Seminar 10/2/2013 - Gregory B. McKenna, Texas Tech University: Upper Bounds to the Relaxation Times in Glass-Forming Systems
Full Title - Upper Bounds to the Relaxation Times in Glass-Forming Systems: Evidence of Non-Diverging Time-Scales from 20 Ma Dominican Amber
Abstract: One perceived important signature of the 'ideal' glass transition and of the complex fluid nature of glass-forming liquids remains the apparent divergence of the dynamics at temperatures above zero Kelvin. Recently, however, this perception has been increasingly challenged both through experiment [1,2], megadata analysis, and in some new theories of the dynamics of glass forming systems[4,5]. In this presentation we summarize some of the prior evidence suggesting that time scales actually do not diverge in glasses that are aged into equilibrium, perhaps 15 K below the conventional glass transition temperature Tg. We then show new results from an extremely densified glass-- 20 Ma old Dominican amber--in which we were able to obtain the upper bounds to the relaxation times at temperatures below the glass transition temperature through a step-wise temperature scan in which the stress relaxation response of the amber was measured both below and above the fictive temperature TF. We find that in the case of the upper bound responses at T>TF, there is a strong deviation of the response from the Super-Arrhenius Vogel-Fulcher extrapolation and this persists to the fictive temperature which is some 43.6 K below Tg. The results are compared to the parabolic model of Chandler and co-workers and the entropy model of Mauro, et al . We find the former model to be consistent with our results if the value of Tx in the model is taken to be the calorimetric glass transition temperature. The latter model, while giving non-diverging time-scales, seems to fall well above the measured upper bound relaxation times. The significance of the results will be discussed.
Bio: Subsequent to receiving his Bachelor's 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 in the area of composite materials 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 as a National Research Council Postdoc and accepted a permanent position as a staff scientist at the then National Bureau of Standards (NBS) (now the National Institute of Standards and Technology, NIST) in 1977. 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 took the position of Professor in the Department of Chemical Engineering and John R. Bradford Endowed Chair in Engineering at Texas Tech University. In 2005 he became a Paul Whitfield Horn Professor at TTU. Dr. McKenna has been a Fellow of the American Physical Society since 1989 and was elected a Fellow of the Society of Plastics Engineers in 1998. He was the recipient of the E.U. Condon Award of the National Institute of Standards and Technology for excellence in technical exposition in 1989 for his classic review article Glass Formation and Glassy Behavior. He is the 2009 recipient of the Bingham Medal of the Society of Rheology and has also received the International Award of the Society of Plastics Engineers and the Mettler Toledo Award from the North American Thermal Analysis Society. Dr. McKenna has served on the Governing Board of the American Institute of Physics, the Executive Committees of the Society of Rheology and The Division of High Polymer Physics (DHPP) of the American Physical Society. He has also served as the Chairman of the DHPP, the Society of Engineering Science, and the Polymer Analysis Division of the Society of Plastics Engineers. He is currently vice-president of the Society of Rheology.
All seminars are held on Wednesdays from 12:00 noon-1:00 p.m. in the Bowen Hall Auditorium Room 222. A light lunch is provided at 11:30 a.m. in the Bowen Hall Atrium immediately prior to the seminar.
Location: Bowen Hall Auditorium
Date/Time: 10/02/13 at 12:00 pm - 10/02/13 at 1:00 pm
Category: PRISM/PCCM Seminar Series