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Electroactive polymers are plastics that expand or contract in the presence of an electric field. Cycle through these shape changes and the materials become actuators or motors that work much like biological muscles. Pump electrons into these polymers, and they can store this electricity.
There are a couple of drawbacks to today's electroactive polymers, however. They require a considerable amount of voltage to change shape. And although some polymers store a useful amount of electricity, finding others that store more would mean being able to make smaller gel-type batteries.
The key is finding materials that have a high dielectric constant, or ability to resist the flow of electric charge. Current practical electroactive polymers like those used in batteries have dialectic constants of around five.
Researchers at Pennsylvania State University have increased the number more than two orders of magnitude with a new composite electroactive polymer that boasts a dielectric constant as high as 1,000.
There are two major uses for the new material, said Qiming Zhang, an engineering professor at Pennsylvania State University. It could be used as artificial muscles or motors that do not have any moving parts, and in super-strength capacitors, or batteries, that can store a lot of charge in a relatively small amount of space.
"When you put it in an electric field it changes shape, so it can push and pull, so it's a kind of motor," said Zhang. Such a simple motor has some interesting advantages, he said. "If you cut a [conventional] motor into two pieces, the motor will not work. But this one -- if you cut it into two pieces, potentially each piece will be working," he said. It is also easy to make very small motors this way. Lower-voltage materials could be used more safely in medical applications and to activate moving parts toys.
The advantage of using a material with a higher dialectic constant for
batteries is that the batteries can be made smaller. If the material has
a dialectic constant a thousand times higher than the regular polymer,
the battery volume will be a thousand times smaller, said Zhang. This
could make for much better electric cars, he said. "If you want... a car
which is very compact, you don't want a battery as big as your house."
In order to hold a charge, a material must block electric current from flowing. This is somewhat like introducing water into a pipe, then blocking it on both ends, said Zhang. If there are two blocked points the water can slosh back and forth in the close section of pipe, but can't flow through. "The charge of the electrons inside the molecules can move within those molecules, but they will not be able to escape," he said. "The dialectic constant is... a kind of measure of how much charge you can move inside the polymer chain. If you can move a lot, you have a high dialectic constant."
The shape change happens because, instead of conducting current, the polymer molecules store the energy by changing the length of the chemical bonds that hold the long, chain-like molecules together. "All the chemical bonds are formed by electrons, so if you can move electrons around you can also change the bonds in length... and the molecule shape can change," said Zhang. "When the polymer changes in shape, you can get a lot of volume change."
The researchers made their more sensitive shape-shifting material by starting with polymer that has a dielectric constant of 60, and adding grains of a second material to the matrix of long, flexible polymer chains. This second material, copper-phthalocyanine, has a dielectric constant in the millions, but is not flexible. The researchers were able to incorporate the tiny grains into the polymer matrix without making the material much less flexible.
Previous attempts at adding materials to polymers to increase their dielectric constants used tiny grains of ceramics, which have higher dialectic constants than copper-phthalocyanine, but made the polymer too stiff. Copper-phthalocyanine is an organic molecule that's also used as a dye, and as a main ingredient for organic transistors.
The research is excellent, and the material has great potential, said Yoseph Bar-Cohen, a senior research scientist at NASA's Jet Propulsion Laboratory and an adjunct professor at the University of California at Los Angeles.
One limitation of today's electronic-type electroactive polymers is it
takes a very high electric field to actuate them, said Bar-Cohen. "Having
to use very high voltage carries risks to the users," he said.
Lower-voltage electroactive polymer materials should find numerous applications, including toys, said Bar-Cohen. Miniature electroactive polymer actuators could also be used in microfluidic devices, which move and mix small amounts of fluids, he said.
The researchers' next step is to use tinier grains of copper-phthalocyanine to boost the dialectic constant. It would be "the same material, but we want to make the filler much smaller," Zhang said. Much smaller particles means more surface area and so more boundaries that will act as barriers to block current, he said. It may be possible to boost the dialectic constant to 5,000 or even 10,000, said Zhang.
The material could be ready for practical use as an artificial muscle material within three years, and for other applications within five years, said Zhang.
Zhang's research colleagues were Hengfeng Li, Martin Poh, Feng Xia, Z.-Y. Cheng, Haisheng Xu, and Cheng Huang. They published the research in the September 19, 2002 issue of Nature. The research was funded by the National Institutes of Health (NIH), the Defense Advanced Research Projects Agency (DARPA), and the Office of Naval Research (ONR).
Timeline: 3 years, 5 years
TRN Categories: Chemistry; Energy; Materials Science and Engineering; MicroElectroMechanical Systems (MEMS); Robotics
Story Type: News
Related Elements: Technical paper, "An All-Organic Composite Actuator Material with a High Dielectric Constant," Nature, September 19, 2002.
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