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--a measurement associated with material strength--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.