Triphenyl amine containing sulfonated aromatic polyimide proton exchange membranes

European Polymer Journal 60 (2014) 235–246

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Triphenyl amine containing sulfonated aromatic polyimide proton exchange membranes Anaparthi Ganeshkumar 1, Debaditya Bera 1, Ershad Ali Mistri, Susanta Banerjee ⇑ Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India

a r t i c l e

i n f o

Article history: Received 22 July 2014 Received in revised form 9 September 2014 Accepted 11 September 2014 Available online 19 September 2014 Keywords: Proton exchange membranes Sulfonated polyimide Triphenylamine Oxygen permeability

a b s t r a c t A series of new sulfonated co-polyimides (co-SPI) were prepared by the polycondensation reaction of two diamines namely; 4,40 -diaminostilbene-2,20 -disulfonic acid (DSDSA) and 4,40 -diaminotriphenylamine (DATPA) with 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA). All the copolymers showed good solubility and flexible membranes were obtained from their DMAc solution. The transmission electron microscopy (TEM) of the polymers revealed microphase separated morphology with well-dispersed hydrophilic (around 5– 100 nm) and hydrophobic domains. The SPI membrane DTN-80 (80% degree of sulfonation) with ionic exchange capacity (IECw) of 2.74 mequiv g1 showed significantly higher proton conductivity (207 mS cm1) at 80 °C in water as compared to the perfluorinated NafionÒ 117 (135 mS cm1) under similar test conditions. All these co-SPI membranes showed lower oxygen permeability (for DTN-80, PO2 = 0.9 barrer) than NafionÒ 117 (PO2 = 3.6 barrer). Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction In the past few decades proton exchange membrane fuel cells (PEMFCs) have emerged as an alternative clean energy conversion devices due to its high power density, high energy conversion efficiency and stationary as well as portable power applications [1–3]. In this context, NafionÒ the perfluorosulfonic acid (PFSA) ionomer membrane developed by DuPont is considered as state-of-the-art membrane for fuel cell applications. However, high production cost due to the difficult synthetic procedure, low glass transition temperature, restricted operation temperature, high fuel crossover and environmental incompatibility of fluorine has triggered the search for an alternative non-fluorinated hydrocarbon membrane [4,5]. Beside thermal and mechanical stability, low gas permeability and high proton conductivity are crucial ⇑ Corresponding author. 1

E-mail address: [email protected] (S. Banerjee). Authors made equal contribution.

http://dx.doi.org/10.1016/j.eurpolymj.2014.09.009 0014-3057/Ó 2014 Elsevier Ltd. All rights reserved.

prerequisites for a proton exchange membrane (PEM) [6–9]. Many research groups have developed various hydrocarbon based PEMs such as sulfonated poly(arylene ether)s (SPAEs) [10–12], sulfonated polyimides (SPIs) [13–20], poly(arylene ether sulfone)s (PAESs) [21–23] and polybenzimidazoles (PBIs) [24,25] to overcome the drawback of the PFSA membranes. Among them, SPIs synthesized from NTDA attracted the researchers worldwide for fuel cell applications due to their excellent set of properties like, high proton conductivity, high thermal stability, excellent mechanical strength, good film forming capability and low methanol permeability [7,19]. However, poor processability of the SPIs caused by low solubility in organic solvents and high melting or softening temperatures limit their many of the practical applications [26]. Considerable efforts have been made to modify their chemical structure to improve their solubility and to enhance the properties prerequisite for a PEM [13–20]. There are few strategies which have been adapted to increase the proton conductivity of the SPI membranes e.g., incorporation of bulky monomer in the polymer

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backbone or bulky substituent as pendent group that enhances the free volume (FV). The enhanced FV helps to increase the water uptake (WU) by the polymer which is responsible for high proton conductivity of the SPIs [27]. The increment in the FV also helps to improve the solubility of the SPIs [28]. Litt and co-workers reported that the bulky or angled monomers can push the polymer rigid rod backbone apart and produce nano-sized pores inside the membrane [29]. They have developed various sequenced and random SPIs using BDSA (4,40 -diaminobiphenyl-2,20 -disulfonic acid), NTDA and non-sulfonated diamines with a bulky or angled structure, and observed their high proton conductivity (comparable to NafionÒ membranes at high relative humidity levels). SPIs derived from diamines bearing N-base groups, such as propeller shaped triphenylamine showed reduction in fuel crossover with comparable proton conductivity in comparison to the SPIs derived from common non-base diamines [20]. It is reported in the literature that incorporation of the triphenylamine (TPA) units into the polyimides helped in improving processability and increasing thermal stability [30]. Lim et al. recently demonstrated that the PEM containing propeller-like structure exhibited good proton conductivity and excellent fuel cell performance due to the good hydrophilic/hydrophobic phase separation and wide channels formation [31]. Considering the positive effect of the propeller like structure on the PEM properties, in the present work we have incorporated the propeller structure in SPIs backbone to prepare low cost (cost effective) PEMs with high proton conductivity and reduced O2 permeability. Accordingly, in the present study, a series of nonfluorinated copolyimides were synthesized by the polycondensation reaction of a combination of a sulfonated diamine (DSDSA) and a propeller shaped TPA moiety containing diamine (DATPA) with an aromatic dianhydride (NTDA). Detailed structural characterization, physical properties, morphology and proton conductivities of the membranes were investigated and correlated with the copolymer structures. 2. Experimental 2.1. Materials 1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTDA), and 4,40 -diaminostilbene-2,20 -disulfonicacid (DSDSA) were purchased from Sigma–Aldrich (U.S.A.) and dried in a vacuum oven for 12 h prior to use. Triethylamine (TEA, 99.0%), m-cresol (99.0%), benzoic acid (>99.5%) and concentrated sulfuric acid (95%) were purchased from E. Merck (India) and used as received. 4,40 -Diaminotriphenylamine (DATPA) was prepared according to the reported literature [32]. N,N-Dimethylacetamide (DMAc), acetone and isopropanol were purchased from Spectrochem (India) and used as received. 2.2. Measurements 1

H NMR of the copolymers were recorded on a Bruker 400 MHz instrument (Switzerland), using DMSO-d6 as a solvent and TMS as a reference. ATR-FTIR spectra of the

copolymers were recorded from a NEXUS 870 FTIR (Thermo Nicolet) spectrophotometer at room temperature under humid free atmosphere. The molecular weights of polymers were measured by size exclusion chromatography (SEC) using DMAc with 3 g/L LiCl as solvent (for TEA salt form) and as eluent. The apparatus consists of an Agilent 1200 Series HPLC pump (Agilent Tech, USA), PolarGel-M column (Agilent Tech, USA), refractive index (RI) detector K2301 (Knauer, Germany). Linear PMMA with molecular weight between 500 and 1,000,000 Da were used for calibration. Thermogravimetric (TGA) measurements were done on a NETZSCH TG 209 F1 thermal analyzer instrument at a heating rate of 10 °C min1 to determine the decomposition temperature under nitrogen and air. Transmission electron microscopy (TEM) of the membranes were performed using a TEM instrument, FEI-TECNAI G2 20S-TWIN. Before the analysis, the samples in acid (H+) form were stained with silver ion (Ag+) by immersing them overnight in 0.5 M AgNO3 aqueous solution followed by rinsed with water and dried at room temperature for 12 h. The stained membranes were embedded with epoxy resin and sectioned to yield 100 nm thick using a Leica Ultracut UCT Leica EM FCS, Austria and placed on copper grids. Atomic force microscope (AFM) analysis was also conducted using AFM 5500 (Agilent technology). Stress–strain behavior of the thin polymer films (10 mm  25 mm) were measured at room temperature using TINIUS OLSEN H5KS at a strain rate of 5% of sample length per minute. The in-plane proton conductivities of the polymer membranes were determined using AC impedance spectroscopy (HIOKI 3550) over a frequency range of 100 Hz–2 MHz. A sample of prehydrated membrane of dimension 2 cm  1 cm was clamped between two platinum electrodes in the conductivity cell. The membrane between the two electrodes was exposed to allow its equilibrium with deionized water during the experiment. The description of conductivity cell assembly and detailed procedure has been reported in our previous literatures [10,11,15]. The concentration of the ion conducting units is usually represented as the molar equivalents of ion conductor per mass of dry membrane and is expressed as IECw, or milliequivalents of ion per gram (mequiv g1 or mmol g1) of polymer (EW = 1000/IECw). The IECw for di-sulfonated copolymers can be calculated from the following relationship (1):

IECw ¼ ð1000=MW repeat unitÞ  DStheo  2

ð1Þ

where DStheo is the degree of sulfonation calculated through monomer feed ratio; it can be defined as the mol fraction of the monomer unit which is sulfonated. The ion exchange capacity (IECw,titr) values of the membranes were determined by acid-base titration. The IECw,titr, WU, swelling ratio, oxidative and hydrolytic stability of the membranes were determined according to the reported protocol [10,11,15]. 2.3. Synthesis of sulfonated polyimides The schematic representation for synthesis of the SPIs is presented in Scheme 1. The prepared co-SPIs are designated as DTN-XX where, -XX represents the mole

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SO 3H SO m H2N

H - N+ 3

o

NH2

N(CH2CH3)3 /m-cresol/80 C/4 h N2 flow

H2N NH2

HO3S

DSDSA

-

+N

O3S

H H2N

NH2 O +

N

(1-m) DATPA

O

O

O

O

+

O

O

N

N

O

O

-

+N m = 0.50, DTN-50 0.60, DTN-60 0.70, DTN-70 0.75, DTN-75 0.80, DTN-80

O

O

N

N

O

O

O 3S

H

O NTDA

NH

SO 3

Benzoic acid 180 oC 16h o 200 C 3h

m

O

O

N

N

O

O

N 1-m

Sulfonated polyimide (DTN-XX), salt form Acid treatment (1.5 M H2SO4, 72 h)

SO 3H

HO3S

m

O

O

N

N

O

O

N 1-m

Scheme 1. Reaction scheme of sulfonated co-polyimide containing DSDSA-co-DATPA/NTDA monomers.

percentage of sulfonated diamine (DSDSA) used for the polymerization reaction. A typical procedure for the preparation of DTN-50 is discussed: A 100 mL 3-necked round bottomed flask equipped with a magnetic stirrer and N2 inlet was charged with DSDSA (0.31 g, 0.845 mmol), 20 mL m-cresol and TEA (0.188 g, 1.86 mmol). The mixture was heated at 80 °C with continuous stirring for 4 h till DSDSA was completely dissolved. Then, DATPA (0.233 g, 0.846 mmol), NTDA (0.454 g, 1.692 mmol), benzoic acid (0.62 g) and m-cresol (30 ml) were successively added in the reaction mixture. The reaction was continued for 16 h at 180 °C and the temperature was raised to 200 °C. Finally, the reaction was continued for another 3 h. After completion of the reaction, the reaction mixture was cooled and diluted by adding extra m-cresol. Finally, the dark brown viscous solution was poured slowly into excess isopropanol under constant stirring. Fibrous precipitates were isolated by filtration and washed thoroughly with acetone to remove any residual solvent. The fibrous polymers were collected after drying in vacuum at 120 °C for overnight.

2.4. Membrane preparation and acidification The salt form of copolymers solution in DMAc (5%, w/v) was casted on a flat bottom Petri dish and dried in an oven by sequential heating at 80 °C (12 h), 100 °C, 120 °C, 140 °C and 160 °C each for 2 h. The average thicknesses of the membranes were in the range 30–62 lm. The membranes in the salt form were converted to acid form by immersing the salt form of membranes in 1.5 M sulfuric acid for 60 h, followed by rinsing with excess deionized water and finally vacuum dried at 100 °C for 24 h. NMR data of the acid form: 1H NMR (DMSO-d6, 400 MHz, d ppm): 8.75 (H1); 8.30 (H5); 8.00–7.70 (H2, H4); 7.60–7.30 (H3, H6, H7); 7.29–7.00 (H8, H9, H10). 3. Results and discussion 3.1. Polymer characterization The scheme of synthesis of the copolymers (SPIs) is shown in Scheme 1. Solubility behavior of the resulting

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SPIs both in TEA salt and acid form were investigated [as 10% (w/v)] in various common organic solvents at room temperature. The SPIs were soluble in polar aprotic solvents like DMAc, dimethyl sulfoxide (DMSO), dimethyl formamide (DMF) and N-methyl pyrrolidone (NMP) but insoluble in tetrahydrofuran (THF) or dichloromethane (DCM). It was also observed that the DSDSA-based SPIs in TEA-salt form showed good solubility in m-cresol, but not in their protonated form. The enhanced solubility of the SPIs was due to the presence of propeller shaped triphenylamine moiety in the polymer backbone which disturbed the chain regularity and hinders the dense chain stacking of polymer chains [30]. The chemical structure of DTN-XX was confirmed by both ATR-FTIR and 1H NMR spectroscopy. In the ATR-FTIR spectra (Fig. 1) the absorption bands of naphthaleneimide rings at 1661 cm1 (AC@O symmetric stretching), 1709 cm1 (AC@O asymmetric stretching) and 1341 cm1 (ACAN asymmetric stretching) confirmed the formation of imide ring for all DTN-XX series. The absence of anhydride carbonyl stretching bands at 1770 and 1743 cm1 and carbonyl stretching band of polyamic acid at around 1780 cm1 indicates complete imidization. The SO2 stretching vibrations of the sulfonic acid group were observed at 1075 cm1 and 1017 cm1. Structural elucidation and quantitative estimation of DS of DTN-XX series was done from the 1H NMR spectra of the SPIs. A representative 1H NMR spectrum of DTN-75 is given in Fig. 2. The samples were measured in their acid form to avoid signal overlap with the -NH proton of triethylamine as reported by Genies et al. [33]. The copolymer compositions for DTN 50–80 calculated from 1H NMR spectra (integrals of signal regions I–III) were in good agreement with the feed ratio. The number average molecular weights of the co-SPIs were in the range of 67,000–77,000 with polydispersity index values from 2.6 to 3.3 (Table 1). 3.2. Thermal properties Thermal stability of the co-SPIs was investigated by thermogravimetric analysis (TGA) under N2 and air. In N2

1661

1341

1075 1017

Absorbance (arbitrary unit)

1709

DTN-80

DTN-75 DTN-70 DTN-60 DTN-50

1800

1600

1400

1200

1000

Wave number (cm-1 ) Fig. 1. ATR–FTIR spectra of DTN-XX membranes.

800

Fig. 3a, the copolymers showed three steps degradation pattern. The initial weight loss started at around 100 °C that is associated with the loss of absorbed water within the membranes. The percent of weight loss (water molecules) increased with the DS of the DTN-XX membranes indicating that the water molecules were strongly connected through hydrogen bonding around the sulfonic acid groups. This is consistent with the amount of WU of these membranes. The second weight loss which started from around 280–320 °C was due to the decomposition of sulfonic acid groups. The third weight loss around 560–580 °C corresponds to the pyrolysis of the polyimide backbone [18,34]. In order to evaluate the thermal stability of the membranes, samples in acid form were pre-heated at 130 °C and maintain an isothermal equilibration for 30 min and cooled down to room temperature before starting the measurements to remove the absorbed moisture (Fig. 3b). In doing this, no weight loss was observed at 100 °C in Fig. 3b. However, the weight loss at 280 °C and 560 °C were observed similar to the degradation under N2 (Fig. 3a). All these co-SPIs did not show any traces of melting or glass transitions in the DSC thermograms, which might be due to the strong interactions of ionic groups, TPA moiety and rigid naphthalene groups in the polymer backbone [15]. Hence, from these results it can be conclude that for these SPIs, the glass transition temperature is higher than the temperature at which the polymer chains starts to decompose by the cleavage of Ar–SO3H bonds (300 °C). 3.3. Mechanical properties, oxidative stability and hydrolytic stability The mechanical properties of dry and hydrated state of the co-SPIs membranes are summarized in Table 2 and the plots are shown in Fig. 4(a) and (b) respectively. All the DTN-XX membranes showed tensile strength in the range of 42–103 MPa (dry state) and 30–55 MPa (hydrated state) which are higher than that of NafionÒ 117 membrane, 38 MPa (dry state) and 22 MPa (hydrated state). However, the tensile strength of all these copolymer membranes decreased with increasing DS values. The Young’s modulus was in the range of 1.81–1.35 GPa (dry state) and 1.03–0.60 GPa (hydrated state), much higher than that of NafionÒ 117 membrane (0.26 and 0.16 GPa for dry and hydrated state respectively). The elongations at break were in the range of 11–66% (dry state) and 29–48% (hydrated state). The much lower elongation at breaks of these membranes compared to the NafionÒ117 membrane (300%) is due to the rigid triphenylamine and naphthalene moiety in DTN-XX copolymers whereas NafionÒ 117 has the soft fluorocarbon backbone. It is a fact that the mechanical properties of the SPIs affected on the water uptake values and swelling ratios of the membranes. Though, the water uptake values of this series of SPI membranes was high enough, but showed higher tensile strength and young modulus than NafionÒ117. The mechanical properties of these SPIs were also comparable to the other SPIs having similar IECw values [15]. The oxidative stability of SPIs was investigated by immersing the films (5 mm  5 mm) into freshly prepared

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1

O

O

N

SO3H

2

5

N

O

3

O

4 HO3S

0.75

O

O

N

N

O

O

6

7 N 8 0.25

9 10

Fig. 2. 1H NMR spectra of DTN-75 (acid form).

Table 1 Degree of sulfonation of the co-sulfonated polyimides. Samples

DTN-50 DTN-60 DTN-70 DTN-75 DTN-80 a

Mn (g mol1)

DSDSA (mol%)

50 60 70 75 80

PDI

77,300 71,800 67,300 70,700 69,900

Degree of sulfonation (DS)

3.20 2.87 3.29 2.60 2.85

Theo.a

NMR

0.50 0.60 0.70 0.75 0.80

0.50 0.59 0.68 0.70 0.80

Calculated from the monomer feed ratio.

100 DTN-50 DTN-60 DTN-70 DTN-75 DTN-80

80

Weight (%)

Weight (%)

80

60

40

20

DTN-50 DTN-60 DTN-70 DTN-75 DTN-80

100

60

40

20

(a)

(b) 0

0 100

200

300

400

500

600

700

800

Temperature (o C)

100

200

300

400

500

600

700

800

Temperature ( o C)

Fig. 3. TGA thermogram of co-polymers DTN-XX in N2.

Fenton’s reagent (2 ppm FeSO4 in 3% H2O2) at 80 °C. The oxidative stability of the membranes decreased with the increase of electron deficient sulfonic acid groups (DS). In the series, DTN-50 showed the highest oxidative stability (t = 2 h) whereas DTN-80 showed the lowest oxidative stability (t = 0.5 h). Also the hydrolytic stability in this series of copolymers decreases with the increase of DS,

for DTN-50 it is 200 h whereas for DTN 80 it is 41 h. The higher stability of the copolymers with lower DS can be attributed to the electron donating TPA core which reduces the electron deficiency at the imide linkage and diminishes the attack of water molecule. The oxidative and hydrolytic stability of these copolyimides were tabulated in Table 3.

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Table 2 Mechanical properties of DTN-XX membranes.

DTN-50 DTN-60 DTN-70 DTN-75 DTN-80 NafionÒ117

Thickness (lm)

Tensile strength (MPa)

Young’s modulus (GPa)

Elongation at break (%)

Dry

Wet

Dry

Wet

Dry

Wet

Dry

Wet

62 60 54 52 52 180

35 51 44 44 40 –

103 70 56 47 43 38

55 44 50 39 30 22

1.81 1.48 1.64 1.19 1.35 0.26

1.03 0.82 0.94 0.71 0.60 0.16

66 42 18 16 11 301

48 43 38 34 29 288

60

120

Tensile Stess (MPa)

100

DTN-50 (1)

DTN-50 (1)

DTN-60 (2)

DTN-60 (2)

50

DTN-70 (3)

Tensile Stress (MPa)

Samples

DTN-75 (4)

80

1

DTN-80 (5)

2

60

3 4

40

5

40

DTN-70 (3)

1 3

DTN-75 (4)

2

DTN-80 (5) 4

30

5

20 5

10

20

(b)

(a) 0

0 0

10

20

30

40

50

60

70

Tensile strain (%)

0

10

20

30

40

50

Tensile strain (%)

Fig. 4. Stress–strain plots of the DTN-XX co-polymers in dry state (a) and wet state (b).

3.4. IECw, microstructure, water uptake and swelling ratio IECw of the membranes in acid form were determined by classical acid-base titration and listed in Table 4. The experimental IECw values were in the range of 1.75–2.66 mequiv g1, almost close to the theoretical data derived from monomer feed ratios, which indicated that ANa form of copolymers were fully converted to their corresponding AH form after the proton-exchange process. These values were also in agreement with the IECw values calculated from NMR spectra, indicating that the sulfonate groups were successfully incorporated into the polymer backbones without any side reactions. The microstructures of the membranes have significant effects on the proton transport. The TEM micrographs (cross-section) of the SPI membranes are shown in Fig. 5.

Table 3 Oxidative and hydrolytic stability of DTN-XX membranes. Polymers

Thickness (lm)

Oxidative stability a (h)

Hydrolytic stabilityb (h)

s DTN-50 DTN-60 DTN-70 DTN-75 DTN-80

48 51 61 40 42

2.0 1.4 0.75 0.6 0.5

200 140 85 70 41

a The time required for the membranes to dissolve completely in 3% H2O2, 2 ppm FeSO4 at 80 °C. b Time that the membrane is completely dissolved in deionized water at 80 °C.

All the copolymers showed a microphase separated morphology. The black spherical region is corresponding to the hydrophilic soft region (sulfonate groups) and the bright or grey region is corresponding to the hydrophobic harder region (polymer backbones). It was observed that the size and number density of the hydrophilic clusters were proportional to the DS of co-SPI membranes. The bigger ionic cluster sizes in DTN-80 even were three times larger (104 nm) than the bigger ionic clusters observed in DTN-50 (34 nm). This also signifies that the sulfonic acid groups aggregate into hydrophilic clusters with the increase of DS and which eventually provide much better proton transport pathways or ionic transport channels by interconnecting these big ionic clusters, as observed for DTN-80 membrane. The AFM phase images (tapping mode) of the co-SPIs were recorded under ambient conditions to investigate their surface hydrophobic/hydrophilic morphology (Fig. 6). The dark regions in the images were assigned to a soft structure, corresponding to the hydrophilic sulfonic acid groups. The bright regions in the images were attributed to a hard structure, corresponding to hydrophobic polymer matrix. The images exhibited clear formation of interconnected hydrophilic network for the higher sulfonic acid containing copolymers (DTN 75 and 80) (Fig. 6). In DTN-50 the hydrophilic ionic clusters were well separated from each other due to its low sulfonic acid content. However, for DTN-80 with higher sulfonic acid content, phase image undergoes a significant change; the hydrophilic ionic domains formed continuous large channels of an ionic rich phase.

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A. Ganeshkumar et al. / European Polymer Journal 60 (2014) 235–246 Table 4 Physical properties of the DTN-XX membranes. Samples

DTN-50 DTN-60 DTN-70 DTN-75 DTN-80 DQN90j NafionÒ117

dMa (g cm3)

1.18 1.10 1.13 1.16 1.17 – 1.98

IECw (mequiv g1)

IECv (mequiv cm3)e

Dimensional changef

WUg (wt%)

WUh (vol.%)

ki[H2O/SO 3]

Theob

Exp.c

Exp.d

Dry

Wet

Dtc

Dl c

30 °C

80 °C

30 °C

80 °C

30 °C

80 °C

1.80 2.13 2.44 2.59 2.74 2.85 –

1.75 2.04 2.32 2.41 2.66 – 0.95

1.94 2.09 2.54 2.59 2.77 – –

2.12 2.34 2.76 3.00 3.20 –

1.70 1.85 1.96 1.97 1.78 –

0.12 0.15 0.25 0.36 0.61 0.125 –

0.01 0.01 0.02 0.03 0.04 0.4 –

21.0 26.0 36.0 45.0 80.0 42.0 18.52

25.0 32.0 47.0 60.0 175.0 – –

24.78 26.52 40.68 52.2 83.2 –

29.5 32.64 53.11 69.6 182 –

6.75 6.83 8.19 9.71 16.24 8.2 10.83

7.95 8.52 10.7 12.91 35.48 – –

a

Density of membrane calculated from the weight and dimension of dry sample. IECw,Theo = (1000/MW repeat unit)  DStheo  2; DStheois calculated theoretically from monomer feed ratio. c Determined by titration. d IECNMR = (1000/MW repeat unit)  DSNMR  2, where DSNMR is calculated from NMR peak ratio. e IECv(dry) = (IECw,Theo  dM); IECv (wet) = (IECv(dry)/(1 + 0.01 WU)) at 30 °C. f Measured at 30 °C. g WU (wt%) = (Wwet  Wdry)/Wdry  100. h WU (vol%) = ((Wwet  Wdry)/dw)/(Wdry/dm)  100. (Wwet and Wdry are the weights of the wet and dry membranes, respectively; dmand dw is the density of the membrane in the dry state and density of water (l g cm3), respectively). i k = WU (wt%)/(100  IECw,Theo.  MW,H2O), where MW,H2O = 18 g/mol. j Ref. [15]. b

Fig. 5. TEM images of silver ion-stained co-SPI membranes.

Weight-based IECs (IECw) and volume-based IECs (IECv) were calculated (Table 4) and correlated with the WU (Fig. 7a–c) [7,35]. The highest WU (wt%) and WU (vol%) was 175% and 182% respectively at 80 °C for DTN-80 with highest IECw (IECw = 2.74). The membranes showed gradual increase (quasilinear) in WU up to 2.59 IECw (3.0 IECv, dry) and after that increased considerably (Fig. 7(a) and (b)). This indicates the distribution of isolated hydrophilic domains in a predominantly hydrophobic matrix up to 2.59 IECw and after that percolation occurred due to expansion of the hydrophilic domains [23]. It can be

concluded that the WU recognized the microstructure of the SPIs and the hydrophilic sulfonic acid was mainly responsible for WU [36]. Again the inflection at 80 °C is much sharper compared to 30 °C. The inflection point indicated that a percolation threshold for DTN-XX is reached at 75 mol% of DSDSA monomer. After the inflection point, for a very small increases in sulfonation percent (80 mol% of DSDSA monomer) a substantial increase in WU occurs. The IECv (wet) of DTN-XX increased from 1.70 to 1.97 mequiv cm3, with the corresponding increase of IECw from 1.80 to 2.59 mequiv g1 at 30 °C. However, it is

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Fig. 6. AFM tapping phase image for sulfonated polyimide copolymers DTN-50, DTN-75 and DTN-80.

200

200 o

30 C

180

80 C

Water uptale (wt.%)

160

(a)

100 80 60 40

DS 80

o

o

140 120

30 C

180

80 C

160

Water uptale (wt.%)

o

DS 80

DS 70 DS 60

DS 50

140 120

(b)

100 80 DS 70

60 40

DS 75

DS 60

DS 50

DS 75

20

20 1.8

2.0

2.2

2.4

2.6

2.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

IEC v (dry) (meq/cm3)

IECw (meq/g) 200

30 oC

DS 80

180

80 oC

Water uptale (vol%)

160 140 120

(c)

100 80

DS 75

60

DS 70

DS 60

40 DS 50 20 1.0

1.2

1.4

1.6

1.8

2.0

IECv (wet) (meq/cm3 ) Fig. 7. Correlation plot of IEC and WU of co-SPI samples at 30 °C and 80 °C.

interesting to note that with the increase in further sulfonation, the membrane (DTN-80) exhibited low IECv (wet) value (1.78) compared to the copolymer with lower DS, i.e. lower IECw (DTN-50 to DTN-75). Here the difference in mass normalized IEC (IECw) is counterbalanced by the

higher percentage of WU. This resulted in excessive swelling and dilution of the ion concentration after equilibration with water. This is observed in Fig. 6c where the slope of the curves assumed a reverse direction due to high WU (vol%) and reduced IECv (wet).

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DTN-50 DTN-60 DTN-70 DTN-75 DTN-80

1400 1200

Table 5 Proton conductivity and oxygen permeability of the DTN-XX membranes. Polymers

Z'' (Ohm)

1000 800

DTN-50 DTN-60 DTN-70 DTN-75 DTN-80 DQN90d NafionÒ117

600 400 200

Proton conductivitya (mS cm1) 30 °C

80 °C

43.1 49.7 52.9 60.3 123.3 81.9 62.8

77.6 84.9 115.6 118.4 207.5 108.2 135.1

PO2 (barrer)b

Selectivity ratioc

1.8 1.6 1.4 1.2 0.9 – 3.6

1.15 1.41 2.2 2.63 6.15 – 1

a

0 1500

2000

2500

3000

3500

Z' (Ohm) Fig. 8. Complex impedance spectra of DTN-XX membranes at 30 °C.

The DTN-XX membranes showed considerable hydrophilicity as observed from their WU values. The hydration number (water molecules per sulfonic acid group, k) also increased with the IECw values as the WU increased. DTN-XX membranes up to DTN-75 showed k values (k < 10), which were lower than that of NafionÒ 117 (k = 10.83) but for DTN-80 the value was considerably higher (16.24). This certain rise in k for DTN-80 over DTN-75 indicates percolation threshold at DTN-75. The dimensional change of these co-SPIs was reported in the Table 4. It can be observed that there is a gradual increase in dimensional change with the increase of DS. It was due to the increased in sulfonic acid groups which enhanced the ionic nature of these co-SPIs [7]. All these SPIs displayed anisotropic membrane swelling, in which the dimensional change was larger in thickness direction than in plane (for, DTN-80 Dtc = 0.61 > Dlc = 0.04). 3.5. Proton conductivity and O2 permeability In plane proton conductivities of DTN-XX membranes were measured as a function of temperature (in the range 30–80 °C) in water using AC impedance spectroscopy. Fig. 8 shows the Nyquist plot of the DTN-XX membranes at 30 °C. The resistance value of all membranes decreased with increasing DSDSA mol% due to increasing amount of proton conducting groups (ASO3H). However, a sudden drop in resistance value was observed for DTN-80 membrane which may be due to exceeding the percolation threshold and that resulted in sudden increase in proton conductivity. The conductivity of all the co-SPI membranes showed dependence upon IECw values as well as temperature. The conductivity values of the membranes were found in the range 43–123 mS cm1 at 30 °C and 77–207 mS cm1 at 80 °C (Table 5). It is observed that for similar IECw, the proton conductivity values of the synthesized co-SPIs are better than many other SPIs under similar experimental condition [15]. The higher conductivity of these copolymers can be due to the hydrophilic/hydrophobic phase separation and wide channels formation because of the presence of the rigid propeller shaped TPA unit [31]. The effect of this propeller shaped TPA structure on the

Measured in water. Measured in 35 °C and 3.5 bar atm pressure. 1 Barrer = 1010 cm3 (STP) cm/cm2 s cm Hg. c Selectivity ratio = [selectivity (polymer)/selectivity (NafionÒ)] and selectivity = (proton conductivity/PO2). d Ref. [15]. b

water uptake and proton conductivity of these SPIs can be realized better once we compare these values with analogues SPI (DQN90) [15] having close by IECw to DTN-80 of this series (Tables 4 and 5). It can be observed from the tables that the analogues SPI has comparatively much lower water uptake (half) and proton conductivity than DTN-80, though they have similar IECw. From this observation, it can be concluded that the triphenyl amine is effective in increasing the water uptake and proton conductivity of the SPIs. A threefold increase in proton conductivity from DTN-50 to DTN-80 at 30 °C can also be correlated with the 3 time increase in the size of the bigger ionic clusters (TEM analysis). The proton conductivity value of DTN-75 was close to NafionÒ 117 whereas DTN 80 showed 1.6 times higher under similar experimental condition (Table 5). The high WU of DTN-75 and 80 under fully hydrated conditions is the probable reason of this behavior [21]. The high hydration number of DTN-80 over NafionÒ 117 also explained this rise in conductivity. However for co-SPIs with lower DS, the conductivity values were still lower as compared to NafionÒ 117. It can be due to acid base interaction (ionic crosslinking) between the nitrogen of TPA unit and the sulfonic acid groups of DSDSA, which restricts the free movement of the sulfonic acid protons [37]. The insufficient connectivity between neighboring hydrophilic clusters as observed from TEM images, also responsible for these lower conductivity. Furthermore, it is also observed from Fig. 9(a) that the proton conductivity value increased linearly with the increase of IECw up to a certain limit (DTN-75, 2.59 IECw) and after the percolation threshold a sudden rise in proton conductivity was observed. Similar trend was also observed for the IECv (dry) and proton conductivity (Fig. 9(b)). From these observations it can be concluded that after a certain DS, the hydrophilic clusters get very close to each other and started to form an interconnected network by absorbing water molecules which resulted in a sharp rise in proton conductivity by the Grotthus mechanism [38]. Similar to Fig. 7(c) (the trend WU and IECv, wet), proton conductivities of the copolymers increased with the

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225

225

o

30 C

o

200

80 C

175 150

(a) 125 100 DS 50

75

DS 60

50

DS 75

DS 70

DS 80

30 C

DS 80

o

Conductivity (S/cm)

Conductivity (mS/cm)

200

o

80 C

175 150

(b)

125 100

DS 50

75 DS 60

50

25

DS 70

DS 75

25 1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

IEC V (dry) (meq/cm3 )

IECw (meq/g) 225 DS 80

o

30 C

Conductivity (mS/cm)

200

o

80 C

175 150

(c) 125 100 DS 50 DS 60

75 50

DS 75 DS 70

25 1.0

1.2

1.4

1.6

1.8

2.0

2.2

IECv (wet)(meq/cm3 ) Fig. 9. IECw (a) IECv (dry) (b) and IECv (wet) (c) dependence of proton conductivity of the co-SPI membranes.

Conductivity (S/cm)

1

DTN-50 DTN-60 DTN-70 DTN-75 DTN-80 Nafion®

0.1

0.01

30

40

50

60

70

80

Temperature ( o C) Fig. 10. Temperature dependence of proton conductivity (s) of DTN-XX membranes.

decreased in IECv (wet) (for DTN-80 at 30 °C and DTN-75, 80 at 80 °C), reflecting the occurrence of ion dilution in the swelled polymer matrix, Fig. 9(c). Temperature dependence of the proton conductivity for all the DTN-XX membranes is presented in Fig. 10. The proton conductivity values of all the membranes increased

with rise in temperature as with the increase of temperature the movements of the protons facilitate [23]. With increasing temperature a change in the inflection point from DTN 75 to DTN 70 in the IEC vs proton conductivity plots (Fig. 9(a)–(c)) were observed. This could be due to the better connection between hydrophilic domains as WU increased with the increase of temperature. Also a sharp transition in the proton conductivity is noticed at the inflection point, because the water activity in the membrane that affects proton transport is drastically enhanced by increasing temperature [7,11]. This fact is better observed in the Fig. 9(c). Membranes intended for PEMFC must have both high proton conductivity and effective oxygen (O2) barrier properties (fuel crossover). Nafion has good proton conductivity due to a strongly interconnected ionic domain structure but it also has high O2 permeability (for NafionÒ 117, PO2 = 3.6 barrer). This high O2 crossover leads to low fuel efficiency, formation of reactive oxygen species. It also depressed the potential of anode and cathode. So, the low gas permeability is one of the important parameter for the development of a better PEM [9,39]. Table 5 shows the O2 permeability coefficient of SPI membranes and NafionÒ 117 under similar test condition. The O2 permeability gradually decreased from DTN-50 to DTN-80 due to increase of the rigid DSDSA unit and decrease of the higher free volume

A. Ganeshkumar et al. / European Polymer Journal 60 (2014) 235–246

element (TPA unit). In addition to it, the higher percentage of sulfonic acid groups (high DSDSA content) induced the strong intermolecular interaction responsible for this low O2 permeability [9]. The selectivity ratio calculated from the proton conductivity and O2 permeability values were higher for all these SPI membranes as compared to NafionÒ 117 (Table 5). 4. Conclusion TPA containing high molecular weight sulfonated copolyimides was successfully synthesized and characterized. The copolyimide membranes showed well-developed hydrophilic/hydrophobic phase separation as confirmed by TEM images. The copolymer, DTN-80 (80% DS) with IECw 2.74 mequiv g1 showed proton conductivity of 207 mS cm1 at 80 °C in water. This value is well above the proton conductivity value of NafionÒ 117 (135 mS cm1). The high proton conductivity values of these copolymers were attributed to the phase separated morphology and formation bigger size ionic clusters, particularly at high DS, and presence of propeller shaped rigid TPA unit in the polymer backbone. AFM images indicated formation of continuous large channels of an ionic rich phase for higher sulfonic acid content copolymer (DTN-80) that is responsible for high proton conductivity. A straight forward synthesis with combined features, such as high thermal and mechanical stability, high proton conductivity and very low O2 permeability made these copolymers as interesting PEM materials.

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[10]

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[17]

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[19]

Acknowledgements A. Ganeshkumar, D. Bera and E.A. Mistri thanks to Indian Institute of Technology, Kharagpur, India for the financial supports in the form of a fellowship to carry out this work.

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