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Stitching together a thin polymer sheet.

It's not exactly quilt making, but materials scientists have now chemically stitched molecules together into polymer sheets. The resulting polymers are considerably tougher than microscopically thin films produced by other means, yet just as flexible.

Samuel I. Stupp and his co-workers at the University of Illinois at Urbana-Champaign describe their technique for fabricating these molecular sheets in the Jan. 1 SCIENCE.

Over the last six decades, polymer scientists have found many ways to link large numbers of small molecular units into lengthy chains, branched structures, and three-dimensional networks. They have succeeded in constructing large molecules in the shape of rings, stars, combs, and ladders -- but not sheets.

"Until now, experimental polymer science has virtually ignored two-dimensional polymers," notes materials scientist Edwin L. Thomas of the Massachusetts Institute of Technology.

In fact, most efforts to synthesize sheet-like objects have involved simply laying down thin films on surfaces or letting certain types of molecules known as amphiphiles assemble themselves into layers. But because their molecular components interact only weakly, these structures have generally proved unstable and fragile.

To synthesize a polymer sheet in which adjacent molecular units are linked by strong chemical bonds, Stupp and his group start with long, relatively stiff molecules specially designed with a group of reactive atoms at one end of each molecule and a different group of reactive atoms in the center. In solution, these molecular units pair up - reactive end to reactive end - into double-length strands.

The strands, in turn, spontaneously organize into a layer, orienting themselves like the evenly spaced bristles of a brush. This puts the set of paired reactive sites in the middle of the layer and the two other reactive sites aligned in planes one-quarter of the distance from the top and bottom of the layer.

The researchers then initiate two independent chemical reactions to create chemical bonds tying the molecular strands together at three different levels. Although neither reaction succeeds in converting all the reactive sites into chemical bonds, there is sufficient redundancy in the chemical stitching to create a robust molecular sheet.

"Stupp's polymers ... are simply connected sheets with boundaries and thus have two distinct surfaces as well as edges," Thomas says.

The resulting sheet -- in effect, a single huge, flat molecule -- is about 10 nanometers thick and covers an area as large as several square microns. Electron diffraction studies confirm the orderliness of molecular units within individual sheets. Moreover, at room temperature these sheets stack neatly to form crystals.

"This remarkable registry among the flat molecular objects suggests that their top and bottom surfaces have the same chemical structure," Stupp and his colleagues report.

The researchers have also discovered that a solid, transparent film made up of two-dimensional polymer patches can double the frequency of laser light passing through it. In other words, the film generates green light from an infrared laser beam. This new material lasts much longer than other types of polymers now used for the same purpose.
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Title Annotation:scientists use double-length strands of molecular units to create layers needed for sheets
Author:Peterson, Ivars
Publication:Science News
Date:Jan 2, 1993
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