Paper PS-FrM8
Synthesis of Polystyrene and Sulfonated Polystyrene Thin Films by Atmospheric Pressure Plasma Enhanced Chemical Vapour Deposition

Friday, November 13, 2009, 10:40 am, Room B2

Thin coatings of pp-PS and pp-sulfonated PS were synthesized by “Plasma Enhanced Chemical Vapour Deposition” (PECVD) in a home-built “Dielectric Barrier Discharge” (DBD) system near atmospheric pressure. Styrene (C₈H₈) and trifluoromethane sulfonic acid (CF₃SO₃H) monomers were used as precursors and were introduced in the discharge using a flux of Ar or He. An atmospheric RF plasma torch was also used for PS deposits. It consists of two closely spaced perforated metallic electrodes. The process gas is Ar. The precursor (styrene vapour) was introduced into the plasma downstream of the electrodes (afterglow). The polymers were deposited on various substrates (PTFE, HDPE, stainless steel, glass, and silicon wafer) and were characterized by FTIR, XPS, SSIMS, WCA, AFM and optical microscopy. The plasma phase was studied by OES.

The pp-sulfonated PS films obtained by plasma copolymerization could present interesting properties as electrolyte membrane for miniaturized fuel cell applications (Polymer Electrolyte Membrane Fuel Cell- PEMFC) using H₂ or CH₃OH (Direct Methanol Fuel Cell-DMFC). CF₃SO₃H allows introduce the sulfonic acid groups for proton conductivity in the membrane whereas the styrene constitutes the backbone of the membrane.

The major features that characterize PS are present in the FTIR, XPS and SSIMS spectra, although some differences between pp-PS films and their conventionally polymerized counterparts are observed (like oxygenation during or after deposition due to the atmosphere environment, branching, degree of cross-linking, and unsaturations). According to the WCA and XPS results, the pp-PS films deposited by the RF plasma torch (placed in a plexiglass chamber) are more oxygenated than those deposited by the DBD operated in a much more controlled atmosphere [1]. The chemical structure of the deposited coatings was investigated by FTIR and TOF-SSIMS as a function of the nature of the carrier gas. The pp-PS films synthesized in the presence of Ar (for both processes) exhibit more branching, more unsaturations, a higher degree of cross-linking, and a lower density of aromaticity than pp-PS synthesized with He as the main plasma gas.

The influence of the nature of the carrier gas, and of the discharge voltage and the ratio of the partial pressures of the monomers on the sulfonic content (therefore on the degree of fragmentation of the CF₃SO₃H monomer) for both carrier gases was investigated for pp-PS sulfonated by XPS, SSIMS and FTIR.

[1] D. Merche, C. Poleunis, P. Bertrand, M. Sferrazza, F. Reniers, IEEE Transactions on Plasma Science (2009), Under Press, Available online on http://www.ieeetps.org/