NMR studies on conformations and interactions with ions of polymers

Abstract

Part A Interaction between halide ions and poly(vinylpyrrolidone) (PVP) in aqueous solution has been shown to cause significant line broadening in $^{35}Cl$, $^{81}Br$, and $^{127}I$ NMR. This excess NMR line width has been used to obtain the intrinsic binding constants of $Cl^-$, $Br^-$ and $I^-$ with PVP. Other anionic species in the aqueous halide-PVP solution have been found to compete with the halide for binding to the polymer, resulting in a decrease of the excess halide NMR line width. Such competition between the halide and other anion has been used to obtain the intrinsic binding constants of various other anions. The binding constants follow the sequence $SO_4^{2-}$ < F < $Cl^-$ < $NO_3^-$ < $Br^-$ < $I^-$ < $SCN^-$, which is nearly identical with lyotropic series. This binding affinity correlates with the polarizability of anions. Part B The interaction between $Na^+$ and polymer was studied by $^{23}Na$ NMR for the aqueous solution of P(HEMA-co-MAANa), sodium salt of poly (2-hydroxyethyl methacrylate-co-methacrylic acid), as a function of the polymer concentration, charge density of the polymer chain and temperature. The NMR line wedth of $^{23}Na$ NMR in 1\% (w/v) aqueous P(HEMA-co-MAANa) narrowed with increasing temperatures due to the rapid exchange of $Na^+$ between free and polymer-bound states with a rate of exchange exceeding the quadrupolar relaxation rate in the latter state. At high concentrations of the polymer above 1.0\% (w/v) at 298 K, the $^{23}Na$ NMR relaxation fits for a single Lorentzian due to the rapid exchange between two $Na^+$ states. However, it follows a biexponential decay of magnetization in dilute solutions of polymer. The biexponential decay character of relaxation increased with the increase of the fraction of the MAANa monomer unit on the polymer chain. This $^{23}Na$ NMR relaxation data was used to deduce the correlation time({\boldmath$\tau_c$}), the degree of binding (pB), and quadrupole coupling constants ({\b...