【作者】 李妮；

【导师】 王善元； 覃小红；

【作者基本信息】 东华大学 ， 纺织材料与纺织品设计， 2008， 博士

【摘要】 静电纺作为一种新型生产纳米纤维的技术,越来越多的受到学者的关注。目前关于静电纺的研究大多集中在纳米纤维以及纤网的用途上,而关于成纤过程和射流的研究相对较少。本文重点研究了成纤过程中的射流,特别对于不稳定射流形态作了较多的研究,通过对射流进行研究从而对纳米纤维的形成过程有了更加深入的了解。同时,本文也对纤网的形态和结构做了部分研究。本文采用传统的静电纺小样机,用聚乙烯醇(PVA)作为研究对象来进行静电纺丝,在影响静电纺的众多参数中,选取了溶液质量分数、溶液导电性和施加电压这三个主要参数,研究了它们对静电纺成纤过程中射流和纤网的影响。首先,本文采用载玻片法截取动态射流,结合光学显微系统,分析了射流在电场中通过不同阶段的拉伸逐渐细化成纳米纤维的过程,射流的细化过程也是一个从液态射流变化成固态纳米纤维的传质过程。同时,观察了成纤过程中射流存在的不同的形态:根据射流的横截面的变化情况把射流分成直径均匀变化和直径不均匀变化的射流;根据射流的弯曲曲率得到弯曲程度不同的射流;根据射流的弯曲形状把射流分成锯齿形、波浪形、正弦形等形状的射流。通过以上的工作,得到了成纤过程和成纤过程中射流的形态,有利于后面分析纳米纤维的变化规律。根据本课题组以往的研究结论,本文通过在静电纺丝液中添加不同含量的LiCl盐来改变高聚物溶液的导电性。由于成纤过程中射流的运动轨迹影响到接受装置上纤网的沉积形态和纳米纤维的形态,本文分别研究了当溶液的质量分数为6～12%、LiCl盐含量为0～1%、施加电压为15～35kV时,成纤过程中稳定段射流长度和以不稳定为顶点的圆锥包络轨迹的变化情况。通过统计分析发现:成纤过程中稳定段射流的长度随着溶液质量分数和施加电压的增加而增加,但随着LiCl盐含量的增加而减少;圆锥包络轨迹的顶角均随着它们的增加而减少,当LiCl盐含量为1%或施加电压为35kV时,在喷头处观察到剧烈振动的射流。稳定长度或圆锥包络轨迹的顶角增大或减少是作用在射流上的纵向拉伸力和径向力相互作用的结果。由于涉及到高压静电场,溶液的导电性对射流以至于最后的纳米纤维影响很大。由此本文进一步研究了不同LiCl盐含量条件下,成纤过程中不稳定点后射流直径同以不稳定点为原点指向接受装置的纵向坐标的关系,对于10%的PVA溶液,得到了LiCl盐含量与射流表面满电荷的关系。分析发现溶液中加入盐可以得到直径均匀变化的射流,同时,盐的加入可以减少射流的直径,加快静电纺成纤过程,当LiCl含量等于或大于0.8%时,射流表面达到满电荷。能量级数b和LiCl含量呈线性关系。以上研究内容为本文的主要实验内容,研究过程逐渐深入,以便对静电纺成纤过程有较全面的认识。由于静电纺中射流在高压静电场中的高速复杂运动和射流本身直径很小,用实验的方法研究相对比较困难,因此本文把数值模拟应用到静电纺中,希望通过软件模拟的方法得到成纤过程中射流在不同静电纺条件下的变化规律,从而通过控制射流来控制接受装置上的纤网,数值模拟的研究对象也是成纤过程中射流。本文通过比较分析,主要采用了专用于粘弹性材料流动仿真的CFD软件POLYFLOW来模拟成纤过程中的稳定射流和不稳定射流在二维空间上的运动状态。由于POLYFLOW软件的局限性和不稳定射流运动的复杂性,本文还采用了另外一个CFD软件FLUENT来辅助模拟不稳定射流在三维空间的运动状态。同时,对以上部分模拟结果进行了验证。在使用POLYFLOW和FLUENT进行模拟时,均采用了GAMBIT前处理软件建立几何模型和进行网格划分。对于稳定射流,模拟结果表明:在溶液流量一定的情况下,随着拉伸力的增加,射流直径的细化速度加快,但拉伸力过大,射流直径会出现粗节或抽拔现象,当拉伸力超过一定数值后,在成纤过程中不能得到稳定的射流;射流的运动速度随着拉伸力的增加而增加,当拉伸力较小时,射流运动速度和运动距离呈线性关系,此时可以得到直径均匀变化的射流,由此在接受装置上可以得到直径分布均匀的纤维,当拉伸力较大时,运动速度和运动距离不再呈线性关系,射流直径变化不均匀。对于不稳定射流,模拟结果表明:对于不同的纵向作用力,射流要形成稳定的弯曲分别存在最小和最大径向速度;增加溶液的质量分数和施加电压相当于增强了使射流沿直线运动的作用因素,而增加LiCl含量相当于增强了使射流发生弯曲运动的作用因素,径向作用力增加,射流的弯曲不稳定性增强;在所研究的三个参数中,LiCl含量对不稳定射流的影响最大;静电纺不同LiCl含量的溶液时,成纤过程中任意时刻射流均有相似的运动轨迹,从而得到直径均匀分布的纤维;施加电压较大时,成纤过程中不同瞬时的射流运动轨迹差异较大,从而使接受装置上纤维直径分布不均匀,纤网中间甚至出现湿斑。结合成纤过程中射流的分析,本文也研究了溶液质量分数、LiCl含量和施加电压对纤网形态(包括纳米纤维沉积形态和纳米纤维形态)的影响。为了提高纳米纤维的质量,本文还研究了溶液性质中溶液质量分数和LiCl含量对纳米纤维直径和串珠的影响。到目前为止,采用静电纺得到的纳米纤维网的机械性能较差,阻碍了纤网的应用,针对这种情况,本文研究了不同静电纺条件下得到的纤网的结晶情况。本部分研究内容对于提高纳米纤维的质量和扩展纳米纤维的用途具有实际的指导意义。研究结果表明:(1)接受装置上纤网的面积随着溶液质量分数的增加而减少;随着LiCl含量的增加而减少,当LiCl含量为0.2%、0.5%和0.8%时,纤网的中间部分会出现一些小凸点;随着施加电压的增加,纤网面积先减小后增大,当施加电压大于或等于25 kV时,纤网中间出现了“小凸点”和圆形湿斑;接受装置上纤网的直径和溶液的质量分数、LiCl含量和施加电压分别为指数函数的关系。静电纺参数对纤网面积的影响规律与静电纺参数对成纤过程中圆锥包络轨迹顶角的影响规律相同。(2)接受装置上纳米纤维的直径随着溶液质量分数的增加迅速增加;随着LiCl含量的增加而减少。(3)溶液质量分数是影响纳米纤维直径大小和串珠的主要因素,溶液导电性是影响纳米纤维直径分布的主要因素。(4)接受装置上纤网的结晶度随着溶液质量分数的增加先增加后减少,随着LiCl含量的增加而减少,随着电压的增加先增加后减少。本文中,在溶液质量分数为10%,LiCl含量为0,施加电压为20kV的条件下静电纺PVA溶液,得到的纤网结晶度最大。更多还原

【Abstract】 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.更多还原