Nature has a way of doing everything first. Bone, mother of pearl and spider silk are examples of natural nanocomposites. But the human hand has never been far behind. In fourth to eighth century A.D., the Mayans mixed indigo with clay in the earliest recorded example of a nanocomposite. Today, researchers are using nanosized building blocks to design and create new materials with potential applications in coatings, adhesives, fire retardants, medical devices, consumer goods and so on.
Left: Mechanism of scCO2-induced surface segregation of nanoparticles. A concentration gradient of fluid scCO2 molecules is developed due to their limited penetration power in a viscous polymer matrix. Nanoparticles coated with a wetting layer of CO2 molecules move to the CO2-rich phase by taking advantage of the steep scCO2 concentration gradient near the surface. Right: Scanning electron microscope image showing the surface segregation of phenyl C61 butyric acid methyl ester nanoparticles embedded in a polystyrene thin film after the scCO2 process.
The materials community is investigating polymer nanocomposites, a new class of multiphase materials containing dispersed nanosized particles. “These materials can have significantly improved physical and mechanical properties by inclusion of a very small amount of nanosized fillers relative to the native polymers,” said Tad Koga, Stony Brook University. “The challenge is to manipulate the dispersions of nanosized particles embedded in nanometer-thick polymer films prepared on solid substrates.”
Koga explained that uniform dispersion of nanosized particles is key for achieving improved properties throughout the polymer thin films. On the other hand, preferential segregation of nanosized particles has been used to inhibit dewetting of polymer thin films at the polymer/substrate interface or to heal crack formation at the polymer/air interface. Achieving the precise control of these dispersions tailored to a specific need is complicated, however, because of a delicate balance of polymer/nanoparticle interactions and constraints placed on configurations of long polymer molecules to accommodate nanoparticles in such a limited space.
Koga and his colleagues discovered a simple, low-temperature route to control spatial distributions of surface modified nanosized particles embedded in nanometer-thick polymer films by using supercritical carbon dioxide (scCO2), a fluid state of carbon dioxide (normally a gas in air at standard temperature and pressure and a solid – dry ice – when frozen).
They found that by “tuning” the CO2 environmental conditions and film thickness, they can make the nanoparticles switch the spatial arrangements between preferential surface segregation at the polymer/air interface and homogenous dispersion within the entire film. “Interestingly,” said Koga, “this scCO2-induced switching occurs regardless of interactions between nanoparticles with different ligands and polymer matrices.”
The resultant structures in CO2 can then be preserved by vitrifying the polymer matrices via rapid pressure quench to atmospheric pressure. Further cross-linking of polymer chains is possible to achieve sufficient thermal stability of the resultant dispersions of the nanoparticles
When polymer nanocomposite thin films are exposed to scCO2 in the highly compressible region near a critical point, two phenomena are induced independently: 1) excess absorption of CO2 molecules into a polymer matrix and 2) excess adsorption of CO2 molecules on the nanoparticle surfaces.
“We found that the excess absorption expands polymer chains homogeneously or heterogeneously, depending on the balance between the penetration length of the fluid molecules into a polymer and the film thickness,” said Koga. He explained that when the film thickness is larger than the penetration length scale of the fluid molecules, a concentration gradient of the fluid is developed at the topmost surface region within the film.
“This concentration gradient seems to provoke instability of the system, leading to migration of the nanoparticles ‘coated’ with a wetting layer of CO2 molecules preferentially to the topmost CO2-rich region,” he said. “By contrast, when the homogenous excess absorption occurs, the resultant expansion of the polymer chains may reduce the entropic penalty of the chains without expelling the nanoparticles to the topmost surface, allowing accommodation of the nanoparticles within the films.”
According to Koga, this is a crucial issue in understanding the control of nanoscopic polymer architectures and provides a potential strategy for creating versatile polymeric nanomaterials with specifically targeted surface properties and/or functionalities.
Part of the research was done at the National Synchrotron Light Source on beamline X10B, where x-ray reflectivity was used to clarify the penetration length of the fluid molecules into the polymer thin films with angstrom-scale spatial resolution.
In addition to Koga, the team includes Mitsunori Asada, a visiting scientist from Kuraray Co., Ltd., in Japan; Peter Gin and Maya K. Endoh, also at Stony Brook; Sushi K. Satija, National Institute of Standards and Technology; and Takashi Taniguchi, Kyoto University.
This work was funded by the National Science Foundation and Kuraray Co., Ltd., Japan. The National Synchrotron Light Source is supported by the Department of Energy.