Poly(styrene-co-acrylonitrile) based proton conductive membranes

Available online at www.sciencedirect.com

EUROPEAN POLYMER JOURNAL

European Polymer Journal 44 (2008) 1462–1474

www.elsevier.com/locate/europolj

Poly(styrene-co-acrylonitrile) based proton conductive membranes Adney L.A. Silva a,b, Iracema Takase b, Robson Pacheco Pereira c, Ana Maria Rocco a,* a

Grupo de Materiais Condutores e Energia, Escola de Quı´mica, Universidade Federal do Rio de Janeiro, Bloco E, Cidade Universita´ria, Rio de Janeiro, RJ, Brazil b Departamento de Quı´mica Analı´tica, Instituto de Quı´mica, Universidade Federal do Rio de Janeiro, RJ, Brazil c Grupo de Materiais Condutores e Energia, Rio de Janeiro, RJ, Brazil Received 28 September 2007; accepted 26 February 2008 Available online 7 March 2008

Abstract Sulfonated poly(styrene-co-acrylonitrile) (PSAN–SO3H) membranes were obtained by sulfonation of the original styrene–acrylonitrile copolymer, in different molar ratios, and characterized by vibrational spectroscopy (FTIR), thermal analyses (TGA and DSC) and electrochemical impedance spectroscopy (EIS). The thermal stability of the sulfonated polymers exhibited a dependence on the sulfonation degree and reached 261 °C for samples up to 1:4 (sulfonating agent to styrene unit). FTIR spectra showed the covalent incorporation of sulfonic groups at the styrene units, confirming the PSAN–SO3H formation. Vibrational spectra also indicated the presence of hydronium ions and dissociated sulfonic groups, indicating the existence of mobile protons for ion conduction. DSC analyses evidenced two glass transition temperatures (Tg), one associated with an ion-water domain and other with the chain backbone glass transition. The maximum conductivity of PSAN–SO3H membranes at ambient temperature was about 103 X1 cm1, achieving 102 X1 cm1 at 80 °C. The conductivity dependency on the temperature was found to be linear, similarly to other sulfonic acid polymers described on the literature, and the water uptake reaches 45.7% of the polymer mass, against 18.9% of the original copolymer. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Polymer membrane; Proton conduction; Electrochemical impedance; Sulfonation reaction

1. Introduction In the last decades, fuel cells (FC) developing and testing have gained crescent attention, especially due to environmental and economical aspects. * Corresponding author. Tel.: +55 21 2562 7595; fax: +55 21 2562 7598. E-mail address: [email protected] (A.M. Rocco).

Among the FC types, proton exchange membrane (also ‘‘polymer electrolyte membrane”) fuel cells (PEMFC) have attained a great interest of researchers worldwide. The main applications for which PEMFC development is crucial include: stationary, automotive and portable power. The development of proton exchange membranes aims to obtain systems with optimized properties and reduced cost, as alternative to NafionÒ, which is the most used

0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.02.025

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membrane for PEMFC applications. Membranes based on styrene or imidazole copolymers have been developed, in an effort to obtain systems with high thermal stability and conductivity. Proton conduction in these membranes are described by a vehicular mechanism in presence of water [1] or hopping, in dry systems [2,3]. Additionally, composite and nanocomposite membranes [4] exhibited enhanced stability and conductivity, and can be considered a good alternative to pure NafionÒ. Poly(styrene-co-acrylonitrile) (PSAN) is a commercially available polymer, which exhibits the ease of processing of polystyrene combined with the rigidity and chemical resistance of polyacrylonitrile. Thus, PSAN is widely used in several applications due to good mechanical properties, chemical resistance and ease of processing [5]. PSAN gel polymer electrolytes doped with NaI/I2 have been studied for dye-sensitized solar cells [6,7], exhibiting ionic conductivity values of about 103 X1 cm1. Sulfonation of commercial polymers is a strategy developed and used during the last three decades [8] to obtain modified polymers for specific applications. Sulfonated polymers have been employed in different technological applications, such as ultrafiltration membranes [9] and PEMFC units [10]. In general, sulfonic acid polymers exhibit hydrophilicity, ion conductivity and mechanical properties highly dependent on the sulfonation degree [11]. Polyacrylonitrile (PAN) grafted with sodium styrene sulfonate were synthesized and characterized by Holdcroft and co-workers [12]. The authors investigated the effect of PAN hydrophilicity on the copolymers properties, such as the ionic domain morphology, water uptake and ion conductivity. However, there are few studies about the application of PAN and its copolymers as ion-conductive membranes [13–15]. The main goal of the present work is to develop and characterize proton-conductive membranes based on poly(styrene-co-acrylonitrile) by acid doping and also by covalent insertion of sulfonic acid groups directly on the styrene units. 2. Experimental 2.1. Materials and samples preparation Poly(styrene-co-acrylonitrile) (PSAN, Mw = 16.5  104 g/mol, 25% AN) was purchased from Aldrich Chem. Co. and utilized as received. Dichloromethane (CH2Cl2), sulfuric (H2SO4) and phos-

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phoric (H3PO4) acid were obtained from Merck; CH2Cl2 was distilled prior to utilization, H3PO4 and H2SO4 were utilized as received. H3PO4-doped PSAN membranes were obtained in mass ratios of 1%, 2%, 5% and 10%. PSAN and H3PO4 were dissolved in CH2Cl2 at 60 °C under stirring during 4 h and membranes were obtained by casting from these solutions onto Petry dishes. The solvent was removed under vacuum at 100 °C until constant weight. The procedure described by Elabd and co-workers [16] was employed to obtain sulfonic acid PSAN (PSAN–SO3H) samples. The sulfonating agent is formed by mixing CH2Cl2 to acetic anhydride and H2SO4 in an ice bath. The sulfonating agent was added to PSAN solutions in adequate concentrations to provide molar ratios of 1:4; 1:2; 1:1; 2:1; 4:1 and 6:1 (moles of sulfonating agent to moles of styrene). Each solution was stirred during 4 h and the reaction was stopped by the addition of distilled water. The precipitate was filtrated, washed five times with methanol and water and dried under vacuum at 60 °C. PSAN–SO3H membranes were obtained by casting from CH2Cl2 solutions onto Petry dishes and the solvent was removed under vacuum at 100 °C until constant weight. 2.2. Spectroscopic characterization Vibrational infrared (FTIR) spectra of PSAN– SO3H membranes were obtained in a Nicolet Magna-IR760 in the spectral region from 4000 to 400 cm1 with 1 cm1 resolution, 128 scans per sample and optimized gain. 2.3. Thermal characterizations Prior to thermal analysis, samples were dried under vacuum at 100 °C during four days. Thermogravimetric (TGA) curves were obtained in a TA2950 TGA equipment, operating under N2 50 mL/min flow from 20 to 800 °C with a heating rate of 20 °C/min. Differential scanning calorimetry (DSC) analyses were performed in a TA2910 MDSC equipment. Samples were heated from 20 to 200 °C at a heating rate of 20 °C/min. After a 3 min isotherm, samples were cooled to 20 °C, kept at this temperature during 5 min and then heated to 200 °C at 20 °C/ min. Thermal data were collected in the last heating scan for all samples.

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2.4. Electrochemical impedance spectroscopy (EIS) Impedance spectra of the membranes were obtained in an AUTOLAB PGSTAT30/FRA equipment in the frequency region from 10 MHz to 1 MHz and 1 cm2 stainless steel blocking electrodes. PSAN/H3PO4 and PSAN–SO3H membranes were studied from 25 to 80 °C under dry conditions and under 100% humidification. The resistance (R) obtained at the semi-circle intercept point on the real axis (Z0 ) of the impedance spectra was used to calculate the proton conductivity (r) values with Eq. (1), in which L is the thickness of the film, A is the geometric area (1 cm2) and R is the resistance, obtained trough simulation of the resistance response in the impedance spectra. r ¼ L=A  R

ð1Þ

2.5. Water uptake PSAN–SO3H membranes (3  1 cm2 strips) were immersed in distilled water at 25 °C during 48 h and then dried with paper tissue, weighted (WS), dried under vacuum at 80 °C and weighted until constant weight (W0). Water uptake (S%) was calculated from the following equation, previously used by other authors [17]: S% ¼

ðW S  W 0 Þ  100 W0

ð2Þ

3. Results and discussion 3.1. PSAN/H3PO4 doped membranes: electrochemical analyses Electrochemical impedance spectra of H3PO4doped PSAN membranes (PSAN/H3PO4) without hydration and after 24 h immersion in water are illustratively shown in Fig. 1. Conductivity (r) values were calculated for different water immersion times using Eq. (1), from resistance (R) values obtained in triplicate from the impedance spectra, and are listed in Table 1. Impedance spectra obtained for PSAN/H3PO4 membranes present the characteristic features of ion-conductive systems, with a semi-circle in the high frequency region and a straight line in the low frequency region. The semi-circle is associated with the resistive response in the bulk of the

membrane, while the straight line reflects capacitive effects in the electrode/electrolyte interface [18]. The maximum conductivity value for acid-doped membranes at ambient temperature was obtained for the membrane containing 10% H3PO4 after immersion in water during 6 h. This membrane exhibited r of (6.55 ± 0.89) 108 X1 cm1, while the series presented values between 1012 and 108 X1 cm1. Increasing acid content and immersion time of PSAN/H3PO4 membranes in water, there is a tendency of increase in r values, evidencing a contribution of the vehicular mechanism in the proton conduction [19]. The proton conduction mechanisms described for acid-doped membranes are strongly dependent on the nanostructure [20], local structure and specific interactions in the solid. PSAN/H3PO4 membranes present a conductivity behavior which is characteristic of proton conduction strongly influenced by a vehicular mechanism. The vehicular mechanism is controlled, at a molecular level, by the interactions between the hydronium ion (H3O+) and the water molecules which surround it. Additionally, the basic nitrile sites in PSAN chain probably allow the proton transport in the absence of water and the resistive behavior of dry PSAN/H3PO4 membranes indicates a contribution of the hopping mechanism to the proton conduction [21]. Despite the basic sites in the molecular structure of PSAN and the amount of acid content in the samples, the conductivity of H3PO4-doped membranes does not reach adequate values for practical applications. In order to achieve PSAN-based membranes with conductivities of about 103 X1 cm1, chemical modification on the PSAN chain is proposed, specifically by introducing sulfonic acid groups on the styrene units, obtaining PSAN–SO3H membranes. 3.2. PSAN–SO3H membranes

3.2.1. FTIR analyses In order to evaluate the extension of the sulfonation reaction in the membrane compared to the original PSAN, vibrational spectra of PSAN and PSAN–SO3H samples in the region from 4000 to 2500 cm1 are depicted in Fig. 2a. Vibrational bands associated with CH stretching from styrene units in PSAN and PSAN–SO3H 1:4 and 1:2 spectra present a progressively decrease in intensity for increasing the sulfonation degree. PSAN–SO3H membranes between 1:1 and 6:1 do not exhibit these

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5

5x10

1% H3PO4 2% H3PO4 10% H3PO4

-Z'' (Ω)

10kHz

10mHz

0

200Hz

1MHz

0.0

5.0x10

5

1.0x10

6

1.5x10

6

Z' (Ω) 4x10

5

-Z'' (Ω)

1% H3PO4 2% H3PO4 10% H3PO4

2x10

5

1MHz

10mHz

27kHz

0 0

2x10

5

4x10

6x10

5

8x10

5

Z' (Ω) Fig. 1. Electrochemical impedance spectra of H3PO4-doped PSAN membranes (PSAN/H3PO4) without hydration (a) and after 24 h immersion in water (b).

Table 1 Conductivity values (r, X1 cm1) of PSAN/H3PO4 membranes under different immersion times in water, at ambient temperature Time (h)

1% H3PO4

2% H3PO4

0 (dry) 2 4 6 8 24

(2.95 ± 0.63) 1010 (6.97 ± 0.61) 1012 (5.36 ± 0.42) 109 (1.73 ± 0.14) 108 (6.51 ± 0.69) 108 (2.39 ± 0.55) 109

(7.66 ± 0.51) (3.96 ± 0.37) (1.68 ± 0.33) (1.68 ± 0.14) (4.66 ± 0.69) (2.87 ± 0.13)

CH peaks, indicating the formation of the sulfonic acid polymer. Fig. 2a also shows the vibrational bands associated with the OH stretching mode, which is present in all PSAN–SO3H samples and absent in PSAN. These bands are formed by a spectroscopic contribution of higher intensity, with maximum between 3420 and 3460 cm1, marked as (1). This broad band is, in general, attributed to hydrated acid groups and reflects the hydrophilicity of the obtained material in comparison to PSAN. Additionally, shoulders with different intensities are located in region (2). The wavenumber of these low-intensity contributions is about 3130 cm1 evi-

5% H3PO4 1010 1012 1012 1011 109 108

(2.57 ± 0.37) (1.85 ± 0.02) (5.27 ± 0.31) (9.05 ± 1.18) (1.80 ± 0.58) (2.16 ± 0.73)

10% H3PO4 1011 1011 1011 1011 108 108

(3.74 ± 0.46) (1.57 ± 0.03) (3.95 ± 0.41) (6.55 ± 0.89) (6.29 ± 1.96) (3.40 ± 0.36)

109 109 109 108 1012 109

dencing, according to Iwamoto and co-workers [22], the presence of hydronium ions (H3O+), originated from the dissociation of sulfonic groups. In their work, the authors also observed a spectroscopic contribution at 3370 cm1, utilizing differential spectroscopy techniques. In the present work, however, this particular contribution was not detected, due to the higher intensity of the OH band, between 3420 and 3460 cm1. Additionally, carboxylic acid groups originated from the hydrolysis of nitrile units may contribute to the band signal, specially for higher sulfonation degrees. FTIR spectra of PSAN and PSAN–SO3H membranes exhibited the presence of a nitrile vibrational

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(I)

1728 cm

(II)

-1

1630 cm

-1

Absorbance (arb. units)

Absorbance (arb. units)

6:1 4:1

2:1

1:1

6:1 4:1

2:1 1:1

1:2

1:2

1:4

1:4

PSAN

PSAN

4000

3500

3000

2500

2000

1900

-1

1800

1700

1600

1500

-1

Wavenumbers (cm )

Wavenumbers (cm )

2236 cm

-1

6:1 4:1

Absorbance (arb. units)

Absorbance (arb. units)

6:1

4:1 2:1 1:1

1:2

2:1 1:1

1:2

1:4 1:4

PSAN PSAN

1600

1400

1200

1000 -1

Wavenumbers (cm )

2300

2275

2250

2225

2200

2175

-1

Wavenumbers (cm )

Fig. 2. FTIR spectra of PSAN and PSAN–SO3H membranes in regions (a) 4000–2500 cm1, (b) 2000–1500 cm1, (c) 1650–950 cm1 and (d) 2300–2175 cm1.

band, centered at 2236 cm1 (Fig. 2d). The nitrile vibrational band presents a markedly intensity decrease for PSAN–SO3H 1:1, resulting in a negligible contribution for samples with higher sulfonation degrees. In Fig. 2b, the FTIR spectra of PSAN and PSAN–SO3H membranes between 2000 and 1500 cm1 are shown.

For all PSAN–SO3H membranes, FTIR spectra exhibited two contributions in this region, at 1728 and 1630 cm1, while in PSAN spectrum both bands are absent. There is a change in the relative intensity between the two contributions with the sulfonation degree of PSAN. For PSAN–SO3H 1:2 and 1:1, the contribution at 1630 cm1 presents

A.L.A. Silva et al. / European Polymer Journal 44 (2008) 1462–1474

higher intensity and, for 2:1, 4:1 and 6:1, there is an inversion, for which the contribution at 1728 cm1 presents progressively higher intensity. The band at 1728 cm1 can be associated with carbonyl (C@O) stretching from a carboxylic acid. Considering the absence of the C„N band for PSAN–SO3H membranes with high sulfonation degrees, the presence of a carboxylic acid is possibly a result of the nitrile group hydrolysis in acid media [23]. The band at 1630 cm1 was attributed, according to Iwamoto and co-workers [22], to a sulfonic acid group vibration, while the 1728 cm1 band could contain a contribution from hydronium ions. In the present work, the carbonyl stretching intensity is probably superimposed to these vibrational contributions, due to concentration effects. Analysis of the vibrational bands associated with water, hydronium and sulfonic group for PSAN– SO3H membranes allows describing the PSAN– SO3H dissociation in water (absorbed by the membrane) by the equation PSANSO3 H þ nH2 O ! PSANSO 3 þ ½H3 O þ ðH2 OÞn1  Fig. 2c shows the vibrational spectra for PSAN and PSAN–SO3H samples in the region between 1650 and 950 cm1. As seen in Fig. 2c, the of C@C stretching modes at 1490 and 1450 cm1, present in PSAN and PSAN–SO3H (1:4, 1:2 and 1:1) spectra are not observed for membranes with sulfonation degrees 2:1, 4:1 and 6:1. This behavior, combined with the other features observed in the FTIR spectra, indicates the complete sulfonation for PSAN–SO3H 2:1, 4:1 and 6:1 membranes. The broad band between 1150 and 1275 cm1 contains contributions from the sulfonic acid group covalently bonded to the styrene units. This band is essentially formed by sulfonic acid in proton (–SO3H) and ionized (–SO3) forms. According to Atorngitjawat and co-workers [24], the bands at 1200 and 1042 cm1 are attributed, respectively, to asymmetric and symmetric sulfur–oxygen stretching modes. Bands at 1128 and 1011 cm1 are attributed to sulfonic acid substituted aromatic ring vibrations [25,26]. 3.2.2. Thermal analyses Fig. 3 shows the TGA curves for PSAN and PSAN–SO3H membranes. The different thermal

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decomposition behavior for each sample can be observed as a function of the sulfonation degree. Pure PSAN exhibits 3% weight loss up to 350 °C, temperature in which the complete decomposition of the sample takes place. Up to 20% weight loss is verified for PSAN–SO3H samples below 350 °C, while sulfonic acid polymers described in the literature, such as SPEEK [13], present similar weight loss below 250 °C. In the present work, PSAN– SO3H membranes with higher sulfonation degree retain, as expected, higher water amounts according to the FTIR analyses. These water molecules may diffuse though the membrane during the TGA heating scan being progressively released, causing a larger decomposition temperature range for PSAN– SO3H samples with higher sulfonation degrees. Despite drying the samples at 100 °C during 96 h prior to TGA analysis, the initial weight loss observed for the membranes is compatible with loss of absorbed water. As observed in the TGA curves, the initial decomposition temperature is inversely proportional to the sulfonation degree. The PSAN–SO3H 1:4 membrane presented 2% weight loss up to 220 °C exhibiting, for higher temperatures, a multi-stage decomposition behavior. Dehydration of PSAN–SO3H 1:2, 1:1, and 2:1 membranes takes place up to 250 °C, while 4:1 and 6:1 membranes exhibited higher weight loss for temperatures below 150 °C. The membranes obtained with higher sulfonation degrees (4:1 and 6:1) are probably in the carboxylic acid form (after the CN group hydrolysis), undergoing decomposition in the temperature range of 120–150 °C. Table 2 lists the decomposition initial temperature (Ti, corresponding to 5% weight loss), the final temperature (Tf) of the weight loss, the temperature range of the decomposition (DT) and the weight loss (Dm), obtained for PSAN and PSAN–SO3H from TGA curves. Decomposition of sulfonated polymers was investigated by Jiang and co-workers [27], who found water elimination from acid samples between approximately 70 and 150 °C, and loss of sulfonic groups in a broad temperature range from 160 to 400 °C. In the present work, PSAN– SO3H decomposition was found to exhibit a multistep behavior and, between 300 and 390 °C, desulfonation (loss of –SO3H groups) probably occurs, simultaneously to other weight loss processes, similarly to NafionÒ membranes [28]. Additionally, desulfonation of SPEEK [29] was described between 280 and 350 °C, similarly to the observed in the present work.

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A.L.A. Silva et al. / European Polymer Journal 44 (2008) 1462–1474 PSAN PSAN-SO3H 1:4 PSAN-SO3H 1:2 PSAN-SO3H 1:1 PSAN-SO3H 2:1 PSAN-SO3H 4:1 PSAN-SO3H 6:1

100

Mass (%)

80

60

40

20

0 0

100

200

300

400

500

600

700

800

Temperature (oC) Fig. 3. TGA curves of PSAN and PSAN–SO3H membranes.

Table 2 Ti, Tf, DTm and Dm values for PSAN and PSAN–SO3H membranes Sample

Ti (°C)

Tf (°C)

DT (°C)

Dm (%)

PSAN 1:4 1:2 1:1 2:1 4:1 6:1

364 261 206 204 215 92 91

442 457 475 519 518 583 592

78 196 269 315 303 491 501

99.5 87.3 82.2 78.3 78.5 67.4 71.6

Thermal decomposition of PSAN was described by Jang and Wilkie [5], which observed approximately 10% weight loss at 419 °C, attributed to the chain backbone decomposition. After this process, the authors detected a 1 wt% residue, at 600 °C. This thermal behavior is coherent with the results obtained in the present work, for which a 0.5% residue was found after 442 °C. From the data presented in Table 2, Ti values are found to decrease with the sulfonation degree up to 2:1. This behavior is probably due to loss of absorbed water by the membranes, which increase with the number of sulfonic groups in the chain. Additionally, for the PSAN–SO3H membranes with sulfonation degrees higher or equal than 4:1, there is probably a carboxylic acid elimination, since these samples presented the characteristic C@O vibrational band at 1730 cm1 and the absence of the 2250 cm1 band of the nitrile group, as described in the FTIR section. There is also an increase in the final decomposition temperature (Tf) and, consequently, DT, with the sulfonation degree. This

increase in Tf and DT indicates a larger number of weight loss steps with the sulfonation degree as a function of the absorbed water and the formation of carboxylic acid, as well as other structural features involving the sulfonic groups in the chain backbone. The decrease in Dm results in higher residual mass, which can be a result of thermalinduced reticulation reactions involving the decomposition products of PSAN–SO3H. Fig. 4 shows the DSC curves of PSAN and PSAN–SO3H membranes in different sulfonation degrees. The DSC curves do not exhibit any exo- or endothermic peak in the temperature range studied, indicating the absence of crystallinity for all the studied membranes. PSAN is a non-crystalline polymer and, as observed in the DSC curves, PSAN–SO3H exhibits a similar behavior. In Fig. 4 the glass transition temperatures (Tg) are marked with asterisks (*) and the values obtained are listed in Table 3. Single Tg values were found for PSAN and PSAN– SO3H 6:1 membranes. In general, each Tg is associated with a certain phase or fraction in which spatially separated nano- and microdomains undergo the glass transition relaxation at a characteristic temperature. Additionally, different molecular arrangements may result in different Tg values depending, e.g., on the hydration degree. For PSAN and PSAN–SO3H 6:1, the presence of a single Tg indicates structural homogeneity in a scale between 109 and 106 m. All PSAN–SO3H membranes, except 6:1, presented two distinct Tg values. The Tg values between 48 and 63 °C are referred as Tg,1 and the values

A.L.A. Silva et al. / European Polymer Journal 44 (2008) 1462–1474

6:1

*

Heat flux (W/g, exotherm up)

4:1

* * 2:1

*

*

*

1:1

*

1:2

* 1:4

* * *

PSAN

* 0

35

70

105

Temperature

140

175

210

(oC)

Fig. 4. DSC curves of PSAN and PSAN–SO3H membranes.

Table 3 Tg,1 and Tg,2 values for PSAN e PSAN–SO3H membranes Sample

Tg,1 (°C)

Tg,2 (°C)

PSAN 1:4 1:2 1:1 2:1 4:1 6:1

– 53 53 63 52 48 –

112 139 152 146 150 115 120

between 112 and 152 °C, Tg,2. PSAN–SO3H membranes with sulfonation degrees 1:4, 1:2, 2:1 and 4:1 presented Tg,1 between 48 and 53 °C, while PSAN–SO3H 1:1 exhibited Tg,1 = 63 °C. PSAN– SO3H samples presented higher Tg,2 values than the observed for original PSAN and these values progressively increase with the sulfonation degree up to 2:1. Under hydration, sulfonic acid membranes, such as NafionÒ, exhibit endo- and exothermic peaks associated with melting and crystallization of water

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present in the membrane. These systems also present different glass transition temperature (Tg) values, which correspond to supramolecular structures with different arrangements and interactions. PSAN–SO3H membranes, despite the absence of endo- or exothermic peaks, exhibit, as shown, two glass transition temperatures in the temperature range studied. According to Eisenberg and co-workers [30], these thermal events, named Ta and Tb, correspond to a glass transition of the polymer backbone and other associated with the ionic domains, formed by partially or fully dissociated –SO3H in water clusters. Moore and Martin [31] also observed for NafionÒ membranes thermal transitions at 150 and 260 °C, attributed to glass transitions of the matrix and of the ionic domains, respectively, following the model proposed by Eisenberg. In the present work, the presence of two distinct Tg values indicates an ordered structure in the range of units to hundreds of nanometers involving the sulfonic groups and water molecules [–SO3H(H2O)n], which present a lower Tg (Tg,1), while the Tg,2 values are associated with the polymer backbone. The presence of these water/acid ordered structures is an advantage in the application of PSAN– SO3H membranes in PEM fuel cells, due to the higher water retention, which allows utilization of the system under different hydration degrees and temperatures. 3.2.3. Electrochemical characterization Electrochemical impedance spectra (EIS) of PSAN–SO3H dry membranes obtained at 25 °C are exhibited in Fig. 5 and the spectra of PSAN– SO3H 1:2 hydrated membranes are shown in Fig. 6. All collected spectra presented a semi-circle in the high frequency region and an inclined line in the lower frequency region. Under the experimental conditions, it is assumed that resistance of PSAN–SO3H membranes is given by the high frequency extrapolation of the Nyquist plot to the real axis (Z0 ) and, in the lowest frequency region, the impedance spectra are dominated by the frequency response of the electrode-membrane interface [18]. The use of blocking electrodes for the impedance spectra results in a polarization phenomenon in the membrane bulk, since there is no ion source or sink. The electrical double layer at each interface possesses infinite resistance against ion transfer, which, in the Nyquist plot of impedance spectra, is represented by a straight line, parallel to the ordi-

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A.L.A. Silva et al. / European Polymer Journal 44 (2008) 1462–1474 6x10 4 PSAN-SO3H - dry membranes 1:2 1:1 2:1 4:1

-Z'' (Ω)

4x10 4

10 mHz

10 mHz

2x10 4

37 mHz

65 mHz

0 1 MHz

2x10 4

0

4x10 4

6x10 4

Z' (Ω) Fig. 5. Impedance spectra of the PSAN–SO3H dry membranes.

1x10 5

PSAN-SO3H (1:2)

3x10

10 mHz

8x10 4

10 mHz

2x10

10 mHz

-Z'' (Ω)

6x10 4

4x10 4

1x10

0 0

1x10

2x10

3x10

51 kHz

2h 4h 6h 8h 24 h 48 h

2x10 4

0

3.7 kHz

1 MHz

0

2x10 4

4x10 4

6x10 4

8x10 4

1x10 5

Z' (Ω) Fig. 6. Impedance spectra obtained for PSAN–SO3H 1:2 membranes in different hydration times.

nate, associated with a limiting capacitance [32]. Ion conductivity (r) values for dry and hydrated membranes at ambient temperature are listed in Table 4. Conductivity of PSAN–SO3H 1:4 and 1:6 membranes was not studied under hydration conditions, since these samples presented high solubility in water. The values listed in Table 4 are in the range from 1010 to 103 X1 cm1, being strongly influenced by both sulfonation degree and hydration time. The maximum conductivity, of 6.75  103 X1 cm1, was achieved for the 2:1 membrane after 48 h

immersed in water. The conductivity increase with the sulfonation degree is a consequence of the number of sulfonic groups in the polymer chain, which, nucleating the water clusters, dissociate into hydronium ions (H3O+). The conductivity increase with hydration time reflects the formation of water clusters around the acid groups, as indicated in the FTIR studies. The polymer structure strongly influences on the nanostructure of the solid and, for polymers with hydrophilic groups and hydrophobic chains, such as SPEEK (sulfonated poly(ether ether ketone)), NafionÒ and possibly PSAN–SO3H, water confine-

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Table 4 Conductivity values obtained for dry and hydrated PSAN–SO3H membranes at ambient temperature T (h)

1:4

0 (dry) 2 4 6 8 24 48

(1.94 ± 0.42) (6.14 ± 2.20) (1.60 ± 0.22) (5.04 ± 0.09) (4.96 ± 0.55) (4.05 ± 0.93) (1.57 ± 0.35)

1:2 1010 106 105 106 105 105 105

(4.13 ± 0.10) (3.96 ± 0.30) (1.14 ± 0.04) (1.45 ± 0.08) (2.56 ± 0.31) (2.84 ± 0.47) (2.56 ± 0.76)

ment takes place, originating water nanodomains or clusters, in which the proton conductivity is similar to that in pure water. For a highly hydrated system, a continuous water phase may be formed, eliminating the confinement and border effects and resulting in a higher conductivity [33]. Asensio and co-workers [34,35] found conductivity values between 103 to 102 X1 cm1 for sulfonated and polybenzimidazole membranes doped with H3PO4 after hydration during 72 h. According to the authors, the high sulfonation degree was the main responsible for the high conductivity values observed. In the present work, PSAN–SO3H membranes presented similar values after 48 h of hydration, without acid doping. Conductivity values for PSAN–SO3H membranes were studied as a function of the sulfonation degree and temperature, and represent average values of three samples, for which the standard deviation was calculated. Ion conductivity (r) as a function of temperature for PSAN–SO3H 2:1 membrane is shown in Fig. 7. As seen in Fig. 7, conductivity of the PSAN– SO3H 2:1 membrane increase with temperature, similarly to SPEEK [36] and other sulfonic acid polymers. The maximum conductivity of 102 X1 cm1 was achieved at 80 °C and the values obtained were reproducible for different samples under similar experimental conditions. The conductivity values obtained for PSAN–SO3H membranes are similar to the ones described for NafionÒ samples [37–39]. Under the experimental conditions employed, in which the samples were kept in high humidity ambient, temperature has a significant influence on the conductivity of PSAN–SO3H membranes, increasing 10-fold for a temperature increase from 30 to 80 °C. Tricoli and Nannetti [40] found increasing conductivity with temperature for NafionÒ composite membranes, however, the conductivity vs temperature behavior does not strictly follow an Arrhenius model, despite being almost linear. In the present work, PSAN–SO3H

108 105 107 107 106 104 105

1:1

2:1

(7.27 ± 1.38) 105 (8.57 ± 0.03) 104 (6.80 ± 0.45) 104 (8.19 ± 1.57) 104 (9.56 ± 1.88) 104 (7.68 ± 0.76) 104 (2.11 ± 0.39) 103

(3.04 ± 0.05)104 (3.79 ± 0.09)106 (2.16 ± 0.13) 103 (2.65 ± 0.14) 103 (2.32 ± 0.39) 103 (5.28 ± 0.69) 103 (6.75 ± 1.01) 103

membranes exhibit an Arrhenius behavior, as seen in Fig. 7. Proton transfer in sulfonic acid membranes such as NafionÒ or SPEEK [41,42], as well as in composite membranes [43] is promoted in the presence of water. The transport mechanisms proposed in the literature for various electrolytes include a vehicular description, where the proton forms a water complex which diffuses through the hydrated membrane [44]. For membranes which present conductivity in the absence of water, such as poly(benzimidazole)/ H3PO4 [2,3], the proton conduction takes place with a non-vehicular mechanism. For both cases, in the presence or absence of water, the proton conduction models reported in the literature usually involve thermodynamic descriptions [45], which consist in macroscopic approaches to the interpretation of the conductivity behavior observed in polymeric membranes. The charge transport mechanisms include proton diffusion (at low temperatures) or migration (at high temperatures) [46]. For PSAN– SO3H membranes, as temperature increases, the local order is reduced probably due to Brownian motion and the water–acid [–SO3H(H2O)n] clusters originally formed by hydrogen bonding interaction are broken, inducing a change in the conduction mechanism. In a molecular level, a phenomenological model for the proton conduction in NafionÒ membranes was developed by Eikerling and co-workers [20], in which the proton mobility occurs by a surface mechanism where the transport takes place along an array of acid groups over the interface, and a bulk mechanism, where the protons are carried in a Grotthus mechanism. Theoretical calculations [47] as well as experimental results [33] indicate the formation of water clusters around sulfonic and phosphonic acid groups, whose number of water molecules vary with hydration degree. Formation of water clusters around sulfonic acid groups should be favored at low hydration degrees, as pointed out

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Temperature (oC) 80

70

60

50

40

30

2.8

2.9

3.0

3.1

3.2

3.3

-2.00

-2.10

-1

log(σ) (σ in Ω cm )

-2.05

-1

-2.15 -2.20 -2.25 -2.30 -2.35 -2.40 -2.45

-1

1000/T (K ) Fig. 7. Arrhenius plot (log(r) vs 1000/T) of conductivity (r) dependence on the temperature for PSAN–SO3H 2:1 membrane.

by Khalatur and co-workers [48]. These studies evidence the molecular structure and behavior of acidbased polymers, which are highly dependent on the water amount in the membranes.

dissociation into hydronium ions, which are favorable characteristics for application of PSAN– SO3H as membrane in fuel cell units. 4. Conclusions

3.2.4. Water uptake Water uptake values for PSAN and PSAN– SO3H membranes are listed in Table 5. All the sulfonated membranes presented progressively higher water uptake than original PSAN, due to the already discussed interaction among the –SO3H group and water molecules. PSAN–SO3H 1:2 membrane achieves 45.7% water uptake, versus 18.9% for the original PSAN. Bouzek and co-workers [49] observed a similar behavior for PPO–SO3H (sulfonated poly(propylene oxide)), in which the increase in the sulfonation degree influenced directly on the water uptake and solubility in water. The water uptake behavior of PSAN–SO3H membranes is coherent with the conductivity increase observed with the sulfonation degree. Additionally, the FTIR studies indicated water–acid interactions and acid Table 5 Water uptake of PSAN and PSAN–SO3H membranes at ambient temperature Membrane PSAN PSAN–SO3H PSAN–SO3H PSAN–SO3H PSAN–SO3H

Water uptake (%) 4:1 2:1 1:1 1:2

18.9 26.5 37.2 45.2 45.7

H3PO4-doped PSAN and PSAN–SO3H membranes were studied in the present work by means of electrochemical impedance spectroscopy, thermal analyses and vibrational spectroscopy. PSAN/ H3PO4 membranes exhibited conductivity values in the range of 1012 to 108 X1 cm1, which increase with the acid concentration in the membrane. However, the maximum conductivity obtained for the PSAN/H3PO4 membranes is not appropriate for its application in FC devices. PSAN–SO3H membranes were obtained by sulfonation reaction of PSAN, which was confirmed by FTIR analyses. FTIR spectra showed vibrational bands associated with the –SO3H group, covalently bonded to the aromatic styrene ring in the polymer chain, and also vibrational modes relative to the dissociated sulfonic group and hydronium ions, evidencing the dissociation of the acid in aqueous media. DSC traces evidenced two Tg values, attributed to the glass transitions of the polymer backbone and the ionic domains, indicating the formation of a nanostructure in which water-ion domains are separated by hydrophobic regions, as in other sulfonated polymers. Thermal stability of PSAN–SO3H was near 200 °C for samples with sulfonation degree up to 2:1 and conductivity values

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