CHE Seminars  




SEMINAR

Speaker Prof. Gregory B. McKenna
Texas Tech University
Topic Equilibrium Behavior of Glass Forming Liquids: Considerations of Fragility and Questioning the Kauzmann Paradox
Date 11 January 2010 (Monday)
Place L-12, Lecture Hall Complex
Time 4.10 to 5.10 p.m.

ABSTRACT

Glass-forming liquids exhibit several fascinating behaviors. In the realm of thermodynamics, it is known that the entropy S of these systems appears to collapse too rapidly as the glass temperature is approached with the result that S can become less than the entropy of the crystal in apparent violation of the third law of thermodynamics. The temperature at which the excess entropy goes to zero is known as the Kauzmann temperature TK that is often associated with an ‘ideal’ glass transition temperature. The apparent violation of the third law is known as the Kauzmann paradox. Here we examine this problem using mixtures of poly(?-methyl styrene) and its oligomers— both non-crystallizing systems. By performing absolute measurements of the heat capacity we can deduce the changing entropy as a function of concentration and temperature. It is found that there is no “ideal” glass-like transition in the entropy upon extrapolation to 100% polymer concentration at temperatures far below the Kauzmann temperature, implying that TK may not be explicitly linked to the glass transition event itself.
Another interesting behavior is the so-called super Arrhenius temperature dependence of the dynamics of complex liquids as the glass temperature is approached. In an attempt to relate this behavior to the thermodynamics of the glass transition event, the concepts of dynamic and thermodynamic fragility were developed. Here we discuss the meanings of dynamic and thermodynamic fragility and examine how they are related (or not) for different classes of glass-forming liquids including inorganic network materials, polymers, small molecule organics and metallic glasses. We find that the nominal correlations between the dynamic fragility index m and the thermodynamics are weak and that the dynamic fragility is probably best correlated to the glass transition temperature itself except for the network glasses. The meaning of these findings is discussed


ABOUT THE SPEAKER
Subsequent to receiving his Bachelors degree in Engineering Mechanics at the U.S. Air Force Academy, Gregory B. McKenna went on to MIT where, in 1971, he earned a Masters Degree before entering on active duty as a test and evaluation engineer at Hill Air Force Base in Ogden, Utah. While in Utah, he completed his higher education by earning a Ph.D. in Materials Science and Engineering at the University of Utah in 1976. Dr. McKenna then moved to the then National Bureau of Standards (now NIST) as a staff scientist. Since then, Dr. McKenna has earned a reputation as a pioneering researcher in four major areas of polymer and plastics science and technology: Physical Aging and Structural Recovery of Polymer Glasses, Solid Mechanics and Nonlinear Viscoelasticity of Polymers, Thermodynamics and Mechanics of Elastomers and Gels, Molecular Rheology. He was the head of the Structure and Mechanics Group in the Polymers Division at NIST from August, 1992 until July, 1999 when he became Professor in Chemical Engineering at Texas Tech University. He is also the editor-in-Chief of Journal of Polymer Science, Polymer Physics Part B. He is a recipient of prestigious Bingham medal by Society of Rheology for the year 2009.


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