UT-ORNL: Small Nanoparticles Have Surprisingly Big Effects on Polymer Nanocomposites

Alexei Sokolov

Polymer nanocomposites mix particles billionths of a meter (nanometers, nm) in diameter with polymers, which are long molecular chains. Often used to make injection-molded products, they are common in automobiles, fire retardants, packaging materials, drug-delivery systems, medical devices, coatings, adhesives, sensors, membranes and consumer goods.

When a team of scientists, including UT’s Alexei Sokolov, tried to verify that shrinking the nanoparticle size would adversely affect the mechanical properties of polymer nanocomposites, they got a big surprise. They found an unexpectedly large effect of small nanoparticles.

The findings were reported recently in the journal ACS Nano.

In addition to Sokolov, the team included scientists from Oak Ridge National Laboratory, and the University of Illinois at Urbana-Champaign. Sokolov is a UT-ORNL Governor’s Chair based in the Department of Chemistry.

Blending nanoparticles and polymers enables dramatic improvements in the properties of polymer materials. Nanoparticle size, spatial organization and interactions with polymer chains are critical in determining behavior of composites. Understanding these effects will allow for the improved design of new composite polymers, as scientists can tune mechanical, chemical, electrical, optical and thermal properties.

At left, small nanoparticles stick to segments of polymer chain about the same size as the nanoparticles themselves. these interactions produce a polymer nanocomposite that is easier to process because nanoparticles move fast, quickly making the material less viscous. At right, many segments of a polymer chain stick to a larger nanoparticle, making it difficult for that nanoparticle to move. Its slower movement results in a viscous material that is more difficult to process. Source: ORNL

Until recently, scientists believed an optimal nanoparticle size must exist. Decreasing the size would be good only to a point, as the smallest particles tend to plasticize at low loadings and aggregate at high loadings, both of which harm macroscopic properties of polymer nanocomposites.

“We see a shift in paradigm where going to really small nanoparticles enables accessing totally new properties,” Sokolov said. That increased access to new properties happens because small particles move faster than large ones and interact with fewer polymer segments on the same chain. Many more polymer segments stick to a large nanoparticle, making dissociation of a chain from that nanoparticle difficult.

“Now we realize that we can tune the mobility of the particles—how fast they can move, by changing particle size, and how strongly they will interact with the polymer, by changing their surface,” Sokolov said. “We can tune properties of composite materials over a much larger range than we could ever achieve with larger nanoparticles.”

Continue reading on the Oak Ridge National Laboratory website.