Sustainable Castor Oil-derived Cross-linked Poly(ester-urethane) Elastomeric Films for Stretchable Transparent Conductive Electrodes and Heaters

Abstract

Substrates are essential for flexible and stretchable devices, requiring sustainability, stretchability, transparency, thermal stability, and chemical stability. This study introduces a sustainable cross-linked poly(castor oil-co-δ-valerolactone) cyclohexyl urethane (PCVU) substrate for flexible, stretchable transparent conducting electrodes (TCEs) based strain sensors and heaters. PCVU is synthesized as a highly transparent (>90%), stretchable (>190%), and thermally stable (~210°C) substrate via thermal cross-link polymerization of poly(castor oil-co-δ-valerolactone) triol and 4,4′-methylenebis(cyclohexyl isocyanate) on a glass mold. PCVU exhibits good chemical stability in various organic solvents and degrades completely in 72 days in an alkaline medium (pH = 14). Using PCVU, we fabricated a robust, flexible, and stretchable TCE with low sheet resistance (<50 Ω sq-1). The TCE fabrication process includes applying an electrospun polyvinyl alcohol (PVA) layer as a leveling agent to improve the adhesion of silver nanowires (AgNW) and utilizing a heat-based nano-welding technique to enhance durability and mechanical stability. The TCE-based strain sensor showed stable and repeatable resistance changes (ΔR/R0) under 5-15% strains, with fast response and consistent signal stability over 100 cycles at 5% strain. The flexible heater reached a maximum temperature of ~150°C at 5.5V, with rapid heating and cooling responses (15 s each). Practical applications include a strain sensor for real-time monitoring of human motion (finger, wrist, elbow, and neck flexion) and a heater used as a thermotherapy pad for the wrist and finger, demonstrating the potential of PCVU-based TCEs for wearable and medical devices.