Dadmun scrutinizes the molecular structure of known polymers in hopes of discovering what causes their distinctive behaviors and how new combinations might improve or combine their best characteristics. "We usually start by saying, okay, we have a molecule that does this. How can we make it better? What can we add or modify to make it do what we want?" he says.
Polymers are made of long chains of repeating molecular units. "We are trying to understand how to bind both ends of a polymer chain onto a surface to create a loop, which is not what the polymer chain wants to do," Dadmun says. If we can create these loops, he says, when we coat the loop-covered surface with a polymer made of loose strands—much like the mesh side of a Velcro fastener—the strands will tangle with the loops and form a strong link between the layers.
If scientists can also create a loop-covered surface on the outside of a mesh layer, the next polymer layer will adhere to the one beneath it, Dadmun says, and so on. "Take the paint on your car, for instance," he says. "First you prime the metal, and then you paint layer after layer on top of each other. All those layers have to bind well to keep the paint from flaking."
The fact that these loop-and-mesh structures are only a fraction of the width of a human hair adds to the challenge. But if Dadmun can devise the right molecular combination, polymer-synthesizing chemists—among them Jimmy Mays, who holds a joint appointment with UT and Oak Ridge National Laboratory—will be able to create paint films that stay hooked together. Such adherent properties would mark a major advancement for the paint industry.
Beyond their ability to adhere, polymers also show potential for use in lightweight antimicrobial protective clothing for soldiers and emergency personnel who may be exposed to hazardous bacteria. "Some polymers are antimicrobial but don't have the other properties needed for some applications," says Dadmun. "For instance, they may not be strong or flexible enough."
One solution is to layer these antimicrobial substances on top of lighter, stronger, more flexible polymers. Bacteria would then be exposed to the antimicrobial surface layer, while the lower fabric layers would add strength and resiliency to the material, which would better protect the skin. "We do this by making branched polymers—structures that Mays is particularly expert at synthesizing. If we design them correctly, the antimicrobial polymers will float to the surface by themselves and stay there, while the rest of the material will retain the desirable properties of the other polymers."
Before Dadmun's new materials are ready for synthesis, they must undergo tests to confirm that they possess the desired properties. "The most basic test we run is called 'contact angle,' " he explains. "We put a drop of water on the [polymer material's] surface and look at the angle the water forms while it sits there, if it sits there at all." The water can spread as the polymer absorbs it; it can create a raised puddle, like water on a tabletop; or it can form a round ball, like a fat raindrop. By studying how the water rests on the material's surface, Dadmun can determine whether the polymer has the requisite properties.
Beyond superior paints and protective clothing materials, Dadmun is devising polymer formulations that are compatible with the body, such as the bone cement used in hip-replacement surgery. The UT chemist is also working on polymers that conduct electrical current, as opposed to plastics and other polymers that function to insulate wires. "NASA needs materials that are both flexible and conductive," he says. "And we're developing polymers that possess both properties."
Dadmun's conductive polymers, known as nanotubes, might also be designed to convert sunlight into energy in solar cells. "Eventually, if we can get similar electrical and conductive results, polymers will replace tasks currently performed by metals, glasses, and ceramics," he says. "These applications are all viable. It's a matter of fine-tuning and understanding molecular-level control."
- - -
For more information, contact Mark Dadmun, (865) 974-6582, e-mail dad@utk.edu.