Shear rheometry characterization of fracture-healing behavior displayed by a physically associating gel Abhishek Bawiskar and Prof. Kendra A. Erk School of Materials Engineering, Purdue University, West Lafayette, IN USA Jamming in concentrated comb-polymer-adsorbed MgO suspensions, Lisa Murray The flow-induced jamming behavior of concentrated suspensions of MgO microparticles containing adsorbed PAA- PEO comb-polymer (ADVA) was studied with shear rheometry. Increased comb-polymer concentrations resulted in larger overshoot peaks which may indicate particle network formation, while multiple overshoots in the wide particle size distribution suspensions may be caused by a hydrocluster aggregates. Narrow MgO Distribution ( microns) Wide MgO Distribution ( microns) Networked Particles Hydrocluster Aggregates Polymers with temperature-responsive gelation are of particular significance as biomedical materials. For certain applications, it is important that the gels have the ability to heal if their structure becomes damaged. Here, we have devised a method to quantify healing kinetics using a shear rheometer. Hold fractured sample for specified aging time Const. shear rate: 1 s -1 Cool sample Const. temp. T = target temperature PMMA: Poly(methyl methacrylate) (9 kg/mol) PnBA: Poly(n-butyl acrylate) (53 kg/mol) Dissolved in 2-Ethylhexanol at high temp; forms a physically associating network at T < 34°C.. Model triblock copolymer: 34°C Endblock aggregates r ~5 nm Midblock bridges Mesh size, ξ ~ 40 nm To fracture, applied shear is fast (relative to relaxation time of the gel). Peak stress just prior to fracture is measure of the gel’s elastic strength. Healing is assessed by the aging time required for the gel to regain total elastic strength upon subsequent re-fracture; quantified by peak stress ratio. Higher temperature results in faster healing: e.g., almost 80% strength restored after 5 min. age at 28°C while it takes over 30 min. at 25°C. Lower temperature results in slower healing kinetics as well as lower strain to fracture, indication of brittle behavior. dry SAP swollen acrylic-acid-based SAP Superabsorbent polymer (SAP) hydrogels are used as internal curing agents to mitigate shrinkage/cracking in high-performance concrete. During the hydration process of concrete, multivalent ions such as Ca 2+ are released into the system. Our results show that when these ions are present, the overall swelling capacity of the SAPs decrease, swelling kinetics change, and stiff- shell/soft-core structures can form. wt% AA-AM17%33%67%83% H 2 O solution Na + solution Ca 2+ solution Al 3+ solution Total swelling of poly(acrylic acid-acrylamide) SAP with 2% xlinker wt% AA-AM17%33%67%83% 1.0% xlinking % xlinking % xlinking Ionic sensitivity of hydrogels used for internal curing of cement, Qian Zhu Stiff-shell / soft-core structure forms in Al 3+ solutions Effect of covalent xlinking on swelling in Ca 2+ Main Conclusions SAP hydrogels are sensitive to the presence of ions, especially Ca 2+ and Al 3+ which can form ionic crosslinks within the gel, leading to reduced swelling capacity and altered swelling kinetics. Gels containing more acrylic acid have more COO - groups available for ionic xlinking and thus, are more sensitive to the presence of ions. 6.0 mg/g Const. shear rate: 1 s -1 Fracture! Re- Fracture!