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  • Articles  (2)
  • Articles: DFG German National Licenses  (2)
  • 1995-1999  (2)
  • Chemistry and Pharmacology  (2)
  • 1
    Electronic Resource
    Electronic Resource
    A positron lifetime and Doppler-broadening study of positron states and free volume in polyethylene (1996)
    Springer
    Journal of radioanalytical and nuclear chemistry 210 (1996), S. 495-503 
    Details
    ISSN: 1588-2780
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Energy, Environment Protection, Nuclear Power Engineering
    Notes: Abstract Positron lifetime (LT) and Doppler-broadening (DB) studies of polyethylene have been performed simultaneously in the temperature range between 80 and 300 K. The LT spectra have been analysed assuming four exponential components. Two long-lived components appear, which were attributed too-Ps pick-off annihilation in crystalline regions (τ3 = 0.9 to 1.2 ns) and at free-volume holes in the amorphous phase ( $$\tau _{\Delta _3 } = 1.5$$ to 2.8), The variation in τ4 correponds to an increase of the mean hole size from 0.053 nm3 at 80 K to 0.188 nm3 at 300 K. From the data the glass transition temperature (T g=195 K), the coefficient of thermal expansion of holes in the glassy and rubbery phase (α h, g = 14.5 · 10−4 K−1 and α h, r = 189 · 10−4 K−1) and the fractional free volume (2.8% to 10.4%) were estimated. The DB curves were fitted by a sum of three Gaussians, the narrowest of which is assumed to represent the self-annihilation ofp-Ps localised at holes. The intensity of the narrow component,I n, varies between 0 and 7.3% in a similar way as the LT intensityI 4/3 varies. From this it was concluded that other Ps reactions beside pick-off are not important. Further, it was shown that the average positron lifetime is dominated by theo-Ps component,T 4 τ g, while the behaviour of the DB peak height is mainly affected by thep-Ps narrow componentI n .
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02056389
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  • 2
    Electronic Resource
    Electronic Resource
    The temperature dependence of the local free volume in polyethylene and polytetrafluoroethylene: A positron lifetime study (1998)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 36 (1998), S. 1513-1528 
    Details
    ISSN: 0887-6266
    Keywords: free volume ; thermal expansion ; positron lifetime measurements ; semicrystalline polymers ; polyethylene ; polytetrafluoroethylene ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Positron lifetime measurements, performed in the temperature range 80-300 K, are reported for polyethylene (PE) and polytetrafluoroethylene (PTFE). The lifetime spectra have been analyzed using the data processing routines LIFSPECFIT and MELT. Two long-lived components appear, which are attributed to pick-off annihilation of ortho-positronium in crystalline regions and at holes in the amorphous phase. The ortho-positronium lifetimes, τ3 and τ4, are used to estimate the crystalline packing density and the size of local free volumes in the crystalline and amorphous phases. The interstitial free volume in the crystals exhibits a weak linear increase with the temperature which is attributed to thermal expansion of the crystal unit cell. In the amorphous phase, the hole volume varies between 0.053 and 0.188 nm3 (PE) and between 0.152 and 0.372 nm3 (PTFE). Its temperature variation may be fitted by two straight lines, the intersection of which is used to estimate a glass transition temperature of Tg = 195 K for both PE and PTFE. The slopes of the free volume in the glassy and crystalline phases with the temperature correlate well with each other. The coefficients of thermal expansion of the hole volume are compared with the macroscopic volume change below and above the glass transition. From this comparison a fractional hole volume at Tg of 4.5 (PE) and 5.7% (PTFE) and a number of 0.73 (PE) and 0.36 (PTFE) × 1027 holes/m3 is estimated. Finally, it is found that the intensity of o-Ps annihilation in crystals shows a different temperature dependence to that in the amorphous phase. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1513-1528, 1998
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
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