ABSTRACT

In this chapter, nanofiber production capability of the electro-centrifuge technique is compared with that of the conventional electrospinning method.

6.1 INTRODUCTION

When the diameters of polymer fibers are shrunk from micrometers to submicro-or nanometers, several new characteristics emerge, including a very high ratio of surface area to volume, flexibility in surface

University of Guilan, Rasht, Iran

*Corresponding author. E-mail: [email protected]

Abstract ................................................................................................. 107 6.1 Introduction .................................................................................. 107 6.2 Experimental Study by Focus on Flow Rate for Both Devices ....114 6.3 Conclusion ................................................................................... 121 Keywords .............................................................................................. 122 References ............................................................................................. 122

functionalities, and superior mechanical performance (e.g., stiffness and tensile strength). These unique properties make polymeric fibers ideal candidates for many important applications.1-3

Although several methods have been proposed for nanofiber manufacturing so far, an efficient and cost-effective procedure of production is still a challenge and is debated by many experts. Electrospinning has been the most successful method to produce nanofibers so far. It is a wellknown and prominent procedure which is based on the electrostatic force that provides the possibility for spinning nanofibers from many types of polymers using melt or solution spinning.4-6

The use of mechanical methods to apply high rates of tension to a polymer solution or melt is only possible for polymers with high extension ability, to prevent uneven transmission of tension and stress concentration. In order to apply high rates of tension to a polymer solution or melt, methods which can apply even distribution of stress during the tensioning process are required. When the centrifugal force acts upon a substance, the particles of that matter experience a force proportional to their distance from the center of rotation. Thus, this force can be used to apply high rates of tension on a polymer solution. During the tensioning process, if the polymer solution has sufficient viscosity, it will be stretched as a string, transforming to a polymeric fiber after drying.7,8

Electro-centrifuge spinning has been recently introduced as a beneficial method for nanofiber production. It is necessary to specify the effective parameters on nanofiber production and diameters. In this research, the effective parameters of the process and the influence of each parameter on fiber diameter are addressed. Also, the fiber production capability of the electro-centrifuge technique is compared with that of the conventional electrospinning method.9-11

6.1.1 NANOFIBERS

Nanofibers are defined as fibres with diameters less than 100 nm which can be produced by different methods such as melt processing, interfacial polymerization, electrospinning, electro-centrifuge spinning, and antisolvent-induced polymer precipitation.5 The fibers which are produced by these methods have a very high specific surface area, unusual strength, high surface energy, surface reactivity, and high thermal and electric

conductivity that could be used in products by special application. These unique properties make the polymeric nanofibers ideal candidates for many important applications, such as nanofiltration,12 nanocatalysis, tissue scaffolds,13 protective clothing, filtration, nanoelectronics,14 highperformance nanofibers,15,16 drug-delivery systems, wound dressings, and composites. Although several methods have been proposed for nanofiber manufacturing so far, an efficient and cost-effective procedure of production is still a challenge and is debated by many experts.17-20

6.1.2 ELECTROSPINNING METHOD

Electrospinning is an efficient and simplicity mean of producing nanofibers by solidification of a polymer solution stretched by an electric field.21 It is a well-known and prominent procedure based on electrostatic forces that provides the possibility for spinning nanofibers from many types of polymers using melt or solution spinning.22,23

In typical electrospinning process, an electrical potential is applied between droplets of polymer solution or melt held through a syringe needle and a grounded target. Afterward the electrostatic field stretches the polymer solution into fibers as the solvent is evaporating. During the process, the polymer jet undergoes instabilities, which together with the solution properties, determine the morphology of the forming nano-sized structures obtained onto the collector. The electrical forces elongate the jet thousands of times causing the jet to become very thin. Ultimately, the solvent evaporates or the melt solidifies, and a web containing very long nanofibers is collected on the target. The fiber morphology is controlled by the experimental design and is dependent on the solution properties and system conditions such as conductivity, solvent polarity, solution concentration, polymer molecular weight, viscosity, and applied voltage.4,24

To overcome various limitations of the typical electrospinning method, researchers have applied some modifications to the original setup.24,25 Some of these reformed techniques which have been done by now include: electrospinning using a collector consisting of two pieces of electrical conductive plates separated by a gap, collecting spun nanofibers on a rotating thin wheel with a sharp edge, fabricating aligned yarns of nylon 6 nanofibers by rapidly oscillating a grounded frame within the jet,26 and using a metal frame as a collector to generate parallel arrays of nanofibers.27 Matthews et

al.26 have applied a rotating drum to collect aligned nanofibers at a very high speed up to thousands of revolution per minute (RPM). In another approach, Deitzel et al.28 have used a multiple field technique which can straighten the polymer jet to some extent. Luming, et al.29 have used this technique to collect aligned nanofibers by increasing the surface velocity of the drum.