Single-layer polymers prefer valleys, say U-M chemists

April 14, 1997

SAN FRANCISCO—In polymer chemistry, as in architecture, it’s important to pay close attention to your building base, say University of Michigan chemists Christine E. Evans and Mark D. Mowery. Just as topography can affect the construction of a new home, peaks and valleys in the underlying substrate have a direct impact on where and how polymers form in single molecular layers. Some 3,000 times thinner than a human hair, these monolayer polymers seem to have a definite preference for valleys.

This new understanding of the importance of substrate “roughness” in polymer formation will be good news to researchers attempting to use ultrathin molecular films and polymers, instead of silicon and microcircuitry, to create the organic equivalent of a semiconducting microchip.

“Varying the surface topography gives us one more tool we can use to direct and control the growth of these single-layer polymers,” said Evans, assistant professor of chemistry, who presented results of the experiments today (April 14) at the American Chemical Society meeting here. “It takes us one step closer to our ultimate goal, which is exploiting polymers’ potential to revolutionize nanoscale or ultrasmall technology.”

In recent experiments, the U-M scientists exposed a single layer of self-assembling molecules called monomers, which were attached to an underlying gold substrate, to ultraviolet light. UV light causes the monomers to link up and form molecular chains called polymers. Evans and Mowery discovered that polymers formed primarily within the valleys of the substrate. By varying the UV light exposure, the substrate’s surface texture and the chemical structure of the monomer layer, they hope to learn how to control the polymerization process.

“These polymers have unique optical and electronic properties that make them very intriguing,” said Mowery, a U-M graduate student. “For instance, diacetylene polymers change color in response to environmental changes, such as increasing temperature, which could prove valuable in a variety of sensing applications.”

According to Evans, these monolayer polymers could be used to create valves and gates small enough to filter individual ions or molecules. They also have the advantage of being durable, robust structures capable of withstanding exposure to a wide range of fluids and chemicals, which makes them especially useful in biological applications.

As an example, Evans cites an ongoing research project, funded by the National Institutes of Health, designed to learn how anesthetics interact with tissue membranes to produce a sedating effect. Evans and her colleagues hope to adapt monolayer polymers to create an artificial membrane which will mimic the surface of membranes in the human body. By exposing the membrane to anesthetics, Evans hopes to learn how molecules in the membrane’s surface interact with individual molecules of anesthetic.

“When they were discovered, no one could have predicted the revolutionary changes common polymers like nylon and polyester would make in our daily lives,” Evans said. “Increasing our understanding of the factors controlling polymerization could lead to new technological developments we cannot even imagine today.”

The research is supported by the Petroleum Research Fund, administered by the American Chemical Society, and the National Institute of General Medical Sciences, National Institutes of Health.