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

The organization of water molecules adsorbed onto cellulose and the supramolecular hydrated structure of microfibril aggregates represents, still today, one of the open and complex questions in the physical chemistry of natural polymers. Here, we investigate by 2H MAS NMR the mobility of water molecules in carefully 2H-exchanged, and thereafter re-dried, microcrystalline cellulose. By subtracting the spectral contribution of deuteroxyls from the spectrum of hydrated cellulose, we demonstrate the existence of two distinct 2H2O spectral populations associated with mobile and immobile water environments, between which the water molecules do not exchange at the NMR observation time scale. We conclude that those two water phases are located at differently-accessible adsorption sites, here assigned to the cellulose surfaces between and within the microfibril aggregates, respectively. The superior performance of 2H MAS NMR encourages further applications of the same method to other complex systems that expose heterogeneous hygroscopic surfaces, like wood cell walls.

Effect of fiber orientation in dynamic FTIR study on native cellulose.

The properties of cellulose materials are highly dependent on the interactions between and within the cellulose chains mainly related to inter- and intramolecular hydrogen bonds. To investigate the deformation behavior of cellulose and its relation to molecular straining, cellulose sheets with different fiber orientations were studied by dynamic FTIR spectroscopy. The sheets were stretched sinusoidally at low strains while being irradiated with polarized infrared light. It is shown that the polarization direction determines the dynamic IR response to a higher extent than the fiber direction in the sample sheets. Different polarization modes give different dynamic signals, allowing conclusions to be drawn on the structural orientation of submolecular groups in the cellulose molecules. The bands in the spectra mainly affected by the deformation of the sheets were derived from skeletal vibrations that include the C-O-C bridge connecting adjacent rings and from the hydrogen bonds. The conclusion that these groups are the ones that are mainly deformed under load has thereby experimentally demonstrated the theoretical calculations from Tashiro and Kobayashi [Tashiro, K.; Kobayashi, M. Polymer 1991, 32, 1516-1526].