Partially fluorinated poly(arylene ether)s: Investigation of the dependence of monomeric structures on polymerisability and degradation during sulfonation
Graphical abstract
Introduction
Against the background of limitations of natural resources, import dependencies of fossil fuels and global environmental problems, the fuel cell technology is thought to be one of the future power sources. Polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) which are based on a proton-conductive membrane material as one key component have attracted much attention in automotive and portable electronic applications in recent years [1]. Many different approaches have been made to overcome the main drawbacks of poly(perfluoroalkyl)sulfonic acids (such as Nafion®) which are reviewed in the literature [2], [3], [4], [5]. These materials show excellent chemical, mechanical and thermal stability and high proton conductivity, but suffer from high methanol permeability (which is relevant for the DMFC mode) and a significant drop in proton conductivity at temperature above 80 °C because of the increased water evaporation. Different strategies have been used in order to overcome the crucial shortcomings of the poly(perfluoroalkyl)sulfonic acids including for example the reinforcement by a porous PTFE matrix [6], [7], the incorporation of hygroscopic oxides [8], [9] (like SiO2, ZrO2 and TiO2) or of inorganic solid proton conductors such as layered metalIV phosphates (e.g. α- or γ-zirconium phosphates or phosphonates) [10]. Much more effort has been invested into the development of alternative proton-conducting membrane materials including many different functionalised polymer families, e.g. sulfonated polysiloxanes [11], polyphosphazenes [12], [13], styrene-grafted (partially) fluorinated polyolefins [14] and aromatic main-chain polymers. The last polymer class covers different chemical structures and a detailed description would go beyond the scope of this article so that the authors would like to refer to review articles for more information [2], [3], [4], [5]. Common structural moieties of aromatic main-chain polymer are phenylene rings connected by chemical bonds and/or linkage groups. Typical examples are sulfonated poly(phenylene ethers)s [15], poly(ether ether ketone)s (sPEEK) [16], poly(ether sulfone)s (sPES) [17], poly(sulfone)s [18] and polyimides (sPI) [19], which are accessible by sulfonation of the corresponding main-chain polymers. Although the sulfonation can be accompanied by polymer backbone degradation as side reaction (depending on the substitution pattern and the nature of chemical bonds and linkage groups in the polymer backbone) [3], [20], this route offers the advantage of an easy and economically reasonable working-up procedure (sulfonation, precipitation, filtration) [21]. The focus of the present work lies on the preparation of partially fluorinated polymeric structures in the first step and of their sulfonated analogues by postsulfonation in the second step as far as the polymerisation led to polymers with a sufficiently high polymerisation degree (Mn > 10,000 Da). In principle fluorinated ionomers are expected to have superior stability and acid strength than their nonfluorinated analogues [20], [21]. Apart from developing partially fluorinated ionomers for fuel cell application [1], [22], [23], there is a strong interest in the preparation of the basic poly(aryl)s for example for polymer optical waveguides [24], [25] for detection of fluoride ions [26], and gas separation membrane materials [27]. These partially fluorinated poly(aryl)s are synthetically accessible by step-growth polycondensation methods which can be realized by reacting an activated difluoro-, dichloro- or dibromoaryl monomer with a diphenol.
The Pd-catalyzed [28] or CuCl-catalyzed [29] Ullmann biaryl polycondensation shows limited applicability to the targeted partially fluorinated polymers. The first method only offers an advantage for the coupling of sterically hindered monomers. The second method is limited to monomers without any Csp2-F bonds due to the applied high temperatures and the risk of side reactions such as branching or eventually cross-linking.
Therefore, the preferred method for the preparation of the targeted partially fluorinated poly(aryl)s is the nucleophilic displacement polycondensation reaction between a difluoro- (or dichloro)aryl monomer and a diphenol in the presence of a deprotonating reagent under mild conditions. This reaction is normally carried out in dipolar-aprotic solvents such as N,N-dimethylacetamide (DMAc) or N-methylpyrrolidinone (NMP) with potassium carbonate as the base without [30] or with water entrainer [31] or in the presence of molecular sieves to remove water formed during the deprotonation reaction of the diphenol [32]. The use of calcium hydride in combination with caesium fluoride as a catalyst can be advantageous in some cases due to the reduction of water formation during the deprotonation step and the concomitant precipitation of CaF2 which facilitates the nucleophilic exchange of fluoride by phenolate during the polycondensation [33].
The main part of this contribution deals with the systematic investigation of how the monomers polymerisability depends on their chemical nature.
The achievable molecular weight will serve as a measure for the polymerisability of various monomers. With this aim, variations in the (bi)phenylene moiety (Scheme 1), in the bisphenol moiety (Scheme 2) as well as fluorine content variations (Scheme 3) were investigated and analyzed.
Providing that the polymers showed good film-forming properties, they were sulfonated (using common sulfonating reagents like concentrated or fuming sulfuric acid [20], [21], chlorosulfonic acid [34] or trimethylsilyl chlorosulfonate) [35], and investigated further in terms of their properties relevant for use as polymer electrolyte membrane in fuel cells.
Section snippets
Instrumentation
NMR spectra were recorded using a Bruker Avance 400 spectrometer at a resonance frequency of 400 MHz or 250 MHz for 1H, 188 MHz for 19F and 250 MHz for 1H-13C HSQC (heteronuclear single quantum coherence).
The molecular weight distributions of the polymers (Mn, PDI) were determined by gel permeation chromatography (GPC) using an Agilent Technology GPC system (Series 1200) coupled with a viscosity detector (PSS ETA-2010) and a refractive index detector (Shodex RI71). A set of three PSS GRAM columns
Structure variations in the (bi)phenylene moiety
Variations in the phenylene (1a–4a) and in the biphenylene (5a–7a) were made by reacting 2,2-bis(4-hydroxyphenyl)hexafluoropropane (BHPHFP) with the corresponding difluoroaryl or difluorobiaryl monomers. Although the nucleophilicity of BHPHFP is less than that of the corresponding nonfluorinated species, it was chosen as model compound in order to avoid any weak points for potential radical attack in the final poly(arylene ether)s. In Table 1, an overview of the molecular weight distribution,
Conclusion
Partially fluorinated poly(arylene ether) backbones with various combinations of different groups Z and L (cf. Scheme 1, Scheme 2, Scheme 3) were synthesized, characterized and evaluated in terms of suitability for postsulfonation and in terms of application as fuel cell membrane (component).
The development of membrane materials for polymer electrolyte membrane fuel cells means that all of the following requirements have to be met.
- (i)
Appropriate monomers yield soluble polymers in high conversion
Acknowledgements
The authors would like to thank the Landesstiftung Baden-Württemberg for financial support of this research effort in the project “Direktmethanol-Mikrobrennstoffzelle mit neuen MEA”. They also would like to thank Dr. Viktor Gogel for MEA preparation and DMFC testing, and Inna Kharitonova and Galina Schumski for their great support in membrane characterization.
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