'Exercise' Can Strengthen New Nanomaterial
Humans strengthen their muscles and bones by pumping iron or playing sports, but physical stress can bend or break many materials. Now NASA-funded research has stumbled across a new material that actually becomes stronger from repeated stress.
The accidental discovery came during stress tests of a material made from rubbery polymers and carbon nanotubes that resemble tiny cylinders. The stiffness of that composite material saw a boost of 12 percent after enduring 3.5 million compressions, or about five per second. Such findings could lead to artificial replacements for the cartilage connective tissue in human bodies.
But the results also puzzle researchers at the lab of Rice University in Houston. They know that certain metals can harden because of repeated stress, but polymers don't tend to do the same.
All that they know is that the strengthening effect came from physical combination of the polymers and the carbon nanotubes, rather than some chemical melding of the materials.
"The data shows that there's very little chemical interaction, if any, between the polymer and the nanotubes, and it seems that this fluid interface is evolving during stressing," said Brent Carey, a graduate student in materials science at Rice University.
The iron man effect for the new material only takes place with repeated stress or compressions, as opposed to simply keeping the material under stress. That effect is similar to how the human body strengthens in response to repeated exercise.
"As long as you're regularly stressing a bone in the body, it will remain strong," Carey said. "For example, the bones in the racket arm of a tennis player are denser. Essentially, this is an adaptive effect our body uses to withstand the loads applied to it."
Many mysteries remain about just how the polymer and carbon nanotubes interact at the nanoscale, where object sizes range from 1 and 100 nanometers. By comparison, a human hair is about 100,000 nanometers wide.
But the early results look great, according to Pulickel Ajayan, professor in mechanical engineering and materials science at Rice University. He heads the lab where Carey continues to investigate the new material.
"For engineered materials, people would love to have a composite like this," Ajayan said. "This work shows how nanomaterials in composites can be creatively used."