Polyaniline nanofibers

Polyaniline nanofibers are a high aspect form of polyaniline, a polymer consisting of aniline monomers, which appears as discrete long threads with an average diameter between 30 nm and 100 nm. Polyaniline is one of the oldest known conducting polymers, being known for over 150 years. Polyaniline nanofibers are often studied for their potential to enhance the properties of polyaniline or have additional beneficial properties due to the addition of a nanostructure to the polymer. Properties that make polyaniline useful can be seen in the nanofiber form as well, such as facile synthesis, environmental stability, and simple acid/base doping/dedoping chemistry. These and other properties have led to the formation of various applications for polyaniline nanofibers as actuators, memory devices, and sensors.

Synthesis

Methods for the polymerization of polyaniline nanofibers seen in literature primarily include [redox|chemical oxidative] polymerization, interfacial synthesis, and "rapid mixing" methods. Other less common methods include nanofiber seeding, electrosynthesis, electrospinning, and preforming polymerization in dilute aniline solutions.

Chemical oxidative polymerization Chemical oxidative polymerization is a traditional and commonly used method for the polymerization of aniline in large quantities. When aniline is mixed with an oxidant in an acidic solution, polymerization will occur. The most important parameter to be controlled in this method for the synthesis of polyaniline nanofibers is the domination of homogeneous nucleation over heterogeneous nucleation. Homogeneous nucleation describes when the nuclei are formed spontaneously in solution while heterogeneous nucleation describes when the nuclei are grown on other species. In the early stages of this polymerization, only nanofibers are formed since there are no heteronuclei available for heterogeneous nucleation. However, if the reaction is left uncontrolled, heterogeneous nucleation will begin to dominate as the polyaniline will preferentially grow on existing particles, leading to irreversible agglomeration. The reaction can be made to favor homogeneous nucleation throughout by increasing reaction speed, temperature of the reaction, and allowing the reaction to proceed without stirring.

The diameter of the polyaniline nanofibers can be controlled with this method through choice of acid. Hydrochloric acid produces nanofibers with a diameter of about 30 nm, while camphorsulfonic acid and perchloric acid produce a diameter of 50 nm and 120 nm respectively. Under normal synthetic methods polyaniline derivatives, such as ones that are alkyl and fluoro substituted, do not exhibit a well-defined fibrous shape, however, in the presence of an aniline oligomer nanofibers of certain derivatives can be synthesized. While the most common oxidant is ammonium peroxydisulfate (APS), various others can be used. One study shows the use of potassium biiodate (KH(IO3)2) as an oxidant, claiming it to lead to polyaniline nanofibers that are longer, have higher crystallinity, and have higher electrical conductivity.

Interfacial synthesis In interfacial synthesis, the polymerization happens at the interface between an aqueous and an organic layer. A typical reaction involves an aqueous solution of acid and oxidant and an organic layer of aniline together. This creates the reactive interface for polymerization to occur. As polymerization proceeds, the polyaniline nanofibers will diffuse into the water layer, leaving the reactive interface. This prevents overgrowth onto the existing wires, allowing for homogeneous nucleation to continue occurring. Conditions in the interfacial synthesis can be tuned, such as the type of acid used as well as the oxidant used.

Rapid mixing reactions Polyaniline nanofibers can also be synthesized through "rapid mixing" reactions. This method attempts to prevent overgrowth that would compromise the nanofiber nature of the polymer by stopping the polymerization immediately after nanofibers have been formed. This is achieved by the rapid mixing of the monomer, aniline, and an initiator solution. At the start of the reaction, the initiator is consumed rapidly and completely depleted when the nanofibers are formed. Without initiator remaining, the synthesis of polyaniline is halted.

Applications

Monolithic actuators Polyaniline nanofibers have been used in the creation of monolithic actuators. They can be used in this application due to their ability to be flash-welded. When exposed to light, polyaniline converts the absorbed energy directly into heat. In a polyaniline film, the heat is dispersed throughout the polymer. In polyaniline nanofibers, however, the heat is trapped within the individual fibers. Therefore, if the intensity of the light is great enough, it will cause the temperature of the nanofibers to rise rapidly, which causes them to weld together or burn. With a moderate flash intensity, the nanofibers will melt rapidly to form a smooth film. Using mask, welds in specific patterns can be made using this technique. In a thick enough sample of nanofibers, only the side exposed to the flash will be welded, creating an asymmetric film where one side remains intact as nanofibers while the other side is effectively crosslinked due to welding. These asymmetric films demonstrate rapid reversible actuation in the presence of acids and bases, in the form of bending and curling. The advantages polyaniline nanofiber asymmetric films have over other actuators include the ease of synthesis, large degree of bending, patternability, and no delamination. These actuators could be used in the development of microtweezers, microvalves, artificial muscles, chemical sensors, and patterned actuator structures.

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