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Single-Molecule Study Exposes Glass Dynamics

Photonics Spectra
Jun 2001
Daniel S. Burgess

Glasses and noncrystalline materials have been important to humans for thousands of years, but scientists have been at a loss to explain what is going on as these materials cool from liquids into amorphous solids. Using a new spectroscopic technique, chemists at the University of Texas are seeking to resolve the debate over the behavior of such substances when near the glass transition temperature.

Single-molecule spectroscopy can monitor the dynamics of fluorescent dyes in a polymer near the glass transition temperature. Imaging the dye molecules at orthogonal polarizations enabled University of Texas researchers to determine their orientations from their intensities on the respective detectors.

Typically, researchers have measured molecular dynamics with spectroscopic techniques that average the motion of, at best, a local subset of the molecules. David A. Vanden Bout, an assistant professor of chemistry and biochemistry at the university, and graduate student Laura A. Deschenes took a different approach.

"We thought, if you want to know if the molecules are all the same or all different, you should simply go and look at the individual molecules one at a time and compare them," Vanden Bout said.

They monitored the behavior of molecules of rhodamine 6G fluorescent dye that they had suspended in a 250-nm film of poly(methylacrylate) at 8 °C, a few degrees above the polymer's glass transition temperature. An Nd:YAG laser operating at the second harmonic served as the excitation source, and a pair of single-photon-counting avalanche photodiodes captured the fluorescence response of the dye at orthogonal polarizations. This enabled them to measure the orientation of the individual molecules in real time.

They found that the rate at which the dye molecules rotated varied by up to a factor of 10 depending on their location in the film, which suggested thermodynamic heterogeneity in the sample. "The system is composed of many environments, all with different time constants," Vanden Bout said. "Some regions are fast and some regions are slow."

Insight and drama

The findings, which appeared in the April 13 issue of Science, confirm the work of other researchers, and the team is happy to have demonstrated an elegant means of offering "insight into a very old problem," Vanden Bout said.

"We had compelling evidence from hole-burning experiments in Mark Ediger's lab [at the University of Wisconsin in Madison] and others that these systems were heterogeneous. However, the demonstration with the single molecules is quite dramatic."

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