Boron nitride nanotubes reinforce polymer materials

One form of boron nitride is a white, lubricious, flaky material whose material properties resemble graphite’s. By contrast, its nanotube form, like carbon’s, is strong, tough, and light. Exploiting those desirable properties in a bulk material entails embedding the nanotubes in a lightweight matrix—provided the nanotubes bind strongly enough to the matrix material that they don’t slip. Quantifying that binding was the goal of an investigation led by Changhong Ke of Binghamton University and Xianqiao Wang of the University of Georgia. The researchers created thin polymer films—of epoxy and of PMMA—and embedded them with boron nitride nanotubes (BNNTs). Cleaving the films left some BNNTs poking out from the surface. By welding a nanotube’s protruding tip to an atomic force microscope, the team could determine how much force was needed to pull it out. For tubes buried deeper than about 200 nm, the force turned out to be 250 nN for the BNNT–epoxy composite and 190 nN for BNNT–PMMA. Less force was needed to extract carbon nanotubes (CNTs) from the same materials. Molecular dynamics simulations revealed the source of the BNNTs’ stronger binding. Because BNNTs are made of two different elements—as opposed to one in the case of CNTs—their bonds are polar. And thanks to that polarity, BNNTs bind to the polymer matrix not just with van der Waals interactions but with Coulomb interactions too. Ke and Wang’s findings suggest that BNNTs are excellent fillers for lightweight, high-strength composites. (X. Chen et al., Appl. Phys. Lett. 107, 253105, 2015, doi:10.1063/1.4936755 .)