Experts are a step closer to working out how long data stored on conventional media will last with the publication of a theory predicting how polymer glasses used in applications including DVDs decay over time.

Researchers at the University of Illinois explained that motions at the molecular level can have macroscopic consequences.

"Glasses, including polymer glasses, are essentially frozen liquids," said Kenneth S Schweizer, the G Ronald and Margaret H Morris Professor of Materials Science at the University of Illinois.

"They appear solid but, because they are frozen liquids, the molecules continually undergo small motions that lead to a time dependence of properties. "

Professor Schweizer and postdoctoral research associate Kang Chen presented the theory to describe the aging process in polymer glasses in the 20 April issue of Physical Review Letters.

The theory predicts not only how polymer molecules move, but the material's properties at a wide variety of times and temperatures.

The researchers explained that polymer glasses have melting points so close to room temperature that many retain some liquid-like properties at room temperature, including motion at the molecular level.

"The movements are so small and so slow that we cannot see them without the aid of sophisticated measuring tools," said Professor Schweizer.

"Nevertheless, this residual motion can significantly change the material's mechanical and thermal properties over time."

As the material gradually reconfigures and approaches equilibrium at room temperature, the movements become slower and slower.

Under sufficiently cold conditions, this "relaxation" time can become astronomically large, even longer than the age of the universe for some materials.

Over time, the molecules crowd closer together, increasing the density and changing the mechanical properties of the material.

"Through our theory we developed a way to relate the physical properties of a polymer glass to the timescale of molecular movement," explained Professor Schweizer.

"This information is especially important in engineering applications where small changes in dimensions, stiffness or other properties can affect long-term performance or reliability."