An Experimental Study by NMR and SANS of the Ambient Hydration of Paper

Abstract

The structural changes in fibre polymers and dispersion of water in the polymer have been studied at length scales less than 400 Å with contrast variation small angle neutron scattering (SANS) and solid state nuclear magnetic resonance (NMR). The SANS of hydrating paper samples is discussed in different angular regions in terms of a scattering wavenumber vector, q (q = 4π/λ . sin θ/2
where λ is the wavelength of the neutrons and θ is the scattering angle). At low q close to the neutron beam, the Guinier region, voids in the structure are found to disappear as the microfibrils
swell with water. The lateral dimensions of the cellulose crystallite are calculated from x-ray diffraction data and there is a good qualitative correlation with relative size of the crystallites and the
appearance of short range of order in the SANS in the mid-range of the q studied. The range of the length scale of the SANS feature is slightly larger than the elementary crystallite which is consistent with layers of swollen cellulose and water around the crystallite. In the high q region, the angular region furthermost from the beam, the scattering is discussed in terms of deviation from Porod scattering. According to this interpretation the interface between cellulose and water is not clearly defined and there is an increase in the amount of surface area for water to bind to.

These results are consistent with water disrupting the hydrogen bonding in fibre polymers. The NMR spin diffusion experiment monitors the exchange of magnetisation between water and polymer protons. A simplistic model of this transfer process is justified and indicates that water is not uniformly dispersed in the polymer as a function of moisture content.