Surface processes occurring on PVDF under energetic ion irradiation

Modifications induced in polyvinylidene fluoride by energetic ions (E ≥ 1 MeV/amu) have been reported in previous papers. It was shown by using methods such as X-ray photoemission spectroscopy (XPS), infrared absorption spectroscopy (IRAS), electron spin resonance (RPE) or differential scanning calorimetry (DSC) that light and very heavy ions lead to opposite modifications of the polymer. Superficial unsaturations characterized as cumulene compounds, and HF gas desorption were produced under oxygen ions. On the other hand, with heavy ions, the polymer surface appears not to be damaged and a rebuilding process was suggested, which involves a migration of reactive species from the bulk to the surface. In this paper, we report desorption experiments and XPS measurements which support a readdition mechanism of HF molecules on cumulene compounds. Gas desorption measurements carried out with a quadrupole mass spectrometer, during the polymer exposure to the ions, lead to a two-step desorption mechanism: migration of HF and F2, followed by desorption of these species from the surface. XPS spectra of PVDF samples irradiated at a high fluence with a focussed krypton ion beam show new structures which correspond to the readdition of reactive species (HF*, H*, F*), on unsaturations produced during the first step of the modification. A PVDF sample of 50 μm thickness previously irradiated with oxygen ions which led to superficial allenic structures was irradiated again with krypton ions. The XPS measurements of the C1s and F1s photoemission lines, the shape of which is similar to that obtained for a pristine PVDF sample, clearly show that a readdition mechanism occurs on the unsaturations previously produced under energetic light ions. (orig.)