Phase separation of poly(N-isopropylacrylamide) in water: a spectroscopic study of a polymer tagged with a fluorescent dye and a spin label

Francoise M. Winnik, M. Francesca Ottaviani, Stefan H. Bossman, Wenseng Pan, M. Garcia-Garibay, Nicholas J. Turro
1993 The Journal of Physical Chemistry  
A spin label (4-amino-2,2,6,6-tetramethylpiperidine 1-oxide, TEMPO) and a fluorescence dye (1-pyrenylbutyl, Py) were attached randomly along the same poly(N4sopropylacrylamide) chain (PNIPAM-Py-T, Mv 2.3 X 106, Py content 1.9 X 10-5 mol g-l), and their interactions in solutions of the polymer were studied using electron paramagnetic resonance (EPR), 13C nuclear magnetic resonance (13C NMR), and fluorescence spectroscopy. The thermoreversible phase transition of aqueous solutions of the polymer
more » ... as examined by these three techniques. EPR spectra of solutions of PNIPAM-Py-T (3 g L-I) were recorded as a function of temperature (20-35 "C) and analyzed in terms of the isotropic hyperfine coupling constant and the correlation time for the reorientation motion. The temperature dependence of the simulated spectra. The fluorescence of PNIPAM-Py-T in cold water (1 g L-*) displayed contributions from isolated excited pyrenes (Py*, monomer emission) and from preassociated pyrenes ("excimer" emission). This excimer emission decreased at the phase transition temperature while the monomer emission increased. Quantum yield determinations and fluorescence lifetime measurements are interpreted in termsof two competing temperature-induced effects: (1) a disruption of the pyrene preassociation and (2) an increase in the efficiency of the quenching of Py* by the TEMPO nitroxide radicals. Supporting evidence was obtained by fluorescence measurements carried out with solutions of PNIPAM-Py-T in methanol and by intermolecular fluorescence quenching experiments using a singly labeled polymer (PNIPAM-Py, M, 2.6 X lo6, Py content 4.2 X mol g-l) and either TEMPO or a TEMPO-labeled polymer (PNIPAM-T, Mv 2.9 X 106, TEMPO content 1 X mol g-l). PNIPAM-Py PNIPAM-T Figure 1. Structure of the polymers used in this study. term of the free energy of mixinn will overcome the negative -_ + Present address: Universitat Karlsruhe, Engler-Bunte Institut, Richard Abstract published in Advance ACS Abstracts, November IS, 1993. enthalpy of solution, resulting in a-positive free energy. Tius at this temperature phase separation occurs. The macroscopic phase separation reflects the collapse of individually solvated coils into dehydrated globules which ag-Willstitter All= 5,
doi:10.1021/j100151a058 fatcat:d4edmz4jrrcnzhiffnrfi7vj6m