Mechanically Robust,Transparent And Selfhealing Conductive Ionic Gels: Preparation,properties And Application In Flexible Sensors

Ionic gels might combine the superior characteristics of hydrogels and conductive materials,such as good flexibility,stretchability,and conductivity,making them“ideal”materials for preparing flexible electronic devices.Despite of many reports about ionic gels during the past years,the application of ionic gels is still limited by a few challenging problems,including weak mechanical strength,poor stretchability,and incompatibility between the conductive fillers and hydrogel matrix.In this Dissertation,a hydrogel with high mechanical strength,excellent stretchability,and good transparency was developed by utilizing the hydrogen bonding and chain entanglement between polyacrylic acid?PAA?and high molecular weight polyethylene oxide?PEO?.At the same time,a small amount of lithium bistrifluoromethylsulfonylimide?Li TFSI?was doped as an electrolyte salt,which further endowed the hydrogel with good ionic conductivity.Flexible sensors based on this ionic gel show good sensitivity,stability and repeatability,which might provide a simple and cost efficient material for applications in the fields of wearable electronic devices,implanted devices,flexible sensors,and actuators.In a typical experiment,PEO was used as the macromolecular physical crosslinker for the hydrogel,2-hydroxy-4'-?2-hydroxyethoxy?-2-methylphenylacetone?I2959?was used as the photo-initiator,and Li TFSI was doped as an electrolyte salt.When mixed with acrylic acid?AA?monomer at a desired composition,the resulting clear precursor solution could be photo-polymerized in situ to form a transparent and stretchable PAA-PEO-Li ionic gel.Characterization results show that the monomer molar ratios of PAA/PEO and the molecular weight of PEO significantly affect the mechanical properties of the ionic gels.For example,when the molecular weight of PEO is 100 kg/mol and the monomer molar ratio of PAA/PEO is0.6:1.0,the tensile strength of the ionic gel is 1.63 MPa and the elongation at break can reach2003%;while when the molecular weight of PEO increases to 600 kg/mol and the monomer molar ratio of PAA/PEO is 1.0:0.6,the tensile strength of the ionic gel increases to 6.37 MPa,and the tensile strain can reach 693%before breaking.At a weight percentage of no more than7wt%,Li TFSI dissolve well in the hydrogel matrix and maintain good compatibility,resulting in ionic gels with ionic conductivity as high as 8.78×10^-3 S/cm.Due to the reversible nature of the hydrogen bonds and physical entanglement,the ionic gels can self-heal to restore their mechanical properties and conductive properties in a humid environment without external stimulation.Flexible sensor prepared by the ionic gels can sense the different movements of human body and mimic the perception of external pressure and temperature change of human skin.