Apr. 11, 2001 A surprisingly simple technique based on physical phenomena first described more than a century ago could provide a new and highly controllable means of producing complex three-dimensional polymer structures. Potentially useful in photonic band gap materials, optical waveguides, lasers and even arrays of tiny chemical beakers, the structures consist of closely-packed air bubbles of uniform size.
The self-assembly technique, reported in the April 6 issue of the journal Science, involves directing moist air across a polymer material dissolved in a fast-evaporating solvent. In its simplest form, the process requires only that researchers exhale across the polymer solution.
"This represents an easy way of making materials with the regular structure needed for optical and photonic applications," said Mohan Srinivasarao, a polymer chemist in the School of Textile and Fiber Engineering at the Georgia Institute of Technology. "This is completely a self-assembly process. We have shown that with very little work you can form nicely-ordered structures whose applications are limited only by your imagination."
Srinivasarao and colleagues David Collings, Alan Philips and Sanjay Patel report that the finished structure consists of interconnected spherical air bubbles arranged in a tightly packed hexagonal pattern. The bubbles can be created in uniform sizes from 0.2 microns up to 20 microns in diameter, allowing a 30-40 micron thick polymer film to contain as many as 15 layers of bubbles.
"The beauty of this process lies in its simplicity," said Andrew Lovinger, director of the National Science Foundation's polymers program, which sponsored the work. "You just let the solvent evaporate at room temperature and in a few minutes you get these beautiful honeycombed polymer films."
Formation of the structure begins by dissolving a coil-like polymer such as polystyrene in a fast-evaporating solvent such as toluene or benzene. The solution containing the dilute (0.1 to 5 percent by weight) polymer is placed on a glass slide, and moist air is directed across it as the solvent evaporates.
Rapid evaporation of the solvent lowers the temperature of the solution by as much as 25 degrees Celsius. Moisture from the warmer air condenses on the surface of the solution, forming a layer of uniform-size droplets packed tightly together like billiard balls. Because the water is denser than the solvent, the layer of droplets sinks into the sample, allowing another layer to quickly form on top of it. The process repeats itself for one to two minutes until all of the solvent is evaporated, producing a three-dimensional pattern of closely packed water droplets preserved in the polymer film.
The water then evaporates layer by layer, leaving an interconnected network of air bubbles that is "as perfect as you could make it," said Srinivasarao, who also holds a adjunct faculty appointment in Georgia Tech's School of Chemistry and Biochemistry.
Though the process appears simple, its success depends on an unusual phenomenon: the willingness of the tiny water droplets to remain separate, not coalescing to form larger drops. The reason this happens is not fully understood, though observations made more than a hundred years ago by British physicist Lord Rayleigh - and work by contemporary scientists including Paul Neitzel of Georgia Tech -- suggest an explanation.
Srinivasarao believes the temperature difference between the warm moist air and the cold solution surface causes the droplets to spin, pulling rapidly moving air with them. The air, he suggests, keeps these tiny droplets apart, preventing them from coalescing into larger drops. The large temperature reduction caused by the evaporating solvent may even turn the droplets into tiny balls of ice, he said.
Srinivasarao has found that the diameter of the water droplets is related to the velocity of air flowing over the polymer solution. As the air flow rate increases from 30 meters per minute to 300 meters per minute, the droplet size decreases from 6 microns to 0.2 microns. He believes higher velocity could produce porous structures as small as 50 nanometers.
The other important condition is humidity, which must be at least 30 percent to produce the tiny water droplets.
The first application for the new structures is likely to be in optics, using structures with pore dimensions comparable to the wavelength of visible light. That makes them of interest as potential photonic band gap materials, optical waveguides, beam-steering systems -- and even arrays of dye lasers.
"We have focused on how to modify the refractive index so we can use these as a photonic band gap material, which is the first application we will go after," Srinivasarao said. "But what we will be able to do is limited only by the imagination."
By modifying the formation process to avoid interconnection of bubbles, the polymer films could also become tiny beakers each holding 10 picoliters of fluid. The bubbles could also be used as templates for the formation of other structures.
Researchers have so far found the technique works with three different types of polymer and several solvents. For the droplets to sink into the solution, the solvent must be less dense than water. The researchers believe the technique may also work with vapors of materials other than water.
Srinivasarao is an NSF CAREER awardee. The CAREER program provides financial support to young investigators during the early years of their faculty positions to encourage not only first-rate research, but also educational commitment and innovation.
The research was sponsored by the National Science Foundation.
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