| As an emerging three-dimensional nanofiber material,electrospinning nanofiber yarn does not only possess the advantages of nanofiber,such as small size,large specific surface area,high porosity,and easy doping and modification,but also has unique optical,electrical,and biological properties that are endowed by its anisotropic structure.Accordingly,its application to wearable sensors,tissue engineering,energy storage,and other fields,seems very promising.More significantly,the nanofiber yarn prepared by electrospinning satisfies the requirements of textile weaving in mechanical properties and can also be used as a conventional textile yarn for secondary processing,due to its excellent orientation,high tensile strength,and ease of weaving.Thereby,it can be applied to a wide range of traditional textiles,upgrading the technology of textile production and improving the added value of traditional textiles.Aiming at the problems of low nanofiber yield and unstable yarn formation process in the current electrospinning nanofiber yarn forming technology,a new method for fabricating electrospinning nanofiber yarn based on a combined control of stepped airflow field and negative-pressure suction was proposed in this paper.First,needleless airflow-electrospinning was used as the nanofiber supplier to meet the needs of nanofiber yarn formation.Then,the directional collection of the formed nanofibers was facilitated by the synergistic effect of stepped airflow and negative-pressure suction to achieve the bundle of nanofibers.Finally,the gathered nanofibers were twisted into yarns by traditional friction twisting method.Furthermore,a continuous nanofiber core-spun yarn could be obtained by pre-introducing a core yarn into the yarn-forming system.On this basis,wearable yarn or fabric sensor were constructed by in-situ polymerization of conductive polymer on the surface of nanofiber in the yarn,and the pressure,stretching and bending sensing properties were further researched.The detailed contents and results were as follows:（1）Based on the combination of airflow electrospinning and traditional friction twisting,a new method for fabricating electrospinning nanofiber yarn controlled by stepped airflow and negative-pressure suction was designed independently.The processing feasibility of the spinning device and spinning feasibility of the yarn preparation were verified,and the spinability of the device as well as the morphology and mechanical properties of the yarn were researched.In addition,the feasibility for fabricating nanofiber core-spun yarn was further verified by pre-introducing a core yarn in the yarn-forming system,and the morphology and mechanical properties of the nanofiber core-spun yarn were characterized and analyzed.The results showed that,the method showed a perfect spinning feasibility of the nanofiber yarn preparation and was suitable to many polymer solutions for the spinning of continuous nanofiber yarn.Prepared nanifiber yarns and nanofiber core-spun yarns displayed good yarn and fiber morphologies.Moreover,the mechanical property of the obtained nanofiber yarn could meet the requirement of textile weaving,which could be further woven into a fabric.（2）The working electric field and airflow field during the spinning process were respectively simulated through using Maxwell electric field software and Fluent airflow field software.The working principles of electric field and airflow field in the yarn forming process were analyzed.The effects of electric field and flow field distribution on nanofiber collecting and gathering were researched.In addition,the kinetic model of nanofiber forming and twisting was established through the mechanical analysis of the nanofiber forming and twisting during the spinning process,and the influencing factors and spinning mechanism on nanofiber yarn preparation were further analyzed and revealed.The theoretical research results showed that,the synergistic effect of stepped airflow and negarive-pressure suction could effectively control the trajectory of the fiber movement,which was helpful for the nanofibers to be collected in a small area of the twist triangle area.Then,the gathered nanofibers could be continuously twisted into yarn through the slubbing action of two friction rollers.（3）PAN model polymer was used as the research object in this chapter and the experimental methods of single factor variables was employed to systematically study the effects of stepped airflow pressure,working voltage,spinning distance,solution flow rate,air pumping volume and friction roller speed on nanofiber yarn yield,nanofiber diameter,yarn twist and mechanical property.The results showed that,the spinning process could be performed stably at the stepped airflow pressure of 0.2,0.4,0.6,0.8 MPa,the working voltage of 34 kV,the spinning distance of 40 cm,the solution flow rate of 48 ml/h,the air pumping volume of 10 L/min and the friction roller speed of 600 r/min.The yield of the yarn prepared under these conditions could reach to 4.207 g/h,and the nanofiber diameter was 192.06 nm.At the same time,the yarn exhibited perfect fiber orientation and twist distribution.The breaking strength and elongation of PAN nanofiber yarn were respectively 23.52 MPa and30.61%.（4）First,a GO-doped PAN nanofiber core-spun yarn with uniformly periodic waves was prepared by changing the pre-tension strain of the core yarn via the nanofiber core-spun yarn forming method designed in this paper.Then,a yarn sensor with ultra-high strain was constructed by in-situ polymerization of PPy conductive polymer on the surface of nanofibers in the yarn.The stretching,bending and pressure sensing properties of the sensor were tested,and its multi-stress sensing mechanism was researched.The results showed that,the yarn sensor exhibited excellent tensile sensing properties and could monitor the relative resistance changes from 0.1%to 500%tensile strain with the highest GF vaule of 34.63.In addition,the yarn sensor could also monitor stable bending deformation,and showed a significant relative resistance changes at different bending degrees.Furthermore,a pressure-sensitive sensing unit could also be constructed by cross-interlacing two yarn sensors.The relative resistance changes of the sensing unit increased as the loaded pressure increased and changed linearly under a small load.Importantly,the sensing unit still had sensitive and fast pressure response and cycle stability（10,000 cycles）under different tensile strains（0%-400%）.（5）Based on the PVDF nanofiber yarn prepared in this paper,PEDOT@PVDF nanofiber yarn composed of PEDOT-coated PVDF nanofibers was obtained by in-situ polymerization of PEDOT conductive polymer on the surface of nanofibers in the yarn.Then,the PEDOT@PVDF nanofiber yarn was woven into a double-layer nanofiber fabric according to the upper and lower cross-interlacing rules.Finally,a flexible,wearable and self-powered pressure-sensitive nanofiber fabric sensor was constructed based on the double-layer nanofiber fabric.The self-powered performance and pressure sensing performance of the sensor was monitored,and the sensing mechanism with high pressure sensitivity of the sensor was researched through comparing with the PEDOT@PVDF conventional fabric sensor.The results show that,the nanofiber fabric sensor could be self-powered,and the output voltage exhibited a distinct switching behavior to applied pressure.In addition,the special multi-level hierarchical structure of the sensor yielded a high sensitivity(18.376 kPa-1 at100 Pa),wide pressure range（0.002-10 kPa）,fast response（15 ms）,and high durability（7500 cycles）.Furthermore,excellent sensing performance of the sensor could also be proved to capture small stress monitoring,subtle human motion monitoring,muscle vibration monitoring,and health monitoring in daily life. |
