Silica-based nanocomposite membranes via the sol gel process of polyethoxysiloxane within a sulfonated poly(ether-ether-ketone) matrix: morphology and proton mobility

This thesis deals with the preparation and characterization of hybrid organic/inorganic membranes obtained from solutions, where the organic phase is the sulfonated poly(ether ether ketone) (SPEEK) and the inorganic phase are silica nanoparticles obtained from the in situ "water free" sol-gel process of polyethoxysiloxane (PEOS). PEOS is a soluble and liquid hyperbranched polymer of low viscosity synthesized in our laboratories via a one pot reaction between tetraethoxysilane (TEOS) and acetic anhydride. The PEOS used within this thesis contains ca 48 wt% of silica. The use of the polymeric silica precursor PEOS instead of the commonly used TEOS offers several advantages among which the need of less water for the conversion into silica, the possibility to modify the ethoxy end-groups so that functional groups can be anchored to the silica particles, and the higher thermal stability of PEOS that allows the application of the sol-gel method not only for the preparation of materials from solution but also from the melt. The aim of this research study was to establish a correlation between membranes’ morphology and proton conductivity, a property relevant for the application that motivated our research: proton exchange membranes for fuel cells. Fuel cells are electrochemical devices that convert chemical energy to electrical energy. Their efficiency depends on the efficiency of the proton transport through the membranes. We postulate that the proton transport in hybrid membranes occurs at the interface between the organic and the inorganic phase. Therefore our efforts were addressed toward the preparation of SPEEK-silica membranes with the highest possible organic-inorganic surface area. In order to create a composite membrane with nanoscopic phase morphology, we followed a semi-interpenetrating network concept, where the phase separation occurs during film formation while one phase becomes highly crosslinked, i.e. the liquid PEOS is transformed into silica. We investigated the parameters that influence the final morphology in order to be able to control it. The morphology of the membranes was determined by transmission electron microscopy, while the proton mobility was measured by electrochemical impedance spectroscopy and 1H solid state NMR. The membranes were prepared from solutions via centrifugal casting and knife coating. The solutions containing from 10 to 50 wt% of PEOS lead to tough samples containing ca 5 to 33 wt% of silica, mostly transparent or opalescent. Higher inorganic contents produced brittle membranes. The particles size can be controlled by selecting carefully the starting polymers: SPEEK prepared from the sulfonation of poly(ether ether ketone) in concentrated sulfuric acid without the addition of oleum and PEOS from which the low volatile fractions had been removed produced samples with smaller sizes and narrower particles size distribution. Other relevant parameters that influence the morphology are: (i) concentration of the solution; (ii) PEOS content; (iii) presence of a compatibilizer that reduces the SPEEK-PEOS phase separation (iv) presence of acidic doping agent that speeds up the conversion of PEOS; (v) presence of water in solution; (vi) stirring time and temperature; (vii) speed of solvent evaporation; (viii) membrane thickness. The non conductive silica generated from PEOS improves the proton mobility within the samples. It is speculated that this effect is due to: (i) the presence of silanol groups that help water retention on the surface of the silica particles, thus facilitating proton hopping between neighboring particles; (ii) the re-arrangements of the hydrophobic/hydrophilic microstructure of SPEEK during the transformation of PEOS into silica. The proton conductivity was enhanced by the introduction of small amounts of phosphotungstic acid, a heteropolyacid that exhibits high conductivity at room temperature. The formation of an ultrafine morphology was achieved by using a compatibilizer - N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole - that reduced the phase separation between SPEEK and PEOS via acid-base interaction with SPEEK and via condensation reaction with PEOS.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2009

Author dc:creator

• Colicchio, Irene

Contributors dc:contributor

• Möller, Martin

• info:eu-repo/classification/ddc/540
• Membran
• Kompositmembran
• Morphologie <Biologie>
• Sol-Gel-Verfahren
• Chemie
• Membranen
• Protonenleitfähigkeit
• Siliciumdioxid
• sulfoniertes Poly(ether ether keton)
• Nanokomposit
• Poly(ethoxysiloxan)
• membrane
• morphology
• proton conductivity
• silica
• sol-gel
• sulfonated poly(ether ether ketone)
• nanocomposite
• polyethoxysiloxane

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-113597%22

OAI record oai:identifier

https://publications.rwth-aachen.de/oai2d?verb=GetRecord&metadataPrefix=oai_dc&identifier=oai%3Apublications.rwth-aachen.de%3A51290