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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." |
