Kink formation in nanocellulose: Comparing TEMPO oxidation and carboxymethylation

• Formation of more kinks by carboxymethylation than TEMPO oxidation. • TEMPO oxidation is prone to break cellulose chains to produce less kinks. • Carboxyl methylation induces bending in amorphous region to create more kinks. • Milder effect of carboxymethylation due to less yet larger carboxymethyl groups. • Negative correlation of kink number to film tensile strength and transparency. This study investigated the kink formation mechanisms in nanocellulose induced by two typical carboxylation methods. TEMPO-mediated oxidation significantly made the nanocellulose chains more water-attracting and easier to dissolve, especially in the less organized areas. The introduction of carboxyl groups made the nanocellulose more susceptible to breakage during subsequent homogenization processes, resulting in shorter and more rigid nanocellulose with fewer kinks. In contrast, the carboxymethyl groups introduced through carboxymethylation exerted a milder effect on the nanocellulose chains. The cellulose microfibrils were more likely bend in the amorphous region, leading to an increase in the formation of kinks along the long and twisted nanocellulose. For TEMPO-mediated oxidation, the number of kinks decreased from 3.23 ± 1.87 to 1.70 ± 0.77 following oxidative gradient treatment, whereas carboxymethylation exhibited an increase in kinks from 4.71 ± 2.97 to 5.52 ± 3.02. These quantitative changes exhibited a negative correlation with the tensile strength and optical transparency of the nanocellulose films. Simultaneously, the thermal stability and contact angles of the films were predominantly governed by the dimensional characteristics and surface properties of the nanocellulose, respectively. This study elucidated how different carboxylation methods affect kink formation in nanocellulose, providing novel insights for enhancing nanocellulose preparation and efficacy.