Journal of Industrial and Engineering Chemistry 15 (2009) 299–303

Preparation and characterization of proton conducting polysulfone graftedpoly(styrene sulfonic acid) polyelectrolyte membranes

Rajkumar Patel, Se Jun Im, Yun Taek Ko, Jong Hak Kim, Byoung Ryul Min *

Department of Chemical and Biomolecular Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, South Korea

A R T I C L E I N F O

Article history:

Received 19 March 2008

Accepted 16 December 2008

Keywords:

Polymer electrolyte membrane

Proton conductivity

Atom transfer radical polymerization

Fuel cell

A B S T R A C T

The syntheses of series of proton conducting comb copolymer membrane involving polysulfone back

bone as main chain and poly(styrene sulfonic acid) (PSSA) being side chain, i.e. polysulfone grafted

poly(styrene sulfonic acid) (PSU-g-PSSA) are presented. Chloromethylation of the polysulfone backbone

was done by Fridel Craft alkylation reaction. Atom transfer radical polymerization was used for control

grafting from the chloromethylated positions. The successful substitution of the chloromethyl group and

its grafting with PSSA was characterized by elemental analysis and proton nuclear magnetic resonance.

Water uptake, electrochemical properties like ion exchange capacity (IEC) and proton conductivities

increase with increase in PSSA contents. Thermal gravimetric analysis (TGA) showed the thermal

stability of membranes up to 250 8C. Proton conductivity for maximum amount of grafting is 0.02 S/cm.

� 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

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1. Introduction

Polymer electrolyte membranes have attracted lots of impor-tance for applications in, dyes-sensitized solar cells [1], facilitatedtransport membranes [2] and lithium rechargeable batteries [3,4]Among them, many investigations have been carried out on protonconducting polymer electrolyte membranes for the applications tofuel cells over the last decade [5–15]. The most common polymerelectrolyte membranes used in fuel cells applications areperfluorinated polymer membranes, e.g. DuPont’s Nafion mem-branes. These membranes consist of a hydrophobic fluorocarbonbackbone and hydrophilic sulfonic pendant side chains. Thesestructures produce a microphase-separated morphology of mem-branes and thus they exhibit excellent thermal, mechanical, andelectrochemical properties. However, these membranes have thefollowing disadvantages: (1) high cost, (2) high methanol cross-over through membranes, and (3) low proton conductivity at hightemperature/low humidity conditions [16]. In order to find analternative, a great deal of research is going on in the developmentof nonflurorinated proton conducting membranes [17–20].

Polysulfone is an amorphous high performance polymer havingexcellent thermal properties, good resistance to inorganic acidsand bases, and outstanding hydrolytic stability against hot waterand steam sterilization. These properties have been exploited since

* Corresponding author. Tel.: +82 2 2123 2757; fax: +82 2 312 6401.

E-mail address: minbr345@yonsei.ac.kr (B.R. Min).

1226-086X/$ – see front matter � 2009 The Korean Society of Industrial and Engineer

doi:10.1016/j.jiec.2008.12.011

very long by functionalization of the backbone with sulfonic group.In this way it is difficult to control the position of sulfonation [20].However, it was developed to the present day well by controllingsulfonated poly(arylene ether sulfone) membranes by variousgroups [15,21–23]. For PEM materials, PSSA and its copolymershave been widely studied because of their synthetic easiness andhigher conductivities, and many studies utilized PSSA in the formof random copolymers, block or graft copolymers [5,24–26].

In this work we report the chloromethylation of the PSUbackbone by Fridel Craft alkylation reaction. Styrene sulfonic acidwas grafted onto the PSU through the chloromethylated group byatom transfer radical polymerization (ATRP) to prepare protonconducting PSU-g-PSSA membranes. Various properties such asion exchange capacity (IEC), water uptake, proton conductivity andthermal properties are reported.

2. Experimental

2.1. Materials

Polysulfone was purchased from BASF (Ultrason S,Mn = 47 � 103 g/mol). 1,1,2,2-Tetrachloroethane (TCE), 4-styrenesulfonic acid (SSA), copper(I) chloride (CuCl), 1,1,4,7,10,10-hexamethyl triethylene tetramine (HMTETA) and dimethyl sulf-oxide (DMSO) were purchased from Aldrich. Stannic chloride(SnCl4) was purchased from Fluka. Chloromethyl methyl ether(CMME) was purchased from TCI. All solvent and chemicals wereregent grade and used as received.

ing Chemistry. Published by Elsevier B.V. All rights reserved.

R. Patel et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 299–303300

2.2. Synthesis of chloromethylated polysulfone (CMPSU)

10 g of PSU was dissolved in 140 mL TCE in a round bottom flaskat 60 8C. 0.2 mL SnCl4 and 20 mL CMME were added followed bynitrogen purging. The reaction was carried out at 60 8C for 40 min.The resultant reaction mixture was precipitated into methanol.CMPSU was purified by dissolving in DMSO, reprecipitating inmethanol and dried under vacuum at room temperature.

2.3. Synthesis of PSU-g-PSSA

1 g CMPSU and 4 g SSA were dissolved in 25 mL and 15 mLDMSO respectively in separate round bottom flasks. After completedissolution 0.2 g CuCl and HMTETA were added followed bynitrogen purging. The reaction mixture was stirred at roomtemperature until the catalyst completely homogenized. Reactionwas carried out at 130 8C for 12 h. The resultant grafted productwas precipitated into methanol and purified by dissolving in DMSOand reprecipitating in methanol followed by during in vacuumoven overnight at room temperature.

2.4. Membrane preparation

PSU-g-PSSA copolymer was dissolved in DMSO and filteredwith 0.45 mm membrane filter followed by casting on a preheatedPetri dish. Petri dish was kept in vacuum oven for 24 h at 80 8C toremove solvent and soaked in distilled water to obtain free-standing film. The film was converted into proton exchangemembrane by immersion in 0.5 M H2SO4 for 2 h, followed byimmersion in boiling deionized water for 2 h. The fabricated filmwas then kept in deionized water.

2.5. Characterization and testing

The 1H NMR spectrometer (Bruker, Avance-600) was used at aresonance frequency of 600 MHz. DMSO-d6 and TMS were used assolvent and internal standard respectively. Substitution degree ofcopolymer was calculated through the added ratio of chemicalelements using EA (model: EA1110, CE Instrument, Italy). Theatomic compositional percentage of carbon, hydrogen, oxygen,sulfur and nitrogen was obtained. IEC of the membranes wasmeasured by the classical titration method. The membranes weresoaked in a 1.0 M NaCl solution for 24 h before IEC was measured.The protons released by the exchange reaction with sodium ionswere titrated against a 0.01 M standardized NaOH solution withphenolphthalein as an indicator. The IEC value of the membraneswas calculated with the following equation:

IEC ðmequiv=gÞ ¼ X � NNaOH

Weight ðpolymerÞ (1)

where X is the volume of NaOH consumed and NNaOH is thenormality of NaOH. The water uptake was determined by theweighing of a vacuum-dried membrane and a fully equilibratedmembrane with water. The surface of the membrane sample wasquickly wiped with absorbent paper to remove the excess of wateradhering to it, and the sample was then weighed. The water uptakeof the membrane was determined as follows:

water uptake ðwt%Þ ¼Ww �Wd

Wd� 100 (2)

where Ww and Wd are the weights of wet and dried membranes,respectively. A four-point probe method was used to measure theproton conductivity of the membranes. Before the measurement ofthe proton conductivity, the prepared membranes were equili-brated with deionized water. Complex impedance measurements

were carried out in the frequency range of 1 Hz to 8 MHz at 25 8Cwith a Zahner IM-6 impedance analyzer (Kronach, Germany). Theimpedance spectra of the membranes were used to generateNyquist plots, and the proton conductivity was calculated from theplots [1,11]. DSC measurement of the dried sample was carried outusing a Differential scanning calorimeter (TA Instrument, DSC2010, USA) at a heating rate of 20 8C/min under nitrogenatmosphere to 200 8C, quenched in liquid nitrogen and reheatedto 350 8C. TGA was performed on TGA 1000, TA instrument at aheating rate of 10 8C/min under nitrogen atmosphere to evaluatethe thermal and thermo-oxidative stability of PSU-g-PSSAcopolymers. Weight loss was measured and reported as a functionof temperature.

3. Results and discussions

3.1. Graft copolymer synthesis

Two steps are involved in the synthesis of polysulfone graftedpolystyrenesulfonic acid for proton conducting membranes, asillustrated in Fig. 1. Polysulfone backbone was functionalized withchloromethyl group by Fridel Craft alkylation reaction using SnCl4

as Lewis acid and CMME as alkalylating agent in TCE at 60 8C for40 min. Chloromethylated group was used as macroinitiator forgrafting of styrene sulfonic acid on to PSU backbone by atomtransfer radical polymerization (ATRP) at 130 8C for 12 h. Beauty ofATRP is that it does not require stringent reaction conditions likeanionic or cationic polymerization [27,28]. Thus the amphiphilicgraft copolymers consist of the hydrophobic PSU main chain andhydrophilic sulfonic groups attached to the grafted aromatic sidechains.

Functionalization of aromatic polymer by chloromethylgroup is an electrophilic Fridel Craft substitution reaction andthe substitution position depends on the type of activatingsubstituent on the phenyl ring. In polysulfone, the ether group isan electron donating substituent leading ortho and parasubstitution. This results in the chloromethylation of orthoposition of the bisphenol moiety in the polysulfone as paraposition is blocked. Successful introduction of chloromethylgroup and grafting of chloromethyl polysulfone was character-ized by 1H NMR spectroscopy. A representative 1H NMRspectrum for CMPSU is presented in Fig. 2. Chloromethylatedprotons appear at 4.3 ppm [29]. Fig. 3 illustrates 1H NMRspectrum for PSU-g-PSSA with 1:2 weight ratio of addedPSU:SSA amount. Peaks at 2.5 and 3.5 ppm correspond toDMSO-d6 solvent and moisture presence, respectively. The paraand ortho protons of the sulfonic group substituted aromaticregion appeared at 6.5 and 7.5 ppm, respectively, due to thedeshielding effect of sulfonic acid substituent.

3.2. Elemental analysis (EA)

Elemental analysis was used as to estimate the degree ofsubstitution through the added ratio of chemical elements usingelemental analysis, as presented in Fig. 4. It indicates the increasein sulfur with increase in grafting percent. The maximum sulfurpercent in the polymer membrane was 9.81% for PSU-g-PSSA (1:4)sample.

3.3. Ion exchange capacity

It has been well known that the IEC values directly depend onthe content of sulfonic acid group incorporated into the polymerand thus they are indicative of the actual ion exchange sitesavailable for proton conduction. Generally high value of IEC isdesirable to achieve higher proton conductivity in polymer

Fig. 1. Chloromethylation of PSU followed by its grafting with stytrene sulfonic acid by atom transfer radical polymerization.

R. Patel et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 299–303 301

electrolyte membranes. However the continuous increase in thecontent of sulfonic acid group may result in the deterioration ofmechanical properties of the membranes because of highlyhydrophilic property of the polymer. Therefore it might beessential to optimize the amount of sulfonic acid group. The IECof the PSU graft copolymers is presented in Fig. 5. As expected, theIEC values increased continuously with increase in the graftingratios of PSSA. PSU-g PSSA with maximum grafting has IEC of0.72 mequiv/g, which is relatively lower than that of Nafion 117which might be due to lower sulfonic group.

3.4. Water uptake

Fig. 6 shows the water uptake of the PSU graft copolymermembranes evaluated at room temperature. Generally, higher IECvalues give higher water uptake because both properties arestrongly related to the amount of sulfonic acid groups. PSU-g-PSSA(1:4) membranes with maximum PSSA grafting exhibited 47.6% ofwater uptake, which is lower than Nafion 117.

3.5. Proton conductivity

Proton conductivity of the PSU graft membranes was deter-mined using four-probe method and compared with that of Nafion117 in Fig. 7. As expected, the proton conductivity increased inproportion to the grafting ratios of PSSA in the membrane. As aconsequence, the polymer becomes more hydrophilic and absorbsmore water, which facilitates proton transport. Therefore, thesulfonation raised the conductivity of the PSU not only increasingthe number of proton site, but also through formation of watermediated pathways for proton. PSU-g-PSSA (1:4) membranes withmaximum PSSA grafting exhibited proton conductivity of 0.02 S/cm which is relatively lower than Nafion 117.

3.6. Differential scanning calorimetry (DSC)

The thermal behavior of the synthesized polymers wasinvestigated using DSC analysis under nitrogen at the heating ratebeing 20 8C/min. The DSC curves of the PSU graft copolymers are

Fig. 2. 1H NMR spectra of chloromethylated PSU.

Fig. 3. 1H NMR spectra of PSU-g-PSSA (1:2).

Fig. 5. IEC (mequiv/g) values of PSU-g-PSSA series.

Fig. 7. Proton conductivity (S/cm) of PSU-g-PSSA series.Fig. 4. Percentage of sulfur in PSU-g-PSSA series and chloromethylated PSU.

Fig. 6. Water uptake (wt%) of PSU-g-PSSA series.

R. Patel et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 299–303302

Fig. 9. TGA diagram of PSU-g-PSSA series.

Fig. 8. DSC curves of PSU-g-PSSA series.

R. Patel et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 299–303 303

presented in Fig. 8. Important aspect to note is the experimentallydetermined specific heat of a vaporization of water. The heat flow forPSU graft membranes was observed around 100 8C, mostlyattributable to the desorption of adsorbed moisture by thehygroscopic sulfonic group. The heat flow is increased by increasingin substitution of PSU. This result suggests that sulfonated PSU withincreasing degree of sulfonation can adsorb more water inintermolecular chain by increasing hydrophilic property.

3.7. Thermogravimetric analysis (TGA)

The thermal stabilities of PSU graft membrane were investi-gated by TGA, as shown in Fig. 9. PSU membranes exhibitedexcellent thermal stability up to 350 8C, above which it started todecompose to around 30 wt%. The first slight weight losses for PSUgraft membranes were observed around 100 8C, mostly attribu-table to the loss of adsorbed moisture due to the hygroscopicproperty of the membrane. The second slight weight losses for PSUmembranes were observed at around 250 8C, mostly attributable

to sulfonic group decomposition. The third weight loss at 350 8Crelates to the degradation of the main polymer chain. But thestability of the Nafion membrane is higher than that of thesulfonated PSU polymer for the second and third weight loss.However, this result suggests that the sulfonated PSU membranesare thermally stable within the temperature range for polyelec-trolyte membrane fuel cell (PEMFC) applications.

4. Conclusion

Two steps synthesis of proton conducting PSU graft copolymerelectrolyte membrane is proposed. In the first step Fridel Craftalkylation reaction was used to introduce chloromethyl pendentgroup onto the PSU backbone. Secondly, a series of PSU-g-PSSAwere synthesized by ATRP. Successful chloromethyl substitutionand grafting of the pendent group were characterized by the 1HNMR and elemental analysis. IEC, water uptake and protonconductivity increased with increasing PSSA contents. Protonconductivity is 0.02 S/cm at room temperature for maximumamount of grafting. TGA showed the thermal stability ofmembranes up to 250 8C.

Acknowledgements

The authors acknowledge the financial support of the Ministryof Education through the second stage Brain Korea 21 Program atYonsei University.

References

[1] J.H. Kim, M.S. Kang, Y.J. Kim, J. Won, N.G. Park, Y.S. Kang, Chem. Commun. 14(2004) 1662.

[2] J.H. Kim, B.R. Min, J. Won, S.H. Joo, H.S. Kim, Y.S. Kang, Macromolecules 36 (2003)6183.

[3] C.Y. Chiu, W.H. Hsu, Y.J. Yen, S.W. Kuo, F.C. Chang, Macromolecules 38 (2005)6640.

[4] J.H. Choi, S.J. Gwon, J.Y. Shon, C.H. Jung, Y.E. Ihm, Y.M. Lim, Y.C. Nho, J. Ind. Eng.Chem. 14 (2008) 116.

[5] B. Smitha, S. Sridhar, A.A. Khan, J. Membr. Sci. 225 (2003) 63.[6] J.M. Bae, I. Honma, M. Murata, T. Yamamoto, M. Rikukawa, N. Ogata, Solid State

Ionics 147 (2002) 189.[7] M.L. Di Vona, Z. Ahmed, S. Bellitto, A. Lenci, E. Traversa, S. Licoccia, J. Membr. Sci.

296 (2007) 156.[8] S. Licoccia, M.L. Di Vona, A. D’Epifanio, Z. Ahmed, S. Bellitto, D. Marani, B. Mecheri,

C. de Bonis, M. Trombetta, E. Traversa, J. Power Sources 167 (2007) 79.[9] K. Ishikawa, K. Kaneko, Y. Takeoka, M. Rikukawa, K. Sanui, I. Ito, Y. Kanzaki, Synth.

Met. 135 (2003) 71.[10] B. Smitha, S. Sridhar, A.A. Khan, J. Membr. Sci. 259 (2005) 10.[11] J.S. Lee, N.D. Quan, J.M. Hwang, S.D. Lee, H. Kim, H. Lee, H.S. Kim, J. Ind. Eng. Chem.

12 (2006) 175.[12] J.H. Kim, H.J. Kim, T.H. Lim, H.I. Lee, J. Ind. Eng. Chem. 13 (2007) 850.[13] K.M. McGrath, G.K.S. Prakash, G.A. Olah, J. Ind. Eng. Chem. 10 (2004) 1063.[14] H.J. Kim, T.H. Lim, J. Ind. Eng. Chem. 10 (2004) 1081.[15] S.J. Im, R. Patel, S.J. Shin, J.H. Kim, B.R. Min, Kor. J. Chem. Eng. 25 (2008) 732.[16] V. Neburchilov, J. Martin, H. Wang, J. Zhang, J. Power Sources 169 (2007) 221.[17] V. Ramani, H.R. Kunz, J.M. Fenton, J. Membr. Sci. 266 (2005) 110.[18] S. Sambandam, V. Ramani, J. Power Sources 170 (2007) 259.[19] P.X. Xing, G.P. Robertson, M.D. Guiver, S.D. Mikhailenko, S.J. Kaliaguine, J. Polym.

Sci. A: Polym. Chem. 42 (2004) 2866.[20] P. Zschocke, D. Quellmalz, J. Membr. Sci. 22 (1985) 325.[21] A. Nohay, L.M. Robeson, J. Appl. Polym. Sci. 20 (1976) 1885.[22] M. Ueda, H. Toyota, T. Ochi, J. Sugiyama, K. Yonetaka, T. Masuko, T. Teramoto, J.

Polym. Sci., Polym. Chem. Ed. 31 (1993) 85.[23] F. Wang, Q. Ji, W. Harrison, J. Mecham, R. Formato, R. Kovar, P. Osenar, J.E.

McGrath, Polym. Preprints 40 (2000) 237.[24] J.F. Ding, C. Chuy, S. Holdcroft, Macromolecules 35 (2002) 1348.[25] H. Okamura, Y. Takatori, M. Tsunooka, M. Shirai, Polymer 43 (2002) 3155.[26] P.D. Iddon, K.L. Robinson, S.P. Armes, Polymer 45 (2004) 759.[27] C.G. Cho, H.Y. Jang, Y.G. You, G.H. Li, S. Guk, High Performance Polym. 18 (2006)

579.[28] K. Matyjaszewski, J.H. Xia, Chem. Rev. 101 (2001) 2921.[29] E. Avram, M.A. Brebu, A. Warshawsky, C. Vasile, Polym Degrad. Stabil. 69 (2000)

175.