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Hexafluoroisopropylidene based sulfonated new copolytriazoles: investigation of proton exchange membrane properties

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e-Polymers
From the journal e-Polymers Volume 17 Issue 2

Abstract

A series of novel sulfonated polytriazole copolymers (PTFOSH-XX) was successfully prepared by the click reaction of 4,4′-(perfluoropropane-2,2-diyl)bis((prop-2-ynyloxy)benzene (TF), 4,4′-diazido-2,2′-stilbene disulfonic acid disodium salt (SAZ) and 4,4′-diazidodiphenyl ether (OAZ). The copolymers were characterized by Fourier transform infrared (FTIR) and proton nuclear magnetic resonance (NMR) spectroscopy. The copolymers showed high mechanical, thermal and oxidative stability and low swelling. The phase separated morphology of the membranes was confirmed from transmission electron microscopy (TEM). The membranes showed proton conductivity as high as 110 and 122 mS cm−1 at 80 and 90°C, respectively depending on the polymer repeat unit structure.

Keywords: click reaction; mechanical properties; polytriazole copolymers; proton conductivity; transmission electron microscopy

1 Introduction

As a core component of the proton exchange membrane fuel cells (PEMFCs), the proton exchange membrane (PEM) acts as a proton transporter and provides a barrier for the reactant and the catalyst support (1), (2), (3), (4). At present perfluoro sulfonated ionomers, for example: Nafion®, Flemion®, Dow® and Aciplex® are commonly used as a PEM material, due to its high chemical and mechanical stability along with its high proton conductivity (5), (6). However, these materials, have some disadvantages such as high cost, high methanol permeability and loss of proton conductivity at high temperature, which restricts their application (7), (8). Therefore, many efforts have been made in order to replace these materials. Several hydrocarbon based-sulfonated aromatic PEM materials, such as sulfonated poly(arylene ether sulfone)s (9), (10), (11), (12), (13), (14), (15), sulfonated poly(arylene ether ketone)s (16), (17), (18), (19), sulfonated polyimides (20), (21), (22), (23), (24), (25), sulfonated poly(benzimidazole)s (26), (27), (28), sulfonated polybenzothiazoles (29), (30), (31) and sulfonated polytriazoles (32), (33), (34), (35), (36), (37), have been developed in the recent years. It is well known, that the hydrocarbon-based sulfonated aromatic copolymers display lower proton conductivity in comparison to the perfluoro sulfonated polymers, due to poor phase separation between the hydrophilic and hydrophobic domains and the weak acidic nature of -SO3H group (38). Typically, the structure-property relationship of the PEMs depends on two factors: the first being the ion exchange capacity (IECw) value, and second being the phase separation between the hydrophilic and hydrophobic domains. The proton conductivity of the PEMs can be enhanced by increasing the IECw values, however, this technique always results in high water uptake and swelling, which consequently leads to poor mechanical properties (39). Many researchers suggest that PEM properties depend on the chemical structure of the polymer and the phase separation between the hydrophilic and hydrophobic domains (10), (11), (12), (22), (23), (24), (40). The hydrophilic domains in the polymers are responsible for proton conductivity and the hydrophobic domains provide mechanical stability (41). Incorporation of fluorine in the polymer backbone is a suitable method to improve the phase separation between hydrophilic and hydrophobic domains in the polymers due to its self-assembling ability. Furthermore, addition of the fluorine in the polymers also helps in processability (42). Recently, many efforts have been made to synthesize the fluorinated sulfonated copolymers (10), (11), (12), (22), (23), (24).

The cuprous ion catalyzed click chemistry reaction between azides and alkynes has become a powerful technique in recent years, because of its remarkable advantages such as, facile reaction, high yields, near perfect reliability, easy product isolation and being tolerant toward a wide range of functional groups (43), (44), (45). This reaction has been widely used in macromolecular science, however, it has not been applied much in the synthesis of PEM materials (46). Ponomarev and coworkers prepared first time sulfonated polytriazoles for the first time, by the click reaction of various bis-alkyne monomers with sulfonated azide and characterized its physicochemical and physicomechanical properties (32). Chang and coworkers prepared sulfonated polytriazoles for PEM application, which showed high proton conductivity (107 mS cm−1) (33). Our group has also prepared several fluorinated sulfonated polytriazoles and their PEM properties have been investigated (35), (36), (37). In the present work, we have increased the IECw values of the polymers by designing the new polytriazole copolymers with smaller repeat unit structures. Therefore, in this study we have prepared a low molecular weight diazide monomer, namely, 4,4′-diazidodiphenyl ether (OAZ). A new series (PTFOSH-XX, where XX: mole percentage of SAZ monomer) of highly fluorinated sulfonated copolymers was synthesized based on the click reaction from a 6F (hexafluoroisopropylidene)-based bis-alkyne monomer, 4,4′-(perfluoropropane-2,2-diyl)bis((prop-2-ynyloxy)benzene (TF), with a mixture of two diazide monomers; 4,4′-diazido-2,2′-stilbene disulfonic acid disodium salt (SAZ) and 4,4′-diazidodiphenyl ether (OAZ). The PEM properties of the resulting PTFOSH-XX copolymers were investigated in detail and the results were compared with analogous polytriazole copolymers (35), (36), (37).

2 Experimental section

2.1 Materials

4,4′-Diaminodiphenyl ether (ODA), tert-butyl nitrite (t-BuONO), azidotrimethylsilane (TMSN3), and 4,4′-diazio-2,2′-stilbene disulfonic acid disodium salt (SAZ) were purchased from Sigma Aldrich (USA) and were used as received. Copper iodide (CuI), and N,N-dimethylformamide (DMF) were purchased from Spectrochem (India). Concentrated sulfuric acid (95%), diphosphorous pentaoxide (P2O5), acetonitrile (CH3CN) and aqueous ammonia solution (25%) were purchased from E. Merck (India). 4,4′-(Perfluoropropane-2,2-diyl)bis((prop-2-ynyloxy)benzene (TF) monomer was synthesized according to reported procedure (36). DMF was purified by stirring with NaOH and distilled from P2O5 under reduced pressure before use.

2.2 Synthesis of, 4,4′-diazidodiphenyl ether (OAZ) monomer

The bis-azide monomer was OAZ was synthesized from ODA, t-BuONO and TMSN3, as shown in the Scheme 1. In a 50 ml round-bottom flask, ODA (1.88 g, 9.43 mmol) was dissolved in 20 ml of CH3CN. After cooling to 0–5°C, t-BuONO (2.91 mg, 28.31 mmol) and TMSN3 (2.60 mg, 22.64 mmol), respectively, were added dropwise into the solution via a syringe. The resulted reaction mixture was stirred at room temperature overnight and concentrated using a rotary evaporator. Finally, the compound was purified by silica gel column chromatography using hexane as an eluent. A brown solid was obtained with a 95% yield (2.26 g). Anal. Calcd. for C12H8N6O (252.23 gmol−1): C, 57.14%; H, 3.20%; N, 33.32%; found C, 57.16%; H, 3.21%; N, 33.35%; 1H-nuclear magnetic resonance (NMR) (600 MHz, CDCl3), δ ppm: 7.00 (s, 8H). Fourier transform infrared (FTIR) spectrometry (KBr, cm−1): 3057 (aromatic C-H stretching), 2120 (azide), 1589 (aromatic C=C stretching), 1500, 1309, (asymmetric C-O-C stretching), 1097 (symmetric C-O-C stretching), 844 (C-N stretching for aromatic azide).

Scheme 1: Synthesis of OAZ monomer.
Scheme 1:

Synthesis of OAZ monomer.

2.3 Synthesis of sulfonated polytriazole copolymers

The PTFOS-XX copolymers were synthesized by the same procedure as described in the earlier reports as shown in the Scheme 2 (35), (36), (37). A typical method for the preparation of PTFOS-60 (TF: OAZ: SAZ) copolymer is described as follows. In a 50 ml three neck round bottom flask, OAZ (0.1272 g, 0.50 mmol), TF (0.5199 g, 1.26 mmol), SAZ (0.4073 g, 0.76 mmol), and CuI (0.0120 g, 0.063 mmol) were added under a nitrogen environment. Ten milliliters of DMF were added via syringe in order to dissolve the monomers. The resulting solution was stirred at 70°C for 12 h, after cooling to room temperature, the polymer solution was precipitated out in 250 ml isopropanol. The resulting fibrous polymers were washed with 20% aqueous ammonia solution and deionized water, and dried at 100°C for overnight under vacuum. The yield of polymers was 98%.

Scheme 2: Synthesis of the sulfonated polytriazoles.
Scheme 2:

Synthesis of the sulfonated polytriazoles.

2.4 Preparation of the sulfonated polytriazole copolymer membranes

The copolymers were dissolved in the DMF at a concentration of 10% (w/v). The polymer solutions were filtered and transferred onto Petri dishes. The Petri dishes were heated in an oven at 80°C for overnight and at 100°C, 120°C, 140°C and 150°C for 2 h at each temperature. The resulting membranes were acidified with 1.5 m H2SO4 at room temperature for 24 h, and washed with deionized water. Finally, the membranes were dried under vacuum at 100°C for 24 h. The thickness of the membranes was in the range of 45–55 μm.

1H-NMR spectra of the copolymers were recorded in their acidified forms. 1H-NMR (DMSO-d6, at room temperature): 8.96–9.05 (H1), 8.36 (H7), 8.26 (H10), 7.97 (H5, 9), 7.86 (H8), 7.31 (H4), 7.20–7.24 (H3, 6), 5.31 ppm (H2).

2.5 Characterization of sulfonated polytriazole membranes

1H NMR spectra of the monomers and the copolymers were recorded on a 600 MHz Bruker instrument (Switzerland) using deuterated DMSO-d6 or CDCl3 as a solvent and tetramethylsilane (TMS) as the internal standard. FTIR spectra of the copolymers were recorded on a Thermo Nicolet NEXUS 870 FTIR spectrophotometer. The thermal stability of all samples was investigated in synthetic air (N2:O2=80:20) at a heating rate of 10°C min−1, by using a TA Instruments thermogravimetric analyzer (TGA) model TGA Q 50. The mechanical properties of polymer membranes (10 mm×63 mm) were measured using a Tinius Olsen H5kS mechanical testing machine, at a crosshead speed of 5 mm/min at room temperature. Transmission electron micrographs (TEMs) of the samples were obtained from JEM-2100 HRTEM, Make-JEOL (Japan) operated at a voltage of 200 kV. Before the experiment, the acidified membranes were stained with lead (Pb++) ions by dipping them in 0.5 m (CH3COO)2Pb aqueous solution followed by rinsing with water and dried at room temperature overnight. The samples were embedded with epoxy resin and sectioned into 100 nm slices, using a Leica Ultracut UCT EM FCS (Austria) and transferred on copper grids for investigation. Water uptake and swelling ratios of the membranes were measured by keeping the membranes for 72 h in deionized water. Oxidative stability of the membranes (5 mm×5 mm) was investigated by putting the samples in Fenton’s reagent at 80°C. Theoretical ion exchange capacities (IECw) of the membranes were derived from the relationship: IECw=(1000/Mwrepeat unit)×DS×2; where DS is the degree of sulfonation. The experimental IECw values of the acidified membranes were also determined by both titrimetric and 1H-NMR analysis. The volumetric ion exchange capacities (IECv) of the membranes in dry and wet states were obtained by multiplying the density of the membrane with the IECw, and by considering the water uptake of the membranes, respectively (35), (36). The water uptake and dimensional change of the samples were measured according to a previously reported method (35), (36). The proton conductivity of all membranes (10 mm×20 mm) in the plane direction was determined by a four-probe conductivity cell attached to the electrochemical impedance spectroscopy in the frequency range 100 Hz to 1 MHz, using a GAMRY reference 3000TM instrument.

3 Result and discussion

3.1 Synthesis and characterization of monomers and polymers

The monomer OAZ was synthesized by the reacting ODA with t-BuONO and TMSN3 in CH3CN as a solvent (Scheme 1). The 6F based bis-alkyne monomer TF was prepared from BPF [4,4′-(perfluoropropane-2,2-diyl)diphenol], and propargyl bromide according to the reported procedure (36). The structures of the both monomers were identified both by 1H-NMR and FTIR spectra. Sulfonated polytriazoles copolymers containing 6F moieties in the main chain were synthesized from TF, SAZ and OAZ, by copper catalyzed click polymerization using DMF as a solvent as shown in Scheme 2. The chemical compositions of the PTFOSH-XX copolymers were controlled by varying the feed ratio of SAZ to OAZ. The molar compositions of the polytriazole copolymers membranes prepared in this study are listed in Table 1. All the PTFOSH-XX copolymers showed high molecular weights (Mn) in the range of 18,700–25,200 g mol−1 with polydispersity indices (PDI) in the range of 2.65–3.54. The molecular weight of the copolymers decreases with the feed ratio of SAZ, the inherent viscosity values of the copolymers showed increasing trend with the feed ratio of SAZ. It is due to the fact that while increasing the degree of sulfonation (DS), the concentration of strong polar -SO3H clusters increases which affects increase in inherent viscosity of the copolymers (40).

Table 1:

Composition and properties of PTFOSH-XX copolymers.

PolymerSAZ (mol%)MnaDbηinhc (dLg−1)Elemental F (%)DS (×2)
Theo.dNMR.e
PTFOSH-606025,2002.651.3514.871.181.19
PTFOSH-707023,3002.951.4314.551.401.40
PTFOSH-808019,8003.281.5214.241.601.58
PTFOSH-909018,7003.541.6113.941.801.80

aMn, Number average molecular weight. bD, Polydispersity index. cInherent viscosity of PTFOSH-XX copolymers in NMP at 30°C. dDegree of sulfonation, calculated from monomer feed ratio. eCalculated from 1H-NMR signal intensities.

The structure and chemical compositions of the PTFOSH-XX copolymers were identified by 1H-NMR spectra. Figure 1, depicts the stacked 1H-NMR spectra of PTFOSH-XX copolymers in the acidified form. The proton signals around 8.96–9.05 ppm were attributed due to the vinyl protons (H1) of the triazole moieties (33), (35), (36), (37). The area under these signals were taken as an integral value=1, and were used for the determination of other proton signals. The proton signals at 8.36 ppm, 7.86 ppm and 8.26 ppm, were ascribed to the aromatic protons (H7 and H8) and vinyl protons (H10), respectively, of the stilbene moiety (33), (35), (36), (37). The signal integral values for stilbene moiety (H7, H8, and H10) in the copolymers were increased with the degree of sulfonation. Similarly, the signal integral values at 7.97 ppm for proton signals H5, and H9, decreases with the degree of sulfonation. The signals for aliphatic protons of the methylene moieties (H2) appeared at 5.31 ppm (35), (36), (37). All proton signals in the copolymers were well assigned. The copolymer compositions and DS were calculated by signal integral values of H1 (TF), H7 and H10 (SAZ), and H5 and H9 (TF and O) (35), (36). The copolymer compositions and DS values calculated from 1H-NMR spectra were in good agreement with the monomer feed ratio (Table 1), which suggest that sulfonated diazide monomer (SAZ) was successfully introduced in the polymer backbone by click polymerization.

Figure 1: 1H-NMR spectra of PTFOSH-XX (XX=60, 70, 80, 90; solvent: DMSO-d6 at room temperature.
Figure 1:

1H-NMR spectra of PTFOSH-XX (XX=60, 70, 80, 90; solvent: DMSO-d6 at room temperature.

The structures of the copolymers were also confirmed from FTIR spectra. The characteristic absorption peaks at around 3300 cm−1, 2123 cm−1 and 2127 cm−1 were assigned due to the stretching vibrations of ≡C-H, C≡C, and –N≡N=N, respectively (not shown) (35), (36), (37). These absorption peaks disappeared after polymerization, which indicates that propargyl groups in the TF monomer and azide groups in the OAZ and SAZ monomers were converted in the triazole ring by the CuAAC reaction.

The FTIR spectra of PTFOSH-XX membranes in their salt form is shown in the Figure 2. The characteristic absorption peaks at around 1081 cm−1, and 1020 cm−1 were assigned to the asymmetric and symmetric stretching vibrations of sodium sulfonate (O=S=O) groups, respectively (10), (11), (12). The peak at around 1050 cm−1 was due to the symmetric stretching band of aromatic ether linkage (12), (35). Furthermore, all polymers exhibited absorption peaks at around 1240–1130 cm−1 attributed to the stretching vibration of C-F bonds in the 6F groups (11), (36).

Figure 2: FTIR spectra of PTFOS-60, PTFOS-70, PTFOS-80 and PTFOS-90 membranes.
Figure 2:

FTIR spectra of PTFOS-60, PTFOS-70, PTFOS-80 and PTFOS-90 membranes.

3.2 Polymer solubility

The solubility behavior of the PTFOSH-XX copolymers in both salt and acid forms was examined in several common organic solvents. The concentration of the polymer solutions was kept at a concentration of 0.01 gml−1. It was observed that all copolymers in both forms were well soluble in polar aprotic solvents such as NMP, DMSO, DMF, and DMAc. However, all were insoluble in THF, DCM, methanol and water.

3.3 Thermal study

The thermal properties of the PTFOSH-XX copolymers were investigated by TGA at a heating rate of 10°C min−1 in synthetic air. The thermal degradation curves of the copolymers in the acid forms are shown in Figure 3. All the copolymers were thermally stable with 10% weight loss temperature in the range of 277–296°C (Table 2). The copolymers showed three step weight loss. Initial weight loss at 100°C was due to the moisture in the membranes. The second weight loss at approximately 190–240°C was as a result of desulfonation of -SO3H groups, and the third weight loss at approximately 250–335°C was because of the fragmentation of the triazole backbone.

Figure 3: TGA thermograms of PTFOSH-XX copolymers.
Figure 3:

TGA thermograms of PTFOSH-XX copolymers.

Table 2:

Thermal and mechanical properties of PTFOSH-XX copolymers.

PolymerTd10a (°C)Tensile strength (MPa)Tensile modulus (GPa)Elongation at break (%)Oxidative stabilityb (h)
PTFOSH-60296532.3020>24
PTFOSH-70288432.0724>24
PTFOSH-80280411.9428>24
PTFOSH-90277341.9518~15

a10 wt% loss temperature determined by TGA, heating rate 10°C min−1 under synthetic air (N2:O2=80:20). bThe dissolve time in Fenton’s reagent [2 ppm of iron (II) sulfate heptahydrate in 3% hydrogen peroxide] at 80°C.

3.4 Mechanical and oxidative stability

The mechanical properties of the PTFOSH-XX copolymer membranes in the acid form were measured at room temperature (Figure 4) and the results are shown in Table 2. The tensile strengths, modulus and elongation at break of these copolymers in the dry state were found in the range of 34–53 MPa, 1.95–2.30 GPa and 18–28%, respectively. The tensile strengths of these copolymer membranes were higher in comparison to analogous SPTA copolymer membranes (21–28 MPa) and Nafion® 117 (22 Mpa) (33), (36). Similarly, these membranes showed higher tensile modulus in comparison to the Nafion® 117 (0.16 GPa) (36). The elongation at the break of PTFOSH-XX copolymer membranes were lower in comparison to Nafion® 117 (288%), this was because of the rigid aromatic structures of these copolymers.

Figure 4: Stress verses strain plot of PTFOSH-XX membranes in dry state.
Figure 4:

Stress verses strain plot of PTFOSH-XX membranes in dry state.

The oxidative stability of the PTFOSH-XX copolymer membranes were checked by putting a small portion of polymer membrane (5 mm×5 mm) into Fenton’s reagent [2 ppm of iron (II) sulfate heptahydrate in 3% hydrogen peroxide] at 80°C. The oxidative stability was estimated by measuring the complete dissolution time (τ) in Fenton’s reagent (10), (12). The oxidative stability of these copolymers was decreased with increasing DS values (35), (36), (37). The PTFOSH-XX (-60, -70, -80) copolymers showed oxidative stability more than 24 h, and PTFOSH-90 copolymer displayed oxidative stability up to 15 h (Table 2). The copolymers in this study exhibited comparable oxidative stability to the previously reported QAZ- and TAZ-based copolymers (PTAQSH-XX, PTFQSH-XX and PTATSH-XX) with the similar DS values (35), (36), (37).

3.5 Ion exchange capacity, water uptake and swelling ratio

Ion exchange capacity (IECw) plays a significant role on the properties of PEM materials, and it denotes the amount of interchangeable protons in the PEM materials (35), (36). The theoretical IECw for the polymer electrolyte could be calculated from the monomer feed value, whereas the experimental IECw can be determined by titrimetry and 1H-NMR spectroscopy (10), (11), (35), (36), (37). The main objective of the current study was to see the impact of increasing the IECw on the PEM properties.

Accordingly, PTFOSH-XX copolymers were prepared by replacing the QAZ by OAZ (low molecular weight) (36). This caused in the rise in IECw from 1.52–2.09 to 1.57–2.20 meq g−1 (Table 3) and which subsequently influences many properties of the PTFOSH-XX copolymers. The experimental IECw values for PTFOSH-XX copolymers matches with the theoretical IECw values (Table 3), which indicates that the -SO3H group was successfully introduced in the copolymers through click polymerization. In proton exchange membranes weight-based IECs (IECw) values are mostly used, the volume-based IECs (IECv) are more important (Table 3) to find a precise comparison of water uptake between various membranes (14). Ion exchange capacity (IECv) is determined by the ratio of number of moles of -SO3H groups and volume of the water absorbed in the membrane (35), (36), (37).

Table 3:

IEC, water uptake and proton conductivity of PTFOSH-XX membranes.

PolymerIECW (meqg−1)IECVc (meqg−1)WU (wt%)dσ (mS cm−1)Eae (kJ mol−1)Ref.
Theo.aTitr.NMR.b30°C80°C30°C80°C30°C80°C90°C
PTFOSH-601.571.551.561.751.7210124141621.94This study
PTFOSH-701.791.761.791.951.7912178283724.36This study
PTFOSH-802.001.961.982.101.86142814597826.43This study
PTFOSH-902.202.182.202.181.9317333211012222.36This study
PTFQSH-902.092.042.002.312.0815303086–19.53(36)
PTAQSH-902.382.342.332.051.9736404776–9.04(35)
PTATSH-902.402.352.382.162.0731365190979.47(37)
SPTA12.352.212.28––26–3072–(33)

aIECw,Theo.=(1000/Mw repeat unit)×DSTheo.×2, where DSTheo. is calculated theoretically from monomer feed ratio. bIECw, NMR=(1000/Mw repeat unit)×DSNMR×2, where DSNMR is calculated from NMR peak ratio. cIECv(dry)=(IECw, Theo.)×dM and IECv(wet)=IECv(dry)/(1+0.01WU), dWU(wt%)=[(Wwet−Wdry)/Wdry]×100. eActivation energy determined in the temperature range 30–80°C, and heating rate 1–2°C min−1.

Water uptake (WU) and the swelling ratio plays a vital role in the proton conductivity in the PEMs, as water molecules act as a carrier for proton conduction. However, excessive water uptake results in too much swelling, which significantly reduces the mechanical properties. Hence, preparation of PEMs with ideal WU and durable mechanical properties are one of the compulsory requirements for their actual utilization in PEMFCs. Basically, water uptake depends on two factors, the first is the ion exchange capacity and the second is the concentration of the -SO3H groups of the sulfonated ionomers. In sulfonated polymers, hydrophilic -SO3H group forms ionic clusters and are dispersed in a continuous hydrophobic region. These hydrophilic ionic clusters in the sulfonated polymers are mainly responsible for WU and proton conduction. The weight-based water uptake (WUw) and swelling ratio of PTFOSH-XX copolymer membranes were evaluated according to previously reported methods, and values are shown in Table 3 (35), (36), (37).

The weight-based water uptake of this series of membranes was also increased with the IECw values at a fixed temperature (Figure 5A and B). The PTFOSH-XX copolymer membranes showed lower water uptake in comparison to the PTAQSH-XX and PTATSH-XX copolymer membranes (Table 3) (35), (37). This was because of the high fluorine percentage (14.87%, 14.55%, 14.24% and 13.94%) in PTFOSH-XX copolymers. However, due to the higher IECw values of these copolymers, the water uptake values were relatively higher in comparison to PTFQSH-XX copolymers (36). The volume-based water uptake (WUv) values were calculated by multiplying the WUw values with the densities of the PTFOSH-XX copolymer membranes.

Figure 5: Correlation diagrams of IECw, water uptake and proton conductivity of PTFOSH-XX membranes (A) at 30°C and (B) at 80°C.
Figure 5:

Correlation diagrams of IECw, water uptake and proton conductivity of PTFOSH-XX membranes (A) at 30°C and (B) at 80°C.

The density values of the PTFOSH-XX copolymer membranes decreases as the percentage fluorine in the copolymers decreases (Table 1). The density values were 1.25, 1.22, 1.17 and 1.15, respectively, for the copolymers with DS 60, 70, 80 and 90. Figure 6A and B shows the effect of volume based ion exchange capacities (dry and wet) on the volume-based water uptake (WUv) of the PTFOSH-XX copolymer membranes. It is obvious from the figures that the volume-based ion exchange capacities (dry and wet) exhibited similar trends at 30°C. However, IECv (wet) shows wide deviation at 80°C in comparison to IECv (dry), due to the percolation effect. This result was significant for PTFOSH-XX copolymers after attaining a definite IECv (wet) value.

Figure 6: Water uptake (vol%) dependence of IECv(dry) (A), and IECv(wet) (B) values of PTFOSH-XX membranes.
Figure 6:

Water uptake (vol%) dependence of IECv(dry) (A), and IECv(wet) (B) values of PTFOSH-XX membranes.

The swelling ratio of PTFOSH-XX copolymer membranes was measured according to the earlier reported methods (35), (36). The through plane swelling ratio of this series of membranes was lower in comparison to the Nafion® 117 (19% at 30°C), for example, 4%, 6%, 10% and 19% at 30°C and 8%, 10%, 22% and 29% at 80°C. These values are comparable to the values for similar type of polytriazoles, i.e. PTFQSH-XX, PTAQSH-XX and PTATSH-XX copolymers (35), (36), (37). However, in plane swelling the ratio was not detected for these PTFOSH-XX copolymer membranes.

3.6 Microstructural analysis

The proton transport properties of the polymer electrolyte are closely associated with the membrane morphology and chemical structure (42). The morphology of PTFOSH-XX copolymer membranes was studied by TEM and the microstructures are shown in Figure 7. The dark spherical areas in the micrographs represent the hydrophilic ionic clusters, and are formed as a result of lead ions exchanged with the protons of -SO3H groups. The bright areas in the copolymers correspond to the formation of hydrophobic domains in the polymer backbone (35), (36), (37).

Figure 7: TEM micrographs of PTFOSH-XX [XX=60 (A), 70 (B), 80 (C); and 90 (D)] copolymers.
Figure 7:

TEM micrographs of PTFOSH-XX [XX=60 (A), 70 (B), 80 (C); and 90 (D)] copolymers.

The interconnection of the ionic clusters gradually increases with DS that eventually helps in better proton transport (47). The TEM micrographs showed small ionic clusters (20–35 nm) in the case of PTFOSH-XX membranes (XX=60, 70). A large number of medium ionic clusters (30–50 nm) were observed in PTFOSH-80 membrane and PTFOSH-90 membranes. The PTFOSH-XX copolymer membranes form good microphase-separated morphology, because of the presence of highly hydrophobic 6F moieties in the polymer backbone (36), (42).

3.7 Proton conductivity measurement

The plane proton conductivity of the PTFOSH-XX copolymers was measured using the electrochemical impedance spectroscopy (EIS) technique. Before doing the measurements, all copolymer membranes were hydrated by dipping in deionized water for 72 h. The resistance of the copolymers was estimated from the Nyquist plot from the higher frequency intercept of the characteristic semicircle in the real axis. Figure 8A displays a typical Nyquist plot for the PTFOSH-60 copolymer at different temperatures and the resistance of the copolymer was regularly decreased with temperature. Furthermore, at a fixed temperature the resistance of the PTFOSH-XX copolymers was also decreased with increasing the degree of sulfonation (Figure 8B).

Figure 8: Nyquist graphs of impedance spectra for the PTFOSH-XX copolymers with (A) temperature at fixed composition and (B) composition at 30°C; Z′, the real part of impedance and –Z″, the imaginary part of the impedance.
Figure 8:

Nyquist graphs of impedance spectra for the PTFOSH-XX copolymers with (A) temperature at fixed composition and (B) composition at 30°C; Z′, the real part of impedance and –Z″, the imaginary part of the impedance.

The proton conductivity of the membranes increases with DS value and temperature (Figure 9). The results are shown in Table 3. The proton conductivity values for PTFOSH-XX copolymers were in the range of 4–32 mS cm−1 at 30°C, 14–110 mS cm−1 at 80°C, and 16–122 mS cm−1 at 90°C. These values were higher than 10 mS cm−1, which is the minimum value required for typical PEM application (20), (21), (33). The proton conductivity value for the PTFOSH-90 copolymer (110 mS cm−1 at 80°C, and 122 mS cm−1 at 90°C) was higher in comparision to the analogous copolymers with same DS value, i.e. PTAQSH-90 copolymer (76 mS cm−1 at 80°C), PTFQSH-90 copolymer (86 mS cm−1 at 80°C), and PTATSH-90 copolymer (90 mS cm−1 at 80°C and 97 mS cm−1 at 90°C) (35), (36), (37). The presence of highly hydrophobic 6F moieties in the copolymers causes a polarity difference between the hydrophilic and hydrophobic segments which become more pronounced in the copolymers with higher DS. This helped in getting a better phase separated morphology in PTFOSH-90 and that finally led to high proton conductivity (35), (37).

Figure 9: Temperature dependence proton conductivity of PTFOSH-XX membranes.
Figure 9:

Temperature dependence proton conductivity of PTFOSH-XX membranes.

Furthermore, PTFOSH-90 (IECw=2.20 meq−1) showed higher proton conductivity (110 mS cm−1 at 80°C) despite its lower IECw in comparison to PTA1 (IECw=2.35 meq−1, 72 mS cm−1 at 80°C) was attributed to the the better phase separated morphology in PTFOSH-90 (33).

The activation energy (Ea) for the proton conduction of the PTFOSH-XX copolymers was calculated from the Arrhenius plot (Figure 10). The activation energies for PTFOSH-XX copolymers were in the range of 21.94–26.43 kJ mol−1 (Table 3), which were close to the similar type of polymers, i.e. PTFQSH-XX copolymers (22.83–16.23 kJ mol−1) and sulfonated polyimides (21.00–23.00 kJ mol−1) (20), (21), (36), (48). However, the activation energies values for PTFOSH-XX copolymers were higher in comparison to Nafion (13.65 kJ mol−1), but smaller in comparison to other sulfonated ionomers (>25 kJ mol−1) (13), (15), (19).

Figure 10: Arrhenius plots for proton conduction of the PTFOSH-XX copolymers.
Figure 10:

Arrhenius plots for proton conduction of the PTFOSH-XX copolymers.

4 Conclusion

A new series of highly fluorinated sulfonated polytriazole copolymers (PTFOSH-XX) with different degrees of sulfonation was synthesized by click polymerization. All copolymers showed good solubility in several aprotic solvents. The good solubility of these copolymers could be attributed to the existence of flexible ether linkages (-O-) and 6F moieties in the polymer backbone (42). The membranes were prepared from these copolymers through the solution casting method. The copolymers were characterized by FTIR and 1H NMR spectroscopy. The copolymers were thermally stable at least 277°C with 10% weight loss. The dry copolymers showed high tensile strength (34–53 MPa) and tensile modulous (1.95–2.30 GPa) which were higher compared to Nafion® 117 (tensile strength 22 MPa, tensile modulus 0.16 GPa). The copolymers showed low water uptake, low swelling and good oxidative stability. The copolymers exhibited phase separated morphology and interconnected channels at higher DS. This was attributed to the presence of highly hydrophobic 6F groups in the copolymers that causes polarity differences between the hydrophilic and hydrophobic segments which became more pronounced at higher DS. This helped in getting a better phase separated morphology in PTFOSH-90 and finally proton conductivity up to 110 and 122 mS cm−1 at 80 and 90°C, respectively.

Acknowledgments

A. Singh thanks the Council of Scientific and Industrial Research (CSIR), New Delhi India for providing a fellowship to carry out this work.

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Received: 2016-10-27
Accepted: 2016-11-24
Published Online: 2017-1-11
Published in Print: 2017-3-1

©2017 Walter de Gruyter GmbH, Berlin/Boston

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https://doi.org/10.1515/epoly-2016-0285
Keywords for this article
click reaction; mechanical properties; polytriazole copolymers; proton conductivity; transmission electron microscopy

Articles in the same Issue

  1. Frontmatter
  2. In this Issue
  3. Full length articles
  4. Hexafluoroisopropylidene based sulfonated new copolytriazoles: investigation of proton exchange membrane properties
  5. Effect of various formulation ingredients on thermal characteristics of PVC/clay nanocomposite foams: experimental and modeling
  6. TFC polyamide NF membrane: characterization, application and evaluation of MTPs and MTC for simultaneous removal of hexavalent chromium and fluoride
  7. Functionalization of epoxy resin and the performance of the resultant magnesium-rich primer
  8. Preparation of a poly(DMAEMA-co-HEMA) self-supporting microfiltration membrane with high anionic permselectivity by electrospinning
  9. Mechanical properties of norbornene-based silane treated glass fiber reinforced polydicyclopentadiene composites manufactured by the S-RIM process
  10. Effect of dip time on the electrochemical behavior of PPy-Cu(OH)2 hybrid electrodes synthesized using pyrrole and CuSO4
  11. Interfacial interaction exploration and oxygen barrier potential of polyethylene/poly(ethylene-co-vinyl alcohol)/clay hybrid nanocomposites
  12. Prediction of tensile modulus of PA-6 nanocomposites using adaptive neuro-fuzzy inference system learned by the shuffled frog leaping algorithm
  13. Molecular design and synthesis of thermotropic liquid crystalline poly(amide imide)s with high thermal stability and solubility
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