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RISS 인기검색어
- 검색키워드
- 저자 : Runt, James (검색결과 4 건)
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Choi, U Hyeok,Liang, Siwei,Chen, Quan,Runt, James,Colby, Ralph H. American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.5
<P>Dielectric relaxation spectroscopy was used to investigate the segmental dynamics of a series of siloxane-based polar copolymers combining pendant cyclic carbonates and short poly(ethylene oxide) (PEO) chains. The homopolymer with cyclic carbonate as the only side chain exhibits higher glass transition temperature T-g and dielectric constant epsilon(s) than the one with only PEO side chains. For their copolymers the observed T-g (agreeing well with the predicted values from the Fox equation) and epsilon(s) decrease with increasing PEO side chain content. These polar polymers exhibit a glassy beta relaxation with Arrhenius character, attributed to local chain motions of side groups attached to the main chain, and a segmental beta relaxation, associated with the glass transition with a Vogel temperature dependence. As PEO side chain content increases, narrowing of the local glassy beta relaxation was observed in the copolymers. The segmental a dynamics were observed to be faster, with an increase in breadth and decrease in strength with increasing PEO side chain content. Owing to the trade-off between T-g and epsilon(s) copolymers of intermediate composition result in the highest ionic conductivity when these copolymers are used to plasticize Li single-ion conducting ionomers.</P>
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Liang, Siwei,Chen, Quan,Choi, U Hyeok,Bartels, Joshua,Bao, Nanqi,Runt, James,Colby, Ralph H. The Royal Society of Chemistry 2015 Journal of Materials Chemistry A Vol.3 No.42
<▼1><P>Copolymerized (CPP) and oligomeric (OP) plasticizers containing ethylene oxide and cyclic carbonate units.</P></▼1><▼2><P>To prepare a safe electrolyte for lithium ion batteries, two groups of novel low-volatility plasticizers combining pendant cyclic carbonates and short ethylene oxide chains have been successfully synthesized, as confirmed by <SUP>1</SUP>H, <SUP>13</SUP>C and <SUP>29</SUP>Si NMR spectroscopy. The Fox equation describes the composition dependence of the glass transition temperature (<I>T</I>g) very well for the random polysiloxane-based copolymer plasticizers (11 000 < <I>M</I> < 13 000) while the smaller oligomer plasticizers have <I>T</I>g as much as 20 K lower than the Fox equation prediction because of their lower molecular weight (450 < <I>M</I> < 700). The Landau–Lifshitz mixing rule describes the dielectric constant of the random polysiloxane-based copolymer plasticizers at all temperatures above <I>T</I>g. Mixing with 20 wt% polysiloxane tetraphenyl borate – Li ionomer (14 mol% borate and 86 mol% cyclic carbonate) increases conductivity relative to the neat ionomer by lowering <I>T</I>g, increasing dielectric constant and providing better solvation of Li<SUP>+</SUP>. The best oligomeric plasticizer only has <I>T</I>g 10 K lower than the Fox prediction but has dielectric constant 30% larger than expected by the Landau–Lifshitz mixing rule, owing to a surprisingly low viscosity, resulting in ambient conductivity 2 × 10<SUP>−5</SUP> S cm<SUP>−1</SUP>. For both groups of plasticizers, the fraction of cyclic carbonates relative to ethylene oxide governs the magnitude and temperature dependence of the ionic conductivity.</P></▼2>
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Molecular Volume Effects on the Dynamics of Polymerized Ionic Liquids and their Monomers
Choi, U Hyeok,Mittal, Anuj,Price Jr., Terry L.,Lee Jr., Minjae,Gibson Jr., Harry W.,Runt Jr., James,Colby Jr., Ralph H. Elsevier 2015 ELECTROCHIMICA ACTA Vol.175 No.-
<P><B>Abstract</B></P> <P>The dependences of the glass transition temperature and the dielectric constant on molecular volume of the repeat unit for polymerized ionic liquids, the monomers they were polymerized from and simple ionic liquids, are reviewed and compared. The ionic conductivities of these materials and the frequencies at which ionic rearrangements occur are shown to be controlled by their glass transition temperatures. The ionic conductivity is proportional to the product of dielectric constant and ionic rearrangement frequency, with the proportionality constant determined by the molecular volume of the repeat unit.</P>
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Dynamics of Precise Ethylene Ionomers Containing Ionic Liquid Functionality
Choi, U Hyeok,Middleton, L. Robert,Soccio, Michelina,Buitrago, C. Francisco,Aitken, Brian S.,Masser, Hanqing,Wagener, Kenneth B.,Winey, Karen I.,Runt, James American Chemical Society 2015 Macromolecules Vol.48 No.2
<P>This paper presents the first findings on the molecular dynamics of the remarkable new class of linear and precisely functionalized ethylene copolymers. Specifically, we utilize broadband dielectric relaxation spectroscopy to investigate the molecular dynamics of linear polyethylene (PE)-based ionomers containing 1-methylimidazolium bromide (<B>ImBr</B>) pendants on exactly every 9th, 15th, or 21st carbon atom, along with one pseudorandom analogue. We also employed FTIR spectroscopy to provide insight into local ionic interactions and the nature of the ordering of the ethylene spacers between pendants. Prior X-ray scattering experiments revealed that the polar ionic groups in these ionomers self-assemble into microphase-separated aggregates dispersed throughout the nonpolar PE matrix. We focus primarily on the dynamics of the segmental relaxations, which are significantly slowed down compared to linear PE due to ion aggregation. Relaxation times depend on composition, the presence of crystallinity, and microphase-separated morphologies. Segmental relaxation strengths are much lower than predicted by the Onsager theory for mobile isolated dipoles but much higher than linear PE, demonstrating that at least some <B>ImBr</B> pendants participate in the segmental process. Analysis of the relaxation strengths using the Kirkwood <I>g</I> correlation factor demonstrates that ca. 10–40% of the <B>ImBr</B> ion dipoles (depending on copolymer composition and temperature) participate in the segmental motions of the precise ionomers under study, with the remainder immobilized or having net antiparallel arrangements in ion aggregates.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/mamobx/2015/mamobx.2015.48.issue-2/ma502168e/production/images/medium/ma-2014-02168e_0011.gif'></P>
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