Molecular insights into reversible and irreversible kinks formed in nanocellulose

Kink defects in nanocellulose are common, yet questions remain open regarding the unclear microstructure-mechanical property relationship. Various kink patterns without molecular-scale resolution cause confusion about how nanocellulose forms different kinks and what the fundamental mechanisms of reversible and irreversible kinks are. In atomic force microscopy images, bent nanofibrils usually exhibit small curvatures, while kinked nanofibrils feature sharp bends, in which kinks are notable due to their distinct disordered configurations. To identify the incipient kink defects formed in nanocellulose, molecular dynamics simulations of cellulose nanocrystals (CNCs) under curvature-dependent bending were subsequently carried out. Five typical bending/kinking modes were found, depending on the anisotropic microstructure and size of CNCs. More importantly, two contrasting cases of kinks were demonstrated, providing evidence that kink defects in nanocellulose depend mainly on the microstructure at the molecular scale. Kinks in CNCs with the (100) and (11¯ 0) crystal plane are recoverable with a few residual defects. While kinks in CNCs with the (010) crystal plane are irreversible with permanent microstructural damage. Compressive stresses accumulated in the bottom chains of CNC contribute to the main mechanism for forming incipient kinks in nanocellulose. The results can fundamentally answer the confusion in recent experiments why bond breakage did not necessarily occur even at high kink angles. The insights present intrinsic deformation mechanisms for understanding the widespread but mysterious kinks arising in nanocellulose.