I’ve written before about the nature of conductive plastics, which are organic polymers that can carry an electric current. Normally plastics are considered insulators, which is why we coat our power lines with plastic so that we can safely handle the material. However, depending on the molecular structure of the plastic, it is possible to have a material which becomes conductive. I’ve also written before about the important implications of electronegativity, which is the property of some atoms to pull in electron density from the other atoms around it. A new discovery recently announced by MIT (published in the journal Nature Materials, link:
http://www.nature.com/nmat/journal/vaop/ncurrent/abs/nmat2792.html) combines these two topics with a third interesting phenomenon: piezoelectricity.
Piezoelectricity is a property shown by some materials, whereby physically stressing the material produces an electric current. The cause of piezoelectricity is difficult to explain, as it’s best expressed mathematically (see attached links), but it depends on the presence of a molecular dipole within the material – which is the result of having electronegative atoms within the material. The important thing to know is that flexing or bending a piezoelectric material generates a small amount of electric charge, which can be harnessed as useful electric current. Like many phenomena in science, this property can manifest itself as the reverse property: upon application of a small electric charge, the material undergoes a physical strain and flexes.
The researchers at MIT have been working for some time to functionalize polymer fibers. Fibers, which are long, drawn-out samples of the plastic, are very useful platforms for functional techniques like analysis and measurement because fibers are durable, flexible, inert, and quite resilient to breakage. The new type of fibers, called acoustic fibers, contain both fluorinated sections and hydrogenated sections. Fluorine is very electronegative, and the result is a dipole across the length of the fiber due to the increased electron density on the fluorine atoms. An additional innovation comes from the generation of the electric field. Normally, metal electrodes would be used – connecting them to a source of electricity completes the circuit. However, the process of heating up a block of plastic and drawing the molten material into a fiber would distort the shape of a metal electrode, and probably break it. To overcome this problem, the researchers used a conductive plastic. That material can be drawn into a thin fiber and still retain its conductive properties without breaking, unlike the metal.
After drawing the fiber, the researchers further aligned the piezoelectric molecules (which increases the amount of physical deflection seen upon application of electricity) by applying a strong electric field to the entire fiber. Slowly lowering the strength of the electric field leaves the molecules permanently aligned, and highly responsive to future applications of electricity. They then tested the fibers by placing them in a tank of water along with a microphone and speaker. By applying an alternating current to the fibers, the piezoelectric material flexes back and forth extremely rapidly, which produces audible sound waves. The reverse was also true; emitting sound through the speaker and into the water would flex the fibers back and forth, which produces an alternating electric current within the conductive polymer fiber core and which can be measured.
These fibers are very promising and may result in tiny microphones, sensors for medical uses, sonar systems to measure water flow in the ocean, and other imaging devices. The low cost and rugged nature of the fibers combines with their high piezoelectric response to make these fibers the perfect choice for these applications.