In-situ FTIR-ATR spectroscopic investigations of atmospheric-pressure plasma modification of polyolefin thin films

Abstract

Surface treatment of polyolefines by atmospheric-pressure dielectric barrier discharges (DBDs) in virtually oxygen-free nitrogen-containing gases was studied in situ byFourier transform infrared spectroscopy in the attenuated total reflection mode (FTIR-ATR)in order to follow the plasma-chemical generation of chemical functional groups and their further temporal development in the presence of inert or reactive atmospheres.

Polyolefin thin films of thicknesses between 50 and 200 nm were prepared directly on ZnS ATR crystals by spin-coating from hot solutions of linear low-density polyethylene (LLDPE), low density polyethylene (LDPE), or polypropylene (PP) in hydrocarbons solvents like xylene or decalin. After the exposure to the afterglows of DBDs in N2 or in mixtures of N2 with H2 or NH3, infrared spectra were taken in situ under inert conditions and after controlled exposure to various reagents, resp., such as water vapor or oxygen. In order to unravel the complex spectra which are generally due to several functional groups with overlapping vibrational bands, exchange reactions with vapor of heavy water (D2O) was applied in order to identify groups which are known to exhibit a rapid H/D exchange like >N=H (imine), -NH2(prim. amine), >NH (sec. amine), -CO-NRH(prim. or sec. amide), using characteristic ratios of wavenumber ratios for corresponding vibrational bands in the deuterated and the protonated moiety.

In addition, reactions of the plasma-treated surface with vapors of4-(trifluoromethyl)-benzaldehyde (TFBA) or4-(trifluoromethyl) phenyl isothiocyanate (TPI) were studied by FTIR-ATR in situ. Based on these experiments, tentative assignments of the observed vibration bands to imino, amino, and amido groups are made and interpreted in terms of feasible or probable chemical mechanisms.