Plasma Surface Modification and Characterization of POSS-Based Nanocomposite Polymeric Thin Films

The effect of a remote oxygen plasma on nanocomposite hybrid polymer thin films of poly[(propylmethacryl-heptaisobutyl-polyhedral oligomeric silsequioxane)-<i>co</i>-(methylmethacrylate)] (POSS-MA) has been examined by advancing contact angle, X-ray photoelectron spectroscopy (XPS), and variable-angle spectroscopic ellipsometry (VASE). Exposure to a 25 W remote oxygen-containing plasma was found to convert the surface of POSS-MA films from hydrophobic to hydrophilic within 20 s. The exposure time needed for this conversion to occur decreased as the O<sub>2</sub>/N<sub>2</sub> ratio in the plasma environment increased, indicating a positive correlation between the hydrophilicity and the presence of oxygen in the plasma. Local bonding information inferred from high-resolution XPS data showed that the isobutyl bonding to the POSS moiety is replaced with oxygen as a result of plasma exposure. Finally, VASE data demonstrates that increasing the weight percent of POSS in the copolymer significantly impedes the oxygen plasma degradation of POSS-MA films. On the basis of these results, a model is presented in which the oxygen plasma removes isobutyl groups from the POSS cages and leaves a SiO<sub>2</sub>-like surface that is correspondingly more hydrophilic than the surface of the untreated samples and is more resistant to oxidation by the plasma. The ability to modify surfaces in this manner may impact the utility of this material for biomedical applications such as microfluidic devices in which the ability to control surface chemistry is critical.