Soft Matter Seminar: The thermomechanics of amorphous polymers near the glass transition
Friday, November 3 at 11:00am to 12:30pm
Regents Hall, 351 3700 O St. NW
Prof. Thao (Vicky) Nguyen, Mechanical Engineering Department at The Johns Hopkins University
Abstract: Glassy polymers are amorphous polymers that have been driven out of equilibrium by cooling to below the glass transition temperature. In this nonequilibrium state, the polymer chains continue to slowly rearrange towards a lower entropy state. This process causes physical properties, such as the viscosity, yield strength, and enthalpy, to change with time in a process commonly referred to as physical aging or structural relaxation. Physical aging can be reversed by plastic deformation, which moves the material further away from equilibrium. This mechanical rejuvenation process is also responsible for post-yield dynamic softening observed in the stress-strain behavior glassy polymers, the extent of which determines the toughness and failure response of the material. Though structural relaxation and viscoplasticity are interdependent phenomenon, they have been treated as separate processes and described by different theoretical approaches. I will present a new theory that strongly couples both viscoplasticity and structural relaxation through an effective temperature thermodynamic framework and a constitutive model based on the Adam-Gibbs model for dependence of the stress relaxation time on the configurational structure. Using this framework, we developed a new unifying thermomechanical theory for amorphous polymers that describes a wide range of nonequilibrium phenomena, including viscoplasticity, physical aging, mechanical rejuvenation, orientation hardening, and the glass transition, using a common set of physically measurable parameters. This presentation will show comparisons of theoretical predictions and experimental measurements of the effect of plastic pre-deformation (cold work) and annealing (aging) on the viscoplastic stress response, including the post-yield softening and orientation hardening, and energy storage as measured by dynamic scanning calorimetry.
Thao (Vicky) Nguyen received her S.B. from MIT in 1998, and M.S. and Ph.D. from Stanford in 2004, all in mechanical engineering. She was a research scientist at Sandia National Laboratories in Livermore from 2004-2007, before joining the Mechanical Engineering Department at The Johns Hopkins University, where she is currently a tenured associate professor and the Marlin U. Zimmerman Faculty Scholar. Dr. Nguyen’s research encompasses the biomechanics of soft tissues and the mechanics of active polymers and biomaterials. Dr. Nguyen has received the 2008 Presidential Early Career Award for Scientists and Engineers (PECASE) and the NNSA Office of Defense Programs Early Career Scientists and Engineer Awards for her work on modeling the thermomechanical behavior of shape memory polymers. She received the 2013 NSF CAREER award and 2016 JHU Catalyst Award to study the micromechanisms of growth and remodeling of collagenous tissues. She was also awarded the inaugural Eshelby Mechanics Award for Young Faculty, the ASME Sia Nemat-Nasser Early Career Award both in 2013, and the ASME Applied Mechanics Division T.J.R. Hughes Young Investigator Award in 2015.
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