SAN FRANCISCO - A research team led by Northwestern University materials scientist Samuel I. Stupp has developed a novel method to improve polymers that could impact not only the plastics industry, but fields as diverse as optical communications, medicine and nanotechnology.
This new method improves polymers by changing the actual organization of the macromolecules using small molecules as additives, rather than changing a polymer's chemical structure as catalysts do.
Stupp, Board of Trustees Professor of Materials Science, Chemistry and Medicine, will present a paper outlining these findings at the 219th American Chemical Society National Meeting in San Francisco at 1 p.m., U.S. Pacific Time, Thursday, March 30.
"Companies are interested in improving mechanical, thermal, transport, flow and other properties of polymers," said Stupp. "To achieve this, they've focused most of their research dollars on the chemistry of catalysts used to make polymers, but this will have limited results. We are using molecular self-assembly to physically change polymers ‹ a completely different direction that holds a great deal of promise."
The researchers have discovered a system of molecules that when dissolved in a liquid monomer, such as styrene, form nanoribbons, reminiscent of DNA strands. Molecules freeze around the ribbons in an orderly fashion, completely changing the physical nature of the liquid monomer and creating a gel with a blue-violet hue, which appears like a liquid crystal when viewed in a microscope. Strikingly, the structural changes are retained when the liquid monomer is polymerized into a solid, such as polystyrene.
Today's polystyrene ‹ used for such common items as food packaging, compact disc jewel boxes, appliances, television cabinets and toys ‹ is inexpensive but has limited toughness. "When manufacturers need increased toughness and stiffness or other special properties, they have to turn to more expensive plastic, such as engineering plastics or liquid crystal polymers," said Stupp. His modified version holds the promise of sophisticated properties at an inexpensive price.
One of the advantages of Stupp's method is the orientation of polymer molecules in the material by a nano-sized and stiff scaffold, formed by self-assembling molecules, in the material's interior. It is well known that polymeric materials are strong along the covalent axis of their molecules because a great deal of energy is required to break covalent bonds, says Stupp. The nanoribbons orient easily along a desired direction and drag the polymer chains around them. Therefore, the method has enormous potential for producing extremely strong polymers without requiring the complex equipment currently necessary to make ultra-strong fibers.
The researchers found that when minute amounts of designed molecules, which they call dendron rodcoils, are dissolved in monomers, the molecules interact with one another, forming weak bonds and assembling into ribbon-like structures. These tiny ribbons ‹ hundreds of nanometers long but only 10 nanometers wide and a few nanometers thick ‹ are scattered throughout the monomer. (By contrast, a human hair is approximately 10,000 nanometers wide.) The final solid polymer contains 108 meters of nanoribbon per cm3, but the ribbons are so thin that they account for only one percent or less of the weight of the entire material.
"The critical next step was to see how the polymerization process, which requires heating the styrene, would affect the self-assembled ribbons," said Stupp. "Would solid polystyrene, the material used in thousands of everyday items, exhibit the same structural properties as the liquid styrene? The answer was a resounding yes."
In the presence of the ribbons, polymer chains line up neatly alongside them. Without the rigid ribbons to guide them, polymer molecules are amorphous coils, resembling a jumbled pile of cooked spaghetti, with chains heading in all directions.
Another advantage of Stupp's method is that the presence of the ribbons also changes polystyrene's optical properties dramatically. The polystyrene becomes strongly birefringent, a property that could be exploited to move light in specific directions. The modified polystyrene also can reflect and transmit certain wavelengths of light. These optical properties are mediated by the nanoribbons, so the material used to make cheap plastic parts also could become a material for advanced photonics.
"Molecular self-assembly has changed the structure of polystyrene completely," said Stupp. "And it doesn't involve searching for new catalysts."
Stupp's team next plans to investigate the mechanical and flow properties in the modified polystyrene, as well as the use of the self-assembled ribbons in other polymers. "We can modify many different materials, and we already know we can modify monomers that polymerize into rubbers with this method," said Stupp. "But the properties exhibited by each may differ." They also will look at using the ribbons in biology for medical purposes, to create structures that direct cells to travel in a certain direction, for example.
The research was funded by the U.S. Army Research Office, the National Science Foundation, the Department of Energy and the Office of Naval Research.
Stupp's research collaborators are postdoctoral researcher Eugene R. Zubarev and graduate student Martin U. Pralle, both of the University of Illinois at Urbana-Champaign, and graduate student Eli D. Sone of Northwestern.
Samuel Stupp can be reached at the conference March 28-30 at (415) 974-6400. At Northwestern, he can be reached at (847) 491-3002 or s-stupp@northwestern.edu .