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Quaternized Polysulfone-Based Nanocomposite Membranes and Improved Properties by Intercalated Layered Double Hydroxide.

Two anions (dodecylbenzenesulfonate anion and stearate anion) are employed to synthesize intercalated layered double hydroxides (LDH) by co-precipitation method. Then the intercalated LDHs are incorporated in the casting solutions of chloromethylated polysulfone (CMPSF) for fabricating quaternized polysulfone/LDH nanocomposite membranes. Fourier transform infrared, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, and mastersizer laser particle size analysis are used to characterize the structure and morphology of LDHs and membranes. The properties of the composite membranes including water uptake, mechanical property, thermal stability, and ionic conductivity are investigated. Compared with other anion exchange membranes, both nanocomposite membranes containing 5% LDH sheets displayed better balanced performance. They exhibit the ionic conductivity of 3.58 x [10.sup.-2] S [cm.sup.-1] and 3.86 x [10.sup.-2] S [cm.sup.-1] at 60[degrees]C, respectively.

INTRODUCTION

Anion exchange membrane (AEM) fuel cells have been attracting more and more attention over the past several years as an alternative to proton exchange membrane fuel cells [1]. The former can offer several advantages over the latter, including faster electrokinetic, increased material stability, and broader choice of fuels [2]. But the AEMs can still not meet key requirements: sufficient ion conductivity, durability, and chemical stability. Two methods have been used to design optimized AEMs. One method is to develop novel polymer materials with functional groups different from quaternary ammonium [3-7]. Another approach is to modify the existing polymers [8-16].

Layered double hydroxides (LDH), known as anionic clays or hydrotalcite-like compounds, consist of positively charged sheets and small, exchangeable interlayer anions. LDHs have attracted considerable attention due to their tunable and versatile properties, such as high bound water content, high anion exchange capacity, reactive surface, pH-dependent solubility, low-toxicity, and low cost. Hence, they have various applications in different fields [17-24], including controlled release of drugs, soil remediation, adsorbents, mechanical reinforcement, fire-retardants, luminescent materials, electrochemical materials, and packaging materials. However, to the best of our knowledge, the incorporation of LDHs into AEMs for fuel cells has scarcely been reported [14], even if the ionic conductivity of LDHs can achieve the order of [10.sup.-3] S [cm.sup.-1] [25].

In this work, two intercalated LDHs were synthesized in-house and were used to prepare casting solutions consisting of dispersed LDH layers and chloromethylated polysulfone (CMPSF) in dimethylacetamide (DMAc). Then the nanocomposite AEMs based on quaternized polysulfone (QPSF) and LDHs were fabricated, and the effects of intercalated LDHs on the properties of nanocomposite AEMs were investigated.

EXPERIMENTAL

Chloromethylation of Polysulfone

Polysulfone was chloromethylated using chloromethyl ether as reported in our previous paper [15]. A typical example is as follows: 11 g polysulfone (Udel P3500, Solvay) was dissolved in 300 mL dichloromethane and then treated with 16 mL chloromethyl ether in the presence of 3 mL anhydrous tin chloride at 30[degrees]C for 30 min to obtain the CMPSF. At last, the mixture was precipitated in 3 L ethanol, followed by filtration, washing with water and evaporation.

Synthesis of LDHs

All the LDHs were prepared by the conventional coprecipitation method [20], 0.03 mol Mg[(N[O.sub.3]).sup.2]-6[H.sub.2]O and 0.01 mol Al[(N[O.sub.3]).sup.3] x 9[H.sub.2]O with 0.01 mol intercalating agent (sodium dodecylbenzenesulfonate or sodium stearate) were dissolved in 300 mL deionized water, the pH value was adjusted to approximately 10 with a 1 mol [L.sup.-1] NaOH aqueous solution. The reaction mixture was subsequently heated at 80[degrees]C for 24 h (Scheme 1). At the end of the co-precipitation process, the precipitated powder was filtered, washed thoroughly with deionized water, and finally vacuum dried. The as-prepared LDHs were designated as LDH-1 (sodium dodecylbenzenesulfonate-intercalated LDH) and LDH-2 (sodium stearate-intercalated LDH), respectively. All reagents were used as received without further purification.

Preparation of Dispersions Consisting of CMPSF and LDH in DMAc

The dispersions consisting of CMPSF and intercalated LDH in DMAc were prepared as follows. At first, the CMPSF was dissolved in DMAc to make a 15% w/v solution. Then an intercalated LDH (5 wt% of the mass of CMPSF) was added in the CMPSF solution. At last, the formed dispersion was stirred for 3 days at room temperature before rheological testing.

Preparation of QPSF Membrane and QPSF/Intercalated LDH Composite Membranes

The QPSF membrane was fabricated by solution casting. At first, the CMPSF was dissolved in DMAc to make a 15% w/v solution, then stirred and filtered. Before casting, N,N,N',N'-tetramethylethylenediamine (TMEDA) was added into the CMPSF/DMAc solution as crosslinking agent (0.2 mL/10 g casting solution) for forming partially crosslink of membranes. Then the film was dried under vacuum at 80[degrees]C for 24 h. The resultant membrane was immersed in 30 wt% trimethylamine (TMA) solution for 24 h to introduce quaternary groups into the membrane, the membrane was put into 1 M KOH solution for 24 h, and the alkaline QPSF membrane was obtained. At last, the alkaline QPSF membrane was washed several times with distilled water and naturally dried under ambient environment to avoid shrinkage due to water loss.

The QPSF composite membranes were also fabricated by solution casting. Firstly, intercalated LDH of a prescribed weight was blended with CMPSF/DMAc (15% w/v) solution, and the mixture was stirred for 24 h at room temperature. After 1 h ultrasonic treatment, the solution was dropped in TMEDA (0.2 mL/10 g casting solution) and kept stirring vigorously for 10 min. Then, it was spread on a glass plate with a glass knife. The film was dried under vacuum at 80[degrees]C for 24 h. The obtained composite membrane was then treated with TMA, KOH solution, and distilled water in turn. The resulting composite membrane was designated as QPSF/y% LDH-x membrane, where x was the same number as LDH-x (x= 1,2), and y was the content of intercalated LDH.

Structure Characterizations

The FT-IR spectra of samples were recorded using a VER-TEX70 spectrometer. The thermal analysis of the samples was performed under nitrogen by using a NETZSCH Instrument. An X-ray diffractometer (X'Pert PRO) was used to determine the change in d-spacing of LDH crystals. Cu-K[alpha] ([lambda] = 1.54 A) was used as an X-ray source at d-generator voltage of 45 kV and current of 80 mA. Samples were scanned in 2[theta] ranges from 2[degrees] to 10[degrees] in steps of 0.02[degrees] and counting time 2 s per step. Rheological behavior of dispersions consisting of CMPSF and intercalated LDH in DMAc were carried out by a rheometer (AR2000EX). The particle sizes of intercalated LDH in DMAc (2 wt%) were examined with a Mastersizer laser particle size analyzer. The surface of samples was examined using a SEM (JEOL JSM-6510). After the sample was dried, a thin gold film was vacuum-deposited for the SEM examination.

Water Uptake

All membranes were vacuum dried at 120[degrees]C for 12 h before water uptake testing. Then the sample films were soaked in deionized water for 24 h at a certain temperature. Weights of dry and wet membranes were measured. The water uptake content was calculated by uptake content

(%) = [[omega].sub.wet] - [[omega].sub.dry]/[[omega].sub.dry] x 100 (1)

where [[omega].sub.dry] and [[omega].sub.wet] are the masses of dried and wet samples, respectively.

Mechanical Properties

Tensile strength was measured by using an INSTRON WN5566 Mechanical Testing Machine. The membrane samples were cut into 0.5 cm X 5.0 cm strips and immersed in water for 24 h at room temperature before testing. The samples were tested using an elongation rate of 10 mm/min. The tensile strength was calculated with the following equation:

Tensile strength = Maximum load/Cross sectional area (N/[mm.sup.2]) (2)

The cross-sectional area is the initial value.

Hydroxide Ion Conductivity

Ionic conductivity (o) of the membranes was measured by a two-probe AC method from 1 Hz to 500 KHz and 10 mV AC perturbation on an Autolab work station. A sample with size of 15 mm x 15 mm was placed in an open, temperature controlled cell where it was clamped between two platinum electrodes. Specimens were soaked in deionized water at least 48 h prior to the test. The impedance measurements were performed in water with 100% relative humidity (RH) at desired temperature. The o out of plane of the membranes was calculated from the impedance data using the formula:

[sigma] = d/R x A (3)

where d and A are the thickness and face area of the sample, respectively, and R is obtained from the real value (Z') of the impedance where the imaginary response (Z") was zero.

RESULTS AND DISCUSSION

XRD Analysis

Figure 1 displays the XRD patterns of LDHs. The characteristic peaks for the basal, second-, and third-order reflections can be easily recognized indicating that these samples have ordered layered structures. Compared with the 26 angles (10[degrees]) of the basal (003) peak of the LDH without intercalation in literature [18], the corresponding peaks of intercalated LDHs shift to lower angles and are located at 3.07[degrees] and 5.64[degrees], respectively. The interlayer distance, [d.sub.003], can be calculated from the first basal reflections by Bragg's equation and is shown in Fig. 1. This presents evidence for the success of intercalation.

FT-IR Spectra Analysis

LDHs are further characterized by FT-IR spectroscopy to confirm the presence of the intercalating agents. In Fig. 2a, all curves show characteristic peaks of LDHs, the strong and broad adsorption peak centered at 3,478 [cm.sup.-1] is related to the overlapped stretching vibrations of the interlayer water and hydroxyl groups. Some bands below 800 [cm.sup.-1] region can be associated with the lattice vibration of the M-0 and O-M-O (M = Mg or Al). The peaks at 2,852 and 2,927 [cm.sup.-1] are the characteristic adsorptions of the C-H stretching vibrations of the alkyl chain in the aliphatic anions. The symmetric and asymmetric stretching vibrations of S=O are observed at 1,186 and 1,037 [cm.sup.-1], respectively. Meanwhile, the symmetric and asymmetric stretching vibrations of CO[O.sup.-] are also observed at 1,414 and 1,558 [cm.sup.-1], respectively. These results are consistent with the literature [26]. The characteristic peaks of LDH-1 are masked by the adsorption peaks of QPSF in QPSF/LDH-1 composite membranes except for the peak at 1,037 [cm.sup.-1], which is insensitive to the content of the LDH-1 in QPSF/ LDH-1 composite membranes. All the characteristic peaks of LDH-2 are clearly overlapped with the adsorption peaks of QPSF in QPSF/LDH-2 composite membranes. The increase of LDH-2 content in QPSF/LDH-2 composite membranes is hinted only through the slightly increased intensity of absorption bands at 2,927 [cm.sup.-1].

Thermal Analysis

In order to study the decomposition behavior of LDHs and QPSF/LDH composite membranes, the TGA curves of them are shown in Fig. 3. As shown in Fig. 3a, it can be very clearly observed that the weight losses of LDH-1 and LDH-2 are mainly undergone via three steps. Below 200[degrees]C, the surface adsorbed and interlayer water molecules are released. From 200[degrees]C to 500[degrees]C, the dehydroxylation of the metal hydroxide layer and the decomposition of the interlayer anions occur.

Above 500[degrees]C, the collapse of the LDH layer happens. Although it is difficult to distinguish the weight loss value of the dehydroxylation of the metal hydroxide layer and the decomposition of the interlayer anions between 200[degrees]C and 500[degrees]C, it is obvious that the content of intercalating agents in LDH-2 is higher than that in LDH-1.

For the TGA curve of QPSF in Fig. 3b, the thermal behavior can be mainly divided into three degradation stages. The first one from room temperature to 165.6[degrees]C can be related to the removal of physically and chemically absorbed water molecules. The second one with a gradual weight loss from 165.6[degrees]C to 373.1[degrees]C may be attributed to the degradation of the quaternary ammonium groups in polymer chains. The third one above 373,1[degrees]C corresponds to the decomposition of the polymer backbones. After incorporation of LDHs, the composite membranes show different weight loss behavior, as seen in Fig. 3b. Before 500[degrees]C, the composite membranes lose weight faster than the QPSF membrane because of the decomposition of the intercalating agents. But the residual parts of composite membranes are higher than that of the QPSF membrane due to the existence of the metal oxides in the LDHs. Furthermore, the degradation temperatures of the quaternary ammonium groups in the composite membranes shift to higher values compared with the QPSF membrane because of the strong interactions between LDH sheets and polymer chains. The higher degradation temperature of the quaternary ammonium groups in the QPSF/ 10%LDH-1 membrane than that in the QPSF/10%LDH-2 membrane suggests better dispersion and stronger interactions between LDH sheets and polymer chains in the QPSF/ 10%LDH-1 membrane.

Morphological Analysis

As seen from the SEM images in Fig. 4, the particles of LDHs exhibited irregular agglomerates with size varying from 0.1 to 10 [micro]m, but the sheet-like morphology of LDHs can be seen. Because of the hydrophobic property, LDH-1 and LDH-2 could not be well dispersed in water during the synthesis, which resulted in bigger particles. After they are adequately subjected to ultrasonic treatment in DMAc, the particles are exfoliated and the particle size distribution of them is measured and shown in Fig. 5. In contrast to the SEM images, both of them show an average particle size in the nanometer scale (the value of average particle size is directly obtained from Malvern mastersizer software). When LDHs are added in the DMAc solutions of CMPSF, the nanoscale particle distribution of LDHs can be maintained even after standing for 24 h (Fig. 6). In order to achieve a good dispersion of inorganic fillers in a polymer membrane, a high viscosity of the casting solution is favorable.

Rheological measurements could present a clue on the stability of a casting solution via the viscosity. The interactions between particles and polymer molecules determine the viscosity. Higher viscosity means stronger interactions. Smaller fluctuation of viscosity corresponds to better stability of the suspension. The influence of LDHs on viscosity with shear rate for CMPSF/LDH dispersions is illustrated in Fig. 7. The CMPSF solution behaves in a non-Newtonian manner but only with very slight shear-thinning phenomenon. After LDHs are added, the viscosity of both dispersions is increased by 50% at least because of the intercalation effects due to the presence of inorganic fillers. The rheological behavior of CMPSF/LDH-1 dispersions is close to Newtonian, while the viscosity of CMPSF/LDH-2 dispersions demonstrates slight shear thinning. However, the viscosity drop is not more than 0.15 Pa s. These results indicate that the casting solutions of CMPSF/LDHs could provide high viscosity and enough stability for the fabrication of AEMs with excellent compatibility between LDH sheets and polymer chains, Fig. 8. As shown in Fig. 8, it could be observed that both of them are transparent, but the transparency of the QPSF/5%LDH-2 composite membrane is inferior to that of the QPSF/5%LDH-1 composite membrane. This phenomenon could be attributed to the differences of dispersion of LDH sheets in the composite membranes, which is also supported by the SEM images of them, Fig. 9. As shown in Fig. 9, the thickness of all LDH sheets is in the nanometer range, but the distribution and size of LDH-1 sheets are better and smaller than those of LDH-2 sheets. The QPSF/5%LDH-1 composite membrane demonstrates smoother morphology than the QPSF/5%LDH-2 composite membrane.

Water Uptake

The anion transference is dependent on water uptake in AEMs, but superfluous water in AEMs would damage their mechanical performance. The water uptakes of QPSF membrane and QPSF/LDH composite membranes are shown in Fig. 10. All membranes display similar increasing curves of water uptake with the increase of temperature. Furthermore, water uptake considerably increases when temperature increases from 80[degrees]C to 95[degrees]C. Comparing the QPSF membrane with QPSF/LDH composite membranes, it is clear that the water uptake decreases after incorporation of LDFI sheets. For every series of composite membranes, the QPSF/LDH composite membrane with 5% content of LDH sheets has the lowest water uptake. For the QPSF/ LDH-1 and QPSF/LDH-2 composite membranes with a same content of LDH sheets, the water uptake of the QPSF/LDH-1 composite membrane is slightly lower than that of the QPSF/ LDH-2 composite membrane due to better distribution of LDH-1 sheets in composite membranes. Table 1 listed the water uptakes of all AEMs at 60[degrees]C. Among them, the water uptakes of the QPSF/5%LDH-1 and QPSF/5%LDH-2 composite membranes are 36.1% and 38.9% at 60[degrees]C, respectively, which could meet the water content requirements of low temperature AEM fuel cells.

Mechanical Properties

Since AEMs are usually hydrated when they work under real running conditions, the QPSF membrane and QPSF/LDH composite membranes were immersed in water for 24 h before the tensile tests. As shown in Fig. 11 and Table 1, the QPSF membrane displayed a tensile strength of around 3.7 MPa, which was close to the tensile strength of the humidified Nafton membrane in literature [26] and an elongation at break of more than 100% due to the plasticization by water molecules. The QPSF/LDH composite membranes demonstrate higher tensile strength than that of the QPSF membrane because of the physical crosslinks induced by the LDH sheets. The tensile strength of QPSF/LDH composite membranes increases with the increase of the LDH sheet content. Similarly, the QPSF/LDH-1 composite membranes have slightly higher tensile strength than that of the QPSF/LDH-2 composite membranes with the same content of LDH sheets. All the composite membranes exhibit a moderate elongation at break, which is in excess of 50%. These results indicate that the QPSF membrane and QPSF/LDH composite membranes are suitable for the AEM fuel cells from the view of mechanical properties.

Ionic Conductivity

It is generally known that the apparent activation energy ([E.sub.a]) of ionic conductivity could be calculated through the Arrhenius relationship between ionic conductivity and temperature. The Arrhenius plots of ionic conductivity of the QPSF membrane and QPSF/LDH composite membranes are displayed in Fig. 12. Compared with the [E.sub.a] of the QPSF membrane, it is apparent that the [E.sub.a] of the composite membranes has slightly increased. More importantly, all these values are comparable to that of AEMs (16.5 kJ mol-1) in literature [28]. The ionic conductivities of the QPSF/ 5%LDH-1 composite membrane and the QPSF/5%LDH-2 composite membrane are the highest one among the corresponding series. It can be explained by two opposite effects of LDH sheets. On the one hand, the incorporation of LDH sheets reduces the water uptake, which results in higher effective concentration of quaternary ammonium groups and consequently higher ionic conductivity. On the other hand, the incorporation of LDH sheets leads to lower ionic conductivity due to their low efficiency of anion transfers. As shown in Table 1, the QPSF/5%LDH-1 and QPSF/ 5%LDH-2 composite membranes show ionic conductivities of 3.58 x [10.sup.-2] S [cm.sup.-1] and 3.86 x [10.sup.-2] S [cm.sup.-1] at 60[degrees]C, respectively. Both of them are higher than the value (3.04 X [10.sup.-2] S [cm.sup.-1]) of the QPSF membrane at 60[degrees]C. The results indicate that the QPSF/ 5%LDH-1 and QPSF/5%LDH-2 composite membranes have great potential for application in the low temperature AEM fuel cells.

CONCLUSIONS

The intercalated LDH with two different types of organic species has been successfully prepared in the present study. When these LDH sheets were dispersed in the DMAc solution of CMPSF, the average LDH sizes are in the nanometer scale, and the viscosity of the dispersions increases, which resulted in optically transparent AEMs. Compared with the QPSF membrane, the QPSF/LDH composite membranes showed higher thermal stability, lower water uptake, enhanced tensile strength, and increased ionic conductivity. The QPSF/5%LDH-1 and QPSF/5%LDH-2 composite membranes displayed favorable performance, suggesting a promising prospect of use as AEMs for AEM fuel cells.

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Na Liang, Wan Liu, Danying Zuo, Pai Peng, Rong Qu, Dongzhi Chen, Hongwei Zhang (iD)

College of Materials Science and Engineering, Wuhan Textile University, Wuhan 430073, People's Republic of China

Correspondence to: H. Zhang; e-mail: hanqiujiang@163.com

Contract grant sponsor: National Natural Science Foundation of China (NSFC); contract grant number: 51503161.

DOI 10.1002/pen.24612

Caption: SCHEME 1. Schematic diagram of LDH synthesis

Caption: FIG. 1. X-ray diffraction patterns of LDHs. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. FT-IR spectra of (a) LDHs, (b) QPSF/LDH-1, and (c) QPSF/LDH-2 composite membranes. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. TGA curves of (a) LDHs, (b) QPSF/10%LDH-1, and (c) QPSF/ 10%LDH-2 composite membranes. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. SEM images of LDHs.

Caption: FIG. 5. Size distribution of LDHs in DMAc. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. Tyndall effect demonstrated by the red light beams from one side and the stability of CMPSF/5% LDFIs dispersions after 24 h. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. Steady viscosity as a function of shear rate at 25[degrees]C for DMAc solution of CMPSF and CMPSF/5% LDHs. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. Images of QPSF/5%LDH composite membranes. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. SEM images of QPSF/5%LDH composite membranes. [Color fig ure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 10. Water uptakes of QPSF membrane and QPSF/LDH composite membranes. [Color figure can be viewed at wileyonIinelibrary.com]

Caption: FIG. 11. Stress-strain curves of QPSF membrane and QPSF/LDH composite membranes. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Water uptake, ionic conductivity, and mechanical properties
of aems.

                  Water            Ionic
                  uptake        conductivity     Tensile    Maximum
                  (%) at      (S [cm.sup.-1])    strength  elongation
Membranes      60[degrees]C   at 60[degrees]C     (MPa)       (%)

QPSF               42.7      3.04 x [10.sup.-2]   3.766      111.34
QPSF/3%LDH-1       39.6      3.28 x [10.sup.-2]   5.946      63.33
QPSF/5%LDH-1       36.1      3.58 x [10.sup.-2]   6.503      67.40
QPSF/10%LDH-1      38.2      2.60 x [10.sup.-2]   6.811      75.33
QPSF/3%LDH-2       40.6      3.66 x [10.sup.-2]   5.569      94.67
QPSF/5%LDH-2       38.9      3.86 x [10.sup.-2]   6.395      87.11
QPSF/10%LDH-2      43.6      3.0 x [10.sup.-2]    6.273      55.33

FIG. 12. Arrhenius plots of ionic conductivity of the QPSF membrane
and QPSF/LDFI composite membranes. [Color figure can be viewed at
wileyonlinelibrary.com]

(a)

Ionic conductivity (ln(S [cm.sup.-1]))

                    [E.sub.a]

QPSF                11.98 kJ [mol.sup.-1]
QPSF/3%LDH-1        13.09 kJ [mol.sup.-1]
QPSF/5%LDH-1        12.66 kJ [mol.sup.-1]
QPSF/10%LDH-1       15.23 kJ [mol.sup.-1]

(b)

Ionic conductivity (ln(S [cm.sup.-1]))

                    [E.sub.a]

QPSF/3%LDH-2        14.87 kJ [mol.sup.-1]
QPSF/5%LDH-2        13.54 kJ [mol.sup.-1]
QPSF 10%LDH-2       13.66 kJ [mol.sup.-1]
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Author:Liang, Na; Liu, Wan; Zuo, Danying; Peng, Pai; Qu, Rong; Chen, Dongzhi; Zhang, Hongwei
Publication:Polymer Engineering and Science
Article Type:Report
Date:May 1, 2018
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