CONONSOLVENCY OF POLY(N-ISOPROPYLACRYLAMIDE) - A LOOK AT SPIN-LABELED POLYMERS IN MIXTURES OF WATER AND TETRAHYDROFURAN

The electron Paramagnetic resonance (EPR) spectra of solutions of a nitroxide-labeled poly-(N-isopropylacrylamide) (PNIPAM-T/880, M(v) 2.9 x 10(-6); label, 4-amino-2,2,6,6-tetramethylpiperidine 1-oxide (TEMPO); label content, 1 x 10(-5) mol g-1) were recorded in mixtures of water and tetrahydrofuran (THF) in order to investigate the phenomenon of conosolvency exhibited by the PNIPAM/water/THF ternary system. A detailed line-shape analysis of the EPR signals was carried out for spectra recorded at constant temperature as a function of solvent composition and as a function of temperature (-10 to +35-degrees-C) for several water/THF mixtures. Changes in the rotational correlation time and in the isotropic hyperfine coupling constant of the label were correlated to the macroscopic polymer phase-separation process. The data support a model involving preferential water adsorption along the polymer chains in mixed water/THF solutions. This situation was compared to that offered by the PNIPAM/water/methanol system, another ternary mixture which exhibits the phenomenon of cononsolvency. Differences between the two systems are described. They are interpreted in terms of the relative hydrogen-bonding abilities of the two organic cononsolvents.