Mechanical and Civil Engineering Seminar

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 LCEsynthesized from a two-stage thiol-acrylate Michael addition and photopolymerization. Our experimental efforts have focused on characterizing the relaxation dynamics of the polymer networkand mesogens,and the effects of mesogen alignment and director orientation on the rate-dependent stress response, hysteresis, and viscoelastic properties.3D digital image correlation was used to characterize the effect of mesogen rotation on the strain field and the soft stress response.We combined the intrinsic dissipation mechanisms of LCEs with the snap-through buckling mechanism of a bistable beam structure to create an extreme energy-absorbing architected material that exhibits an order of magnitude higher specific energy dissipation than previously reported for the same design using PDMS. Our current efforts are focused on developing a nonlinear viscoelastic constitutive model that incorporates the relaxation mechanisms of the polymer network and the mesogens to investigate how their interactions may lead to the enhanced dissipation behavior of LCEs.

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 softtissues 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 Facultyin 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 SESin 2020 and is currently aneditor for the journal Mechanics of Materials and an associate editor for the Journal of Biomechanical Engineering.