Zhaoxu Meng

Designing multi-layer graphene-based assemblies for enhanced toughness in nacre-inspired nanocomposites

Polymers reinforced with multi-layer graphene (MLG) phases are promising candidates for new materials with high modulus, strength and toughness. Drawing inspiration from nacres layered “brick and mortar” structure, here we propose molecular scale design strategies to improve the mechanical performance of MLG–polymer layer-by-layer nanocomposites. We present a coarse-grained molecular dynamics (CG-MD) study of interfacial failure mechanisms of MLG domains embedded in a poly(methyl methacrylate) (PMMA) matrix through pull-out simulations. Our simulations reveal two distinct deformation and failure mechanisms that greatly influence the toughness and energy dissipation of the system: pull-out failure, which occurs along the MLG–PMMA interface, and yielding failure, which occurs within the graphitic phase through the sliding of staggered graphene sheets. For any length of the graphitic assembly, the energy dissipated per layer from MLG yielding is greater than that of MLG pull-out. Theoretical continuum analysis further reveals that there exists a critical number of layers of graphene, beyond which the failure mode changes from yielding to pull-out. Our modeling framework provides effective strategies to design graphene–polymer layered nanocomposites with optimal toughness, and advance the mechanical performance of nanomaterials.

Optimizing the mechanical properties of cellulose nanopaper through surface energy and critical length scale considerations

Cellulose nanopaper exhibits outstanding stiffness, strength, and toughness that originate from the exceptional properties of constituent cellulose nanocrystals (CNCs). However, it remains challenging to link the nanoscale properties of rod-like CNCs and their structural arrangements to the macroscale performance of nanopaper in a predictive manner. Here we address this need by establishing an atomistically informed coarse-grained model for CNCs via a strain energy conservation paradigm and simulating CNC nanopaper properties mesoscopically. We predict how the mechanical properties of CNC nanopaper with nacre-inspired brick-and-mortar structure depend on CNC overlap length and interfacial energy. We show that the modulus and strength both increase with increasing overlap length, but saturate at different critical length scales where a transition from non-covalent interfacial sliding to CNCs fracture is the key influencing mechanism. Maximum toughness is achieved when the interface and CNC failure are tuned to occur at the same time through balanced failure. We propose strategies for maximizing nanopaper mechanical performance by tuning interfacial interactions of constitutive CNCs through surface modifications that improve shear transfer capability. Our model generates broadly applicable insights into factors governing the performance of self-assembling paper materials made from 1D nanostructures.