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

: Khosravi, Z.; Kotula, S.; Lippitz, A.; Unger, W.E.S.; Klages, C.-P.


Plasma Processes and Polymers 15 (2018), Nr.1, Art. e1700066, 15 S.
ISSN: 1612-8850
ISSN: 1612-8869
Fraunhofer IST ()
amine; direct plasma; FTIR-ATR; gas phase derivatization; imine; NEXAFS; polymer plasma treatment; remote plasma; TFBA

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.