Dimensions of biological cellulose nanocrystals maximize fracture strength

Cellulose nanocrystals (CNCs) exhibit outstanding mechanical properties exceeding that of Kevlar, serving as reinforcing domains in nature's toughest biological nanocomposites such as wood. To establish a molecular-level understanding of how CNCs develop high resistance to failure, here we present new analyses based on atomistic simulations on the fracture energy of Ibeta CNCs. We show that the fracture energy depends on the crystal width, due to edge defects that significantly reduce the fracture energy of small crystals but have a negligible effect beyond a critical width. Additionally, collective effects of sheet stacking and stabilization by van der Waals interactions saturate at a critical crystal thickness that we predict with an analytical relationship based on a physical model. Remarkably, ideal dimensions optimizing fracture energy are found to be 4.8-5.6 nm in thickness (approximately 6-7 layers) and 6.2-7.3 nm in width (approximately 6-7 cellulose chains), which correspond to the common dimensions of CNCs found in nature. Our studies shed light on evolutionary principles that provide guidance toward high mechanical performance in natural and synthetic nanobiocomposites.