The Evolution of Carbon Nanotube Network Structure in Unidirectional Nanocomposites Resolved by Quantitative Electron Tomography

Carbon nanotube (CNT) reinforced polymers are next-generation, high-performance, multifunctional materials with a wide array of promising applications. The successful introduction of such materials is hampered by the lack of a quantitative understanding of process-structure-property relationships. These relationships can be developed only through the detailed characterization of the nanoscale reinforcement morphology within the embedding medium. Here, we reveal the three-dimensional (3D) nanoscale morphology of high volume fraction (V_f) aligned CNT/epoxy-matrix nanocomposites using energy-filtered electron tomography. We present an automated phase-identification method for fast, accurate, representative rendering of the CNT spatial arrangement in these low-contrast bimaterial systems. The resulting nanometer-scale visualizations provide quantitative information on the evolution of CNT morphology and dispersion state with increasing V_f, including network structure, CNT alignment, bundling and waviness. The CNTs are observed to exhibit a nonlinear increase in bundling and alignment and a decrease in waviness as a function of increasing V_f. Our findings explain previously observed discrepancies between the modeled and measured trends in bulk mechanical, electrical and thermal properties. The techniques we have developed for morphological quantitation are applicable to many low-contrast material systems.