Sequentially bridged graphene sheets with high strength, toughness, and electrical conductivity

Significance

There is a continuing search for manufacturable sheets having high strength and toughness in all sheet directions for diverse applications, from airplanes to windmills. Cross-plied carbon fibers in a polymer resin requiring high-temperature cure presently provide the common solution. We demonstrate cross-linked graphene sheets that are manufacturable from graphene platelets, which are resin-free, processable at low temperature, contain less than 10 wt % additives, and provide high strength and record toughness in all in-plane directions. This advance results from successive use of π–π and covalent cross-linking agents. Simultaneous enhancement of strength, durability, and electrical conductivity are demonstrated. Spectroscopic measurements, including Raman studies of interplatelet stress transfer, elucidate the chemical nature and physical consequences of these dual cross-linking agents.

Abstract

We here show that infiltrated bridging agents can convert inexpensively fabricated graphene platelet sheets into high-performance materials, thereby avoiding the need for a polymer matrix. Two types of bridging agents were investigated for interconnecting graphene sheets, which attach to sheets by either π–π bonding or covalent bonding. When applied alone, the π–π bonding agent is most effective. However, successive application of the optimized ratio of π–π bonding and covalent bonding agents provides graphene sheets with the highest strength, toughness, fatigue resistance, electrical conductivity, electromagnetic interference shielding efficiency, and resistance to ultrasonic dissolution. Raman spectroscopy measurements of stress transfer to graphene platelets allow us to decipher the mechanisms of property improvement. In addition, the degree of orientation of graphene platelets increases with increasing effectiveness of the bonding agents, and the interlayer spacing increases. Compared with other materials that are strong in all directions within a sheet, the realized tensile strength (945 MPa) of the resin-free graphene platelet sheets was higher than for carbon nanotube or graphene platelet composites, and comparable to that of commercially available carbon fiber composites. The toughness of these composites, containing the combination of π–π bonding and covalent bonding, was much higher than for these other materials having high strengths for all in-plane directions, thereby opening the path to materials design of layered nanocomposites using multiple types of quantitatively engineered chemical bonds between nanoscale building blocks.