Influences of Annealing on the Perfluorosulfonate Ion-Exchanged

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Influences of Annealing on the Perfluorosulfonate Ion-Exchanged Membranes Prepared by Melt Extrusion Jing Wang, Miaokun Yang,* Peng Dou, Xuejun Wang, and Heng Zhang Shandong Dongyue Polymer Materials Company Ltd., Zibo, Shandong 256400, China ABSTRACT: Perfluorosulfonate ion-exchanged membranes (PFSIEMs) have been prepared by the melt-extrusion method. Subsequently, the effect of annealing on the properties of PFSIEMs was studied. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) analysis suggested that −SO3− groups could be brought to the membrane surface, presumably by side chain movement during the course of thermal annealing. In the presence of coexisting ion clusters and crystallization, area resistance increased with the rise of the treatment temperature. X-ray diffraction (XRD) analysis further showed that the annealing treatment should increase the crystallinity of membranes, markedly. The equivalent weight (EW) and water uptake were also employed to evaluate the effect of annealing on membranes. For membranes prepared by melt extrusion, abnormal fluctuations are solely evident in the thermogravimetric (TG) traces of annealed specimens. In addition, two glass transition temperatures, ca. 120 and ca. 250 °C, have been observed in the differential scanning calorimetry (DSC) analysis. When the thermal annealing temperature was increased, however, the lower endothermic peak shifted to the high temperature region, mostly because water molecules can serve as the plasticizer to reduce Tg. In addition, we also found that the dynamic mechanical property of membranes can be substantially impacted by annealing. Changes in the temperature relaxation have been explored with dynamic mechanical analysis (DMA).

1. INTRODUCTION Nafion membranes are ion-exchange membranes commercialized by E. I. du Pont de Nemours and Company. Typically, these polymers are composed of polytetrafluoroethylene backbones, perfluorinated pendant side chains, and terminal sulfonate groups. Due to their unique physicochemical characteristics, chemical structure, and morphology, perfluorosulfonated ionomers (PFSIs) have attracted enormous attention. In particular, these polymers have found broad application in diverse fields such as fuel cells,1−3 including proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), the chlor-alkali process, and the vanadium redox flow battery (VRB), exhibiting high conductivity, sufficient thermal stability, and excellent chemical inertness.4,5 Annealing, a simple but effective processing method in material science, has been employed to improve the performance of perfluorosulfonate ion-exchanged membranes (PFSIEMs). Although there are some reports on annealing treatment of solution-cast membranes6−8 and meltextrusion membranes,9,10 studies on annealing treatment are mostly focused on H-type Nafion membranes,6−10 especially solution-cast membranes, and the research on Na-type meltextruded membranes is scarce. Notably, it appears that the properties of melt-extrusion membranes are distinctively different from those of solution-cast membranes. For instance, crystallinity is generally not evident in the low temperature solution-cast polymers, but it is often present in the meltextrusion membranes. Moreover, the solution-cast membranes are brittle and soluble in a variety of polar organic solvents at ambient temperature, which is opposite the melt-extrusion membranes.11 Therefore, it is necessary to study the direct effects of annealing on the performance of melt-extrusion PFSIEMs, especially Na-type melt-extrusion membranes. © 2014 American Chemical Society

In the present work, we have prepared Na-type perfluorosulfonate ion-exchanged membranes utilizing the meltextrusion method. Subsequently, extensive studies were carried out to elucidate the influence of annealing, by chemical analyses and microstructural evolution. Naturally, our work should yield new insight into the theoretical foundation of annealing, especially regarding its effects on Na-type melt-extrusion membranes.

2. EXPERIMENTAL SECTION 2.1. Materials. Perfluorosulfonated ionomer (PFSI) precursors were supplied by Shandong Dongyue Godboat New Material Co. Ltd. (Zibo, China). All other reagents and chemicals were of analytical grade and used as received. 2.2. Preparation of Perfluorosulfonate Ion-Exchanged Membranes (PFSIEMs). The melt flow index (MFI) of PFSI precursors was obtained with a CS-127 melt flow rate meter. For the charging barrel, its diameter, length, and die diameter are 9.55 mm, 8 mm, and 2.1 mm, respectively. The conditions used for measurement are a temperature of 260 ± 0.2 °C and a weight of 1.20 kg. According to the MFI, all PFSI precursors were processed by a single screw extruder (L/D = 30/1, D = 30 mm) with a temperature profile of 260, 270, 270, and 280 °C. PFSI precursor membranes terminated with sulfuryl fluoride groups were obtained. Subsequently, samples were hydrolyzed by soaking in a mixture of 13% NaOH and 30% dimethyl sulfoxide (DMSO) (w/w) at 70 °C for 2 h, affording the desired sulfonate products.12 The Na-type perfluorosulfonated Received: Revised: Accepted: Published: 14175

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and derivative thermogravimetric (DTG) curves. Differential scanning calorimetry (DSC) analysis was conducted on a TA Instruments Q2000 within the temperature range −50 to 380 °C at a heating rate of 10 °C/min under N2 purge. 2.8. Dynamic Mechanical Properties of PFSIEMs. Dynamic mechanical analysis was carried out on a TA Instruments Q800. Membrane samples were measured in the tensile mode at 1 Hz frequency with a heating rate of 3 °C/min. The storage modulus (G′), loss modulus (G″), and loss factor tan δ (=G″/G′) were recorded as a function of temperature. 2.9. X-ray Diffraction (XRD) Analysis. X-ray diffraction (XRD) characterization of perfluorinated membranes was carried out on a D8 diffractometer (Bruker, Germany) with Cu Kα radiation of wavelength λ = 1.540 56 Å at ambient temperature. Membrane samples were equilibrated with room temperature air for 24 h beforehand. Diffraction patterns were recorded according to Bragg’s angle (2θ), and the range was 8− 80°, with a scan increment of 0.02°. Plus, the working voltage and current were 40 kV and 40 mA, respectively. Theoretically, data obtained from the X-ray diffraction analysis of membrane samples can be employed to calculate the degree of crystallinity. Specifically, background scattering was corrected in diffraction patterns, and the crystalline/ amorphous peaks were fit to Pearson VII distribution functions, affording correlation coefficients greater than 98%. The assignments of crystalline and amorphous peaks were based on the position of crystalline and noncrystalline peaks in polytetrafluoroethylene (PTFE). Relative crystallinity was calculated using the following equation reported by Alexander:15

ionomers were rinsed with deionized water until they were neutral, followed by drying at room temperature with a drum wind dryer for at least 24 h. Finally, the pristine membranes were prepared from the corresponding precursors, by the hydrolysis procedure described above. 2.3. Annealing Process of PFSIEM. The annealing of polymers has been carried out using the previously reported conditions.13 Pristine membranes were annealed in a vacuum oven at different temperatures. Specifically, the oven was first heated to the desired temperature and then maintained at the same temperature for 1.5 h. Subsequently, the heater was turned off, and the oven was allowed to slowly cool to room temperature. The pristine membranes are referred to as D0, and the samples annealed at X °C are referred to as DX accordingly. 2.4. FTIR Measurements. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) analysis was performed on a Bruker Tensor-27 instrument, equipped with a Pike Miracle single-bounce attenuated total reflection cell, a DTGS detector, and a ZnSe single crystal. All IR spectra were collected at 4 cm−1 resolution, with 128 scans, and within the frequency range 650−4000 cm−1. The integrated area of the SO3− group was determined by calculating with Origin 8.0 software. 2.5. Equivalent Weight (EW) Experiments. The ionexchange capacity (IEC) of membranes was determined by a titration method. All samples were first immersed in sulfuric acid solution to convert the sodium salt form to the acidic form. Subsequently, the dry samples obtained were placed in 1.0 M NaCl solution to leach off H+ incorporated in membranes, and the protons released were neutralized with 0.01 M NaOH solution.14 The IEC and equivalent weight (EW) can be calculated using the following equations: IEC (mequiv/g) = EW (g/mol) =

∞

xc =

Δ(NaOH)C NaOH m

1000 IEC

Wwet − Wdry Wdry

∞

∫0 [Icr(s) + Iam(s)]s 2 ds

·100

Herein, xc is the polymer crystalline (%), Icr is the sum of the intensities of the fitted crystalline peaks, Iam is the sum of the intensities of the fitted amorphous peaks, and s is the magnitude of the reciprocal-lattice vector, which is given by

Here, Δ(NaOH) is the volume of NaOH solution consumed (mL), CNaOH is the concentration of NaOH solution (mol/L), and m is the dry mass of the membrane (g). 2.6. Water Uptake Measurements. The water uptake of specimens was determined by the weight difference between the wet membrane (Wwet) and the dry membrane (Wdry). Initially, the dry weight of membranes was determined, and then samples were immersed in deionized water at room temperature for at least 24 h. Subsequently, the wet weight was measured after water was rapidly wiped off from the specimen surface with filter papers. The water uptake can be calculated using the following equation: water uptake =

∫0 Icr(s)s 2 ds

s=

2 sin(θ ) Λ

where 2θ is the diffraction angle (degrees). Plus, integrals were numerically approximated following the trapezoid rule.12 2.10. Area Resistance Test. Perfluorosulfonate ionexchanged membranes can serve as polyelectrolytes and cation-conducting polymers.16 Generally, the electrical resistivity ρ is often used to evaluate the conductivities of conductors. However, because the thicknesses of different perfluorosulfonate ion-exchanged membranes vary, it is difficult to compare their conductivities using the corresponding ρ. According to the transposed equation of Pouillet’s law (R = ρl/ A, RA = ρl), we could intuitively contrast the conductivity of various PFSIEMs on the premise of employing the same area. RA is referred to as the area resistance. To determine the area resistance of membranes, a certain concentration of aqueous sodium chloride solution was prepared. Subsequently, the PFSIEM specimens were immersed in the sodium chloride solution for at least 24 h, until a balance was achieved. The resulting membranes were rinsed with deionized water and then cut into rectangular pieces (10.0 cm × 5.0 cm), which were positioned between two electrodes at ambient temperature. The experimental device shown in

·100%

2.7. Thermal Analysis. Prior to thermal analysis, membrane samples were predried at 30 °C for at least 24 h in a drum wind dryer. Thermogravimetric analysis (TGA) of specimens was performed on a TA Q500 thermogravimetric ananlyzer (TA Instruments Co., USA). At first, samples were heated from ambient temperature to 600 °C at a rate of 10 °C/ min, protected by dry nitrogen under atmospheric pressure. The resulting data were subsequently compiled to generate TG 14176

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Figure 3 shows the DSC of the Na+ neutralized membranes. For samples without extra drying, in order to retain original

Figure 1 was designed by our research group. The values of area resistance are reported as the average of separate measurements, with an error of one standard deviation.

Figure 3. DSC curves of Na-type PFSIEMs. The black circles refer to membranes predried at 30 °C for at least 24 h in a drum wind dryer; the red circles refer to membranes further dried at 120 °C under vacuum for at least 48 h.

thermal history and avoid introducing new thermal history, two endothermic events are present between 50 and 200 °C and between 200 and 260 °C. In particular, the endothermic process within the lower temperature range has produced a fairly distinctive peak (ca. 100 °C), which we suspected could contain the evaporation peak of inner water molecules, due to the weakening of the endothermic peaks in the second and third heating runs. To eliminate possible interference from water, membranes were dried in a 120 °C oven under vacuum for at least 48 h according to Moore’s method,11 and the resulting materials were subjected to DSC analysis under the same conditions. As shown in Figure 3 (red circles), the intensity of the lower temperature peak at 100 °C is decreased so much that it develops into a broad but weak peak. Simultaneously, the endothermic peak at ca. 150 °C starts to emerge, which is considered to be related to the thermal transition of ionic clusters,18,19 the matrix,11 or the melting of imperfect crystallites.17 Details of the low temperature peak will be further discussed in section 3.7. The endothermic peak at ca. 250 °C is attributed to the ionic cluster thermal transition or melting of polytetrafluoroethylene-like crystallites. The two endotherms are similar to Moore’s study11 on Na-form Nafion (ca. 150 °C and ca. 260 °C). Therefore, we conclude that the lower temperature endothermic process should involve a previously reported thermal transition process11,16,20 after eliminating the influence of the water molecules. The low temperature peak will also be further discussed in section 3.7. On the basis of these preliminary results, an annealing procedure involving heating at 170 and 270 °C for 1.5 h has been developed to ensure the promotion of polymer reorganization, along with a series of comparative tests at lower temperature. 3.2. FTIR Measurements. Figure 4 shows the results obtained from ATR-FTIR analysis. Notably, the absorption between 1300 and 1100 cm−1 seems to be highly visible, presumably due to the strong C−F antisymmetric and symmetric vibrations. The membrane precursors containing −SO2F groups are referred to as “DF”. The strong band around 1470 cm−1 can be assigned to the sample before hydrolysis treatment.21 After hydrolysis, −SO2F groups are converted to sulfonates; thus the absorption peak of the −SO2F group should disappear. At the same time, the characteristic

Figure 1. Schematic diagram of the area resistance experiment device.

3. RESULTS AND DISCUSSION 3.1. Determination of Annealing Temperature. A survey of the literature shows that thermal transition and relaxation could be present in the solution-cast membranes prepared from commercial Nafion ionomers11,17 and Dow ionomers.16 However, research aiming to investigate the morphology and properties of Na-type melt-extrusion membranes is fairly limited. To ensure the reorganization of the molecular structure, and to provide sufficient thermal energy for the movement of molecular chains, an appropriate annealing temperature should be selected, possibly based on the results obtained from the primary thermoanalysis before annealing. Figure 2 shows the TG and DTG curves of the Na+-form perfluorinated membranes. Notably, it appears that the initial

Figure 2. TG and DTG traces of Na-type PFSIEMs.

decomposing temperature is ca. 443 °C based on 5% mass loss (T5%), and a sharp decomposition peak is evident at 505 °C (62.61%). Therefore, it was decided to use the temperature range of −50 to 380 °C for DSC analysis, so that the weight loss is not too significant, while the majority of the membrane sample is barely degraded. 14177

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Figure 5. Area resistance of PFSIEM as the function of annealing temperature.

Figure 4. ATR-FTIR spectra within the range 2000−900 cm−1.

the conductivity of polymers, and the results indicated that annealing could substantially increase their proton conductivity. Possibly, this could be due to the fact that heat treatment can release the sulfonic acid groups buried inside the backbone and side chains, affording more compact clusters. However, it appears that annealing treatment not only induces the change of ionic clusters, but also promotes the development of crystalline structures. Theoretically, chain mobility can play a significant role in the formation of ionic clusters.27 However, the development of crystalline structure can lead to higher chain packing density, which would substantially reduce chain mobility. As a result, the impact originated from crystallinity seems to be stronger than the one due to the formation of ionic clusters, which would lead to the increase of area resistance, in agreement with the results reported by Jung.28 3.4. X-ray Diffraction. Next, X-ray diffraction (XRD) analysis was employed to investigate the morphological properties of membranes. As illustrated in Figure 6, XRD

absorption band around 1060 cm−1 would appear, which can be attributed to the symmetric stretching frequency of SO3−. The absorption band at ∼980 cm−1, however, can be assigned to the symmetric stretching frequency of the ether linker next to the backbone,22,23 or the C−F stretching of the −CF2−CF(CF3)− group in the side chain.24,25 The split band at 980−960 cm−1 was not observed in the IR spectra of Na-type PFSIEM.26 Subsequently, the spectra of annealed and unannealed PFSIEM samples were subject to direct comparison, but no significant difference was observed. Therefore, we conclude that annealing would not alter the chemical compositions of membranes. Liang et al. suggested that polymer side chains could relocate from the bulk of membranes to the surface; meanwhile, the degree of such relocation should increase with the rise of the treatment temperature.24 Hence, it is highly desirable to study the impact of SO3− groups by integral calculation, which is generally related to ionic clusters and the electrical properties of PFSIEMs. As shown in Figure 4, it appears that the integrated area of SO3− groups gradually increases with the increase of the annealing temperature. ATR is a surface-based infrared technology, and its penetration depth is only several micrometers. Therefore, we conclude that SO3− groups have been brought out of the bulk and eventually reach the membrane surface. Similar results were also observed by Liang et. al, and they have provided a plausible mechanism. Theoretically, annealing treatment would shrink the size of clusters but increase the total numbers. Due to the high surface energy upon thermal treatment, aggregation of clusters promoted by side chain movement could occur to form larger clusters. However, this would not take place in the bulk because the main chain crystallinity is the governing vector that decreases the overall system energy, which would prevent the clusters from aggregating. Consequently, the side chain movement should bring the −SO3− group out of the bulk to the surface, ultimately decreasing the surface energy. 3.3. Area Resistance Analysis. Area resistance analysis has been carried out using the device developed in our laboratory, as illustrated in Figure 1. The effect of annealing on area resistance (AR) has been examined, and the results are shown in Figure 5, suggesting that area resistance often changes with the variation of annealing temperature. In this study, the annealing temperature has been maintained within the range 70−270 °C. Besides, 0 °C means that this sample has not gone through any annealing process. Notably, it appears that the area resistance grows with the increase of the annealing temperature, inconsistent with a previous report.13 Luan et al. have extensively studied how the annealing temperature can affect

Figure 6. X-ray diffraction profile for precursor membranes and annealed membranes.

patterns of different membranes have been obtained, in which two broad scattering peaks at 2θ = 12−20° and 2θ = 40° are present, corresponding to the amorphous halo (2θ = 16, 40°) and crystalline part (2θ = 17.5°), respectively. Notably, these results are in excellent agreement with the ones obtained from Nafion membranes.29−31 In addition, it appears that peak intensity decreases after hydrolyzation, which clearly indicates that the PTFE-like crystallinity has been partly disrupted when −SO2F is converted to −SO3Na. It is highly possible that ionic aggregates could be formed during the course of hydrolyzation; thus, the lamellar ordering of the tetrafluoroethylene backbone 14178

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would be severely damaged, affording lower crystallinity and thicker interlamellar amorphous layers.19,32 Crystalline morphology (2θ, ca. 18°) is usually evident in Nafion membranes, which are known to exhibit broad peaks at 2θ = 12−20°, representing the (100) plane of crystalline PTFE backbone33 and the amorphous halo (2θ, ca. 16°).14,28 The increase of crystalline morphology after annealing treatment has been shown in Table 1. It appears that annealing can induce a Table 1. Degree of Crystallinity for PFSIEM Samples sample

cryst area

amorph area

deg crystallinity/%

D0 D70 D170 D270

1199.08 2660.87 1490.06 2720.22

4676.32 7141.07 3191.09 5113.61

20.41 27.15 31.83 34.72

Figure 8. TG curves of D0 and annealed samples (D70, D170, and D270).

The initial decomposition temperature seems to be ca. 445 °C, based on 5% mass loss (T5%). Surprisingly, unusual fluctuation is evident in the curves of annealed samples, especially compared with the unannealed ones. Because such fluctuation appears to be derived from the mass change (y-axis), rather than the temperature change (xaxis), we conclude that temperature instability should not be the reason. The repeated TG analysis of D0 membranes does not display obvious fluctuation. However, when annealed samples were subject to TG analysis under the same conditions, curve fluctuation emerged again, clearly suggesting that such fluctuation should not be originated from instrument instability. In the TG curves of annealed membranes, the abnormal fluctuation often appears at ca. 475 °C, at which samples might be already decomposed and start to release gas. That said, when time is not enough for gas release, the gas density would be instantaneously increased, which should in turn increase the air pressure inside the furnace chamber to generate a downward force, resulting in mass increase in the initial stage of fluctuation. Naturally, this unusual pressure buildup process can be employed to explain the fluctuation observed in TG analysis. On the basis of the difference observed between D0 and annealed specimens, we conclude that annealing treatment can substantially influence the properties of PFSIEMs. It was speculated that side chain movement could relocate −SO3− groups from the bulk to the membrane surface, which would decrease the degree of hydration in annealed samples, ultimately weakening pristine stability. It was reported that the formation of ionic pairs such as −SO3−Na+ can stabilize C− S bonds;34 however, when −SO3− groups were moved to the membrane surface, the stability due to the ionic pair should be attenuated. In addition, the stability of clusters can be altered by varying the content of counterions, or the degree of hydration34 which could be reduced by annealing treatment. Thus, the thermal stability of annealed membranes should decrease. As a result, gases such as CO2 and SiF435 are released quickly from the annealed samples within the temperature range 470−600 °C, which seems to be a one-step process accompanied by abnormal fluctuations. Figure 9 shows the DSC curves of D0 and annealed samples, in which two endothermic peaks are present, presumably due to the glass transition temperature (Tg) at ca. 120 °C and ca. 250 °C. Kawano et al. attempted to investigate the thermal behavior of Nafion-117 membranes in their sodium form (Nafion−Na) by DSC analysis.34 In particular, two endothermic peaks, near 122 and 250 °C, are present in the first heating DSC curves,

high degree of crystallinity within the temperature range of chain rearrangement. Meanwhile, increasing the annealing temperature could lead to the increase of chain mobility, which would promote the development of crystalline morphology, supporting the results described in section 3.3. Overall, we found that the development of crystalline structure should be beneficial to the increase of area resistance. 3.5. EW and Water Uptake Experiments. EW and water uptake are deemed as important parameters for PFSIEMs; therefore, the effect of annealing on them should also be evaluated. Figure 7 illustrates the change of EW and water

Figure 7. Water uptake and equivalent weight change with increasing annealing temperature.

uptake with the rise of annealing temperature. It can be seen that water uptake gradually decreases with the increase of annealing temperature. On the basis of the results obtained from IR analysis, annealing treatment does not alter the chemical compositions of membranes; however, it was found that the values of EW increased with the increase of annealing temperature, exhibiting the same trend as the change of area resistance. Combining this with results from XRD analysis, we expected that the annealing process probably could lead to the development of crystalline morphology, which would substantially dampen water absorption. Consequently, due to the low water uptake of membranes, the corresponding area resistance seems to be increased, as shown in Figure 5. Moreover, the decrease of the water uptake also reflects the reduction of IEC, which corresponds to the increase of EW. 3.6. Thermal Analysis Experiment. The thermogravimetric (TG) behavior of unannealed (D0) and annealed samples is shown in Figure 8. Notably, a similar degradation pattern is present in the TG curves of all specimens examined. 14179

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Figure 9. Comparison of DSC curves of different samples.

which is similar to what has been observed in this work. In addition, the low temperature peak at 120 °C should be originated from the evaporation of water molecules. We also found that the lower Tg should be ca. 150 °C rather than 120 °C; thus the results obtained by Kawano could have been influenced by the evaporation of water molecules, which was proved in their second heating studies. As previously reported, the low temperature peak can be assigned to the transition to ionic clusters, and the cluster stability is related to hydration.34 Moreover, the water content of membrane samples seems to decrease with the increase of annealing temperature. The lower transition temperature increases with the rise of annealing temperature, mostly because water molecules can act as a plasticizer, effectively reducing Tg.36,37 Notably, it was found that the high temperature endothermic peak gradually shifts to the left, and it eventually disappears in the DSC curve of sample D270. Because DSC is a thermoanalytical technique to measure the change of specific heat, it could be reduced by the development of crystallization. Moreover, the glass transition seems to be completely originated from the amorphous state of materials. If the crystallinity is adequately high, the thermal transition would be absent due to the lack of pronounced heat changes. Hence, the disappearance of a peak in the DSC spectrum of sample D270 indicated that its crystallinity should be higher than those of other specimens, which was confirmed in section 3.4 by Xray diffraction. Generally, decreased heat change would lower the sensitiveness of DSC analysis, making it difficult to observe the glass transition process. Because DMA analysis examines the viscoelastic behavior of polymers based on the variations of modulus or mechanical loss, its sensitiveness should be much higher than those of the former. Therefore, the occurrence of the glass transition could be further verified in section 3.7. 3.7. Dynamic Mechanical Studies. To investigate the effect of annealing on the mechanical properties of PFSIEMs, the storage modulus (G′), the loss modulus (G″), and the loss factor (tan δ) have been determined as a function of temperature, as summarized in Figure 10. Notably, it appears that a similar profile is evident in the curves of log G′ (Figure 10a), whereas a pronounced difference is present in the curves of log G″ (Figure 10b) and tan δ (Figure 10c). As shown in parts b and c of Figure 10, two distinct relaxation regions including the low temperature peak at ca. 160 °C and the high temperature relaxation at ca. 250 °C are present in the plots, which are labeled as β and α, respectively. Notably, these two peaks are ubiquitously evident in the spectra of all samples examined. Possibly, the high temperature peak may correspond to the glass transition process. When thermal transition (Tg or

Figure 10. Variation of storage modulus G′, loss modulus G″, and loss factor tan δ with the rise of temperature for D0, D70, D170, and D270 membranes.

α transition) occurs, the storage modulus decreases by several orders of magnitude (Figure 10a), affording the maximum tan δ (Figure 10c). The appearance of β relaxation at ca. 160 °C seems to be in excellent agreement with the results reported by Eisenberg,38 which should be associated with water content, as indicated by previous DSC experiments. When water content is reduced, the lower transition temperature would increase. For Nafion−H membranes, the β peak shifted to the lower temperature region while water content was increased, which has also been assigned as the glass transition of the matrix in Eisenberg’s early study. With the increase of annealing temperature, the two peaks in log G″ curves become more distinctive, and the intensity of the 175 °C peak in tan δ curves also increases markedly. As mentioned in section 3.4, increasing the annealing temperature can improve the chain mobility, leading to the formation of crystalline morphology. As a result, 14180

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Notes

the enhanced motion of molecular chains could further suppress internal resistance; thus the elastic modulus would decrease in the material with a high degree of crystallinity. Therefore, the internal energy loss and tan δ (=G″/G′) should dramatically increase, as illustrated in Figure 10c. Interestingly, a mechanical relaxation event has been observed at ca. 70 °C in the profile of loss modulus, and this peak was labeled as γ. To the best of our knowledge, such an event is very rare in the literature. On the basis of the spectral decomposition of the tan δ curves, the mechanical relaxation event at lower temperatures can be ascribed to the relaxation mode of the PTFE domain of the Nafion host polymer, as reported by Di et al.39 However, this rationale does not seem to be applicable to our experimental results, especially given the fact that membrane crystallinity is increasing. In addition, the same γ relaxation peak40 is evident in both Nafion−H and Nafion−Na samples, because it is not affected by the counterion and attributed to local −CF2− backbone motions.25 However, the γ relaxation peaks that were previously observed are located at ca. −100 °C, which is markedly distinct from the result obtained from our study (ca. 70 °C). Further, we found that log G′ decreases with the rise of temperature in the γ region, and a peak centered at ca. 70 °C is evident in the tan δ plots (slight) with the corresponding log G″ for D170 and D270 specimens, respectively. Even though some experts think that this relaxation peak could be attributed to water moisture, more supporting evidence is still needed.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Key Projects in the National Science & Technology Pillar Program during the Twelfth Fiveyear Plan Period (Grant 2011BAE08B00) and International Science & Technology Cooperation Program of China (2011DFA52110). The authors also acknowledge Shandong Dongyue Godboat New Material Co. Ltd. for providing raw materials.

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4. CONCLUSIONS In this work, the effects of annealing on Na-type melt-extrusion membranes have been extensively investigated, and the annealed PFSIEMs seem to possess different physicochemical properties. Results from ATR-FTIR analysis indicated that polymer side chains might have relocated from the bulk to the surface after annealing treatment. When the annealing temperature was increased, the intensity of −SO3− groups on the surface was also increased; because heat treatment can release −SO3− groups buried inside the backbone and side chains, more compact clusters would be obtained. Plus, the area resistance should also be reduced. XRD spectra and the increased area resistance both suggested that obvious crystallization impact should be present, which is generally stronger than the one from ionic clusters. On the other hand, EW and water uptake seem to alter with the increase of annealing temperature. Unusual fluctuations in TG curves were only observed in the annealed specimens, which are scarcely reported in the literature. Interestingly, it was found that DSC curves have exhibited two glass transition temperatures. With the increase of annealing temperature, the lower endothermic peak shifts to the high temperature region, possibly because water molecules can act as a plasticizer, effectively reducing Tg. Plus, the dynamic mechanical properties of PFSIEMs can also be substantially affected by annealing treatment.

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REFERENCES

(1) Zhu, Y.; Pei, S.; Tang, J.; Li, H.; Wang, L.; Yuan, W. Z.; Zhang, Y. Enhanced chemical durability of perfluorosulfonic acid membranes through incorporation of terephthalic acid as radical scavenger. J. Membr. Sci. 2013, 432, 66. (2) Yang, L.; Tang, J.; Li, L.; Ai, F.; Chen, X.; Yuan, W. Z.; Zhang, Y. Properties of precursor solution cast PFSI membranes with various ion exchange capacities and annealing temperatures. RSC Adv. 2013, 3, 7289. (3) Thiam, H. S.; Daud, W. R. W.; Kamarudin, S. K.; Mohamad, A. B.; Kadhum, A. A. H.; Loh, K. S.; Majlan, E. H. Performance of direct methanol fuel cell with a palladium-silica nanofibre/Nafion composite membrane. Energy Convers. Manage. 2013, 75, 718. (4) Alberti, G.; Narducci, R.; Sganappa, M. Effects of hydrothermal/ thermal treatments on the water-uptake of Nafion membranes and relations with changes of conformation, counter-elastic force and tensile modulus of the matrix. J. Power Sources 2008, 178, 575. (5) Mauritz, K. A.; Moore, R. B. State of understanding of nafion. Chem. Rev. 2004, 104, 4535. (6) Moore, R. B.; Martin, C. R. Procedure for preparing solution-cast perfluorosulfonate ionomer films and membranes. Anal. Chem. 1986, 58, 2569. (7) Collette, F. M.; Thominette, F.; Mendil-Jakani, H.; Gebel, G. Structure and transport properties of solution-cast Nafion® membranes subjected to hygrothermal aging. J. Membr. Sci. 2013, 435, 242. (8) Vengatesan, S.; Cho, E.; Kim, H. J.; Lim, T. H. Effects of curing condition of solution cast Nafion® membranes on PEMFC performance. Korean J. Chem. Eng. 2009, 26, 679. (9) DeLuca, N. W.; Elabd, Y. A. Nafion/poly(vinyl alcohol) blends: Effect of composition and annealing temperature on transport properties. J. Membr. Sci. 2006, 282, 217. (10) Alberti, G.; Narducci, R.; Di Vona, M. L.; Giancola, S. Annealing of Nafion 1100 in the Presence of an Annealing Agent: A Powerful Method for Increasing Ionomer Working Temperature in PEMFCs. Fuel Cells 2013, 13, 42. (11) Moore, R. B.; Martin, C. R. Chemical and morphological properties of solution-cast perfluorosulfonate ionomers. Macromolecules 1988, 21, 1334. (12) Hensley, J. E.; Way, J. D.; Dec, S. F.; Abney, K. D. The effects of thermal annealing on commercial Nafion® membranes. J. Membr. Sci. 2007, 298, 190. (13) Luan, Y.; Zhang, Y.; Zhang, H.; Li, L.; Li, H.; Liu, Y. Annealing effect of perfluorosulfonated ionomer membranes on proton conductivity and methanol permeability. J. Appl. Polym. Sci. 2008, 107, 396. (14) Teng, X.; Sun, C.; Dai, J.; Liu, H.; Su, J.; Li, F. Solution casting Nafion/polytetrafluoroethylene membrane for vanadium redox flow battery application. Electrochim. Acta 2013, 88, 725. (15) Alexander, L. X-ray diffraction methods in polymer science. J. Mater. Sci. 1971, 6, 93. (16) Moore, R. B., III; Martin, C. R. Morphology and chemical properties of the Dow perfluorosulfonate ionomers. Macromolecules 1989, 22, 3594.

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(17) Page, K. A.; Cable, K. M.; Moore, R. B. Molecular origins of the thermal transitions and dynamic mechanical relaxations in perfluorosulfonate ionomers. Macromolecules 2005, 38, 6472. (18) Lage, L. G.; Delgado, P. G.; Kawano, Y. Thermal stability and decomposition of nafion® membranes with different cations. J. Therm. Anal. Calorim. 2004, 75, 521. (19) Starkweather, H. W. Crystallinity in perfluorosulfonic acid ionomers and related polymers. Macromolecules 1982, 15, 320. (20) Luan, Y.; Zhao, J.; Yang, J.; Ma, X. Thermal analysis of perfluorosulfonated ionomers. J. Therm. Anal. Calorim. 2012, 110, 1119. (21) Bian, Z. The research of fluorinated surfactants. Contemp. Chem. Ind. 2008, 37, 484 (in Chinese). (22) Falk, M. An infrared study of water in perfluorosulfonate (Nafion) membranes. Can. J. Chem. 1980, 58, 1495. (23) Cable, K. M.; Mauritz, K. A.; Moore, R. B. Effects of hydrophilic and hydrophobic counterions on the Coulombic interactions in perfluorosulfonate ionomers. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1065. (24) Liang, Z.; Chen, W.; Liu, J.; Wang, S.; Zhou, Z.; Li, W.; Sun, G.; Xin, Q. FT-IR study of the microstructure of Nafion® membrane. J. Membr. Sci. 2004, 233, 39. (25) Moukheiber, E.; De Moor, G.; Flandin, L.; Bas, C. Investigation of ionomer structure through its dependence on ion exchange capacity (IEC). J. Membr. Sci. 2012, 389, 294. (26) Heitner-Wirguin, C. Infra-red spectra of perfluorinated cationexchanged membranes. Polymer 1979, 20, 371. (27) Jung, H. Y.; Cho, K. Y.; Lee, Y. M.; Park, J. K.; Choi, J. H.; Sung, Y. E. Influence of annealing of membrane electrode assembly (MEA) on performance of direct methanol fuel cell (DMFC). J. Power Sources 2007, 163, 952. (28) Jung, H. Y.; Kim, J. W. Role of the glass transition temperature of Nafion 117 membrane in the preparation of the membrane electrode assembly in a direct methanol fuel cell (DMFC). Int. J. Hydrogen Energy 2012, 37, 12580. (29) Teng, X.; Dai, J.; Su, J.; Zhu, Y.; Liu, H.; Song, Z. A high performance polytetrafluoroethene/Nafion composite membrane for vanadium redox flow battery application. J. Power Sources 2013, 240, 131. (30) Xu, W.; Lu, T.; Liu, C.; Xing, W. Low methanol permeable composite Nafion/silica/PWA membranes for low temperature direct methanol fuel cells. Electrochim. Acta 2005, 50, 3280. (31) Chen, Z.; Holmberg, B.; Li, W.; Wang, X.; Deng, W.; Munoz, R.; Yan, Y. Nafion/Zeolite Nanocomposite Membrane by in Situ Crystallization for a Direct Methanol Fuel Cell. Chem. Mater. 2006, 18, 5669. (32) Fujimura, M.; Hashimoto, T.; Kawai, H. Small-angle X-ray scattering study of perfluorinated ionomer membranes. 1. Origin of two scattering maxima. Macromolecules 1981, 14, 1309. (33) Song, M. K.; Kim, Y. T.; Fenton, J. M.; Kunz, H. R.; Rhee, H. W. Chemically-modified Nafion®/poly(vinylidene fluoride) blend ionomers for proton exchange membrane fuel cells. J. Power Sources 2003, 117, 14. (34) de Almeida, S. H.; Kawano, Y. Thermal behavior of nafion membranes. J. Therm. Anal. Calorim. 1999, 58, 569. (35) Wilkie, C. A.; Thomsen, J. R.; Mittleman, M. L. Interaction of poly(methyl methacrylate) and nafions. J. Appl. Polym. Sci. 1991, 42, 901. (36) Kim, Y. S.; Dong, L.; Hickner, M. A.; Glass, T. E.; Webb, V.; McGrath, J. E. State of water in disulfonated poly(arylene ether sulfone) copolymers and a perfluorosulfonic acid copolymer (nafion) and its effect on physical and electrochemical properties. Macromolecules 2003, 36, 6281. (37) Wang, F.; Hickner, M.; Kim, Y. S.; Zawodzinski, T. A.; McGrath, J. E. Direct polymerization of sulfonated poly(arylene ether sulfone) random (statistical) copolymers: candidates for new proton exchange membranes. J. Membr. Sci. 2002, 197, 231.

(38) Kyu, T.; Hashiyama, M.; Eisenberg, A. Dynamic mechanical studies of partially ionized and neutralized nafion polymers. Can. J. Chem. 1983, 61, 680. (39) Di Noto, V.; Boaretto, N.; Negro, E.; Stallworth, P. E.; Lavina, S.; Giffin, G. A.; Greenbaum, S. G. Inorganic−organic membranes based on nafion, [(ZrO2)·(HfO2)0.25] and [(SiO2)·(HfO2)0.28] nanoparticles. Part II: Relaxations and conductivity mechanism. Int. J. Hydrogen Energy 2012, 37, 6215. (40) Yeo, S. C.; Eisenberg, A. Physical properties and supermolecular structure of perfluorinated ion-containing (nafion) polymers. J. Appl. Polym. Sci. 1977, 21, 875.

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