Exploring the Structure of the Modified Top Layer of Polypropylene During Plasma Treatment

Plasma modifications of polypropylene (PP) surfaces are analyzed by means of vacuum beam experiments. A plasma source provides Ar ion beams and a background of UV photons. Additionally, neutral oxygen beams are sent to perform reactive sputtering of PP. The etch rate and chemical state are monitored in real time by in situ Fourier transform infrared (FTIR) spectroscopy. At the onset of Ar bombardment, PP shows high sputter yields, which decrease down to a constant etch rate indicating the formation of a modified top layer. The stationary top layer is modeled as combination of a pristine fraction plus a crosslinked fraction of amorphous hydrocarbon. Photonand ion-dominated etch processes provide different cross-linking fractions, whereas the sputter efficiency is maximized at intermediate ion energies (200 eV).