A review on polymer nanofibers by electrospinning and their applications in nanocomposites
Introduction
When the diameters of polymer fiber materials are shrunk from micrometers (e.g. 10–100 μm) to submicrons or nanometers (e.g. 10×10−3–100×10−3 μm), there appear several amazing characteristics such as very large surface area to volume ratio (this ratio for a nanofiber can be as large as 103 times of that of a microfiber), flexibility in surface functionalities, and superior mechanical performance (e.g. stiffness and tensile strength) compared with any other known form of the material. These outstanding properties make the polymer nanofibers to be optimal candidates for many important applications. A number of processing techniques such as drawing [118], template synthesis [[45], [108]], phase separation [106], self-assembly [[104], [161]], electrospinning [[29], [49]], etc. have been used to prepare polymer nanofibers in recent years. The drawing is a process similar to dry spinning in fiber industry, which can make one-by-one very long single nanofibers. However, only a viscoelastic material that can undergo strong deformations while being cohesive enough to support the stresses developed during pulling can be made into nanofibers through drawing. The template synthesis, as the name suggests, uses a nanoporous membrane as a template to make nanofibers of solid (a fibril) or hollow (a tubule) shape. The most important feature of this method may lie in that nanometer tubules and fibrils of various raw materials such as electronically conducting polymers, metals, semiconductors, and carbons can be fabricated. On the other hand, the method cannot make one-by-one continuous nanofibers. The phase separation consists of dissolution, gelation, extraction using a different solvent, freezing, and drying resulting in a nanoscale porous foam. The process takes relatively long period of time to transfer the solid polymer into the nano-porous foam. The self-assembly is a process in which individual, pre-existing components organize themselves into desired patterns and functions. However, similarly to the phase separation the self-assembly is time-consuming in processing continuous polymer nanofibers. Thus, the electrospinning process seems to be the only method which can be further developed for mass production of one-by-one continuous nanofibers from various polymers.
Although the term “electrospinning”, derived from “electrostatic spinning”, was used relatively recently (in around 1994), its fundamental idea dates back more than 60 years earlier. From 1934 to 1944, Formalas published a series of patents [51], [52], [53], [54], [55], describing an experimental setup for the production of polymer filaments using an electrostatic force. A polymer solution, such as cellulose acetate, was introduced into the electric field. The polymer filaments were formed, from the solution, between two electrodes bearing electrical charges of opposite polarity. One of the electrodes was placed into the solution and the other onto a collector. Once ejected out of a metal spinnerette with a small hole, the charged solution jets evaporated to become fibers which were collected on the collector. The potential difference depended on the properties of the spinning solution, such as polymer molecular weight and viscosity. When the distance between the spinnerette and the collecting device was short, spun fibers tended to stick to the collecting device as well as to each other, due to incomplete solvent evaporation.
In 1952, Vonnegut and Neubauer were able to produce streams of highly electrified uniform droplets of about 0.1 mm in diameter [153]. They invented a simple apparatus for the electrical atomization. A glass tube was drawn down to a capillary having a diameter in the order of a few tenths of millimeter. The tube was filled with water or some other liquid and an electric wire connected with a source of variable high voltage (5–10 kV) was introduced into the liquid. In 1955, Drozin investigated the dispersion of a series of liquids into aerosols under high electric potentials [37]. He used a glass tube ending in a fine capillary similar to the one employed by Vonnegut and Neubauer. He found that for certain liquids and under proper conditions, the liquid was issued from the capillary as a highly dispersed aerosol consisting of droplets with a relatively uniform size. He also captured different stages of the dispersion. In 1966, Simons patented an apparatus for the production of non-woven fabrics of ultra thin and very light in weight with different patterns using electrical spinning [136]. The positive electrode was immersed into the polymer solution and the negative one was connected to a belt where the non-woven fabric was collected. He found that the fibers from low viscosity solutions tended to be shorter and finer whereas those from more viscous solutions were relatively continuous. In 1971, Baumgarten made an apparatus to electrospin acrylic fibers with diameters in the range of 0.05–1.1 microns [6]. The spinning drop was suspended from a stainless steel capillary tube and maintained constant in size by adjusting the feed rate of an infusion pump. A high-voltage dc current was connected to the capillary tube whereas the fibers were collected on a grounded metal screen.
Since 1980s and especially in recent years, the electrospinning process essentially similar to that described by [6] has regained more attention probably due in part to a surging interest in nanotechnology, as ultrafine fibers or fibrous structures of various polymers with diameters down to submicrons or nanometers can be easily fabricated with this process. A survey of open publications related with electrospinning in the past 10 years is given in Fig. 1(a), whereas those publication distributions all over the world are shown in Fig. 1(b). These literature data were obtained based on a SciFinder Scholar search system. The data clearly demonstrated that the electrospinning has attracted increasing attentions recently. Up to date, it is generally believed that nearly one hundred different polymers, mostly dissolved in solvents yet some heated into melts, have been successfully spun into ultrafine fibers using this technique (though only half of them have been found by us from the open literature, see subsequently). Strangely enough, although the electrospinning process has shown potential promising and has existed in the literature for quite several decades, its understanding is still very limited. In this paper, a systematic review is made on the researches and developments related to electrospun polymer nanofibers including processing, structure and property characterization, applications, and modeling and simulations. Other issues regarding the technology limitations, research challenges, and future trends are also addressed in the paper.
Section snippets
Fundamental Aspect
A schematic diagram to interpret electrospinning of polymer nanofibers is shown in Fig. 2. There are basically three components to fulfill the process: a high voltage supplier, a capillary tube with a pipette or needle of small diameter, and a metal collecting screen. In the electrospinning process a high voltage is used to create an electrically charged jet of polymer solution or melt out of the pipette. Before reaching the collecting screen, the solution jet evaporates or solidifies, and is
Composite application
One of the most important applications of traditional (micro-size) fibers, especially engineering fibers such as carbon, glass, and Kevlar fibers, is to be used as reinforcements in composite developments [20]. With these reinforcements, the composite materials can provide superior structural properties such as high modulus and strength to weight ratios, which generally cannot be achieved by other engineered monolithic materials alone. Needless to say, nanofibers will also eventually find
Other applications
In addition to composite reinforcement, other application fields based on electrospun polymer nanofibers have been steadily extended especially in recent years. One of the best representatives in this regard is shown by relevant US patents, in which most applications are in the field of filtration systems and medical prosthesis mainly grafts and vessels. Other applications which have been targeted include tissue template, electromagnetic shielding, composite delamination resistance, and liquid
Geometrical characterization
Geometric properties of nanofibers such as fiber diameter, diameter distribution, fiber orientation and fiber morphology (e.g. cross-section shape and surface roughness) can be characterized using scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) [32], [102], [114], [140]. The use of TEM does not require the sample in a dry state as that of SEM. Hence, nanofibers electrospun from a
Modeling of electrospinning process
The electrospinning process is a fluid dynamics related problem. In order to control the property, geometry, and mass production of the nanofibers, it is necessary to understand quantitatively how the electrospinning process transforms the fluid solution through a millimeter diameter capillary tube into solid fibers which are four to five orders smaller in diameter. When the applied electrostatic forces overcome the fluid surface tension, the electrified fluid forms a jet out of the capillary
Concluding remarks
Electrospinning is an old technology, which has existed in the literature for more than 60 years, and yet is an immature but the most possible method for the fabrication of continuous nanofibers. A comprehensive as well as state-of-art review on this technique together with applications of polymer nanofibers produced by it has been made in this paper. So far only relatively small number of polymers have been tried to be electrospun into nanofibers, and the understanding in electrospinning
Acknowledgements
The authors would like to acknowledge Professor A.L. Yarin, Technion-Israel Institute of Technology, Dr. Krishnan Jayaraman, University of Auckland, and Dr. Xiumei Mo, National University of Singapore, for their kind help and assistance.
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