New insights into nano-crystalline cellulose structure and morphology based on solid-state NMR

Abstract

The phase structure of nanocrystalline cellulose (NCC) is studied with solid-state NMR experiments on NCC samples subjected to 2H/1H exchange in order to label regions accessible to water. These novel results show partial but similar exchange in portions of the 13C spectrum previously assigned to both highly crystalline and disordered/surface cellulose. Exchange in liquid D2O occurs rapidly, faster than the time-scale required to dry the samples, while exposure to D2O vapor reveals a single time constant of ∼30 h to build up to the liquid exposed exchange level. In both cases ca. 37 % of the expected exchangeable sites are found to contain 2H. These results, along with those from relaxation experiments requiring multi-component or stretched-exponential fits, together with X-ray diffraction results suggest the NCC particles contain distributions of environments distinguished by their structures and crystalline perfection. The incomplete exchange of the disordered/surface regions suggests that the NCC crystallites contain abundant disordered domains in their interiors that are protected from exchange. Preliminary 31P/13C REDOR experiments on phosphoric-acid hydrolyzed NCC samples provide a 13C spectrum of accessible surface sites, but are of relatively low signal-to-noise. These experiments suggest a phosphate coverage of 2.6 per 100 anhydroglucose monomers, similar to sulfate coverage measured previously for sulfuric acid hydrolyzed samples.