Structures, molar mass distributions, and morphologies of TEMPO-oxidized bacterial cellulose fibrils

2,2,6,6-Tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation has been applied to bacterial cellulose (BC). The TEMPO-oxidized BC (TO-BC) gel particles were subjected to ultrasonication in water to prepare mechanically fibrillated TO-BC (TO-BC-U) samples. The carboxy contents of the TO-BC samples were 1.5‒1.6 mmol/g. X-ray diffraction patterns and solid-state 13 C-nuclear magnetic resonance (NMR) spectra of the BC, TO-BC, and TO-BC-U samples showed that cellulose I α was the dominant crystalline structure. The crystallinities of the samples calculated from the carbon signal areas in the NMR spectra were approximately the same between the BC and TO-BC samples, showing that TEMPO-mediated oxidation selectively occurred on the crystalline BC fibril surfaces. However, the crystallinities of the TO-BC-U samples were lower than those of the BC and TO-BC samples, indicating that ultrasonication of the TO-BC samples in water caused partial decreases in crystallinity. The TO-BC-U samples contained both single fibrils and fibril bundles; completely individualized TO-BC-U fibrils with homogeneous widths was not obtained. The average widths of the single TO-BC-U fibrils were ~ 3 nm, which are close to those of TO-cellulose nanofibrils prepared from wood-cellulose samples. Thus, the crystalline BC fibrils with widths of ~ 3 nm were the smallest crystalline elements. The lengths of the TO-BC samples were greater than 2‒3 µm, whereas the weight-average cellulose chain lengths of the cellulose/TEMPO-oxidized cellulose molecules in TO-BC-U samples were < 800 nm. Hence, each TO-BC-U fibril consisted of multiple cellulose and oxidized cellulose molecules, which were packed along the longitudinal direction. Graphical abstract