Mineral Hydrogel-based Thermal-stiffening And Self-sensing Sheath-core Elastic Fiber

Stiffness,reflected by elastic modulus,is a measure of the ability of a material to resist deformation when subjected to external forces.People often design materials with specific stiffness according to different tasks.However,materials with fixed stiffness are no longer sufficient for many emerging applications such as smart wearables,3D printing,and soft robots.Inspired by various modulus-switchable biomaterials like sea cucumber skin,octopus tentacle,and elephant nose,a few modulus-adjustable materials have been extensively reported,but their stretchability and elasticity are often poor,which greatly limits their future applications in soft devices.Generally,most hydrogels are thermal-softening due to the weakened intermolecular interactions.In contrast,our group previously reported a mineral plastic hydrogel that exhibits novel thermal-stiffening behavior,whose modulus increases ca.1000 times as temperature increases from 25 to 80 ^oC.The hydrogel also possesses many other intriguing properties such as self-healing,plasticity,and bio-degradability,which has been applied as ionic skins,stretchable sensors,and organic-inorganic adhesives.Nevertheless,the mineral plastic hydrogel is difficult to maintain its shape and easily loses water as exposed in air;so far,such excellent thermal-stiffening behavior has not been really utilized in fabricating functional materials/devices.In this thesis,we first studied the molecular mechanism of the thermal-stiffening behavior of mineral plastic hydrogel.Aided with several characterizations like small-angle X-ray scattering,two-dimensional correlation spectroscopy,and low-field NMR,thermal-stiffening is ascribed to the strengthened interaction between polyacrylic acid and amorphous calcium carbonate clusters,which causes significant dehydration and microphase-separated glassy state.The mineral plastic hydrogel is further incorporated into the elastic polyacrylamide network to form an interpenetrating hydrogel that integrates good thermal-stiffening and elastic properties.Further,a water-impermeable fluoroelastomer is coated on the hybrid hydrogel to form an air-stable sheath-core fiber,whose modulus increases 30 times as temperature increases from 25 to 80 ^oC.By incorporating gold nanorods into the mineral plastic phase,the resulting hybrid sheath-core fiber is also able to stiffen at specific sites.Furthermore,owing to the ionically conducting nature of mineral plastic hydrogel with mobile Ca²⁺ions,the sheath-core fiber may be used as strain sensors.Demonstrations are shown in the real-time monitoring of the fiber-controlled slow descent of loads.It is finally shown that,the sheath-core fiber can be further woven into various threads and textiles,which may greatly expand its application scenarios.