Preparation And Application Of Environmental Stimulus Responsive Flexible Actuators

With the rapid development of electronic information technology,environmental stimulus responsive intelligent materials emerge,and gradually develop towards the direction of intelligent,flexible wearable and multifunctional integration.Flexible actuators,as an important branch of intelligent material,have rapid response to external environmental stimuli,and can also meet the requirements of large-scale bending deformation,excellent cycle stability and multiple stimulus responsiveness.Therefore,flexible actuators have broad application prospects in emerging fields such as rehabilitation physiotherapy,intelligent switch,artificial muscle and flexible robot.Some of the currently reported flexible actuators have various problems,including complex process flow,high cost,large environmental pollution,unstable driving performance,single stimulus selectivity,etc.There is still a certain distance to meet the current high standards of flexible actuators with high flexibility,rapid and multiple stimulus response,large-scale bending deformation,excellent cycle stability,environmental protection,safety and pollution-free actuating process,which greatly limits the application and development of flexible actuators in future emerging intelligent systems.Therefore,it is imperative to select some environmentally friendly materials and use simple and convenient processing methods to prepare flexible actuators with multiple stimulus response behaviors.This thesis reviewed the development of intelligent materials and flexible actuating materials,elaborates on the current research status of some flexible actuators and the preparation and application of flexible actuators with different stimulus responsiveness.Our research started from some new functional materials that can quickly perceive and respond to stimuli such as light,heat and moisture in the environment.Physical blending or structural assembly were adopted to endue materials with multiple stimulus responsiveness.Then,one-dimensional fiber-based and two-dimensional membrane-based actuators with high flexibility were prepared by traditional wet spinning method and roto-coating method.Finally,through innovative design,a variety of bionic robots and intelligent control devices were prepared,which have a great application prospect in the intelligent control system in the future.The main research contents of this thesis are as follows:(1)Sodium alginate fiber was prepared from natural sodium alginate by wet spinning method.Through the analysis and characterization of mechanical properties,it is concluded that when the concentration of sodium alginate spinning solution was 4.0 wt%and the concentration of calcium chloride coagulation bath was 4.0 wt%,the sodium alginate fiber prepared by wet spinning process had the best mechanical strength.In addition,directional twisting operation was used to deform the physical structure of alginate fiber in the gelatinous state,and the twisted alginate fiber with controllable fiber twists and fiber length was obtained.Since the directional twisting operation only changed the physical shape and did not affect the chemical structure of the fiber,the twisted alginate fiber still had excellent hygroscopicity and could undergo rapid and reversible swelling-deswelling behavior under the stimulation of external water.The influence of gel fiber diameter and fiber number on fiber twists limit and twist number on fiber moisture absorption,swelling,crystallinity and orientation were studied.The results showed that with the increase of fiber diameter and gel fiber twists,the twist angle of the twisted alginate fiber increased,and the twist limit decreased,that is,the thicker the gel fiber was,the less twist was obtained.With the increase of fiber twists,micro-nano cracks on the fiber surface gradually increased and the roughness increased,which greatly improved the swelling degree and adsorption-desorption rate of water molecules on the fiber surface.Moreover,directional twisting operation had great effect on improving the mechanical strength and orientation of the fiber,but had little effect on the crystallinity.(2)Sodium alginate gel fiber was obtained by wet spinning,and then the twisted alginate fiber with different twist number and different length was prepared by directional twisting method.Under the influence of swelling-deswelling behavior,the twisted alginate fiber can produce rapid and reversible rotational motion under the stimulation of external water and moisture.The surface morphology of twisted alginate fiber was observed by scanning electron microscope.It was found that the surface roughness of the twisted fiber increased significantly with the increase of fiber twists,which increased the swelling degree of the twisted alginate fiber to some extent.Infrared spectrometer was used to record the structural changes of the twisted alginate fiber before and after it was exposed to heavy water stimuli.Results show that the heavy water in the twisted fiber would undergo a rapid and reversible adsorption-desorption process,leading to swelling-deswelling of the fiber volume,and finally,under the action of swelling-contraction force,the twisted alginate fiber would produce a rapid and reversible rotary motion behavior.Rotation test results show that with the increase of fiber twists and fiber length,the revolution and rotation speed of the twisted alginate fiber were greatly increased,with the maximum rotation speed up to 13000 rpm(1361 rad/s)and the number of single revolution up to 420 turns.In addition,the rotational motion of twisted fiber under water or moisture stimulation can show that:during the untwisting movement,the rotation speed of the fiber under the stimulation of water was much higher than that under the stimulation of moisture,while the recovery process was completely opposite.This can be explained that the water can make the fiber swell faster and thus produce a faster untwisting speed,while the moisture can evaporate quickly and thus produce a faster recovery speed.Moreover,the twisted alginate fiber could convert water stimulation into mechanical energy output in the process of rotation,and the calculated kinetic energy output was up to 1.48×10~4 W/kg and the gravitational potential energy was up to 6.47×10~2 W/kg due to the fiber's contraction movement during the rotational motion behavior.Furthermore,the twisted alginate fiber showed almost no deterioration in rotation speed under continuous water stimulation of 400 times,demonstrating excellent cyclic stability.Finally,the twisted alginate fiber was designed into"hydraulic generator","intelligent curtain","breathing fabric",and"intelligent crane",realizing its potential application in the field of intelligent controller,smart textile and garment.(3)The twisted GO/SA composite fiber was obtained by wet spinning and directional twisting method using a mixed spinning solution containing GO and sodium alginate.The addition of GO has two distinct advantages:Firstly,the excellent hydrophilicity of GO and water-solubility of sodium alginate enabled the two main materials to form a uniform and stable mixed spinning solution in aqueous.Secondly,the addition of GO not only had no effect on the hygroscopic of GO/SA composite fiber,but also enabled it with excellent photothermal response,promoting the twisted GO/SA fiber to generate fast,reversible and stable rotational motion behavior under external light and moisture stimulation.The actuating mechanism of the twisted GO/SA composite fiber under infrared light and moisture stimulation was emphatically studied.Under near-infrared light,water molecules in the composite fiber and the environment had reversible desorption and adsorption behavior,thus generating reversible contraction-swelling force and inducing reversible twisting and recovering rotational motion of fiber.The rotational motion mechanism of the twisted GO/SA fiber under moisture stimulation was the same as that explained in(2),which was caused by the adsorption-desorption behavior of water molecules in the twisted fiber.Rotation test results show that:with the increase of GO content,the rotation speed and revolution of the twisted GO/SA fiber increased significantly when it exposed to infrared light or moisture stimulation.In addition,by applying a stable high-low moisture stimulation,one single twisted GO/SA fiber could produce up to 5 MPa of actuating stress,far greater than the contractile force of mammalian skeletal muscle.The twisted GO/SA fiber can be used in intelligent network,remote controller and intelligent suspension bridge,etc.(4)First,GO film and CNT/PDMS composite film were prepared by solution evaporation method and rotary coating method respectively.Then GO layer and CNT/PDMS layer were compounded by secondary curing method to obtain GO-CNT/PDMS bilayer membrane-based actuator.Due to GO's excellent hygroscopic property and CNT's excellent photothermal property,the bilayer membrane was capable of fast,reversible and stable bidirectional bending deformation under the stimulation of infrared light,temperature and moisture.The actuating principle were as follows:Under photothermal stimulation,the rapid heat absorption of CNT in the CNT/PDMS layer leaded to thermal expansion of PDMS,and the CNT/PDMS layer generated a large expansion force.At the same time,the temperature rise made the water molecules between GO layers increase,leading to the volume contraction of GO layer,thus generating a large contraction force.Finally,under the combined action of expansion force and contraction force,the GO-CNT/PDMS bilayer membrane quickly bended to the side of GO layer.Under the stimulation of moisture,the hydrophobicity of CNT/PDMS layer made it basically unaffected by environmental moisture,while the excellent hydrophilicity of GO layer would rapidly adsorb water molecules,resulting in rapid volume expansion of the GO layer.Finally,under the action of swelling force of the GO layer,the GO-CNT/PDMS bilayer membrane would rapidly bend and deform to the side of the CNT/PDMS layer.Test results showed that with the increase of CNT content and light intensity,the bending angle and response speed of the bilayer membrane increased greatly.When the CNT content was controlled at 5.0 wt%and the light intensity was 0.5 W/cm~2,the bilayer membrane can reach the maximum bending angle of 90°at 2.48 s,and after 180 times of continuous bent-recovery deformation tests,the bilayer membrane still had stable actuating behavior.In addition,changing the contact temperature would lead to a great increase in the bending speed of the bilayer membrane.When the external temperature reached 80?,the membrane could reach the maximum bending angle of 180°at 1.72 s.Then,gradually increasing the relative humidity of the environment,the bending angle of the bilayer membrane increased dramatically.When the ambient humidity reached90%,the bilayer membrane could reach its maximum bending angle of 137°,and when the ambient relative humidity dropped to 30%,the membrane could quickly return to its initial state,and the whole actuating process could be completed within 3s.In addition,directional arrangement of GO layer can make the GO-CNT/PDMS bilayer membrane with controllable bending direction.Finally,through a series of bionic designs,the GO-CNT/PDMS bilayer membrane has been prepared into intelligent fingers,smart tweezers,smart switches,etc.,which has a great application prospect in the field of bionic actuating devices and flexible robots in the future.