Why do nano-sized particles change the electrical insulating properties of polymers? The simple answer is that there is more surface or interfacial area in the composite. If you take a large rock and break it into smaller rocks you increase the surface area. The same is true for the metal oxides that are made into nanoscale particles, only it’s called interfacial area because the nanoparticles are within another material.
“One thing we know is that we’ve added a huge amount of interface,” says Schadler, referring to those places where polymer meets nanoparticle. “Something interesting happens in this interfacial region that fundamentally changes how this material behaves in an electric field.”
Interfacial regions may be the focus of investigation, but why increasing that area affects the property of the material is still an open question. One idea is that injected “hot” electrons might scatter off the interface thus interfering with their travel across the material. Because nanocomposites have more interface, you get more scatter. That’s the simplest hypothesis, says Schadler. Another way to think about it, she says, is to consider that local conductivity may be different in those areas that are filled (occupied by a nanoparticle) as compared to unfilled polymer. A third hypothesis is that electrons get trapped in the interfacial regions.
Nelson introduces the idea of free volume or space “not fully accounted for” that is a recognized property of polymers. “When you start putting nanomaterials into polymers, you change the free volume you make it much more closely packed, as it were,” he says. By reducing the amount of free volume, perhaps you’re limiting the space where electrons can become “more active and therefore more damaging,” says Nelson.
“We don’t have all the answers by any means. We’re still groping with this ourselves,” says Nelson. Intrigued by the phenomena and tantalized by the performance of early nanocomposites, Nelson and Schadler have both the motivation and the funding to delve deeper into the fundamentals. “Only by understanding it, can we tailor these materials to what we need. At the moment, it’s a trial-and-error sort of process,” says Nelson. “If we understood some of these questions, then we’d be in a better position to engineer these materials from a point of knowledge rather than just guesswork.”
Moreover, dielectric integrity is not the only property that changes when making composite materials, says Nelson. “We can make thermal and mechanical changes to the properties, which normally go hand in hand with a reduction in the electrical properties,” he says. “I think the indications are now that we can have our cake and eat it too. We can improve the thermal and mechanical properties without having an enormous impact on the electrical properties.”
This is a hallmark of nanocomposites, says Schadler, the ability to alter one property without diminishing others. In addition to playing with different ingredients, like substituting titania for silica, Schadler thinks tinkering with these metal oxides directly may be a way to engineer desired material properties. “There are a myriad of opportunities for tailoring that interface,” she says. “If we understand how the surface of the fillers affects properties, then we can improve the properties even further,” she says. “In some ways I feel like we’re just at the beginning of this.”