Polymer Gels: Fracture, Friction, Healing and Superabsorbency
Dr. Kendra Erk (School of Materials Engineering, Purdue University, West Lafayette, Indiana)
Polymer gels have many industrial uses, from injectable drug-delivery and self-healing materials to the superabsorbent gels used in baby diapers as well as in high-performance concrete. Model gels with well-defined, custom-synthesized network structures can be investigated to determine the fundamental structure-property relationships that control the gel's overall performance. The first half of this presentation will describe a variety of rheometry-based characterization methods to investigate the shear-induced fracture, friction, and healing behavior of model physically associating polymer gels. Gels deformed at different shear rates displayed evidence of shear banding and fracture, which was directly observed using simultaneous particle tracking flow visualization. Using scaling laws from traditional sliding friction experiments, the rheological response following instability formation was power-law shear thinning with a fixed slope regardless of the type of instability, illustrating that fracture and banding are molecularly and rheologically similar. When fractured gels were allowed to rest undisturbed for set time durations, timescales to achieve complete healing were 2-3 orders of magnitude greater than the gel's characteristic relaxation time. The second half of this presentation will describe the use of superabsorbent polymer gels as concrete internal curing agents. When incorporated into cementitious mixtures, the swollen gels release their stored water to fuel the curing reaction, resulting in reduced mixture shrinkage and cracking and thus increasing the concrete's overall durability and service life. However, the gel's swelling performance and mechanical properties were strongly sensitive to multivalent ions naturally present in the mixture. By synthesizing model acrylic gels with different ionic characters and exposing gel particles to controlled ionic solutions, it was discovered that the presence of Ca2+ and Al3+ decreased swelling capacity and altered swelling kinetics to the point where some gel compositions displayed significant deswelling behavior. Interestingly, when incorporated into mortar mixtures, the gel particles which displayed rapid water release due to ionic interactions with the cement were found to be the most effective at reducing the overall shrinkage of the mortar.
Biography: Dr. Erk is an Assistant Professor of Materials Engineering at Purdue University. Before joining Purdue in March 2012, she was a National Research Council Postdoctoral Research Associate in the Polymers Division of the National Institute of Standards and Technology (NIST, Gaithersburg, MD). She received her Ph.D. in 2010 from Northwestern University (Materials Science and Engineering) and her B.S. in Materials Engineering in 2006 from Purdue University. Prof. Erk was the recipient of an NSF CAREER Award in 2015 for her work on hydrogel-based internal curing agents for high-performance concrete. She was also named a 2014 “Distinguished Young Rheologist” by TA Instruments for her work on characterizing the fracture and healing behavior of polymer gels using rheo-physical experiments. Her overall research interests are on characterizing and synthetically controlling the mechanical properties and deformation responses of soft materials and complex fluids, with an emphasis on polymer physics and rheology. @PurdueSoftMSE, http://soft-material-mechanics.squarespace.com/
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