Non-destructive assessment of aligned carbon nanotube structures through correlation of polymer nanocomposite properties via ultrasonic testing

Evaluation of nanoparticle distribution and orientation within polymer nanocomposites is critical to ascertaining structure–property relationships but has been a challenge. Nanoparticles form multi-scale structures consisting of nanometer- and micrometer-scale agglomerations, requiring inspection of high resolution and large field of view (FOV) at the same time. Electron microscopy provides high-resolution 2D images of small FOV. Micro-computed tomography provides 3D images of moderate resolution and FOV, but is limited in its ability to resolve regions of similar elements such as polymers and carbon-based nanoparticles. In this work, an ultrasonic testing (UT) technique of a moderate resolution (sub-millimeter) was used to indirectly assess microstructures of carbon nanotubes (CNTs) within an epoxy matrix over a sizable volume (~ mm in all directions). CNTs were magnetically aligned and agglomerated using two different field strengths, and such CNT micro-structure change affected the fracture toughness data of CNT-epoxy nanocomposites. The propagation speed and energy loss (attenuation) of the reflected wave were correlated to changes of CNT orientation and distribution by magnetic field application.