Polymers plus quasicrystals — A puzzling interaction
Sometimes trying something that really shouldn't work can lead to an amazing discovery. That's what happened to Valerie Sheares, an Ames Laboratory associate and Iowa State University assistant professor of chemistry. The discovery, a polymer-quasicrystal composite, has the best characteristics of each of the constituent parts. It's opened the door for a variety of innovative uses. Why it works, however, remains a puzzle—one that Sheares is eager to solve.
It was her expertise with polymers combined with an introduction to quasicrystals that kindled Sheares' curiosity. Shortly after arriving at Iowa State in 1996, she heard Pat Thiel, Ames Lab Materials Chemistry Program director, discuss quasicrystals and their unique properties.
Discovered in the 1980s, quasicrystals typically are aluminum-rich alloys of specific compositions. They are extraordinarily hard, have low coefficients of friction under certain test conditions, and don't conduct heat as well as most metals. "All things that in one material are pretty outstanding, especially considering that the material is a combination of common metals," Sheares says. "They exhibit properties not found in the constituent metals, but when you put them together in the right combination, under the right conditions, you get this outstanding set of properties."
As Thiel described some of the unique properties of quasicrystals, Sheares started thinking about the possible benefits of combining quasicrystals with polymers. Polymers—large molecules that consist of repeating chemical units joined together—typically are filled with all kinds of materials, such as calcium, silica, alumina or carbon black. "Almost no commercial plastic is unfilled," Sheares explains. "There are all kinds of things mixed into a polymer. The polymer really becomes the matrix that is holding the filler together. These other things are dispersed throughout to alter the properties." As a result, polymers are used in a wide array of products, everything from grocery sacks to automobile tires.
In addition, polymer processing is well understood, according to Sheares. They are processed into films, fibers and shapes used in thousands of products. Even in the planning stages, the benefits of a material with the unique quasicrystal properties that could be processed as easily as polymers had enticing possibilities.
An improbable mixture
The fundamental question of whether polymers and quasicrystals would mix, however, was not guaranteed. "We tried things that shouldn't have worked, but we tried them because we were just thinking from a polymer standpoint," Sheares explains.
In particular, polymers don't like to mix with each other let alone with other materials. Coupling agents generally are used to increase the adhesion or interaction between the polymer and the filler by attaching to both surfaces, but the nature of the quasicrystal surface prevents the coupling agent from being able to attach.
"Quasicrystals have a low-surface energy—they don't like contact with other things. It's like Teflonâ , that's a low-surface energy material, so things do not stick. That gave us the idea that these things aren't going to want to mix with themselves, much less with polymers," she notes.
Considering these barriers, Sheares' experiments showed surprising results. "We dissolved the polymer in a solvent and added it to a quasicrystal powder. We wanted to see it disperse in the liquid," she says. Even though the heavier, metallic powder sank, it was evenly dispersed on the bottom with no clumping together. That was the first indication that the two materials would blend rather than run away from each other.
The next step was to mix a polymer powder with a quasicrystal powder and to process the mixture using compression molding, one of the most common techniques for fabricating materials. The mixture was poured into a mold, and heat and pressure were applied causing the polymer to become molten or soft. The research team of Sheares, Paul Bloom, an ISU graduate student, and K.G. Baikerikar, an Ames Lab scientist, worked diligently adjusting the timing and temperature to maximize how evenly the particles dispersed before the material was cooled down. Using electron microscopy, the researchers examined the new composite, slicing into it to confirm that the quasicrystals were evenly dispersed throughout. This characteristic is key because the closer the interaction between the polymer and the filler, the stronger the material. Tests confirmed that the composite's mechanical properties were increased, yet the thermal properties remained unchanged, verifying that in these tests, the quasicrystals behaved just like other hard fillers used in polymers.
Amazing combination of properties
Next, the researchers examined the wear properties of the composite. Essentially, the tests involve a steel ball held in contact with the composite while the composite rotates on a turntable. Then the composite and steel ball are examined for wear. The results were impressive—quasicrystals, as a filler, significantly improved the wear-resistance of the polymers.
Even more amazing, the composite exhibited a nonabrasive characteristic—neither the composite nor the steel ball showed significant signs of wear. "Almost everything that is hard is also abrasive. If the material itself doesn't wear, it's probably wearing very hard on the other surface. Quasicrystals in polymers are not like that. They are not scratched, and they don't scratch other surfaces," Sheares explains.
While the first polymers tested were high-performance, subsequent tests with commodity polymers also have been positive. "We have gone through every type of polymer you can imagine. It doesn't really matter what the matrix is. It performs similarly," Sheares adds.
With the success in the laboratory, Sheares applied for a composition of matter patent. "We are the first people on record to make a polymer quasicrystal composite, so we want to own that composition," she says. Approval on the patent application is pending.
Thiel, who also is chair of the ISU chemistry department, calls Sheares' accomplishments very exciting. "Valerie is very enthusiastic and has clear expectations of what she wants to do. She wasn't afraid to try something that nobody expected to work. As a result, she's opened up a whole new area of study both in terms of the practical ramifications and the fundamental questions related to the nature of the interactions between polymers and quasicrystals," Thiel says.
New hope for hip replacements
One application Sheares and Ames Lab colleagues currently are working on grew out of conversations between Bloom and another graduate student, Brian Anderson. Surya K. Mallapragada, Anderson's major professor, is a member of the Materials Chemistry Program and an ISU associate professor of chemical engineering.
One of Mallapragada's research areas involves the use of ultra-high-molecular-weight polyethylene in replacement hip joints. "The big problem with polyethylene is that there is a lot of wear. The polyethylene is in the form of a cup. A metal ball rotates in the polyethylene socket, and there is constant friction causing wear on both materials," Mallapragada explains. Not only does this impact the mechanical integrity of the replacement joint, but as the ball and socket wear, particles from these materials lead to bone loss causing the joint to loosen, she says.
With its nonabrasive, wear-resistant characteristics, Sheares' composite appears to offer a solution. Her research group filled the ultra-high-molecular-weight polyethylene with quasicrystals and did the wear test. "They performed just beautifully, just like they had done in every other polymer," Sheares says.
The next hurdle was determining how the composite behaved with living cells. The initial test to establish its biological compatibility was positive, i.e., it's not causing cell death. Sheares emphasizes that this was a quick, short-term test, but it means they can move on to much more extensive biological testing. "It's pretty exciting. It is an application that you can see immediately how it could be utilized," she says.
Searching for clues
The investigation into polymer-quasicrystal composites is far from over. Even as Sheares gets more feedback from industry with ideas for applications, she is searching for clues that will help her answer some very fundamental questions about polymer quasicrystal interactions at surfaces. What is that interaction? Why do the quasicrystals disperse?
In some respects, the success in the laboratory has hindered finding the answers. "If it only worked with certain types of polymers, then we could look at why one works, but another one doesn't. It works with every polymer. It's great, but it doesn't help us explain why," she says.
Unraveling this mystery is a top priority. After all, if you know how and why something works, you can make it work even better.
The Department of Energy's Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time.