Ex Situ Chemical Determination of Free Radicals and Peroxides on Plasma Treated Surfaces

Ex situ determinations of free radicals by 2,2-Diphenyl-1-pricylhydrazyl (DPPH) method and of peroxides by the iodine method applied to plasma treated surfaces were examined. Plasma polymers and plasma-treated polymers, which are already exposed to ambient oxygen, the original free radicals could be detected as peroxides. It was found that the specificity of the both methods was not high enough for quantitative determination. Chemical cleaning of aluminum alloy surfaces, such as alkaline cleaning and deoxidizing treatment, was found to leave large amount of DPPH scavengers, which could be removed by argon or oxygen plasma treatment, implying that the reactivity of DPPH is not exclusive to free radicals. DPPH method should be viewed as an analytical tool to detect scavengers of DPPH (rather than free radicals) existing on the surface region accessible to the DPPH solution. The iodine method should be likewise viewed as an analytical tool for iodine-liberating species existing on the surface region. Plasma treatment (O₂ or Ar) of aluminum surface did not remove the species, and in some cases increased the amount. The both methods could be used as tools to assess the overall surface reactivity, particularly of metals, which could influence the adhesion of coatings.