Graphene Oxide Modified Polymer-based Flexible High Dielectric Nanocomposites For Sensoring Application

With the rapid development of microelectronic technology and flexible wearable devices,the demand for flexible sensors is dramatically increased.Among the materials for preparing flexible sensors,polymers have attracted extensive attention because of their low cost and excellent mechanical properties.However,commercial polymer materials usually possess very low dielectric constant（<10）,which severely hinders their further application in the sensing field.In addition,although traditional modification technology by preparing organic/inorganic composites endow the polymers with high dielectric constant,the flexibility of the material usually deteriorates due to the high concentration（>50 vol%）of nanofillers.Therefore,preparing polymer composites with both high dielectric constant and excellent flexibility is of great significance to the development of flexible sensors.In this work,we combine the inherent excellent properties of graphene oxide（GO）such as controllable electrical conductivity under heat treatment,good dispersibility,large layer structure and catalytic effect with advantages of other inorganic nanoparticles to fabricate polymer-based composite materials via electrospinning,electrostatic self-assembly and in-situ polymerization technique.By constructing the“micro-capacitor”structure in the polymer matrix,high dielectric constant is achieved while the flexibility of the material is maintained.These materials show excellent properties on the capacitive sensing and tribo-nanogenerator devices,which hold promising application in the flexible wearable sensors.（1）Polyacrylonitrile（PAN）nanofiber mats containing GO nanoribbons with core-shell microstructure are firstly prepared via coaxial electrospinning and then hot-pressed into dense composite films.It is revealed that hot-pressing assisted by a stretching force under appropriate temperature and pressure can generate local conformational changes of PAN,leading to the formation of an electroactive phase with dielectric constant increasing by 60%.Meanwhile,the GO transformed into reduced graphene oxide（rGO）under heat reduction,serving as conductive nanofillers to further promote the increase of dielectric constant.Consequently,the resultant materials realize various applications through morphology regulation,i.e.porous electrospinning films can be processed into flexible capacitive sensing materials with high sensitivity(0.81 k Pa^-1),while the dense films displaying thermally stable dielectric properties with a high dielectric constant（?_r=33.5,25℃;?_r=42,150℃）and low loss（tanδ=0.09,25℃;tanδ=0.55,150℃）over a broad temperature range can be applied in energy storage field.（2）A functional nanofillers,halloysite nanotubes（HNTs）decorated rGO hybrid microstructures（HNTs@rGO）is successfully prepared via controllable electrostatic self-assembly and in-situ heat reduction method.These hybrid microstructures combine characteristics of natural 1D ceramic nanotubes with large aspect ratio and high electric conductivity of rGO micro-sheets,which provide ideal material collocation in the construction of microcapacitors.The HNTs not only effectively prevent direct contact between the rGO micro-sheets in the composites but also play an important role in forming dielectric interface within microcapacitors.Consequently,an HNTs@rGO/polyvinyl butyral（PVB）composites containing a very low content of 5wt%rGO exhibits an ultra-high dielectric constant of 150 and extremely low loss of 0.12 at 1 k Hz.The optimized HNTs@rGO/PVB films acting as tribo-negative layer can light up 124 LEDs under“contact-separation”mode with 4×4 cm area size.And because of the flexibility of this sort of material,it can be assembled into various passive devices to monitor human motion signals,which shows promising application in the sensing field.（3）To further solve the problem that high content of rigid particles deteriorates the mechanical properties of polymer matrix,we adopt a liquid metal（LM）called eutectic gallium indium（EGa In）to prepare functional flexible materials with high dielectric constant.In this part of work,an elastomer,poly（n-butyl methacrylate）,coated liquid metal（LM）nanodroplets（EGa In@PBMA）is successfully fabricated via a facile in-situ free radical polymerization method.The realization of this reaction is attributed to the use of oleic acid（OA）molecules which not only improve the dispersion state of liquid metal nanodroplets（LMNPs）in the organic solvent,but also introduce unsaturated double bonds on the surface of LMNPs.The as prepared soft nanoparticles can be directly hot-pressed into nanocomposites which exhibit excellent mechanical and dielectric properties.The results indicate that when the LM volume fraction is up to 40%,the composites still maintain a very high breaking elongation of 442%,and the dielectric constant of the material can reach 22 while the dielectric loss is only 0.068 at 1k Hz.In addition,by incorporating GO into EGa In@PBMA,the as prepared EGa In@PBMA-rGO composites show further improvement of dielectric properties and can be used as tribo-positive friction layer combined with nitrile rubber to form a single-electrode nanogenerator（S-TENG）.The flexible TENG devices can monitor human motion signals under various modes,which hold great potential in the application of sensing field.（4）Gel materials are widely used in the field of flexible sensor materials because of their excellent resilience.In this chapter,it is initially discovered that multifunctional hydrogels are fabricated via ultra-fast polymerization（～seconds/minutes）by graphene oxide-adsorbed liquid metal nanodroplets（LMNPs@GO）vs.by conventional approaches（～hours/days）.Graphene structures have never been found to play a role in accelerating fabrication of functional polymer materials.LMNPs@GO are used to rapidly initiate and further cross-link polyacrylic acid（PAA）chains into a three-dimensional（3D）network without any extra molecular initiators,cross-linkers,heat source,and/or protective gas.The polymerization process with LMNPs@GO is extremely faster than that without GO involved（20 seconds vs.4 hours of prepolymer formation,and then 10 minutes vs.3 days of crosslinking）for free radical polymerization of PAA hydrogels.The resulting hydrogel with 2wt%reduced graphene oxide（rGO）exhibits 600%increase in tensile strength and950%enhancement in conductivity,as well as excellent self-healing capabilities,in comparison with that of the pure PAA.The sensitivity studies show its great potential for the application of flexible sensors.Furthermore,combined with solvent replacement method,the hydrogel material can be converted into a dielectric gel material with high dielectric constant,which is applicable for fabricating stretchable capacitors.This creative study not only broadens a novel application of GO for making advanced multifunctional polymer materials,but also provides a brand-new route to realization of ultra-fast manufacturing technology that is significantly promising for industrial production in wearable devices.