Clarifying the viscous relaxation mechanism for twisting carbon nanotube ribbons

Twisted carbon nanotube (CNT) fibers have emerged as promising candidates for actuators and artificial muscles due to their excellent mechanical properties, offering significant potential in mechanical engineering and intelligent medical applications. Despite this promise, the effects of twisting on the viscous mechanical behavior of CNT fibers and their composites, particularly the evolution of their microstructure and time-dependent relaxation behavior, remain insufficiently understood. This study addresses this gap by conducting single and multiple stress relaxation experiments on untwisted ribbons and twisted CNT fibers. The findings reveal greater stress relaxation in CNT ribbons compared to twisted fibers, attributed to the microstructural constraints introduced by twisting. A viscoelastic model was developed to accurately capture the experimental stress-time curves and provide theoretical derivations of the stress-strain relationships for CNT ribbons and fibers. Additionally, numerical simulations elucidate the underlying viscous mechanisms, demonstrating that the intrinsic viscosity driving stress relaxation governs the time-dependent stress behavior and strain-rate sensitivity of the assembly. The study further highlights the critical role of twisting in shaping relaxation behavior, emphasizing the influence of enhanced interface interactions. Loading-unloading experiments on single and quadruple CNT fibers reveal that interface constraints significantly affect stress relaxation and rate sensitivity, offering new insights into the interplay between microstructural dynamics and mechanical performance. This work advances our understanding of the time-dependent properties of CNT fibers and provides a foundation for designing high-performance CNT-based materials for long-term applications.