1H and 13C solid-state NMR of Gossypium barbadense (Pima) cotton

The interaction of water with cellulose and its influence on the nuclear spin dynamics in Gossypium barbadense (Pima) cotton were investigated by 1H and 13C solid-state NMR techniques. 1H spin diffusion results from a Goldman-Shen experiment indicate that the water is multilayered. 1H MAS experiments provide evidence of a range of correlation times for the water, indicative of molecular motion ranging from restricted to relatively mobile. The 1H spin-lattice relaxation time varies with water content and is different for static and MAS conditions. By coupling the Goldman-Shen sequence with 13C CP/MAS, cross-polarization from the molecularly mobile water protons distributes magnetization throughout the cellulose (as opposed to enhancing 13C resonances from only the crystalline or the amorphous domains or from only the surface of the cellulose). However, spatial localization of the combined Goldman-Shen-13C CP/MAS experiment using both short mixing and contact times yields a spectrum consistent with predominantly the Iβ polymorph of cellulose. Longer mixing times and the same, short contact time yield a spectrum that is indicative of an increased Iα polymorph content in the crystallite interiors relative to the smaller values found with short mixing times.