Topochemistry of Carboxylated Cellulose Nanocrystals Resulting from TEMPO-Mediated Oxidation

Abstract

Surface carboxylated cellulose nanocrystals with different sizes and degrees of oxidation were prepared by TEMPO-mediated oxidation of cotton linters and microfibrils of parenchyma cell cellulose (PCC). The size of the oxidized crystals depended on (i) the starting material, (ii) an eventual acid prehydrolysis, and (iii) the oxidation conditions. The oxidized cellulose nanocrystals were characterized by transmission electron microscopy, conductometric titration, and solid-state NMR spectroscopy. During TEMPO oxidation, the main reaction corresponded to a selective oxidation of surface primary hydroxyl groups into carboxylic groups. At the same time, a decrease of the crystal size occurred, resulting from some degradation in the amorphous areas of the starting material. The introduction of negative charges at the interface of the crystalline domains induced a better individualization of the crystallites. The degrees of oxidation (DO) determined by conductometric titration were in agreement with those deduced from solid-state NMR data. The DO values reached 0.4 and 0.24 for PCC microfibrils and cotton linters, respectively. In the case of HCl-hydrolyzed samples, these values reached 0.23 for PCC microfibrils and 0.15 for cotton linters. When dispersed in water, these carboxylated cellulose crystallites led to birefringent suspensions that did not flocculate nor sediment, due to their polyelectrolyte character created by the presence of surface negative charges.

TEMPO-mediated surface oxidation of cellulose whiskers

“…The use of this technique has been attracting a number of investigations since the first reports of De Nooy et al (1994), which showed that only the hydroxymethyl groups of polysaccharides were oxidized, whereas the secondary hydroxyls remained unaffected. For cellulose, the TEMPO-mediated oxidation was applied to different cellulose samples, ranging from cotton linters, to wood pulp, parenchyma cell cellulose, rayon and cellulose III (Isogai and Kato 1998;Tahiri and Vignon 2000;Araki et al 2001;da Silva Perez et al 2003;Saito and Isogai 2004;Montanari et al 2005;Saito et al 2005Saito et al , 2006. These studies have led to the preparation of a series of products, ranging from water-soluble polyuronic acid to partially derivatized cellulose products.…”

Electric Interfacial Layer of Modified Cellulose Nanocrystals in Aqueous Electrolyte Solution: Predictions by the Molecular Theory of Solvation

“…15,16 Large-scale application of CNC as a reinforcing filler in composite materials requires modifications to tune up its surface properties, thus enhancing its compatibility with polymer matrices and solubility in nonpolar solvents. 17 Several methods for CNC surface modification have been developed, including grafting, 18,19 polymer self-assembly, and surfactants. 20 Suspensions of cellulose nanocrystals display a phase separation into clear isotropic and structured chiral nematic phases above a critical concentration, 21 and upon drying form iridescent chiral nematic films.…”

Bacterial Cellulose-based Biomimetic Composites

“…Surface modification was performed by TEMPO (2,2,6,6-tetramethylpyperidine-1-oxyl)-mediated oxidation to introduce carboxyl functional groups, which are known to be effective inducers of apatite nucleation. TEMPO, a watersoluble and stable nitroxyl radical, is well-known for its catalytic and selective oxidation of primary hydroxyl groups of polysaccharides under aqueous conditions (Saito & Isoagi, 2004;Saito & Isogai, 2005;Montanari et al, 2005). Culture medium modification was performed by the addition of an amino sugar (N-acetylglucosamine) in the culture medium normally used for BC production.…”