Local chemical transformations in poly(dimethylsiloxane) by irradiation with 248 and 266 nm

Poly(dimethylsiloxane) (PDMS) has been irradiated with a frequency quadrupled Nd:YAG laser and a KrF *-excimer laser at a repetition rate of 1 Hz. The analysis of ablation depth versus pulse number data reveals a pronounced incubation behavior. The thresholds of ablation (266 nm: 210 mJ cm −2, 248 nm: 940 mJ cm −2) and the corresponding effective absorption coefficients α eff (266 nm: 48900 cm −1, 248 nm: 32700 cm −1, α lin = 2 cm −1) were determined. The significant differences in the ablation thresholds for both irradiation wavelengths are probably due to the different pulse lengths of both lasers. Since the shorter pulse length yields a lower ablation threshold, the observed incubation can be due to a thermally induced and/or a multi-photon absorption processes of the material or impurities in the polymer. Incubation of polymers is normally related to changes of the chemical structure of the polymer. In the case of PDMS, incubation is associated with local chemical transformations up to several hundred micrometers below the polymer surface. It is possible to study these local chemical transformations by confocal Raman microscopy, because PDMS is transparent in the visible. The domains of transformation consist of carbon and silicon, as indicated by the appearance of the carbon D- and G-bands between 1310 and 1610 cm −1, a band appearing between 502 and 520 cm −1 can be assigned to mono- and/or polycrystalline silicon. The ablation products, which are detected in the surroundings of the ablation crater consist of carbon and amorphous SiO x ( x ≈ 1.5) as detected by infrared spectroscopy.