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Browsing by Author "He, Yadong"

Now showing 1 - 3 of 3
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    Double helical conformation and extreme rigidity in a rodlike polyelectrolyte
    Wang, Ying; He, Yadong; Yu, Zhou; Gao, Jianwei; ten Brinck, Stephanie; Slebodnick, Carla; Fahs, Gregory B.; Zanelotti, Curt J.; Hegde, Maruti; Moore, Robert Bowen; Ensing, Bernd; Dingemans, Theo J.; Qiao, Rui; Madsen, Louis A. (Nature Publishing Group, 2019-02-18)
    The ubiquitous biomacromolecule DNA has an axial rigidity persistence length of ~50 nm, driven by its elegant double helical structure. While double and multiple helix structures appear widely in nature, only rarely are these found in synthetic non-chiral macromolecules. Here we report a double helical conformation in the densely charged aromatic polyamide poly(2,2′-disulfonyl-4,4′-benzidine terephthalamide) or PBDT. This double helix macromolecule represents one of the most rigid simple molecular structures known, exhibiting an extremely high axial persistence length (~1 micrometer). We present X-ray diffraction, NMR spectroscopy, and molecular dynamics (MD) simulations that reveal and confirm the double helical conformation. The discovery of this extreme rigidity in combination with high charge density gives insight into the self-assembly of molecular ionic composites with high mechanical modulus (~ 1 GPa) yet with liquid-like ion motions inside, and provides fodder for formation of other 1D-reinforced composites. © 2019, The Author(s).
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    Nano-Confined Room-Temperature Ionic Liquids for Electrochemical Applications
    He, Yadong (Virginia Tech, 2018-02-28)
    Room-temperature ionic liquids (RTILs) and their derivatives are promising electrolytes for electrochemical devices including supercapacitors. Understanding the behavior of RTILs in these devices is critical for improving their performance. The energy density of supercapacitors can be improved greatly by using RTILs as electrolytes and nanoporous carbon as electrodes, but the mechanism of the charge storage using these materials is not well understood. In this dissertation, the diffusion and charging dynamics of RTILs in nanopores are studied. The results show that ion packing typically plays the most important role in ion diffusion. The study also demonstrates that the cyclic charging and discharging of a pore can exhibit a number of interesting features (e.g., sloshing of ionic charge along the pores during cyclic scans), which help explain experimental observations such as the negligible contribution of co-ions to charge storage at high scan rates. Solid electrolytes with both high ionic conductivities and excellent mechanical strength are needed in many electrochemical devices. The invention of ion gels featuring aligned polyanions immersed inside RTILs has shown promise in meeting this demand, but the mechanism behind their superior mechanical strength remains elusive. Using molecular simulations, it is discovered that the high elastic moduli of model PBDT ion gels originate from the RTIL-mediated interactions between the polyanions. This insight is useful for future design of ion gels to improve their transport and mechanical properties.
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    Probing Nanoscale Thermal Transport in Surfactant Solutions
    Cao, Fangyu; Liu, Ying; Xu, Jiajun; He, Yadong; Hammouda, B.; Qiao, Rui; Yang, Bao (Springer Nature, 2015-11-04)
    Surfactant solutions typically feature tunable nanoscale, internal structures. Although rarely utilized, they can be a powerful platform for probing thermal transport in nanoscale domains and across interfaces with nanometer-size radius. Here, we examine the structure and thermal transport in solution of AOT (Dioctyl sodium sulfosuccinate) in n-octane liquids using small-angle neutron scattering, thermal conductivity measurements, and molecular dynamics simulations. We report the first experimental observation of a minimum thermal conductivity occurring at the critical micelle concentration (CMC): the thermal conductivity of the surfactant solution decreases as AOT is added till the onset of micellization but increases as more AOT is added. The decrease of thermal conductivity with AOT loading in solutions in which AOT molecules are dispersed as monomers suggests that even the interfaces between individual oleophobic headgroup of AOT molecules and their surrounding non-polar octane molecules can hinder heat transfer. The increase of thermal conductivity with AOT loading after the onset of micellization indicates that the thermal transport in the core of AOT micelles and across the surfactant-oil interfaces, both of which span only a few nanometers, are efficient.