Preparation And Properities Of Polymer Composites Based On Liquid Metal Phase Transition

With the continues development of electronic field,soft electronic materials hasattracted increasing interest of researchers due to their wide application in wearable devices,electronic skins,soft robots and other fields.Metal-polymer composites（MPCs）,as an important component of flexible electronic materials,have always been the attracted focus of research.However,traditional MPCs prepared by simply blending conductive fillers（such as solid metal particles,carbon materials and conductive polymers,etc.）with polymers couldn’t meet the needs of the development of modern soft electronic materials because of the damage to the matrix caused by the rigid conductive fillers.Therefore,developing new MPCs is an urgent problem.Compared with rigid conductive fillers,liquid metals（LM）have attracted much attention due to their metallic properties and fluidity at room temperature,which widely applied in thermal management,catalysis,soft robotics,and electronic skin.Liquid metal polymer composites（LMPCs）with combined advantages of liquid metals and polymers have been broadly applied in advanced electronic industry.Currently,researchers usually focus on the conductivity,thermal conductivity,fluidity,self-healing or biocompatibility of LM,some characteristics of LM have been ignored and not been fully utilized.It is known that the transition of LM between solid and liquid（phase transition）can be easily realized under the influence of thermal energy in response to thermal energy at room temperature.The properties of LM are different in liquid and solid state.For example,LM loses its fluidity after solidification.At the same time,due to the different densities of LM in solid and liquid state,the volume of LM will change during the phase transition.Especially,the volume of Ga and Bi will decrease after melting,which is different from most metals.Such difference between the solid and liquid state of LM may bring unexpected merits to LMPCs which have been rarely investigated in researches of LM composites.Therefore,exploring the phase transition behaviour of LM in LMPCs will bring new development opportunities for LM composites.The details are as follows:（1）Pure Ga and PDMS were firstly mixed by simple mechanical blending.Then,the mixture was injected into the heat shrinkable tube with a syringe,and the LM-PDMS conductive fiber was obtained by heating and curing after standing at room temperature.LM particles were gradient dispersed in the polymer matrix through characterizations.The gradient distribution of LM particles endowed the LM-PDMS fibers with distinctive properties.The shapes and electrical conductivities of LM-PDMS fibers could be tuned through the phase transition（the absorbed thermal energy）of LM particles in LM-PDMS fiber.The fibers can be programmed into straight,bent or helical shapes,and the bending degree depended on the heat absorbed by the LM inside the fiber.In addition,the resistance of the LM-PDMS fibers could also be tuned by the thermal energy.Upon heating,the LM-PDMS fibers showed a transition from an insulator（10⁸Ω）to a conductor（10Ω）with programmed conductivity along the axial direction.More importantly,both the shape programmability and conductivity transition were highly reversible by heating and cooling.The programmed helical LM-PDMS fibers successfully mimicked natural plant tendrils with excellent mechanical performance and acted as smart strain-mediated conductors.The LM-PDMS fibers could also be applied as flexible temperature electrical switches in high-tech devices.In the last chapter,the effect of phase transition of pure Ga on the properties of LM-PDMS fiber was studied.Although the shape and electrical properties of LM-PDMS fiber can be controlled by adjusting the phase transition of Ga,the phase transition could only proceed at a specific temperature（melting point）due to the fixed melting point of Ga.In order to further explore the influence of metallic phase transition on LMPCs,this chapter introduces Ga-In binary metal（BM）into polymer matrix according to the knowledge of phase diagram in physical chemistry textbooks,and binary metal polymer composites（BMPCs）was prepared.It is found that the BMPCs exhibit temperature-driven or deformation-driven reversible transition between insulator and conductor,which is closely related to the phase transition of BM in BMPCs.More importantly,this phase transition process of BM can be precisely regulated according to the lever rule of the phase diagram,and the performance of the LMPCs can also be precisely controlled.This work would build the intimate and interesting connection between metal phases and polymer science to inspire next-generation soft conductors.（3）The binary metal stretchable electronics（BMSEs）was prepared by simple blending of the BM and polymer,which is more convenient in comparation to those structure-based stretchable conductors.Compared with the rigid conductive fillers,BMSEs with the BM conductive fillers had lower percolation threshold.Meanwhile,introducing BM into polymers would not cause obvious damage to the matrix,so that BMSEs maintained excellent conductivity under large strain（the resistance is still less than 10Ωwhen the strain exceeds 600%）.In addition,the BMSEs exhibited excellent stability,and no significant relative resistance（R/R₀≈1.09）was observed after 2000loading cycles at a strain of 100%.More important,the resistance of BMSEs strongly dependent on temperature,the value of R/R₀ for BMSEs decreased under heating,which can be reversed with remarkable stability.The BMSEs has many potential applications in the wearable health monitoring devices,human-machine interaction and software robotics.