Performance And Constructions Of Surface Microstructure Based On Conductive Polymer For Interlocking Structure Flexible Piezoresistive Materials

A flexible piezoresistive material refers to a flexible conductive composite material that has good flexibility and is capable of freely bending,folding,and even stretching and converting external mechanical stress into an electrical impedance signal.In order to overcome the problems of low sensitivity,slow response,severe hysteresis and poor stability of traditional flexible piezoresistive materials,Interlocking Structural Flexible Piezoresistive Sensing Materials?IFPSM?have been widely studied,whose mechanism is mainly based on conversion of the conductive contact point or contact area on the interlocking surface microstructure under pressure.Therefore,the type and size of the surface microstructure are the main influencing factors of piezoresistive performance.At present,surface microstructures are mostly realized by micro-nano processing template such as photolithography,whose cost is high,efficiency is low and it is difficult to prepare in a large area and adjust the size.In this thesis,the relationship between structural design and performance regulation is studied based on the construction of surface microstructures with easy-to-adjust size and its regulation mechanism for IFPSM sensing.The templating method is still the main way to construct microstructures.However,this process cannot achieve microstructure construction on curved surfaces.The fibrous IFPSM is one of the most important wearable flexible piezoresistive sensing materials because of its spinnable and woven characteristics that can be integrated into clothing or fabrics for body bonding.At present,the preparation of fiber surface microstructures,especially the regulation of the surface microstructure size,is still a great challenge.In order to solve this problem,a two-dimensional polypyrrole wrinkles structure having a wavelength of 1.2-10.2?m was constructed on the surface of the TPU fiber by means of pyrrole as a solvent and also as a reactant in the manner of swelling-induced TPU expansion.This process allows the production of size adjustable conductive wrinkles on very fine?<50?m?and coarser?>500?m?TPU fibers.The piezoresistive properties of the conductive fibers interlocking with copper sheets were studied.The results show that the smaller wrinkle size,the higher the sensitivity.When the wrinkle wavelength is 1.2?m,the sensitivity can reach 0.4 kPa^-1,response time is less than53 ms,and the stability of use is higher than 4000 times.The two conductive fibers"cross"were interlocked and pressed,and it was found that the single conductive fiber did not have the piezoresistive effect,and the double-conductive conductive fiber had a piezoresistive sensitivity of 0.1kPa^-1.The conductive fiber can be woven and cannot be damaged within 70%stretch,so that it has a good application prospect in the field of wearable electronics.The thin-film IFPSM rich microstructural construction method makes it more excellent.A polyurethane film with a Ra of 10-45?m roughness was constructed by using industrial sandpaper as a template.Polypyrrole conductive wrinkles with a wavelength of 0.5?m were prepared by a“swelling-deposition”method on the rough surface,and a secondary rough structure IFPSM was constructed.By constructing a"bridge"type IFPSM with the PET interdigital electrode,the sensitivity of the IFPSM to pressure is up to 6.1 kPa^-1?0-2 kPa?due to the presence of the secondary roughness and the"bridge"assembly.,the stability of use is greater than 3500 times,and the response and relaxation times are slightly 18ms and 35ms respectively.Further,we found that the initial resistance of the"bridged"IFPSM increases with the increase of roughness.The resistance reaches a"saturated"state with pressure within 5 kPa,and the"saturation"resistance at different roughness levels is basically the same.Therefore,its sensitivity is determined by the initial resistance.With the developments of IFPSM,the focus of research has gradually evolved from simple improvement to multi-functionality.A high specific surface area?>105 m²/g?polyaniline nanofiber having a length of about 1?m and a diameter of about 50 nm was prepared by an organic/water interfacial polymerization.A dense,uniform and stable hierarchical conductive network structure with polyaniline nanofibers coating at a cotton fiber nonwoven fabric with a diameter of 10?m by a simple dip-drying process and a dual sensing material with pressure sensitive and gas sensing functions is prepared.The structure ensures sufficient gas escape channels and adsorption specific surface area,and also ensures that sufficient variable conductive contact points are formed.The influence of the loading content of polyaniline nanofibers on pressure sensitivity and gas sensitivity was further studied.It was found that the optimal pressure-sensitive load was near the conductive“percolation threshold”?0.36 wt%?,and the gas sensing sensitivity increased with the increase of load.In order to solve the contradiction of the load amount,multiple layers of sensing film with loading near"percolation threshold"were assembled to form a multi-layer interlocking IFPSM.It was found that the multi-layer interlocking structure can simultaneously improve the pressure sensitivity and gas sensitivity performance.The sensitivity of the pressure sensing is 0.89 kPa^-1,and the sensitivity is 18.8?50 ppm NH₃?.In combination with the structural adjustment of micron-scale in pressure sensing and nano-scale in gas sensing,the gas sensitivity and pressure sensitivity are simultaneously improved.