Mechanical Properties of Graphene Origami and Its Reinforced Polyethylene Nanocomposites

<p dir="ltr">Polymeric nanocomposites reinforced with two-dimensional (2D) nanosheet fillers, such as graphene, exhibit excellent mechanical properties for load bearing and impact-resistant applications due to the remarkable properties of graphene. However, graphene’s intrinsic 2D structure leads to brittle fracture under external loading and significantly limits its reinforcement effectiveness in polymeric nanocomposites due to low interfacial load transfer efficiency. To address this challenge, this study focuses on the three-dimensional (3D) graphene origami (GOri) structure, to uncover its high flexibility under nanoindentation, reveal its evolution patterns under impact loading, elucidate the strengthening mechanism in polyethylene (PE) nanocomposites, and explore design strategies for GOri-based composites. </p><p dir="ltr">The specific research works are as follows: </p><p dir="ltr">(1) The high flexibility of GOri was significantly enhanced under nanoindentation by introducing the origami pattern. The results show that GOri sustains significantly higher indentation loads at larger depths than pristine graphene, with further improvements achieved in bilayer formats. The deformation capacity and strength of GOri can be effectively tuned by origami morphology, surface roughness, and stacking configurations, while maintaining its intrinsic stiffness. In addition, the auxetic characteristics of GOri enhance bending stiffness without compromising strength. These findings highlight the potential of GOri as a promising candidate for graphene-based impact protection applications and provide practical guidelines for the design of advanced protective systems. </p><p dir="ltr">(2) In addition to static mechanical evaluation, the dynamic performance of GOri was also assessed under high-velocity ballistic impact. The results show that the threshold penetration velocity of GOri is significantly improved by 46.27% compared with pristine graphene. Depending on the initial impact velocity, three distinct penetration phenomena (i.e. bounce-back, capture, and perforation) are observed. At low impact velocities, GOri undergoes a unique unfolding process that increases the buffer distance and enhances impact resistance. The impact performance and energy absorption capacity of GOri can be effectively tuned by its morphology, surface roughness, as well as by impact velocity, projectile radius, and layer number. The investigation on the perforation behaviors and energy absorption of GOri offers useful guidelines for graphene applications in impact protection. </p><p dir="ltr">(3) When 2D graphene transforms into the 3D origami structure of GOri, the resultant GOri/PE nanocomposite exhibits higher interfacial shear strength (IFSS), owing to the larger surface roughness of GOri and the associated stronger van der Waals (vdWs) interactions. The IFSS can be tuned by GOri morphology, with the roughest GOri achieving improvements of 680.5% and 551.8% along the zigzag and armchair directions, respectively, and showing sensitivity to matrix density but little dependence on polymerization degree. Beyond interfacial enhancement, GOri nanofillers endow PE nanocomposites with ultrahigh flexibility and a negative Poisson’s ratio (NPR), providing them with superior energy absorption and impact resistance for protective application. The unique combination of high IFSS, high flexibility, and intrinsic auxeticity makes GOri/PE nanocomposites an ideal candidate for many practical applications, such as impact protection, flexible electronics, and soft robotics.</p><p dir="ltr">(4) Moreover, the out-of-plane impact resistance performance of GOri/PE nanocomposites under static and dynamic impact conditions was investigated. The results show that GOri can significantly enhance the fracture strength and specific penetration resistance of the nanocomposites, which is attributable to the strengthening effect induced by the rough surface of GOri. This roughness-induced improvement over conventional graphene can be further tuned by increasing the roughness of GOri. In addition, the presence of GOri restricts matrix diffusion during penetration, thereby stabilizing the nanocomposite under dynamic loading. These findings highlight the potential of GOri/PE nanocomposites as lightweight, high-performance protective materials and provide key insights into the nanoscale mechanisms governing impact resistance in polymer-based systems. </p><p dir="ltr">This thesis systematically investigates the mechanical performance of GOri and GOri/PE nanocomposites using molecular dynamics (MD) simulations. The results show that the origami structure of graphene enhances its flexibility, IFSS, and impact resistance, while maintaining its intrinsic stiffness. GOri’s tunable morphology, roughness, and auxetic characteristics contribute to significant improvements in fracture strength, penetration resistance, and energy absorption under both static and dynamic loading. Furthermore, GOri restricts matrix diffusion during penetration, enhancing nanocomposite stability. These insights advance the understanding of nanoscale reinforcement mechanisms and provide effective guidelines for designing next-generation lightweight, high-performance polymer nanocomposites for protective and multifunctional applications.</p>