Transparent And Healable Ionogels With Excellent Mechanical Properties For Flexible Strain Sensors

In recent years,ionogels have become one of the ideal choices for the preparation of flexible electronics due to their tunable mechanical and electrical properties.However,since the mechanical properties of ionogels are generally weak,and inevitably be damaged in daily use.Therefore,there is an urgent need to prepare ionogel materials with both excellent mechanical properties,including high mechanical strength or tear resistance ability,and damage repair capability.Thus,on the one hand,the probability of mechanical damage to the material is reduced.On the other hand,even if the ionogels are mechanically damaged,the wound can be repaired by external stimulation to restore the mechanical and electrical properties of the ionogels,which will effectively improve the safety and durability of the ionogels in practical uses.In this thesis,a series of transparent and repairable ionogel materials with excellent mechanical properties are designed and prepared to address the above-mentioned problems in the field of ionogels research,which provides new research ideas and methods to solve the current problems of poor mechanical properties and non-tear resistance of ionogels,and further expands the application of ionogels in the field of flexible strain sensors.It is hoped that the research in this thesis can contribute to the large-scale preparation of flexible ionogel strain sensors with specific excellent mechanical properties and healability.1.How to prepare flexible ionogel strain sensors with high mechanical properties(fracture strength>10 MPa)is a great difficulty in the current research field of ionogel sensors.We firstly selected two poly(ε-caprolactone)diols with different molecular weighes that have good compatibility with ionic liquids as flexible segments;2,2-bis(hydroxymethyl)propionic acid and N,N-di-tert-butylethylenediamine were chosen as the chain extenders to synthesize linear poly(urea-urethane).Then,the linear poly(urea-urethane)was further cross-linked by Desmodur N3300 curing agent so as to prepare covalently crosslinked high strength poly(urea-urethane)(PUU)with high mechanical strength.Within the covalently crosslinked network of as-prepared PUU,poly(ε-caprolactone)diols worked as the flexible chains to provide soft phases of PUU;while the large hydrogen-bond domains that were formed through the hierarchical hydrogen bonds,including the double hydrogen bonds between urethane and urethane groups of the chain extender 2,2-bis(hydroxymethyl)propionic acid and the hydrogen bonds between carbamates,worked as the hard phases of PUU.The density of the large hydrogen-bond domains within the PUU can be finely tailored by tuning the ratio of two poly(ε-caprolactone)diols with different molecular weights,so as to control the mechanical properties of the PUU.After the successful synthesis of PUU,ionic liquid was further dispersed to the PUU network to obtain the ionogels that simultaneously possessed superhigh mechanical strength of 19.2 MPa,large stretchability(～1200%),high transparency,and good elasticity.Besides the prominent mechanical properties,the ionogels also had high efficient healability attributed to the multiple reversible dynamic interactions,including the hierarchical hydrogen bonds as well as the large steric-hindrance urea bonds formed by the reaction of N,N-di-tert-butylethylenediamine with isocyanates.These multiple reversible dynamic interactions can be opened and reformed under the thermal stimulation,endowing the ionogel with efficient healability.Moreover,the ionogels cross-linked by Desmodur N3300 curing agent also have excellent elasticity.When the ionogels were encapsulated into a flexible strain sensors,they have remarkable ensitivity,high tensile stretchablity,low hysteresis,and can sensitively detect different human activities.2.How to endow the ionogel strain sensors with both excellent tear resistance and elasticity is a huge challenge due to the naturally contradiction betwwen these two mechanical properties.On the basis of the above work,we used poly(ε-caprolactone)diols as the flexible chains,4,4-diaminodiphenyl ether and piperazine as the chain extenders,and Desmodur N3300 curing agent as the cross-linker,and finally synthesize a covalently cross-linked poly(urea-urethane)with excellent tear resistance.After that,ionic liquid was dispersed into the poly(urea-urethane)networks to obtain the ionogels.4,4-Diaminodiphenyl ether can form urea bonds upon reaction with isocyanate and together form dynamic hydrogen bonding hard domains through multiple hydrogen bonding interactions between urea-urea groups andπ-πstacking between benzene rings,thus imparting tear resistance to the ionogels while improving its mechanical strength.The resultant ionogels possess excellent tear resistance(fracture energy of about 48.68k J/m~2),ultra-high mechanical properties(fracture strength of about 22 MPa),and high stretchability(strain of about 1700%).Besides the excellent mechanical properties,the ionogels also have high efficient healability owing to the multiple reversible dynamic interactions,including the multiple hydrogen bonds between urea groups as well as the large steric-hindrance urea bonds formed upon the reaction of piperazine with isocyanate.After being chemically cross-linked by Desmodur N3300 curing agent,the ionogels also exhibit excellent elasticity.We encapsulated it into a flexible strain sensor,and the sensor has high sensitivity,large tensile stretchability,and low hysteresis,which can detect different human activities.