Atomistic Simulation of a Two-Dimensional Polymer Tougher Than Graphene
A graphene/polyethylene hybrid 2D polymer, “graphylene”, exhibits a higher theoretical fracture toughness than graphene, while remaining 2× stiffer and 9× stronger than Kevlar®, per mass. Within the base structure of graphylene, the sp3-bonded polyethylen
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Atomistic Simulation of a Two-Dimensional Polymer Tougher Than Graphene Emil Sandoz-Rosado, Todd D. Beaudet, Radhakrishnan Balu, and Eric D. Wetzel Abstract A graphene/polyethylene hybrid 2D polymer, “graphylene”, exhibits a higher theoretical fracture toughness than graphene, while remaining 2 stiffer and 9 stronger than Kevlar®, per mass. Within the base structure of graphylene, the sp3-bonded polyethylene linkages provide compliance for ductile fracture, while the benzene rings contribute to high stiffness and strength. Combining stiff and compliant units to achieve enhanced mechanical performance demonstrates the potential of designing 2D materials at the molecular level.
1.1
Introduction
The extraordinary in-plane stiffness and intrinsic strength of graphene [1] in its pristine state have made it a desirable candidate as a structural material. Chemical vapor deposition of large-area graphene has been refined [2] to the point that grain boundaries of graphene approach the breaking strength of perfect crystalline graphene [3], a phenomenon that has been supported by atomistic simulations [4]. Graphene has the theoretical potential to enable ballistic barriers that have 10–100 less weight than barriers composed of Kevlar with the same ballistic limit [5], and has also demonstrated a specific kinetic energy of penetration an order of magnitude greater than steel and 2–3 greater than Kevlar, as measured by microscale ballistic experiments [6]. However, because graphene is a network of very stiff sp2 bonds, it is highly resistant to fracture initiation but, once formed, a crack will propagate in a brittle manner [7, 8]. This brittle behavior may limit graphene’s potential as a structural engineering material, as local failure due to a flaw or stress concentration is likely to trigger a sudden and catastrophic global failure. To demonstrate a two-dimensional (2D) material with a more ductile fracture response compared to graphene, we propose a new family of 2D polymer which we refer to as “graphylene.” This 2D covalent polymer network can be conceptually described as a graphene/polyethylene hybrid comprising benzene rings linked by short polyethylene chains. These short polyethylene links give graphylene in-plane stiffness and strength values that are somewhat lower than graphene. However, we demonstrate that the flexibility of the sp3 bonded carbon atoms in the polyethylene chains leads to ductile fracture propagation behavior, with significantly higher energy required to propagate cracks relative to graphene. 2D polymers in the form of hexagonal carbon rings connected by linear carbon links have been recently described. Graphyne [9] and related allotropes [10] are composed strictly of extremely stiff carbon–carbon double and triple bonds, likely leading to brittle behavior. Graphane [11] adds single hydrogen bonds to each carbon atom in graphene, resulting in a hexagonal network of sp3 bonds. Studies have also examined carbon allotropes that are randomly hydrogen functionalized [12]. Stiffness and strength i
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