The process of making a stack of parallel sheets of graphene starts with a chemical vapor deposition process (I) to make a graphene sheet with a polymer coating; these layers are then stacked (II), folded and cut (III) and stacked again and pressed, multiplying the number of layers. The team used a related method the team to produce scroll-shaped fibers.
The process of making a stack of parallel sheets of graphene starts with a chemical vapor deposition process (I) to make a graphene sheet with a polymer coating; these layers are then stacked (II), folded and cut (III) and stacked again and pressed, multiplying the number of layers. The team used a related method the team to produce scroll-shaped fibers.

Particles – or fillers – are used in composites to bring strength and stiffness to otherwise soft materials like polymers. The result is lightweight yet sturdy materials that combine the best of both components. But when filler particles are on the nanoscale, it can be tricky to get them to stay put in a composite.

Now researchers from Massachusetts Institute of Technology (MIT), US Army Research Laboratory, and Nanjing University in China have come up with a new way of keeping nanoadditives organized in a composite by using stacks of graphene sheets or rolled up scrolls [Liu et al., Science 353 (2016) 6297].

Like the layers in puff pastry that provide the crunch, the team led by Michael S. Strano at MIT incorporated graphene sheets into polycarbonate composites either in simple stacks or as scrolls of rolled up sheets (Fig. 1). Up to 320 layers of graphene a few atoms thick are incorporated into the polycarbonate, spaning the full width of the material.

“[Our] new concept creates nanocomposites with perfectly aligned semi-infinite graphene,” explains first author of the study, Pingwei Liu. “We used large-area, high-quality graphene grown by chemical vapor deposition (CVD) and maintain the quality of the graphene during the process, which gives a maximum reinforcement with minimal addition.”

The reinforcing fibers are evenly spaced in an aligned fashion within the composite, significantly increasing the strength of the material at an exceptionally low volume fraction. In fact, the graphene makes up only 0.082% of the volume of the nanocomposite.

The team also formed composites another way using rolled up sheets of graphene. These scrolled fibers can stretch to 110% of their original length before breaking – 30 times better than super-strong materials like Kevlar.

“All the reinforcement comes from the direct load transfer to graphene filler itself,” explains Liu. “The scrolled fibers, meanwhile, demonstrate exotic elongation with a telescoping mechanism, which offers a completely new way to for composite materials to dissipate energy during deformation.”

The combination of properties could open the way to woven protective materials that ‘give’ without breaking. Not only that, but the use of graphene as an additive – in either form – also conveys electrical conductivity and optical transparency on the final composite.

“These new materials combining mechanical reinforcement, electrical, optical, and thermal properties with additions of nanofillers may have applications in energy generation and storage (e.g. supercapacitors), protection(e.g. lightening protectionand electromagnetic shielding), optical devices, electronic systems, and more,” adds Liu.

The challenge now, he says, is to obtain large-area, high-quality CVD graphene at a cost low enough to make the new composites economic. Angelos Kyrlidis, research and development manager at Cabot Corporation, believes the work represents a unique and creative way to make composites with large-area graphene.

“This work assembles the composites from CVD graphene, where a very high aspect ratio can be obtained, while still maintaining many of the features and properties of the single layer graphene,” he states.

This article was originally published in Nano Today (2016), doi:10.1016/j.nantod.2016.08.005