Regensburg 2004 – scientific programme

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CPP: Chemische Physik und Polymerphysik

CPP 16: POSTER: Computational Physics, Complex Systems

CPP 16.27: Poster

Tuesday, March 9, 2004, 17:00–19:00, B

Sum-Frequency Generation Spectroscopy: Monitoring the Hydrophobic Recovery of PDMS Stamps after Plasma Etching — •Dominik M.P. Hoffmann, Klaus Kuhnke, and Klaus Kern — Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart

Polydimethylsiloxane (PDMS) is a polymer widely employed in the fabrication of stamps for micro-contact-printing, as substrates for micro-fluidic cells, and water-proof protection layers. The PDMS surface, which is hydrophobic in its native state, becomes hydrophilic by oxygen plasma-etching. Contact angle measurements have established that the hydrophobicity recovers on the time scale of a few weeks.

Sum-frequency generation (SFG) spectroscopy can access the preferential orientation of a specific chemical group near an interface. We monitor the hydrophobic recovery by SFG vibrational spectroscopy of the methyl groups. For typical plasma etching parameters (15 s etching time at nominally 190 W plasma power) the contact angle decreases from 110^∘ to 0^∘ reflecting the substantial decrease of hydrophobicity while SFG spectroscopy finds that only one third of the preferentially oriented methyl groups disappear. In addition, the characteristic time for the hydrophobic recovery observed by SFG is shorter than the time observed by contact angle measurements. The observations will be discussed in terms of a model with two different recovery processes.