 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.
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 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.
[Text copied from
University
of Illinois news release]
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