Thursday, March 01, 2007

From the Labs: Nanotechnology

New publications, experiments and breakthroughs in nanotechnology--and what they mean.

By Kevin Bullis

Microscopic structures in a new, ultratough material that is reinforced with disc-shaped nanoparticles change shape under stress, altering the way they refract light. Credit: Mckinley Lab

Tough Nanomaterials
Potential applications include tear-resistant fabrics and fuel-saving car parts

Source: "High-Performance Elastomeric Nanocomposites via Solvent-Exchange Processing"
Shawna M. Liff et al.
Nature Materials 6: 76-83

Results: Researchers have found that using clay nanoparticles to reinforce a polyurethane material makes it 20 times as stiff and twice as resistant to heat. The polyurethane is composed of two different types of monomers--molecules linked up into polymer chains. The monomers don't mix well, so they locally separate into hard organized regions and soft amorphous regions. A new dispersion process ensures that the nanoparticles preferentially reinforce the hard regions, making the polyurethane stiffer. Since the process also leaves the soft, amorphous areas free to flex, the material can still stretch substantially without breaking.

Why it matters: To date, most attempts to use nanoparticles to stiffen elastomers such as polyurethane have also resulted in undesired decreases in flexibility, which can mean increases in brittleness. The new process not only makes the material stiffer but also makes it much tougher. The material could be used in lightweight, resilient packaging or spun into fibers to make tear-resistant clothing. Or, in an application that takes advantage of its heat resistance, it could replace some metal car body parts exposed to elevated temperatures, such as the hood. The general processing method could also be used to make a wide range of other new elastic materials.

Methods: The process uses two solvents. In one, the clay nanoparticles are dispersed; the other dissolves the polyurethane. The two solvents are then mixed until the suspended nanoparticles spread evenly throughout the dissolved polymer. When the second of the solvents is removed or evaporates, the clay particles are trapped within a tangle of polymer chains. The clay nanoparticles are selected to have a chemical affinity for the crystalline hard structures within the polyurethane, so those are what they preferentially aggregate with, rather than with the soft, amorphous regions.

Next Steps: Reducing the amount of solvent used could make the manufacturing process cleaner and easier. Making actual products from the material may require adjusting manu­facturing techniques: too much heat during processing may reduce the material's stiffness.