actoth


 * Multimedia Component:** [|Link]

=Electrospinning of Polytetrafluoroethylene=

Abstract
Electrospinning is a method of producing non-woven fibrous mats with fiber diameters on the micrometer to nanometer scale. Interest in electrospinning has been steadily increasing over the past two decades due to increased reactivity and porosity of the fibers. Various applications of electrospun fibers have been investigated, including use in membrane filtration technologies, tissue scaffolds, and textiles. Polytetrafluoroethylene (PTFE) is a fluoropolymer with unique properties, such as its extremely low coefficient of friction and chemical inertness. PTFE has been used in numerous applications since its discovery in 1938. Recent studies in electrospinning of polytetrafluoroethylene have proven successful and ultimately open the door for new applications of polytetrafluoroethylene.

Introduction
Electrospinning is a simple, inexpensive technique that is used to produce polymeric fibers on the micro- and nanometer scale. Electrospun nanofibers are of interest due to the intrinsically high specific surface area-to-volume ratio and small pore size, ultimately leading to increased surface reactivity. [1] To create electrospun fibers, an electric field is applied to a polymer solution in order to create a charge within the polymer. As the charge repulsion of the polymer solution overcomes the surface tension, a polymer jet, or Taylor cone, is formed. While the polymer is being pulled from the initial jet, the solvent evaporates, leaving a solid fiber deposited on the collection plate. [2] A typical electrospinning setup is shown in Figure 1, where the polymer is pushed out of a syringe using a pump, with the applied voltage connected to the needle tip. The typical distance between the needle tip and collection plate is approximately 10-20 centimeters.


 * Figure 1.** Typical electrospinning setup

Processing parameters during the fabrication of electrospun fibers play a major role when it comes to end fiber diameter and morphology. Three distinct groups of processing parameters that have shown to have an effect on the end fiber diameter and morphology have been identified, as shown in Table 1. [3] By varying the processing parameters, fiber size and shape will be affected. In general, increasing the applied voltage, volume feed rate, and needle diameter will increase the end fiber diameter while increasing the collection distance will decrease end fiber diameter. Due to the scale of electrospun fibers, scanning electron microscopy must be used in order to evaluate the fiber morphology. Ultimately, electrospun fiber diameter and morphology should be optimized for the application of interest. [4-5]

Polymer concentration Molecular weight of polymer Electrical conductivity Elasticity Surface tension || Distance from needle to collector Volume feed rate Needle diameter || Humidity Atmospheric pressure ||
 * Table 1.** Processing parameters that affect fiber morphology in electrospinning
 * Solution properties: || Viscosity
 * Processing conditions: || Applied voltage
 * Ambient conditions: || Temperature

The Development of Electrospinning
The concept of electrospinning was originally described by Formhals in his 1934 patent US 1,975,504. [6] Formhals took the idea of applying an electrical field to a solution to form threads, which was an already established method of production but not commercially practical, and developed a method to collect usable threads through the use of a rotating drum. Although this method drastically improved upon previous attempts to use an applied electrical field to form fibers, there were still issues with solvent evaporation due to the short distance between the polymer solution and collector drum. Formhals continued to improve his process, ultimately increasing the collection distance to allow more time for the solvent to evaporate. [7] Next, Formhals patented a new method of electrostatically spinning composite fibers onto his developed rotating drum. [8] The improvements that Formhals made to the electrospinning process ultimately increased commercial interest in the method, leading to more research on the concept.

During the 1960s, Taylor initiated studies on the jet stability during electrospinning. [9] It was determined that jet stability is dependent on the geometry of the electrode, which was ultimately mechanically dependent rather than electrically dependent. Taylor’s findings showed the shape of the solution jet during fiber formations, as shown in Figure 2. This jet shape is now known as a Taylor cone.


 * Figure 2.** Taylor cone shape as demonstrated by Sir Geoffrey Taylor [9]

The study of electrospinning essentially remained stagnant until the 1990s, when Reneker and Doshi studied the effect of electric field on fiber formation and ultimately discussed potential applications. [10-12] The since increasing interest in electrospinning is apparent from the number of publications relating to electrospinning each year, as shown in Figure 3.


 * Figure 3.** Number of “electrospinning” publications per year (Source: Scifinder Scholar)

More recently, the idea of coaxial electrospinning has become of interest due to the potential to have an inner core and outer clad shell using two different materials. This method was first investigated by Li in 2004, where a poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) core was spun with a poly(vinyl pyrrolidone) (PVP) shell in order to improve the morphology of the MEH-PPV fibers. [13] The general schematic of a coaxial electrospinning setup is shown in Figure 4.


 * Figure 4.** Coaxial electrospinning setup to create “core-clad” fibers

Polymers that typically cannot be electrospun due to various materials properties, such as a low dielectric constant, can be electrospun using a coaxial system. This is done by making the polymer with the low dielectric constant the inner core while using an electrospinnable polymer as the outer shell. Coaxial electrospun fibers can also be used as a method of drug delivery. [16] For example, Maleki et. al. produced tetracycline hydrochloride (TCH) core-poly(lactide-co-glycolide) (PLGA) clad fibers and reported that the drug release profile was controllable by varying the shell polymer and electrospinning parameters. This technology proves hopeful in provided a more controlled method in drug delivery since other bioactive agents such as antibiotics, DNA, proteins, or growth factors, can also be incorporated into the fibers. [14]

History of Polytetrafluoroethylene
Polytetrafluoroethylene (PTFE) is a polymeric material discovered in 1938 by Roy Plunkett, a scientist employed by DuPont. [15] Although it was discovered accidently via the unplanned polymerization of tetrafluoroethylene, PTFE, shown in Figure 5, has been used in all types of applications including biomedical devices, cooking appliances, and fabrics. [16-18] Its unique properties are what attracted Plunkett to the discovered material, including its inertness and non-stick abilities.


 * Figure 5.** Chemical structure of PTFE

Controversies over the use of Teflon in products such as non-stick cooking pans have come about due to the said risk of exposure to perfluorooctanic acid (PFOA). PFOA is a known carcinogen and a class action lawsuit has been filed again DuPont, stating that it should have released warnings about the side effects of Teflon products. [19]

In the 1930s, it was proven that chlorofluorocarbon gases (CFCs) used as refrigerants were extremely hazardous. For this reason, two General Motors’ scientists developed a new inert, non-toxic, and odorless gas to replace the original CFCs called Freon. General Motors teamed up with DuPont to manufacture the new Freon products in a new plant in Deepwater, New Jersey. [20] This is where Roy Plunkett, a young scientist who recently graduated from Ohio State University, was employed. [15] In 1938 and at the age of 27, Plunkett was investigating Freon gases when a sample froze overnight. Rather than discarding the unknown sample, Plunkett and his assistant ran tests on the whitish wax. [15] Although it was believed that tetrafluoroethylene could not be polymerized but after a couple of days of testing, Plunkett determined that his newly discovered material was in fact a thermoplastic. In 1939, Plunkett filed for a patent under the joint General Motors/DuPont company, Kinetic Chemicals. By 1941, the patent was accepted and Teflon began to be used in everyday applications.

It turned out that the new material had some interesting properties: it was very slippery and inert to basically all materials tested on it. [20] After the accepted patent in 1941, Teflon was first used in military applications. [20] After the war, PTFE resins were starting to be used in other applications, such as consumer products. [17] PTFE has been used in a variety of applications, such as automotive, electrical, aerospace, and cable industries. [21] More recently, PTFE coatings have been used on pots and pans for non-stick cooking applications and even for coatings on medical implants. [21] The low coefficient of friction of PTFE is one major factor that makes it so advantageous. Against steel, PTFE has a dynamic coefficient of friction of 0.05 to 0.10. [22]

Due to the unique properties of PTFE much research is still being conducted on the polymer. For example, heparin-bonded PTFE vascular prostheses were investigated to determine whether these grafts were better than the basic PTFE grafts. [23] The heparin coated vascular graft prevents thrombus formation on the surface of the graft while remaining bioactive. [25] Ultimately polytetrafluoroethylene proves to be an advantageous material in multiple applications. For these reasons, the electrospinning of PTFE has been investigated in order to combine the unique properties of both PTFE and electrospun fibers.

Electrospinning of PTFE
Combining the advantageous properties of nanofibers with the unique characteristics of PTFE make the idea of electrospun PTFE seem pretty straightforward, however, the low dielectric constant of PTFE make is nearly impossible to electrospin. The low dielectric constant prevents sufficient charging of the polymer solution prohibiting the electrospinning process. [24] Nonetheless, with the recent advances in coaxial electrospinning, it has been demonstrated that a normally unspinnable polymer can be electrospun by using this polymer as the core with an electrospinnable polymer shell. [25] For these reasons, the electrospinning of PTFE as the core in a coaxial electrospun fibers has been recently investigated by multiple groups.

Han et. al. successfully coaxially electrospun amorphous PTFE (Teflon AF) core/poly(caprolactone) (PCL) clad fibers, as shown in Figure 5. [24] It was determined that by minimizing the pump rate of the Teflon AF, loss due to dripping was minimized as well as beading on the final fibers. Minimization of beading on the fibers is desirable in order to improve mechanical properties for various applications. [24]


 * Figure 6.** 10wt% PCL/1wt% PTFE fibers [24]

Following the fabrication of the electrospun PTFE, techniques to characterize the hydrophobicity of the fibers were performed, such as contact angle measurements. The increased surface roughness of the PTFE fibers ultimately led to an increase in the contact angle from 120° as a film to 158° as fibers, deeming the fibers superhydrophobic, as shown in Figure 6. The fibers also demonstrated an extremely low tilt angle required for a water droplet to roll off of the material. [24]


 * Figure 7.** Contact angle measurements for (left) PCL fiber mat and (right) PCL-Teflon AF coaxial fiber mat [24]

In a similar study, Muthiah et. al. demonstrated the superhydrophobicity of amorphous PTFE core (Teflon AF)/poly(vinylidene fluoride) (PVDF) clad electrospun fibers. [25] Fiber diameter of the coaxial fibers ranged from less than 100 nanometers to around 500 nanometers. Transmission electron microscopy (TEM) allowed the distinct core/clad structure to be visualized, as shown in Figure 7. This proved that the method developed by Muthiah et. al. was successful in coaxially electrospinning Teflon AF/PVDF. As shown by Han et. al. the Teflon AF/PVDF fibers also demonstrated superhydrophobicity through contact angle testing by obtaining contact angles greater than 150°. [25]


 * Figure 8.** TEM (30,000x magnification) micrograph of coaxially electrospun Teflon AF/PVDF fibers [25]

Applications of Electrospun PTFE
The success of electrospun PTFE opens up a door for a wide variety of applications, especially relating to superhydrophobic surfaces. [24-26] Superhydrophobicity has recently gained increased interest due to the attempt to engineer a material similar to that of the lotus leaf. [26] Water and oils on the lotus leaf surface simply roll out with little to no residue or contamination left behind.30 Multiple methods of producing superhydrophobic surfaces have been investigated, including template synthesis, phase separation, electrochemical deposition, electrohydrodynamics, crystallization control, chemical vapor deposition, and self-assembly. [26] However, electrospinning has proven to be successful in producing superhydrophobic surfaces and is an inexpensive and controllable technique to do so. [24-26]

Superhydrophobic surfaces can be used in multiple industries, including energy, textiles, microfluidics, and construction. [24-26] More specifically, coaxially electrospun PTFE has been investigated for use as a semipermeable membrane in Li-air batteries. [25] In a Li-air battery, a semipermeable membrane is required between the Li metal anode and the air cathode, as shown in Figure 9.By utilizing an electrospun superhydrophobic PTFE membrane in a Li-air battery, oxygen should be able to pass through the membrane while atmospheric moisture will be unable to pass through the membrane. [25] This will help address any safety issues that may arise from atmospheric water entering into the battery.

Figure 9. Schematic of a Li-air battery

Conclusion
Electrospun fibers have had increasing interest in the past two decades due to their increased surface area, porosity, and surface reactivity. As demonstrated throughout the past seventy years, PTFE has many unique properties that allow it to be adapted to multiple applications. Although the investigation of electrospinning PTFE has just recently begun, these fibers could be a simple solution for a superhydrophobic material with porosity.