A new material design could reduce pollution where the rubber meets the road.

    Strategically adding weak points along microscopic chains called polymers actually makes them harder to tear, researchers report in the June 23 Science. Because polymers are used in car tires, the findings could help reduce plastic pollution as tires wear down over time.

    When tires scrape against the road, they drop tiny particles of rubber and plastic polymers, which pollute waterways and contaminate the air (SN: 11/12/18). Every year, tires release an estimated 6 million metric tons of these microplastics into the environment. Stronger polymers that break apart less easily could limit the amount of particles shed annually.

    To make such tough materials, Stephen Craig, a chemist at Duke University, and colleagues added molecules called cross-linkers to the polymers. These cross-linkers connected jumbled-up polymer chains to their many neighbors, and they were specifically designed to break apart easily. At the microscopic scale, the polymers act like a tangle of spaghetti strands with the cross-linkers holding them all together and helping them retain their shape, says Craig’s collaborator Shu Wang, a chemist at MIT.When the team stretched the polymer spaghetti, the individual cross-linkers broke easily, as expected. But the bulk material required more force to rip than they expected.

    The secret to the increased toughness lies in the path the tear has to take, Craig says. The tear propagates through the easy-to-break cross-linkers rather than through the tougher polymer strands. Each broken connection follows the path of least resistance but dodging the long polymer strands means breaking many cross-linkers, which requires more stretching force overall.

    This isn’t the first time researchers have used weak connectors to make polymers stronger. But unlike in similar materials, the increased toughness doesn’t come at the expense of other beneficial properties like stiffness.

    Craig says he hopes the findings will help extend the lifetimes of car tires and plastics, potentially limiting annual microplastic pollution.