Interaction between Na+ and polymer in aqueous solution of sodium salts of poly(2-hydroxyethyl methacrylate-co-methacrylic acid) studied by 23Na-NMR

Abstract

The interaction between Na+ and polymer was studied by Na-23-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 width of Na-23-NMR in 1% (w/v) aqueous solution of the P (HEMA-co-MAANa) narrowed with increasing temperature 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 Na-23-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 feature of Na-23-NMR relaxation was used to deduce the correlation time (tau(c)), the degree of binding (pB), and the quadrupole coupling constants (chi) of the polymer-bound counterion. The chi and tau(c) values show that the mobilities of the polymer chain are correlated with the motion of Na+ in aqueous solution of the polymer and there is a small degree of the specific binding between COO- and Na+. No evidence in support of the intramolecular conformational change by the charge density variation in P(HEMA-co-MAANa) was obtained.