Topological Defect-Induced Conformational Disorder Governs Urbach Tail States and Charge Transport in Epoxy Networks

Abstract The relationship between cross-linked network topology and charge transport remains poorly understood in polymer dielectrics, despite its critical importance for the electrical reliability of epoxy-based insulation materials. Here, we provide the first in-depth investigation demonstrating that topological defects inherently present in epoxy networks can induce conformational disorder, which governs the formation of Urbach tail states and consequently controls charge transport behavior. By regulating the level of hydrolyzable chlorine during resin synthesis, epoxy networks with different densities of dangling chain ends were obtained. These dangling chain ends, which are intrinsically unavoidable in practical epoxy systems, act as topological defects that disrupt the integrity of the cross-linked network and introduce additional conformational degrees of freedom within the polymer network. Such conformational freedom further reshapes the energy landscape of condensed polymer dielectrics. The increase in Urbach energy indicates a higher density of localized electronic states near the band-tail edge. As these tail states deepen and become more populated, carrier transport gradually evolves toward field-assisted hopping through the tail states under high electric fields, which ultimately deteriorates the dielectric strength. These findings provide insight into the microscopic origin of electrical conduction in polymer dielectrics and offer guidance for the molecular-level design of high-reliability insulation materials.