Scattered light reveals polymer wave motion.

Thin polymer films form the basis of products ranging from biological glues to nonstick coatings. In many applications, the polymer molecules stick out from a surface like grasses growing in a microscopic field. Rooted at one end, the molecules reach up and undulate as if blown by the wind.

A new method of characterizing these waves could lead to better understanding of the properties of polymer films. Researchers at the Institute of Electronic Structure and Laser in Heraklion, Greece, tracked the movements of tethered polymer molecules by measuring how they scatter light. The motions of the polymers can either enhance or interfere with the way they stick to surfaces, affecting their performance as paint primers and other coatings.

Although scientists have looked at the static properties of what they call polymer brushes-measuring their density, for example-this new technique is "definitely one of the first" ways to study their collective motion, says Anna C. Balazs of the University of Pittsburgh. "It's a very elegant technique. It's clever in that it isolates information that you're looking for, and it doesn't destroy the sample." The study appears in the Dec. 20 Science.

The team immersed one side of a glass prism in a solution containing suspended polymer molecules, which coated the surface. Each molecule consisted of a short segment of polyethyleneoxide, which stuck to the glass, and a longer chain of polystyrene, which stretched out into the solution.

The researchers then shone a laser beam through the prism so that it hit the interface between the glass and the solution at an angle shallow enough to reflect most of the beam. A little light got through the glass, however, and passed through the undulating polymer molecules, says study coauthor Bradford J. Factor, now at the Chandler, Ariz., offices of Intel, an electronics company. By measuring how that little bit got scattered, the researchers could record the molecules' wavy movements.

The waves of the brush are complex, Factor says, but they can be thought of as combinations of several distinct patterns of motion. The experimental technique lets researchers analyze each pattern separately and track how the movement dies down over time.

The technique also ensures that not much light is scattered off the polymer molecules remaining in the solution. Such noise would overwhelm the signal created by the fine movements of the tethered molecules. "We concentrate the light very close to the [prism] surface," Factor says. "The molecules that are far away from the surface don't see the light." The researchers can control the depth of the light's penetration by varying the angle at which the laser beam strikes the interface.

Balazs would like to see the technique used to investigate what happens when two polymer brushes come into contact, as when two coated surfaces are brought together.

The technique could also characterize biological systems, such as cell membranes or proteins adhering to surfaces.