North Carolina State University
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Media Contacts:
Dr. Harald Ade, 919/515-1331 or [email protected]
D. Andrew Winesett, 919/515-8147 or [email protected]
Tim Lucas, News Services, 919/515-3470 or [email protected]
Oct. 5, 1999
Physicists' Findings Aid in Creation of New Thin-Film Materials
FOR IMMEDIATE RELEASE
Even if you've never heard of polymer thin-films, chances are you benefit nearly every day from products made from these high-tech coatings.
They're key ingredients in the slow-release fertilizer you apply to your lawn and the timed-release pills in your medicine cabinet. 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, where 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, a native of Rocky Mount.
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.
-- lucas --
NOTE TO EDITORS: Copies of Dr. Ade's team's abstracts in Nature and Europhysics Letters follow. For copies of the full articles, contact Tim Lucas at (919) 515-3470 or [email protected]
"Confinement Induced Miscibility in Polymer Blends"
published July 7, 1999, in Nature
by S. Zhu, Y. Liu, M.H. Rafailovich, J. Sokolov and D. Gersappe of the SUNY-Stony Brook, and D.A. Winesett and H. Ade of NC State University
ABSTRACT: Phase separation between the immiscible polymer components in a thin-film polymer blend results in large surface modulations, thus inhibiting their use in modern thin-film applications where increasing miniaturization is required and tight tolerances exist. The challenge is hence twofold: One, can complete mixing in thin-film polymer blends be achieved; and two, can this be done without limiting the types of polymers used. Until now, the method of adding copolymer compatibilizers to the blend has only been able to induce complete miscibility in polymer mixtures that are either close to a critical point in their phase diagram or have an attractive interaction between the components. Here, we present theoretical and experimental studies that demonstrate complete mixing in compatibilized thin-film blends that would otherwise be highly immiscible in the bulk state. Rather than manipulating the chemical composition of the blend, we exploit the reduction in entropy caused by the co
"Phase Segregation in Polymer Thin Films: Elucidations by X-ray and Scanning Force Microscopy"
published Feb. 15, 1999, in Europhysics Letters
by H. Ade, D..A. Winesett and A.P. Smith of NC State University, and S. Qu, S. Ge, J. Sokolov and M. Rafailovich of SUNY-Stony Brook
ABSTRACT: We have used quantitative X-ray microscopy in combination with Scanning Force microscopy to monitor the phase segregation of spun-cast thin films of polystyrene and poly(methyl methacrylate) blends upon annealing. Both techniques complement and enhance each other in elucidating the complicated structures that develop as a function of annealing time. We have determined the composition of the mixed phases that result from solvent spin casting. We subsequently observe the sudden rearrangement into domains much smaller than those originally formed. Unique, intricate hydrodynamic mass flow patterns form during coarsening which are in qualitative agreement with recent simulations of phase segregation in two-dimensional viscous fluids. Complicated polymer-polymer interfaces persist even in the later stages that are explained in terms of the geometric constraints of a thin film and the dependence of polymer viscosity on film thickness.
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