A step toward stronger polymers

Many of the objects we encounter are made of polymers - long chains of repeating molecules. Networks of polymers form man-made materials such as plastics, as well as natural products such as rubber and cellulose.

Within all of these polymeric materials, there are structural flaws at the molecular level. To form an ideal network, each polymer chain would bind only to another chain. However, in any real polymeric material, a significant fraction of the chains instead bind to themselves, forming floppy loops.

''If your material properties depend on having polymers connected to each other to form a network, but you have polymers folded around and connected to themselves, then those polymers are not part of the network. They weaken it,'' says Jeremiah A. Johnson, an assistant professor of chemistry at MIT.

Johnson and his colleagues have now developed, for the first time, a way to measure how many loops are present in a given polymer network, an advance they believe is the first step toward creating better materials that don't contain those weak spots.

Huaxing Zhou, an MIT postdoc, is the lead author of a paper describing the new technique in this week's issue of the Proceedings of the National Academy of Sciences. Other authors are visiting researcher Jiyeon Woo, chemistry graduate student Alexandra Cok, chemical engineering graduate student Muzhou Wang, and Bradley Olsen, an assistant professor of chemical engineering.

Although polymer chemists have known about these loops since the 1940s, they have had no way to count them until now. In the new paper, the researchers measured the percentage of loops in a gel, but their approach could be used for nearly any type of polymer network, Johnson says.