Styrene grafted ethylene chlorotrifluoroethylene (ECTFE‐g‐PSSA) protonic membranes: Preparation,characterization, and transport mechanism

The main goal of the present work is the development of partially fluorinated, low‐cost proton exchange membranes. The styrene grafted onto commercial ethylene chlorotrifluoroethylene (ECTFE) membranes using solution grafting technique, and after that the membranes were sulfonated. Diluting styrene on ECTFE with a solvent mixture of methanol plus methylene chloride (1:1) was highly effective in promoting the grafting reaction as indicated by the increase in the degree of grafting (DG) to 21.3% compared to other solvents. The DG in ECTFE membranes increased with an increase in the monomer concentration up to 60% and then declined. Fourier transform infrared spectroscopic analysis confirmed grafting and sulfonation onto ECTFE films. The maximum value of proton conductivity for ECTFE‐g‐PSSA film with DG = 21.3% was observed to be 141 mS cm⁻¹, which is also higher than those of Nafion 212 membrane. Furthermore, the activation energy of ECTFE‐g‐PSSA membranes was obtained ranging from 8.27 to 9.726 kJ mol⁻¹. So both proton transport mechanisms (hopping and vehicle) have been commonly accepted. The mobility of the charge carriers calculated from proton conductivity data has robust dependence on the grafting yield and temperature. Moreover, the tensile strength and elongation at break ratio decreases with the increase in DG. The water and methanol uptakes increase up to 0.97% and 30%, respectively, for the highest DG value. Finally, the ECTFE‐g‐PSSA has lower cost and higher conductivity they could be better used instead of Nafion in direct methanol fuel cells.     

ASAXS study of styrene‐grafted sulfonated poly(vinylidene fluoride) membranes

Small‐angle and wide‐angle X‐ray scattering and anomalous small‐angle X‐ray scattering were used to investigate proton‐conducting membranes prepared by radiation‐induced styrene grafting and sulfonation of commercial poly(vinylidene fluoride) (PVDF‐g‐PS) films. The membranes retain the lamellar and highly oriented structure of the original PVDF films even through excessive grafting and sulfonation. The sulfonate groups aggregate in the central part of the amorphous layers, where they form a weakly ordered structure that does not show any preferred orientation. This structure is suggested to be lamellar with alternate metal‐sulfonated hydrate and PVDF‐g‐PS layers. The lamellar period is 15.1 Å. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1734–1748, 2000     

Sorption and diffusion of methanol and water in PVDF‐g‐PSSA and Nafion® 117 polymer electrolyte membranes

Sorption and diffusion properties of poly(vinylidene fluoride)‐graft‐poly(styrene sulfonic acid) (PVDF‐g‐PSSA) and Nafion® 117 polymer electrolyte membranes were studied in water/methanol mixtures. The two types of membranes were found to have different sorption properties. The Nafion 117 membrane was found to have a maximum in‐solvent uptake around 0.4 to 0.6 mole fraction of methanol, while the PVDF‐g‐PSSA membranes took up less solvent with increasing methanol concentration. The proton NMR spectra were recorded for membranes immersed in deuterated water/methanol mixtures. The spectra showed that the hydroxyl protons inside the membrane exhibit resonance lines different from the resonance lines of hydroxyl protons in the external solvent. The spectral features of the lines of these internal hydroxyl groups in the membranes were different in the Nafion membrane compared with the PVDF‐g‐PSSA membranes. Diffusion measurements with the pulsed field gradient NMR (PFG‐NMR) method showed that the diffusion coefficient of the internal hydroxyl groups in the solvent immersed Nafion membrane mirrors the changes in the diffusion coefficients of hydroxyl and methyl protons in the external solvent. For the PVDF‐g‐PSSA membranes, a decrease in the diffusion coefficient of the internal hydroxyl protons was seen with increasing methanol concentration. These results indicate that the morphology and chemical structure of the membranes have an effect on their solvent sorption and diffusion characteristics. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3277–3284, 2000     

Phase separation and crystallinity in proton conducting membranes of styrene grafted and sulfonated poly(vinylidene fluoride)

A series of proton exchange membranes have been prepared by the preirradiation grafting method. Styrene was grafted onto a matrix of poly(vinylidene fluoride) (PVDF) after electron beam irradiation. Part of the samples was crosslinked with divinylbenzene (DVB) or bis(vinylphenyl)ethane (BVPE). Subsequent sulfonation gave membranes grafted with poly(styrene sulfonic acid) and marked PVDF‐g‐PSSA. It was found that the intrinsic crystallinity of the matrix decreased in both the grafting and the sulfonation reaction in all the membranes. The graft penetration and the ion conductivity are influenced strongly by the crosslinker. The ion conductivity is considerably lower in crosslinked membranes than in noncrosslinked ones. Generally, the mechanical strength decreases with crosslinking. The membranes show a regular phase separated structure in which the sulfonated grafts are incorporated in the amorphous parts of the matrix polymer. The phase separated domains are small, of the order of magnitude of 100–250 nm. These were resolved on transmission electron micrographs and on atomic force images but could not be resolved with microprobe Raman spectroscopy. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1741–1753, 1999     

Dependence of methanol permeability on the nature of water and the morphology of graft copolymer proton exchange membranes

Novel polymers with controlled microstructures were prepared and studied to further advance the understanding of structure–property relationships of proton conducting membranes. PAN‐g‐macPSSA membranes, which contained poly(styrenesulfonic acid) (PSSA) grafts of defined graft length, are compared with PVDF‐g‐PSSA membranes, prepared by radiation‐grafting, and Nafion® 117. The intrinsic properties of PAN‐g‐macPSSA membranes are insensitive to the macromonomer graft length but are highly dependent on the ion exchange capacities (IEC). Increasing the IEC increases the content of free water absorbed by the membrane. Self‐diffusion coefficients of water in water‐swollen PAN‐g‐macPSSA were found to be similar to that of N117, despite PAN‐g‐macPSSA's higher water content. Of the polymers studied, PAN‐g‐macPSSA exhibited the lowest methanol permeability, which is explained on the basis of it containing a more tortuous ionic network. Methanol permeability decreased with decreasing volume of free water. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2240–2252, 2006     

Preparation and characterization of PVDF‐based blend membranes as polymer electrolyte membranes in fuel cells: Study of factor affecting the proton conductivity behavior

There is a huge demand especially for polyvinylidene fluoride (PVDF) and its copolymers to provide high performance solid polymer electrolytes for use as an electrolyte in energy supply systems. In this regard, the blending approach was used to prepare PVDF‐based proton exchange membranes and focused on the study of factor affecting the ir proton conductivity behavior. Thus, a series of copolymers consisting of poly (methyl methacrylate) (PMMA), polyacrylonitrile (PAN), and poly(2‐acrylamido‐2‐methyl‐l‐propanesulfonic acid) (PAMPS) as sulfonated segments were synthesized and blended with PVDF matrix in order to create proton transport sites in PVDF matrix. It was found that addition of PMMA‐co‐PAMPS and PAN‐co‐PAMPS copolymers resulted in a significant increase in porosity, which favored the water uptake and proton transport at ambient temperature. Furthermore, crystallinity degree of the PVDF‐based blend membranes was increased by addition of the related copolymers, which is mainly attributed to formation of hydrogen bonding interaction between PVDF matrix and the synthesized copolymers, and led to a slight decrease in proton conductivity behavior of blend membranes. From impedance data, the proton conductivity of the PVDF/PMMA‐co‐PAMPS and PVDF/PAN‐co‐PAMPS blend membranes increases to 10 and 8.4 mS cm⁻¹ by adding only 50% of the related copolymer (at 25°C), respectively. Also, the blend membranes containing 30% sulfonated copolymers showed a power density as high as 34.30 and 30.10 mW cm⁻² at peak current density of 140 and 79.45 mA cm⁻² for the PVDF/PMMA‐co‐PAMPS and PVDF/PAN‐co‐PAMPS blend membranes, respectively. A reduction in the tensile strength was observed by the addition of amphiphilic copolymer, whereas the elongation at break of all blend membranes was raised.     

Effect of the initial matrix material on the structure of radiation‐grafted ion‐exchange membranes: Wide‐angle and small‐angle X‐ray scattering studies

Seven different fluoropolymer films were used as matrix materials for radiation‐grafted ion‐exchange membranes. The crystallinity and preferred orientation of these membranes were studied with wide‐angle X‐ray scattering, and the lamellar structure of the membranes was examined with small‐angle X‐ray scattering. The crystallinity of poly(vinylidene fluoride) (PVDF)‐based matrix materials varied between 57 and 40%, and the crystallinity of the sulfonated samples varied between 34 and 23%. The lamellar periods of PVDF‐based matrix materials were about 115 Å, and the lamellar periods of poly(ethylene‐alt‐tetrafluoroethylene) and poly(tetrafluoroethylene‐co‐hexafluoropropylene) were 250 and 212 Å, respectively. When the samples were grafted, the lamellar periods increased. Correlation function analysis showed very clearly that the long‐range order decreased because of grafting and sulfonation processes. For those samples that showed good proton conductivity, the lamellar period also increased because of sulfonation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1539–1555, 2002     

Proton conducting membranes based on poly(vinyl chloride) graft copolymer electrolytes

The direct preparation of proton conducting poly(vinyl chloride) (PVC) graft copolymer electrolyte membranes using atom transfer radical polymerization (ATRP) is demonstrated. Here, direct initiation of the secondary chlorines of PVC facilitates grafting of a sulfonated monomer. A series of proton conducting graft copolymer electrolyte membranes, i.e. poly(vinyl chloride)‐g‐poly(styrene sulfonic acid) (PVC‐g‐PSSA) were prepared by ATRP using direct initiation of the secondary chlorines of PVC. The successful syntheses of graft copolymers were confirmed by ¹H‐NMR and FT‐IR spectroscopy. The images of transmission electron microscopy (TEM) presented the well‐defined microphase‐separated structure of the graft copolymer electrolyte membranes. All the properties of ion exchange capacity (IEC), water uptake, and proton conductivity for the membranes continuously increased with increasing PSSA contents. The characterization of the membranes by thermal gravimetric analysis (TGA) also demonstrated their high thermal stability up to 200°C. The membranes were further crosslinked using UV irradiation after converting chlorine atoms to azide groups, as revealed by FT‐IR spectroscopy. After crosslinking, water uptake significantly decreased from 207% to 84% and the tensile strength increased from 45.2 to 71.5 MPa with a marginal change of proton conductivity from 0.093 to 0.083 S cm^?1, which indicates that the crosslinked PVC‐g‐PSSA membranes are promising candidates for proton conducting materials for fuel cell applications. Copyright © 2008 John Wiley & Sons, Ltd.     

Cross‐linked poly(aryl ether sulfone) membranes for direct methanol fuel cell applications

Partially sulfonated poly(aryl ether sulfone) (PESS) was synthesized and methacrylated via reaction with glycidyl methacrylate (PESSGMA) and cross‐linked via radical polymerization with styrene and vinyl‐phosphonic acid (VPA). The chemical structures of the synthesized pre‐polymers were characterized via FTIR and ¹H NMR spectroscopic methods and molecular weight was determined via GPC. Membranes of these polymers were prepared via solution casting method. The crosslinking of the PESS polymer reduced IEC, proton conductivity, swelling in water, and methanol permeability of the membranes while increasing the modulus and the glass transition temperature. However, the introduction of the VPA comonomer increased the proton conductivity while maintaining excellent resistance to methanol cross‐over, which was significantly higher as compared with both PESS and the commercial Nafion membranes. Membranes of PESSGMA copolymers incorporating VPA, exhibited proton conductivity values at 60 °C in the range of 16–32 mS cm⁻¹ and methanol permeability values in the range of 6.52 × 10⁻⁹ – 1.92 × 10⁻⁸ cm² s⁻¹. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 558–575     

Synthesis and properties of novel sulfonated polyimides containing binaphthyl groups as proton‐exchange membranes for fuel cells

A novel sulfonated diamine monomer, 2,2′‐bis(p‐aminophenoxy)‐1,1′‐binaphthyl‐6,6′‐disulfonic acid (BNDADS), was synthesized. A series of sulfonated polyimide copolymers containing 30–80 mol % BNDADS as a hydrophilic component were prepared. The copolymers showed excellent solubility and good film‐forming capability. Atomic force microscopy phase images clearly showed hydrophilic/hydrophobic microphase separation. The relationship between the proton conductivity and degree of sulfonation was examined. The sulfonated polyimide copolymer with 60 mol % BNDADS showed higher proton conductivity (0.0945–0.161 S/cm) at 20–80 °C in liquid water. The membranes exhibited methanol permeability from 9 × 10^?8 to 5 × 10^?7 cm²/s at 20 °C, which was much lower than that of Nafion (2 × 10^?6cm²/s). The copolymers were thermally stable up to 300 °C. The sulfonated polyimide copolymers with 30–60 mol % BNDADS showed reasonable mechanical strength; for example, the maximum tensile strength at break of the sulfonated polyimide copolymer with 40 mol % BNDADS was 80.6 MPa under high moisture conditions. The optimum concentration of BNDADS was found to be 60 mol % from the viewpoint of proton conductivity, methanol permeability, and membrane stability. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 222–231, 2007     

Partially Sulfonated Polystyrene and Poly(2,6‐dimethyl‐1,4‐phenylene oxide) Blend Membranes for Fuel Cells

Summary: Based on Flory–Huggins parameters (χ), the miscibility and the effect of morphological change on proton conductivity and methanol permeability of partially sulfonated polystyrene (SPS) and partially sulfonated poly(2,6‐dimethyl‐1,4‐phenylene oxide) (SPPO), having an identical ion exchange capacity, were investigated. When 50 wt.‐% of SPPO was blended, both the proton conductivity and methanol permeability had the highest values, which resulted from the change of amorphous domains and the hydrogen bonding between the two ionomers.

The proton conductivities, water uptake and methanol permeability for the SPPO/SPS blend membranes studied here. The membranes with 50 wt.‐% SPPO clearly showed the greatest increase in these properties.     

Synthesis and Characterization of Sulfonated Polyimides Containing Trifluoromethyl Groups as Proton Exchange Membranes

A novel sulfonated diamine, 4,4′‐bis(4‐amino‐3‐trifluoromethylphenoxy) biphenyl 3,3′‐disulfonic acid (F‐BAPBDS), was successfully synthesized by nucleophilic aromatic substitution of 4,4′‐dihydroxybiphenyl with 2‐chloro‐5‐nitrobenzotrifluoride, followed by reduction and sulfonation. A series of sulfonated polyimides of high molecular weight (SPI‐x, x represents the molar percentage of the sulfonated monomer) were prepared by copolymerization of 1,4,5,8‐naphathlenetetracarboxylic dianhydride (NTDA) with F‐BAPBDS and nonsulfonated diamine. Flexible and tough membranes of high mechanical strength were obtained by solution casting and the electrolyte properties of the polymers were intensively investigated. The copolymer membranes exhibited excellent oxidative stability due to the introducing of the CF₃ groups. The SPI membranes displayed desirable proton conductivity (0.52×10⁻¹–0.97×10⁻¹ S·cm⁻¹) and low methanol permeability (less than 2.8×10⁻⁷ cm²·s⁻¹). The highest proton conductivity (1.89×10⁻¹ S·cm⁻¹) was obtained for the SPI‐90 membrane at 80°C, with an IEC of 2.12 mequiv/g. This value is higher than that of Nafion 117 (1.7×10⁻¹ S·cm⁻¹). Furthermore, the hydrolytic stability of the obtained SPIs is better than the BDSA and ODADS based SPIs due to the hydrophobic CF₃ groups which protect the imide ring from being attacked by water molecules, in spite of its strong electron‐withdrawing behaviors.     

Synthesis and Characterization of Phenolphthalein‐Containing Sulfonated Poly(arylene ether phosphine oxide)s by Direct Polycondensation for Proton Exchange Membrane

A series of novel phenolphthalein‐containing sulfonated poly(arylene ether phosphine oxide)s (sPAEPP) with various sulfonation degrees were synthesized by direct polycondensation. The structure of sPAEPP was confirmed by ¹H‐NMR, ¹³C‐NMR, and IR spectroscopy. The high‐molecular weight of these polymers was determined by gel permeation chromatography (GPC). The transparent, tough, and flexible membranes could be achieved by solution casting. The macroscopic properties and microstructure of the obtained membranes were investigated in detail. The results showed that these sPAEPP membranes displayed excellent properties in terms of swelling, proton conductivity, and methanol permeability. For example, sPAEPP‐100 membrane exhibited an appropriate water uptake of 33.1%, a swelling ratio of only 11.7% (lower than 20.1% of Nafion 117), a proton conductivity of 0.11 S cm^?1 (similar to that of Nafion 117) at 80 °C, and a methanol permeability of 4.82 × 10^?7 cm² s^?1. Meanwhile, it also presented outstanding oxidative stability. Atomic force microscope (AFM) micrographs showed that the hydrophilic domains of the sPAEPP‐100 membrane formed connected and narrow ionic channels, which contributed to its high proton conductivity and good dimensional stability. As a result, sPAEPP‐100 membrane displays excellent application prospect for fuel cells. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 1097–1104     

Investigation of proton conductivity of anhydrous proton exchange membranes prepared via grafting vinyltriazole onto alkaline‐treated PVDF

This work uses a simple “grafting through” approach in the preparation of anhydrous poly(vinylidene fluoride) (PVDF)‐g‐PVTri polymer electrolyte membranes (PEMs). Alkaline‐treated PVDF was used as a macromolecule in conjunction with vinyltriazole in the graft copolymerization. The obtained polymer was subsequently doped with triflic acid (TA) at different stoichiometric ratios with respect to triazole units and the anhydrous PEMs (PVDF‐g‐PVTri‐(TA)_x) were prepared. All samples were characterized by FTIR and ¹H NMR. The composition of PVDF‐g‐PVTri was determined by energy dispersive spectroscopy. Thermal properties of the membranes were examined by thermogravimetric analysis and differential scanning calorimetry. The surface roughness and morphology of the membranes were studied using atomic force microscopy, X‐ray diffraction, and scanning electron microscopy. PVDF‐g‐PVTri‐(TA)₃ (C3‐TA₃) with a degree of grafting of 47.22% showed a maximum proton conductivity of 0.09 S cm^?1 at 150 °C and anhydrous conditions. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1885–1897     

Synthesis of proton‐conducting membranes by the utilization of preirradiation grafting and atom transfer radical polymerization techniques

The atom transfer radical polymerization (ATRP) of styrene onto poly(vinylidene fluoride)‐graft‐poly(vinylbenzyl chloride) (PVDF‐g‐PVBC) membranes was investigated. Novel membranes were designed for fuel‐cell applications. The benzyl chloride groups in the PVDF‐g‐PVBC membranes functioned as initiators, and a Cu‐based catalytic system with the general formula Cu(n)X_n/ligand [where X is Cl or Br and the ligand is 2,2′‐bipyridyl (bpy)] was employed for the ATRP. In addition, 10 vol % dimethylformamide was added for increased solubility of the catalyst complex in styrene. The system was homogeneous, except for the membrane, when the initiator/copper halide/ligand/monomer molar ratio was 1/1/3/500. As anticipated, the fastest polymerization rate of styrene was observed with the copper bromide/bpy‐based catalyst system. The reaction rate was strongly temperature‐dependent within the studied temperature interval of 100–130 °C. The degree of grafting increased linearly with time, thereby indicating first‐order kinetics, regardless of the polymerization temperature. Furthermore, 120 °C was the maximum polymerization temperature that could be used in practice because the membrane structure was destroyed at higher temperatures. The degree of styrene grafting reached 400% after 3 h at 120 °C. Such a high degree of grafting could not be reached with conventional uncontrolled radiation‐induced grafting methods because of termination reactions. On the basis of an Arrhenius plot, the activation energy for the homogeneous ATRP of styrene was 217 kJ/mol. The prepared membranes became proton‐conducting after sulfonation of the polystyrene grafts. The highest conductivity measured for the prepared membranes was 70 mS/cm, which is comparable to the values normally measured for commercial Nafion membranes. The scanning electron microscopy/energy‐dispersive X‐ray results showed that the membranes had to be grafted through the matrix with both PVBC and polystyrene to become proton‐conducting after sulfonation. In addition, PVDF‐g‐[PVBC‐g‐(styrene‐block‐tert‐butyl acrylate)] membranes were also synthesized by ATRP. © 2002 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 591–600, 2002; DOI 10.1002/pola.10146     

Nanofiltration membranes based on poly(vinylidene fluoride‐co‐chlorotrifluoroethylene)‐graft‐poly(styrene sulfonic acid)

Graft copolymers comprising poly(vinylidene fluoride‐co‐chlorotrifluoroethylene) backbone and poly(styrene sulfonic acid) side chains, i.e. P(VDF‐co‐CTFE)‐g‐PSSA were synthesized using atom transfer radical polymerization (ATRP) for composite nanofiltration (NF) membranes. Direct initiation of the secondary chlorinated site of CTFE units facilitates grafting of PSSA, as revealed by FT‐IR spectroscopy. The successful “grafting from” method and the microphase‐separated structure of the graft copolymer were confirmed by transmission electron microscopy (TEM). Wide angle X‐ray scattering (WAXS) also showed the decrease in the crystallinity of P(VDF‐co‐CTFE) upon graft copolymerization. Composite NF membranes were prepared from P(VDF‐co‐CTFE)‐g‐PSSA as a top layer coated onto P(VDF‐co‐CTFE) ultrafiltration support membrane. Both the rejections and the flux of composite membranes increased with increasing PSSA concentration due to the increase in SO₃H groups and membrane hydrophilicity, as supported by contact angle measurement. The rejections of NF membranes containing 47 wt% of PSSA were 83% for Na₂SO₄ and 28% for NaCl, and the solution flux were 18 and 32 L/m² hr, respectively, at 0.3 MPa pressure. Copyright © 2008 John Wiley & Sons, Ltd.     

Highly proton conducting proton‐exchange membranes based on fluorinated poly(arylene ether ketone)s with octasulfonated segments

A new bisphenol monomer containing a pair of electron‐rich tetra‐arylmethane units was designed and synthesized. Based on this monomer, along with commercial 4,4′‐(hexafluoroisopropylidene)diphenol A and 4,4′‐difluorobenzophenone, a series of novel poly(arylene ether ketone)s containing octasulfonated segments of varying molar percentage (x) (6F‐SPAEK‐x) were successfully synthesized by polycondensation reactions, followed by sulfonation. Tough, flexible, and transparent membranes, exhibiting excellent thermal stabilities and mechanical properties were obtained by casting. 6F‐SPAEK‐x samples exhibited appropriate water uptake and swelling ratios at moderate ion exchange capacities (IECs) and excellent proton conductivities. The highest proton conductivity (215 mS cm⁻¹) is observed for hydrated 6F‐SPAEK‐15 (IEC = 1.68 meq g⁻¹) at 100 °C, which is more than 1.5 times that of Nafion 117. Furthermore, the 6F‐SPAEK‐10 membrane exhibited comparable proton conductivity (102 mS cm⁻¹) to that of Nafion 117 at 80 °C, with a relatively low IEC value (1.26 meq g⁻¹). Even under 30% relative humidity, the 6F‐SPAEK‐20 membrane (2.06 meq g⁻¹) showed adequate conductivity (2.1 mS cm⁻¹) compared with Nafion 117 (3.4 mS cm⁻¹). The excellent comprehensive properties of these membranes are attributed to well‐defined nanophase‐separated structures promoted by strong polarity differences between highly ionized and fluorinated hydrophobic segments. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 25–37     

Synthesis of sulfonated poly(imidoaryl ether sulfone) membranes for polymer electrolyte membrane fuel cells

New sulfonated poly(imidoaryl ether sulfone) copolymers derived from sulfonated 4,4′‐dichlorodiphenyl sulfone, 4,4′‐dichlorodiphenyl sulfone, and imidoaryl biphenol were evaluated as polymer electrolyte membranes for direct methanol fuel cells. The sulfonated membranes were characterized with Fourier transform infrared spectroscopy, thermogravimetric analysis, and proton nuclear magnetic resonance spectra. The state of water in the membranes was measured with differential scanning calorimetry, and the existence of free water and bound water was discussed in terms of the sulfonation level. The 10 wt % weight loss temperatures of these copolymers were above 470 °C, indicating excellent thermooxidative stability to meet the severe criteria of harsh fuel‐cell conditions. The proton conductivities of the membranes ranged from 3.8 × 10^?2 to 5 × 10^?2 S/cm at 90 °C, depending on the degree of sulfonation. The sulfonated membranes maintained the original proton conductivity even after a boiling water test, and this indicated the excellent hydrolytic stability of the membranes. The methanol permeabilities ranged from 1.65 × 10^?8 to 5.14 × 10^?8 cm²/s and were lower than those of other conventional sulfonated ionomer membranes, particularly commercial perfluorinated sulfonated ionomer (Nafion). The properties of proton and methanol transport were discussed with respect to the state of water in the membranes. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5620–5631, 2005     

Synthesis and characterization of sulfonated naphthalenic polyimides based on 4,4′-diaminodiphenylether-2,2′-disulfonic acid and bis[4-(4-aminophenoxy)phenylhexafluoropropane] for fuel cell applications

The paper describes the synthesis and characterization of sulfonated polyimides based on 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA), 4,4′-diaminodiphenylether-2,2′-disulfonic acid (ODADS) and (bis[4-(4-aminophenoxy)phenylhexafluoropropane] (BDAF)). Several copolymer samples were prepared by varying the molar ratio of ODADS: BDAF (0.5:1.50, 0.75:1.25, 1:1 and 1.50: 0.5) in the initial monomer feed. Structural characterization of the copolymers was done using FT-IR and ¹H NMR. ¹H NMR was also used to calculate the copolymer composition. Thermal characterization was done using thermogravimertry and dynamic mechanical analysis. Polymer films were prepared by solution casting using m-cresol as solvent. The membranes thus prepared were characterized for water uptake, water stability, methanol permeability and proton conductivity. The obtained sulfonated polyimides (SPI’s) had proton conductivities in the range of 0.137-3.94 mS/cm. SPI’s with 50% degree of sulfonation had proton conductivity comparable to that of Nafion with methanol permeability lower than that of Nafion. It was found that the degree of sulfonation of polyimide had a large effect on the thermal stability, water uptake, ion-exchange capacity and proton conductivity.     

Synthesis of poly(arylene ether sulfone)‐block‐sulfonated polybutadiene: the selective post‐sulfonation of block copolymers as proton exchange membranes

A series of poly(arylene ether sulfone)‐block‐sulfonated polybutadiene (PAES‐b‐sPB) with different ion exchange capacities (IECs) were synthesized and evaluated as proton exchange membranes (PEMs) for possible applications in fuel cells. These sulfonated block copolymers were synthesized via condensation reaction between modified PAES and PB prepolymers, followed by selective post‐sulfonation of PB blocks using acetyl sulfate as the sulfonating reagent. The sulfonic groups were only attached onto PB blocks due to the high reactivity of double bonds to acetyl sulfate. The success of synthesis and selective post‐sulfonation were all confirmed by the Fourier transform infrared (FT‐IR) and nuclear magnetic resonance (NMR) spectra. PAES‐b‐sPB had good film‐forming ability and thermal stability. Mechanical properties of membranes varied with the sulfonation. The presence of sulfonic groups increased the tensile strength and Young's modulus but decreased the elongation at break. Transmission electron microscopy (TEM) images showed large ionic aggregates in membranes. Phase separation as well as the interconnected sulfonate groups which only localized on flexible PB blocks led to these ionic domains. The proton conductivity increased with the increasing IEC and temperature. With relatively low IEC, most membranes still exhibited sufficient proton conductivity. The above results indicated this strategy could be a prospective choice to prepare novel PEMs. Copyright © 2010 John Wiley & Sons, Ltd.