Engineering Ionic Liquid-Modified Silicon Carbide Elastomer Composites for Enhanced Thermochromic Responsiveness in Smart Flexible Sensors

Flexible thermochromic sensors emerge as next-generation smart materials for adaptive temperature sensing, thermal management, and multifunctional device applications. In this study, nitrile butadiene rubber (NBR) composites incorporating a thermochromic pigment, silicon carbide (SiC), and two distinct ionic liquids (ILs) were systematically investigated. Rheometric analysis at 160 °C revealed that SiC promotes network formation by reducing scorch and vulcanization times (t0.5, t90) and increasing torque increment (ΔM), which correlates with enhanced cross-link density. The introduction of ILs provided a tunable effect on both cure kinetics and network architecture: [bmim][BF4] acted as a catalytic accelerator, shortening curing times and increasing ΔM, while PMIMTFSI exhibited a plasticizing effect, resulting in lower cross-link density. Stress–strain analysis confirmed these trends, with SiC- and [bmim][BF4]-containing composites displaying higher tensile strength and modulus, whereas [C3mim][TFSI] reduced stiffness but increased extensibility. Thermo-oxidative aging studies over 1, 2, and 3 weeks demonstrated superior aging factors (AF) for SiC- and [bmim][BF4]-based systems, underscoring their resilience against oxidative degradation. Importantly, it was observed that the choice of ionic liquid allows direct control over the thermochromic response: [bmim][BF4] enabled effective and reversible color transitions at elevated temperatures, while [C3mim][TFSI] suppressed pigment activity within the elastomeric matrix. These results highlight the critical role of ionic liquid chemistry in tailoring both functional and structural properties of thermochromic elastomer sensors.