Simple Polymer Moves With Electricity

"This polymer is not new, but we can now alter it so it moves much more when an electric field is applied," says Dr. Qi-Ming Zhang, associate professor of electrical engineering and an associate at Penn State's Materials Research Laboratory. "The larger motion is an order of magnitude improvement in performance in acoustics, biomedical instrumentation and artificial organs possible."

Poly(vinylidene fluoride-trifluoroethylene) Copolymer was developed for sturdy, thin-film bags to store blood and other liquids. Researchers have long known that it has weak piezoelectric characteristics. When an electric voltage was placed on the film, the film moved, slightly. When pressure deformed the film, it produced electricity.

"As a piezoelectric material, this polymer was not very promising, the response was very small," says Zhang. "But as an electrostrictive material, the response is much larger and we can actually see it move under a voltage."

Electrostrictive materials are similar to piezoelectric materials, but are not polarized.

Zhang, working with Vivek Bharti and Xin Zhong Zhao, Penn State Postdoctoral Fellows, found a way to alter the polymer and create a material that moved 40 times more than some of the best known materials and is much easier and less expensive to manufacture.

Reporting in today's (June 26) issue of Science, the researchers explained how bombarding the material with electrons altered both the molecular conformation of the material and created new chemical bonds. We insert defects into the material and it becomes more compliant and flexible and has a higher dielectric constant, says Zhang.

Polymer materials consist of long chains that usually look like strands of spaghetti tangled around each other. The electron bombardment causes crosslinks to form between nearby strands and changes the molecular conformation. The altered material has a deformation rate under a high electric field of 4 percent or a 4-inch change for every 100 inches.

"This material is very sturdy, biologically neutral, can be molded in many shapes, is flexible and pliable," says Zhang. "Most electrostrictive materials are brittle ceramics that cannot move very far without breaking. Other known polymers can exhibit similar behavior, but they are very soft."

The flexibility, pliable and ease of manufacture of Poly(vinylidene fluoride-trifluoroethylene) Copolymer make it ideal for improved acoustic transducers for use in medical imaging equipment, underwater detectors and stereo speakers. Because of its high dielectric constant, the material could also be used for capacitors. In the long term, applications as artificial skin that senses touch, drug delivery systems, artificial muscles and organs may all be possible.

While easy to manufacture, pretreatment of the material does makes a difference. The initial material needed to be purer than that used to manufacture thin-film bags. The treatment temperature, annealing, quenching and whether or not the material was stretched also influenced the outcome.

"Eventually, we should be able to fine tune the properties over a large range of values to tailor the material to specific applications," says Zhang.

The researchers would also like to try working with strands or wires of the polymer in addition to thin films.