SURFACE FLUORINATION OF POLYETHYLENE FILMS BY DIFFERENT GLOW-DISCHARGES - EFFECTS OF FREQUENCY AND ELECTRODE CONFIGURATION

Polyethylene has been treated in a CF4 plasma using two configurations for the electrodes: parallel-plate electrodes at 13.56 MHz, and a nonsymmetrical configuration of electrodes at 70 kHz. The chemical species involved in the plasmas and the subsequent surface modifications were characterized. Comparison of the emissions in the range 200-450 nm was made with optical emission spectroscopy. The excited radicals' CF2 bands and the CF+ broad continuum were detected and a comparison of their relative intensities indicated a more energetic aspect to the low-frequency discharge. This was confirmed by comparing the ratio of N2+/N2 in the two cases. Analysis of the surface properties by means of X-ray photoelectron spectroscopy (XPS), static secondary ion mass spectrometry (SSIMS), and contact angle measurements showed rapid fluorination of the surface (t = 0.1 s) for the nonsymmetrical electrodes. Longer treatment times (up to 15 s) led to more fluorinated surfaces, thereby decreasing the wettability of the treated substrates; fluorine incorporation was, however, more significant at equivalent treatment times in the diode reactor. SSIMS analysis indicated the formation of more complex fluorine compounds with an increase in the treatment time with no notable modification other than replacement of H by F. Further increases in the treatment time for low-frequency treated samples in the nonsymmetrical configuration of electrodes caused an increase in the surface roughness, as observed by SEM (scanning electron microscopy) analysis. RBS (Rutherford backscattering spectroscopy) measurements showed that beyond 0.1 s, which corresponds to the time required for rapid fluorination, fluorine diffuses through the polymeric matrix and its concentration is enhanced with increasing treatment time.