TEMPO-mediated oxidation has opened new chemistry field of cellulose science and technology. This is because C6-OH groups exposed on native cellulose microfibril surfaces are position-selectively oxidized to sodium C6-carboxylate groups in water at room temperature under moderate conditions. Osmotic effects and electrostatic repulsions efficiently work between the anionically charged TEMPO-oxidized cellulose microfibrils, resulting in complete individualization of TEMPO-oxidized cellulose nanofibrils by gentle mechanical disintegration in water. Therefore, individualized and isolated cellulose microfibrils without any aggregation are visualized as TEMPO-oxidized cellulose nanofibrils by TEM or AFM. In this study, distributions of carboxy groups among the molecules in TEMPO-oxidized cellulose nanofibrils prepared from wood, cotton, and algal celluloses were investigated. Most C6-carboxy groups in TEMPO-oxidized cellulose nanofibrils were esterified with anthracene-methyl groups, showing an UV absorption peak at 365 nm. The anthracene-methylated TEMPO-oxidized cellulose nanofibrils were dissolved in 8% LiCl/DMAC. After dilution to 1% LiCl/DMAc, the solutions were subjected to size-exclusion chromatography with multi-angle laser-light scattering, refractive index, and UV detection. For TEMPO-oxidized algal cellulose nanofibril, C6-carboxy group-rich molecules were present predominantly in the low-molar-mass region, which was consistent with the core-clad cellulose chain packing structures in individual algal cellulose microfibrils and partial depolymerization of the oxidized cellulose molecules. In contrast, TEMPO-oxidized wood and cotton cellulose nanofibrils had almost homogeneous distributions of C6-carboxy groups in all molar mass regions, indicating that each cellulose microfibril in higher plants consisted of interdigitated cellulose chain packing structures. Because formation of the interdigitated packing structures are possibly caused by the formation of periodically disordered regions present along the longitudinal direction of each cellulose microfibril, never-dried plant cellulose samples were used as starting materials for TEMPO-mediated oxidation, disintegration in water to prepare nanofibrils, and distribution analysis of C6-carboxy groups in the TEMPO-oxidized cellulose nanofibrils.

Microfibril structures based on distribution of carboxy groups in tempo-oxidized celluloses

ACS Spring Meeting 2021

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