Distribution of carboxy groups in TEMPO-oxidized cellulose nanofibrils prepared from never-dried Japanese cedar holocellulose, Japanese cedar-callus, and bacterial cellulose

Abstract

We prepared 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized samples from never-dried Japanese cedar (JC) holocellulose, JC-callus, and bacterial cellulose (BC). The original never-dried samples and their TEMPO-oxidized products were characterized by neutral sugar composition analysis. TEMPO-oxidized cellulose nanofibrils (TEMPO-CNFs) were prepared from the TEMPO-oxidized samples by ultrasonication in water. The carboxy groups in TEMPO-CNFs were position-selectively esterified with 9-anthryl diazomethane (ADAM) to prepare TEMPO-CNF-COOCH2-C14H9 samples, which had UV absorption peak at 365 nm. The mass-average degree of polymerization (DPw) values of 1% lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) solutions of the original samples were determined by size-exclusion chromatography in combination with multi-angle laser-light scattering, ultraviolet absorption, and refractive index detection (SEC/MALLS/UV/RI), and were 5490, 2660, and 2380 for the JC holocellulose, JC-callus, and BC samples, respectively. The TEMPO-CNF-COOCH2-C14H9 sample solutions in 1% LiCl/DMAc were analyzed by SEC/MALLS/UV/RI to obtain SEC elution patterns. The patterns corresponded to the molar mass and carboxy group distributions of the samples, which were detected by RI and UV absorption of anthryl groups, respectively. The carboxy groups existed in the entire molar mass distribution regions of all the TEMPO-CNF samples, although their lower molar mass regions contained higher carboxy group densities. The obtained results indicate that random depolymerization occurred on the cellulose microfibril surfaces at the initial stage of TEMPO-catalyzed oxidation and/or ultrasonication in water. This depolymerization mechanism can explain all the obtained SEC-elution patterns of the TEMPO-CNFs, without considering the presence of periodically disordered regions in the cellulose microfibrils of the never-dried cellulose samples.Graphical abstract