Toughening up Graphene Oxide Sheets with Polymer

Two-dimensional materials — materials that are only one atom thick — have been touted as the building blocks of next-generation sensors and wearable electronics.

While these materials, which include graphene and graphene oxide, are valued for their strength and high surface-to-volume ratio, they are also usually brittle. Scientists and engineers have been searching for a way to increase their toughness and their resistance to cracks and piercings by combining them with other materials without introducing inherent weaknesses in the combinations.

Northwestern Engineering professors have now developed a way to toughen up graphene oxide by layering it with polyvinyl alcohol, a synthetic polymer. In doing so, they have a created a material that is two times tougher and, because of the way the two materials bind, greatly reduces failures due to cracks and piercings.

The results, published in the journal Matter on May 29, could help the design of advanced composite materials.

 “We found a way to develop and study materials that effectively negate the defects that prevented their full implementation in the past,” said Horacio Espinosa, James N. and Nancy J. Farley Professor in Manufacturing & Entrepreneurship, professor of mechanical engineering, and co-author of the research. “This could serve as a model for two-dimensional materials going forward.” Other co-authors include Jiaxing Huang, professor of materials science and engineering, and SonBinh Nguyen, professor of chemistry in the Weinberg College of Arts and Sciences.

The material design was proposed by Espinosa, Huang, and Nguyen and fabricated through Langmuir-Blodgett deposition, a technique that tiles graphene oxide sheets on water surface to create monolayer thin films. Here, they used the technique to layer a monolayer of polyvinyl alcohol’s polymer chains on top of graphene oxide sheets. The technique creates a near-ideal model system of a polymer-graphene oxide nanocomposite.

The polymer chains interact with the graphene oxide through hydrogen bondings, forming reinforcements that strengthen the weak spots on the latter. As cracks develop, the hydrogen bonds break and reform, effectively resist the spreading of the cracks. “The polymer bridges the cracks and makes the material much tougher, and potentially self-healing,” Espinosa said. The researchers confirmed the finding with imaging and computational simulations.

The layered material is only about 5 nanometers thick, and retains the native strength, stiffness, and low weight of graphene oxide. “We hope that our findings will serve as a model for how other nanocomposites could be created going forward,” Nguyen said.

Next, researchers must determine how to scale-up the processing of these nanocomposites while retaining their unique features.

“One needs to be very cautious when extrapolating discoveries made on the nanoscaled building blocks to their bulk forms,” Huang said. “Scaling of nanoscale properties to bulk requires quite significant research effort.”