Synthesis and characterization of proton‐conducting copolyimides bearing pendant sulfonic acid groups

A series of sulfonated copolyimides (co‐SPIs) bearing pendant sulfonic acid groups were synthesized from 1,4,5,8‐naphthalenetetracarboxylic dianhydride (NTDA), bis(3‐sulfopropoxy) benzidines (BSPBs), and common nonsulfonated diamines via statistical or sequenced polycondensation reactions. Membranes were prepared by casting their m‐cresol solutions. The co‐SPI membrane had a microphase‐separated structure composed of hydrophilic and hydrophobic domains, but the connecting behavior of hydrophilic domains was different from that of the homo‐SPIs. The co‐SPI membranes displayed clear anisotropic membrane swelling in water with negligibly small dimensional changes in the plane direction of the membrane. With water uptake values of 39–94 wt %, they showed dimensional changes in membrane thickness of about 0.11–0.58, which were much lower than those of homo‐SPIs. The proton conductivity σ values of co‐SPI membranes with ion exchange capacity values ranging from 1.95–2.32 meq/g increased sigmoidally with increasing relative humidity. They displayed σ values of 0.05–0.16 S/cm at 50 °C in liquid water. Increasing temperature up to 120 °C resulted in further increase in proton conductivity. The co‐SPI membranes showed relatively good conductivity stability during the aging treatment in water at 100 °C for 300 h. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1545–1553, 2005     

Effects of tetracarboxylic dianhydrides on the properties of sulfonated polyimides

A series of sulfonated polyimides (SPIs) were synthesized from a sulfonated diamine of 4,4′‐bis(4‐aminophenoxy) biphenyl‐3,3′‐disulfonic acid (BAPBDS), common nonsulfonated diamines, and various tetracarboxylic dianhydrides including 1,4,5,8‐naphthalene tetracarboxylic dianhydride (NTDA), 3,4,9,10‐perylene tetracarboxylic dianhydride (PTDA), 4,4′‐binaphthyl‐1,1′,8,8′‐tetracarboxylic dianhydride (BTDA), 4,4′‐ketone dinaphthalene 1,1′,8,8′‐tetracarboxylic dianhydride (KDNTDA), and isophthatic dinaphthalene 1,1′,8,8′‐tetracarboxylic dianhydride (IPNTDA). Their membrane properties were investigated to clarify the effects of the dianhydrides. They displayed reasonably high mechanical properties, thermal stability, and proton conductivity. The dianhydrides with flexible and non‐coplanar structure (IPNTDA > KDNTDA > BTDA) led to the better solubility of the SPIs than those with rigid and coplanar one (NTDA, PTDA). The dianhydride with the smaller molecular weight led to the larger value of the number of sorbed water molecules per sulfonic acid group (λ) in membrane, that is, NTDA (λ: 17) > PTDA (15) > BTDA (14) > KDNTDA (12) > IPNTDA (10), and as a result let to the larger proton conductivity in water. All of the BAPBDS‐based SPIs showed the anisotropy in membrane swelling and in proton conductivity, of which the degree hardly depended on the dianhydride moieties. The water stability of SPI membranes against the aging in water at 130 °C for 192 h was in the order, PTDA = NTDA ≧ BTDA > KDNTDA > IPNTDA. The hydrolysis stability of polymer chain was similar between the BTDA‐ and KDNTDA‐based SPIs. These results are discussed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 905–915, 2010     

Substituents effect on the properties of sulfonated polyimide copolymers

A series of sulfonated polyimide (SPI) copolymers containing methyl, methoxy, or fluorine groups were synthesized to elucidate the substituents effect on their proton conducting properties as well as thermal, hydrolytic, and oxidative stability for polymer electrolyte membrane fuel cell applications. SPIs of high molecular weight (M_w > 200 kDa, M_n > 80 kDa) along with the ion exchange capacity (IEC) varying between 1.34 and 1.91 mequiv/g were obtained, which gave tough, ductile, and flexible membranes by solution casting. The thermal properties of the SPIs were dominated by the electronic structure of the sulfonated aromatic rings. The electron‐donating methyl groups lowered the thermal decomposition temperature. The hydrolytic and oxidative stability was roughly in the order of IEC (the higher IEC membranes were less stable). Fluorine groups, either as ? F or ? CF₃, had negative effect on the hydrolytic and oxidative stability. In the water uptake and proton conductivity, hydrophobic components are rather more influential than the substituents. It was found out that the SPI(5, 8, 0.7) containing bis(phenoxy)biphenylene sulfone moieties as a rigid hydrophobic component showed the best balanced properties in terms of the stability and the proton conductivity for its rather low IEC. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4469–4478, 2008     

Synthesis and properties of novel sulfonated polyimides from 2,2′‐bis(4‐aminophenoxy)biphenyl‐5,5′‐disulfonic acid

Novel sulfonated polyimides (SPIs) were prepared from 1,4,5,8‐naphthalenetetracarboxylic dianhydride (NTDA), 2,2′‐bis(4‐aminophenoxy)biphenyl‐5,5′‐disulfonic acid (oBAPBDS) with nonlinear configuration, and common nonsulfonated diamines. Water uptake (WU) in liquid and vapor, water stability, and proton conductivity σ of the resulting SPI membranes were investigated. They were soluble in m‐cresol and dimethylsulfoxide, and their WUs in liquid were much larger than those of the SPIs from other sulfonated diamines with linear configuration such as 4,4′‐bis(4‐aminophenoxy)biphenyl‐3,3′‐disulfonic acid (BAPBDS). NTDA‐oBAPBDS membrane was soluble in water at room temperature, whereas all the oBAPBDS‐based copolyimide membranes were insoluble in water and maintained mechanical strength after being soaked in distilled water at 80 °C for 40–1000 h. This much improved water stability was due to the enhanced solubility stability of membrane toward water. The water vapor sorption isotherms were rather similar between the SPIs with the nonlinear and linear configurations of sulfonated diamine moieties. The present SPIs with IECs of 1.8–2.6 meq/g, including NTDA‐BAPBDS, showed reasonably high proton conductivities under the highly humid conditions and roughly fell on the same σ–WU relation line. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1432–1440, 2004     

Preparation and characterization of sulfonated polyimide ionomers via post‐sulfonation method for fuel cell applications

This paper describes our work on the synthesis of a series of sulfonated homo‐/co‐polyimides (SPI) which were obtained by post‐sulfonation method over three steps. In the first step, 4,4′‐oxydianiline (ODA) and 4,4′‐diaminodiphenylsulfone (DDS) dissolved in N‐methyl pyrrolidone (NMP) were reacted with benzophenonetetracarboxylic dianhydride (BTDA) in order to yield poly(amic acid) (PAA). Secondly, precipitated PAA was sulfonated via concentrated sulfuric acid (95–98%) at room temperature to give post‐sulfonated PAA (PSPAA). Finally, PSPAA was converted into post‐sulfonated PI (PSPI) by the thermal imidization method. PSPIs with ion exchange capacity (IEC) ranging from 0.20 to 0.67 meq/g were prepared. The thermal properties of the PSPIs were evaluated and high desulfonation temperature was found in the range of 190–350°C, suggesting the high stability of sulfonic acid groups. In water, PSPI‐5 membrane displayed similar proton conductivity to Nafion®117, whereas this membrane showed poor conductivity in dry state. All PSPIs displayed good solubility in common polar aprotic solvents such as NMP and dimethylacetamide (DMAc). Furthermore, the effects of post‐sulfonation reaction on chemical structure, thermal oxidative behavior, and physical properties of the PSPI membranes such as membrane quality/stability and water uptake were discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

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 properties of novel polyimides from sulfonated binaphthalene dianhydride for proton exchange membranes

A sulfonated dianhydride monomer, 6,6′‐disulfonic‐4,4′‐binaphthyl‐1,1′,8,8′‐tetracarboxylic dianhydride (SBTDA), was successfully synthesized by direct sulfonation of the parent dianhydride, 4,4′‐binaphthyl‐1,1′,8,8′‐tetracarboxylic dianhydride (BTDA), using fuming sulfuric acid as the sulfonating reagent. A series of sulfonated homopolyimides were prepared from SBTDA and various common nonsulfonated diamines. The resulting polymer electrolytes, which contain ion conductivity sites on the deactivated positions of the aryl backbone rings, displayed high proton conductivities of 0.25–0.31 S cm^?1 at 80 °C. The oxidative stability test indicated that the attachment of the ? SO₃H groups onto the dianhydride units did not deteriorate the oxidative stability of the SPI membranes. The better membranes were achieved by the copolymerization of nonsulfonated diamine, SBTDA, and BTDA. Copolymer membrane synthesized from hexane‐1,6‐diamine, SBTDA, and BTDA displayed excellent water stability of more than 1000 h at 90 °C, while its proton conductivity was still at a high level (comparable to that of Nafion 117). Furthermore, the novel block copolymer ( II‐b ) displayed higher proton conductivity compared with the random one ( II‐r ) obviously, probably due to the slightly higher water uptake and better microphase separated morphology. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2820–2832, 2008     

Preparation and properties of ionic cross‐linked sulfonated copolyimide membranes containing pyrimidine groups

A series of sulfonated copolyimides containing pyrimidine groups (SPIs) were synthesized by random copolymerization of 1,4,5,8‐naphthalenetetracarboxylic dianhydride (NTDA), 2‐(4‐aminophenyl)‐5‐aminopyrimidine (PAPRM), and 4,4′‐diaminodiphenyl ether‐2,2′‐disulfonic acid (ODADS). Proton exchange treatment in 1.0 M sulfuric acid solution resulted in ionic cross‐linking of the sulfonated copolymers due to the acid (sulfonic acid)‐base (pyrimidine group) interactions and the membrane with more basic PAPRM moiety could absorb sulfuric acid to favor the proton transfer. The effects of the structure of the diamines on the properties of SPI membranes were evaluated by studying the membrane parameters including water uptake, proton conductivity, water stability, and methanol permeability. The basic pyrimidine groups introduced in the main chains could effectively resist membrane swelling due to the strong interchain interactions through basic pyrimidine groups and sulfonic acid groups. Compared with the corresponding uncross‐linked copolyimides (NTDA/ODADS/ODA), the acid–base copolyimides displayed excellent water stability. The SPI membranes also exhibited improved mechanical properties and decreased methanol permeability. However, the cross‐linked membranes showed lowered proton conductivities than the uncross‐linked ones because a small part of the sulfonic acid groups had been consumed during the cross‐linking process. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis and properties of novel sulfonated (co)polyimides bearing sulfonated aromatic pendant groups for PEFC applications

Novel sulfonated diamines bearing aromatic pendant groups, namely, 3,5‐diamino‐3′‐sulfo‐4′‐(4‐sulfophenoxy) benzophenone (DASSPB) and 3,5‐diamino‐3′‐sulfo‐4′‐(2,4‐disulfophenoxy) benzophenone (DASDSPB), were successfully synthesized. Novel side‐chain‐type sulfonated (co)polyimides (SPIs) were synthesized from these two diamines, 1,4,5,8‐naphthalene tetracarboxylic dianhydride (NTDA) and nonsulfonated diamines such as 4,4′‐bis(3‐aminophenoxy) phenyl sulfone (BAPPS). Tough and transparent membranes of SPIs with ion exchange capacity of 1.5–2.9 meq g^?1 were prepared. They showed good solubility and high thermal stability up to 300 °C. They showed isotropic membrane swelling in water, which was different from the main‐chain‐type and sulfoalkoxy‐based side‐chain‐type SPIs. The relative humidity (RH) and temperature dependence of proton conductivity were examined. At low RH, the novel SPI membranes showed much higher conductivity than the sulfoalkoxy‐based SPIs. They showed comparable or even higher proton conductivity than Nafion 112 in water at 60 °C (>0.10 S cm^?1). The membrane of NTDA‐DASDSPB/BAPPS (1/1)‐s displayed reasonably high proton conductivities of 0.05 and 0.30 S cm^?1 at 50 and 100% RH, respectively, at 120 °C. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2862–2872, 2006     

Synthesis and characterization of epoxy‐based semi‐interpenetrating polymer networks sulfonated polyimides proton‐exchange membranes for direct methanol fuel cell applications

Novel epoxy‐based semi‐interpenetrating polymer networks (semi‐IPNs) of aromatic polyimide, derived from 2,2‐benzidinedisulfonic acid (BDSA), were prepared through a thermal imidization reaction. Dynamic scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) were utilized to verify the synchronization of the imidization of sulfonated poly(amic acid) (SPAA) and the crosslinking reactions of epoxy. The semi‐IPNs of epoxy/sulfonated polyimides (SPI‐EPX) exhibit excellent film‐forming characteristics and mechanical integrity at room temperature. Conductivities at 100 °C of 0.0243 S cm^?1 (SPI‐EP30) and 0.0141 S cm^?1 (SPI‐EP50) were obtained, which are similar to that of the Nafion 117 (0.0287 S cm^?1). The increase in the conductivity of SPI‐EP(30,40) with temperature is more rapid than that of Nafion 117. The SPI‐EPX exhibited lower methanol permeability than did Nafion117. The hydrolytic stability of the SPI‐EPX was followed by FTIR spectroscopy at regular intervals. SPI‐EPX prepared using epoxy‐based semi‐IPNs of sulfonated polyimide, SPI‐EP(40,50), exhibited higher hydrolytic stability than the phthalic polyimides (five‐membered ring polyimides).The microstructure was analyzed using atomic force microscopy (AFM) in the tapping mode, which demonstrated that SPI‐EP50 exhibited a nanophase that was separated into an essentially reticulated and venous hydrophobic and hydrophilic domains. Transmission electron microscopy (TEM) confirmed widespread and well‐connected hydrophilic domains, proving the higher hydrolytic stability and strong proton‐transporting properties of the SPI‐EPX membrane. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2262–2276, 2008     

Synthesis and properties of novel sulfonated polyimides containing phthalazinone moieties for PEMFC

A novel sulfonated diamine, 1,2-dihydro-2-(3-sulfonic-4-aminophenyl)-4-[4-(3-sulfonic-4-aminophenoxy)-phenyl]-phthalazin-1-one(S-DHPZDA), was successfully synthesized and two series of six-membered sulfonated polyimides (SPIs) were prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), S-DHPZDA, and nonsulfonated diamines DHPZDA or 4,4′-diaminodiphenyl ether (ODA). The chemical structure of the S-DHPZDA and the SPIs were characterized by ¹H NMR and FT-IR. Tough, brownish and transparent membranes were cast from SPIs’ solution in NMP. The water uptake, swelling ratio, chemical and thermal stability, hydrolytic and oxidative stability as well as proton conductivity of these new polymers were investigated systematically. Compared with Nafions, the obtained SPI membranes have onset decomposed temperatures of these two series SPIs were above 318 °C and decomposed temperature of main chain were 565 °C and excellent dimension stabilities on similar IECs. Introduction of phthalazinone moieties had improved the copolyimides’ solubility in polar aprotic organic solvents like m-cresol, NMP, DMSO, DMF etc. The SPIs had high proton conductivity (σ) in the order of magnitude of 10⁻³ to 10⁻² S cm⁻¹ depending on the degree of sulfonation (DS) of the polymers.     

Synthesis and properties of sulfonated polyimides from homologous sulfonated diamines bearing bis(aminophenoxyphenyl)sulfone

Three homologous sulfonated diamines bearing a bis(aminophenoxyphenyl)sulfone structure, namely, bis[4‐(4‐aminophenoxy)phenyl]sulfone‐3,3′‐disulfonic acid (pBAPPS‐3DS), bis[4‐(4‐aminophenoxy)phenyl]sulfone‐2,2′‐disulfonic acid (pBAPPS‐2DS), and bis[4‐(4‐aminophenoxy)‐2‐(3‐sulfobenzoyl)phenyl]sulfone (pBAPPS‐2DSB), were synthesized. A series of sulfonated polyimides (SPIs) were synthesized from 1,4,5,8‐naphthalene tetracarboxylic dianhydride, these sulfonated diamines, and nonsulfonated diamines, and their properties were investigated in comparison with those reported for the SPIs from another homologous diamine or bis[4‐(3‐aminophenoxy)phenyl]sulfone‐3,3′‐disulfonic acid (mBAPPS‐3DS). These SPIs were soluble in common aprotic solvents and showed reasonably high proton conductivity, except for pBAPPS‐2DS‐based SPIs, the conductivity of which was slightly lower because of the lower water uptake. The water stability of these SPIs considerably depended on the structure of the sulfonated diamines and was in the order of pBAPPS‐2DSB ≈ pBAPPS‐2DS > pBAPPS‐3DS ? mBAPPS‐3DS. Their water stability was much lower than that of the SPIs from 4,4′‐bis(4‐aminophenoxy)biphenyl‐3,3′‐disulfonic acid. The reason was discussed on the basis of the basicity of the sulfonated diamine and the solubility property of the SPIs. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2797–2811, 2007     

Synthesis and characterization of novel sulfonated naphthalenic polyimides as proton conductive membrane for DMFC applications

To prepare proton conductive membrane for direct methanol fuel cells (DMFC), a novel sulfonated aromatic diamine monomer, 1,4-bis(4-amino-2-sulfonic acid-phenoxy)-benzene (DSBAPB) was synthesized and characterized by ¹H NMR and FT-IR. Then a series of sulfonated polyimides (SPIs) were prepared from DSBAPB with 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA) and a non-sulfonated diamine, 4,4′-oxydianiline (ODA) via one-step high-temperature polymerization method. The sulfonation degree of the SPIs can be controlled by changing the mole ratio of sulfonated monomer to non-sulfonated monomer. The obtained SPI membranes exhibit desirable proton conductivity ranged from 7.9 × 10⁻³ to 7.2 × 10⁻² S cm⁻¹ and low methanol permeability of less than 2.85 × 10⁻⁷ cm² s⁻¹. Furthermore, the hydrolysis stability of the obtained SPIs is better than the BDSA based SPIs caused by the flexible structure.     

Crosslinked sulfonated polyimide networks as polymer electrolyte membranes in fuel cells

Sulfonated polyimides with tertiary nitrogen in the polymer backbone were synthesized with 1,4,5,8‐naphthalenetetracarboxylic dianhydride, 4,4′‐diaminobiphenyl 2,2′‐disulfonic acid, 2‐bis[4‐(4‐aminophenoxy)phenyl]hexafluoropropane, and diaminoacrydine hemisulfate. They were crosslinked with a series of dibromo alkanes to improve the hydrolytic stability. The crosslinked sulfonated polyimide films were characterized for their thermal stability, ion‐exchange capacity (IEC), water uptake, hydrolytic stability, and proton conductivity. All the sulfonated polyimides had good thermal stability and exhibited a three‐step degradation pattern. With an increase in the alkyl chain length of the crosslinker, IEC decreased as 1.23 > 1.16 > 1.06 > 1.01, and the water uptake decreased as 7.29 > 6.70 > 6.55 > 5.63. The order of the proton conductivity of the crosslinked sulfonated polyimides at 90 °C was as follows: polyimide crosslinked with dibromo butane (0.070) > polyimide crosslinked with dibromo hexane (0.055) > polyimide crosslinked with dibromo decane (0.054). The crosslinked polyimides showed higher hydrolytic stability than the uncrosslinked polyimides. Between the crosslinked polyimides, the hydrolytic stability decreased with an increase in the alkyl chain length of the crosslinker. The crosslinked and uncrosslinked sulfonated polyimides exhibited almost the same proton conductivities. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2370–2379, 2005     

Thermal and hydrolytic stability of sulfonated polyimide membranes with varying chemical structure

Many important properties required for fuel cell applications including hydrolytic stability, depend on various factors like flexibility of the polymer backbone, ring structure and phase separation. This paper is primarily focused on studying the effect of the chemical backbone structure on the hydrolytic stability and other properties. To study the difference in the hydrolytic stability with change in the chemical backbone structure of sulfonated polyimides we synthesized phthalic sulfonated polyimides and naphthalenic sulfonated polyimides. Two series of phthalic sulfonated polyimides were prepared using 4,4′-oxydiphthalic anhydride (ODPA) and 4,4′-methylene dianiline (MDA), and 4,4′-(hexafluoroisopropylidine) diphthalic anhydride (6FDA) and oxydianiline (ODA). 4,4′-Diaminobiphenyl-2,2′-disulfonic acid (BDSA) was used to introduce sulfonic acid group into both series. Naphthalenic polyimides were synthesized from 1,4,5,8-naphthalenetetra-carboxylic dianhydride, BDSA, MDA and ODA. Also to observe other properties according to variation of sulfonic acid content, the degree of functionalisation was effectively controlled by altering the mole ratio between the sulfonated and non-sulfonated diamine monomers in phthalic sulfonated polyimides. The hydrolytic stability of the polyimides was followed by FT-IR spectroscopy at regular intervals. Polyimides prepared using naphthalenic dianhydride, NTDA, exhibited higher hydrolytic stability than the phthalic dianhydrides. The proton conductivity, ion exchange capacity (IEC) and water uptake measurements revealed the dependence on the molecular weight of the repeating unit. The proton conductivity of the sulfonated polyimides was found to vary with chemical backbone structure.     

Synthesis and properties of sulfonated polyimides derived from bis(sulfophenoxy) benzidines

A series of aromatic sulfonated polyimides (SPIs) bearing sulfophenoxy side groups have been successfully synthesized and evaluated as polymer electrolyte membranes for fuel cell applications. The SPIs had high viscosity and gave tough and flexible membranes. The SPI membranes showed anisotropic membrane swelling in water with much larger dimensional change in thickness direction than in plane one. They showed the better proton‐conducting performance even in the lower relative humidity (RH) range than the other SPI membranes, for example, a high proton conductivity of 0.05 S/cm at 50 % RH and 120 °C. They maintained high mechanical strength and conductivity after aging in water at 130 °C for 500 h, showing much better water stability compared with the main‐chain‐type SPI and side‐chain‐type SPI membranes reported so far. In polymer electrolyte fuel cells (PEFCs) operated at 90 °C and 84–30%RH, they showed fairly high cell performances and have high potential for PEFC applications. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1463–1477, 2009     

Synthesis, water stability and proton conductivity of novel sulfonated polyimides from 4,4′-bis(4-aminophenoxy)biphenyl-3,3′-disulfonic acid

A novel sulfonated diamine monomer, 4,4′-bis(4-aminophenoxy)biphenyl-3,3′-disulfonic acid (BAPBDS) with the high basisity and flexible structure was synthesized by direct sulfonation of 4,4′-bis(4-aminophenoxy)biphenyl (BAPB). Sulfonated polyimides (SPIs) were prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), BAPBDS and nonsulfonated diamines such as BAPB. The SPI membranes showed much higher water stability at high temperatures than other sulfonated diamine-based SPIs reported so far. Their water vapor sorption isotherm, water uptake (WU), density, dimensional change and proton conductivity σ were investigated. The SPIs showed rather isotropic dimensional changes with WU and the volume increases were slightly smaller than those estimated from the additivity. The SPIs with ion exchange capacities (IECs) of 1.9–2.7 meq/g displayed the similar relationship between σ and WU each other, which was different from those of Nafion 117 and also of the SPIs with the lower IECs. The former SPIs showed reasonably high σ values of 10⁻² S/cm or more even at WU of 25 g/100 g dry polymer under 70% RH at 50 °C, whereas the latter showed the similarly high σ values only in liquid water, but not in the nearly saturated water vapor.     

Structure–property correlations of sulfonated polyimides. I. Effect of bridging groups on membrane properties

A series of six‐membered sulfonated polyimides were synthesized using 1,4,5,8‐naphthalenetetracarboxylic dianhydride, 4,4′‐diaminobiphenyl 2,2′‐disulfonic acid as the sulfonated diamine, and various nonsulfonated diamine monomers having different bridging groups. These bulky bridging groups have the capacity to increase hydrolytic stability and proton conductivity. Polyimides with bulky bridging groups showed increased solubility but exhibited lower thermal stability. The ion exchange capacity and water uptake reduced with increase in the bulkiness of the bridging group. This was attributed to the increase in the molecular weight of the repeating unit and hence effectively reduced the sulfonic acid content. In low temperatures, the conductivity was lower than Nafion®115 and, with increase in temperature, the conductivity rapidly increased and exhibited better conductivity than Nafion®115. Polyimides with bulky bridging groups 4‐amino phenyl sulfone, and 2‐bis[4‐(4‐aminophenoxy)phenyl]hexafluoropropane showed higher conductivity than other polyimides and Nafion®115 despite low ion exchange capacity. The hydrolytic stability of the polyimides with bulky bridging groups was higher than the polyimides with less bulky atoms because of the imparted flexibility. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3612–3620, 2004     

Synthesis and properties of polyimide ionomers containing sulfoalkoxy and fluorenyl groups

A series of sulfopropylated and sulfobutylated polyimide copolymers containing fluorenyl groups, SPI‐4, were synthesized to investigate the effect of alkyl side chains on the properties (stability, mechanical strength, water uptake, and proton conductivity) of the polymimide electrolyte membranes. SPI‐4 showed much better hydrolytic stability (in 10% MeOH aq at 100 °C) than the main chain sulfonated polyimide, SPI‐1. Tough, flexible, and ductile membranes were obtained from these copolymers. At high relative humidity all the SPI‐4 membranes showed high mechanical properties (>34 MPa of the maximum stress) and proton conductivity (>0.1 Scm^?1). These properties are comparable to or even better than those of the perfluorosulfonic acid ionomer (Nafion 112). The new polyimide ionomers have proved to be a possible candidate as polymer electrolyte membrane for PEFCs and DMFCs. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4439–4445, 2005     

Proton conductive polyimide ionomer membranes: Effect of NH,OH, and COOH groups

A new series of sulfonated polyimide (SPI) copolymers containing NH, OH, or COOH groups were synthesized by the polycondensation of 1,4,5,8‐naphthalnetetracarboxylic dianhydride, 3,3′‐bis(sulfopropoxy)‐4,4′‐diaminobiphenyl, and 3‐(4‐aminophenyl)‐5‐(3‐aminophenyl)‐1H‐1,2,4‐triazole (SPI‐8‐m), 3,5‐bis(4‐aminophenyl)‐1H‐1,2,4‐triazole (SPI‐8‐p), 3,6‐diaminocarbazole (SPI‐9), 3,5‐diamino‐1H‐1,2,4‐triazole (SPI‐10), bis(3‐aminopropyl)‐amine (SPI‐11), 2,6‐diaminopurine (SPI‐12), 2,4‐diamino‐6‐hydroxyprymidine (SPI‐13), or 3,5‐bis(4‐aminophenoxy)benzoic acid (SPI‐14). The obtained SPIs were soluble in polar organic solvents and gave tough and flexible membranes by solution casting. The SPI membranes having NH and COOH groups showed high thermal (decomposition temperature ≈200 °C) and mechanical (maximum stress >22 MPa) stability. Introducing NH groups, especially triazole and carbazole groups, was effective in improving proton conductive properties of SPI membranes at low humidity. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2846–2854, 2010