Ultrastructural Characterization of the Core–Shell Structure of a Wide Range of Periodate-Oxidized Cellulose from Different Native Sources by Solid-State ¹³C CP-MAS NMR
2018
Leguy, Julien | Nishiyama, Yoshiharu | Jean, Bruno | Heux, Laurent
The periodate oxidation of cellulose, which leads to the opening of the glucosyl rings to yield two aldehyde groups in the so-called dialdehyde cellulose, stands as one of the major and promising pathways for the derivatization of cellulose for the design of new biosourced materials. Despite the wide use of this method and the simplicity of its chemistry, the measurement of the degree of oxidation (DO) of the periodate-oxidized cellulose is not straightforward due to the concomitant side reactions and heterogeneity of the oxidized products. Here, we present a new simple method based on solid-state ¹³C CP-MAS NMR spectroscopy relying on the quantification of the intricate signals of the washed oxidized sample, using the sharp C1 signal of intact cellulose as an internal standard, allowing the determination of degrees of substitution from 0 to 2. This new NMR method, applied first to microfibrillated cellulose, is compared to the current conventional methods, namely, the UV absorbance monitoring of the periodate consumption and the oxime formation protocol. Agreement between the DONMR and DOUV values can be obtained if one assumes a full oxidation for the solubilized part of the product. Regarding the DO deduced from the oxime formation, it agrees with the present DONMR only if buffered conditions are used during the reaction to prevent hydrolysis. Solid-state NMR experiments also provided information on the heterogeneous character of the reaction in favor of a core–shell model of an oxidized cellulosic material skin surrounding an intact core of unmodified native cellulose. This method could be applied on a wide range of substrates with a different morphology and crystallinity.
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