-isopropylacrylamide) in mixtures of water and methanol: A spectroscopic study of the phase-transition process with a polymer tagged with a fluorescent dye and a spin label

The thermoreversible phase transition of poly(N-isopropylacrylamide) random ly labeled with a spin label, 4-amino-2.2',6,6'-tetramethylpiperidine 1-oxi de (TEMPO), and a fluorescent dye, 4-(pyren-1-yl)butyl (PNI-PAM-Py-T). in d ifferent H2O/MeOH mixtures was studied by turbidimetry. continuous-wave ele ctron paramagnetic resonance spectroscopy (CW-EPR), and fluorescence spectr oscopy. The macroscopic phase diagram of PNIPAM-Py-T in H2O/MeOH measured b y turbidimetry was identical to those of poly(N-isopropylacrylamide) (PNIPA M) and of TEMPO-labeled PNIPAM (PNIPAM-T) in H2O/MeOH mixtures. However, di stinct differences among the three polymers were detected in their solvent- dependent EPR and fluorescence-spectroscopic properties. The EPR spectra we re analyzed in terms of the isotropic hyperfine coupling constants, which m onitor the variation in environmental polarity of the radical labels occurr ing for the conformational transitions of the polymer as a function of temp erature, as well as the correlation time for reorientation motion. the incr ease of which is indicative of the increased viscosity of the radical envir onment and interactions occurring between the radical and other surface gro ups of the precipitated polymer. if compared to the soluble polymer. The fl uorescence of Py in PNIPAM-Py-T displayed contributions from isolated excit ed pyrenes (monomer emission) and from preformed pyrene ground-state aggreg ates excimer emission. The quantum efficiencies of monomer and excimer emis sion were monitored as a function of solvent composition. By the two experi mental approaches, we demonstrate the profound influence of the PNIPAM-atta ched pyrene units in increasing the hydrophobicity of the nanodomains forme d upon heal-induced precipitation of PNIPAM-Py-T.