IR- and NEXAFS-spectroscopic characterization of plasma-nitrogenated polyolefin surfaces

Modification of polyethylene and polypropylene surfaces by atmospheric-pressure plasmas using mixtures of nitrogen and hydrogen was studied using Fourier-transform infrared spectroscopy in the attenuated total reflection mode (FTIR-ATR) and by near-edge x-ray absorption fine structure spectroscopy (NEXAFS) in order to shed some light on the chemical nature of nitrogen-containing functional groups on the polymer surface. Using FTIR-ATR spectroscopy combined with hydrogen-deuterium isotope exchange of active hydrogen atoms, it was shown that the direct treatment of PE foils by dielectric barrier discharges (DBDs) in N2/H2 mixtures and a subsequent exposure of the samples to the ambient air results in the formation of -NH2 moieties of primary amides on the polymer surface. Corresponding in situ experiments with streaming N2/H2 DBD post-discharges virtually free of H2O and O2, on the other hand, showing the absence of -NH2, proving that no primary amines or amides are formed by this treatment although substantial amounts of nitrogen are incorporated. Moreover, directly N2/H2-plasma-treated polymer surfaces, similar to afterglow-treated low-density polyethylene (LDPE), show amphiphilic character as to be seen by chemical derivatization with nucleophilic reagents 4-(trifluoromethyl)phenylhydrazine and 4-(trifluoromethyl)benzyl­amine, in addition to electrophilic aromatic aldehydes normally used to derivatize such surfaces. The presence of imines or other functional groups with C=N moieties which may be invoked to explain the dual (amphiphilic) reactivity is proven by NEXAFS studies on ultrathin plasma-treated PE films, confirming significant amounts of nitrogen in C=N bonds and carbon in C=C bonds.