Molecular Mechanism Underlying the Crystallinity Changes of Cellulose upon Fibrillation and Reassembly Revealed via Two-Dimensional Solid-State NMR

The crystallinity of cellulose decreases as the cell wall structure of pulp fibrillates to cellulose nanofibers (CNFs) in water. The decreased crystallinity is partially recovered when the CNFs are reassembled into bulk structures through dehydration. We analyzed the molecular mechanism that underlies these changes in crystallinity via two-dimensional ¹³C-¹³C solid-state nuclear magnetic resonance (NMR) spectroscopy. A cellulose sample was extracted from a ¹³C-labeled Norway spruce (<i>Picea abies</i>) and then subjected to 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidation. This oxidation aims to regioselectively convert the C6 hydroxymethyl groups exposed on microfibril surfaces to carboxylate groups, which enables these oxidized glucuronate residues to function as surface markers. The C4 carbons within the oxidized residues remained noncrystalline through both fibrillation and reassembly. Other internal carbons of nonoxidized glucose residues underwent reversible and coordinated transitions between the crystalline and noncrystalline states, accounting for the changes in crystallinity detected via NMR. The presence of two surface environments, i.e., interfibril interfaces and air-exposed surfaces, was also suggested.