Study On Electrospun Jet, Fiber-forming Process And Morphologies Of Electrospun Fabric

As a new method to produce nanofibers,electrospinning is attracting more and more attentions from the scholars.Now,the study on electrspinning mainly focuses on the nanofiber and the fabric applications.But,the studies on the fiber-forming process and the jet are very limited.To have a better understand to the fiber-forming process, electrospun jet,especially unstable jet,is the emphasis in this paper.At the same time, the morphology and the structure of electrospun fabric were investigated.Traditional electrospinning set-up was employed to electrospin Polyvinylalcohol (PVA) solution.There are many parameters which affect electrospinning.The effects of following most important parameters:solution concentration,solution conductivity and applied voltage on the electrospun jet and fabric were evaluated.Firstly, fiber-forming process was analyzed with a glass slide,adhered to a long insulating rod, which was swept below the jet at different locations and an optical microscope which was attached to a computer.During fiber-forming process,jet diameter became smaller and smaller and liquid jet changed into solid nanofiber with solvent evaporation.Meanwhile,different jet morphologies were observed:the jet with various cross-section,the jet with various curvatures and the jet with various shapes. According to the above work,fiber-forming process and jet morphology during electrospinning were obtained which favored to the study on the nanofibers produced under different electrospun conditions.Because our previous study,solution conductivity was improved through adding different LiCl concentrations into the solution.Since there are direct relations between jet trajectory and the morphology of the fabric and nanofiber,when PVA solution concentration between 6 and 12 wt%, LiCl concentration between 0.2 and 1 wt%,or applied voltages between 15 and 35 kV was employed,the length of stable jet(L) and the vertex angle of envelope cone (θ) were studied in the paper.Results indicated that L increased with the increase of solution concentration and applied voltage,but decreased with the increase of LiCl concentration.Results also indicated thatθdecreased with solution concentration, LiCl concentration and applied voltage.When LiCl concentration was 1 wt%or applied voltage was 35kV,stable jet disappeared,resulting in a whipping jet over the whole needle-to-collector distance.The change of L orθwas decided by the competition between longitudinal draw force and radial draw force acted on electrospun jet.Due to high-voltage electrostatic field,solution conductivity has a significant effect on electrospun jet and nanofiber.So,the effect of LiCl on the variation of jet diameter with longitudinal coordinate after the onset of whipping instability was investigated carefully.The following results were obtained.The jet with uniform diameter was observed after adding LiCl into electrospun solution and jet diameter decreased with the increase of LiCl concentration.In other words,the addition of salt in polymer solution made electrospun jet easy to thin and solidify. When LiCl concentration was equal or larger than 0.8 wt%,surface charge became saturated and full charge on jet surface was obtained.The power exponent b in the allometric law had an approximate linear relation with salt concentration.Above experiments were primary experiments in the paper.Because of jet high -speed and complex movement in high-voltage electrostatic field and very small jet diameter,it is hard to study electrospun jet with experiments. So,numerical simulation was used and aimed to understand jet movement under different spinning conditions.Thus,nanofiber quality on the collector was controlled by controlling electrospun jet.Compared with other software used in computational fluid dynamics(CFD),POLYFLOW was chose preferentially to simulate jet movement in 2D space.Due to the limitation of POLYFLOW and the complexity of unstable jet movement,the software FLUENT was used to simulate the movement of unstable jet in 3D space.Meanwhile,experimental verifications were carried out for some simulation results.When POLYFLOW and FLUENT were used,GAMBIT was choose as pre-processing software to build geometric model and plot grid.For stable jet,results indicated that when solution volumetric flow rate was kept constant,jet diameter became thin easily with increasing draw force.But when draw force was too great,there were rough knot and suddenly thinning appeared along jet.When draw force reached a value,stable jet disappeared during electrospinning.Jet velocity during fiber-forming process increased with the increased of draw force.When draw force was not big,there was a linear relationship between jet velocity and longitudinal coordinate.To this case,electrospun jet and nanofiber with uniform diameter were obtained.For unstable jet,results indicated that LiCl concentration was the most important parameter.For different longitudinal draw force,there were biggest and lest radial velocity to form unstable jet separately.The increase of solution concentration and applied voltage resulted in longitudinal draw force increased which favored to stable jet and the increase of LiCl concentration made radial draw force increase which favored to unstable jet.When polymer solution with the addition of LiCl was electrospun,at any time,there were similar jet trajectories during fiber-forming process which produced nanofiber with narrow diameter distribution.When applied voltage was big,there were very different jet trajectories which produced nanofiber with wide diameter distribution.There even was a wet spot in the middle of electrospun fabric.Following the analyses of the jet during fiber-forming process,the effects of solution concentration,LiCl concentration and applied voltage on electrospun fabric, including the morphology of nanofiber deposition and nanofiber,were also investigated in the paper.To improve nanofiber quality,the effect of solution concentration and LiCl concentration on nanofiber diameter and beaded fiber was also studied.By far,the mechanical properties of electrospun fabric are poor which hinders its application.So,fabric crystal under different electrospun condition was investigated which had practical meaning to improve nanofiber quality and expand fabric application.The following results were obtained:(1) Nanofiber deposited area decreased with the increase of solution concentration and LiCl concentration.When LiCl concentration was 0.2,0.5 and 0.8 wt%separately,there were little protuberances in the middle of electrospun fabric.Nanofiber deposited area decreased with the increase of applied voltage firstly,and then suddenly increased.When applied voltage was equal or bigger than 25 kV,little protuberances and wet spot were observed in the middle of electrospun fabric.There was an exponential function between the diameter of nanofiber deposited area and the three parameters mentioned above separately which was similar as the effect onθ.(2) Nanofiber diameter increased with the increase of solution concentration and decreased with the increase of LiCl concentration.(3) Solution concentration was the most significant parameter which affected nanofiber diameter and beaded fiber and solution conductivity was the most effective parameter which affected nanofiber diameter distribution.(4) Fabric crystallinity increased with the increase of solution concentration and applied voltage firstly separately,and then decreased.It decreased with the increase of LiCl concentration.In the paper,when solution concentration was 10 wt%,LiCl concentration was 0wt%and applied voltage was 20 kV,electrospun fabric with best crystal was obtained.