The hydration of paper studied with solid-state magnetisation-exchange 1H NMR spectroscopy

Abstract

The wide-line 1 H nuclear magnetic resonance (NMR) spectrum of paper in equilibrium with ambient humidity consists of super-imposed relatively broad and narrow lines. The narrower line is of the order of 2 kHz wide at half the maximum height, while the broader line is of the order of 40 kHz in width at half height. On the basis of these line widths, the narrow line is assigned to water sorbed to the paper, and the broad line to the polymeric constituents of the paper. It was not possible to distinguish between the various polymeric components of paper contributing to the 1 H NMR spectra. A modified Goldman-Shen pulse sequence was used to generate a spatial magnetisation gradient between the polymer and water phases. The exchange of magnetisation between protons associated with water and those associated with the macromolecules in paper was observed. The exchange of magnetisation is discussed within a heat transfer model for homonuclear dipolar coupling, with exchange being characterised by a spin-diffusion coefficient. Consideration of the magnitude of the initial rate of the exchange process and estimates of the spinspin relaxation times based on 1 H line widths indicate that some water must exist in a sufficiently immobile state as to allow homonuclear dipolar interactions between adjacent polymer and water protons. Thus, water sorbed onto paper must exist in at least two states in mass exchange with each other. This observation allows certain conclusions to be drawn about the ratio of free/bound water as a function of moisture content and the dispersal of water within the polymer matrix.

The study of water behaviour in regenerated cellulosic fibres by low-resolution proton NMR

“…cellulose protons [41]. In addition, a separate intra-molecular dipolar relaxation pathway might also be possible for water at cellulose surfaces, due to static dipolar interaction between proton pairs on the same molecule.…”

Water in cellulose: evidence and identification of immobile and mobile adsorbed phases by 2H MAS NMR

“…26,28,29 The most direct precursors and motivators to our present work are both 2 H and 1 H spectroscopic studies of WAC. [30][31][32] Regarding 2 H NMR spectroscopy directed toward detecting line shapes, there are two main approaches. In conventional 2 H single-pulse experiments, broad spectral components (say, 10-20 kHz wide or wider) are not detected for instrumental reasons.…”

“…Radloff et al 31 detected water molecules in sparsely hydrated cellulose samples that, at low temperature, exhibited dominantly broad 2 H spectra and, thereby, very slow molecular dynamics. More recently, Garvey et al 30 used a more complex experimental scheme and detected, by 1 H NMR, magnetization exchange between water and cellulose in paper. The magnitude of the observed exchange rate led the latter authors to the conclusion that some water molecules could be in a rather immobile state.…”

Equilibrium moisture content (EMC) in Norway spruce during the first and second desorptions

“…The study establishes 1 st and 2 nd desorption iso-therms for a variety of initial conditions and it covers the 60.1%-99.9% RH range. Advanced versions of both high and low field NMR relaxometry have been introduced to the wood and paper sciences during the last decade (see for example Topgaard and Södermann 2001;Garvey et al 2006;Johannessen et al 2006;Kekkonen et al 2009;Cox et al 2010). One series covers the RH range from 60.1% to 98.4% and thus includes the full RH range for which Spalt (1958) reported differences between the 1 st and the 2 nd desorption curves.…”