3D transmission electron microscopy to quantify the morphology of aligned nanofiber nanocomposites

Recent advances in fabricating controlled-morphology, aligned carbon nanotube (A-CNT) assemblies with high (20% and above) volume fraction (V_f) create unique opportunities for markedly improving the performance of the next-generation advanced materials. These performance improvements are due to the scale-dependent and intrinsic properties of the CNTs themselves, and the ability to transfer their anisotropic properties to the polymer nanocomposites (PNC). However, a range of experimentally determined properties of such advanced PNCs are not in agreement with theoretical predictions. This fact strongly suggests that the initial assumptions about the PNC structure may be inaccurate. Here we present new 3-dimensional (3D) measurements of the nanoscale morphology of polymer materials containing high V_f of A-CNTs as revealed using energy-filtered electron tomography. We quantify the evolution of CNT morphology and state of dispersion with increasing V_f in terms of network structure, alignment, bundling and waviness and find that increasing the A-CNT V_f results in a nonlinear increase in bundling and alignment and a decrease in waviness. These new and rich data sets may be used to re-visit prior experimental measurements of these PNC electrical, thermal and mechanical properties, e.g., the large and nonlinear increase in thermal conductivity at high packing is attributable to decreased waviness and increased bundling. The results reported here show that electron tomography is a powerful tool in investigating the 3D morphology of nanocomposites, making it an essential tool for finding pathways to optimize the multifunctional properties of smart, hierarchical composites.