Distinguished Seminar in Computational Science and Engineering
May 5, 2022, 12 PM
Hybrid Event
In-Person: Allen Room, 36-462 │ Zoom Webinar: Registration Link
Viscoelastic Behavior of Liquid Crystal Elastomers
Thao (Vicky) Nguyen
Professor and Marlin U. Zimmerman, Jr. Faculty Scholar
Johns Hopkins Whiting School of Engineering
Abstract:
Liquid-crystalline elastomers (LCEs) are a class of soft stimuli-responsive materials composed of stiff mesogens bound to an elastomeric network of flexible polymer chains. The mesogens can order and disorder in response to an external stimulus, such as temperature and mechanical deformation. This allows LCEs to undergo reversible phase transitions between the polydomain, monodomain, and isotropic states. The motion of the mesogens relative to the polymer network also leads to unusual behavior, including large reversible actuation in response to temperature, soft-elasticity, and enhanced dissipation. The latter includes an elevated loss factor (tan δ) over a wide range of frequencies and temperatures and large hysteresis that increases with strain rate. In this presentation, I will describe our efforts to investigate the viscoelastic behavior and underlying relaxation mechanisms of a main-chain LCE synthesized from a two-stage thiol-acrylate Michael addition and photopolymerization using a combined experimental and constitutive modeling approach. Our experimental efforts focused on characterizing the effects of mesogen alignment and director orientation on the rate-dependent stress response, hysteresis, and viscoelastic properties. We applied 3D digital image correlation was to characterize the effect of mesogen rotation on the strain field and the soft stress response. The experimental results showed both viscous director rotation and viscous network deformation contributed significantly to the stress response. We next developed a nonlinear micropolar theory to describe the contributions of these two mechanisms on the stress response of monodomain LCEs and investigate how their interactions leads to the enhanced dissipation behavior observed in experiments. Finally, I will present recent computational modeling studies of the energy absorption behavior of an architected LCE material.
Bio:
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 Professor and Marlin U. Zimmerman Faculty Scholar in the Department of Mechanical Engineering with secondary appointments in Materials Science and Ophthalmology. 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 to study the micromechanisms of growth and remodeling of collagenous tissues. She was also awarded the inaugural Eshelby Mechanics Award for Young Faculty in 2013, the 2013 ASME Sia Nemat-Nasser Early Career Award for research excellence in mechanics and materials, and the T.J.R. Hughes Young Investigator Award from the ASME Applied Mechanics Division in 2015. She served as the President of the Society of Engineering Science (SES) in 2020 and is currently an Editor-in-Chief of the Journal of Biomechanical Engineering.
Viscoelastic Behavior of Liquid Crystal Elastomers
Thao (Vicky) Nguyen
Johns Hopkins Whiting School of Engineering