Construction Of Zeolite Imidazole Framework/Polyvinylidene Fluoride Composite Fiber For Enhanced Pressure Sensing Performance

Flexible pressure sensors have made great process in wearable electronic devices,human-machine interaction and other fields.The market demand for flexible devices with high sensitivity and skin friendliness is gradually increasing.Fiber/textile materials have attracted extensive attention because of their good breathability and skin-friendly properties.However,due to the low dielectric constant and relatively complex reprocessing of traditional fiber materials,it is difficult to achieve high sensitivity of the fiber-based sensor,thus hindering their detection ability forsmall physiological signals such as pulse.Therefore,this work proposes to use electrospinning technology and in-situ growth method to construct a composite nanofiber membrane,with a metal organic framework（MOF）,zeolitic imidazolate framework-8（ZIF-8）particles,evenly distributed on the polyvinylidene fluoride（PVDF）fiber surface.The introduction of MOF not only increases the dielectric constant of the fiber,but also increases the contact area of the microstructure obtained on the surface of the fiber,thereby significantly improving the sensitivity of the fiber-based sensor.Next,the composite fiber membrane was used as a functional dielectric layer to fabricate flexible capacitive pressure sensors,and the sensing sensitivity of both and the practical application of human physiological signal monitoring were investigated in detail.The main research content and results are as follows:（1）Construction of ZIF-8/PVDF composite fiber membraneFirstly,PVDF fiber membrane was prepared by electrospinning,and ZIF-8 powder was prepared by hydrothermal method.The ZIF-8 powder and PVDF spinning solution were uniformly mixed to obtain the blended ZIF-8/PVDF composite fiber membrane,but there was an obvious agglomeration of ZIF-8 particles in the fiber membrane,lead to ununiformly composites.In order to obtain uniformly composites,we added the components of MOF in two steps.Firstly metal salts（Zn（NO₃）₂·6H₂O）were added to PVDF solution and then electrospinning,the obtained nanofiber was soaked in organic ligand（dimethylimidazole,2-Hmim）solution,and ZIF-8 particles could be grown in-situ on the fiber surface.The ligand content and soaking time had a great influence on the in-situ growth of MOF particles.Therefore,by adjusting the organic ligand content and in-situ growth time,ZIF-8/PVDF composite fibers under optimal in-situ growth conditions（140 mg/12 h）were obtained.This in-situ grown ZIF-8/PVDF composite fiber membrane not only improves the dielectric constant of the composite fiber membrane,but also obtains a fiber membrane with excellent surface microstructure due to the dense and uniform adhesion of ZIF-8 particles on the fiber surface.（2）Fabrication of flat capacitive pressure sensor and research on its sensing performancePure PVDF fiber membranes,blended ZIF-8/PVDF composite fiber membranes with different ZIF-8 powder mass ratios,and ZIF-8/PVDF composite fiber membranes under different in-situ growth conditions were used as the functional dielectric layer to prepare pressure sensors,respectively.Noted that conductive silver fabrics were used as electrodes.It was found that the sensing sensitivity of the capacitive pressure device with the ZIF-8/PVDF composite fiber membrane as the dielectric layer had the highest sensitivity.In particular,in the small pressure range（0-0.3 k Pa）,the sensing sensitivity of in-situ grown ZIF-8/PVDF composite fiber membrane as dielectric layers is three times higher than that of sensor devices with blended ZIF-8/PVDF composite membranes as dielectric layers,and four times higher than that of sensor devices with pure PVDF fibre membranes as dielectric layers.In addition,the sensor has a minimum detection capability of 1.0 Pa and exhibits good cycling stability（>10,000 cycles）.This capacitive pressure sensor can work well on the surface of the human body to monitor physiological signals such as pulse,respiration,and swallowing in real time,providing data support for human health.（3）Fabrication of capacitive pressure sensor yarn and research on its sensing performanceThe flat pressure sensors have the disadvantages of limited gas permeability and difficult integration into wearable electronics.Therefore,a fiber shape capacitive pressure sensor was designed.Copper wire was used as the electrode 1 for electro spinning,the ZIF-8/PVDF composite fiber with optimal in-situ growth conditions was used as the functional dielectric layer,and the conductive silver fiber was adopted as the electrode 2.The cross-sectional microstructure of the sensor,the sensing sensitivity of the device with different number of copper wires and electrospinning time have also been investigated.The results show that with the increase of the number of copper wires,the sensing sensitivity tends to increases and then decreases.With the increase of spinning time,the sensing sensitivity showed an increasing trend.Considering the sensing application requirements and volume requirements of the device comprehensively,it is determined that the electrode 1 uses 4 copper wires and the spinning time of 8 minutes was regarded as the optimal production parameters.The sensor has good sensing linearity and cyclic stability,and can be woven and bent at will,so the capacitive pressure sensor with fibrous structure can be integrated into knitted clothing for monitoring human respiration and joint activities.