Hydration in polymer studied through magic angle spinning nuclear magnetic resonance and heteronuclear ¹³C{¹H} Overhauser enhancement spectroscopy: cross-relaxation and location of water in poly(acrylamide)

Abstract

A combination of magic angle spinning (MAS) and heteronuclear ¹³C{¹H} Overhauser enhancement spectroscopy (HOESY) is shown to be a powerful technique for studying hydration in polymers. This is demonstrated in poly(acrylamide)-water system. The increased spectral resolution due to MAS is shown to resolve polymer-polymer and polymer-water dipolar correlations in the two dimensional HOESY experiment. The 2D experiment is thus shown to lead to an indirect detection of water interacting with the polymer. The one dimensional transient Overhauser experiment involving selective inversion of water allows the study of cross-relaxation between water protons and carbonyl carbon in the polymer side chain. The cross-relaxation rate is rationalized in terms of a direct dipole-dipole interaction between the carbonyl carbon and the hydrated bound water. Based on temperature dependent ¹⁷O spin-lattice relaxation time measurements and a two-step motional model for water, we gather that water molecules close to the observed polymer site reorient anisotropically, typically an order of magnitude slower than in pure water. The correlation time for bound water mobility has been estimated to be 0.58×10^-10 s at 298 K, and, in turn, has been used to locate hydrated water at a distance of 3.45 Å from amide carbonyl. This is the first time such an estimate has been made for hydrated water in a polymer using HOESY data.