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Functionalization of Cellulose—Chemical Approach
Published in Narendra Pal Singh Chauhan, Functionalized Polymers, 2021
Merin Sara Thomas, Prasanth K.S. Pillai, Sabu Thomas, Laly A. Pothen
Surface modification of cellulosic materials by oxidation is a conventional technique, and the oxidized cellulose is widely used in agriculture, cosmetics and medicine (Coseri et al. 2013). It is highly suitable for making functional hemostatic scaffolding material that is bioresorbable and easily degradable under different physiological conditions. The hydroxyls present in the cellulosic structures are oxidized into the carboxylic acid or aldehyde functional groups, though for the carboxylic moiety, nitroxyl radicals-based oxidation is required, and for aldehyde, periodate oxidation is required (Isogai et al. 2018). Periodate oxidation selectively cleaves the vicinal diols, resulting in two aldehyde functionalities. This type of oxidation reaction cellulose suspension is performed using sodium metaperiodate (NaIO4) solution at elevated temperatures, which proceeds through the formation of a cyclic intermediate of diol and periodate anion to form aldehydes (Kim and Choi 2014, Syamani et al. 2018). 2,2,6,6-Tetramethylpiperidinyloxyl radical (TEMPO) oxidation of cellulose is an alternative promising route to convert surface hydroxyl of cellulose into charged carboxyl units. Several studies were reported based on TEMPO oxidation functionalization. This method selectively oxidizes the primary alcoholic group only; the secondary alcoholic group remains intact. Usually, sodium hypochlorite or sodium chlorite or sodium bromide is used as a secondary oxidant to recycle the TEMPO. Mechanism of periodate oxidation of cyclic 2,3-diols (Bunton and Shiner 1960)Catalytic oxidation mechanism of C6–OH groups of cellulose by TEMPO/NaBr/NaClO in water at pH 10 (Isogai et al. 2011)
Oxidized demethylated lignin as a bio-based adhesive for wood bonding
Published in The Journal of Adhesion, 2021
Xinyi Chen, Xuedong Xi, Antonio Pizzi, Emmanuel Fredon, Guanben Du, Christine Gerardin, Siham Amirou
Oxidation is a commonly used method in starch-based wood adhesives production.[33] Recently, the specific oxidation of plant flours has attracted the attention of some researchers as regards their effectiveness for adhesives application.[34,35] Periodate is a specific oxidant cleaving carbon to carbon bonds presenting vicinal aliphatic hydroxyl groups to form aldehydes and dialdehydes.[34–38] Often the structural units within lignin presents on their aliphatic part both hydroxyl groups vicinal to the β-O-4 bonds to another lignin unit. The β-O-4 linkage is easily cleaved by an oxidant so that aldehydes and possibly also dialdehydes are generated. In addition, the greater proportion of aromatic hydroxyl groups have generated by demethylation thereby increasing aromatic ring reactivity. Hence, the aldehydes generated by periodate oxidation are likely to react with hydroxyl-activated reactive phenolic sites contributing to form a stronger hardened network. Such an adhesive could be classified as a biobased product.
Enhancement of the mechanical properties of chitosan
Published in Journal of Biomaterials Science, Polymer Edition, 2020
Another approach to chemically cross-link CS and boost its mechanical stability without subsuming baleful chemicals involved utilizing oxidized polysaccharides. The oxidized polysaccharides are concocted via the reaction of natural polysaccharides with sodium periodate. Sodium periodate attacks adjacent hydroxyl moieties of glucose residues, and transforms them into aldehyde moieties. The aldehyde moieties of oxidized polysaccharides could then mediate chemical cross-linking with the CS’s amine moieties via the construction of Schiff’s base [14, 40, 41]. On one occasion, oxidized cellulose (O–C) was subsumed with CS. The inclusion of 5% O–C (relative to CS’s weight) into CS films escalated their tensile strength from 51.9 to 74.7 MPa and concurrently diminished their elongation percent. The authors regarded these alterations to the establishment of both the chemical Schiff’s base and also the physical hydrogen bonds. These linkages strengthened the CS film but limited the motion of its macromolecular chains causing it to elongate less [14]. Oxidized hyaluronic acid (O–HA) was also subsumed with CS at varying ratios of 10:1, 10:3 and 10:5 CS/O–HA. Escalating the amount of subsumed O-HA altered the Young’s modulus of the concocted hydrogels where values of 130.78, 199.35, and 181.47 kPa were recorded, respectively. This indicated that the stiffest hydrogel was the one concocted with 10CS:3O-HA. This gel exhibited the upmost cross-linking among CS and O-HA where 88.71% of the CS’s available amine moieties were cross-linked forming Schiff’s base (Figure 7). The copious amount of Schiff’s base (–CH = N–) stiffened the gel and escalated its Young’s modulus [41]. CS derivatives were also cross-linked with oxidized polysaccharides. N-Succinyl CS (N–S–CS) is a chemical CS’s derivative concocted through introducing succinyl moieties to a portion of the CS’s amine moieties. This adds carboxylic moieties to the CS’s construction and permits its solubility at diverse pHs. Oxidized alginate (O–Alg) was subsumed with N–S–CS. The established Schiff’s bases triggered the gelation of the N–S–CS/O–Alg mixture. Moreover, escalating the O-Alg concentration from 5 to 16% established more Schiff’s bases and further strengthened the gel [42].