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Polymers grafted by interlocking strands.

They run along the edges of outdoor gear, the tops of sneakers, the borders of bags. These fasteners -- Velcro being the best-known brand -- have two strips: one bristly and one fuzzy. Pressed together, they stick, as tiny hooks on the rough side grasp the soft side's looping threads.

Designed properly, polymers can join in the same way: Their surfaces can bear the molecular equivalents of hooks and threads.

Dilip Gersappe, a materials scientist at the University of Pittsburgh, and his colleagues describe a new method for grafting polymers with interlaced strands. A report on what they call "molecular Velcro" appears in the Aug. 19 SCIENCE.

"At the molecular level, it's like joining your hands together by interlocking your fingers," says coauthor Anna C. Balazs, also a Pittsburgh materials scientist. "Think of each hand as a different polymer, and think of your fingers as the strands that hook them together."

To create such a polymer concoction, the researchers used three compounds that otherwise do not mix: two "homopolymers," A and B, and a third "copolymer," C. When blended together, polymers A and B each form tiny globules surrounded by polymer C. The grafts occur when polymer C binds the surfaces of A and B.

The researchers first modeled the hook and loop interaction on a computer. Then they mixed up suitable polymers, Balazs says. For homopolymers A and B, they used deuterated poly(ethyl acrylate)(d-PEA) and poly(methyl methacrylate)(d-PMMA), respectively. For copolymer C they combined PEA-poly-styrene(PS) and PMMA-PS. They subsequently blended, cooked, and cooled various combinations of the polymers and then tested them for their physical properties.

Interestingly, the scientists found that these bonds "significantly improved the structural integrity and mechanical properties" of the materials. The new polymer blend proved stronger, more pliant, and less likely to snap than either of the homopolymers.

"What's nice about this technique is that it's very general," Balazs says. "It will work for a whole class of materials."
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Title Annotation:new method for grafting polymers discovered
Author:Lipkin, Richard
Publication:Science News
Article Type:Brief Article
Date:Aug 20, 1994
Words:326
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