Synthesis and Characterization of Phenolphthalein-based Poly

Oct 22, 2010 - Hydrophilic-hydrophobic alternating multiblock poly(arylene ether sulfone) copolymers containing 4,4′- biphenol (BP) or phenolphthale...
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Ind. Eng. Chem. Res. 2010, 49, 12125–12134

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Synthesis and Characterization of Phenolphthalein-based Poly(arylene ether sulfone) Hydrophilic-Hydrophobic Multiblock Copolymers for Proton Exchange Membranes Ruilan Guo, Ozma Lane, Desmond VanHouten, and James E. McGrath* Macromolecules and Interfaces Institute, Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24061, United States

Hydrophilic-hydrophobic alternating multiblock poly(arylene ether sulfone) copolymers containing 4,4′biphenol (BP) or phenolphthalein (PPH) were synthesized via a coupling reaction between phenoxide-terminated hydrophilic oligomers (BPS100 and PPH100) and highly reactive decafluorobiphenyl end-capped hydrophobic oligomers (BPS0 and PPH0). The block length and block combination of copolymers were varied by precisely controlling the molecular weight Mn and end group functionality of oligomers. The resulting hydrophilic-hydrophobic sequenced multiblock copolymers afforded transparent, ductile, and tough membranes by solution casting from DMAc. Membrane properties of these copolymers were characterized including intrinsic viscosity, thermal stabilities, morphology, water uptake, and proton conductivity. Results were compared among copolymers with various block lengths and block types. Proton conductivity measurements revealed that PPH0BPS100 copolymers having high IEC values showed better performance than copolymers with PPH100 as hydrophilic blocks. Well-defined nanophase separated morphologies of the multiblock copolymers were demonstrated by tapping mode atomic force microscopy (AFM), confirming that the hydrophilic domains provide a pathway for water and proton transport. It was also shown that the volume fraction of hydrophobic/ hydrophilic domains, as well as IEC values, played a critical role in determining the morphological structures and thus the proton transport. Preliminary studies on exploring the film drying temperature and annealing effect on the membrane properties were also reported. 1. Introduction Most current proton exchange membrane fuel cell (PEMFC) technology is based on perfluorosulfonic acid (PFSA) polymer membranes, represented by Dupont’s Nafion. Nafion exhibits generally good chemical stability and proton conductivity at high relative humidity (RH) and low temperature.1,2 However, there are several obvious drawbacks of Nafion which limit its PEM application, such as high cost, high methanol permeability, insufficient thermomechanical properties above 80 °C, and the environmental hazards associated with its disposal. These limitations have stimulated many efforts in the development of new hydrocarbon-based membrane materials. Recently, wholly aromatic hydrophilic-hydrophobic multiblock copolymers have been considered as strong and promising PEM membrane candidates for overcoming limited proton conduction under partially hydrated conditions.3-10 Hydrophilic-hydrophobic multiblock copolymers have attracted increasing attention in PEM application because the morphology of the copolymer membrane can be easily tailored by varying the type and length of two sequences in the multiblock structures. Multiblock copolymers synthesized by coupling fully disulfonated poly(arylene ether sulfone) hydrophilic blocks (BPSH100) with different types of hydrophobic blocks were largely developed in our research group.11-24 Membranes based on these multiblock copolymers exhibit unique nanophase separated morphologies and greatly improved proton conduction under partially hydrated conditions by forming well-connected continuous hydrophilic domains. Various engineering materials were used as the hydrophobic blocks to provide dimensional and mechanical stability, such as poly(arylene ether sulfone),11-13 fully or * Corresponding author e-mail: [email protected]; tel: 540-231-5976; fax: 540-231-8517.

partially fluorinated poly(arylene ether sulfone),14-17 poly(arylene ether ketone),18 poly(arylene ether)-polyimides,19-21 polybenzophenone,22,23 and polybenzimidazole.24 To extend previous work, a new kind of membrane, sulfonated poly(arylene ether sulfone) multiblock copolymers containing phenolphthalein, was designed and synthesized as reported here. Primarily known and used as a pH indicator, phenolphthalein (PPH) is a bisphenol that contains a heterocyclic pendant lactone (Figure 1). It has been used in polymers such as polycarbonates, polyesters, and poly(arylene ether)s.25-28 Due to its fully aromatic and bulky structure, PPH has contributed significantly to the mechanical properties, thermal and environmental stability of these resins. However, much of its utility as a rigid heterocyclic bisphenol has still been overlooked. An additional advantage of phenolphthalein into polymers backbone is that the heterocyclic pendant lactone provides chemically reactive sites for further derivatization or grafting reactions to achieve functional polymeric materials, which has not been fully recognized and explored. This paper, therefore, describes the design, synthesis, and characterization of multiblock poly(arylene ether sulfone) copolymers based on phenolphthalein-containing hydrophilic and hydrophobic oligomers. Three groups of multiblock copolymers (PPH0-PPH100, PPH0-BPS100, and BPS0-PPH100)

Figure 1. Structure of phenolphthalein (PPH).

10.1021/ie100785t  2010 American Chemical Society Published on Web 10/22/2010

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with varying block lengths were synthesized via coupling different types of hydrophilic and hydrophobic oligomers. The influence of the block length and block combination on the fundamental membrane properties (including water uptake, proton conductivity), thermal properties, and morphology will be described. 2. Experimental Section 2.1. Materials. Monomer-grade 4,4′-dichlorodiphenyl sulfone (DCDPS) and 4,4′-biphenol (BP) were provided by Solvay Advanced Polymers, and dried in vacuo at 110 °C for 24 h prior to use. 3,3′-Disulfonated-4,4′-dichlorodiphenylsulfone (SDCDPS) was provided by Akron Polymer Systems using a published procedure from our lab29 and dried in vacuo at 150 °C for 24 h prior to use. Phenolphthalein (PPH) was purchased from Sigma-Aldrich and recrystallized with ethanol/water. Decafluorobiphenyl (DFBP) was used as received from Lancaster. Toluene, cyclohexane, and anhydrous N,N-dimethylacetamide (DMAc) were purchased from Sigma-Aldrich and used as received. Potassium carbonate, methanol, and 2-propanol were purchased from Sigma-Aldrich and used without further purification. 2.2. Synthesis of Phenoxide Terminated Fully Disulfonated Hydrophilic Oligomer PPH100 and BPS100. Two series of fully disulfonated poly(arylene ether sulfone) hydrophilic oligomers (BPSH100 and PPH100) with molecular weights ranging from 5 to 15 kg/mol were synthesized via nucleophilic aromatic substitution. Molecular weight control and telechelic phenoxide functionality of hydrophilic blocks were achieved by using stoichiometrically adjusted amounts of monomers (PPH in excess for PPH100, or BP in excess for BPS100). Synthesis of BPS100 oligomers followed the same procedures as reported earlier.12 A sample synthesis of 5000 g/mol PPH100 hydrophilic oligomer is provided as follows: 3.8211 g (12.0 mmol) of PPH, 5.0952 g (10.4 mmol) of SDCDPS and 1.9079 g (13.8 mmol) of potassium carbonate were charged into a 150-mL three-necked flask equipped with a condenser, a Dean-Stark trap, a nitrogen inlet, and a mechanical stirrer. Then, anhydrous DMAc (50 mL) and toluene (25 mL) were added to the flask and the trap was filled with toluene. The reaction was heated to 150 °C with stirring and N2 purge. The solution was allowed to reflux at 150 °C while the toluene azeotropically removed the water produced during reaction. After 4 h, toluene was drained from the trap and removed from the reaction as well by slowly increasing the temperature to 185 °C. The reaction was allowed to proceed for another 72 h. The resulting viscous solution was then cooled to room temperature and filtered to remove salt. After filtration, the solution was coagulated in isopropanol. The collected oligomer was washed with isopropanol and then dried at 120 °C in vacuo for at least 24 h. 2.3. Synthesis of Phenoxide Terminated Unsulfonated Hydrophobic Oligomers PPH0 and BPS0. Two series of unsulfonated hydrophobic oligomers (BPS0 and PPH0) were synthesized with molecular weights ranging from 5 to 15 kg/ mol. Molecular weight control of hydrophobic blocks, as well as the telechelic phenoxide functionality, was achieved by using stoichiometrically adjusted amounts of monomers (PPH in excess for PPH0, BP in excess for BPS0). Synthesis of BPS0 oligomers followed the same procedures as described in reference.12 A sample synthesis of 5000 g/mol PPH0 hydrophobic oligomer is provided as follows: 5.7320 g (18.0 mmol) of PPH, 4.6428 g (16.2 mmol) of DCDPS, and 2.8621 g (20.7 mmol) of potassium carbonate were charged to a 150-mL three-

necked flask equipped with a condenser, a Dean-Stark trap, a nitrogen inlet, and a mechanical stirrer. Anhydrous DMAc (50 mL) and toluene (25 mL) were added to the flask and the solution was heated to 145 °C with stirring. The solution was allowed to reflux at 145 °C while the toluene azeotropically removed the water in the system. After 4 h, the toluene was removed by slowly increasing the temperature to 180 °C. The reaction was allowed to proceed for another 48 h. The resulting viscous solution was filtered and coagulated in methanol. The oligomer was dried at 110 °C in vacuo for at least 24 h. 2.4. End-Capping of Phenoxide Terminated Hydrophobic Oligomers with DFBP. Phenoxide terminated BPS0 and PPH0 oligomers were end-capped with DFBP via a nucleophilic aromatic substitution reaction. End-capping of BPS0 followed the same procedures as reported earlier.12 A typical end-capping reaction of 5000 g/mol PPH0 oligomer is as follows: 5.0000 g (1.0 mmol) of PPH0 oligomer and 0.5528 g (4.0 mmol) of potassium carbonate were charged to a three-necked 150-mL flask equipped with a condenser, a Dean-Stark trap, a nitrogen inlet, and a mechanical stirrer. Anhydrous DMAc (50 mL) and cyclohexane (15 mL) were added to the flask. The solution was allowed to reflux at 100 °C to azeotropically remove the water. After 4 h, the cyclohexane was removed from the system. The reaction temperature was set to 105 °C after 2.0047 g (6.0 mmol) of DFBP was added. The reaction was allowed to proceed for 12 h. Same isolation and purification process of PPH0 oligomer was applied. 2.5. Synthesis of Hydrophilic-Hydrophobic Multiblock Copolymers PPH0-PPH100, PPH0-BPS100, and BPS0PPH100. Three series of multiblock copolymers with different block combinations were synthesized via a coupling reaction between phenoxide terminated BPS100 or PPH100 hydrophilic oligomers and DFBP end-capped BPS0 or PPH0 hydrophobic oligomers. A typical coupling reaction to afford PPH0-PPH1005K-5K multiblock copolymer was performed as follows: 5.0000 g (1.0 mmol) of PPH100-5K oligomer (Mn ) 5000 g/mol), 0.5528 g (4.0 mmol) of potassium carbonate, 100 mL of DMAc, and 30 mL of cyclohexane were added to a three-necked 250mL flask equipped with a condenser, a Dean-Stark trap, a nitrogen inlet, and a mechanical stirrer. The reaction mixture was stirred at 100 °C for 4 h to dehydrate the system with refluxing cyclohexane. After removing the cyclohexane, 5.0000 g (1.0 mmol) of DFBP end-capped PPH0-5K oligomer (Mn ) 5000 g/mol) was added. The coupling reaction was conducted at 105 °C for 24 h. The resulting yellowish viscous polymer solution was filtered and precipitated in 2-propanol. The copolymer was purified in a Soxhlet extractor with methanol for 24 h and with chloroform for another 24 h before being dried at 120 °C in vacuo for 24 h. Coupling reactions to afford PPH0-BPS100 and BPS0-PPH100 multiblock copolymers followed the similar procedures as describe above. 2.6. Characterization. 1H and 13C NMR spectroscopy were conducted on a Varian INOVA 400 MHz spectrometer with DMSO-d6 to confirm the chemical structures of the oligomers and copolymers. 1H NMR was also used to determine number average molecular weights (Mn) of the oligomers via end group analysis. Intrinsic viscosities (IV) and number-average molecular weight (Mn) were determined by size exclusion chromatography (SEC) with polystyrene gel columns on the basis of polystyrene calibration at 30 °C in NMP containing 0.05 M LiBr.30 2.7. Film Casting and Membrane Acidification. The copolymer in salt form was dissolved in DMAc (∼7% w/v) and filtered through a 0.45-µm Teflon syringe filter. The filtered solution was then cast onto dry, clean glass substrate. The films

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Scheme 1. Synthesis of Fully Disulfonated Hydrophilic Oligomers (BPS100 and PPH100) with Phenoxide Telechelic Functionality

Scheme 2. Synthesis of Unsulfonated Hydrophobic Oligomers (BPS0 and PPH0) with Phenoxide Telechelic Functionality

were dried for 24 h with an infrared lamp at ∼45 °C. The residual solvent was removed by drying the film in a vacuum oven at 110 °C for 24 h. Other film drying conditions were also used and are indicated specifically in the paper. The membranes in the salt form were converted to acid form by boiling in 0.5 M sulfuric acid solution for 2 h, followed by boiling in deionized water for 2 h as reported earlier.31 2.8. Determination of Ion Exchange Capacity (IEC), Proton Conductivity, and Water Uptake. Ion exchange capacity (IEC) values were determined based on the integrations of protons related to sulfonated groups in 1H NMR spectra. The measured values were crosschecked with calculated target values according to designed copolymer structures. Proton conductivities under fully hydrated conditions were evaluated in liquid water at 30 °C. The conductivity of the membrane was determined from the geometry of the cell and resistance of the film which was taken at the frequency that produced the minimum imaginary response. A Solartron (1252A + 1287) impedance/gain-phase analyzer over the frequency range of 10 Hz to 1 M Hz was used. In determining proton conductivity in liquid water, membranes were equilibrated at 30 °C in DI water for 24 h prior to the testing. For determining proton conductivity under partially hydrated conditions, membranes were equilibrated in a humidity-temperature oven (ESPEC, SH-240) at the specified relative humidity (RH) and 80 °C for 24 h before measurements. The water swelling ratio was determined by measuring the size difference between dry and fully hydrated membranes in

three dimensions (x, y refer to in-plane dimensions, z is the thickness direction). Swelling ratios were obtained by averaging at least three measurements for each sample. The water uptake of membrane was determined by the weight difference between dry and hydrated membranes. The vacuum-dried membranes were weighed (Wdry), and then immersed in deionized water at room temperature for 24 h. The wet membrane was blotted dry and immediately weighed (Wwet). The water uptake was calculated according to the following equation and averaged from 3 measurements: water uptake (%) )

Wwet - Wdry × 100% Wdry

3. Results and Discussion 3.1. Synthesis of Hydrophilic and Hydrophobic Oligomers: BPS100, PPH100, BPS0, and PPH0. Fully disulfonated hydrophilic oligomers, BPS100 and PPH100 (Scheme 1), and unsulfonated hydrophobic oligomers, BPS0 and PPH0 (Scheme 2), with phenoxide telechelic functionality were synthesized via step growth polymerization. The molecular weight and end group functionality of the oligomers were precisely controlled by offsetting the molar feeding ratios of monomers. In all cases, the molar feed ratios of phenols (BP or PPH) over SDCDPS or DCDPS were greater than 1 to give phenoxide telechelic functionality and the target number-average molecular weights ranging from 5 to 15 kg/

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Figure 2. 1H NMR spectrum of phenoxide terminated BPS100 hydrophilic oligomer. Figure 4. 1H NMR of spectrum of phenoxide terminated PPH100 hydrophilic oligomer.

Figure 3. 1H NMR spectrum of phenoxide terminated BPS0 hydrophobic oligomer.

mol. The number-average molecular weights Mn of the oligomers were determined by 1H NMR end group analysis. Figures 2 and 3 show the 1H NMR spectra of BPS100 and BPS0 oligomers, respectively. The number-average molecular weights of BPS100 oligomers were determined from the integration ratios of the end group protons of BP moieties (peaks a and c in Figure 2) and the main chain protons (peaks i and h in Figure 2) on BP moieties. Similarly, for BPS0 oligomers, the number-average molecular weights were calculated by comparing the integration ratios of the end group protons of BP moieties (peaks a and b in Figure 3) and the main chain BP protons (peaks h and g in Figure 3). On the 1H NMR spectra of telechelic PPH100 (Figure 4) and PPH0 (Figure 5) oligomers, small peaks at ∼6.7 and 6.95 ppm were assigned to the protons on the PPH moieties which are located at the end of the oligomers. The peaks at ∼7.3 and 7.05 ppm were assigned to the main chain protons of PPH moieties. The number average molecular weight of PPH100 oligomers were determined by comparing the integrations of the end group PPH protons (peak a in Figure 4) and main chain PPH protons (peak h in Figure 4). Similarly, the molecular weights of PPH0 oligomers were calculated from the integration ratios of protons at end groups (peak a in Figure 5) and main chain protons (peak f in Figure 5). The designed target and measured molecular weights of these four oligomers are summarized in Table 1. The measured

Figure 5. 1H NMR of spectrum of phenoxide terminated PPH0 hydrophobic oligomer. Table 1. Number Average Molecular Weights of Telechelic Oligomers measured Mn (g/mol)a hydrophilic oligomers

hydrophobic oligomers

target Mn (g/mol)

BPS100

PPH100

BPS0

PPH0

5,000 7,500 10,000 15,000

5,160 7,310 9,940 15,250

5,060 7,590 9,760 15,070

5,060 7,550 10,860 15,200

5,170 7,650 10,180 14,920

a

Measured from 1H NMR.

number average molecular weights of oligomers from 1H NMR were in good agreement with the designed target values indicating the right end group functionality and successful control of molecular weights of both hydrophilic and hydrophobic oligomers. 3.2. End-Capping of the Hydrophobic BPS0 and PPH0 Oligomers Using DFBP. To facilitate the coupling reaction between hydrophilic and hydrophobic blocks, the hydrophobic oligomers needed to be modified on the phenoxide end groups

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Scheme 3. End-Capping of Hydrophobic Oligomers (BPS0 and PPH0) with DFBP

to afford multiblock hydrophilic-hydrophobic copolymers. DFBP was chosen as the end-capping reagent to produce fluorine-terminated hydrophobic oligomers due to its high reactivity (Scheme 3). The end-capping reagent DFBP was used in large molar excess (600%) to prevent interchain extension reactions and ensure complete end-capping. Due to the high reactivity of DFBP, the end-capping reactions were conducted at relatively low temperature (105 °C) with short reaction time (12 h). The disappearance of the phenoxide end group peaks in 1H NMR spectra (Figure 6) confirmed that all of the phenoxide groups

in hydrophobic oligomers had reacted with DFBP. In addition, only a slight increase in the intrinsic viscosity of hydrophobic oligomers was observed by SEC after end-capping reactions, suggesting that the possible interchain coupling reactions or extension reactions were minimized. 3.3. Synthesis of PPH-BPS Multiblock Copolymers (PPH0PPH100, PPH0-BPS100, and BPS0-PPH100). PPH-containing hydrophilic-hydrophobic multiblock copolymers were synthesized by coupling phenoxide-ended hydrophilic oligomers (BPS100 or PPH100) and DFBP-ended hydrophobic oligomers

Figure 6. 1H NMR of DFBP end-capped BPS0 (left) and PPH0 (right) oligomers. Arrows indicate where the end-group peaks previously presented before DFBP end-capping.

Scheme 4. Synthesis of PPH-BPS Multiblock Copolymers through Coupling Reaction

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Figure 7. 1H NMR of PPH0-PPH100 (top), PPH0-BPS100 (middle), and BPS0-PPH100 (bottom) multiblock copolymers. Arrows indicate the disappearance of the end groups on the hydrophilic blocks after the coupling reaction.

(BPS0 or PPH0) as shown in Scheme 4. Three types of PPHcontaining copolymers, PPH0-PPH100, PPH0-BPS100, and BPS0-PPH100, were produced by coupling different types of hydrophilic and hydrophobic oligomers. For example, PPH0BPS100 copolymer was synthesized by coupling PPH0 hydrophobic oligomer and BPS100 hydrophilic oligomer. In 1H NMR spectra of multiblock copolymers, the disappearance of the phenoxide end group peaks in the hydrophilic blocks confirmed that the coupling reactions were complete (Figure 7). The coupling reactions end-linked the telechelic hydrophilic and hydrophobic oligomers and produced the hydrophilic-hydrophobic multiblock copolymers. As reported earlier,12 13C NMR could be used to confirm that the low temperature coupling reaction minimized the ether-ether chain exchange reactions, i.e., chain extension reaction via coupling telechelic oligomers dominated in the system. Figure 8 shows a comparison of the 13C NMR spectra of the PPH35 random (statistical) copolymer and PPH0-BPS100 multiblock copolymer, which possessed the same chemical composition while having different degrees of order in sequences. As shown, peaks from the random copolymer show multiplets, indicating

a random sequence of sulfonated moieties along the main chain. On the other hand, sharp and narrow peaks present in multiblock copolymer, suggesting an ordered sequence in the copolymer. These observations further confirmed the prevention of randomization via low temperature coupling. In the following discussions, acronyms of the multiblock copolymers, PPH0-PPH100-x-y, PPH0-BPS100-x-y, and BPS0PPH100-x-y are used, where x and y denote the molecular weights of the hydrophobic and hydrophilic blocks, respectively. For example, PPH0-BPS100-5K-5K indicates that the copolymer was produced by coupling PPH0 hydrophobic oligomer of 5000 g/mol molecular weight with BPS100 hydrophilic oligomer of 5000 g/mol molecular weight. 3.4. Characterization of Membrane Properties of PPHBPS Multiblock Copolymers. PPH-BPS multiblock copolymers with varying hydrophilic and hydrophobic block combinations and lengths were synthesized and the fundamental membrane properties of the PPH-BPS multiblock copolymers were examined to determine their potential fuel cell application, as summarized in Table 2. The copolymers are categorized into three groups depending on the combination of blocks. The first group includes PPH in both hydrophilic and hydrophobic blocks. The second group includes PPH in hydrophilic block and the third group has PPH in hydrophobic block instead. In each category, the multiblock copolymers varied in block lengths from 5K-5K to 15K-15K. All multiblock copolymers displayed high intrinsic viscosities ranging from 0.61 to 2.02 dL/g, from which tough, ductile membranes were solution-cast in salt-form and subsequently acidified using 0.5 M H2SO4 to convert to acid-form membranes. IEC values, which were determined by 1H NMR, were close to the theoretical values calculated from designed copolymer composition, indicating a high degree of coupling between hydrophilic and hydrophobic oligomers. For the PPH0-PPH100 and BPS0-PPH100 copolymers with equal block length, the IEC values ranged from 1.38 to 1.46 meq/g. Since BPS100 is a higher IEC hydrophilic block system compared with PPH100 block, i.e., BPS100 incorporates more sulfonic acid moieties than PPH100 block given the same block length, PPH0-BPS100 copolymers possessed high IEC values ranging from 1.62 to 1.73 meq/g. It is known that the water uptake of sulfonated polymers generally has a profound effect on proton conductivity and mechanical properties. Higher water uptake generates a more solvated species, which is necessary for high conductivity. On the other hand, too much water swelling may dilute the proton concentration and produce mechanically less stable membranes. As a general trend, the water uptake of the PPHBPS multiblock copolymers roughly increased with increasing block lengths. It is of interest to observe the effect of block combination on the proton conductivity and water uptake. As shown, PPH0-BPS100 copolymers outperformed the copolymers in the other two categories in terms of proton conductivity. Their proton conductivities were very high and reached up to 0.154 S/cm with an IEC of 1.73 meq/g. BPS0-PPH100 series showed proton conductivities comparable with those of PPH0-PPH100 copolymers, however, it had the lowest water uptake among three categories of copolymers. These observations seemed to suggest that high proton conductivity mostly relied on high IEC values; while water uptake can be managed by incorporation of various hydrophobic blocks (PPH0 or BPS0) as well as varying in block length. Another unexpected observation is that the proton conductivity decreased with increase in block length in the cases of all three types of copolymers, particularly in the PPH0-BPS100 copolymers. As the block length tripled (from

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Figure 8. 13C NMR spectra of PPH35 random copolymer (top) and PPH0-BPS100 multiblock copolymer (bottom). Table 2. Membrane Properties of PPH-BPS Multiblock Copolymers in Acid Form IEC (meq g-1) a

sample

Nafion 112 PPH0-PPH100 5K-5K 7K-7K 10K-10K 15K-15K BPS0-PPH100 5K-5K 7K-7K 10K-10K 15K-15K PPH0-BPS100 5K-5K 7K-7K 10K-10K 15K-15K

intrinsic viscosity (dL g )

calculated

measuredc

water uptake (wt %)

proton conductivity (S cm-1)d

0.66 0.61 0.71 0.88 1.15 0.74 0.69 0.93 1.23 1.63 1.10 2.02

1.43 1.44 1.41 1.45 1.44 1.45 1.40 1.44 1.78 1.74 1.76 1.80

0.91 1.42 1.43 1.38 1.39 1.41 1.46 1.44 1.46 1.73 1.65 1.62 1.67

25 38 93 103 217 33 68 54 72 113 96 102 259

0.090 0.098 0.120 0.076 0.064 0.075 0.110 0.092 0.090 0.154 0.148 0.117 0.083

b

-1

a Films were in acid form. b Measured using SEC in NMP with 0.05 M LiBr at 30 °C. c Determined from peak integrations in 1H NMR. d Measured in liquid water at 30 °C.

Figure 9. Tapping mode AFM phase images of PPH0-BPS100-5K-5K (left) and PPH0-BPS100-10K-10K (right) multiblock copolymer films in acid form.

5K-5K to 15K-15K) in the PPH0-BPS100, the membrane lost almost half of its performance (from 0.154 S/cm to 0.083 S/cm). These results could be ascribed to (1) high degree of water swelling of copolymers with long hydrophilic blocks, resulting in low ion concentration; (2) the morphological change with increase in block length. Figure 9 shows the tapping mode AFM phase images of PPH0-BPS100 multiblock copolymers with different block lengths. The bright regions in the images correspond to hard hydrophobic domains and the dark areas correspond to soft

hydrophilic domains. Multiblock copolymers with hydrophilic and hydrophobic blocks are known to have an ability to form phase separated morphologies, which provides the continuous channels and thus facilitates the transport of protons. As shown, a well-defined, nanophase separation morphology was formed in PPH0-BPS100 copolymer with shorter block length (5K-5K). The well-connected, continuous hydrophilic channels afforded high proton conductivity. In the case of PPH0-BPS100 copolymer with longer block length (10K-10K), the regularity of this phase separation morphology was disrupted. The hydrophobic domains seemed to expand in volume more quickly with increase in block length due to the bulky structure of PPH. In other words, the hydrophilic blocks were not sufficient in volume fraction to form well-connected channels, resulting in lower proton conductivities. This observation suggests that the volume fraction of the hydrophilic/hydrophobic segments should be considered as well as IEC values in molecular design to achieve high proton conductivity. In the practical application of PEM, films are not in fully hydrated state; instead, they are partially hydrated. Therefore the properties of PEM membranes at various levels of relative humidity (RH) have more practical importance. The humidity dependence of proton conductivity was measured at 80 °C for PPH0-BPS100-5K-5K, PPH0-BPS100-7K-7K, and PPH0-

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Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010 Table 3. Water Uptake of PPH0-BPS100 Multiblock Copolymers Subjected to Different Thermal Treatments water uptake (wt %) samplea PPH0-BPS100

5K-5K 7K-7K 10K-10K 15K-15K

110 °C (45 °C)b

130 °C (75 °C)b

250 °C (75 °C)b

113 96 102 259

55 34 39 76

44 30 34 41

a

Films were in acid form. b Temperatures in parentheses are the initial drying temperatures using IR lamp. Final drying time at 110 and 130 °C was 24 h. 250 °C annealing was conducted for 30 min under N2.

Figure 10. Proton conductivity behavior of PPH-BPS multiblock copolymers at various levels of relative humidity (RH) at 80 °C.

Figure 11. Film drying conditions and annealing effect on the water swelling ratio of PPH0-BPS100-5K-5K multiblock copolymer.

PPH100-15K-15K copolymers to explore the effects of block length and block type (Figure 10). At high hydration levels, proton transport can be facilitated through water assisted hydrophilic domains. At partially hydrated states, however, there is not sufficient amount of water retention in the membranes to develop these domains. As a result, a quick drop in proton conductivities was observed with decrease in RH due to increase in the morphological barrier for proton transport, as similar trends were observed in most of current PEM materials. As the same trend in fully hydrated states, copolymers with BPS100 as hydrophilic blocks (PPH0-BPS100) showed better performance than copolymer with PPH100 as hydrophilic blocks (PPH0-PPH100) over the full RH range due to their high IEC values. In addition PPH0-PPH100 copolymer seemed to be more dependent on humidity as its conductivity dropped more quickly with decrease in RH although it had the similar starting value (0.157 S/cm) as PPH0-BPS100 copolymers at the highest RH level. Increase in the block length of PPH0-BPS100 from 5K5K to 7K-7K improved the performance at low RH levels. Considering the fact that thermal treatments normally have strong impact on the morphology development in polymer membranes, preliminary studies on the effects of film drying temperature and annealing were conducted on PPH0-BPS100 series copolymers. The salt form membranes were subjected to three different thermal treatments before being converted to acid form: (1) initially dried at 45 °C overnight under IR lamp before final drying in vacuum oven at 110 °C for 24 h; (2) initially dried at 75 °C overnight under IR lamp and final drying in

vacuum oven at 130 °C for 24 h; (3) initially dried at 75 °C overnight under IR lamp and annealed at 250 °C for 30 min with protection of N2. The followed acidification process and measurements of membrane properties were the same for all membranes. Surprisingly, the water uptake was significantly reduced by increasing the initial drying temperature and annealing as shown in Table 3. Comparing the samples treated at 110 °C (45 °C) and 130 °C (75 °C), by simply increasing the initial drying temperature from 45 to 75 °C and slightly increasing final drying temperature from 110 to 130 °C, water sorption was reduced dramatically. The reduction in wt% of water sorption was as high as 2.5 times. Furthermore, annealing the membranes at 250 °C further reduced the water uptake for all membranes. More interestingly, the annealed membranes seemed to have similar water uptake regardless of block length. The thermal treatments showed the similar effects on the water swelling ratio of the membranes, as shown in Figure 11. The x and y represent the in-plane swelling, and z represents the through-plane swelling. As a general observation, the multiblock copolymers showed anisotropic swelling behavior, where membrane swelling occurred mainly in the thickness direction suggesting the nanophase separated blocky structure. Upon higher temperature drying and annealing treatment, the throughplane swelling ratio of the membrane was greatly reduced. Combined with significantly reduced water uptake after annealing, the much less swelling ratio is considered as important improvements in addressing the durability and dimensional stability issues under low relative humidity cycling operation of fuel cells. The effects of drying temperatures and annealing on the proton conductivity of the membranes under partially hydrated conditions were also studied with PPH0-BPS100-7K7K copolymer. The results are compared according to different thermal treatments as shown in Figure 12. As a general observation, membranes treated at higher temperatures (either initial drying temperature or annealing) showed much improved proton conductivity in the full RH range measured. By increasing the initial drying temperature, the typical quick-drop curve of proton conductivity with RH was obviously improved to be comparable to Nafion 212. Particularly, the membrane sample annealed at 250 °C outperformed even Nafion 212 in RH levels between 70% and 30%. The slope of proton conductivity drop with RH was flattened to certain extent upon annealing. All these encouraging observations suggested that film drying temperature and annealing had major impact on the proton conductivity of membranes under partially hydrated conditions. In principle, these greatly improved results are expected to originate from how the favorable bicontinuous nanophase separated morphology developed in the membranes during the film formation process. Systematic studies on the film drying conditions and annealing effects on the membrane

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Figure 12. Film drying conditions and annealing effect on the proton conductivity behavior of PPH0-BPS100-7K-7K multiblock copolymer (measured at 80 °C).

morphology and properties are being actively pursued and will be reported in the future. 4. Conclusions Telechelic hydrophilic (BPS100 and PPH100) and hydrophobic (BPS0 and PPH0) oligomers with controlled molecular weights were successfully synthesized via nucleophilic aromatic substitution. Three types of sulfonated multiblock PPH-containing poly(arylene ether sulfone) copolymers varying in block lengths and block types were produced via a coupling reaction between phenoxide-terminated hydrophilic oligomers and DFBP end-capped hydrophobic oligomers. All the copolymers produced tough, ductile membranes when cast from DMAc solution. Incorporation of PPH into copolymers contributes to excellent thermal stability of the membranes applicable for PEM application. The copolymers were systematically characterized in terms of several fundamental membrane parameters including IEC, water uptake, and proton conductivity. Copolymers featuring BPS100 as hydrophilic blocks (PPH0-BPS100) outperformed the copolymers with PPH100 as hydrophilic segments (PPH0PPH100 and BPS0-PPH100) under both fully and partially hydrated conditions via the formation of a nanophase separated morphology. Morphology studies also revealed that the volume fraction of the hydrophilic/hydrophobic segments, as well as high IEC values, was important for achieving high proton conductivities. Under partially hydrated conditions all the multiblock copolymers showed similar trend that the proton conductivities decreased with decrease in relative humidity due to the low water retention and the increase in morphological barrier to proton conduction in the membranes. These results confirmed that in multiblock copolymers, the hydrophilic domains provide channels for water and proton transport while the hydrophobic domains maintain the mechanical stability. Preliminary results suggested that film drying conditions and annealing had major effects on the membrane properties, namely, significantly reduced water uptake and swelling ratio, and greatly improved proton conductivity under partially hydrated conditions. Acknowledgment We thank the Department of Energy (DE-FG36-06G016038) for its support for this research.

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ReceiVed for reView April 1, 2010 ReVised manuscript receiVed September 28, 2010 Accepted October 4, 2010 IE100785T