New Blending Method for Thin-Film Polymers

Even people who have never heard of polymer thin-films benefit nearly every day from products made from these high-tech coatings. They're key ingredients in the slow-release fertilizer they apply to their lawns and the timed-release pills in their medicine cabinets. They're also used in multicolor photographic printing, biomedical membranes, anti-reflective coatings, LCDs and other useful products.

As scientists work to create new polymer thin-films by using blends of polymers, however, they often face a perplexing problem: Like oil and water, many polymers simply don't mix.

Now, research by physicists at North Carolina State University, in partnership with materials scientists at the State University of New York (SUNY) at Stony Brook, is helping solve this problem. Their work is shedding new light on what happens to polymer blends as the dimensions shrink, and how scientists can exploit these changes to create new and better thin-film materials.

"We know that as a material shrinks, its large-chain molecules -- its polymers -- no longer have room to 'stretch out' as they ordinarily would. This affects their spatial relationship to other polymers and, in some cases, the 'mix-ability' of the polymers themselves," says Dr. Harald Ade, associate professor of physics at NC State.

"The challenge," Ade says, "is to learn how to understand and control these effects and, if possible, turn it to our advantage so we can promote a consistent mixture of polymers throughout a thin-film blend, without limiting the types of polymers used."

In an article published this July in the science journal Nature, Ade, doctoral student D.A. Winesett and their colleagues from SUNY-Stony Brook took a giant step toward this goal. They showed, for the first time, that highly dissimilar polymers can be completely blended into a thin film by exploiting reduction in entropy -- a measure of the number of possible molecular arrangements in a material -- that occurs as a result of miniaturization.

"It's sort of like getting water and oil to mix," Ade says. "The beauty of nature is that if a polymer blend is shrunk small enough, the emulsifier utilized is essentially prevented from associating with other emulsifier molecules. There's no room for the emulsifier to arrange itself in such a way due to the confined space."

(Emulsifiers are agents that mediate between different polymer types and act like a "detergent," stabilizing the polymer mixture. If, however, the emulsifier molecules associate with other emulsifiers, they lose much of their stabilizing ability and the polymers they once held together can separate, causing unacceptably large modulations on the surface of the material and inconsistent structure within it.

Such flaws would render a material useless for most modern applications, in which a perfectly flat surface is required and tight structural tolerances exist.

In contrast, "the thin-film polymer blend we created was made from very dissimilar polymers but had a perfectly flat surface and a completely mixed, uniform structure when reduced to nanoscale," Ade says. "This is the first time we've seen that in highly immiscible systems."

Because the new blending technique doesn't depend on chemistry, scientists should be able to use it on nearly any polymer blend. "This will have a significant impact on any technological process that relies on ultra-thin polymer coatings, such as photolithographic printing and magnetic disk coating," says Winesett.

To conduct their research, Ade and Winesett and their colleagues employed high-powered X-ray microscopes and scanning force microscopes to analyze thin-film polymer blends developed at Stony Brook. By using such advanced imaging tools, the NC State researchers were able to detect structural changes that occurred within the materials -- insights other, more commonly used types of microscopes and imaging devices can't provide.

Ade's team is one of the only scientific teams worldwide with the equipment and expertise to use X-ray microscopy for such advanced applications.

In addition to the Nature article, earlier this year Ade and Winesett also collaborated with the SUNY-Stony Brook scientists on a paper in the journal Europhysics Letters. - By Tim Lucas

Contact: Dr. Harald Ade, D. Andrew Winesett, Tim Lucas