Design,Fabrication And Application Of Flexible,Transparent And Solvent-free Ionic Conductor Electrodes For Stretchable Electroactive Devices

Flexible and stretchable electronics is an important area of emerging technologies.Stretchable electroactive devices represented by flexible electroactive actuators,stretchable electroluminescences and stretchable transistors are expected to be widely applied in soft robots,artificial skin and flexible display.Stretchable electroactive devices consist of one strechable electroactive dielectric layer sandwiched by two flexible electrodes to form layer-by-layer structures.Structure and functions of flexible electrodes exert significant influenece on performance,stability and application areas of stretchable electroactive devices.The ideal flexible electrodes should possess multifunctions including high stretchability,conductivity,transparence as well as good compatibility and adhesion to the dielectric active layer.Unfortunately,existing flexible electrodes are far from the ideal ones,which hinders the further development of stretchable electroactive devices.The objective of the current dissertation is to explore a new path to multifunctional stretchable electrodes for stretchable electroactive devices via the controllable synthsis of a block copolymer by rational design at a molecular level.A novel class of ionic conductive diblock copolymers poly[(ethylene glycol)9 methyl ether acrylate-b-n-butyl acrylate](P(mPEGA-b-nBA))was designed and synthesized.The relationship between their chain structures and properties was systematically investigated.Also,a coating technique to form flexible transparent solvent-free electrodes was studied.Thus,high performance flexible dielectric elastomer actuators(DEAs)and stretchable alternating current electroluminescences(ACELs)were realized.Main conclusions and creative results are as follow.(1)P(mPEGA-b-nBA)with different structures were designed and controllably synthesized via RAFT solution polymerization,then was mixed with LiClO4 to fabricate a new class of polymeric conductive electrolyte,which was flexible transparen and solvent-free.It was found that the chain structure of P(mPEGA-b-nBA)and the molar ratio of[Li+]/[EO]significantly influenced the performance of the electrolyte.LiClO4/P(mPEGA140-b-nBA80)with[Li+]/[EO]=1/8 possessed conductivity of 1.25×10-4 S/cm,which was high enough for the electrodes of stretchable electroactive devices.(2)Post-crosslinkable cyclohexenyl-functionalized P(mPEGA-b-nBA)was synthesized via copolymerizing with an asymmetric divinyl monomer cyclohex-3-enylmethyl acrylate(CEA).The fabricated electrodes were able to be cured via thiol-ene UV reactions.The crosslinking density affected mechanical properties and conductivity of the cured electrolyte.Modulus and elongation at break both increased with higher content of CEA,while conductivity decreased due to the lower mobility of polymer chains and the decreased conductive volume fraction in the polymer matrix.(3)P(mPEGA-b-nBA)nanodispersion self-dispersing in water/ethanol was made to fabricate flexible stretchable transparent solvent-free electrodes to avoid mass diffusion between electrode and dielectric active layers.The influence of P(mPEGA-b-nBA)chain structure,molar ratio of[Li+]/[EO],thickness of the electrodes and crosslinking density on the actuation performance of DEAs were investigated.The DEA with LiClO4/P(mPEGA140-b-nBA80)([Li+]/[EO]=1/8)as electrode materials possessed the highest area actuation strain of 96%at 4.2 kV.The DEA with electrodes of 3 ?m in thickness could exhibit 60%actuation area strain and 87%light transmittance.However,further decrease in electrode thickness led to more defects and lower transmittance.Integration of electrodes and dielectric elatomer active layer was fullfilled by UV curing the electrodes of cyclohexenyl-functionalized P(mPEGA-b-nBA).In such cases,the maximum actuation area strain could be as high as 90%when the gel fraction of the cured electrodes was 0.49 while the actuation area strain of the DEA reduced with higher gel fraction.Focus tunable lens was directly made of the transparent DEA.The lens possessed high transparence to visible light and high fidelity to all colors.(4)A novel stretchable ACEL was designed and fabricated with ZnS:Cu particles/poly(styrene-b-n-butyl acrylate-styrene)(SBAS)as an emissive layer and LiClO4/P(mPEGA140-b-nBA80)as flexible transparent electrodes.The dielectric constant of emissive layer matrix,ZnS:Cu particle concentration and thickness of an emissive layer,the molar ratio of[Li+]/[EO]in electrodes was found to have significant influence on the performance of the stretchable ACEL.SBAS endowed the ACEL the record high luminance due to its high dielectric constant and high dielectric breakdown strength.The low modulus and large elongation at break of SBAS made the ACEL highly stretchable and conformable,which is particularly attractive for wearable applications.The block copolymer of P(mPEGA140-b-nBA80)electrodes offered good compatibility and adhesion between different layers so that the stretchable ACEL worked well even when being stretched,bended,folded or twisted.Solvent-free electrodes allowed the ACEL to remain the high performance for long time.The highest luminance reached as high as 450 cd/m2 at 1 kHz 22 kV/mm.Once 500%being stretched,the stretchable ACEL could still remain 35%luminance of its initial state.After 1000 cyclings stretched from 150%to 300%strain,85%initial luminance remained.The luminance maintained 95%after exposing to ambient environment for 10 days.Compared with the reported ZnS:Cu-based stretchable ACELs,the fabricated ACEL represents a significant advance in terms of the record high luminance,high stretchability and excellent performance stability.