Synthesis and properties of poly(ether ether ketone)-block-sulfonated polybutadiene copolymers for PEM applications

European Polymer Journal 46 (2010) 592–601

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Synthesis and properties of poly(ether ether ketone)-block-sulfonated polybutadiene copolymers for PEM applications Yuan Zhao, Jie Yin * School of Chemistry & Chemical Engineering, the State Key Lab. of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 22 August 2009 Received in revised form 12 October 2009 Accepted 9 November 2009 Available online 14 November 2009 Keywords: Proton exchange membrane Fuel cell Block copolymer Selective sulfonation Proton conductivity

a b s t r a c t A novel series of sulfonated block copolymers were successfully synthesized by the condensation of modified poly(ether ether ketone) (PEEK) and polybutadiene (PB), followed by the selective post-sulfonation of PB blocks using acetyl sulfate as the sulfonating reagent. The sulfonic acid groups were only attached onto PB segments due to the high reactivity of double bonds to sulfonating reagent. The degree of sulfonation was controlled by changing the feed ratio of sulfonating reagent to block copolymer. PEEK-b-sPB could be easily cast into flexible and transparent membranes. The obtained membranes exhibited good thermal stability and satisfied mechanical properties. Tensile test showed the incorporation of sulfonate groups into PB blocks resulted in an increase in tensile strength and a decrease in elongation at break. TEM images revealed the existence of ionic spherical domains with the average sizes of 50–100 nm. Some of these small domains further aggregated to form large hydrophilic regions. The proton conductivity values were measured in the range of 102 S/cm in water and increased with increasing IEC and temperature. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Proton exchange membrane fuel cells (PEMFCs) have attracted considerable attention for vehicular transportation and portable applications due to their high energy efficiency and environmental friendship [1–3]. As one of the key components of PEMFCs, proton exchange membrane (PEM) supports proton transfer and the study of PEM is of great interest. Current state-of-the-art PEM commercially used is Dupont’s Nafion, which shows excellent chemical stability as well as high proton conductivity at moderate temperature and high relative humidity [4,5]. Many researchers have investigated Nafion on microstructure and morphology [6]. Due to the connected ionic channels formed by micro-phase separation structure between hydrophobic backbones and hydrophilic sulfonic acid groups, Nafion possesses high proton conductivity even * Corresponding author. Tel.: +86 21 54743268; fax: +86 21 54747445. E-mail address: [email protected] (J. Yin). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2009.11.007

with low ion exchange capacity (IEC) [7,8]. However, high cost and other drawbacks have limited its further applications [9]. Many efforts have been devoted to develop alternative proton conductive materials [10–18]. One alternative approach is to prepare sulfonated block copolymers. The incorporation of soft aliphatic units into stiff polymer backbones can provide membrane with both flexibility and mechanical stability. The sulfonation of rigid segments is responsible for proton conduction while flexible aliphatic chains help forming phase-separated structure [19]. There are several reports about PEM based on sulfonated block copolymers, such as sulfonated poly(styrene-b-isobutylene-b-styrene) block copolymer [20–23], sulfonated polystyrene-b-poly(ethylene-r-butylene)-bpolystyrene (SSEBS) [8,24–26], sulfonated polysulfoneblock-PVDF copolymers [27,28], P(VDF-co-HFP)-b-SPS diblock copolymers [29,30], and sulfonated aliphatic/aromatic polyimide ionomers [31]. Although phase-separated morphologies of these block copolymers can be more precisely controlled [19], the motion of the sulfonic acid groups

Y. Zhao, J. Yin / European Polymer Journal 46 (2010) 592–601

attached onto aromatic rings are still restrained due to the stiffness of aromatic chains, and not easy to form large ionic channels which are preferable for proton conduction [32]. In addition, some difficulties have limited the application of block copolymer as the PEM. For instance, the preparation of prepolymers, the selection of mutual solvent for different blocks, and the processing of block copolymers. Therefore, it is a challenge to design and prepare suitable sulfonated block copolymers which could further promote the aggregation of sulfonic acid groups. Recently, we synthesized block copolymers containing aromatic poly(ether ether ketone) (PEEK) and polybutadiene (PB) segments. Sulfonic groups were later selectively introduced into PB chains. In this article, we report the detailed preparation of PEEK-b-PB block copolymers with different compositions and the selective post-sulfonation of PB. Two series of products possess good film-forming ability and thermal stability. Tensile test exhibits the change of mechanical properties because of the sulfonated PB blocks involved. TEM shows ionic spherical domains aggregate to form large ion-rich regions. With relatively low water uptake and IEC, the proton conductivities are acceptable in water at ambient and elevated temperatures. 2. Experimental 2.1. Materials 4,40 -Difluorobenzophenone was purchased from J&K Chemical Ltd., Shanghai. Carboxyl-terminated polybutadiene (CTPB, Mn = 3250) was kindly supplied by Chinese Lanzhou Latex Research Center. Bisphenol A, 4-aminophenol, N-methyl-2-pyrrolidone (NMP), toluene, tetrahydrofuran (THF), triethylamine (Et3N), acetic anhydride, concentrated sulfuric acid (98%) and potassium carbonate were purchased from China National Pharmaceutical Group Corporation. NMP, toluene and Et3N were distilled under reduced (for NMP and toluene) or normal (for Et3N) pressure and dried with 4A molecular sieve prior to use. Potassium carbonate was dried at 150 °C in vacuo for 10 h before use. Other reagents were used as received. 2.2. Synthesis of acyl chloride terminated polybutadiene (PB-COCl) A typical procedure was as follows (Scheme 1). Carboxyl-terminated polybutadiene (6.5 g, 2 mmol) and

HOOC

H2 C

C H

H2 H2 C C C H p

HC CH2 C H

COOH + Cl

O S Cl

ClOC

C H

50 mL of toluene were added into a dry two-neck flask equipped with a magnetic stir bar and a nitrogen inlet. Five milliliters of thionyl chloride (SOCl2) was then added dropwise into the solution. The mixture was heated under reflux at 70 °C for 2 h and 90 °C for 18 h. After distilled to remove SOCl2, the mixture was stored in desiccator for later use. 1 H NMR spectrum (in CDCl3): d (ppm): 4.96, 5.41 (2H, CH@CH), 2.03 (4H, CH2). FT-IR (KBr pellet, cm1): 1802 (AC@O); 1640 (AC@C); 996, 912 (@CAH). 2.3. Synthesis of fluoro-terminated poly(ether ether ketone) (PEEK-F) Fluoro-terminated polymer was prepared as shown in Scheme 2, following the procedure described in the literature [33]. To a three-neck flask 4,40 -difluorobenzophenone (X g, Table 1), bisphenol A (4.56 g, 20 mmol) and potassium carbonate (3.312 g, 24 mmol) were added into a mixture solution of NMP (40 mL) and toluene (20 mL) under the nitrogen flow. The reaction mixture was stirred at room temperature for a few minutes, and then heated under reflux at 160 °C for 4 h and 190 °C for 16 h. After cooled to room temperature, the mixture was diluted with 10 mL of NMP and poured into large excess of ethanol. The red precipitate was filtered, washed with hot deionized water three times to remove salts and dried in vacuo at 120 °C for 24 h. 1 H NMR spectrum (in d6-DMSO): d (ppm): 6.92–7.85 (16H, aromatic), 1.65 (6H, CH3). FT-IR (KBr pellet, cm1): 1654 (C@O of ArAC(@O)AAr), 1240 (OAAr). 2.4. Synthesis of amino-terminated poly(ether ether ketone) (PEEK-NH2) To a dry three-neck flask equipped with a magnetic stir bar and nitrogen inlet, PEEK-F (3.8 g), specific weight of 4aminophenol, potassium carbonate, NMP (40 mL) and toluene (20 mL) were added. The reaction mixture was heated to 170 °C and maintained for 20 h. Then it was cooled to room temperature and poured into 200 mL of ethanol with stirring. After concentrated hydrochloric acid (3 mL) was added dropwise, brown precipitate was formed. The precipitate was filtered, washed with hot deionized water several times till neutral and dried in vacuo at 80 °C for 24 h. 1 H NMR spectrum (in d6-DMSO): d (ppm): 6.92–7.85 (aromatic), 5.06 (NH2), 1.65 (CH3). FT-IR (KBr pellet, cm1): 3370, 3445 (ANH2). 2.5. Synthesis of PEEK-PB block copolymer (PEEK-b-PB)

q

70oC 2h, 90oC 18h H2 C

593

H2 H2 C C C H p

HC CH2 C COCl H q

Scheme 1. Synthesis of acyl chloride terminated polybutadiene (PBCOCl).

To a completely dry three-neck flask were charged 1.34 g of PEEK-NH2, 30 mL of NMP, 12 mL of toluene and several drops of Et3N. After PEEK-NH2 was completely dissolved, 6.7 g of PB-COCl was added dropwise. The reaction mixture was stirred at 60 °C for 6 h in nitrogen, and then poured into 200 mL of ethanol. The precipitate was filtered, washed with ethanol twice and dried in vacuo at 60 °C for 24 h.

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CH3

O C

n+1 F

F + n HO

C

OH

CH3

NMP/Toluene/K 2CO3 160oC Reflux 4 h, 190oC 16h CH3

O C

F

O

C

H2N

O

CH3 O

C

O C

O

ClOC

H2 H2 C C C H p

C H

H N

H N

PEEK

O

n

CH3 H2 C

PEEK-F

NMP/Toluene/K 2CO3 170oC 20h

NH2

O C

F

n

CH3 HO

O C

O

HC CH2 C COCl H q O

H2 C

C

C H

CH3COOSO3H

NH2

PEEK-NH2

NMP/Toluene/Et 3N 60oC 6h O H2 H2 C C C CH C H q p m HC CH2

PEEK-b-PB

THF 75oC

NaOH H N PEEK

H N

O C

H2 C

C H

H2 C C H

H2 C

H2 C CH

q-l

p-k

HC CH2

OOCCH3 O H2 H H2 C C C C CH C H k l m SO3Na CH CH2 NaO3S

OOCCH3

PEEK-b-sPB Scheme 2. Synthesis of poly(ether ether ketone)-block-sulfonated polybutadiene.

Table 1 Synthesis of prepolymers and block copolymers. X (g)

I II

4.578 4.796

Molar ratioa

21:20 11:10

PEEK-F

PEEK-NH2 4

PEEK-b-PB 4

Yield (%)

Mn (10 )

Yield (%)

Mn (10 )

Yield (%)

Mn (104)

92.8 89.0

1.00 0.75

76.5 71.4

0.92 0.44

83.3 80.8

33.49 26.24

a The molar ratio of 4,40 -difluorobenzophenone to bisphenol A. The ratio is 21:20 in I or 11:10 in II, that leads to PEEK prepolymers with different molecular weights.

1 H NMR spectrum (in CDCl): d (ppm): 6.56–7.85 (H, aromatic), 4.96, 5.40 (CH@CH), 2.03 (CH2), 1.70 (CH3). FT-IR spectrum is shown in Fig. 1.

2.6. Synthesis of sulfonated PEEK-block-PB (PEEK-b-sPB) A literature method [20,34] was employed to prepare acetyl sulfate. Acetic anhydride (15.3 g, 0.15 mol) was added into a round-bottom flask and stirred at 0 °C for

0.5 h. Concentrated sulfuric acid (98%, 15.0 g) was added dropwise in 0.5 h. The mixture was stirred at 0 °C for 6 h and then kept in refrigerator before use. To a two-neck flask equipped with a magnetic stir bar and a nitrogen inlet were charged PEEK-b-PB (1.5 g) and THF (40 mL). After PEEK-b-PB was dissolved, acetyl sulfate (Y g, Table 2) was added dropwise. The mixture was heated under reflux at 80 °C for 12 h. Then half of THF was removed by distillation. The concentrated mixture was

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Y. Zhao, J. Yin / European Polymer Journal 46 (2010) 592–601

Fig. 1. FT-IR spectra of PEEK-b-PB (II-0) and PEEK-b-sPB (II-5, salt form).

poured into large excess of ethanol. NaOH solution was used to adjust pH to 7. The precipitate was filtered, washed with ethanol and deionized water twice and dried in vacuo for 24 h. FT-IR spectrum is shown in Fig. 1. 1H NMR spectrum is shown in Fig. 2.

ular weight was calculated with respect to linear polystyrene (PS) standards. Thermogravimetric analysis (TGA) was performed under nitrogen atmosphere on a TA Q5000IR thermogravimetric analyzer, at a heating rate of 20 °C/min. Samples were preheated at 120 °C for 10 min, then cooled to room temperature.

2.7. Membrane formation and proton exchange

2.9. Tensile test

PEEK-b-sPB (0.8 g) was dissolved in 10 mL of NMP (8%, w/v) and filtered. The filtered solution was cast onto glass plate and dried at 80 °C for 12 h. Crude membrane in salt form was immersed in methanol for 24 h to remove the residual solvent, and then soaked in 2.0 N hydrochloric acid at room temperature for 48 h for proton exchange. The acidic membrane was washed with deionized water several times and dried in vacuo at 80 °C for 12 h.

The mechanical properties were measured using Instron tensile tester (model 4465) at 20 °C and 50% relative humidity at a crosshead speed of 2 mm/min. At least three measurements were performed for each membrane. Membrane bars with a typical size of 40 mm  4 mm (test area) were dried at the same condition (20 °C, 50% RH) for 24 h before testing. The thickness was around 0.05 mm. It was used for tensile strength calculation.

2.8. Measurements

2.10. TEM observation

1

H NMR spectra were recorded on a Varian Mercury Plus 400 MHz spectrometer. The samples were dissolved in deuterated chloroform (CDCl3) or deuterated dimethyl sulfoxide (d6-DMSO). Fourier transform infrared (FT-IR) spectra were obtained on a Perkin-Elmer Paragon 1000PC FT-IR spectrometer. Gel permeation chromatography (GPC) measurements were carried out using a Perkin-Elmer Series 200 apparatus with DMF as eluent. The molec-

Membrane morphology was studied by transmission electron microscopy (TEM) using a JEOL2100F microscope operated at 200 kV. Membranes were immersed in 1.0 N AgNO3 aqueous solution for 24 h, rinsed with water, and dried at room temperature for 12 h. The ultrathin Ag+ stained membrane samples were prepared at 90 °C by an ultramicrotome of model Ultracut-R made by Leica using a diamond knife.

Table 2 The weight of acetyl sulfate used in post-sulfonation of PEEK-b-PB.

a b

Polymers

Ia-0

Y (g) Molar ratiob IEC (mmol/g)

0 0 0

b

I-1

I-2

I-3

I-4

IIa-0

II-1

II-2

II-3

II-4

II-5

1.55 1:1 0.09

3.09 2:1 0.15

4.64 3:1 0.19

6.18 4:1 0.23

0 0 0

2.36 1:1 0.10

4.72 2:1 0.22

7.08 3:1 0.31

9.44 4:1 0.46

11.8 5:1 0.64

The molar ratio of monomers for the preparation of block PEEK is: 21:20 in I, and 11:10 in II. The molar ratio of acetyl sulfate to double bonds in PB.

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Y. Zhao, J. Yin / European Polymer Journal 46 (2010) 592–601

2

1

1

O

2

4

3

C

O

5

CH3 4 3 C O n CH3

H-2,3,4 H-5

H-1

A

9

8

7

6

5

4

3

2

PPM 2

1

O

1

2

3

C

O

6

6

H2 C

H2 C

12

H2 C

C H

C H

7

7

13

14

15

H C

H2 C

H C

H3CCOO

4

5

CH3 4 3 C O n CH3 8

p-k

H2 C

9

CH

q-l CH2

HC 17

k

10

H2 C

11 18

CH 19

SO3Na

l

HC

16

OOCH3

NaO3S

H-2, 3, 4

H-6,9,16,21

20

CH2

H-5

21

H-12,15,18

H-1

H-7

B

H-11

H-10

9

8

7

6

H-13,20

5

4

H-14,19

H-8,17

3

2

1

PPM Fig. 2. 1H NMR spectra of (A) PEEK-F (prepolymer, II) and (B) PEEK-b-sPB (II-5, salt form) in d6-DMSO.

2.11. Water uptake

2.12. Ion exchange capacity

Membranes were dried at 80 °C in vacuum for 24 h and immersed into deionized water at room temperature for 12 h. Then the membranes were taken out, wiped with tissue paper to remove the excessive water and quickly weighed on a balance. Water uptake of membrane was calculated from:

Ion exchange capacity (IEC) was determined by titration. Membrane was immersed in 50 mL of 1.0 N NaCl solution for 24 h, and then titrated with 0.01 N NaOH. At least three measurements were performed for each membrane and the average was used. 2.13. Proton conductivity

WU ¼ ðW w  W d Þ=W d

ð1Þ

where Ww and Wd are the weight of wet and corresponding dry membranes, respectively.

Proton conductivity measurements were carried out on AUTOLAB PGSTA302 electrochemical test system coupled with a computer, over a frequency range from 100 Hz to 1 MHz with an oscillating voltage of 10 mV. The tempera-

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ture range is 20–80 °C at the increment of 15 °C. The test cell was similar to that showed in the literature [35]. The distance of two gold electrodes mounted on Teflon plate was 12 mm. The cell was immersed in deionized water for measurement in liquid water. The resistance value associated with the membrane conductance was determined from high frequency intercept of the impedance with the real axis. Proton conductivity was calculated from the following equation:

r ¼ D=ðTWRÞ

ð2Þ

where D is the distance between the two electrodes, T and W are the thickness and width of the membrane, respectively, and R is the resistance value measured. All membrane bars with a typical size of 20 mm  7 mm were soaked in deionized water for 24 h before testing. At least three measurements were performed for each membrane bar and the average was used.

Table 3 Thermal properties of membranes I and II. Polymers

Td (°C)

Ia-PEEKb I-0c I-1 I-2 I-3 I-4 IIa-PEEKb II-0c II-1 II-2 II-3 II-4 II-5

Td1

Td2

Td3

– – 212.6 203.4 201.3 221.2 – – 218.8 205.6 212.9 211.0 204.2

– 429.2 426.4 411.7 409.5 415.2 – 416.6 406.4 412.0 406.9 407.8 418.5

530.2 518.9 488.8 496.9 485.6 488.3 525.0 517.2 508.6 511.3 504.3 512.9 496.1

a The molar ratio of monomers for the preparation of block PEEK is: 21:20 in I, and 11:10 in II. b PEEK-F prepolymers. c The molar ratio of acetyl sulfate to double bonds in PB.

3. Results and discussion

Poly(ether ether ketone) (PEEK) was prepared through nucleophilic substituted polycondensation of 4,40 -difluorobenzophenone and bisphenol A in the NMP/toluene solvent system. The temperature was kept at 160 °C for 4 h to azeotropically remove all the water from the reaction mixture with toluene. By controlling the molar ratio of two monomers, fluoro-terminated PEEK copolymers with different molecular weights were obtained (I and II, Table 1). To increase the activity of block copolymerization, the end groups of PEEK and polybutadiene (PB) were replaced by amino group and acyl chloride, respectively (Schemes 1 and 2). Special sulfonating reagent which only reacted with C@C in PB block was synthesized and employed. By changing the molar ratio of acetyl sulfate to double bonds in PB, two series of sulfonated block copolymers were prepared by selective post-sulfonation (Table 2). Fig. 1 shows the FT-IR spectra of PEEK-b-PB (II-0) and PEEK-b-sPB (II-5, salt form). In PEEK-b-sPB, new absorption band at 1043 cm1 was assigned to the symmetric stretching vibration of sulfonate groups. The band at 1734 cm1 was ascribed to carbonyl group in acetate ester. There was no corresponding signal in PEEK-b-PB. Both spectrum showed characteristic absorption bands at 1499, 1593 cm1 (C@C in aromatic rings) and 1652 cm1 (C@O in benzophenone units). The 1H NMR spectra of PEEK-F (prepolymer, II) and PEEK-b-sPB (II-5, salt form) are shown in Fig. 2. There was no new peak in the range of 7–9 ppm, indicating that no sulfonation reaction occurred on aromatic rings. Peak H-14 and peak H-19 were possibly involved in the peak of d6-DMSO. All products were soluble in polar organic solvents such as NMP and DMSO. They were successfully cast into flexible and transparent membranes. The thermal stability of the polymers was measured at a heating rate of 20 °C/min under N2 by thermogravimetric analysis, as reported in Table 3. TGA curves of PEEK-F (IIPEEK), PEEK-b-PB (II-0) and PEEK-b-sPB (II-5, salt form)

are given in Fig. 3. PEEK-F showed excellent thermal stability which had only one weight loss step with an onset at around 530 °C, corresponding to the degradation of the aromatic backbone. There were distinct two steps of degradation in PEEK-b-PB. The first step in the range of 350– 450 °C was attributed to the degradation of PB block. The second major weight loss step begun at around 500 °C was due to the decomposition of the PEEK backbone. Sulfonated block copolymer PEEK-b-sPB exhibited three-step degradation patterns. The new weight loss at 180–230 °C was assigned to the desulfonation of sulfonate groups.

3.2. Mechanical properties Good physical strength and ductility are required in proton exchange membrane fabrications to survive the stress of electrode attachment. The stress–strain data of membranes I and II are summarized in Table 4. Fig. 4 shows the representative stress–strain behavior of membranes II. Due to the flexibility of the soft PB blocks, unsulf-

100

80

Weight (%)

3.1. Synthesis and characterization

60

PEEK-F 40

PEEK-b-PB

20

PEEK-b-sPB

0 0

100

200

300

400

500

600

700

o

Temperature ( C) Fig. 3. TGA curves of PEEK-F (prepolymer, II), PEEK-b-PB (II-0) and PEEKb-sPB (II-5, salt form).

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Table 4 Mechanical propertiesa of I and II.

a b c

Samples

Thickness (lm)

Tensile strength (MPa)

Young’s modulus (GPa)

Elongation at break (%)

Ib-0c I-1 I-2 I-3 I-4 IIb-0 II-1 II-2 II-3 II-4 II-5

49 45 58 70 70 40 50 48 40 50 41

26.0 32.8 47.7 54.8 60.0 15.6 22.0 28.2 31.2 41.7 47.1

0.57 1.21 2.00 2.20 2.38 0.35 0.47 0.63 0.87 1.45 1.83

30.6 4.4 3.3 4.1 3.8 186.3 49.1 30.1 8.4 10.8 4.3

Membrane bars were dried at 20 °C, 50% RH for 24 h before testing. The molar ratio of monomers for the preparation of block PEEK is: 21:20 in I, and 11:10 in II. The molar ratio of acetyl sulfate to double bonds in PB.

onated membrane II-0 showed a maximum stress of 15.6 MPa and an elongation at break of 186.3%, while Young’s modulus was less than 0.4 GPa. These data were similar with Nafion. As shown in Fig. 4, with the increased sulfonation level, the tensile strength and Young’s modulus of membranes II increased obviously while elongation at break mainly decreased. That was attributed to the strong ionic interactions of sulfonic groups, which increased the interaction between molecular chains, restricted the stretch of backbone and hindered the strain during extension. In general, membrane I-x (x = 1, 2, 3, 4) had higher tensile strength than its counterpart (membrane II-x, x = 1, 2, 3, 4) because of relatively higher content of rigid PEEK blocks. However, most test bars of I-2, I-3, and I-4 broke up easily during the measurement, which suggested that they were too stiff and brittle to be treated subsequently and might not adaptable in PEM production. II-4 and II-5 exhibited much better maximum tensile stress (41.7 MPa, 47.1 MPa) and Young’s modulus (1.45 GPa,

1.83 GPa) than II-0. They were strong enough for fuel cell applications. 3.3. TEM observation The electrochemical and physical behaviors of proton exchange membrane closely related to their internal structures, especially the distribution of ionic sites. The morphologies of PEEK-b-sPB membranes have been investigated by transmission electron microscopy performed on ultra thin films on the copper grid. TEM images of membranes II-4 and II-5 are shown in Fig. 5. Dark regions were assigned to the hydrophilic domains and the bright regions to hydrophobic matrix. Both spherical ionic clusters with average sizes of 50–100 nm and large ionic aggregates were observed in two samples. The small ionic spheres embedded in nonionic matrix were formed by nanophase separation due to the mutual repulsion between hydrophilic sulfonate groups and hydrophobic blocks [36]. As

50

II-0 II-1 II-2 II-3 II-4 II-5

Tensile Stress (MPa)

40

30

50 40 30 20 10 0

0

5

10

15

20

20

10

0 0

50

100

150

Strain (%) Fig. 4. Representative stress–strain behavior of membranes II.

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Y. Zhao, J. Yin / European Polymer Journal 46 (2010) 592–601

599

Fig. 5. TEM images of sulfonated block copolymer membranes: (A) II-4 and (B) II-5.

clearly shown in the enlarged images, ionic aggregates consisted of these ionic spheres. That was possibly because the soft PB chains facilitated the motion of ionic clusters and promoted their further aggregation during membrane formation. In addition, impurities might form the aggregates as well, such as residual Ag+ or sulfonated PB segments broken down from the block copolymers. 3.4. Water uptake and proton conductivity Table 5 shows the water uptake, IEC and proton conductivity of membranes I and II in water at 20 °C. The sulfonated block copolymer membranes showed increased water uptake with increasing IEC due to the strong water absorption of the sulfonic groups. However, all membranes exhibited relatively lower water uptake (less than 30%). Low IEC led to this result. In addition, it might due to high hydrophobicity of both PEEK and PB as well. Although PB blocks were selectively post-sulfonated, the highest degree

of sulfonation is only 0.087. Therefore, only a few parts of hydrophobic backbone were sulfonated and hydrated. The proton conductivity of membrane increased with increasing IEC as well (Table 5). Except for II-1, other membranes II were more conductive than membranes I and showed proton conductivities higher than 0.01 S/cm at 20 °C, which is the lowest value of practical interest for use as PEM in fuel cells. Proton conductivities of II-4 and II-5 at different temperatures were also measured (Fig. 6). The range was from 20 °C to 80 °C at increments of 15 °C. The proton conductivities of both membranes increase with the increasing temperature, which was the typical behavior observed in many other sulfonated polymer membranes. II-4 and II-5 exhibited conductivities of 0.035 S/cm and 0.037 S/cm at 80 °C, respectively. In this study, considering the mechanical properties, membranes II-4 and II-5 have a good prospective usage as PEM in fuel cells. To elevate the proton conductivity, further work will focus on increasing the IEC. Improving the efficiency of

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Table 5 Ion exchange capacity, water uptake and proton conductivity of I and II.

a b c d

Polymer

IEC (mmol/g)

Water uptake (wt.%)

DSa

Proton conductivityd (mS/cm) (20 °C)

Ib-1c I-2 I-3 I-4 IIb-1 II-2 II-3 II-4 II-5

0.09 0.15 0.19 0.23 0.10 0.22 0.31 0.46 0.64

7.4 9.7 12.6 18.4 8.0 12.4 21.0 25.6 28.2

0.017 0.030 0.038 0.046 0.012 0.029 0.041 0.061 0.087

5.6 7.6 9.1 10.5 6.1 10.2 14.3 20.0 22.5

Calculated from IEC [sulfonate groups]/[C@C bonds in polybutadiene]. The molar ratio of monomers for the preparation of block PEEK is: 21:20 in I, and 11:10 in II. The molar ratio of acetyl sulfate to double bonds in PB. Membrane bars were soaked in deionized water for 24 h before testing.

Acknowledgment

0.04

Proton conductivity (S/cm)

II-4 II-5

We thank the staff of Instrumental Analysis Centre of Shanghai Jiao Tong University for the measurements.

0.03

References

0.02

0.01 20

30

40

50

60

70

80

90

o

Temperature ( C) Fig. 6. Temperature dependence of the proton conductivities for II-4 and II-5 in water. Membrane bars were soaked in deionized water for 24 h before testing.

selective post-sulfonation of PB and quantitatively sulfonating PEEK will be two reasonable approaches. 4. Conclusions PEEK-b-PB block copolymers were successfully synthesized by copolymerization of modified poly(ether ether ketone) and polybutadiene. Two series of PEEK-b-sPB were prepared by selective post-sulfonation of PB blocks. Varying the feed ratios of sulfonating reagent could change the sulfonation degree. All sulfonated block copolymers were easily cast into flexible and transparent membranes. PEEK-b-sPB membranes exhibited good thermal stability and mechanical properties. TEM observation showed significant hydrophilic/hydrophobic nanophase separation and large ionic aggregates which consisted of spherical ionic clusters (50–100 nm). The proton conductivity of membranes was dependent on the IEC and temperature. Therefore, incorporating PB can improve the phase separation and the flexibility of membrane, while sulfonic groups localized on PB can control the mechanical properties and afford proton conductivity. This method could be one prospective choice to improve both processability and proton conductivity of PEM.

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