Shogo Takamuku

Fully aromatic block copolymers for fuel cell membranes with densely sulfonated nanophase domains

Two multiblock copoly(arylene ether sulfone)s with similar block lengths and ion exchange capacities (IECs) were prepared by a coupling reaction between a non-sulfonated precursor block and a highly sulfonated precursor block containing either fully disulfonated diarylsulfone or fully tetrasulfonated tetraaryldisulfone segments. The latter two precursor blocks were sulfonated via lithiation-sulfination reactions whereby the sulfonic acid groups were exclusively placed in ortho positions to the many sulfone bridges, giving these blocks IECs of 4.1 and 4.6 meq·g⁻¹, respectively. Copolymer membranes with IECs of 1.4 meq·g⁻¹ displayed well-connected hydrophilic nanophase domains and had decomposition temperatures at, or above, 300 °C under air. The copolymer with the tetrasulfonated tetraaryldisulfone segments showed a proton conductivity of 0.13 S·cm⁻¹ at 80 °C under fully humidified conditions, and surpassed that of a perfluorosulfonic acid membrane (NRE212) by a factor of 5 at -20 °C over time.

Properties and degradation of hydrocarbon fuel cell membranes: a comparative study of sulfonated poly(arylene ether sulfone)s with different positions of the acid groups

Sulfonated fully aromatic polymers with different positions of the acid groups, but with an identical polymer backbone, have been investigated and compared with respect to their properties as proton-exchange membranes. Three different series of sulfonated poly(arylene ether sulfone)s (SPAES) having the same backbone structure were prepared from 4,4′-dichlorodiphenyl sulfone (DCDPS) and 4,4′-dihydroxybiphenyl (BP), each series comprising three levels of the ion exchange capacity (IEC). The first series of SPAES had the sulfonic acid groups placed in ortho positions to the ether bridges (oeSPAES) and were prepared through post-polymerization sulfonation. The second and third series of SPAES carried the sulfonic acid groups in meta (msSPAES) and ortho (osSPAES) positions to the sulfone bridges, respectively, and were prepared by polycondensations of mixtures of BP, DCDPS and either 3,3′-disulfonated or 2,2′-disulfonated DCDPS, respectively. The latter monomer was synthesized via lithiation–sulfination–oxidation of DCDPS. Analysis of solvent cast membranes showed that the osSPAES copolymers had a high dimensional stability in water up to 100 °C, even at high ionic contents. Moreover, the osSPAES and the msSPAES membranes were better at retaining conductivity at reduced relative humidity than the oeSPAES membranes. Thermogravimetry of the membranes in the acid form under air indicated no significant differences based on the placement of the sulfonic acid groups. The resistance towards radical attack was analyzed by 1H NMR spectroscopy after immersing the membranes in Fentons reagent at 60 °C, and the ability to retain IEC was found to increase in the order oeSPAES < msSPAES < osSPAES. Membrane stability was also studied in an accelerated hydrolysis test by immersing the membranes in 0.1 M aq. HCl at 200 °C in a sealed vessel. The results showed a dramatic loss of sulfonic acid in the oeSPAES membranes, presumably activated by the proximity of the ether bridges. Careful 1H NMR analysis also suggested that the osSPAES polymers degraded by scission of the sulfone bridges. The position of the sulfonic acid groups ortho to the sulfone bridges seemingly destabilized the latter under the severe conditions employed in the test.

Sulfonated poly(arylene ether sulfone) ionomers containing di- and tetrasulfonated arylene sulfone segments

Poly(arylene ether sulfone) (PSU) ionomers containing disulfonated aryl-SO2-aryl and tetrasulfonated aryl-SO2-aryl-aryl-SO2-aryl segments, respectively, were synthesized and studied to establish their structure–property relationships as proton-exchange membranes. High molecular weight PSUs with different distributions of sulfone bridges in the backbone were prepared by nucleophilic aromatic substitution reactions involving 4,4′-dichlorodiphenyl sulfone (DCDPS), 4,4′-bis[(4-chlorophenyl)sulfonyl]-1,1′-biphenyl (BCPSB), 4,4′-isopropylidenediphenol (bisphenol A), and 4,4′-(1,4-phenylenediisopropylidene)bisphenol (bisphenol P). The polymers were sulfonated viametallation and reaction with sulfur dioxide, followed by oxidation of the resulting sulfinates. This procedure allowed the introduction of two sulfonic acid units on electron-deficient aryl rings in ortho positions to each sulfone bridge of the PSUs. Analysis by small angle X-ray scattering of solvent cast membranes showed that ionic clustering was promoted in ionomers containing sulfonated BCPSB residues and flexible bisphenol P residues. The fully sulfonated PSUs had ion-exchange capacities (IECs) of 3.3–4.1 meq g−1 and were water soluble. However, partly sulfonated polymers with IECs of approx. 1.7 meq g−1 showed high proton conductivity at moderate water uptake and decomposed only above 240 °C during heating 1 °C min−1 under air. This work demonstrated that BCPSB residues can be conveniently and fully tetrasulfonated, which opens possibilities to prepare various aromatic copolymers and membranes with locally very high densities of hydrolytically stable sulfonic acid groups.

Block selective grafting of poly(vinylphosphonic acid) from aromatic multiblock copolymers for nanostructured electrolyte membranes

Alternating aromatic multiblock copolymers have been structurally designed to enable selective lithiation and subsequent anionic graft polymerization from only one of the two block types. The multiblock copolymers were prepared by coupling polyfluoroether (PFE) and polysulfone (PSU) precursor blocks under mild conditions. The judicious combination of blocks allowed for block selective lithiation of the PSU blocks to obtain a macroinitiator for anionic polymerization of diethyl vinylphosphonate. The block selective grafting was confirmed by 1H and 19F NMR spectroscopy. After hydrolysis to obtain poly(vinylphosphonic acid) (PVPA) side chains, mechanically stable transparent electrolyte membranes were cast from 1-methyl-2-pyrrolidinone solutions. Analysis by atom force microscopy showed that the copolymers self-assembled to form nanostructured membranes with continuous proton conducting PVPA phase domains. Calorimetry showed separate glass transition temperatures from the PFE and PVPA phases, with the latter increasing with increasing annealing temperatures as a result of anhydride formation. Fully hydrated multiblock copolymer membranes reached proton conductivities above 80 mS cm−1 at 120 °C. The approach of block selective lithiation and modification of aromatic block copolymers can be used as a general strategy to prepare durable and functional nanostructured polymer membranes and materials.

Hypersulfonated polyelectrolytes: preparation, stability and conductivity

Specially tailored polyelectrolytes are becoming important as energy-related materials. Here we explore a synthetic strategy to prepare fully aromatic polymers containing single phenylene rings in the backbone functionalized with four sulfonic acid groups. Thioether bridges of semifluorinated poly(arylene thioether)s were oxidized to sulfone bridges, followed by substitution of all fluorines by NaSH and quantitative oxidation of the resulting thiol groups. This gave poly(arylene sulfone)s containing octasulfonated biphenyl units, reaching ion exchange capacities up to 8 meq g−1 and unprecedented high local sulfonic acid concentrations. These polyelectrolytes are stable up to 300 °C under air and achieve proton conductivities of up to 90 mS cm−1 at 120 °C and 50% relative humidity. Despite the excellent performance of this unique new class of hypersulfonated polymers, our data suggests that incomplete proton dissociation may ultimately limit the conductivity of highly sulfonated polymers.

Enzymatic synthesis of poly(aniline) particles

The template-guided enzymatic polymerization of conducting poly(aniline) particles was investigated. In order to clarify the role of templates in this polymerization, various template polymers such as poly(ethylene oxide) (PEO), poly(vinyl alcohol) (PVA). poly(vinyl pyrrolidone) (PVP), and poly(sodium 4-styrenesulfonate) (SPS) were used in this study. The enzymatic synthesis of poly(aniline) was successful with a stoichiometric amount of hydrogen peroxide and a catalytic amount of the enzyme (horseradish peroxidase). The chemical structures of template polymers provided difference in particle size of obtained poly(aniline).