Toughened Fiber for Artificial Muscle Based on Carbon Nanotube And Graphene Oxide

Abstract

Fiber/yarn types of carbon nanotubes (CNT) and graphene-based artificial muscles have recently attracted attention because of their remarkable physical properties (strength, lightweight, electrical and thermal conductivity, and flexibility). In particular, the toughen property is being actively researched because it has essential properties for various applications such as bulletproof vest, wearable devices, and artificial muscles. Therefore, the research areas presented in this thesis show the toughen fibers based on CNT and graphene and applications of the artificial muscle. First, the toughened CNT fiber was developed through the inspiration of the bundle structure of muscle. The toughened CNT yarn, which is simultaneously increased in mechanical strength and elongation, is made by poly(vinyl) alcohol (PVA) infiltration and a coiled ply structure. This toughened CNT fiber shows high-energy absorption (357.2 J/g), the ability to absorb mechanical energy before fracture. Also, not only does it demonstrate the ability to maintain high toughness in water and high temperature but also shows the ease in shape change by using water. Second, graphene oxide (GO)-based fibers which are fabricated by wet-spinning techniques have been developed as high toughened fibers. The high strength GO-based fiber is increased by compositing a polyacrylonitrile (PAN) binder, a surface coating with polydopamine (PDA), and through a heat treatment. GO/PAN fibers, which are composed of liquid-crystalline graphene oxide (LCGO) and PAN, were prepared by a wet-spinning method. The LCGO/PAN fiber of mechanical strength (220 MPa) and modulus (19 GPa) was optimized at a composition of LCGO (80 wt%) and PAN (20 wt%). Through pyrolysis of the LCGO/PAN fiber, the mechanical strength was significantly increased to 526 MPa. Third, LCGO-based actuators are demonstrated by the simple fabrication of LCGO-based fibers with an infiltrated nylon-6,6 polymer by wet-spinning. These toughened LCGO/nylon fibers could be twisted to form torsional actuators and coiled to form tensile actuators. The reversible tensile actuation of contraction or elongation was demonstrated by strokes as high as -80 and 75%, respectively, by controlling the relative twisting and coiling direction of the LCGO/nylon fiber. Moreover, the LCGO/nylon actuator showed remarkably little hysteresis, could lifting loads over 100 times heavier than itself, and generate stability at high temperatures. Lastly, the carbon-based hybrid actuator is developed as an inspired nano-structure of spider silk. Spider silks are well known as being stretchable and contractible fibers with are very tough. Those tough fibers with stretchability and contractibility are attractive for use as energy absorption materials, and they are needed for wearable applications, artificial muscles, and soft robotics. Therefore, the stretchable and contractible toughened fiber is inspired by the structure of spider silk. The spider silk-inspired hybrid fiber demonstrates 495 J/g of gravimetric toughness, which exceeds the 165 J/g of spider silk. Besides, the hybrid fiber was reversibly stretched to 80% strain without deformation. These mechanical properties (toughness and stretchability) are realized by the hybridization of aligned GO/CNT in a polyurethane matrix as elastic amorphous regions and β-sheet segments of spider silk. In addition, similar to the supercontraction of spider silk, the bio-inspired hybrid fiber contracted up to 60% in response to water and humidity. It exhibited 610 kJ/m3 of contractile energy density, which is higher than previously reported moisture driven actuators.