Theory predicts aging process in DVDs, plexiglas, other polymer glasses
James E. Kloeppel, Physical Sciences Editor
217-244-1073;
kloeppel@uiuc.edu
Photo by L. Brian Stauffer
Researchers at the U. of I., led by Kenneth S. Schweizer, the G. Ronald and Margaret H. Morris Professor of Materials Science have developed a theory that predicts how polymer glasses age.
Released 4/23/2007
CHAMPAIGN, Ill. —
Polymer glasses are versatile plastics widely used in applications ranging
from aircraft windshields to DVDs. Researchers at the University of
Illinois have developed a theory that predicts how these materials age.
The theory also explains why 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.”
Three years ago, Schweizer and graduate student Erica Saltzman developed a theory that described the transition upon cooling of a polymeric material from a liquid to an amorphous solid or glass. The theory explained how the viscosity of a polymer glass changes dramatically over a narrow temperature range. The researchers reported that work in the July 22, 2004, issue of the Journal of Chemical Physics.
Now, in the April 20 issue of Physical Review Letters, Schweizer and postdoctoral research associate Kang Chen present a theory to describe the aging process in polymer glasses. The new theory predicts not only how polymer molecules move, but also the material properties, at a wide variety of times and temperatures.
Polymer glasses are plastics that possess unusual and technologically useful mechanical properties. Unlike most other types of solids, polymer glasses can possess high impact resistance and, even though they are stiff, can often be significantly deformed without breaking. They are usually inexpensive to make, and easily melted and molded into many shapes.
And, they're always on the move.
Unlike window glass, which
melts at roughly 1,200 degrees above room temperature, polymer glasses have melting
points much closer to room temperature. So close, in fact, that many polymer
glasses retain some liquid-like properties at room temperature, including motion
at the molecular level.
“The movements are so small and so slow, we can’t see them without
the aid of sophisticated measuring tools,” Schweizer said. “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.
“Among other possible effects, the aging process causes polymer glasses
to become stiffer and often more brittle,” said Schweizer, who also is
a professor of
chemistry, of
chemical
and biomolecular engineering, and a researcher at the university’s
Frederick
Seitz Materials Research Laboratory.
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 time scale of molecular movement,” Schweizer
said. “This information is especially important in engineering applications
where small changes in dimensions, stiffness or other properties can affect long-term
performance or reliability.”
The work was funded by the National Science Foundation.
