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 4 h 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.