Depicted at left, small nanoparticles stick to segments of polymer chain that are 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. (Image credit: Oak Ridge National Laboratory, U.S. Dept. of Energy, created by Jan-Michael Carrillo)
Polymer nanocomposites combine particles measuring about billionths of a meter (nanometers, nm) in diameter with polymers, which are lengthy molecular chains.
Frequently used to manufacture injection-molded products, they are widespread in coatings, automobiles, packaging materials, adhesives, fire retardants, medical devices, drug-delivery systems, membranes, sensors, and consumer goods.
A research team led by the Department of Energy’s
Oak Ridge National Laboratory got a huge surprise when they tried to validate that decreasing the nanoparticle size would unfavorably influence the mechanical properties of polymer nanocomposites,
We found an unexpectedly large effect of small nanoparticles.
Shiwang Cheng, ORNL
The collaborative team of researchers at ORNL, the University of Illinois at Urbana-Champaign (Illinois) and the University of Tennessee, Knoxville (UTK) reported their findings in the ACS Nano journal.
Remarkable improvements can be seen in the properties of polymer materials when nanoparticles are mixed with polymers. Spatial organization, nanoparticle size, and interactions with polymer chains are important to establish the behavior of composites.
Understanding these effects will pave the way for the enhanced design of new composite polymers, as researchers can tweak mechanical, electrical, chemical, thermal, and optical properties.
Researchers believed, until recently, that an optimal nanoparticle size must be in existence. Downsizing would work only to a point, as the tiniest particles are likely to aggregate at high loadings and plasticize at low loadings, both of which damage macroscopic properties of the polymer nanocomposites.
The ORNL-led research compared polymer nanocomposites comprising of particles measuring 1.8 nm in diameter against those with particles measuring 25 nm in diameter. Most conventional polymer nanocomposites comprise of particles measuring 10 - 50 nm in diameter.
In the future, it is possible that novel polymer nanocomposites could contain nanoparticles measuring a lot less than 10 nm in diameter, enabling new properties not viable with larger nanoparticles.
Well-dispersed small “sticky” nanoparticles enhanced properties, one of which shattered records: Raising the temperature of the material lower than 10 °C resulted in a rapid, million-fold drop in viscosity. A pure polymer (without nanoparticles) or a composite with large nanoparticles would require a temperature increase of no less than 30 °C for a similar effect.
We see a shift in paradigm where going to really small nanoparticles enables accessing totally new properties.
Alexei Sokolov of ORNL and UTK
That boost in access to new properties occurs because small particles travel faster than big ones and interact with fewer polymer segments on the same chain. Several more polymer segments adhere to a large nanoparticle, causing dissociation of a chain from that nanoparticle to become hard.
“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.”
The ORNL-led research needed expertise in materials science, physics, chemistry, computational science and theory.
“The main advantage of Oak Ridge National Lab is that we can form a big, collaborative team,” Sokolov said.
Cheng and UTK’s Bobby Carroll performed experiments they engineered with Sokolov. They used broadband dielectric spectroscopy to track the movement of polymer segments linked with nanoparticles. Calorimetry exposed the temperature at which solid composites converted to liquids. With small-angle X-ray scattering, Halie Martin (UTK) and Mark Dadmun (UTK and ORNL) were able to characterize nanoparticle dispersion in the polymer.
To understand the experimental results and associate them to primary interactions, structure, and dynamics in a better manner, the researchers looked at large-scale modeling and simulation (by ORNL’s Bobby Sumpter and Jan-Michael Carrillo), which was enabled by the Oak Ridge Leadership Computing Facility, a DOE Office of Science User Facility at ORNL.
It takes us a lot of time to figure out how these particles affect segmental motion of the polymer chain. These things cannot be visualized from experiments that are macroscopic. The beauty of computer simulations is they can show you how the chain moves and how the particles move, so the theory can be used to predict temperature dependence.
Shiwang Cheng, ORNL
University of Illinois researchers Shi-Jie Xie and Kenneth Schweizer developed a new fundamental theoretical description of the collective activated dynamics in such nanocomposites and quantitatively used it to comprehend new experimental phenomena. The theory helps to predict physical behavior that can be used to put together design rules for enhancing material properties.
Titan, America’s most powerful supercomputer was used by Carrillo and Sumpter to develop and run simulations. They also wrote codes to examine the data on the Rhea cluster. The LAMMPS molecular-dynamics code calculated the speed at which nanoparticles travelled relative to polymer segments and the amount of time polymer segments adhered to nanoparticles.
We needed Titan for fast turn-around of results for a relatively large system (200,000 to 400,000 particles) running for a very long time (100 million steps). These simulations allow for the accounting of polymer and nanoparticle dynamics over relatively long times. These polymers are entangled. Imagine pulling a strand of spaghetti in a bowl. The longer the chain, the more entangled it is. So its motion is much slower.
Jan-Michael Carrillo, ORNL
Molecular dynamics simulations of extensive, entangled polymer chains were required to calculate time-correlation functions similar to experimental conditions and discover agreements or connections between the theories proposed and experiments by colleagues at Illinois.
The simulations also visualized the way in which nanoparticles traveled relative to a polymer chain. Corroborating experiment and theory helps scientists to move closer to validating predictions and creates a better understanding of how nanoparticles alter behavior, such as how altering nanoparticle–polymer interactions or nanoparticle size will influence the temperature at which a polymer loses sufficient viscosity to become liquid and begin to flow.
Large particles are comparatively immobile on the time scale of polymer movement, while small particles are more mobile and inclined to detach from the polymer a lot faster.
The research paper is titled “Big Effect of Small Nanoparticles: A Shift in Paradigm for Polymer Nanocomposites." The research was supported by DOE’s Office of Science.