New Class of Ultrathin Polymers Discovered

Scientists have defied the textbooks and transformed 1,2-disubstituted ethylene molecules into polymers, discovering in the process a new class of ultrathin polymer films with potential applications ranging from coating tiny microelectronic devices to creating plastic solar cells.

University of Delaware polymer chemist Chris Snively and chemical engineering professor Jochen Lauterbach said they were motivated by the fact that polymer chemistry textbooks said it couldn't be done.

University of Delaware researchers Chris Snively (right) and Jochen Lauterbach have discovered a new class of ultrathin polymer films with potential applications ranging from coating tiny microelectronic devices to plastic solar cells. (Photo by Kathy F. Atkinson)

The research, which also involved doctoral student Seth Washburn, focused on formerly nonpolymerizable ethylenes. Among them are several compounds that are derived from natural sources, such as cinnamon, and are approved by the Food and Drug Administration for use in fragrances and foods. One of the compounds is found in milkshakes, according to the scientists.

“There's been a rule that these molecules wouldn't polymerize,” said Snively, who is a research associate in Lauterbach's laboratory group. “When I first saw that in a textbook when I was in graduate school, I said to myself, 'Don't tell me I can't do this.' That's when he started the quest to disprove the widely accepted scientific rule of thumb.

Polymerization is a chemical reaction in which monomers, which are small molecules with repeating structural units, join together to form a long chain-like molecule -- a polymer. Each polymer typically consists of 1000 or more of these monomer “building blocks.”

There are lots of natural polymers in the world, ranging from DNA to chewing gum. Plastics are one of the most common groups of man-made polymers. These synthetic materials were first created in the mid-1800s and are found today in a wide range of applications, including foam drinking cups, carpet fibers, epoxy and PVC pipes.

Since the late 1990s, Lauterbach and Snively have been developing a method to make extremely thin polymer layers on surfaces. These nanofilms -- at least 1000 times thinner than a human hair -- are becoming increasingly important as coatings for optics, solar cells, electrical insulators, advanced sensors and numerous other applications.

Formerly, to make a pound of polymer, scientists would take a monomer and a solvent and subject them to heat or light. Recently, Lauterbach and Snively developed a new polymer-making technique that eliminates the need for a solvent.

Since the late 1990s, Lauterbach and Snively have been developing a method to make extremely thin polymer layers on surfaces. The film covering the surface of these metal samples is at least 1000 times thinner than a human hair. (Photo by Kathy F. Atkinson)

Their deposition polymerization process takes place in a vacuum chamber, where the air is pumped out and the pressure is similar to outer space. The material to be coated, such as a piece of metal, is placed in the chamber, and the metal is cooled below the monomer's freezing point, which causes the monomer vapor to condense on the metal. Then the resulting film is exposed to ultraviolet light to initiate polymerization.

The two-step process allows for the formation of uniform, defect-free films with thicknesses that can be controlled to within billionths of a meter. The process is fairly “green,” in that not only are no solvents used, but there also is very low energy consumption using this method, according to Lauterbach.

“You also can do photolithography with it,” he said, meaning that the polymer will appear only where the light hits the monomer film.

While their polymerization technique was reported a few years ago, the class of materials the UD scientists have applied it to lately is new and unique.

“We can make nanometer-thick films, but we can't make a gram of the material yet,” Snively said. “We're working on ways to scale up the process.”

The scientists also want to find out if the materials may be stronger, tougher or possess unique properties compared to other polymers.

“It's exciting because you don't really know what all their properties are yet,” Snively said.

As for all the potential applications, Lauterbach said, “we're kind of in the discovery phase, looking to see where all these materials could be used.”

Soon, the UD researchers may be applying the new class of polymers they've discovered to plastic solar cells through collaborative research in UD's new Sustainable Energy from Solar Hydrogen program, which is supported by a five-year, $3.1 million grant from the National Science Foundation's Integrative Graduate Education and Research Training (IGERT) program.

An ultrathin polymer film of fumaronitrile is shown, which formerly was believed to be "unpolymerizable," on a tantalum foil. The film is circular due to the shape of the window where ultraviolet light is emitted into the vacuum chamber during the film deposition process. (Photo courtesy of Lauterbach Laboratory, University of Delaware)

UD's program, which began this fall, includes students and faculty from electrical and computer engineering, mechanical engineering, chemical engineering, materials science, chemistry, physics, economics and policy.

“We're looking forward to participating with our students in that program,” Lauterbach said.

And until the current polymer textbooks are revised, Lauterbach and Snively also enjoy asking their students to make note of the change in the margins.

“Right now, the excitement for us is that we've proven textbooks wrong,” Lauterbach said.

The discovery was reported as a “communication to the editor” in the Nov. 28 edition of Macromolecules, a scientific journal published by the American Chemical Society.

For more information, visit: www.udel.edu