Topochemistry of Carboxylated Cellulose Nanocrystals Resulting from TEMPO-Mediated Oxidation

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.