Electrospinning of polymeric nanofiber materials : process characterization and unique applications

Abstract

Electrospinning or electrostatic fiber spinning employs electrostatic force to draw a fiber from a spinneret. This fiber solidifies and lies down on a collector in the form of a non-woven fiber mat. Electrospinning has attracted much attention recently due to the ease with which fine fibers about 10 nanometers to 10 microns in diameter can be produced from both natural and synthetic polymers. Despite the large volume of publications on this technology, few publications discuss the mechanics of electrospinning. Most publications deal with the exploratory works on what material can be electrospun and the potential applications of the electrospun fibers. This work examined the electrohydrodynamics of the electrospinning process and developed this technology for making functional materials. The first part of this dissertation deals with the electrohydrodynamics of the process. The effects of processing parameters and material properties on the size and structure of electrospun fibers were studied. The experimental findings validated the analytical scaling model developed by Fridrikh and co-workers to predict how the final radius or "the terminal jet radius" of the electrospun fiber depends on the processing parameters.

(cont.) The scaling formula is derived from the force balance between surface tension and surface charge repulsion. The scaling model provides a powerful tool for controlling the fiber diameter just by adjusting the surface tension, the flow rate, and the electric current on the jet. The next part of this dissertation describes the role of fluid elasticity in the formation of fibers from polymer solution by electrospinning. Obtaining a uniform electrospun fiber can become problematic when the polymer solution is too dilute. In this case, experience suggests that the lack of elasticity prevents the formation of uniform fibers; instead, droplets or necklace-like structures know as "beads-on-string" are formed. Model fluids were prepared by blending small amounts of high molecular weight polyethylene oxide (PEO) with concentrated aqueous solutions of low molecular weight polyethylene glycol (PEG). The formations of beads-on-string and uniform fiber morphologies were observed in a series of solutions having the same polymer concentration, surface tension, shear viscosity, and conductivity but with different degrees of extensional viscosity. A high degree of extensional stress was observed to arrest the breakup of the jet, which was due to the Rayleigh instability.

(cont.) In some cases, the extensional stress was able to suppress the Rayleigh instability altogether. The susceptibility of the jet to the Rayleigh instability was examined in two ways. First, a Deborah number was defined as the ratio of the fluid relaxation time to the instability growth time and was shown to correlate with the arrest of droplet breakup, giving rise to electrospinning rather than electrospraying. Second, the critical extensional stress on the jet was shown to be large enough in some cases to completely suppress the Rayleigh instability. The next part of this dissertation describes ways to produce functional electrospun fibers for potential applications. It presents a method to electrospin into fibers materials that would otherwise be difficult or impossible to process using conventional extrusion or electrospinning. This method involves electrospinning two materials into fibers with core-and-shell morphology. The "electrospinnable" shell fluid serves as a processing aid to electrospin the core fluid. The shell of the fiber can be removed during post processing, while the core of the fiber remains intact. Several types of core/shell fibers were produced for the first time to illustrate the versatility of this technique.

(cont.) Finally, the dissertation presents a process to make transparent, electrospun-fiber reinforced-composite that has the ability to change reversibly, its color and transparency in response to stimuli, such as irradiation. Matching the refractive indexes of the fiber and the matrix is important in order to reduce haze and poor visibility. Any large mismatch will contribute to haze and poor visibility. Electrospinning is chosen to produce the reinforcing fiber for the following reasons. Electrospinning produces very fine fibers (average diameter ranging from 100 nm to 500 nm) that can minimize the scattering of light in case there is a slight mismatch in refractive indexes. Electrospinning can produce a non-woven mat that has a large ratio of surface area to mass for better bonding to the matrix material. It also allows the dye to disperse in the spin solution without compromising the chemical stability of the dye during electrospinning. The result is a mechanically tough, highly transparent composite that has low haze, the ability to change color, and selective transmittance.