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

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.