Effect of Discharge Gases on Microwave Plasma Reactions of Imidazole on Poly(dimethylsiloxane) Surfaces: Quantitative ATR FT-IR Spectroscopic Analysis

Although our recent studies revealed that under Ar microwave plasma conditions reactions of imidazole with poly(dimethylsiloxane) (PDMS) result in the formation of Si-CH2-imidazole species, the issue of the discharge gas effect on microwave plasma reactions remain unanswered. This study examines how these reactions in the presence of Ar, O2, and CO2 gases under microwave plasma conditions will affect surface reactions on PDMS in the presence of imidazole vapors. When Ar microwave plasma reaction conditions are employed, imidazole molecules react to the PDMS surface through hydrogen abstraction of the N-H bonds to form Si-CH2-imidazole and Si-CH2-CH3 linkages. When O2 microwave plasma reactions are conducted for 20 s or less discharge times, Si-O-imidazole and Si-O-CH3 species on the PDMS surface are formed. On the other hand, for discharge times above 20 s, the Si-O-CH3 linkages are converted to Si-O-CH2-imidazole entities as a result of hydrogen abstraction. The CO2 microwave plasma reactions in the presence of imidazole vapors result in the formation of Si-O-imidazole-CH3 species on the PDMS surface, followed by hydrogen abstraction, resulting in the formation of Si-O-imidazole-CH2‚ radicals, which react with subsequent imidazole molecules through the formation of CH2-N linkages. Similarly to the previous studies, quantitative ATR FT-IR surface analysis showed that the highest yields of imidazole reactions occur under O2 microwave plasma conditions. All experiments utilized in this study allowed surface analysis at 1.3 μm from the surface.