Browsing by Author "Park, MJ"

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    Effect of molecular weight on the mechanical and electrical properties of block copolymer electrolytes.
    (American Chemical Society, 2007-06-26) Singh, M; Odusanya, O; Wilmes, GM; Eitouni, HB; Gomez, ED; Patel, AJ; Chen, VL; Park, MJ; Fragouli, P; Latrou, H; Hadjichristidis, N; Cookson, DJ; Balsara, NP
    The relationship between ionic conductivity, morphology, and rheological properties of polystyrene-block-poly(ethylene oxide) copolymers (SEO) doped with a lithium salt, Li[N(SO2CF3)(2)], is elucidated. We focus on lamellar samples with poly(ethylene oxide) (PEO) volume fractions, phi, ranging from 0.38 to 0.55, and PEO block molecular weights, M-PEO, ranging from 16 to 98 kg/mol. The low-frequency storage modulus (G') at 90 degrees C increases with increasing M-PEO from about 4 x 10(5) to 5 x 10(7) Pa. Surprisingly, the conductivity of the SEO/salt mixtures with the molar ratio of Li to ethylene oxide moieties of 0.02 sigma, also increases with increasing M-PEO, from 6.2 x 10(-5) to 3.6 x 10(-4) S/cm at 90 degrees C. We compare sigma with the conductivity of pure PEO/salt mixtures, sigma(PEO), and find that sigma/[phi sigma(PEO)] of our highest molecular weight sample is close to 0.67, the theoretical upper limit for transport through randomly oriented lamellar grains. © 2007, American Chemical Society
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    Humidity-induced phase transitions in ion-containing block copolymer membranes.
    (American Chemical Society, 2008-03-25) Park, MJ; Nedoma, AJ; Geissler, PL; Balsara, NP; Jackson, A; Cookson, DJ
    The phase behavior of ion-containing block copolymer membranes in equilibrium with humidified air is studied as a function of the relative humidity (RH) of the surrounding air, ion content of the copolymer, and temperature. Increasing RH at constant temperature results in both disorder-to-order and order-to-order transitions. In-situ small-angle neutron scattering experiments on the open block copolymer system, when combined with wafer uptake measurement, indicate that the disorder-to-order transition is driven by an increase in the partial molar entropy of the water molecules in the ordered phase relative to that in the disordered phase. This is in contrast to most systems wherein increasing entropy results in stabilization of the disordered phase. © 2008, American Chemical Society
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    Increased water retention in polymer electrolyte membranes at elevated temperatures assisted by capillary condensation
    (American Chemical Society, 2007-11) Park, MJ; Downing, KH; Jackson, A; Gomez, ED; Minor, AM; Cookson, DJ; Weber, AZ; Balsara, NP
    We establish a new systematic methodology for controlling the water retention of polymer electrolyte membranes. Block copolymer membranes comprising hydrophilic phases with widths ranging from 2 to 5 nm become wetter as the temperature of the surrounding air is increased at constant relative humidity. The widths of the moist hydrophilic phases were measured by cryogenic electron microscopy experiments performed on humid membranes. Simple calculations suggest that capillary condensation is important at these length scales. The correlation between moisture content and proton conductivity of the membranes is demonstrated. © 2007, American Chemical Society