| Recently, with the rapid development in nanoscience and nanotechnology, nanofibersbecome a hot topic in the fields of textile and material science.Based on different principles, lots of methods can produce fibers in nano-scale, suchas stretching, template polymerization, phase separation, self-organization andelectrospinning. Compared them, electrospinning has been recognized as the most feasibletechnique for the fabrication of continuous polymeric nanofibre, besides that, it also canproduce nanofiber bundles and nanofiber yarns by changing the negative pole. Althoughthe shaping mechanism of electrospinning, product structure control and its functionalapplication from various polymeric materials have developed well, the basic performanceof nanofiber bundles and the continuity still need to be intensively studied.Single wall nanotubes(SWNTs) were acidized to purify the inclusions and thenamidated to graft amide group. Add the functionalized SWNTs into PA6/formic acidsolution, and consecutively produce PA6/SWNTs nanofiber bundles by uniqueelectrospinning. After that, study the nanofiber bundles shaping mechanism by discussingthe morphology of the Taylor cone and the trajectory of the jet flow, also by studying thespinning conditions of producing nanofiber bundles steadily and continuously. Finally,PA6/SWNTs composite nanofiber bundles with different length-diameter ratios anddifferent contents of SWNTs were made by the optimized parameters. Study thedistributions, status and the electrical properties enhancement effects of SWNTs inside ofnanofibers, meanwhile, discuss the transfer mechanism of the charge and the strain sensingmechanism.The results show that the length of SWNTs decreased and dispersed well in thePA6/formic acid solution after amidating. The optimized parameters to produce nanofiberbundles were20kV,0.09ml/h and5.4m/min. Mechanical properties results show that compared to the filaments with acided SWNTs, the breaking stress of PA6/SWNTsnanofiber filaments with amide functionalized SWNTs increased obviously, up to79.75MPa from43.42MPa, furthermore, the initial module was also increased about100%.The stretching curve indicated that all the nanofibers in the bundle breakdown in thesame time and the yield point without intensifying area after that exist in the curve. TheFast Fourier Transform (FFT) results show that the serrated waves in the curve are noisewhich generated from the stretching process, really not from the asynchronous fracture.The Taylor cone and the trajectory of the flow jet were recorded by a high speedcamera and the shaping mechanism was investigated by studying the relationships betweenthe winding speeds and the diameters of the nanofibers and bundles. The stream of liquiderupting from the spinneret firstly, and then deposited onto the surface of the negativecollector forming randomly-oriented nonwoven fiber mats, at this point, the surface of thenanofiber is solidified, but the inner of it is still viscoelastic state, the solvent wouldevaporate in the follow-up drying and winding process. After that, under the bundlingeffect of the bath, nanofiber filaments were converted into bundls and wound onto apackage. The morphological structure and viscoelasticity of the nanofiber in differentstages were tested by scanning electron microscope (SEM) and Atomic ForceMicroscope(AFM). The AFM results show that the mechanical property of the nanofiberafter drying is the best, and after that are the bath stage and the flying stage, respectively.At last, the easy backing-off bundles comprised with high alignment and orientationdegree of nanofibers were obtained by controlling the winding speed. When the windingspeed is lower than7.2m/min, the main role of the winding process is rearrange therandomly-oriented nanofibers without stretching it. However, when the winding speed isfaster than7.2m/min, the quantities of the nanofibers in the bundle is already fixed and thehook can no longer unbend. So the winding process can be regarded as hot-stretching, themolecular chain would be rearranged and the mechanical property of it would beimproved.The intensity and distribution of the electric fields in the electrospinning process have be simulated by using the3D Maxwell software, the results show that with the increase ofthe spinning space, the electrical intensity close to the spinneret tip weakened anddistributed more separately, the electrical field of the negative pole move to the central partfrom left. The conductivity of the negative pole can relieve the electrical voltage by theweak current way, when the conductivity of the bath decreased, the volume and the angleof the Taylor cone increased, while the straight segment change little firstly, but decreasedsuddenly when the bath conductivity down to0S/cm.The conductivity of the as-spun nanofiber bundles with different length-diameter ratioand contents of SWNTs were studied and there is almost109order of magnitudeimprovement by adding SWNTs. The percolation threshold of it is about0.8wt%and theelectrical conductivity increased directly with adding SWNTs up to that threshold and.The conductivity of the PA6/SWNTs nanofiber bundles with length-diameter of10000~15000SWNTs were larger than with length-diameter of1000~1500under the samecontents, and the percolation threshold came out earlier. The testing length has little effecton its conductivity and the biggest difference was7.8%when changing the testing lengthfrom2mm~2cm.A custom-made test fixture was used to extend the as-spun bundles and measure thestrain sensitivity of the PA6/SWNTs nanofiber bundles. The results show that there is apositive relationship between△R/R and△L/L when the strain varied between10%and60%. Also, higher CNT concentration and length-diameter yield lower△R/R, and hencehigher sensitivity, while the sensitivity varied little when changing the initial testing length. |
PDF Full Text Request