Multiscale Mechanics And Design Of Noncovalent Interface Layered Nanocomposites

Significant engineering applications and sustainable development demands pro-vide strategic opportunities and challenges for developing novel green and environment-friendly high-performance structural and functional nanocomposites.Such prominent characteristics of nanocomposites are mainly derived from the excellent properties of their nano-functional units,while the primary challenge is how to effectively transfer the outstanding nanoscale properties of their functional units to the macroscale.Addi-tionally,how to overcome the contradiction between strength and toughness to realize the strengthening-toughening design of nanocomposites has always been the pivotal scientific issue in the cross-field of mechanics and materials frontiers.Structural biological materials often exhibit outstanding mechanical properties and versatility due to their sophisticated hierarchical architectures with limited organizations ranging from the molecular to the macroscopic scale and fine-designed interfaces that join building bricks.In these hierarchical materials,weak interfaces operate in synergy with micro-architectures concurrently at multiple scales to govern inelastic deformation and toughening mechanisms,for structural biomaterials to generate unique and attrac-tive combinations of stiffness,strength,and toughness,encouraging the development of high-performance bioinspired composites.Noncovalent interfaces,a class of weak interfaces,can break and re-form dynamically during deformation,providing load trans-fer capability over long sliding distances and enabling large inelastic deformation at the interface.Therefore,it is a competent approach to incorporate noncovalent interfaces within materials to modulate toughening mechanisms and balance stiffness,strength,and toughness,thereby promising to address prime challenges and pivotal scientific is-sues in nanocomposites.This paper focuses on three critical scientific projects,i.e.,the multiscale mechanical framework of the noncovalent interface in nanocomposites,the mechanical optimization design based on interface regulation,and the interfacial mechanical behavior of two-dimensional materials.By extending interfacial constitutive relations condensed from various atomic in-teractions and distribution of interlayer functional groups,we proposed a multiscale analysis framework from the bottom-up for brick-interface systems.Due to the peri-odicity of interface constitutive relation,there are three deformation modes for regular interface under different overlapping lengths,i.e.,uniform,localization,and kink.Two critical transition length parameters were defined to describe the deformation trans-formation in noncovalent interfaces.The interfacial kink exhibits multiple topologi-cal defects nucleating and propagating across the interface,thereby strengthening and toughening nanocomposites simultaneously.We identified that the deformation of com-mensurate interface behaves similarly to the regular one through the discrete shear-lag analysis for different interfacial stacking configurations,while the linear-sliding model can well describe that of incommensurate and random interfaces.Interestingly,it found that the load transfer capability of a random interface exceeds that of a regular one as the overlapping length is sufficiently long because of resistance to sliding.Our theoretical predictions and mechanical framework were validated through large-scale molecular dynamics simulations.More importantly,using a few universal characteristic parame-ters,a deformation-mode phase diagram was proposed to give the landscape for hierar-chical materials under different noncovalent interfaces.We then investigated the optimal strategy of simultaneously strengthening and toughening for graphene-based nacre-like materials reinforced by strong noncovalent interfacial interactions.By coupling the interlayer sliding and structural stability,we modified the classical shear-lag model to characterize the toughening effect during the pullout process,demonstrating that the interfacial toughness and shear strength signifi-cantly impact the strength of graphene-based nacre-like materials,while the toughness is mainly dominated by the interlayer energy dissipation during the pullout process of graphene oxide(GO)sheets.Melamine molecule was chosen as the interlayer crosslink agent owing to the ultrastrong noncovalent interaction between melamine and GO.The melamine molecule bound to GO by anomalous hydrogen bonding can significantly improve the interfacial shear stress and maintain the interlayer energy-dissipation effi-ciency resulting from the breaking and reforming of hydrogen bonds.An optimal range of melamine content and GO oxidation degree was then explored for synchronously su-perior strength and toughness by balancing the competitions among the tensile strength of GO sheets,the interlayer shear strength,and the interfacial toughness.In particular,a scaling law was proposed as the evaluation criterion to correlate the internal inelas-tic deformation of graphene-based nacre-like materials and their mechanical proper-ties,revealing the advantages of interlayer noncovalent interaction compared with other crosslinks.Finally,we explored the mechanical behaviors and mechanisms of two-dimensional(2D)material assemblies under van der Waals interactions.Integrating the nonlinear shear-lag model and molecular dynamics simulations,we demonstrated that the classical shear-lag model could not quantificationally describe the in-plane de-formation of multilayer 2D material assemblies due to the edge effect induced by the interlayer van der Waals attraction.Then,a modified shear-lag model,accounting for the edge effect through two characteristic constants under the framework of the shear-lag model,was proposed to quantitatively reveal the respective contributions of the in-terlayer sliding,edge-effect shear stress,and the elasticity of 2D material platelet on the deformation of multilayer 2D material assemblies.Furthermore,the peeling and tearing of self-folded graphene from the substrate under external force and thermal activation were studied It found that the tearing taper angle of graphene follows a scaling law dif-ferent from the macroscale to the adhesion strength of the substrate due to the van der Waals effect of the substrate;under the thermal activation,self-folded graphene can peel and tear through the interlayer van der Waals interaction,then realized self-assembly on the substrate.In summary,this paper systematically investigated the multiscale mechanics and design of noncovalent interface layered nanocomposites,which not only shed new light on pivotal scientific issues in the cross-field of mechanics and materials frontiers but also provides guidelines for the design and optimization of advanced layered nanocom-posites toward engineering applications.