Studies of EPES REELS spectra of polyethylenes aided by line shape analysis—Effect of electron irradiation

Abstract

Polyethylenes of different density, branching structure, crystallographic order, and their degradation due to electron beam were studied using elastic peak electron spectroscopy (EPES) and reflection electron energy loss spectroscopy (REELS) aided with line shape analysis by the pattern recognition (PR) method. This approach offers an algorithm of classification derived from a reference set, i.e. set of spectra recorded from standards exposed to low electron dose (about few C m −2), i.e. polyethylene (100% of C sp 3 bonds) and polystyrene (75% of C sp 2 bonds). Then, the obtained classifier is applied for identification of spectra recorded from polyethylenes exposed to electron beam (doses from 40 to 60 C m −2). The EPES REELS spectra are analyzed in the vicinity of electron quasi-elastic and inelastic losses. Due to electrons undergoing a quasi-elastic scattering from atoms of different atomic numbers, i.e. carbon and hydrogen, for undamaged polymers the surface hydrogen content can be evaluated. Changes due to electron irradiation in polyethylenes are indicated by decreasing content of hydrogen, increasing C sp 2 content and changes in the π loss peak in the REELS spectra. Results of PR method are consistent with results obtained from the C 1s XPS spectra fitting and the width of C KLL XAES spectra (parameter D). Highest stability under electron irradiation was shown by polyethylene of largest molecular weight and most linear branching structure. Application of the PR method to EPES REELS spectra allows to distinguish different polyethylenes and quantify the C sp 2 content.