Relationship of Distribution of Carboxy Groups to Molar Mass Distribution of TEMPO-Oxidized Algal, Cotton, and Wood Cellulose Nanofibrils.

Distributions of carboxy groups among the molecules in 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCNs) prepared from wood, cotton, and algal celluloses were investigated. Most C6-carboxy groups in TOCNs were esterified with anthracene-methyl (-CH2C14H9) groups, showing an ultraviolet light (UV) absorption peak at 365 nm. The anthracene-methylated TOCNs were dissolved in 8% (w/w) lithium chloride/N,N-dimethylacetamide (LiCl/DMAc). After dilution to 1% LiCl/DMAc, the solutions were subjected to size-exclusion chromatography with multiangle laser-light scattering, refractive index, and UV detection. For algal TOCN, 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, wood and cotton TOCNs had almost homogeneous distributions of C6-carboxy groups in all molar mass regions, which could not be explained in terms of the simple core-clad cellulose chain packing structures.

Molar Masses and Molar Mass Distributions of Chitin and Acid-Hydrolyzed Chitin.

Never-dried and dried crab shell chitin and squid pen chitin samples were acid-hydrolyzed in 1 M HCl at 85 °C for up to 2 h. The crystallinities, crystal sizes, and degrees of N-acetylation of the acid-hydrolyzed chitin samples are almost unchanged the same before and after acid hydrolysis. The original and acid-hydrolyzed chitin samples were dissolved in 8% (w/w) lithium chloride/N,N,-dimethylacetamide, and the solutions were subjected to size-exclusion chromatography with multiangle laser-light scattering analysis to determine their molar masses and molar mass distributions. The molar mass of each chitin sample decreases with increasing acid hydrolysis time, and the weight-average degree of polymerization (DPw) becomes constant after acid hydrolysis for 0.5 to 2 h. However, the DPw values of the chitin samples after acid hydrolysis for 2 h (DPw-2h) are different: Never-dried squid pen chitin has the highest DPw-2h of 1530, whereas the DPw-2h values of other chitin samples are in the range 220-410.

Different Conformations of Surface Cellulose Molecules in Native Cellulose Microfibrils Revealed by Layer-by-Layer Peeling.

Layer-by-layer peeling of surface molecules of native cellulose microfibrils was performed using a repeated sequential process of 2,2,6,6-tetramethylpiperidine-1-oxyl radical-mediated oxidation followed by hot alkali extraction. Both highly crystalline algal and tunicate celluloses and low-crystalline cotton and wood celluloses were investigated. Initially, the C6-hydroxy groups of the outermost surface molecules of each algal cellulose microfibril facing the exterior had the gauche-gauche (gg) conformation, whereas those facing the interior had the gauche-trans (gt) conformation. All the other C6-hydroxy groups of the cellulose molecules inside the microfibrils contributing to crystalline cellulose I had the trans-gauche (tg) conformation. After surface peeling, the originally second-layer molecules from the microfibril surface became the outermost surface molecules, and the original tg conformation changed to gg and gt conformations. The plant cellulose microfibrils likely had disordered structures for both the outermost surface and second-layer molecules, as demonstrated using the same layer-by-layer peeling technique.

Changes in the degree of polymerization of wood celluloses during dilute acid hydrolysis and TEMPO-mediated oxidation: Formation mechanism of disordered regions along each cellulose microfibril.

Most commercially available plant celluloses, such as kraft pulps and cotton celluloses, have so-called leveling-off degrees of polymerization (LODPs) when subjected to dilute acid hydrolysis. The formation of LODPs is hypothesized to be caused by disordered regions that are present periodically along each cellulose microfibril in plant celluloses. Here, we prepared never-dried wood cellulose, and wood celluloses at different drying stages, and subjected them to dilute acid hydrolysis. The viscosity average degrees of polymerization (DPv) of the wood celluloses decreased in DPv with increasing dilute acid-hydrolysis times. However, the DPv values after 4h of acid hydrolysis differed from those of softwood bleached kraft pulp (SBKP), which showed typical patterns of LODPs. Thus, the disordered regions corresponding to LODPs that were observed for SBKP are probably not synthesized in the native wood. Instead, such disordered regions are formed secondarily or artificially during the isolation/purification and/or drying processes of plant cellulose fibers. The results of the dilute acid hydrolysis and the 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation of SBKP and softwood unbleached soda-anthraquinone pulp showed that the structures of disordered regions can be controlled via the preparation and drying conditions of wood cellulose used as starting materials.