Battery acid, plastic containers and windowpanes are among the many glassy materials whose molecular properties the new study quantifies. Application of the findings could help manufacturers improve the design of such materials from the ground up. images ©Shutterstock/collage K. Talbott |
Researchers at the National Institute of Standards and
Technology (NIST) and Wesleyan Univ. have used computer simulations to gain
basic insights into a fundamental problem in material science related to
glass-forming materials, offering a precise mathematical and physical
description* of the way temperature affects the rate of flow in this broad
class of materials—a long-standing goal.
Manufacturers who design new materials often struggle to
understand viscous liquids at a molecular scale. Many substances including
polymers and biological materials change upon cooling from a watery state at
elevated temperatures to a tar-like consistency at intermediate temperatures,
then become a solid “glass” similar to hard candy at lower
temperatures. Scientists have long sought a molecular-level description of this
theoretically mysterious, yet common, “glass transition” process as
an alternative to expensive and time-consuming trial-and-error material
discovery methods. Such a description might permit the better design of
plastics and containers that could lengthen the shelf life of food and drugs.
A fundamental question is why many materials behave
differently when temperature changes. In some “fragile” glass-forming
materials, a modest variation in temperature can make the material change from
highly fluid to extremely viscous, while in “strong” fluids this
change in viscosity is much more gradual. This effect influences how long a
manufacturer has to work with a material as it cools. “For decades,
material scientists have heavily relied on empirical rules of thumb to
characterize these materials,” says NIST theoretician Jack Douglas. “But
if you want to design a material that does precisely what you want, you need a
molecular understanding of the underlying physical processes involved.”
According to Douglas, the
increasingly viscous nature of glass-forming liquids is related to molecules
that move together in long strings around other atoms that are almost frozen in
their motion. The growth of these snake-like structures leads to an increase in
the viscosity of the liquid: the lower the temperature, the longer the chains,
and the more viscous the fluid. The team found that the rate at which these
spontaneously organizing snake-like strings grow in size as the material cools
is quantitatively related mathematically to the fluid fragility—confirming
intuitive arguments made nearly half a century ago by physicists G. Adams and
J.H. Gibbs, but now bolstering them with a firm computational underpinning.
Douglas and his collaborator Francis Starr of Wesleyan Univ. achieved a large variation of
fluid fragility through use of a computer model, which mimics a polymer fluid
that includes tiny nanometer-sized particles. Portraying the addition of
various amounts of nanoparticles and varying their interaction with the
polymers, Starr says, gave the team a sort of “knob to tweak” to
reveal how the fluidity changed with temperature and how the motion of the
clusters was quantitatively related to changes in the fluid’s properties. This
tuning of cooperative motion in glass-forming liquids and fragility should be
crucial in material design. Douglas says.
* F.W. Starr and J.F. Douglas. Modifying fragility and
collective motion in polymer melts with nanoparticles. Physical Review
Letters, week ending March 18, 2011, Vol. 106, 115702 pp. 1-4, DOI:
10.1103/PhysRevLett.106.115702.