Abhishek Bawiskar and Prof. Kendra A. Erk

1. 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 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. Model triblock copolymer: PMMA: Poly(methyl methacrylate) (9 kg/mol) PnBA: Poly(n-butyl acrylate) (53 kg/mol) • 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. Dissolved in 2-Ethylhexanol at high temp; forms a physically associating network at T < 34°C.. Mesh size, ξ ~ 40 nm Endblock aggregates r ~5 nm 34°C Midblock bridges Relaxation time (determined from step-strain experiments) can be approximated to an Arrhenius plot: Healing time (aging time required for 100% recovery) also fits the same plot with different constant for parallel shift: Eacorresponds to activation energy associated with polymer chain pull out Const. shear rate: 1 s-1 Const. shear rate: 1 s-1 Re-Fracture! Fracture! Hold fractured sample for specified aging time Cool sample Const. temp. T < 34C, gel formation T > 34C T = target temperature Jamming in concentrated comb-polymer-adsorbed MgO suspensions, Lisa Murray Shear Start-Up = 0.001, 0.01, 0.1, 1.0 s-1 Shear Start-Up = 1.0 s-1 , comb-polymer concentrations The flow-induced jamming behavior of concentrated suspensions of MgOmicroparticles 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 hydroclusteraggregates. 6.0 mg/g Narrow MgO Distribution (0.5-40 microns) Networked Particles 6.0 mg/g Wide MgODistribution (0.5-400 microns) Hydrocluster Aggregates Ionic sensitivity of hydrogels used for internal curing of cement, Qian Zhu 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 Ca2+ 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. Effect of covalent xlinking on swelling in Ca2+ Stiff-shell / soft-core structure forms in Al3+ solutions • Main Conclusions • SAP hydrogels are sensitive to the presence of ions, especially Ca2+ and Al3+ 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. Total swelling of poly(acrylic acid-acrylamide) SAP with 2% xlinker dry SAP swollen acrylic-acid-based SAP