Chemical modification of polystyrene by low-energy (<100 eV) electron irradiation studied by mass spectrometry

Abstract

Dip-coated thin films of polystyrene were chemically modified (degraded) by low-energy electron irradiation (0-100 eV). The resulting degradation was studied by mass spectrometry with variable electron energy. In the negative-ion mode, H- was observed and was associated with two phenomena. The resonant structure at 9.1 ± 0.4 eV was associated with dissociative electron attachment. The energy of the resonance peak agreed with the results obtained for organic molecules. At a higher energy, a monotonie rise of the emission yield was observed and was associated with nonresonant dipolar dissociation. More mass fragments were observed in the positive-ion mode. The predominant ion was H+ at 100 eV of electron irradiation, and alkyl ions were detected at higher masses. Emission thresholds were measured for the most predominant positive ions. The shape of the emission curve showed that there was no resonant process in the formation of these ions. The energy threshold was measured at about 20 eV, and this meant that the carbon structure of the polymer broke with a recombination of the degradation products with hydrogen atoms before the emission occurred. Below 19 eV, there was only hydrogen loss; this implied that the possible degradation mechanisms did not break the general carbon structure of the material, leading to cross-linking. © 2008 Wiley Periodicals, Inc.