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Authors
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Dlubek, G. ; De, U. ; Pionteck, J. ; Arutyonov, N. Yu. ; Edelmann, M. ; Krause-Rehberg, R.
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Title
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Temperature Dependence of Free Volume in Pure and Silica-Filled Poly(dimethyl siloxane) from Positron Lifetime and PVT Experiments
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Date
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25.04.2005
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Number
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12860
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Abstract
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<B>Summary:</B>The microstructure of the free volume and its temperature dependence in two poly(dimethyl siloxane)s (PDMS), one in the pure state and the other filled with 35 wt.-% of an in situ hydrophobized fumed silica with a specific surface area of 200-300 m<SUP>2</SUP> · g<SUP>-1</SUP>, were studied by pressure-volume-temperature experiments (PVT, <I>T</I> = 22-156 °C, <I>P</I> = 10-200 MPa) and positron annihilation lifetime spectroscopy (PALS, <I>T</I> = -173-100 °C, <I>P</I> = 10<SUP>-5</SUP> MPa). The Simha-Somcynsky equation of state was used to estimate the hole free volume fraction, <I>h</I>, and 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>, from the specific total volume, <I>V</I>. The PALS spectra were analyzed with the routine LT9.0, which allowed for a dispersion, <I><IMGSRC="/giflibrary/12/sigma.gif"BORDER="0"></I><SUB>i</SUB>, in all three of the lifetimes: the para-positronium (<I>p</I>-Ps, <I><IMGSRC="/giflibrary/12/tau.gif"BORDER="0"></I><SUB>1</SUB>), positron (e<SUP>+</SUP>, <I><IMGSRC="<br />/giflibrary/12/tau.gif"BORDER="0"></I><SUB>2</SUB>), and ortho-positronium (<I>o</I>-Ps) lifetime (<I><IMGSRC="/giflibrary/12/tau.gif"BORDER="0"></I><SUB>3</SUB>). This kind of analysis delivered correct <I>p</I>-Ps lifetime parameters, <I><IMGSRC="/giflibrary/12/tau.gif"BORDER="0"></I><SUB>1</SUB>, <I><IMGSRC="/giflibrary/12/sigma.gif"BORDER="0"></I><SUB>1</SUB>, and <I>I</I><SUB>1</SUB>. It was speculated that e<SUP>+</SUP>, like <I>o</I>-Ps, undergoes Anderson localization at empty sites of the, static or dynamic, disordered structure. The hole size distribution, its mean value, <IMGSRC="/giflibrary/12/lang.gif"BORDER="0"><I>v</I><SUB>h</SUB><IMGSRC="/giflibrary/12/rang.gif"BORDER="0">, and dispersion, <I><IMGSRC="/giflibrary/12/sigma.gif"BORDER="0"></I><SUB>h</SUB>, were calculated from the <I>o</I>-Ps lifetimes. A comparison of <IMGSRC="/giflibrary/12/lang.gif"BORDER="0"><I>v</I><SUB>h</SUB><IMGSRC="/giflibrary/12/rang.gif"BORDER="0"> with <I>V</I><SUB>f</SUB> was used to estimate the specific hole number, <IMGBORDER="0"ALIGN="ABSMIDDLE"SRC="http://www3.interscience.wiley.com/cgi-bin/TeX2gif.pl?%24N'_%7B%5Crm%20h%7D%24">. During melting of the semicrystalline samples at-38 °C (<I>T</I><SUB>m</SUB>), <IMGSRC="/giflibrary/12/lang.gif"BORDER="0"><I>v</I><SUB>h</SUB><IMGSRC="/giflibrary/12/rang.gif"BORDER="0"> increased abruptly, and <I><IMGSRC="/giflibrary/12/sigma.gif"BORDER="0"></I><SUB>h</SUB> suddenly decreased. Both effects are explained by the disappearance of the rigid-amorphous fraction (RAF) and, thus, a reduction in the dynamic heterogeneity. The following leveling-off in <IMGSRC="/giflibrary/12/lang.gif"BORDER="0"><I>v</I><SUB>h</SUB><IMGSRC="/giflibrary/12/rang.gif"BORDER="0"> and the low value of <I><IMGSRC="/giflibrary/12/sigma.gif"BORDER="0"></I><SUB>h</SUB> are attributed to the fast segmental relaxation in the PDMS melts which leads to a smearing of the molecule density distribution around a hole during the <I>o</I>-Ps lifetime.
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Publisher
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Macromolecular Chemistry and Physics
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Wikidata
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Citation
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Macromolecular Chemistry and Physics 206 (2005) 827-840
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DOI
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https://doi.org/10.1002/macp.200400546
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Tags
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