  Citation: Hubbard, J.; Tirano, J.; Zea, H.; Luhrs, C. Effects of Thermal Activation on CNT Nanocomposite Electrical Conductivity and Rheology. Polymers 2022, 14, 1003. https:// doi.org/10.3390/polym14051003 Academic Editor: Giulia Fredi Received: 3 February 2022 Accepted: 25 February 2022 Published: 2 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Effects of Thermal Activation on CNT Nanocomposite Electrical Conductivity and Rheology Joel Hubbard 1, * , Joaquin Tirano 2 , Hugo Zea 2 and Claudia Luhrs 1 1 Mechanical and Aerospace Engineering Department, Naval Postgraduate School, Monterey, CA 93943, USA; ccluhrs@nps.edu 2 Departamento de Ingeniería Química y Ambiental, Universidad Nacional de Colombia, Bogotá 111321, Colombia; jetiranov@unal.edu.co (J.T.); hrzear@unal.edu.co (H.Z.) * Correspondence: joel.hubbard@nps.edu Abstract: Carbon-based nanocomposites featuring enhanced electrical properties have seen increased adoption in applications involving electromagnetic interference shielding and electrostatic dissipation. As the commercialization of these materials grows, a thorough understanding of how thermal activation affects the rheology and electrical performance of CNT–epoxy blends can inform quality decisions throughout the production process. The aim of this work was the identification of the effects that thermal activation has on the electrical and rheological properties of uncured epoxy mixtures and how those may be tied to the resulting cured composites. Herein, three distinct CNT-loaded composite mixtures were characterized for changes in electrical resistivity and viscosity resulting from varying activation times. Electrical conductivity decreased as activation time increased. Uncured mixture viscosity exhibited a strong dependence on CNT loading and applied strain, with activation time being found to significantly reduce the viscosity of the uncured mixture and surface profile of cured composite films. In all cases, cured composites featured improved electrical conductivity over the uncured mixtures. Factors contributing to the observed behavior are discussed. Raman analysis, optical microscopy of CNT networks, and data from silica bead mixing and dispersion studies are presented to contextualize the results. Keywords: CNT composites; viscosity; electrically conductive; thermal activation; rheology 1. Introduction Many industries are now utilizing nanocomposites due to the enhanced material prop- erties achieved with relatively low nanofiller loadings. One such nanocomposite, carbon nanotube (CNT) epoxy composites, is particularly attractive to the aerospace industry, where favorable electrical properties can be incorporated into structural and adhesive components. CNT’s high aspect ratios enable the generation of electrically conductive composites at extremely low loadings [1–4]. The reduced resistivity of these materials makes them appealing to a wide variety of industries where electrostatic dissipation (ESD) or electromagnetic interference (EMI) solutions are needed [5–10]. The conductivity of the finished material is primarily a function of nanofiller loading and its dispersion within the surrounding matrix.. Inconsistent dispersion resulting in agglomerated areas of CNT bundles or areas devoid of CNTs can occur during the mixing and stages of production. The resulting inhomogeneity in these localized areas can have detrimental effects on the material, potentially causing unreliable performance, hotspots, or premature failure. Efforts to validate complete CNT dispersion in the finished composite product have employed various techniques. Several groups utilized scanning electron microscopy (SEM) employing voltage or charge contrast imaging to identify CNT bundles within a composite but are limited in practicality for commercial use due to the small sample area and destruc- tive nature of the sample preparation for analysis [11–15]. In [16] rare-earth oxides are Polymers 2022, 14, 1003. https://doi.org/10.3390/polym14051003 https://www.mdpi.com/journal/polymers