Construction And Force Sensing Performance Of Ionic Conductive Gel Composites With Hierarchical Responsive Networks

Flexible force sensors are indispensable information collection devices in the Internet of Things（Io T）system.As an important flexible electronic,flexible mechanical sensor can collect and convert external mechanical stimuli into electrical signals,having broad application prospects in human-machine interactions,intelligent robots,and personal healthcare.Currently,as flexible mechanical sensors have put forward higher requirements for sensitivity,stability,and measurement range,the development of flexible mechanical sensing materials with high flexibility and high conductivity has become an urgent demand.Ionic conductive gel is a kind of flexible conductive material with ions as carriers,having similar composition,structure and mechanical properties to human tissue.Due to their high stretchability,high conductivity and biocompatibility,ionic conductive gels are considered potential flexible mechanical sensing materials.However,due to the lack of efficient structural processing methods,flexible mechanical sensors based on bulk ionic conductive gels show some problems,such as low sensitivity and poor limit of detection.In addition,flexible mechanical sensors inevitably suffer from scraping,puncture,or cutting damage in practical applications.It is easy to cause device failure due to the rupture of the ionic conductive gel structure.Existing ionic conductive gels are often cross-linked by high-density covalent cross-linking agents,and their network structure has poor dynamic reversibility.Therefore,it is difficult to repair or recycle the equipment after failure,resulting in the waste of resources and e-waste.Herein,we proposed a hierarchical cross-linking strategy and developed the structural processing methods of ionic conductive gel.The relationships between the composition,network structure and corresponding mechanical and sensing properties are deeply studied.A series of ionic conductive gels with high sensing performance were fabricated.The main results of this dissertation are as follows:（1）A stretching/competitively-coordinating/releasing（SCR）strategy was developed to design and prepare a polyacrylamide/alginate hydrogel（SPAH）with a self-wrinkled structure.Compared with traditional microstructural processing methods,the SCR strategy realizes controllable adjustment of the amplitude and period of the wrinkling structure on the surface of the SPAH.Moreover,this method can be extended to hydrogels with different geometries to realize the controllable construction of wrinkling structures on the surface of complex geometries.In addition,the SCR strategy also constructs soft/hard cross-linking networks from the outer layer to the inner layer for the SPAH,which can realize multi-level dissipation of energy and make the SPAH have excellent mechanical performance.Due to its unique wrinkling structure,excellent stretchability and high conductivity,SPAH can work as a mechanical stimulation-sensitive ionic conductive gel in a flexible capacitive sensor,showing high sensitivity(3.19 k Pa^-1),wide detection range（0.025 to 20 k Pa）and low detection limit（<25 Pa）.（2）A gradient-responsive cross-linking strategy was developed to fabricate a thermoplastic engineering ionogel（TPEI）.TPEI is composed of polyvinyl alcohol/polyacrylamide（PVA/PAAm）and choline chloride/ethylene glycol（Ch Cl-EG）.Hydrogen bonds and microcrystalline domains act as weak dynamic bonds and strong dynamic bonds to cross-link the TPEI.The microcrystalline domains ensure the mechanical strength and structural stability of the TPEI at ambient temperature,and high-density hydrogen bonds make the TPEI have fast self-healing ability.At high temperatures,the microcrystalline domains of the TPEI are destroyed,resulting in the sol-gel transition of the TPEI,which makes it possible to recover materials and design structures through thermal processing technology.As an ionic conductive gel,TPEI can work as a flexible mechanical sensing material,showing a short response time（～140ms）,wide-temperature tolerance（-20～80 ^oC）and excellent durability.（3）A competitively-gradient cross-linking strategy was developed to fabricate a polyacrylic acid/hydroxypropyl trimethylammonium chloride chitosan（PAA/HACC）ionic conductive elastomer（PHIE）.Hydrogen bonds and electrostatic interactions act as weak physical cross-linking networks and strong physical cross-linking networks in the PHIE,respectively.Notably,hydrogen bonds and electrostatic interactions are in a competitive-cooperative relationship in the PHIE.The proportion of hydrogen bonds and electrostatic interactions in the PHIE can be regulated by the p H value of the system to fabricate PHIE with different mechanical properties.The gradient physically dual-cross-linking network enables PHIE to have excellent mechanical properties and self-healability at ambient temperature,and show a thermoplasticity at high temperature.In addition,PHIE also shows temperature-dependent transient-response rheology.Compared to TPEI,the unique rheological property of PHIE enables it to be processed and structurally designed by fused deposition modeling 3D printing.Printed PHIE can be used as a stress stimulation-sensitive ionic conductor to assemble a capacitive sensor,showing high sensitivity(2.46 k Pa^-1),low limit of detection（11 Pa）and a wide response range（0.01-10 k Pa）.（4）A hyperbranched gradient cross-linking strategy was developed to fabricate a hyperbranched polyethyleneimine/thioctic acid（HPEI/TA）ionogel（HPTI）.HPTI has a hyperbranched molecular structure with a large number of end groups and branched chains,which makes it possible to design and construct more cross-linked structures to achieve the high mechanical strength of the HPTI.Partially uncross-linked branched chains can make HPTI molecules have good mobility even under high-density cross-linking.Hyperbranched molecular structure combined with the gradient dynamic cross-linked design of hydrogen bonds and disulfide bonds makes HPTI have high mechanical strength and fast self-healing performance,overcoming the problem of the low mechanical strength of PHIE.In addition,the HPTI can be recycled and remolded by fused deposition modeling 3D printing.The intriguing properties of HPTI with excellent mechanical properties,self-healability and 3D printability,make it a potential ionic conductor for using flexible mechanical sensors.The assembled capacitive sensor based on 3D printed HPTI shows a high sensitivity(1.32 k Pa^-1)and a low limit of detection（<0.032 k Pa）.