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Authors
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Dlubek, G. ; Wawryszczuk, J. ; Pionteck, J. ; Goworek, T. ; Kaspar, H. ; Lochhaas, K.H.
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Title
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High-Pressure Dependence of the Free Volume in Fluoroelastomers from Positron Lifetime and PVT Experiments
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Date
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07.02.2005
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Number
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12370
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Abstract
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The microstructure of the free volume and its high-pressure dependence in fluoroelastomericcopolymers of tetrafluoroethylene (TFE) and perfluoro(methyl vinyl ether) (PMVE), PFE, as well asvinylidene fluoride (VDF) and hexafluoropropylene (HFP), VDF/HFP<SUB>22</SUB>, was studied by pressure-volume-temperature experiments (<I>PVT</I>, <I>P</I> = 0.1-200 MPa) and positron annihilation lifetime spectroscopy (PALS,<I>P</I> = 0.1-448 MPa, all at 22.5 <IMG SRC="/images/entities/deg.gif">C). Employing the Simha-Somcynsky equation of state the excess (hole)free volume fraction <I>h</I> and the specific free and occupied volumes, <I>V</I><SUB>f</SUB> = <I>hV</I> and <I>V</I><SUB>occ</SUB> = (1 - <I>h</I>)<I>V</I>, wereestimated from the specific total volume <I>V</I>. The pressure variation of these volumes and theircompressibilities are discussed. We found that the occupied volume <I>V</I><SUB>occ</SUB> exhibits a remarkable compressibility, <I><IMG SRC="/images/gifchars/kappa.gif" BORDER=0 ></I><SUB>occ</SUB> <IMG SRC="/images/entities/ap.gif"> 2.4 × 10<SUP>-4</SUP> MPa<SUP>-1</SUP>, while it shows (for <I>T</I> > <I>T</I><SUB>g</SUB>) almost no thermal expansion. The PALSspectra were analyzed using the ne<br />w routine LT9.0 assuming dispersion in both the positron (<I><IMG SRC="/images/gifchars/tau.gif" BORDER=0 ></I><SUB>2</SUB>) andorthopositronium (o-Ps) lifetimes (<I><IMG SRC="/images/gifchars/tau.gif" BORDER=0 ></I><SUB>3</SUB>). We speculate that positrons (e<SUP>+</SUP>) may show Anderson localizationat sites of the entire empty volume possibly assisted by an affinity to fluorine atoms. From the o-Pslifetime parameters the (excess) free volume hole size distribution, its mean value <<I>v</I><SUB>h</SUB>>, and dispersion <I><IMG SRC="/images/gifchars/sigma.gif" BORDER=0 ></I><SUB>h</SUB>were calculated. From a comparison of <<I>v</I><SUB>h</SUB>> with <I>V</I><SUB>f</SUB> the specific hole number <I>N</I><SUB>h</SUB><I>'</I> was estimated. <I>N</I><SUB>h</SUB><I>'</I> wasdetermined to be constant and the same in compression and thermal expansion experiments. When takingcorrectly into account the compressibility of the occupied volume <I>V</I><SUB>occ</SUB> the discrepancy in <I>V</I> vs <<I>v</I><SUB>h</SUB>> plotsbetween compression and thermal expansion experiments discussed in the literature disappears.
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Publisher
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Macromolecules
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Wikidata
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Citation
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Macromolecules 38 (2005) 429-437
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DOI
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https://doi.org/10.1021/ma048310f
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Tags
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