STRUCTURAL-MODIFICATION MECHANISM FOR POLYIMIDE-DOPED POLY(TETRAFLUOROETHYLENE) AT SUBTHRESHOLD FLUENCES USING 248 NM RADIATION

Single-photon excimer laser ablation of neat poly(tetrafluoroethylene) (PTFE) is not observed at emissions in the ''quartz'' UV, i.e., from about 190-380 nm. However, it has been successfully demonstrated that, when the fluoropolymer is doped with small quantities of polyimide (PI), ablation in the quartz UV, e.g., at 248 and 308 nm and pulse widths of about 25 ns, is readily achieved. When PI-PTFE blends are exposed to subthreshold fluences, considerable changes in surface topography occur although clearly defined structures, e.g., pits, are not formed. Using photoacoustic infrared spectroscopy to evaluate surface and bulk chemical changes to blends exposed to subthreshold excimer laser fluences, <100 mJ/cm2, it is shown that PI (1) is distributed throughout the bulk and resides at the surface and (2) is selectively absorbing the high-energy photons and as a result being preferentially removed from the surface.