We have so far found that the C6-hydroxyl groups exposed on the microfibril surfaces can be regioselectively oxidized to carboxylates by TEMPO-mediated oxidation. In this study, we develop a method to peel the surface molecules of the TEMPO-oxidized native cellulose fibrils in a layer-by-layer manner, and the obtained results are discussed in terms of crystalline structures of native cellulose microfibrils. Purified Cladphora sp., cotton lint and softwood bleached kraft pulp were oxidized using the TEMPO/NaClO/NaBr system with NaClO of 3.8 or 10 mmol per gram of cellulose. A solution of 2 M aqueous sodium hydroxide (200 mL) was slowly poured into the 0.2% TEMPO-oxidized cellulose suspension (200 mL) including sodium borohydride (0.01 g). The mixed suspensions were kept at 105 deg C for 12 h after bubbling N2 gas for 10 min. After washed with water by centrifugation, the water-insoluble residues were oxidized again and treated under the same alkali conditions. All the samples were freeze-dried and analyzed by electric conductivity titration, X-ray diffraction and 13C solid-state nuclear magnetic resonance (NMR). The layer-by-layer peeling of the surface molecules of the cellulose microfibrils was achieved by the successive process of the TEMPO-mediated oxidation and hot-alkaline extraction of the surface-oxidized molecules. 13C Solid-state NMR analysis suggested that the C6-hydroxyl groups facing outside of the surface chains are in the gg conformation, while those facing inner side of the surface molecules are in the gt conformation. In contrast, the C6-hydroxyl groups in the core regions are all in the tg conformation. When this peeling method was applied to low crystalline cellulose microfibrils from cotton lint and softwood bleached kraft pulp, these thin microfibrils seemed to have somewhat disordered molecules of two layers from the surfaces, which was in contrast with highly crystalline microfibrils from Cladophora sp.

Structural analysis of the cellulose microfibrils from algae and higher plants via layer-by-layer peeling of their surface molecules

251st National Meeting (2016)

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