Stressing Strengthens Nanocomposites

Repeatedly stressing polymer nanocomposites can cause the materials to become stronger, according to a study by materials scientists (ACS Nano, DOI: 10.1021/nn103104g). That property, which is one of the key characteristics of bone and other biomechanical tissues, is highly unusual in synthetic materials. The discovery may lead to new types of nanostructured hybrid materials for use in load-bearing applications, including artificial connective tissues. Brent J. Carey and Pulickel M. Ajayan of Rice University and coworkers used a chemical vapor deposition method to infuse freestanding carbon nanotube (CNT) forests with polydimethylsiloxane (PDMS) and then subjected samples of the nanocomposite to repeated cycles of compressive stressing. Unlike the neat polymer, which exhibited little change in response to compression cycles, the CNT/PDMS nanocomposite’s stiffness began increasing immediately. The stiffness increased 7.5% after one day of dynamic stressing and a total of 12% by the end of a week—3.5 million stress cycles—with no indication of the effect plateauing, the team reports. The increased stiffness, though not yet fully understood, appears to result from stress-induced polymer chain alignment at the CNT/PDMS interface, which enhances interaction between the components, the researchers suggest.