Researchers have used clay nanoparticles to modify a polymer material, making it 20 times stiffer, 4 times tougher, and able to withstand temperatures that are more than twice as hot. The new materials could eventually be used in rugged lightweight fabrics, less-bulky packing materials, and much lighter car parts.
The work is part of a growing effort to design materials with nanoscale structures that mimic those found in nature, such as those in ultra-strong seashells. (See "Silicon and Sun.") In the current work, researchers at MIT's program in polymer science and technology greatly improved the properties of an elastic polyurethane used in biomedical applications by dispersing tiny clay particles throughout it.
The elastic polyurethane is ordinarily made of two types of polymers, one hard and crystalline, the other a soft, tangled polymer. The researchers developed a method for reinforcing the rigid structures with thin, flat, nanoscale clay platelets. The clay nanoparticles link the hard polymer chains into a continuous network running throughout the soft polymer.
The result is a material that has properties that are typically hard to combine: stiffness and stretchiness. In the past, others have found ways to make the material stiffer, but that came with a trade-off, says lead researcher Gareth McKinley, a professor of mechanical engineering at MIT. In previous attempts, a material made seven times stiffer "became more brittle--it snapped," he says. McKinley has made the material stronger still--23 times stronger--without making it brittle. "We are able to make it both stronger as well as keeping it nice and stretchy," he says.
Since the new material is stiff, it takes a significant amount of energy to deform it. But even once the material starts to deform, it doesn't break. Instead, it absorbs yet more energy as it stretches. Indeed, the nano-reinforced material will absorb as much as four times the amount of energy as the original material without breaking.
The greater toughness means that much less material can be used--as much as 75 percent less. Thin sheets of the material, while being resistant to tearing, would be flexible enough to serve as packaging, such as for the military's meals-ready-to-eat (MREs), McKinley says. The material could also be spun into fibers to make flexible yet tear-resistant fabrics.
The new material is also resistant to heat: the clay particles "improve the high-temperature strength of these polymers immensely," McKinley says. The original polyurethane starts to soften at around 100 °C, losing its stiffness and breaking easily. But the new material is heat resistant to 200 degrees, which means it could be used in applications such as the hood of a car. Because the materials are light, the fuel savings "could potentially be very large," McKinley says.
While Evangelos Manias, a professor of materials science and engineering at Pennsylvania State University, says that the new material is impressive, he cautions that the process limits the ways the material can be used. If it is heated too much while being incorporated into a product, the clay particles might clump together, causing the enhanced properties to be lost.
Manias says that even more significant than the new material is the process used to make it. It's been difficult to uniformly disperse nanoparticles such as the clays throughout polymers because they have incompatible chemical properties: the clay attracts water, while the polymers repel it. The problem is made more challenging in this case because the clay nanoparticles must connect only with the hard segments of the polyurethane and not with the soft, stretchy polymer mesh. Otherwise the material will lose its stretchiness.
To make it possible to locate the clay nanoparticles at just the right places, McKinley and his colleagues at MIT developed a system that uses two solvents, one to disperse the clay nanoparticles and the other to dissolve the polymer. These two solvents are then mixed until the suspended nanoparticles are spread evenly throughout the dissolved polymer. The solvent that dissolved the polymer is then evaporated, leaving behind a tangle of polymer that traps the clay particles. Because this method does not chemically alter the nanoparticles, as has been done in other approaches, the particles retain a chemical affinity to the rigid structures within the polyurethane, which causes them to connect to these and not to the soft parts of the structure.
Manias says that this process could apply to a wide variety of systems, using different nanoparticles, such as nanotubes, to make even more remarkable materials. "The most important thing is that this can be applied more broadly than just polyurethane," he says. "There are whole fields of science where this can be applied."