Soft and Hybrid Materials
For centuries, humans have relied on natural polymers, long-chain molecules, in the form of rubber, an elastic hydrocarbon polymer, and cellulose, a plant-based material used to make paper. But in the nineteenth century, scientists began to study and then blend natural polymeric materials with other compounds to produce functional products. By combining nitric acid and cellulose, for instance, they created nitrocellulose, a highly flammable substance, and later, through the addition of camphor, they created the films used in early still and motion-picture cameras.
In the early twentieth century, scientists took over where nature left off and began to synthesize polymers in the laboratory. Nylon, the first human-made polymer molecule that could reliably be fabricated, was created in the 1930s and is still in use today.
Synthetic polymers, including plastics, are ubiquitous in our modern world, forming everything from the soles of our sneakers to the side panels of our automobiles. For JIAM research affiliates, the ever expanding range of available polymers provides the ingredients for new soft and hybrid materials that may soon revolutionize energy production and storage, provide for the capture of carbon from combustion of fossil fuels, and turn heat into electricity.
By combining a polymer with nanoscale components, JIAM research affiliates are able to dramatically change material properties. For instance, by mixing polymers with clay, researchers are creating plastics with improved heat deflection, resistance to softening, and superior ability to block diffusion of gasses. Such materials can be used as components in the high-heat environment of a car engine or to create plastic beverage bottles that better retain carbonation. Plastic bottles are more durable and much lighter than glass, which can significantly reduce transportation costs.
When mixed with polymers, carbon nanoparticles that are in the shapes of nanotubes, fullerenes (structures that resemble soccer balls), and graphene (one-atom-thick carbon sheets) create a range of useful materials.
Carbon-reinforced polymers, which may contain 20 to 40 percent carbon, are currently used as strong, lightweight structural materials. But formulations containing smaller amounts of carbon can serve different functions. Polymers tend to be electrical and thermal insulators, while carbon is an electrical and thermal conductor. Though modern computer chips continue to get smaller, they still give off significant amounts of heat. A polymer blended with 1 to 3 percent carbon nanotubes retains the polymer’s adhesive properties, but the carbon tubes can draw off heat and protect a computer’s electronic components from damage.
However, a homogeneous mixture is required to create adequate pathways for the escape of thermal energy. Therefore an even dispersion of the carbon particles in the polymer is needed and JIAM research affiliates are carefully controlling the interface between two blended constituents to create materials with the targeted structure and properties.
Diblock copolymers are materials that contain two polymer blocks chemically bonded at the ends. Though the two polymers are securely bonded end to end, the two blocks want to phase separate. The result is a microphase separation, like oil and water, forming into desirable nanoscale shapes and structures.
Depending on the lengths and connectivity of the copolymers, for instance, JIAM research affiliates can exploit microphase separation to form upright cylinders in a hexagonal arrangement. By first degrading the cylinders with UV radiation, the scientists can then use acetic acid to wash away the material in the tubes and create a nanoporous template on top of a substrate. Picture a regular pattern of holes left in a sheet of dough by a small round cookie cutter. The tubes provide the means for the orderly deposition of particles on the substrate—for instance, to grow vertically aligned carbon naonstructures. This technique can also be used to deposit particles on a silicon substrate to create tiny computer chips.
Organic Solar Cells
Combinations of conjugated polymers and carbon fullerenes can be used to create organic photovoltaic cells (OPVs) capable of converting sunlight into electricity. Conventional photovoltaic cells rely on semiconductors—chiefly silicon—instead of carbon.
Most polymers are solution processable, which means they can be dissolved and blended with other components—in this case, carbon fullerenes—to create the thin films necessary for use in OPVs.
OPVs approach conversion efficiencies of about 8 percent—substantially less than the most efficient conventional silicon-based solar cells. In part, OPVs’ efficiency is limited by their inability to absorb light across the entire solar spectrum. JIAM research affiliates are working to increase OPVs’ ability to absorb a wider range of solar light and to enhance the pathways for charge generation and transport.
Despite their current limitations, OPVs offer numerous advantages over conventional solar cells: They are significantly lighter and thinner, much more flexible, and potentially much cheaper to produce.
Lighter means that roofs could be blanketed with OPVs without need to reinforce roof structures. Thin and flexible mean that OPVs could be applied as coatings or blended into the materials of portable structures like military tents or used as mobile power sources for soldiers, who now may carry twenty pounds or more of batteries to run their equipment.
Regardless of how efficient solar panels become, there’s little doubt that society will, for the foreseeable future, continue to rely to some extent on carbon-based fuels—coal, petroleum, natural gas. It’s unlikely, for instance, that jet aircraft will ever derive their power from solar cells.
Fossil fuels are known for their high energy conversion rate, but they also are associated with a range of negative environmental impacts. Carbon dioxide (CO2), a product of combustion of these fuels, contributes to global climate change. Sequestering and storing CO2 is a way to reduce the amount of greenhouse gases released into the atmosphere, but carbon capture remains a complex and expensive process.
JIAM research affiliates are exploring ways to craft polymer membranse that will selectively extract CO2 from the gases issuing from the flues of coal-fired power plants. Once extracted and captured, the CO2 could be used in chemical processes or injected into depleted oil and gas wells. Sequestered CO2 is prevented from entering the atmosphere and, thus, does not affect global climate.
From Heat to Electricity
Automobile tail pipes, hot-water heaters, and other appliances, electric lights, and computers and other electronic devices generate heat, a form of energy. JIAM researchers are exploring thermoelectric materials capable of converting that wasted heat into electricity.
Ceramic materials have been used for this purpose, but ceramics are brittle, heavy, and expensive to produce on a large scale. Thermoelectric polymers would be inexpensive to produce, and, because they’re flexible and thin, they could be used to coat many devices such as hot water heaters.
To work, thermoelectric materials must be good conductors of electricity but poor thermal conductors. Polymers are poor thermal conductors, but they’re also poor electrical conductors. A blend of carbon nanotubes and a polymer demonstrates dramatically improved electrical conductivity but only slightly increased thermal conductivity, making it suitable for use as a thermoelectric material. Candidate polymers for this purpose include those used in OPVs as well as styrene acrylonitrile, which tend to disperse nanoparticles well.
Fuel cells use a chemical process to convert a fuel—typically hydrogen, an abundant resource—and oxygen into electricity. The byproduct of this process is water. There are no products of combustion, including greenhouse gases, which makes fuel cells environmentally preferable power sources.
In polymer electrolyte membrane (PEM) fuel cells, the hydrogen enters the cell on the anode side, and a catalyst divides the hydrogen into positively charged ions and negatively charged electrons. The polymer electrolyte membrane allows hydrogen ions to pass through to the cathode, but blocks the electrons, creating an electrical charge.
Currently, fuel cells are not sufficiently efficient or cost effective enough to allow them to serve as practical energy sources. JIAM research affiliates are studying the elementary processes taking place in fuel cell materials, working to make the ions and electrons inside the cells move faster and the chemical reactions occur more quickly. These and other improvements in the technology could make fuel cells ideal for use in automobiles.
Reduction Oxidation (Redox) Flow Battery
While some JIAM researchers are devising means for creating energy, others are at work developing ways to store it. Akin to the fuel cell is the reduction oxidation (redox) flow battery.
These batteries, which produce energy by dissolving electrically active materials in an electrolyte polymer solution, also could power tomorrow’s automobiles. After the redox battery’s electrolytes have been depleted, they can be replenished, similar to the way conventional automobiles take on fuel at filling stations.
These batteries could also be used to store wind and solar power for later use. Recent JIAM research has led to a ten-fold increase in efficiency of these batteries.
The behavior of polymer blends and combinations of soft materials and hard substances (hybrids) commonly results from how the materials interact at their interfaces. Dramatic advances in the tools available to visualize and characterize these materials allow JIAM’s materials researchers to study and manipulate these interfaces on the nanoscale.
UT and Oak Ridge National Laboratory (ORNL), together, possess the full complement of these essential tools. ORNL’s Spallation Neutron Source, the best neutron-scattering facility in the world, allows JIAM research affiliates to “decorate” polymer molecules. This, in turn, allows the molecules to be visualized as if they are in contrasting colors. From there, scientists are able to modify and change the structure of individual molecules.
JIAM is in the process of building a specialized microscopy laboratory for use by its soft and hybrid materials research affiliates. The facility will include atomic force microscopes, transmission electron microscopes, and other high-power electron microscopes. These devices provide detailed resolution to less than a nanometer.
Meanwhile, JIAM’s team of soft and hybrid materials researchers possesses the full range of skills necessary to synthesize new polymer molecules, study their behavior, and characterize their properties. JIAM’s theorists, with access to the Kraken and Jaguar supercomputers—among the world’s fastest—can help explain unexpected results produced by experiments on new soft and hybrid materials and direct efforts to exploit their unique properties.