A Study On Preparatrion Of Ordered Nanomaterials By Electrospinning

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.