Surface treatment of polyolefines by ambient-pressure discharges in N2-H2 mixtures - steps towards a new perspective

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In the preceding DFG project (ATR-FTIR studies in situ of plasma-based amination of polymers using atmospheric-pressure barrier discharges) (Kl1096/19-1; 24 months) we demonstrated that the derivatization with aromatic aldehydes, hitherto commonly applied for quantification of primary amino groups -NH2 on plasma treated polymer surfaces, is by no means specific to amines but will also work with other functional groups such as imines >C=N-. Therefore the role of amino groups has probably been overestimated in the past, compared with alternative nitrogen-bearing functional groups. Applying FTIR-ATR in situ to the study of nitrogen-plasma treatment of polyethylene surfaces and the subsequent H/D isotope exchange or chemical derivatization with 4 trifluoromethyl-benzaldehyde we showed that a derivatization reaction with the aldehyde may take place although primary amines could not be detected with an independent method (exchange of N-H vs. N-D and IR spectroscopy of the region of expected ND2 deformation vibrations). These findings bear consequences for the applications of such surfaces as well as for the understanding of their chemical properties and formation mechanism.The purpose of the proposed project is to fathom these consequences, using physico-chemical investigations of the composition and reactivity of the nitrogen-plasma-treated surfaces. In order to test the hypothesis that the surface chemistry is dominated by imino groups, featuring both nucleophilic and electrophilic properties, rather than by amines, chemical gas-phase derivatization reaction with electrophilic and nucleophilic reagents will be studied in-situ using FTIR-ATR spectroscopy. Obtained spectra will be compared with those measured on suitable model compounds. In addition ex-situ measurements will be performed on plasma-treated polymer foils and wet-chemical reactions (reductions, hydrolyses) shall be included to obtain a comprehensive picture of these surfaces from a new perspective. The chemical studies will be complemented by physical measurements using NEXAFS, UPS and MIES jointly with research partners in Berlin and Clausthal, respectively; these methods offer additional possibilities to gain information with respect to the degree of unsaturation of chemical moieties on the surface ( CH=CH- vs. -CH2-CH2- and -CH=N- vs. -CH2-NH-).A question concerning the mechanism of plasma nitrogenation is the role of nitrogen atoms and hydrogen in the N2-H2 mixtures normally used. To this end FTIR-ATR studies in situ are planned during plasma treatment with varying H2 content between 0 and 4 %. These investigations shall be complemented by optical emission measurements aiming at a quantitative study of N atoms in the discharge afterglow. Based on these data it should be possible to assess if N atoms and excited N2 molecules formed therefrom may be responsible for the surface modification or if alternatively hydrogen-bearing species NHx have to be taken into consideration.