Perfluoropolyethers: Analysis by TOF-SIMS

Abstract

TOF-SIMS spectra of the following perfluoropolyethers (PFPEs) were analyzed: Demnum-S, Krytox Fomblin-Z, perfluoropoly(dioxolane), perfluoropoly(tetramethylene oxide), and perfluoropoly-(ethylene oxide). In each case, the PFPE produced an intense negative ion spectrum with simple patterns persisting into the high-mass region surpassing the mean MW of the polymer. In contrast, the positive ion spectra are much more complex and compressed toward the low-mass end. Totally unrelated formation mechanisms for the positive and negative ions are thus indicated. It is proposed and substantiated that the negative ion spectra are dominated by anions resulting from one-step dissociative electron capture at ether oxygens R-O-R′ + e- → R-O- + •R′. The Demnum molecule, having locally symmetric ether oxygens, produces two major peaks in each repetitive fragmentation packet, equal in intensity but displaced by the mass difference of the two end groups. PFPEs with asymmetric ether oxygens produce negative ion spectra resulting from asymmetric cleavage. The negative ion pattern of Fomblin-Z can be accurately modeled with the dissociative electron capture at ether oxygens linking randomly-distributed methylene and ethylene units. Other less intense peaks in the negative ion spectra of PFPEs result from multiple electron capture events and from polymer chains with anomalous end groups or intrachain units.