The mechanical properties of epoxy nanocomposites with graphene platelets, single-walled carbon nanotubes, and multi-walled carbon nanotube additives were compared at a nanofiller weight fraction of 0.1 ± 0.002%.
* graphene platelets significantly out-perform carbon nanotube additives.
* The Young’s modulus of the graphene nanocomposite was 31% greater than the pristine epoxy as compared to 3% increase for single-walled carbon nanotubes.
* The tensile strength of the baseline epoxy was enhanced by 40% with graphene platelets compared to 14% improvement for multi-walled carbon nanotubes.
* The mode I fracture toughness of the nanocomposite with graphene platelets showed 53% increase over the epoxy compared to 20% improvement for multi-walled carbon nanotubes.
* The fatigue resistance results also showed significantly different trends
The above is one of three new studies from researchers at Rensselaer Polytechnic Institute illustrate why graphene should be the nanomaterial of choice to strengthen composite materials used in everything from wind turbines to aircraft wings.
Graphene equal to 0.1 percent of the weight of the composite boosted the strength and the stiffness of the material to the same degree as adding carbon nanotubes equal to 1 percent of the weight of the composite. This gain, on the measure of one order of magnitude, highlights the promise of graphene, Koratkar said. The graphene fillers also boosted the composite’s resistance to fatigue crack propagation by nearly two orders of magnitude, compared to the baseline epoxy material.
Though graphene and carbon nanotubes are nearly identical in their chemical makeup and mechanical properties, graphene is far better than carbon nanotubes at lending its attributes to a material with which it’s mixed.
“Nanotubes are incredibly strong, but they’re of little use mechanically if they don’t transfer their properties to the composite,” Koratkar said. “A chain is only as strong as its weakest link, and if that link is between the nanotube and the polymer, then that is what determines the overall mechanical properties. It doesn’t matter if the nanotubes are super strong or super stiff, if the interface with the polymer is weak, that interface is going to fail.”
Koratkar said graphene has three distinct advantages over carbon nanotubes. The first advantage is the rough and wrinkled surface texture of graphene, caused by a very high density of surface defects. These defects are a result of the thermal exfoliation process that the Rensselaer research team used to manufacture bulk quantities of graphene from graphite. These “wrinkly” surfaces interlock extremely well with the surrounding polymer material, helping to boost the interfacial load transfer between graphene and the host material.
The second advantage is surface area. As a planer sheet, graphene benefits from considerably more contact with the polymer material than the tube-shaped carbon nanotubes. This is because the polymer chains are unable to enter the interior of the nanotubes, but both the top and bottom surfaces of the graphene sheet can be in close contact with the polymer matrix.
The third benefit is geometry. When microcracks in the composite structure encounter a two-dimensional graphene sheet, they are deflected, or forced to tilt and twist around the sheet. This process helps to absorb the energy that is responsible for propagating the crack. Crack deflection processes are far more effective for two-dimensional sheets with a high aspect ratio such as graphene, as compared to one-dimensional nanotubes.
Koratkar said the aerospace and wind power industries are seeking new materials with which to design stronger, longer-lived rotor and wind turbine blades. His research group plans to further investigate how graphene can benefit this goal. Graphene shows great promise for this because it can be produced from graphite, which is available in bulk quantities and at relatively low cost, he said, which means mass production of graphene is likely to be far more cost effective than nanotubes
The Two Other Research Papers
Fracture and Fatigue in Graphene Nanocomposites,” published in Small(5 page pdf)
Only 0.125% weight of functionalized graphene sheets was observed to increase the fracture toughness of the pristine (unfilled) epoxy by 65% and the fracture energy by 115%. To achieve comparable enhancement, carbon nanotube (CNT) and nanoparticle epoxy composites require one to two orders of magnitude larger weight fraction of nanofillers. Under fatigue conditions, incorporation of 0.125% weight of functionalized graphene sheets drastically reduced the rate of crack propagation in the epoxy 25-fold.
An experimental study on buckling of graphene/epoxy nanocomposite beam structures is presented. Significant increase (up to 52%) in critical buckling load is observed with addition of only 0.1% weight fraction of graphene platelets into the epoxy matrix. Based on the classical Euler-buckling model, the buckling load is predicted to increase by ∼ 32%. The over 50% increase in buckling load observed in our testing suggests a significant enhancement in load transfer effectiveness between the matrix and the graphene platelets under compressive load. Such nanocomposites with high buckling stability show potential as lightweight and buckling-resistant structural elements in aeronautical and space applications.