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Authors
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Dlubek, G. ; Gupta, A. Sen ; Pionteck, J. ; Krause-Rehberg, R. ; Kaspar, H. ; Lochhaas, K.H.
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Title
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Temperature Dependence of the Free Volume in Fluoroelastomers from Positron Lifetime and PVT Experiments
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Date
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18.11.2004
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Number
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12148
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Abstract
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The microstructure of the free volume and its temperature dependence in fluoroelastomericcopolymers of tetrafluoroethylene (TFE) and perfluoro(methyl vinyl ether) (PMVE), PFE, as well asvinylidene fluoride (VDF) and hexafluoropropylene (HFP), VDF<SUB>78</SUB>/HFP<SUB>22</SUB>, were studied by pressure-volume-temperature experiments (PVT, <I>T</I> = 300-485 K, <I>P</I> = 0-200 MPa) and positron annihilationlifetime spectroscopy (PALS, <I>T</I> = 100-473 K, <I>P</I> = 0 MPa). Employing the Simha-Somcynsky equationof state (S-S eos), the excess free (hole) 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>, were estimated from the specific total volume <I>V</I>. The temperature and pressurevariation of these volumes and their expansivity and compressibility will be discussed. The PALS spectrawere analyzed using the routine LT9.0 assuming a dispersion in both the positron (<I><IMG SRC="/images/gifchars/tau.gif" BORDER=0 ></I><SUB>2</SUB>) and orthopositronium (o-Ps) lifetime (<I><IMG SRC="/images/gifchars/tau.gif" BORDER=0 ></I><SUB>3</SUB>). From the lifetime pa<br />rameters the 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> and <I>V</I><SUB>f</SUB> the specific hole number<I>N</I><SUB>h</SUB>' was estimated. <I>N</I><SUB>h</SUB>' was determined to be independent of the temperature. Indications were foundthat o-Ps may prefer larger holes with a weight approximately proportional to the hole volume. Apparentdiscrepancies between S-S modeling and the conclusions from PALS are discussed. From the comparisonof the hole size distribution with the theory of thermal fluctuation a fluctuation volume <<I>V</I>> is estimatedwhich decreases above <I>T</I><SUB>g</SUB> with increasing temperature. Attempts are made to interpret the PALS resultsin terms of the theory of structural and dynamic heterogeneity of glass-forming liquids.
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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 37 (2004) 6606-6618
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DOI
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https://doi.org/10.1021/ma049067n
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Tags
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