Temperature Dependence of the ESR Spectrum of Irradiated Oriented Polyethylene

Temperature dependence of the ESR spectrum in irradiated polyethylene has been studied using stretched specimens. Among the observed spectra the septet spectrum with doublet substructure is known to be due to an ``allyl type'' –CH2–ĊH–CH=CH–CH2– radical. Although the spectrum observed at 142°C is such a septet (splitting 21.3 G) with doublet substructure (splitting 5.5 G), it changes with temperature in a complicated way. Positions of the peaks are displaced, and their apparent number changes, while the spectrum below — 180°C remains unchanged. Such changes were found completely reversible with temperature, and the magnitude of the over-all splitting as well as the outermost doublet remained constant. The analysis of this complicated temperature dependence has been carried out on the assumption that the hyperfine coupling constant of the β protons changes with temperature, while that of the α protons remains constant. By choosing an appropriate set of coupling constants (Aβ) of β protons, the observed spectrum could be reproduced. From the spectrum at — 180°C were deduced Aβ1 = 30 G and Aβ2 = 11 G. Such inequivalence of the coupling might suggest that the methylenic group in the ``allyl type'' radical should be twisted with respect to the nodal plane of the unpaired π electron. As the temperature rises, a hindered oscillation of the methylenic group will take place between two equivalent twisted positions with respect to the same nodal plane. Such behaviors have been interpreted using a theory of time-dependent phenomena and the activation energy was obtained to be 410 cal/mole, the frequency factor being 4.6×108 sec—1.