Chlorination of irradiated polyethylene single crystals: 1: Analysis of alkyl radical decay kinetics |
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Authors: | H.J. Grimm E.L. Thomas |
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Affiliation: | Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, USA |
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Abstract: | The chlorination of electron beam-irradiated polyethylene (PE) single crystals was studied for a range of irradiation doses, temperatures and chlorine interaction times. The results of this work are presented in two parts: (1) Electron paramagnetic resonance (e.p.r.) analysis of irradiated PE without chlorination and (2) characterization of chlorine uptake in irradiated PE by use of X-ray energy of spectroscopy (XES) and electron spectroscopy for chemical analysis (e.s.c.a.). Spectra were obtained for 75 – 600 Mrad doses over 136 to 333 K temperature range. Alkyl radical concentrations and decay kinetics were derived from e.p.r. spectra and provided a basis for the chlorination analysis in Part II. A first-order alkyl radical decay constant of 4.3 × 10?5s?1 at 298 K was determined in agreement with previous research. Alkyl radical migration was modelled as a one, two or three-dimensional unsteady-state Fickian diffusion process. E.p.r. alkyl radical concentration data was used to estimate alkyl radical diffusion coefficients yielding values of 10 to 4 × 10?18cm2?1 at 298 K for the three models evaluated, respectively. These values are consistent with a diffusivity estimated by Seguchi and Tamura based on a 3-D model. A three-dimensional diffusion model seems to be the most realistic since it permits a zig-zag radical trajectory due to the staggered arrangement of ?CH2- units in crystalline PE. Although a 3-D model would allow diffusion in all directions, it is still consistent with the observation that consecutive migration steps are most likely intermolecular. |
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Keywords: | chlorination polyethylene single crystals alkyl radical decay kinetics |
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