New Sulfonated Polystyrene and Styrene−Ethylene/Butylene−Styrene Block Copolymers for Applications in ElectrodialysisFrancielli Muller,† Carlos A. Ferreira,*,† Lourdes Franco,‡,§ Jordi Puiggalí,‡,§ Carlos Aleman,*,‡,§

and Elaine Armelin*,‡,§

†Departamento de Engenharia de Materiais, PPGEM, Universidade Federal do Rio Grande do Sul, Av. Bento Goncalvez, 9500,Setor 4, Predio 74- 91501-970, Porto Alegre (RS), Brazil‡Departament d’Enginyeria Química, ETSEIB, Universitat Politecnica de Catalunya, Av. Diagonal 647, 08028, Barcelona, Spain§Centre for Research in Nano-Engineering, Universitat Politecnica de Catalunya, Campus Sud, Edifici C′, C/Pasqual i Vila s/n,Barcelona E-08028, Spain

*S Supporting Information

ABSTRACT: In this study we prepared blends of polystyrene (PS)and high-impact polystyrene (HIPS) with poly(styrene−ethylene−butylene) (SEBS) triblock copolymer. After sulfonation, blends wereused to fabricate ion-exchange membranes by solvent-casting andsubsequent thermal treatment to obtain homogeneous packingdensities. The morphology and structure of the blends wereinvestigated by scanning electron microscopy, atomic force micros-copy, and FTIR spectroscopy. Furthermore, the thermal transitionsand stability of all the blends were characterized using calorimetrictechniques and compared with those of the individual polymers.Analyses of the physical properties (i.e., ionic conductivity, ion-exchange capacity, water uptake, dimensional stability, mechanicalproperties, etc.) showed that the performance of the PS-containing membranes is, in general, higher than that of the HIPS containingone. Furthermore, the highest sulfonation degree was also found for the PS/SEBS membranes. The capabilities of the membraneswere tested by investigating the extraction of Na+ by electrodyalisis. Comparison of the percentage of extracted ions indicates that theincorporation of SEBS results in a significant improvement with respect to membranes made of individual polymers.

■ INTRODUCTION

Membranes made of both natural and synthetic polymers havebeen developed for over 50 years and currently comprise a widerange of applications, such as water purification,1 gas separation,2

solvent dehydration,3 fuel cells,4,5 lithium batteries,6,7 biosensorsand biomaterials,8−11 and medical dialysis.12 In comparison withtraditional industrial separation processes, such as adsorption,extraction, and distillation, membranes technology holds inherentadvantages (e.g., less reagent and solvent consumption, no chemicaladditives, and recyclability), leading to reduced energy consump-tion while the performance of separation processes increases.A membrane is defined as an interface between two adjacent

phases acting like a selective barrier that regulates mass transferbetween two compartments. Among various kinds of mem-branes, ion exchange membranes constitute the most usefulcategory. In particular, proton exchange membranes (PEMs) areextensively used as electrolytes in electrochemical devices, suchas fuel cells, batteries, and electrolyzers, as well as barriers inseparation processes, such as filtration, gas separation, anddialysis. PEMs are typically phasing segregated materials, wherea percolated network of a hydrophilic domain conducts protonswhile the hydrophobic phase confers not only mechanicalstrength but also dimensional and hydrolytic stability duringelectrodialysis operation.

The performance of ion exchange membranes can be assessedon the basis of the following aspects: flux and selectivity, cost ofproduction, thermal and chemical stabilities, and mechanicalstrength.13 These properties are essentially related to the chemicaland physical nature of polymers. In the last few decades, thedesign of materials able to combine high ionic conductivity anddurability, good mechanical strength, reduced permeability, andlow cost for high-volume production has become one the majorchallenges in the field of new polymer materials for electrodialysisapplications.14−17

Sulfonated hydrocarbons are very promising ion exchangemembranes.18−23 Compared to Nafion, a commercially availableand widely studied fluorocarbon PEM manufactured by DuPont,sulfonated hydrocarbon PEMs are generally easy to produce andrecyclable, relatively free from environmental pollution problems,and can be synthesized using cheap monomers. Indeed, theirunique inconvenience is that, after a few months of use, theyneed to be replaced due to porous obstruction.Within this context, amorphous sulfonated polystyrene

copolymers (SPS), high-impact polystyrene (HIPS), and

Received: July 10, 2012Revised: September 4, 2012Published: September 19, 2012

Article

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© 2012 American Chemical Society 11767 dx.doi.org/10.1021/jp3068415 | J. Phys. Chem. B 2012, 116, 11767−11779

poly(styrene−ethylene−butylene) triblock copolymer (SEBS)are promising membrane materials, especially for dialysis applica-tions. This is because of their high thermal and mechanicalstabilities and their relatively low production cost with respect tothat of Nafion. Furthermore, they maintain their mechanicalproperties and have high water uptakes over a wide temperaturerange.In this study, we investigate and characterize SEBS-containing

blends that has been mixed with polystyrene (PS) and HIPS ascation-exchange membranes (CEMs) for electrodialysis appli-cations. Furthermore, a conducting polymer, polyaniline (PAni)doped with camphorsulfonic acid (CSA), was added to enhanceboth the conductivity and stability of these CEMs, the resultsbeing compared with those obtained for other organicmembranes modified with conducting polymers.24−28 Figure 1

depicts the chemical structure of SEBS triblock copolymer,PS, HIPS graft copolymer, and PAni-CSA. In order to conferionic conductivity to the hydrophilic domain of the membrane,the aryl groups of SEBS have been sulfonated using theprocedure described by Chlanda and Cooke.29 The properties ofthese polymeric membranes are largely influenced by both thephase morphology and the interfacial properties, which in turndepend on the composition, the processing conditions, and thecompatibility between the components of the blend.30−34

Moreover, the impact of homogeneity on surface layers, whichis obtained through a thermal treatment after solvent-cast filmpreparation, on the efficiency and resistance of the membranes

has been also evaluated. Thus, the membranes reported in thiswork have been found to exhibit a valuable combination ofmechanical, chemical, and electrochemical properties. The useof PS and SEBS block copolymer is also expected to provide asignificant cost reduction compared to conventional technolo-gies for the fabrication of commercial PEMs (e.g., Selemion).35

■ EXPERIMENTAL SECTIONMaterials. PS (Mw= 27.5 × 103 g/mol) and HIPS (Mw =

16 × 103 g/mol) homopolymers, were kindly supplied byInnova S.A. (PS-N2380 and HIPS-SR550). The SEBS, whichwas purchased from Kraton Co. (catalog number G-1650M),is a linear triblock copolymer with two PS end blocks (Mw=10.3 × 103 g/mol) and a poly(ethylene-co-butylene) (PEB)midblock (Mw = 53.3 × 103 g/mol). Solvents, metal salts,and acid solutions employed were: 1,2-dichloroethane (Vetec,ACS reagent, ≥99.0%), dimethylformamide (Vetec, ACSreagent, ≥99.8%), methanol (Aldrich, anhydrous, 99.8%),ethanol (Panreac, ACS reagent, ≥99.5%), acetone (Panreac,NMR reagent, 99.8%), tetrahydrofuran (Panreac, PAI reagent,99.5%), trichloromethane (Panreac, PAI reagent, 99.95%),dimethyl sulfoxide (Panreac, PAI reagent, 99.95%), trifluoro-acetic acid (Sigma, ReagentPlus, 99%), dichloroacetic acid(Sigma, ReagentPlus, 99%), tetrachloromethane (Sigma,anhydrous, ≥99.5%), o-xylene (Panreac, PS reagent 99%),sodium chloride (Sigma, BioXstra, ≥99.5%), sulfuric acid(Nuclear, ACS reagent, 95−98%), and acetic anhydride (Synth,99.5%). Aniline (Nuclear, ACS reagent, 99.9%), ammoniumpersulfate (Synth, reagent grade), hydrochloric acid (Sigma,ACS reagent, 37%), and ammonium hydroxide (Synth, ACSgrade) were used for the synthesis of PAni emeraldine base,which was performed using a standard method reported byMacDiarmid and Epstein.36 The resulting polymer was dopedwith CSA (Aldrich, purum, ≥98.%).

Blends and Membrane Preparation. Acid-bearingpolymers were prepared by sulfonation of the PS segments.Sulfonation was carried out in 1,2-dichloroethane using theprocedure described in ref 29. PS (or HIPS) and SEBS blends,in the ratio of 75% of homopolymer and 25% of triblockcopolymer, were prepared. In a 2 L three-necked flask equippedwith a dropping funnel, thermometer, and mechanical stirring,60 g of PS (or 60 g of HIPS) was dissolved in 195 mL of1,2-dichloroethane. In a 1 L Erlenmeyer flask equipped with amagnetic stirring, 15 g of SEBS was stirred in 1,2-dichloro-ethane until complete dissolution. After this, 19 mL of aceticanhydride was added to the last solution, the mixture wasplaced in the reaction flask containing PS (or HIPS), and thecontents were heated to 40 °C. Then, 6 mL of sulfuric acid wasadded dropwise and the reaction was maintained at 40 °C, witha controlled oil bath, for 5 h. Then, the reaction was cooled toroom temperature and the mixture was precipitated in a 50/250(v/v) methanol/dimethylformamide solution. The precipitatewas washed with water until the residual water was pH 7, beingsubsequently reduced in a rotary evaporator. The remainingpartially sulfonated PS/SEBS (or sulfonated HIPS/SEBS)blend solution in dimethylformamide was poured on a glassplate, spread to uniform thickness (100−150 μm) with aspreader, and dried for 15 min at 110 °C in an oven.Blends with low resistivity were prepared considering 5 wt %

of PAni doped with CSA (PAni-CSA) in the composition.Aniline was polymerized according to the standard methoddescribed in ref 36. Doped PAni was first filtered and thentreated with 1.0 M NH4OH to obtain the emeraldine base

Figure 1. Chemical formula of the systems used in this work: (a)polystyrene-b-(ethylene-co-butylene)-b-styrene triblock copolymer(SEBS), (b) polystyrene homopolymer (PS), (c) high-impactpolystyrene (HIPS), (d) polyaniline doped with camphorsulfonicacid (PAni-CSA), (e) Selemion AMV commercial membranes, and (f)Selemion CMT.

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form. Subsequently, PAni emeraldine base was doped with1.5 M CSA and the solution stirred for 24 h.37 The conductingpolymer was added to the reaction after 5 h of sulfonation, andthe reaction remained under agitation for a further 30 min forits complete dissolution. The blends modified with Pani-CSAhave been labeled as PS/SEBS/PAni and HIPS/SEBS/PAni.The influence of the drying procedure on the membranes

properties has been evaluated for HIPS/SEBS blendsconsidering two different methods: (a) films dried at roomtemperature by solvent casting and (b) films dried at 110 °C for15 min in an oven and subsequently annealed overnight at60 °C under vacuum. The membranes made of all the othercomposition blends were prepared using procedure b.Water Uptake Measurement and Ion-Exchange

Capacity (IEC). In order to evaluate the water absorption,the “wet” and “dry” weights (Wwet and Wdry, respectively) wereobtained as follows. Membranes were equilibrated in deionizedwater overnight at room temperature, blotted with a Kimwipeto remove surface water, and finally weighted to obtain theWwet. Then, the membranes were kept in an oven at 80 °C for12 h and weighted to obtain Wdry.

38,39 The water uptake wasdetermined by the mass difference between the wet and thedried membranes. The water absorption was calculated as thepercentage increase in mass over the dry weight:

=−

×W W

Wwater uptake 100%wet dry

dry (1)

To determine the ion-exchange capacity (IEC), the membraneswere equilibrated in 100 mL of 1 M HCl solution for 72 h. Afterthat, they were removed from the solution, and the excess ofacid was eliminated by washing the films repeatedly with distilledwater. Next, membranes were immersed in 1 M NaCl toexchange protons by Na+, three renewed solutions being used inthis process. The amount of H+ in such three solutions wasdetermined by titration with 0.005 M NaOH to a phenolph-thalein end point. The IEC was expressed in milliequivalents ofH+ per gram (mequiv·g−1) of the dry weight of the membrane:40

=V M

WIEC NaOH NaOH

dry (2)

where VNaOH and MNaOH are the blank-corrected volume (mL)and molar concentration (mol/L) of NaOH solution, respectively.Both the IEC and the water uptake values were taken as the

average values of five membrane samples.Electrodialysis. Tests were conducted using a three-

compartment cell described in our previous works.25,41

Platinized titanium electrodes were used as anode and cathode.The volume in each compartment was 200 mL. Membraneswere immersed in the working solutions for 48 h to reach anequilibrium state. A pseudostationary state was achieved withpre-electrodialysis for 15 min. After this period, solutions werereplaced by new ones, and the experiment was restarted.Solutions of NaCl (0.1 M) were prepared with deionized waterand poured on the working compartment cell located on themiddle. In the compartments located on the right and on theleft of the working compartment cell, solutions of Na2SO4 0.1 Mwere poured. The anionic membrane was the commercialSelemion AMV, which was purchased from Asahi Glass Co.(Figure 1e), and the cationic membranes were those preparedin this study. The effective area of the membranes was 10 cm2.Tests were conducted by applying a current density of 3.5 mA/cm2

for 4 h at 20 ± 2 °C.

The limiting current density (ilim) was derived from thevariation of the membrane potential (φm) against the appliedcurrent density (i).42 The i−φm polarization curves wereperformed with all the membrane systems described above. Thevalue of i was increased every 2 min, and the corresponding φmwas determined using two platinum contact electrodes adheredto the surface of the membrane. Flame photometric determina-tions with a Digimed DM-61 flame photometer were used toobtain the concentration of Na+.

Instrumentation and Techniques. The sulfonationdegree (SD%) of the membranes was determined by theStandard Test Method ASTM International D5453,43 the resultsbeing corroborated by 1H NMR (in DMSO-d6 or CDCl3) usinga 300 MHz Bruker AMX300 spectrometer operating at300.1 MHz. Intrinsic viscosities were measured from polymersolutions in toluene or dimethylformamide, using an Ubbelohdeviscometer thermostated at 25.0 ± 0.1 °C. Structural character-ization of membranes was performed using a Thermo ScientificNicolet 6700 FTIR spectrophotometer. Samples were placedin an attenuated total reflection (ATR) accessory (ThermoScientific Smart Orbit) with a diamond crystal. The FTIRspectra were obtained after 32 scans at a resolution of 4 cm−1,in a spectral range of 400−4000 cm−1, in transmittance mode.Scanning electron microscopy (SEM) studies were carried outusing a focused ion beam Zeiss Neon 40 microscope equippedwith an energy dispersive X-ray (EDX) spectroscopy system andoperating at 30 kV. Samples were mounted on a double-sidedadhesive carbon disk and sputter-coated with a thin layer of carbongraphite to prevent sample charging problems. Topographic AFMimages were obtained with a Molecular Imaging PicoSPM using aNanoScope IV controller under ambient conditions. The root-mean-square (rms) roughness (r) was determined using thestatistical application of the Nanoscope software, whichcalculates the average considering all the values recorded inthe topographic image with the exception of the maximum andthe minimum. AFM measurements were performed on variousparts of the films, which produced reproducible images similarto those displayed in this work.Differential scanning calorimetry (DSC) was performed on a

TA Instruments Q100 differential scanning calorimeter usinga heating rate of 100 °C/min under nitrogen gas. Data werecollected from −90 to 200 °C. Due to the amorphous behaviorof polymers employed in this work, melting peaks are notexpected, and the Tg values were determined from the secondheating run after quenching the sample from the melt state.Thermogravimetric analysis (TGA) of the membranes was per-formed using a Q50 thermogravimetric analyzer (TA Instru-ments) at a heating rate of 20 °C/min under a nitrogenatmosphere. Prior to DSC and TGA measurements, all sampleswere dried under vacuum at 60 °C overnight.Ionic conductivity was measured by ac impedance spectros-

copy with an AutoLab PGSTAT302N frequency responseanalyzer (FRA) employing a two-electrode configuration anda procedure described elsewhere.44,45 A 10 mV sinusoidal acvoltage over a frequency range from 10 kHz to 10 mHz wasapplied. Conductivity values reported in this work correspondto the average of five membrane samples.Mechanical properties were evaluated using a universal

testing machine (Zwick GmbH & Co., model Z2.5/TN1S)with integrated testing software (testXpert, Zwick). Samplesused for the test stress−strain assays consist of rectangularspecimens with dimensions 30 mm long × 4 mm wide. Theinitial grip separation was set at 10 mm, and the cross-head

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speed was 0.80 mm/min. Mechanical properties of themembranes were determined from stress−strain curves derivedfrom uniaxial tensile tests. Specifically, these properties were theYoung’s modulus (E), the elongation or strain at break (σmax),the yield strength or stress at maximum force (εσ max), and thetensile strength or stress at break (εb). All the mechanical assayswere conducted at room temperature. The mechanical parametersreported in this work were averaged over 10 independentmeasurements for each dry and wet membrane. The drymembranes correspond to polymer films with a post-thermalannealing, whereas the wet membranes refer to dry membranesafter their immersion in deionized water at room temperaturefor 24 h.

■ RESULTS AND DISCUSSIONSulfonation Degree, IEC, Water Uptake, Conductivity,

and Solubility Parameters. Membrane sulfonation wascarried out by reacting acetic anhydride and sulfuric acid witha mixture of PS or HIPS and SEBS. Thus, the reaction of aceticanhydride with sulfuric acid produces acetylsulfuric acid, whichin turn reacts with the aryl groups to produce the sulfonatedmembrane.29 The reaction time is a crucial factor since,although the number of sulfonic acids in the styrene aromaticring increases with the reaction time, the chemical stability andlife performance of the membranes used in electrodialysisdecrease with increasing reaction time. In this work the sulfona-tion reaction time was 5 h in all cases, which was reported to besuitable for the lifetime of the membrane performance.The resulting partially sulfonated binary blends were soluble

in common polar aprotic solvents (e.g., chloroform, tetrahy-drofurane, dimethyl sulfoxide, and dimethylformamide) andcompletely insoluble in apolar solvents. PAni-CSA-containingblends were partially soluble in all the solvents mentionedfor binary blends with the exception of chloroform. Solubilitywas also highly dependent on the number of sulfonic groupsincorporated to the polymer matrix. Due to the chemicalstructure of the blends, the solvent-cast films exhibited relativelygood dimensional stability. They were tough and flexible,being excellent candidates for the fabrication of electrodialysismembranes.Table 1 summarizes the main properties of the polymer

membranes prepared in this study. HIPS/SEBS presents lowintrinsic viscosity in toluene solution, while PS/SEBS showedthe highest intrinsic viscosity determined by capillary viscosimetry

in dimethylformamide (i.e., this blend was insoluble in toluene).The reduced and inherent viscosities (ηred and ηinh, respectively)of PS/SEBS blends increase progressively with decreasingconcentration. This unusual behavior has been attributed to theionic strength promoted by the mobile ions located inside thepolymer matrix, due to the presence of the counterions ofthe sulfonate groups. PS/SEBS blends exhibited the highestsulfonation degree, having better polyelectrolyte properties thanHIPS/SEBS copolymers. Thus, polyelectrolytes expand in polarsolvents like dimethyl sulfoxide, dimethylformamide, or watersolutions. The viscosity values showed in Table 1 for PS/SEBScopolymers in dimethylformamide solution were calculated usingthe Fuoss and Strauss equation46 and cannot be taken by simpleextrapolation of ηsp/c to zero concentration.Water sorption increased with concentration of sulfonate

groups due to the strong hydrophilicity of this functionality,as described in other works.47,48 The water uptake and thenumber of water molecules coordinated by the −SO3

− group ofthe sulfonated HIPS/SEBS blends were reduced after the heattreatment. This result is associated with both an increase of thepacking density and a reduction of the free volume for thetransport of proton and water molecules, as will be discussed indetail in the next subsection.The dimensional stability of the membranes increases as the

blend affinity for water decreases. Accordingly, as the blendaffinity for water increases, ionic transport resistance decreases.Water absorption is slightly higher for the membranes preparedwith PS (e.g., 18% and 14% for PS/SEBS and PS/SEBS/PAni,respectively) than for those obtained using HIPS (e.g., 13% and8% for HIPS/SEBS and HIPS/SEBS/PAni, respectively). Thelatter values refer to membranes dried in an oven, even althoughmembranes dried at room temperature displayed higher wateruptakes (not included in Table 1). This must be attributed tothe fact that the latter present a more porous morphology thanthe former, as observed by SEM. In particular, the highest waterabsorption was obtained for the HIPS/SEBS membrane dried atroom temperature (36%).The IEC measured for different membranes are included in

Table 1. The IEC is higher for the PS-containing membranesthan for those made of HIPS. Indeed, the IEC of the HIPS/SEBS membrane is higher before annealing (0.42) than afterannealing (0.36), evidencing the influence of the dryingprocess. The IECs found for the PAni-containing membranesis higher than that found for membranes without conductingpolymer. This has been attributed to both the hydrophilicity ofPAni-CSA, which facilitates the ions’ transport, and the higherionic conductivity achieved for conducting polymer-containingmembranes. Thus, PS/SEBS/Pani shows the highest ionic con-ductivity (Table 1) and IEC. Similarly, the values of these twoproperties are higher for HIPS/SEBS/PAni than for HIPS/SEBS.The sulfonation degree (SD), which depends on the reaction

time, was quantified by UV fluorescence and confirmed by NMRspectroscopy. As in this study, the reaction time was maintainedas a constant, so slight differences in the SD should be essentiallyattributed to the polymer matrix composition. Moreover, the SD,which ranged from 9.3% to 13.1%, was smaller than thatreported for commercial and noncommercial membranes.49,50

1H NMR analysis (see Supporting Information) confirms thelow sulfonation degree of polymer blends. The main peaks,attributed to the aromatic rings from polystyrene units, appearat 6.4−7.2 ppm. NMR spectra of PS/SEBS and PS/SEBS/PAnipartially sulfonated graft polymers exhibit a broad peak at7.4 ppm corresponding to the C−H aromatic protons adjacent

Table 1. Chemical Composition, Intrinsic Viscosity,Sulfonation Degree (SD), Water Uptake, Ion-ExchangeMembrane (IEC), and Conductivity Parameters Measuredfor the Blends Made of PS or HIPS and SEBS, with andwithout Conducting Polymer

sample

intrinsicviscositya

(dL/g)SDb

(%)

wateruptake(wt %)

IECc

(mequiv·g‑1)conductivityd

(S/cm)

HIPS/SEBS 0.02* 11 13 0.36 1.6 × 10−9

HIPS/SEBS/PAni

0.08* 9 8 0.51 7.1 × 10−9

PS/SEBS 1.43# 13 18 0.86 6.3 × 10−9

PS/SEBS/PAni 1.32# 11 14 1.24 1.5 × 10−7

aMeasured by capillary viscosimetry in *toluene and #dimethylforma-mide at 30 °C. The intrinsic viscosity of PS, HIPS, and SEBS measuredin toluene is 0.23, 0.16, and 0.15 dL/g. bMeasured according to theprocedure indicated in the ASTM D5453-09.43 cMeasured by titration.dMeasured by ac impedance in the solid state and at room temperature.

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to the sulfonic groups and one additional peak at 8.1 ppmcorresponding to the OH dynamic group. However, HIPS/SEBS blends show a modest shoulder of C−H aromatic peakclose to 7.2 ppm, indicating a very low ratio of C−H adjacentsulfonic groups compared to PS/SEBS copolymers. Weconclude that HIPS graft polymer needs higher reaction timewith acetic anhydride and sulfuric acid for effective sulfonationdegree in polymer chains. Quantification of SD from C−Haromatic peaks was not possible due to the overlapping ofaromatic signals of nonsulfonated rings.51−53

Structural and Thermal Characterization. FTIR spec-troscopy was used to corroborate the sulfonation of the blends.The FTIR-ATR spectra of the individual components (PS,SEBS, and HIPS) are compared with those of the sulfonatedPS/SEBS and HIPS/SEBS in Figure 2. The mean peaks of PS(Figure 2a) are observed at 3079, 3058, and 3025 cm−1 (C−Haromatic stretching, triple bands); 2917, 2848 cm−1 (C−H

aliphatic stretching); 1600, 1490, and 1450 cm−1 (CCaromatic stretching); and 1027 and 904 cm−1 (C−H in-planebending); the strong absorption bands at 748, 694, and538 cm−1 are attributed to C−H out-of-plane bending. Thebands of the SEBS copolymer (Figure 2a) are detected at 3060,3025, 2918, 2850, 1602, 1493, 1456, 1375, 756, 698, 543 cm−1;comparison with PS evidences a significant enhancement of thebands associated with the C−H aliphatic groups at ∼3000cm−1. The sulfonation reaction produces two new intense peaksin the PS/SEBS blends at 1215 and 1161 cm−1 (Figure 2a),which correspond to the asymmetric and symmetric SOstretching vibrations, respectively, indicating the success of thesulfonation reaction.54 Another strong absorbance, which hasbeen attributed to the C−S stretching, is detected at 1124 cm−1.The appearance of characteristic peaks associated with thesymmetric stretching of the −SO3

− group and the in-planebending of the para-substituted phenyl ring at 1030 and 1004 cm−1,

Figure 2. FTIR-ATR spectra of (a) PS and SEBS individual polymers and the sulfonated PS/SEBS blend and (b) HIPS and SEBS individualpolymers and the sulfonated HIPS/SEBS blend. Asterisks indicate C−H out-of-plane bending vibrations.

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respectively, confirm the sulfonation of the PS/SEBS blend.Furthermore, the broad and weak absorption band between3600 and 3300 cm−1 in the sulfonated membrane indicatesthe presence of a significant number of −OH from the −SO3Hgroups, corroborating that the polymer blend was successfullysulfonated. Comparison of the HIPS, SEBS, and HIPS/SEBSFTIR spectra (Figure 2b) allowed us to conclude that, asexpected, the latter blend was also successfully sulfonated. Thus,the main absorption bands from −SO3H groups detected in thespectrum of the blend, and not identified in those of individualHIPS and SEBS, were used as the reference.On the other hand, we should mention that FTIR spectro-

scopy was not useful for distinguishing the presence of PAni inthe polymeric matrix, due to the very low intensity of CC orCN groups. In spite of this, we detected the presence of PAniin PS/SEBS/PAni and HIPS/SEBS/PAni by Raman spectros-copy (not shown). The PAni-CSA Raman spectrum was similarto those described in the literature.55,56 Peaks at 1168 and1339 cm−1 are related to C−H bonds of quinoid rings and theformation of the polaronic C−N+ group typically found indoped PAni, respectively. The peaks associated with the CNand CC stretching appeared at 1506 and 1580 cm−1,respectively, while the SO groups of CSA were identified bya broad band at 1034−1000 cm−1. In the PS/SEBS/PAni andHIPS/SEBS/PAni membranes spectra peaks of PAni wereobserved, demonstrating the incorporation of the conductingpolymer into the blend. Moreover, the incorporation of PAniwas also visually corroborated because of the change to a darkcolor (i.e., the addition of conducting polymers provokes a

change of color, even when a very small concentration is used,because of the electrochromic properties of these materials).Most sulfonated hydrocarbon CEMs are derived from

amorphous glassy polymers in the nonequilibrium state.Amorphous glassy polymers generally encompass a free volumethat is higher than the minimal free volume, which is obtainedfrom the difference in specific volume when the transition fromthe liquid to the hypothetical equilibrium glassy state occurs.Thus, the thermal history of glassy polymers can significantlyinfluence the packing density of their polymer chains. All themembranes synthesized in this study were previously annealedovernight at 60 °C under vacuum before DSC and TGAanalysis, unless other conditions are explicitly stated.No endothermic peak from a fusion process was detected in

the DSC curves of the blends, which has been related with amaterial completely amorphous and a pronounced Tg response,with the exception of SEBS. The latter copolymer exhibits threetransitions that have been assigned to α relaxation of the PSsegment (Tgα = 93 °C), the β relaxation (Tgβ = −57 °C), andmelting temperature (Tm ∼ 17 °C) of the elastomeric block,the latter corresponding to the maximum endotherm peak justafter the first Tg. The small area of the melting endotherm(ΔH = 20.5 J/g) indicates that there is a very small percentageof crystallinity in the material (Figure 3).57 The addition of PS,which shows only one glass transition close to 104 °C, to SEBSresults in a blend with glass transition temperature (Tg1 = 127 °C)higher than those of the two individual components.Incorporation of PAni-CSA 5 wt % to PS/SEBS does not affectsignificantly the thermal properties of the blend, as is reflected inTable 2. On the other hand, the two sulfonated blends retain

Figure 3. DSC curves of sulfonated PS/SEBS and HIPS/SEBS blends compared to nonsulfonated PS, SEBS, and HIPS. All curves are referred to thesecond heating run with the membranes previously annealed overnight at 60 °C and under vacuum.

Table 2. Thermal Properties of Sulfonated Blends and Nonsulfonated Individual Polymers (PS, HIPS, and SEBS)

sample Tg1a (°C) Tgβ

a (°C) Tgαa (°C) Tm

a (°C) ΔH (J/g) Td,5%b (°C) Td,50%

b (°C) Tonsetb (°C) char yieldc (%)

PS 104 − − − − 388 426 428 0.04k 95 − − − − 393 437 437 1.10SEBS − −57 93 17 20.5 406 450 454 0.16PS/SEBS 127 −60 95 17 0.72 303 403 411 3.24PS/SEBS/PAni 130 −63 95 17 0.68 281 403 410 5.46HIPS/SEBS 104 −61 97 16 0.79 247 436 446 4.43HIPS/SEBS/PAni 104 −56 96 18 0.58 246 434 439 10.7

aGlass transition (Tg) and melting (Tm) temperatures were obtained by DSC.bDecomposition temperatures were obtained by TGA for 5% and 50%

of weight loss. Tonset is the temperature at which the sample shows a significant mass loss. cAt 600 °C and under nitrogen gas flow.

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signs of α and β relaxations from SEBS copolymer, which shouldbe attributed to phase separation. The Tg1 transition of theblends varies with the composition, whereas Tm is a modestshoulder with no practical significance. Substitution of PS byHIPS in the blend to extend the chains and incorporate therubbery segment provokes a higher mobility of the chains. Thisproduces a slight increase in the Tg1 with respect to that of theHIPS homopolymer (Figure 3).TGA of PS, HIPS, and SEBS, which are displayed in Figure 4a,

exhibit one degradation step only. The highest decompositiontemperature (Tonset) was obtained for SEBS (Table 2).However, blends present three well-defined degradationsteps, as shown in Figure 4b. According to the literature,48,52

the first step (between 50 to 200 °C) has been related to theloss of residual water that was retained in the samples becauseof the interactions with the sulfonic groups. The second weightloss, which occurs at around 250−340 °C, has been attributedto the elimination of sulfonic acid groups. Finally, the thirdstep, which depends on the blend composition, corresponds to

the polymer chain degradation. Comparison of the thermog-ravimetric curves of sulfonated HIPS/SEBS samples dried atroom temperature or under vacuum reveals that membranessubmitted to the latter drying process suffered a partial elim-ination of sulfonic acid. Thus, the weight loss observed ataround 250−340 °C is lower for samples dried under vacuumthan for those dried at room temperature. Shi and Holdcroft52

reported not only that highly sulfonated membranes increasethe loss weight in the 50−200 °C region but also that thedegradation steps of these samples are sharper than those withlow degree of sulfonation. These features corroborate ourhypothesis about the formation of strong intermolecularinteraction between sulfonic acid groups and water molecules.On the other hand, the presence of a low concentration(5 wt %) of PAni-CSA does not influence the thermal stabilityof the blends, even though the residual char at 600 °C is higherthan for blends without conducting polymer (i.e., 5.5% and10.7% for PS/SEBS/PAni and HIPS/SEBS/PAni, respectively).Degradation temperatures are listed in Table 2.

Figure 4. Thermogravimetric analysis of membranes prepared by solvent casting: (a) PS, SEBS, and HIPS (nonsulfonated) and (b) PS/SEBS, PS/SEBS/PAni, HIPS/SEBS, and HIPS/SEBS/PAni blends (sulfonated). All samples were previously annealed overnight at 60 °C under vacuum withthe exception of that explicitly indicated.

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Mechanical Properties. The stress−strain curves of PS/SEBS, PS/SEBS/PAni, HIPS/SEBS, and HIPS/SEBS/PAniblends are displayed in Figure 5, while the Young’s modulus,tensile strength, and elongation at break are listed in Table 3. Inorder to avoid the negative influence of the packing densityassociated with the free-volume inside glassy polymer chains,samples were submitted to thermal annealing before stress−strainstudies. Thus, the annealing process is supposed to stabilize theglassy polymers, inducing densification of the polymer matrix.This restricts the polymer chain mobility, and the membraneplasticization effects induced by the absorbed small molecules(e.g., water or solvents) are minimized.58 Mechanical assaysindicate that all blends undergo yielding at roughly the samestress level and present brittle−ductile behavior. Thus, theelongation of the samples is not accompanied by any significantreduction in the cross-sectional dimensions (macroscopicnecking) up to fracture. The flow stress follows the sametrend, with the exception of the wet sulfonated PS/SEBS blend,which showed a slight neck section with 5−8% of elongation(Figure 5a). The highest and the lowest tensile strengths were

obtained for the dry sulfonated PS/SEBS (28 ± 2 MPa) and thedry sulfonated HIPS/SEBS/Pani (7 ± 0.3 MPa). As expected,the incorporation of PAni-CSA produced deterioration of themechanical properties, even in the wet samples (Table 3 andFigure 5b,d). This effect was previously described for otherrelated systems.59,60

Regarding to the Young’s modulus, PS-containing blendsshowed the highest modulus (1046 and 779 for dry and wet,respectively). The presence of rubber phases in HIPS increasesthe deformation and reduces the flow resistance of the PS matrix.Thus, the elastomer supports part of the load in the stage priorto rupture, which is displayed for both dry and wet HIPS/SEBSmembranes (Figure 5c).The Young’s modulus was lower for the wet membranes

than for the dry ones in all cases, indicating that the rigiditydecreases when the membrane hydration increases. Hydrationaffects the membranes in the following way: (1) membranesswell as the ionic regions grow with the incorporation of waterand (2) molecular fragments with charged end groups becomemore mobile.61 However, these effects were relatively low in the

Figure 5. Stress−strain curves of sulfonated membranes annealed at 60 °C under vacuum overnight (dry) and exposed to a high humidity chamber(RH 100%, wet): (a) PS/SEBS, (b) PS/SEBS/PAni, (c) HIPS/SEBS, and (d) HIPS/SEBS/PAni blends.

Table 3. Mechanical Properties Obtained for Polymer Blends

E (MPa) σmax (MPa) εσ max (%)

polymer film dry wet dry wet dry wet

PS/SEBS 1046 ± 4 779 ± 6 28 ± 2 19 ± 0.6 4.4 ± 0.4 8.1 ± 2PS/SEBS/PAni 685 ± 7 592 ± 8 19 ± 0.3 12 ± 0.8 4.7 ± 0.6 3.8 ± 0.3HIPS/SEBS 811 ± 7 545 ± 6 21 ± 0.3 17 ± 0.5 12.3 ± 1.3 11.6 ± 1.1HIPS/SEBS/PAni 280 ± 4 235 ± 3 7 ± 0.3 4 ± 0.05 6.4 ± 0.6 4.4 ± 0.2

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systems studied in this work because of the applied thermalannealing treatment. Thus, hydration only caused a slight reduc-tion of Young’s modulus and tensile strength with respect to thevalues determined for dry films. PS/SEBS is an exception to thisbehavior, its higher hydration being attributed to the lowerdensification of the polymeric matrix compared to HIPS/SEBS.SEM and AFM Studies. In addition to the chemical

structure, composition, and thermal properties, preparationconditions are of fundamental importance in CEMs design. Thethermal annealing applied on sulfonated hydrocarbon polymershas been found to alter the macroscopic characteristics ofCEMs studied in this work. The homogeneity and porosityof membranes prepared with partially sulfonated blends andPAni-CSA have been investigated with SEM, the resultingmorphologies of films prepared by solvent casting and afterannealing treatment being compared.Figure 6 displays SEM micrographs for PS, HIPS, and SEBS

as reference. Low-magnification images show smooth andregular surfaces. However, before annealing, PS presents atypical matrix droplet structure at the surface with high free-volume density and some porosity, as is reflected in the high-magnification micrograph (Figure 6a, inset). HIPS presentsthe most homogeneous surface with nondroplet structure(Figure 6b), whereas a phase separation between the PS andthe −EB− block has been found in SEBS. This separation isevidenced by the appearance of white zones randomly distributedon the film surface (Figure 6c).Prior to annealing, the PS/SEBS blend does not present well

distributed phases, indicating immiscibility between the triblockcopolymer and the homopolymer. The −EB− block separationphase is clearly identified in the micrometric (Figure 7a) andnanometric images displayed in Figure 7b, respectively. On theother hand, the miscibility of SEBS with HIPS is better thanwith PS, even before annealing. Thus, HIPS/SEBS micrographs(Figure 8a,b) show a free-volume surface density with biggerand coarser droplet structure than PS (Figure 6a,b) but withoutwhite regions like those identified in PS/SEBS films.After annealing, polymer blends present higher homogeneity

and nondroplet structures, corroborating that thermal treatmentsignificantly improves the packing density of thermoplasticblends (Figures 7c and 8c). On the other hand, sulfonatedPS/SEBS/PAni membranes show a homogeneous and compactstructure (Figure 7d), whereas incorporation of PAni toHIPS/SEBS results in an immiscible system as is evidencedby the appearance of numerous particles at the film surface(Figure 8d).Differences in the membrane surface morphology can be

expressed in terms of roughness. AFM was used to measure theaverage roughness (r) of all membranes investigated in thiswork, surface topography images being displayed in Figure 9.The roughness values determined for PS/SEBS and PS/SEBS/PAni membranes are very similar (r = 17 and 15 nm, res-pectively), proving that PAni-CSA is well-dispersed in thepolymer matrix, which is fully consistent with the SEMmicrographs. Substitution of PS by HIPS in the binary blendcomposition promotes a higher roughness (r = 55.0 nm) due tothe volume density of the grafted polymer. In contrast, theroughness of the HIPS/SEBS/PAni membrane (r = 19.6 nm)was significantly lower than that of HIPS/SEBS. This is probablybecause the regions analyzed by AFM do not represent theternary blend topography (i.e., the particles observed by SEMwere presumably PAni clusters not detected by the AFM tip, andpolymer surface is deformed). Finally, we should mention that

AFM measures on membranes exhibiting droplet structures (i.e.,prior to the annealing treatment) were difficult due to the largesize of height profile.

Electrodialysis Study for Na+ Extraction. Limitingcurrent density (ilim) values were determined for the polar-ization curves (i.e., variation of the membrane potential, ϕm,against the applied current density, i) using the Cowan−Brownprocedure.62,63 At low i values, the linear relation with ϕmindicates the quasi-equilibrium state in the interface betweenthe membrane and the solution. Such linear behavior is lost toreach a plateau region, which allows us to identify the ilim valueat the inflection point of the curve. The ilim values of PS/SEBS,

Figure 6. SEM micrographs of the individual polymers used asreference: (a) PS, (b) HIPS, and (c) SEBS. The inset in part a denotesa high-magnification image showing the porosity inside the PS filmprepared by solvent-casting and without annealing treatment (scalebar: 2 μm).

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Figure 7. SEM micrographs of PS/SEBS blends: (a) low magnification (before annealing), (b) high magnification (before annealing), (c) lowmagnification (after annealing); and (d) PS/SEBS/PAni (after annealing).

Figure 8. SEM micrographs of HIPS/SEBS blends: (a) low magnification (before annealing), (b) high magnification (before annealing), (c) lowmagnification (after annealing), and (d) HIPS/SEBS/PAni (after annealing).

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HIPS/SEBS, PS/SEBS/PAni, and HIP/SEBS/PAni mem-branes ranged from 3.9 to 5.1 mA·cm−2. Electrodialysis assayswere carried out using ilim = 3.5 mA·cm−2, which is relativelyclose to the values derived from the polarization curves, andmembranes with an effective area of 10 cm2 in all cases.Table 4 shows the percentage of Na+ extracted through the

membranes fabricated in this work. For comparison, Na+

extraction results recently obtained for HIPS/PAni membranesusing the same experimental conditions (i.e., PAni doped withCSA, ilim = 3.5 mA·cm−2 and area of the membrane = 10 cm2)have been included in Table 4.28 Furthermore, electrodialysiscontrol measurements have been also performed using

Selemion CMT (Figure 1f), which is currently consideredamong the most advanced commercial membranes. For themembranes prepared in this work, the largest transport of Na+

was found for the PS/SEBS (24.8%), the percent extractionobtained for HIPS/SEBS being ∼5% smaller. This should beattributed to the fact that, even after annealing, the partialimmiscibility between the triblock copolymer and thehomopolymer favors the formation of a porous interface regionin the former. The addition of PAni-CSA to PS/SEBS andHIPS/SEBS membranes does not have any positive effect in theextraction. Indeed, the percentage of Na+ extracted decreasesby only 1.3% and 0.2%, respectively.

Figure 9. AFM topographic image of the membrane surfaces: (a) PS/SEBS, (b) PS/SEBS/PAni, (c) HIPS/SEBS, and (d) HIPS/SEBS/PAni. Left:front image. Right: 3D image.

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Table 4 clearly shows that SEBS improves significantly thetransport of Na+ found in our previous study for HIPS/PAnimembranes, which ranged from 0.3% to 10.0% depending onthe experimental procedure used for their fabrication.28 Thisshould be attributed to the fact that the impact resistance ofthe HIPS/SEBS blend is smaller than that of HIPS, allowingincreasing the permeability of ions transport in the former. Onthe other hand, inspection of the results obtained using theSelemion CMT indicates that, although the blends prepared inthis work represent an important advance in the field, morework is still needed to reach the performance of the commercialmembranes. Thus, the membranes fabricated in this work arestill ∼2.5 times less effective than the commercial SelemionCMT. This difference has been attributed to the fact that thelatter presents a significantly higher concentration of sulfonicgroups than the SEBS-containing membranes. Accordingly, ourfuture research in this field will be essentially oriented towardthe search of effective methods to increase the concentrationof sulfonic groups in HIPS/SEBS and, especially, PS/SEBSmembranes.

■ CONCLUSIONSSulfonated PS/SEBS, HIPS/SEBS, PS/SEBS/PAni, and HIPS/SEBS/PAni membranes have been fabricated and characterized.The sulfonation degree, which ranged from 9.3% to 13.1%, wasof fundamental importance for the dimensional stability andwater uptake. Thus, the dimensional stability and affinity forwater decreases and increases, respectively, with increasingsulfonation degree. However, the ion-exchange capacity wasalso influenced by the ionic conductivity of the membranes,which was increased by the addition of PAni-CSA. Thus, themembrane with the highest ionic conductivity, PS/SEBS/PAni,was found to present the highest ion-exchange capacity.The chemical structural, thermal, and mechanical properties

of the blends have been characterized using FTIR spectroscopy,calorimetric studies, and stress−strain assays, respectively.PS-containing membranes present higher tensile strength andYoung modulus than the HIPS-containing one. The annealingtreatment has been found to induce densification of the polymermatrix, producing a reduction in the mobility of the molecules.SEM micrographs show that the miscibility of SEBS is betterwith HIPS than with PS, phase separation being observedin PS/SEBS copolymers. However, homogeneity improves afterannealing, evidencing the influence of the thermal treatment in

the packing density. On the other hand, PAni-CSA has beenfound to be completely immiscible with HIPS/SEBS, while itforms homogeneous and compact mixtures when mixed withPS/SEBS.Electrodialysis assays showed that transport of Na+ decreases

as follows: PS/SEBS > PS/SEBS/PAni > HIPS/SEBS > HIPS/SEBS/PAni. Thus, the Na+ extraction ability is higher for thePS-containing membranes than for the HIPS-containing ones,incorporation of PAni-CSA having a negative effect in both cases.Moreover, the percentage of Na+ extracted using the membranesfabricated in this study ranges from 24.8% to 19.9%, whichrepresents a very noticeable improvement with respect to HIPS/PAni membranes previously studied.28 Thus, SEBS modulatesthe impact resistance of HIPS facilitating the ion transport and,therefore, this triblock copolymer should be considered as a veryeffective material for the fabrication of CEMs.

■ ASSOCIATED CONTENT*S Supporting Information1H NMR spectra of PS, SEBS, HIPS, PS/SEBS, and PS/HIPS.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: ferreira.carlos@ufrgs.br (C.A.F.), carlos.aleman@upc.edu (C.A.), and elaine.armelin@upc.edu (E.A.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work has been supported by CAPES-MICINN InternationalCooperation Program from Brazilian and Spanish Education andScience Ministries (PHB2007-0038-PC), MICINN and FEDERfunds (MAT2009-09138 and MAT2009-11513) and by theDIUE of the Generalitat de Catalunya (2009SGR925,2009SGR1208). We are indebted to Mr. D. Aradilla for hisassistance in AFM analyses. F.M. acknowledges financial supportfrom the CAPES agency (grant no. BEX 4463/10-2) for her oneyear stay at UPC. Support for the research of C.A. was receivedthrough the prize ‘‘ICREA Academia’’ for excellence in research(funded by the Generalitat de Catalunya).

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Table 4. Percent Extraction (E%) and Extraction in mg/L(Emg/L) of Na

+ for Membranes Prepared in This Work andCommercial Selemion CMT (control)b,a

membrane E% Emg/L

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