A ToF-SIMS study of the deuterium—hydrogen exchange induced by ammonia plasma treatment of polyolefins

Abstract

ToF-SIMS has been used to study the surface functionalization of polypropylene and polyethylene samples by exposure to low-pressure ammonia plasma. Static secondary ion mass spectrometry is an appropriate tool to determine hydrogen isotopes as elements and in fragment ions with high sensitivity and selectivity. Specifically the exchange of hydrogen isotopes and the incorporation of N-containing moieties in the near-surface layer of the polyolefins have been studied in ND₃ plasma experiments with conventional polypropylene (h-PP) and polyethylene (h-PE) and NH₃ plasma experiments with deuterated polyethylene (d-PE). Considering the exchange of hydrogen between the plasma and the polymer surface studied by using deuterated ammonia and polyolefin samples the conclusion has been derived that polypropylene with its side chain methyl groups is more susceptible to hydrogen exchange reactions. For ND₃ plasma treatment of polyethylene and polypropylene similar N-containing fragments were obtained and the measured semi-quantitative ToF-SIMS N-uptake data are rather similar. The observation of a wide range of characteristic mixed C_kN_lH_mD_n⁺ secondary fragment ions suggests complex and manifold reaction pathways at the polymer–plasma interface besides simple grafting of –ND₂ or –NH₂ moieties formed by ammonia fragmentation in the plasma. Finally, indications of an isotopic effect for hydrogen isotopes in the plasma process have been observed by comparison of ND₃/h-PE results with those of NH₃/d-PE.