Our research is focused on polymer nanocomposites with properties that rival metals and ceramics (e.g., high electrical conductivity and/or thermal stability), while maintaining beneficial polymer properties (e.g., low density).
Although we are interested in all aspects of polymer nanocomposites, we are currently focusing our efforts in three areas: layer-by-layer assembly of multifunctional thin films, thermoelectric polymer composites, and nanoparticle stabilization.
A variety of functional thin films can be produced using the layer-by-layer (LbL) assembly technique. Thin films, typically < 1µm thick, are created by alternately exposing a substrate to positively- and negatively-charged molecules or particles.
The movie below illustrates the layer-by-layer assembly process using a polycation (positively-charged polymer) and anionic clay platelets, which is the basis for our gas barrier and anti-flammable technologies. Each individual layer may be 1 – 100+ nm thick depending on: chemistry, molecular weight, charge density, temperature, deposition time, counter ion, and pH of the species being deposited.
The ability to control coating thickness down to the nm-level, easily insert variable thin layers without altering the process, economically use raw materials (due to thin nature), self-heal, and process under ambient conditions are some of the key advantages this deposition technique has.
The PNC Lab is currently studying LbL-based thin films as a super gas barrier (i.e. foil replacement material), an anti-flammable coating for foam and fabric, antimicrobial surfaces, electrochromic thin films and electrically conductive layers that can be patterned using traditional photolithography.
We are always looking for collaborators that can benefit from these types of functionalities or the LbL process more generally.
Conductive polymer composites can be used as electromagnetic interference (EMI) shielding, heat dissipation films, chemical sensors, actuators, and have potential as thermoelectric materials (Nano Lett. 2008). Despite the significant potential for these composite systems, high conductive filler concentrations are often required to achieve sufficient electrical conductivity. High filler loadings can lead to processing difficulties and loss of polymer-like properties.
Using water-based emulsion polymers, or other exclusionary volume systems, as the composite matrix to generate a segregated network of conductive filler is a promising method to dramatically reduce the amount of filler needed to achieve good conductivity.
The figure above schematically outlines the process of generating a segregated network of carbon nanotubes and the SEM image shows a poly(vinyl acetate) emulsion-based composite containing 5 wt% single-walled carbon nanotubes.
Our goal is to develop composites with very low percolation thresholds (< 0.1wt%) and high electrical conductivity (> 100 S/cm). We've already made significant strides working with both carbon black (Polymer 2008 and Macromol. Chem. Phys. 2008) and carbon nanotubes (Adv. Mater. 2004 and Nano Lett. 2008) in latex and will continue to work with these systems, but we are interested in a variety of conductive nanoparticles (metal nanowires, carbon nanofibers, ceramic nanotubes, etc.).
In addition to segregated network composites, we are studying ways to improve the performance of epoxy and other solution-processed nanocomposites. Significant reductions in percolation threshold and improvements in electrical conductivity have been achieved by using clay to disperse carbon nanotubes in epoxy (Adv. Func. Mater. 2007).
We've also studied the impact of covalent and noncovalent stabilization of nanotubes in epoxy by using polyethylenimine as a model stabilizer (Macromol. Rapid Comm. 2009). These studies not only provide methodology for tailoring electrical conductivity, but also thermo-mechanical composite behavior (modulus, expansion coefficient, etc.).
One of the greatest challenges to the use of nanoparticles (carbon nanotubes, metal nanowires, clay platelets, etc.) is control of microstructure during processing and use. We are exploring a variety of ways to tailor the dispersion and structure of nanoparticles in liquid environments in an effort to improve the behavior of suspensions and composite materials that contain these nanoparticles.
The figure shown above highlights one approach using poly(acrylic acid) to stabilize single-walled carbon nanotubes (SWNT) in water (J. Coll. Interf. Sci. 2008)
At low pH, PAA is uncharged and highly coiled and is able to completely exfoliate SWNT. At high pH, PAA is highly charged and straightened out as a result. This allows SWNT to be more bundled and networked. These microstructural changes influence suspension viscosity and electrical conductivity of composites made with this system (Nano Lett. 2006).
We are investigating a variety of stimuli-responsive polymers (pH, temperature, light and chemical) to further extend this ability to tailor nanoparticle dispersion and microstructure. Additionally, we are studying the use of nanoparticles to stabilize one another, such as clay acting as a solid surfactant to better disperse carbon nanotubes (Adv. Func. Mater. 2007). These methods of nanoparticle stabilization should offer new ways to obtain a desired microstructure and improve our ability to use these particles in useful products (composites, sensors, etc.).
B. S., 1997, North Dakota State University
Ph. D., 2001, University of Minnesota