Studies of polymer-surface nitrogenation using dielectric-barrier postdischarges (“NitroPol”)

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Plasma-nitrogenation of polymer surfaces has recently gained much interest for applications such as adhesion enhancement, electroless metal deposition, or biomedical purposes, to name important examples. Plasma-nitrogenated polymer surfaces differ from plasma-oxidized ones by their basiscity and nucleophilicity, introduced by N-functional groups with unbonded electron pairs, while oxygen-containing polymer surfaces generally are acidic and electrophilic. In spite of the importance of plasma-nitrogenated polymer surfaces the current understanding regarding plasma treatment of polymer surfaces with nitrogen or nitrogen-containing gases is much less advanced than that of oxygen- or air-plasma treatment. In the project proposed here, atmospheric-pressure DBDs in N2 shall primarily be studied with respect to their interaction with polyolefin surfaces and with model compounds. In the first place these investigations shall be performed utilizing flowing postdischarges fed by dielectric-barrier discharges in N2, serving as sources of active species which are supposed to play a dominant role for the surface modification. The project aims at the identification of products of the mentioned interaction, i. e., functional groups introduced to the polymer surface, as well as on mechanism and kinetics of the product formation, based on gas-phase densities of reactive species mainly involved: groundstate and metastable excited atoms, N(4S) and N(2P), and metastable excited molecules, N2(A). The densities can be determined with sufficient accuracy from optical emission spectroscopy and chemical-kinetic calculations. To analyze the surfaces, Fourier-transform infrared spectroscopy in the attenuated reflection mode or immersion-IRRAS (infrared reflexion-absorption spectroscopy) will be applied, as well as X-ray photoelectrons spectroscopy. Use will also be made of chemical-derivatization and trapping techniques to classify and quantify products and to stabilize labile moieties. Experiments with model alkanes will additionally be evaluated by gas chromatography coupled with high-resolution mass spectrometry. Scientific objectives of this project are the answers to three fundamental questions: (1) What is the exact chemical nature of polyolefin surfaces after nitrogenation by an atmospheric-pressure afterglow, what kind of functional groups are formed? (2) What are the gas-phase species involved in formation of a specific functional group, can a plausible chemical pathway be demonstrated for its formation, in agreement with state-of-the-art knowledge about chemical mechanisms in general? (3) How does the rate of product formation depend on species densities, can a set of rate equations be formulated? From a more practical standpoint of plasma-based surface technology, the project will provide an exact description of discharge and postdischarge parameters to obtain a controlled surface modification of polyolefins with well-known nitrogen-containing functional Groups.