Photochemical modification of cross-linked poly(dimethylsiloxane) by irradiation at 172 nm

Cross-linked poly(dimethylsiloxane) (PDMS) was irradiated with a Xe-2*-excimer lamp (172 nm) under ambient conditions. The irradiation in combination with the formed ozone results in an oxidation of PDMS to SiO2 at the polymer-air interface. The surface properties of the irradiated surfaces were studied by means of contact angle measurements, infrared spectroscopy, and X-ray photoelectron spectroscopy. The photochemical conversion of surface methylsilane groups to silanol groups is responsible for the large increase in surface free energy. Subsequent degradation of the polymer and formation of SiOx was monitored by infrared spectroscopy. As determined by X-ray photoelectron spectroscopy, the binding energy shifts reach values corresponding to SiO2. The atomic ratio concentration O:Si changes from about 1:1 (PDMS) to about 2:1 (SiO2). On the basis of the XPS and IR results, the photochemical reaction pathway from PDMS to silicon oxide via surface silanol groups is discussed. The strict linearity of the contact angle versus irradiation time and the clear dependence from irradiation intensity allows the tuning of the chemical surface functionalities.