NATURE OF PLASMA-INDUCED SURFACE RADICALS OF POWDERED POLYETHYLENE STUDIED BY ELECTRON-SPIN-RESONANCE

Plasma-induced polyethylene (PE) radicals were studied in detail by electron spin resonance (ESR). The room temperature ESR spectrum of plasma-irradiated PE exhibits an apparent sextet spectrum, which is virtually identical with that of gamma-irradiated PE at 77 K. It was found with the aid of systematic computer simulations, however, that the room temperature ESR spectrum consists of three kinds of spectral components: a sextet spectrum (I) as a major spectrum, a septet spectrum (II), and a smeared-out broad line (III). The sextet and septet spectra were assigned to the midchain alkyl radical, -CH2CHCH2-, and the allylic radical, -CH2CHCH=CHCH2-, respectively, as in the case of those in gamma-irradiated PE. The smeared-out broad line, thought to be an intermediate level of conversion to a broad single line, was assigned to an immobilized dangling-bond site (DBS) at the surface cross-linked region, not to a polyenyl radical, -CH2CH-(CH=CH)nCH2-, as in most studies of gamma-irradiated PE. This indicates that studies of temperature-dependent ESR spectra of peroxy radical probe as a direct evidence of the molecular motion of a linear PE chain should be carefully discussed. The observation of a well-defined sextet spectrum of plasma-irradiated PE at room temperature can be ascribed to the fact that the radical formation of PE has been achieved with a brief plasma irradiation using the sample which is completely unsaturated bond-free. Thus, the nature of radical formation of PE was found to be reflected by the presence or absence of unsaturated bonds in the virgin sample in a very sensitive manner.