【作者】 Javed Rafique；

【导师】 于杰；

【作者基本信息】 哈尔滨工业大学 ， 材料物理与化学， 2008， 博士

【摘要】 近年来,纳米科学和技术已经引起遍及世界的广泛兴趣,预期其将带来全面的技术和工业革命。在不同的纳米制备技术中,静电纺丝是一种工艺简单功能多变的制备连续纳米纤维的方法。但是,由于电纺法制备的纤维定向性差以及现有定向技术产量低的缺陷而限制了这项技术的商业化进程。目前,电纺技术的研究仍处于起步阶段,仍有大量阻碍电纺技术应用的迫切问题需要进一步研究解决,其中尤为突出的就是纳米纤维的定向技术。显而易见,定向纤维在其应用方面具有极其重要的意义,但是,到目前为止,定向纤维的电纺工艺仍然没有很好地建立。另一方面,迄今为止对电纺工艺的研究大部分集中在溶液的高浓度范围内,由于溶液的高粘度在这个浓度范围得到的电纺产物一般为纳米纤维。低浓度溶液的电纺产物是纳米颗粒,在很多领域都有潜在的应用。然而,人们对于低浓度溶液的电纺行为研究很少,知识很有限。本研究旨在解决电纺领域存在的上述问题。本论文建立了一种非常简单的电纺定向技术,利用这种技术我们制备了长达25厘米,横向覆盖宽度高达63厘米的高定向聚合物纳米纤维。这项技术基于一种改进的电纺设备、点电极收集器的采用以及纤维的侧向喷射。这种电纺过程的显著特征在于纤维的喷出是单根不连续的,这个现象已经由高速像机拍摄的实时图像所证实。制备过程中纤维的定向是借助于点电极产生的汇聚电场的诱导作用实现的。为了提高定向纤维的产量,我们将这种技术从单喷头进一步扩展到多喷头(3个纺丝喷口)。通过对如上定向技术的改进我们还开发了一种制备定向螺旋纳米纤维的方法,其中螺旋纤维的收集是通过一个倾斜的玻璃片进行的。利用这种方法我们成功制备了定向的聚己内酯(PCL)螺旋纤维,其形貌结构和螺旋直径依赖于溶液浓度,在4.7%-10%的浓度范围内获得的螺旋纤维的螺旋直径在6.9-14.9μm,其中在溶液浓度10%时获得了三维的螺旋纤维。这种螺旋纤维的形成是由于高速液流撞击基材表面时会产生机械不稳定性,这种机械不稳定性导致液流的弯曲折叠,使液流沿螺旋路径沉积在基材表面。同样,螺旋纤维的定向也是通过尖端电极所产生的汇聚电场的作用实现的。通过适当控制收集基材的位置和倾斜度利用这种方法可以直接在基材表面制备由纳米纤维组成的图案。利用上述方法我们制备了定向的PAN原纤维,通过随后的碳化反应制备了平均直径为80nm的高定向纳米碳纤维(CNFs)。同时,我们利用传统的电纺装置制备了平均直径为60nm的非定向碳纳米管纤维(CNFs)。研究了稳定化温度、碳化温度、升温速率以及衬底对纳米碳纤维形貌和结构的影响。利用多种分析手段如XRD、Raman、SEM、TEM、FTIR和TG/DTA等对样品进行了分析表征。另一方面,本论文研究了低浓度范围的电纺行为,通过电纺荷电粒子的自组装获得了蜂窝状多孔结构。通过控制溶液浓度与纺丝电压可以在某种程度上控制这种结构的形貌。我们利用浓度范围在8到13% (w/v)的聚氧乙烯(PEO)水溶液制备了这种蜂巢状多孔结构,通过变化溶液浓度可以实现对其孔壁结构的控制,获得密实和多孔的壁结构。微孔倾向于呈现多边形尤其是六边形,微孔直径的尺寸在15到80μm,组成蜂巢结构的纳米颗粒尺寸范围为50到200nm,组成蜂巢结构的纳米纤维的直径分布为50到100nm。形成此种微观结构的纳米粒子和纳米纤维的形貌取决于溶液浓度和纺丝电压。我们研究了衬底的性质对于自组装微观结构形态的影响,结果表明在铝箔和玻璃衬底上的多孔结构形貌明显不同,与铝上的样品相比玻璃上样品的孔径更大,孔壁更直。这可能是因为由于玻璃不导电而铝是导体,因而在沉积过程中玻璃上的电荷积累更多的缘故。同时发现在铝箔表面沉积时一般要先形成一层连续的聚合物薄膜,而后才开始自组装过程,而在玻璃表面沉积时自组装可以在基材表面直接进行。其原因可能同样是由于这两种基材导电性的差异。我们进一步研究了基材的放置位置对自组装行为的影响,发现大颗粒倾向于落在距离注射器喷口近的位置,组装成垂直于表面的柱状物,而小的颗粒则落在距离远的位置,自组装成蜂巢状多孔结构。电纺过程中产生的液滴的表面张力以及彼此之间的静电排斥力在蜂窝结构的形成中起主要作用。基于以上两种作用力的竞争行为,我们建立了一个生长模型并成功解释了这种荷电液滴的自组装行为。更多还原

【Abstract】 Nanoscience and nanotechnology have attracted worldwide interest in recent years with the expectation that it will bring about a global technological and industrial revolution. Amongst different nano fabrication techniques, electrospinning is a very simple and versatile method for the fabrication of continuous nanofibers. However, the lack of orientation and low throughput of electrospinning aligned nanofibers has hindered the commercialization of this technology. The research on electrospinning technology is still in its infancy and further work is urgent to solve the numerous problems obstructing its applications especially the techniques of preparing aligned nanofibers. Alignment of the nanofibers is obviously important for their applications but the techniques of electrospinning aligned nanofibers are not well established so far. On the other hand, up to now, the research on the electrospinning is mostly in the range of high solution concentration, where the electrospun products are nanofibers due to the high solution viscosity. The electrospun products in the low concentration range are nanoparticles, which have many potential applications in various fields. However, the understanding on electrospinning in the low solution concentration range is limited. The present study aims at solving the above problems existing in the area of electrospinning.In this dissertation, a very simple alignment technique is presented, by which highly aligned polymer nanofibers of more than 25 cm in length were electrospun over a lateral range as large as 63 cm. This technique is based on a modified configuration, application of a tip collector, and sideward ejection. The salient feature of the electrospinning process is the production of single nanofibers one by one, which was confirmed by real-time images taken by a high-speed camera. The alignment of the nanofibers is realized with the aid of a converging electric field generated by the tip collector. This technique was further employed with multi jets (3 spinning nozzles) to scale up the production rate of highly aligned nanofibers. Based on the above modified electrospinning technique, a new and simple electrospinning method has been developed for producing aligned helical polymer nanofibers. The helical fibers were collected by a tilted glass slide. By this method the aligned helical PCL nanofibers were prepared successfully. The morphology and loop diameters of the helical structures depend on the PCL solution concentration and the loop diameters are in the range of 6.9-14.9μm for the concentration range of 4.7%-10%. The three-dimensional helical structures were obtained at the high solution concentration of 10%. These helical structures were formed by jet buckling due to mechanical instability when hitting collector surface. Similarly, the converging electrical field generated by the tip collector plays an important role in the alignment of the helical structures. This technique was also utilized to prepare nanofiber patterns directly on different substrates by appropriately selecting the substrate position and obliquity.Highly aligned carbon nanofibers (CNFs) with average diameter of about 80 nm were prepared from polyacrylonitrile (PAN) nanofibers. The alignment of the precursor nanofibers was achieved by using the above mentioned electrospinning technique. Random CNFs with average diameter of about 60 nm were also prepared by conventional electrospinning setup. The effects of the stabilization and carbonization temperature, temperature-increasing rates, and substrate on the morphology and structure of the CNFs were investigated. Different techniques such as XRD, Raman, SEM, TEM, FTIR, and TG/DTA were used to characterize the structure and morphology of the PAN, stabilized, and carbonized nanofibers.This study also includes the self-assembly of honeycomb microporous structures from electrospun charged polymeric nanoparticles during electrospinning at low solution concentration. Certain control on the morphology of the honeycomb structures was achieved through solution concentration and applied electrospinning voltage. The honeycomb microporous structures were prepared with polyethylene oxide (PEO) aqueous solutions in the concentration range from 8 to 13% (w/v) with solid to porous walls. The micro holes tend to be polygonal especially hexagonal in shape. The size of the micropores was in the range from 15 to 80μm and the size of the nanoparticles forming the honeycombed structures was from 50 to 200nm. The diameter of the nanofibers forming the honeycomb structure was from 50 to 100nm. It was found that morphology and structure of the nanoparticles and nanofibers forming the honeycomb structures depend upon the solution concentration and applied electrospinning voltage. The effects of substrate nature on the morphology of self assembled honeycomb micrporous structures was studied and obvious difference was observed with the hole larger and hole walls straighter for the films on glass than that on aluminum,which is because the charge density accumulated on the insulating glass is higher than that on the conducting alluminum. It was also found that on aluminum substrate an insulating layer of nanoparticles is formed initially before the self-assembly phenomenon starts, while on the glass substrate the self-assembly starts directly on the substrate surface. This is possibly due to the difference in conductivity for the two substrates also. Further the effects of the substrate positions on the self-assembly phenomenon was studied. It was observed that the large particles tends to land on the positions close to the needle nozzle and self-assemble into vertical rods while the small particles tend to land on the positions far from the needle nozzle and self-assemble into honeycomb microporous film. The surface tension of the solution droplets and electrostatic repelling forces between them play key roles in the formation of the honeycomb structures. Based on the competitive actions of the above two forces we established a model for explaining the self-assembling behavior of the solution droplets.更多还原