Designing polymers for structural jobs.

Designing polymers for structural jobs

Materials such as wood and steel, which can withstand sustained loads and stresses, provide sturdy skeletons for bridges, buildings, cars and other structures. But scientists keep their eyes open for new materials, such as polymers, that might be tough enough for structural roles, yet lighter, cheaper or easier to process.

Although reinforced polymers have already made inroads for certain specialty applications, polymers suited for larger-scale structural uses remain uninvented. The delay stems from a design tradeoff: With polymers, improvements in load-bearing strength nearly always bring an increase in brittleness.

New insights into what makes polymers tough -- i.e., strong and nonbrittle -- could help researchers get around this tradeoff, says materials scientist Albert Yee of the University of Michigan in Ann Arbor. His team recently gained one such insight while examining the molecular structure of polycarbonate, a material serendipitously discovered in the 1950s and long ranked as the toughest polymer, Yee says.

He and his co-workers found that rigid groupings of six to nine of the monomeric units in this polymer link together to form flexible segments in a chain. Mechanical stresses appear to dissipate through spatial shifting of these groupings, Yee says, rather than through cleavage of chemical bonds -- the precursor to material failure.

Previous attempts to strengthen polycarbonates by chemically modifying the monomeric units have increased the polymer's brittleness, he says. But by taking advantage of the cooperative behavior of larger monomeric groupings, the Michigan team has designed and synthesized a polycarbonate that remains strong at higher temperatures (over 150[degrees]C) without gaining brittleness, Yee reports.