Northwestern University |
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Polymer Surfaces and Interfaces & Mechanics of Soft Materials – Shull Research Group |
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Wrinkling-cracking (WC) Test |
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A combined wrinkling–cracking (WC) technique is used to characterize the fracture behavior and mechanical properties polymeric thin films deposited on a elastomeric substrate. In brittle materials, the layered structures will form parallel channel crack in the direction of the applied strain when subjected to uniaxial tension. Independent of these crack formation events, periodic surface instabilities in the form of wrinkles will appear in the direction perpendicular to the direction of the applied strain. The investigation of the mechanisms behind ductile fracture of polymers is still in its infancy. Ductility is observed in materials that undergo large, permanent deformation under an applied tensile load, and typically uniaxial tensile test and indentation tests are used. However, these traditional mechanical tests do not fully describe the mechanism of ductile fracture. Ductile fracture can be differentiated into two pathways: (a) Plastic deformation, or yielding, of the polymer chains, which could lead to strain hardening or strain softening, and (b) through the formation of fracture surfaces. These fracture events do not always end in catastrophic failure, as further deformation blunts the crack-tip and hinders the growth of the cracks. Due the yielding of the material, surface wrinkling and fracture become dependent variables. As a result, the analysis of the wrinkling wavelength enables the characterization of a material post-yield. The prescribed testing geometry is novel for the characterization of ductile polymers. |
Biaxial Membrane Inflation Test |
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A non-traditional biaxial membrane inflation technique is employed to characterize the mechanical behaviors polymeric thin films. The technique is unique in that it requires very small amount of sample, and it enables testing in hydrated conditions that are environmentally relevant for water filtration membrane applications. Abstract: A membrane inflation experiment was used to measure the mechanical response of ABCBA pentablock copolymer membranes designed to transport water through the coalescence of water-swollen polyelectrolyte domains within the polymer. Creep resistance of the block copolymers originates from glassy A blocks of either poly(p-methyl styrene) or poly(t-butyl styrene), and water permeability is attributed to the polystyrene C block that is partially sulfonated. The creep resistance of the membranes in the wet state was enhanced by replacing the poly(t-butyl styrene) A blocks with poly(p-methylstyrene), or by decreasing the molecular weight of the polyelectrolyte C block. Both of these substitutions increased the creep resistance of the membranes in a wet state while retaining a high degree of water permeability. Publication: “Water transport and mechanical response of block copolymer ion-exchange membranes for water purification.” Chen, Shawn H., Carl Willis, and Kenneth R. Shull. Journal of Membrane Science 544 (2017): 388-396. Abstract: Biaxial membrane inflation experiments were employed to characterize the creep response and mechanical properties of stoichiometric polyelectrolyte complex films of the strong polyelectrolyte poly(styrenesulfonate) (PSS) and poly(diallyldimethylammonium) (PDADMA). PSS-PDADMAC films were inflated in NaCl solutions at increasing concentrations, from 0 M to 0.5 M. Viscous creep and stress-strain behaviors were measured to determine the breaking and re-forming of the polymer/polymer ion pairs. The dependence of creep compliance and biaxial viscosity on the cross-link density of the complex is linked to classical theories of rubber elasticity, and provides insights to the interal structures of polyelectrolyte complexes. Publication: In preparation. |
Grazing-Incidence Small-Angle X-Ray Scattering |
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The high brilliance x-ray source at the Advanced Photon Source at Argonne National Lab provides a unique platform for characterizing the structural-composition-property relationship of materials. The highly collimated and highly energetic X-ray beams at APS is well suited for polymer systems that scatter weakly, and large signals are necessary to obtain any useful information on the structural and form factors. Moreover, the morphological evolution of our membranes occur at very short time scales. At the APS beamline 8ID-D, a grazing-incidence x-ray scattering technique has been developed to study the evolution of ionic domains in charged block polymers cast from solution in real-time. The grazing geometry allows for a larger scattering, which reduces beam damage and the amount of sample needed. |
University of California – San Diego |
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Moore’s Cancer Center – Esener Research Group |
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Project: Template-Growth of Calcium Phosphate Nano-Wiffle Balls |
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Scanning electron microscope images of silicon nano-wiffle-balls for targeted delivery of cancer therapies. Image Credit: Inanc Ortac, Ph.D. student in electrical engineering at UCSD. |
Research goal: Synthesis of nano-scale, mesoporous nano-wiffle balls using bio-compatible materials deposited on polymeric templates for drug delivery in cancer treatment. The motivation is to hide foreign enzymes used for cancer treatment from the body’s natural immune response by encapsulation within the nano-wiffle-balls. The nano-wiffle balls filled with foreign enzymes can then be directly injected near the tumor site, or be delivered systemically through the blood stream and accumulate at the target sites. |
Laboratory for Energy Storage and Conversion – Meng Research Group |
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Project: Metal-organic Frameworks (MOF) for Rechargeable Lithium Ion Batteries |
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Abstract: The electrochemical performance of a MIL-101(Fe) metal–organic framework (MOF) as a lithium ion battery electrode is reported for the first time. Iron metal centers can be electrochemically activated. The Fe3+/Fe2+ redox couple is electrochemically active, but not reversible over many cycles. A comparison between ex situ and in operando X-ray absorption spectroscopy (XAS) on the Fe K-edge is presented. Our results indicate that the capacity fade is related to a time dependent, irreversible oxidation of Fe2+ to Fe3+. These results are key in proving the importance of in operando XAS measurements. The MOF side reaction with an electrolyte has been computationally modeled. These results provide further insights on the mechanism responsible for the MOF lack of reversibility. Future guidelines for improving the reversibility of MOFs used as electrodes in Li-ion batteries based on the fine-tuning of the electronic structure of the material are proposed. |
