Comparative study on kink formation and properties of nanocellulose prepared by different acid hydrolyses of H2SO4 and HCl

Cellulose is the most abundant carbohydrate polymer. Kink formation during acid hydrolysis critically affects the structural and functional properties of nanocellulose, but its mechanism is still unclear. In this study, microcrystalline cellulose was hydrolyzed using hydrochloric acid (2, 3, 4 mol/L) or sulfuric acid (40, 45, 50 wt%) at 50 °C, followed by centrifugation, dialysis and homogenization to prepare nanocellulose. Both acid type and concentration significantly influenced kink formation. Atomic force microscopy showed that nanocellulose from hydrochloric acid hydrolysis (CNCH) had longer, wider fibers with higher number of kinks (N K ), while sulfuric acid-derived nanocellulose (CNCS) exhibited tighter chain packing, higher crystallinity, and fewer kinks. Amorphous regions were more prone to deformation and preferentially formed kinks to release stress. The lower amorphous content in CNCS resulted in lower kink density. Higher acid concentration increased crystallinity and reduced N K within a certain concentration range. The N K is one of the potential factors that contribute to the regulation of the properties of nanocellulose films. This study proposes a preliminary chemical–mechanical synergistic hypothesis to interpret the possible origins of kink formation, which can provide theoretical references for tailoring kink structures by modulating acid hydrolysis conditions.