Impact resistance of nanocellulose films with bioinspired Bouligand microstructures

Abstract

The Bouligand structure features a helicoidal (twisted plywood) layup of fibers that are uniaxially arranged in-plane and is a hallmark of biomaterials that exhibit outstanding impact resistance. Despite its performance advantage, the underlying mechanisms for its outstanding impact resistance remain poorly understood, posing challenges for optimizing the design and development of bio-inspired materials with Bouligand microstructures. Interestingly, many bio-sourced nanomaterials, such as cellulose nanocrystals (CNCs), readily self-assemble into helicoidal thin films with inter-layer (pitch) angles tunable via solvent processing. Taking CNC films as a model Bouligand system, we present atomisticallyinformed coarse-grained molecular dynamics simulations to measure the ballistic performance of thin films with helicoidally assembled nanocrystals by subjecting them to loading similar to laser-induced projectile impact tests. The effect of pitch angle on the impact performance of CNC films was quantified in the context of their specific ballistic limit velocity and energy absorption. Bouligand structures with low pitch angles (18-42 ) were found to display the highest ballistic resistance, significantly outperforming other pitch angle and quasi-isotropic baseline structures. Improved energy dissipation through greater interfacial sliding, larger in-plane crack openings, and through-thickness twisting cracks resulted in improved impact performance of optimal pitch angle Bouligand CNC films. Intriguingly, decreasing interfacial interactions enhanced the impact performance by readily admitting dissipative inter-fibril and inter-layer sliding events without severe fibril fragmentation. This work helps reveal structural and chemical factors that govern the optimal mechanical design of Bouligand microstructures made from high aspect ratio nanocrystals, paving the way for sustainable, impact resistant, and multifunctional films.

pH‐Controlled network formation in a mixture of oppositely charged cellulose nanocrystals and poly(allylamine)

“…As an example of such rods with negative surface charges, cellulose nanocrystals (CNCs) have recently attracted great interest due to their environmental friendly and abundant nature, biocompatibility and biodegradability, and their ability to self‐assemble into liquid crystalline order, combined with outstanding mechanical properties that make them attractive candidates as fillers in composites . CNC rods consist of parallel‐packed cellulose molecules held together by hydrogen bonds and hydrophobic interactions and thus can be considered as amphiphilic .…”

Microstructure and Size Effects on the Mechanics of Two Dimensional, High Aspect Ratio Nanoparticle Assemblies

“…The two dimensional, neat CNC films used in this study were constructed using a previously validated CG model (Qin et al, 2017). Briefly, this CG framework is a bead-spring, mesoscopic model of CNCs that is characteristic of cellulose I with a Iβ crystal structure.…”

“…Detailed atomistic mechanical assessments (uniaxial extension, three point bending, etc.) were performed to inform the CG force field parameters following the strain energy conservation principle (Qin et al, 2017). Intra-CNC bonds with local strains >5% engineering strain were deemed failed and removed from the simulation (Wu et al, 2014;Qin et al, 2017).…”

“…were performed to inform the CG force field parameters following the strain energy conservation principle (Qin et al, 2017). Intra-CNC bonds with local strains >5% engineering strain were deemed failed and removed from the simulation (Wu et al, 2014;Qin et al, 2017). More information on the specifics of the CG model form, its validation, or potential applications can be found in previous work (Qin et al, 2017(Qin et al, , 2019Natarajan et al, 2018).…”

“…Between atomistic MD and continuum models lie mesoscopic, or coarse-gained (CG), MD models. These models have been highly successful at describing the mechanical properties of all CNC materials, both during quasi-static (Qin et al, 2017;Natarajan et al, 2018) and dynamic (Qin et al, 2019) deformation processes. Furthermore, MD simulations do not require explicit failure criteria and can capture discrete and non-affine deformation events, like bond breakage and molecular sliding.…”