PHASE-SEPARATION OF POLY(N-ISOPROPYLACRYLAMIDE) IN WATER - A SPECTROSCOPIC STUDY OF A POLYMER TAGGED WITH A FLUORESCENT DYE AND A SPIN-LABEL

A spin label (4-amino-2,2,6,6-tetramethylpiperidine I-oxide,TEMPO) and a fluorescence dye (1-pyrenylbutyl, Py) were attached randomly along the same poly(N-isopropylacrylamide) chain (PNIPAM-Py-T, M(v) 2.3 X 10 (6), Py content 1.9 x 10(-5) mol g-1), and their interactions in solut ions of the polymer were studied using electron paramagnetic resonance (EPR), C-13 nuclear magnetic resonance (C-13 NMR), and fluorescence s pectroscopy. The thermoreversible phase transition of aqueous solution s of the polymer was examined by these three techniques. EPR spectra o f solutions of PNIPAM-Py-T (3 g L-1) were recorded as a function of te mperature (20-35-degrees-C) and analyzed in terms of the isotropic hyp erfine coupling constant and the correlation time for the reorientatio n motion. The temperature dependence of the simulated spectra. The flu orescence of PNIPAM-Py-T in cold water (I g L-1) displayed contributio ns from isolated excited pyrenes (Py, monomer emission) and from prea ssociated pyrenes (''excimer'' emission). This excimer emission decrea sed at the phase transition temperature while the monomer emission inc reased. Quantum yield determinations and fluorescence lifetime measure ments are interpreted in terms of two competing temperature-induced ef fects: (1) a disruption of the pyrene preassociation and (2) an increa se in the efficiency of the quenching of Py by the TEMPO nitroxide ra dicals. Supporting evidence was obtained by fluorescence measurements carried out with solutions of PNIPAM-Py-T in methanol and by intermole cular fluorescence quenching experiments using a singly labeled polyme r (PNIPAM-Py, M(v) 2.6 X 10(6), Py content 4.2 X 10(-5) mol g-1) and e ither TEMPO or a TEMPO-labeled polymer (PNIPAM-T, M(v) 2.9 X 10(6), TE MPO content 1 X 10(-5) mol g-1).