Highly hydrophobic and partially conductive polydimethylsiloxane surface produced by direct fluorination and subsequent annealing

Highly hydrophobic materials are of interest in many scientific and technological areas. To improve surface properties of polydimethylsiloxane (PDMS), PDMS sheets were surface fluorinated in a laboratory vessel using a F₂/N₂ mixture with 12.5% F₂ by volume at about 55 °C under 0.1 MPa for different times of 15, 30, and 60min. Surface wettability and surface energy were evaluated by contact angle measurements and surface energy calculations, and surface electrical properties were investigated by measurements of surface conductivity and surface potential decay. These results show that direct fluorination produced a highly hydrophobic and partially conductive PDMS surface with low surface energy, which had little correlation with fluorination time in the investigated range of 15 to 60 min. Moreover, annealing of the surface fluorinated PDMS sheets at 150 °C for 60 min caused a further increase in surface hydrophobicity, while reduced surface conduction of the fluorinated PDMS sheets. Attenuated total reflection infrared analyses and scanning electron microscope surface and cross-section observations reveal substantial changes in physicochemical characteristics of the sheet surface layers due to direct fluorination. The high hydrophobicity or low surface energy is attributed to the changes in chemical composition and structure of the surface layers and in their surface roughness. The partial conductivity of the fluorinated PDMS surfaces is a result of the competition between the compositional change and the structural change.