Theoretical nanotube structure inspires super-hard polymer material

A lightweight polymer material full of holes, developed by researchers at Rice University's Brown School of Engineering and their colleagues, is nearly as hard as diamond, as proved by the mere dents left by speeding bullets.

This polymer material is based on tubulanes, theoretical structures of crosslinked carbon nanotubes predicted to have extraordinary strength. In the Rice lab of materials scientist Pulickel Ajayan, they found tubulanes can be mimicked as scaled-up, 3D-printed polymer blocks, which proved to be better at deflecting projectiles than the same material without holes. These blocks can also be highly compressed without breaking apart.

As detailed in a paper in Small, this discovery could lead to printed structures of any size with tunable mechanical properties.

Tubulanes were predicted in 1993 by chemist Ray Baughman of the University of Texas at Dallas and physicist Douglas Galvão of the State University of Campinas in Brazil, both co-principal investigators on the new paper. Tubulanes themselves have yet to be made, but their polymer cousins may be the next best thing.

Rice graduate student and lead author Seyed Mohammad Sajadi and his colleagues built computer simulations of various tubulane blocks, printed the designs as macroscale polymers and subjected them to crushing forces and speeding bullets. The best proved to be 10 times better at stopping a bullet than a solid block of the same material.

The Rice team fired projectiles into patterned and solid cubes at 5.8 kilometers per second. According to Sajadi, the results were impressive: "The bullet was stuck in the second layer of the structure. But in the solid block, cracks propagated through the whole structure." Tests in a lab press showed how the porous polymer lattice lets tubulane blocks collapse in upon themselves without cracking.

The Ajayan group made similar structures two years ago when it converted theoretical models of schwarzites into 3D-printed blocks. But according to Sajadi, the new work is a step toward what materials scientists consider a holy grail.

"There are plenty of theoretical systems people cannot synthesize," he said. "They've remained impractical and elusive. But with 3D printing, we can still take advantage of the predicted mechanical properties because they're the result of the topology, not the size."

Sajadi said that tubulane-like structures of metal, ceramic and polymer are only limited by the size of the printer. Optimizing the lattice design could lead to better materials for civil, aerospace, automotive, sports, packaging and biomedical applications.

"The unique properties of such structures comes from their complex topology, which is scale-independent," explained Rice alumnus Chandra Sekhar Tiwary, co-principal investigator on the project and now an assistant professor at the Indian Institute of Technology, Kharagpur. "Topology-controlled strengthening or improving load-bearing capability can be useful for other structural designs as well."

According to co-authors Peter Boul and Carl Thaemlitz of Aramco Services Co, a sponsor of the research, potential applications span many industries, but the oil and gas industry will find tubulane structures particularly valuable as tough and durable materials for well construction. Such materials must withstand impacts that can transform standard cements into rubble, particularly during hydraulic fracturing. "The impact resistance of these 3D-printed structures puts them in a class of their own," Boul said.

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.