X-Message-Number: 13548
From: Brent Thomas <>
Subject: glass transition info
Date: Thu, 13 Apr 2000 15:15:17 -0400

snagged from science daily web site:
interesting info about glass transitions in liquid mediums...

Source:
               
              University Of Illinois At Urbana-Champaign
(http://www.uiuc.edu)

            Date:
               
              Posted 4/13/2000


           Heat Capacity Of Glassy Substance Holds
           Key To Its Transition Kinetics 

           CHAMPAIGN, Ill. -- The idea that rigidity and orderliness go
together is
           a triumph of modern theoretical physics. But how these two
properties
           interrelate when a liquid is cooled and becomes solid-like -- a
           phenomenon called the glass transition -- has been less clear.
Now,
           University of Illinois chemical physics professor Peter Wolynes
and
           graduate student Xiaoyu Xia have found a way to explain the odd
           behavior of glassy materials. 

           "A periodic array of atoms in a crystal behaves differently than
a fluid,"
           said Wolynes, who holds the James R. Eiszner Chair in chemistry
at the
           UI. "For example, you can't move just one atom in an array
without
           displacing the entire structure. The rigidity of glass, an
amorphous solid,
           is more mysterious. Without any apparent order, this chaotic
jumble of
           atoms behaves as if rigidly frozen." 

           Glassy phenomena typically occur on long time scales. "How
rapidly the
           time scale increases as the material is cooled is what determines
the
           'fragility,' " Wolynes said. "The fragility differentiates rapid
glass-formers
           -- like polymers -- from slow ones -- such as ordinary window
glass.
           Quantitatively relating fragility to other glass-forming
characteristics has
           been an elusive goal, however." 

           Ten years ago, Wolynes developed a theory called the Random First
           Order Transition Theory of Glasses that qualitatively described
the
           glass-transition phenomenon. The resulting mathematical
expression was
           based upon microscopic theories of freezing. 

           Unlike ordinary freezing -- which typically involves only a few
orderly
           patterns -- it appeared there were many patterns into which a
liquid could
           freeze and still be called disordered, Wolynes said. The number
of
           possible freezing patterns seemed to be correlated with the
material's
           rigidity. 

           "In the intervening years, we realized we could take our theory
and
           quantitatively explain the one number that was needed to
distinguish one
           glassy substance from another -- the fundamental flow
characteristic
           called fragility," Wolynes said. "We could then correlate a
material's
           fragility with thermodynamic measurements of its heat capacity." 

           While the heat capacity of a liquid is rather high, at the glass
transition it
           falls to a value more typical of the crystalline state. The heat
capacity is
           especially interesting because it is related to the amount of
disorder, or
           entropy, in a substance: A liquid with a large heat capacity
loses entropy
           much more rapidly as it cools. By measuring the heat capacity of
a
           substance, and then plugging it into their equation, the
researchers can
           predict the speed at which the molecular motion changes with
           temperature. 

           "The fact that all glassy materials can now be expressed in a
universal
           form gives us much greater confidence that we truly understand
the
           glass-transition phenomenon," Wolynes said. "This knowledge will
be
           useful in many other fields of study, including protein folding."


           Wolynes and Xia described their theory in the March 28 issue of
the
           Proceedings of the National Academy of Sciences. 

           Editor's Note: The original news release can be found at
           http://www.admin.uiuc.edu/NB/00.04/glasstip.html 

Brent Thomas
Muscato Corporation

407-774-7800 (phone)
407-774-7801 (fax)

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