The molecular byproduct inevitably contained in oxidized cellulose nanofibers (CNFs)/water dispersions was identified in this study to amorphous and water-soluble oxidized cellulose molecules with the major skeleton of glucose/glucuronate alternating co-polysaccharides. These oxidized molecules also possessed short segments of polyglucuronate, and the molar ratio of major glucose/glucuronate skeleton to the polyglucuronate segment by the number of monomer units was approximately 4:1 to 2:1 depending on the degree of oxidation (DO) of cell wall cellulose. Their weight-average molecular weights were approximately 8000‒15000. The amount of the byproduct reached as high as 16% when the cell wall cellulose with DO = 0.31 was disintegrated by sonication. Meanwhile, the byproduct amount stayed at approximately 4% when the cellulose with DO = 0.24 was disintegrated in the same manner.
We found that the molecular-chain length distributions of this byproduct were in good agreement with the length distributions of the dent defects formed on CNF surfaces. We thus conclude that the oxidized surface molecules of CNFs were stripped off as the byproduct in the disintegration process of cell wall cellulose, and such peeled parts on the surfaces were recognized as the dent defects of CNFs.
The generation of glucose/glucuronate alternating co-polysaccharides in the disintegration of oxidized cell wall cellulose relies on its crystallite structure; in a single crystallite, cellulose molecules having a two-fold helix structure are uniaxially oriented, so that the ones of every two glucose units, with the exocyclic C6 hydroxymethyl groups pointing outside of the crystallite, were selectively converted to the glucuronate units by the oxidation in this study. The implication in this study is thus that chemical functionalization of the crystallites in cell wall cellulose utilizing the two-fold helix structure of surface molecules will lead to the novel production of functional polymers that cannot be artificially synthesized.