Design And Properties Of Flexible Polymer-Based Gel Strain Sensing Materials

In recent years,the emergence of ever-changing flexible sensing materials/concepts has opened up new prospects in the future electronics field,and has shown great application potential in the fields of information,energy,medical and national defense.Compared with traditional electronic devices,flexible sensing materials have obvious advantages in terms of deformability,irregular surface fitting,portability,decoration,etc.Ideally,new flexible electronics can be bent,stretched,compressed,twisted,and deformed into complex,non-planar shapes while maintaining their good sensitivity,reliability,and integration.Polymer-based gel sensing materials that integrate flexibility and electronic/ionic conductivity have become frontier materials for flexible conductive polymers.Optimizing the structure and composition of polymer-based gels can enhance the performance of flexible strain sensors.In this study,the reduced graphene oxide@polyamide-polyether block copolymer(RGO@Pebax)aerogel-like nanocomposite was first prepared by supercritical CO2 foaming,physical dip-coating,and in-situ reduction,which endowed the gel sensing material with light weight and compressibility.Second,the polyacrylic acid/polyvinyl alcohol/carbon black lithium chloride(PAA/PVA/CBLiCl)nanocomposite hydrogel was prepared by the polymerization-soaking method,which endowed the gel sensing material with high mechanical,photothermal conversion and conductive sensing.Polyacrylic acid-catechin@liquid metal(PAA-Catechin@LM)ionogel prepared by self-triggered polymerization,which endows the gel with room temperature formability,stretchability,and wide temperature adaptation range and excellent sensing performance.Finally,the PAA-Catechin@LM ionogel was used as the conductive medium and applied on the leather substrate to realize the sensing performance of the leather.The above gel sensing materials were assembled into strain sensors,and the influence of strain on the sensing performance of the gel sensing materials was explored,further revealing the gel sensing mechanism.The specific research content and results are as follows:(1)A flexible,lightweight and highly sensitive RGO@Pebax aerogel-like nanocomposite was prepared by supercritical CO2 foaming,physical dip-coating,and in-situ reduction.Pebax was composed of polyamide as the hard segment and polyether as the soft segment.Hard segment polyamide has physical crosslinking effect and soft segment polyether has CO2 adsorption effect,which is conducive to the preparation of porous structure by supercritical CO2 foaming,giving the nanocomposite flexible compressibility.The RGO conductive medium was successfully introduced through dip-coating and in-situ reduction,giving aerogellike excellent conductivity.Fourier transform infrared spectroscopy(FTIR),X-ray diffraction(XRD),ultraviolet-visible-near-infrared spectroscopy(UV-vis-NIR),transmission electron microscopy(TEM)and scanning electron microscopy(SEM)were used to analyze the characterization of the material.Through mechanical,electrical conductivity and sensing performance tests,the effects of expansion rate and RGO addition on the mechanical,electrical and sensing properties of aerogellike nanocomposite were investigated.The expansion ratio of the aerogel-like nanocomposite was as high as 5.1 when the forming time was controlled to 60 s.The larger the expansion rate,the better the compressibility,and the maximum compressive strain can reach 50%.Benefiting from the advantages of microcellular structure and 3D interconnected channels,aerogel-like nanocomposites are highly flexible and lightweight.The aerogel-like nanocomposite exhibited excellent electrical conductivity(0.053 S/m)and sensing performance(GF=13.27).Due to the synergistic effect of dual mechanisms:the"disconnection-connection" transition of microcracks and the "increase-decrease"of contact area in 3D interconnected conductive channels.In addition,the aerogellike nanocomposite exhibited excellent reliability and stability during continuous cyclic loading-unloading.The application of aerogel-like nanocomposite strain sensor has been proved to good responsiveness in variable conductor and human motion detection.(2)PAA/PVA/CBLiCl nanocomposite hydrogels with excellent mechanical,photothermal conversion,electrical conductivity and sensing properties were successfully prepared by thermally induced radical polymerization and physical immersion.The gel was characterized by FTIR,XRD,UV-vis-NIR,TEM,and SEM.The effects of CB addition amount and LiCl concentration on the mechanical properties,evaporation properties,electrical properties and sensing properties of hydrogels were investigated through performance tests.When the concentration of LiCl was 12 mol/L and the addition of CB was 0.025 wt%,the tensile strength reached 16.2 MPa,the elongation at break was 13.85,and the elastic modulus was 1.24 MPa.The main reason for the excellent physical properties is that multiple hydrogen bonds were formed between the polymer chains of PAA and PVA.In addition,the salting-out effect reduced the distance between molecular chains and strengthened the hydrogen bond interaction between polymer chains.When the addition of CB was 0.2 wt%,the gel had a high absorbance(96%).The gel showed the excellent seawater desalination performance,with an evaporation capacity of 2.53 kg m-2,an evaporation rate of 1.25 kg m-2 h-1,an evaporation efficiency of 90.3%.Moreover,the concentration of metal salt ions in the desalinated water was lower than the drinking water standard set by the World Health Organization.At the same time,the salting-out effect endowed the gel with a conductivity of up to 0.5 S/m.The assembled strain sensor was used to monitor the movement of prosthetic limbs,which broadened the application of high-strength hydrogels in the direction of flexible electronics.(3)APAA-Catechin@LM ionogel with toughness,self-healing,adhesion,and 3D formability was successfully prepared by self-triggered free radical polymerization.The gelation time of the ionogel was optimized by adjusting the dosage of the nanocatalyst(Catechin@LM).The effects of monomer content on the mechanical,electrical and sensing properties of ionogels were investigated through performance tests.The gel was characterized by FTIR,XRD,UV-vis-NIR,TEM and SEM.When the content of catechin@LM was 9.5 wt‰,the gel could be formed at room temperature during shorter gelation time(30 s).When the monomer content was 20 wt%,the gel could withstand tensile strain up to 2268%,Young’s modulus as low as 0.08 kPa,high self-healing efficiency(97%),adhesion(102 N/m),3D formability and other advantages.The above advantages could be attributed to the multiple interactions in the gel:including the carboxyl group(COOH)on the polyacrylic acid chain and the gallium ion(Ga3+)to form a dynamic coordination bond,the hydrogen bond interaction between the polymer chains,and the loose covalent chemical crosslinks.In addition,the conductivity of the gel could reach 0.27 S/m.At the same time,the assembled strain sensor could detect small changes in the upper limbs(including the response of fingers at different bending angles).Furthermore,the sensor also has a large co-deformation sensing capability(perception of balloon deformation)and the ability to realize the perception of manipulator motion.(4)PAA-Catechin@LM ionogel was assembled into leather collagen fibers by self-triggered free radical polymerization to prepare ionogel/leather sensing material,also known as "leather skin".The leather with a threedimensional hierarchical structure was selected as the matrix,and then the mass ratio of the ionogel to the leather(Wi:WL)was adjusted.The influence of mass ratio on the mechanical,electrical and sensory properties of leather skin was investigated through performance tests.The leather skin was characterized by FTIR,XRD,UV-vis-NIR,TEM and SEM.The results showed that when Wi:WL was 7:1,the leather skin had mechanical properties(17.8 MPa),air permeability(745 mL/cm2/h),water vapor permeability(75 g/cm2/h)and extensive weather resistance(-80-100℃).Leather skin successfully combines the advantages of leather matrix and ionogel.Multiple strong interactions endowed leather skin with stable structure and versatility.The leather skin had high ionic conductivity(0.25 S/m).Additionly,the leather skin strain sensor was assembled,which restored the sensing performance of leather.Inspired by leather skin,a biomimetic ion leather skin glove was designed to control a robotic arm and achieve complex hand action interaction.This is of great significance for remote online detection and noncontact control systems.