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Journal of Membrane Science 369 (2011) 536–544

Contents lists available at ScienceDirect

Journal of Membrane Science

journa l homepage: www.e lsev ier .com/ locate /memsci

omposite polymer electrolytes of sulfonated poly-ether-ether-ketone (SPEEK)ith organically functionalized TiO2

.L. Di Vonaa,∗, E. Sgrecciaa,e, A. Donnadiob, M. Casciolab, J.F. Chailanc, G. Auerd, P. Knauthe,∗∗

Università di Roma Tor Vergata, Dip. Scienze Tecnologie Chimiche, Roma, ItalyUniversità di Perugia, Dip. Chimica, Via Elce di Sotto 8, Perugia, ItalyMAPIEM, Université du Sud Toulon-Var, La Garde, FranceCrenox GmbH, Uerdingen, GermanyUniversité de Provence-CNRS: UMR 6264 Laboratoire Chimie Provence, Centre St. Jérôme, Marseille, France

r t i c l e i n f o

rticle history:eceived 4 August 2010eceived in revised form1 December 2010

a b s t r a c t

Synthesis and properties of proton-conducting composites of SPEEK with organically functionalized TiO2

are described. Composites with hydrophilic titania particles present an inhomogeneous microstructurewith agglomeration of TiO2 particles, high strength and low ductility, high water uptake and protonconductivity. Composites with hydrophobic titania particles have a very homogeneous microstructure,

ccepted 21 December 2010vailable online 31 December 2010

eywords:ulfonated aromatic polymersroton conductors

very reproducible mechanical properties, lower water uptake and proton conductivity.© 2011 Elsevier B.V. All rights reserved.

ybrid materialsuel cells

. Introduction

Proton exchange membrane fuel cells (PEMFCs) are devices thatmploy as electrolyte a solid polymer. This energy conversion tech-ology is one of the most promising in the field of electric vehiclesnd portable applications.

In order to achieve high PEMFC efficiency, the polymericembrane should satisfy several requirements: high proton con-

uctivity (typically 0.1 S/cm under operation conditions), goodhemical, thermal and mechanical stability, and low permeabilityo reactants. Low cost and ready availability are important econom-cal requirements. Furthermore, the membrane should work at anperative temperature around 120 ◦C for long time [1–3]. Althoughhere is much interest in the development of an “ideal” mem-rane, there is nowadays no material that can completely satisfyhe required performances [4].

The current materials research explores different fields:lend of polymers, cross-linked membranes, ionic liquids,lock-copolymers, and organic/inorganic hybrids [5–9].rganic/inorganic hybrids can offer opportunities in many

∗ Corresponding author. Tel.: +39 0672594385; fax: +39 0672594328.∗∗ Corresponding author.

E-mail addresses: divona@uniroma2.it (M.L. Di Vona),hilippe.knauth@univ-provence.fr (P. Knauth).

376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2010.12.044

areas and not last in the field of fuel cell materials [10]. Their maincharacteristic is the capability to combine the properties of thecomponents. Choosing suitable materials, it is thus possible toreach the right features for different applications [11,12].

Several techniques can be used to obtain organic/inorganichybrids [13–16]. In this work, we have dispersed an inorganic com-ponent in an organic polymer, obtaining a composite that belongsto Class I hybrids, according to the classification by Judeinstein andSanchez [17]. The organic matrix is sulfonated poly-ether-ether-ketone (SPEEK). PEEK is an inexpensive, fully aromatic polymercharacterized by high thermal resistance, mechanical strength andoxidation stability [18] that needs to be sulfonated for achievingthe proper proton conductivity [19]. Increasing the degree of sul-fonation, one enhances the conductivity of the polymer, but, themechanical properties deteriorate due to a large hydrophilicity ofthe membrane [20].

The inorganic components added to the organic matrix areorganically functionalized TiO2 nano-particles. Incorporation ofnanosized binary oxide materials (SiO2, TiO2, ZrO2) in SPEEKmembranes has several attributes of interest, including decreasedmembrane swelling, reduced permeability towards methanol and

improved morphological stability without compromising protonconductivity at high degree of sulfonation [21–23]. NanostructuredTiO2, with a typical dimension less than 100 nm, is used in manyapplications ranging from UV shielding to solar cells and photo-catalysts, owing to its peculiar properties. Its chemical stability,

embrane Science 369 (2011) 536–544 537

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M.L. Di Vona et al. / Journal of M

ven under strongly acidic or basic conditions, and its capabil-ty to modify the hydrophilic/hydrophobic balance in the hybridystems make the material suitable to be used as filler in poly-eric electrolyte membranes [24,25]. The presence of the inorganic

ller is expected to accentuate the phase separation between theydrophobic and hydrophilic domains, which is a factor controllinghe water channeling and proton conductivity in PEMFCs [26,27].

To produce high-performance composites, the realization ofwell-stabilized dispersion in the organic matrix is paramount.hen ceramic powders are dispersed, it is difficult to avoid spon-

aneous agglomeration and undesired clustering, because of theresence of attractive van der Waals forces between oxide parti-les, which are the primary cause of the inhomogeneous dispersionf nanosized particles [28].

Different procedures can be used to generate Class I hybrid poly-ers; among them the formation of inorganic components in situ

n a polymer matrix seems to be very promising. Recently, weeported the formation of SPEEK/TiO2 hybrid membranes using aon hydrolytic sol–gel route. The results were very encouragingith respect to the homogeneity and properties of the composites,

ut the synthetic procedure was too complicated for applying athe industrial level [29].

It was also very difficult to achieve homogeneous dispersionithin the SPEEK matrix when SiO2 particles [30], generated in situ

y a hydrolytic sol–gel procedure, were used as filler. The level ofdhesion between the inorganic domains and the polymer matrixn the obtained composite membranes was generally very low,

orsening the mechanical properties of the composites. Properontrol at an atomic level and dispersion of the inorganic com-onent is instead feasible using organically functionalized oxides31]. The functionalization can improve the homogeneity of theystem, enhancing the compatibility between the components,voiding agglomeration of oxide nanoparticles and modifying theydrophilic/hydrophobic character of TiO2 surfaces.

The objective of this work is to determine if simple, inexpensiveolecules, readily used for industrial purpose can be applied for the

unctionalization of inorganic fillers in PEM membranes. This wouldave a significant cost-saving impact. We have chosen two modelases of functionalization: a typical hydrophilic surface modifiers tri(hydroxymethyl)-propane and a typical hydrophobic surface

odifier is silicone oil. In that way, we want to verify the influencen the homogeneity of filler particles distribution and on relevantroperties sensitive to the presence of water. The used chemicals,specially silicone oil, are well known to be extremely stable (ateast kinetically).

These two different TiO2 particles with organically modifiedurfaces are used as inorganic filler in SPEEK [32]. The hybridembranes are characterized and compared with pure SPEEKembranes. The swelling and thermal behaviour, mechanical

trength, and electrical properties are discussed.

. Experimental

.1. Membrane synthesis

Poly-ether-ether-ketone (PEEK, Victrex, 450P, MW = 38,300),unctionalized titanium dioxide (Hydrous titanium dioxide,natase, 350 m2/g, Crenox GmbH, Germany) and all other chemicalsAldrich) were reagent grade and were used as received. SulfonatedEEK (SPEEK) was prepared by reaction of PEEK with concentrated

ulphuric acid at 50 ◦C for 2 h [33]. The solution was poured, underontinuous stirring, into a large excess of ice-cold water. Aftertanding overnight, the white precipitate was filtered and washedeveral times with cold water to neutral pH. The sulfonated polymerSPEEK) (Scheme 1) was dried over night at 80–85 ◦C. The degree

Scheme 1. Formula of SPEEK.

of sulfonation (DS) was evaluated both by 1H NMR and by titra-tion, according to published procedures [34]. Both methods gaveaccording results, indicating a DS = 0.75 (Ion exchange capacity,IEC = 2.2 meq/g).

The TiO2 was obtained from hydrolysis of a TiOSO4 solution ina conventional industrial process (sulphate process). The result-ing nano-sized TiO2 (“titanium hydrate”; average diameter 5 nm)with anatase structure was neutralized using ammonia, thoroughlywashed with deionized water, and dried subsequently. Residualsulphuric acid in the TiO2 was 0.3% (SO4/TiO2).

Two types of functionalized TiO2 (F-TiO2) were used asfiller. One was functionalized with hydrophilic molecules: 10%tri(hydroxymethyl)-propane was added to the powders (called inthe following THP–TiO2 sample); the other was functionalized withhydrophobic molecules: 10% silicone oil (Soil–TiO2 sample). Thefunctionalization of the TiO2 surface was achieved by spraying theorganic compounds (50% tri(hydroxymethyl)-propane in water and50% polymethylhydrosiloxane MH 15, Momentive PerformanceMaterials, Leverkusen, CAS-Number 63148-57-2) onto the titaniumhydrate under vigorous mixing and subsequent drying at 110 ◦C.Final carbon analysis revealed 8.9% of tri(hydroxymethyl)-propaneand 9.5% of polymethylhydrosiloxane on TiO2.

SPEEK-based composite membranes containing 5 wt.% of F-TiO2were prepared by dissolving 250 mg SPEEK in 20 ml of dimethylsulfoxide (DMSO) and adding 12.5 mg F-TiO2 to the solution. Theresulting mixture was stirred for 4 h, evaporated to 5 ml, cast onto aPetri dish and heated to dryness. After cooling to room temperature,the resulting membranes were peeled off and dried under vacuumat 80 ◦C for 24 h and then further dried in the oven at 140 ◦C for64 h to remove the solvent. Pure SPEEK membranes were preparedfollowing the same procedure for sake of reference. The thicknessof the resulting membranes was in the range ∼60–90 �m.

2.2. Membrane characterisation

2.2.1. Structure and microstructureX-ray diffraction (XRD) patterns were recorded at room temper-

ature using a Siemens D5000 diffractometer with CuK� radiation(� = 0.1540 nm). Scanning electron microscopy (SEM) images weremade with a Philips environmental SEM and Atomic ForceMicroscopy (AFM) with an Autoprobe apparatus.

Fourier-transform infrared (FTIR) spectra of membrane sampleswere collected in transmission mode in the range 4000–400 cm−1

(32 scans, 2 cm−1 resolution) with a Bruker Equinox 55. The mem-brane thickness was ca. 60 �m in all cases. A background spectrumwas run and sample spectra were normalized against the back-ground spectrum.

2.2.2. Thermal and water uptake propertiesHigh resolution thermogravimetric analysis (TGA Q500, TA

Instruments) was performed under air flux in the temperature◦ ◦

range between 25 C and 550 C with a maximum heating rate of

5 K/min.Water uptake was measured by full immersion in deionized

water. Excess water was removed with absorbing paper and thenthe mass change of the samples was measured. The experiments

538 M.L. Di Vona et al. / Journal of Membrane Science 369 (2011) 536–544

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ig. 1. X-ray diffraction patterns of: (a) SPEEK; (b) composite of SPEEK and tri(hydrof SPEEK and silicone oil-functionalized (Soil)–TiO2 (Blue: TiO2 anatase, JCPDS file 0n this figure legend, the reader is referred to the web version of the article.)

ere performed at different water temperatures in the range5–145 ◦C. After immersion in water at 145 ◦C one sample was driedver P2O5 and weighted. The final weight was compared with thenitial one: no mass loss was observed, indicating that TiO2 did noteach-out.

Water sorption isotherms were determined after equilibrationith water vapor at different temperatures under 0–95% RH. Theater sorption isotherms were recorded using a TA5000 thermo-

ravimetric analyzer. Prior to all experiments, the membranes wererst dried in situ 3 h at 80 ◦C under 0% RH. RH was then modified inor 10% steps and the water uptake recorded at each step during 2r 3 h. The reversibility of water uptake was checked by systematicesorption experiments, reducing RH with same steps from 95 to%.

.2.3. Mechanical propertiesStress–strain tests were performed using an ADAMEL Lhomargy

Y30 test machine at room temperature at a constant crossheadpeed of 5 mm/min with aluminium sample holders as describedn Ref. [35].

Dynamic mechanical analysis (DMA) was performed with aMA 2980 apparatus from TA instruments in tension mode with

amples of approx. 15 × 7 mm2 size and 90 �m thickness. DMAas operated in air at a fixed frequency of 1 Hz with oscillation

mplitude of 10 �m. This last value was chosen to keep the lineariscoelastic response of samples during experiments. The stor-ge (E′) and loss modulus (E′′) spectra versus temperature werebtained at 3 K/min between 50 and 250 ◦C.

.2.4. Electrical propertiesDielectric analysis (DEA) measurements were performed using

DEA 2970 dielectric analyzer from TA instruments with ceramicarallel plate (CPP) configuration of electrodes. This apparatusllows a frequency scan ranging from 10 Hz to 100 kHz. All experi-ents were performed on 25 × 25 mm2 membrane samples under

ry argon atmosphere and with heating rate of 2 K/min. Pressure

lamping between electrodes was about 50 N cm−2. The measuredurrent was separated into its capacitive and conductive com-onents. An equivalent capacitance and conductance were thenalculated and used to determine the dielectric permittivity ε′ andhe dielectric loss factor ε′′, which is proportional to conductance.

thyl)-propane (THP)–TiO2 (Blue: TiO2 anatase, JCPDS file 021-1272); (c) composite72, green: SiO2 JCPDS file 045-0112). (For interpretation of the references to color

Ionic conductivity � was calculated as follows:

� = ε0ε′′ω (1)

where ε0 is the absolute permittivity of the free space(ε0 = 8.85 × 10−12 F/m) and ω is the angular frequency of the appliedsinusoidal voltage.

Through-plane conductivity measurements were carried outon membranes, 8 mm in diameter and 90 �m thick, sandwichedbetween gas diffusion electrodes (ELAT containing 1 mg cm−2 Ptloading), which were pressed on the membrane faces by means ofporous stainless steel discs. The pressure clamping the membranebetween the electrodes (60 kg cm−2) was applied before startingthe measurements and not controlled during the experiment. Themembrane conductivity was determined as a function of temper-ature and relative humidity by impedance spectroscopy with aSolartron Sl 1260 Impedance Analyser in the frequency range 10 Hzto 1 MHz at a signal amplitude ≤100 mV. Relative humidity wascontrolled as described in Ref. [19]. The conductivity � of the sam-ples in the transverse direction was calculated from the impedancedata, using the relation:

� = d

RS(2)

Here d and S are the thickness of the sample and the electrode area,determined before the measurements. The resistance R was derivedfrom the high frequency intercept with the real axis on a complexplane impedance plot.

3. Results and discussion

3.1. Structure and microstructure

X-ray diffraction patterns of SPEEK and composite membranesare reported in Fig. 1. The SPEEK reference membrane is fully amor-phous: the broad signal around the reflections of crystalline SPEEKis indicative of the lack of crystallinity. Composite membranesshow a clear amount of crystalline anatase TiO2 phase within the

majority amorphous polymer, as can be immediately concludedfrom the reflections in the diffraction pattern. Furthermore, onecan observe a small amount of crystalline silicon dioxide whenusing hydrophobic TiO2, due to partial oxidation of silicone oil dur-ing heat treatment in air. This partial crystallization is expected

M.L. Di Vona et al. / Journal of Membrane Science 369 (2011) 536–544 539

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ig. 2. Typical SEM micrographs of SPEEK/TiO2 composites and corresponding EDoil–TiO2. EDX analysis: (a1) composite with THP–TiO2; (a2) focus on a TiO2 agglom

o influence the mechanical and electrical properties of the hybridembranes.SEM images (Fig. 2) show typical microstructures of SPEEK/TiO2

omposites. Fig. 2a shows a membrane with hydrophilic TiO2:he significant agglomeration of titania particles leads to an inho-

ogeneous membrane. The corresponding EDX analysis (a2) neargglomerated TiO2 particles shows a large amount of S, due to sul-onic acid groups, and Ti from the inorganic oxide. One can assume

egregation of sulfonic acid head groups near the hydrophilic TiO2articles, which enhances agglomeration. Fig. 2b shows a mem-rane with hydrophobic TiO2, which is more homogeneous, dueo smaller interaction between TiO2 particles. Here, the presencef Si is evidenced by EDX analysis (b1) near the TiO2 parti-

lysis: (a) composite with hydrophilic THP–TiO2; (b) composite with hydrophobic; (b1) composite with Soil–TiO2.

cles, due to the silicone oil coating. With both filler types, thechemical nature of the functionalized surface is clearly revealedand has a strong influence on the microstructure of the mem-branes.

The following AFM images (Fig. 3) show the characteristics ofSPEEK/TiO2 composites annealed at 140 ◦C for 64 h and untreated.All the surfaces are without pores, but the presence of the sec-ond phase makes the surfaces inhomogeneous with a higher mean

roughness for the annealed membranes (Rms = 49 nm for annealedand Rms = 15 nm for non-annealed hydrophilic TiO2 composites).With hydrophobic TiO2, the surface is less rough, especially afterannealing, due to lower agglomeration (Rms = 20 nm for annealedand Rms =14 nm for non-annealed).

540 M.L. Di Vona et al. / Journal of Membrane Science 369 (2011) 536–544

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ig. 3. Typical AFM images of SPEEK/THP–TiO2 (a) annealed at 140 ◦C for 64 h and (

Fig. 4 shows a comparison between typical FTIR spectra of SPEEKnd of SPEEK/THP–TiO2 and SPEEK/Soil–TiO2 composites. In allpectra, aromatic groups from PEEK backbone and sulfonic acidroups are observed. The hydrophobic composite exhibits some dif-erences around 2350 cm−1. There is also a clear difference betweenhese spectra around 3500 cm−1, the region of OH absorption. Fur-

−1

hermore, the transmittance between 800 and 400 cm is lower inomposites. The broad peak in the region of 400–600 cm−1 is dueo the transverse vibration of the Ti–O bonds; it is associated withhe longitudinal vibrational mode in the region of 700–950 cm−1.

ig. 4. Comparison of FTIR spectra: SPEEK/hydrophilic THP–TiO2 composite (black line)ine). (For interpretation of the references to color in this figure legend, the reader is refer

reated and SPEEK/Soil–TiO2 (c) annealed at 140 ◦C for 64 h and (d) untreated.

3.2. Thermal stability

The thermogravimetric analysis (Fig. 5A) shows that SPEEK andSPEEK/F-TiO2 composite membranes have a similar decompositionprofile. In Fig. 5B is reported a magnification of the three mass losscurves in the range 25–300 ◦C. At low temperature, two mass losses

can be observed for all samples corresponding to about 7% of theirinitial mass. The first loss (below 100 ◦C) can be attributed to watermolecules sorbed by hydrophilic groups and lost until the dry stateof the sample is reached [36,37].

SPEEK/hydrophobic Soil–TiO2 composite (red line) and single-phase SPEEK (greenred to the web version of the article.)

M.L. Di Vona et al. / Journal of Membrane Science 369 (2011) 536–544 541

Fig. 5. (A) TGA (solid line) and DTG (dashed line) curves for (a) SPEEK (black), (b)SPEEK/THP–TiO2 (blue) and (c) SPEEK/Soil–TiO2 (red) membranes. (B) Zoom in thetemperature range of 25–300 ◦C. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of the article.)

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uptake obtained is consistent with that obtained by immersionin liquid water. A moderate hysteresis is observed between watersorption and desorption isotherms.

Table 1Water uptake coefficients � in deionized water at different temperatures.

T [◦C] SPEEK SPEEK/THP–TiO2 SPEEK/Soil–TiO2

25 4 4 4105 103a 77 46125 114a 90 62

a

A small loss around 190 ◦C is due to loss of residual solventMSO. A major mass loss starts approximately at 210 ◦C and isttributed to the decomposition of the sulfonic acid groups ofPEEK. The whole weight loss can be evaluated knowing the degreef sulfonation of SPEEK. It is expected to be around 16% and Fig. 5Aonfirms a weight loss close to this value for all membranes. Theresence in the membranes of titanium dioxide slightly modifieshe temperature range of decomposition of sulfonic groups, whichccurs between 205 and 360 ◦C for the SPEEK membrane, between15 ◦C and 325 ◦C for SPEEK/THP–TiO2 and between 210 ◦C and30 ◦C for SPEEK/Soil–TiO2. If we relate the width of the tem-erature range where sulfonic acid loss occurs to a distributionf sulfonic acid groups in various environments, it would meanhat the incorporation of TiO2 reduces this distribution, possiblyy segregation of sulfonic acid head groups on the oxide surfaces evidenced earlier. It is well-known that the affinity of titaniumydrate to sulphate groups is high.

The last mass loss is attributed to PEEK main chain decomposi-ion. For SPEEK membranes, it is observed between 360 and 500 ◦C,hile for SPEEK/THP–TiO2 and SPEEK/Soil–TiO2 it is recorded in

he temperature range 325–465 ◦C and 330–460 ◦C, respectively.lthough the decomposition profiles are not modified by theddition of TiO2, its interactions with SPEEK modify the temper-ture range where the main decomposition reaction occurs. Theain chain decomposes at lower temperature in composites. The

resence of second phase particles destabilizes the polymer back-

one versus decomposition, possibly by reduction of interactionsetween macromolecules.

Fig. 6. Water uptake coefficients (�) obtained for SPEEK (1 h of immersion),SPEEK/hydrophilic (hphi) THP–TiO2 and SPEEK/hydrophobic (hpho) Soil–TiO2 com-posites (steady-state).

3.3. Water uptake behaviour

The solubility properties and the water uptake behaviour of themembranes are important parameters to take into considerationfor the performance in PEMFCs. In sulfonated aromatic polymers,such as SPEEK, excessive water uptake may lead to swelling andmechanical degradation, but in contrast a too low water uptakedoes not permit good conductivity. The formation of the compositedrastically modified the water absorption characteristics of SPEEK.

The values of water uptake coefficient � (the number of mol ofwater absorbed per mol of acid groups) were obtained using theequation:

� = (mwet − mdry)mdry

· 1000IEC · M(H2O)

(3)

mwet and mdry are the weight of the samples after and before theimmersion in water and M(H2O) is the molar mass of water.

Fig. 6 shows � values obtained by full immersion in water atdifferent temperatures for the three membranes (Table 1). We canobserve that at 25 ◦C the values of the water uptake coefficient arenot influenced by the presence of F-TiO2 in the matrix. The pres-ence of functionalized titanium dioxide is instead of fundamentalimportance at higher temperature; it enhances the stability of themembranes reducing their tendency to absorb water. While SPEEKis soluble in water after more than 1 h at a temperature greater then75 ◦C, the two composites reach a stable � value even at 145 ◦C.Obviously, the nature of the chemical modification influences thebehaviour of the membranes: the water uptake values are higherif the surface additive is hydrophilic tri(hydroxymethyl)-propanerather than hydrophobic silicone oil.

The results of water vapour sorption experiments at 25 ◦C forcomposites with hydrophilic TiO2 are shown in Fig. 7. The water

145 219 100 69

a After 1 h of immersion.

542 M.L. Di Vona et al. / Journal of Membrane Science 369 (2011) 536–544

Fig. 7. Water sorption isotherm (�, sorption; , desorption) at 25 ◦C for aSPEEK/THP–TiO2 composite annealed at 140 ◦C for 64 h.

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ig. 8. Stress strain curves of SPEEK/hydrophobic Soil–TiO2 (red line) andPEEK/hydrophilic THP–TiO2 (black line) and SPEEK annealed at 120 ◦C for 168 h.For interpretation of the references to color in this figure legend, the reader iseferred to the web version of the article.)

.4. Mechanical properties

Typical stress–strain tests for single-phase and composite mem-ranes are presented in Fig. 8. The static mechanical properties ofPEEK [38] and SPEEK with hydrophobic TiO2 are very reproducible,hich is in accordance with a very homogeneous membrane. In

ontrast, the mechanical properties determined for the membraneith hydrophilic TiO2 show a large scatter, probably related to the

nhomogeneous nature of the membrane, as shown in the SEMmage. The data reported in Table 2 have therefore considerably dif-erent standard deviations. Provided this limitation, it seems thathe composite with hydrophilic TiO2 shows the highest strengthnd lowest ductility. Both composite are considerably less duc-ile than pure SPEEK, which is in accordance with the considerablencrease of the glass transition temperature.

Fig. 9 presents DMA experiments made on annealed and nonnnealed SPEEK/F-TiO2 composites. As expected, the thermal treat-ent increases both storage modulus in the glassy state and glass

ransition temperature. This can be explained by two effects:

able 2oung’s modulus (E), ultimate strength (�), elongation at rupture (ε) and glass tran-ition temperatures (Tg) of SPEEK/TiO2 composites and pure SPEEK annealed at40 ◦C for 64 h.

Membrane E [MPa] � [MPa] ε [%] Tg [◦C]

SPEEK/THP–TiO2 1400 ± 500 41 ± 13 7 ± 4 200 ± 5SPEEK [38] 1240 ± 120 43 ± 4 29 ± 13 190 ± 10SPEEK/Soil–TiO2 880 ± 20 27 ± 2 8 ± 2 185 ± 5

Fig. 9. (a) Storage modulus (E′) and (b) tan ı of SPEEK/TiO2 composites as functionof temperature from DMA experiments.

removal of residual solvent which has a plasticising effect and mor-phological stabilization as reported for PEEK [39].

However, one can note that modulus increase is more pro-nounced for SPEEK/THP–TiO2 composite than for SPEEK/Soil–TiO2composite. These results in agreement with stress–strain tests canbe explained by the functionalization type of TiO2. As mentionedfor SEM results, THP–TiO2 leads to important agglomeration, butwith strong interactions between titania particle agglomerates andSPEEK, which leads to an important mechanical reinforcement. Onthe opposite, silicone oil coating of Soil–TiO2 allows good parti-cle dispersion, but decreases SPEEK/TiO2 interactions, which leadsto a poor mechanical reinforcement. The composites made usinghydrophobic TiO2 have a slightly higher glass transition than theones made using hydrophilic TiO2, probably due to the better dis-persion of TiO2 particles.

3.5. Electrical measurements

Fig. 10 shows the ionic conductivity of SPEEK/TiO2 compos-ites annealed at 140 ◦C for 64 h obtained from dielectric analysis.During heating, an increase of proton conductivity was observedin both samples until reaching a maximum around 95 ◦C. Prob-

ably the hydrophilic nature of the second phase allows reachinghigher proton conductivity. Above 110 ◦C, the proton conductiv-ity decreases until a minimum around 170 ◦C for Soil–TiO2 and160 ◦C for THP–TiO2 (at 10 kHz). The lower temperature of the min-imum for hydrophilic TiO2 composite can be related to its lower

M.L. Di Vona et al. / Journal of Membrane Science 369 (2011) 536–544 543

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Fig. 12. Proton conductivity at 100 ◦C as function of relative humidity for aSPEEK/Soil–TiO2 composite (�) and pure SPEEK (�) annealed at 140 ◦C for 64 h. Thearrow indicates the conductivity change occurred within 12 h after the conductivityhad reached a maximum.

Fig. 13. Proton conductivity at 90% RH as function of temperature for a

ig. 10. Ionic conductivity of (a) SPEEK/Soil–TiO2 and (b) SPEEK/THP–TiO2 compos-tes annealed at 140 ◦C for 64 h.

g revealed by DMA. Above this temperature an increase of ioniconductivity is observed due to chain-assisted motion.

The proton conductivity of SPEEK/Soil–TiO2 determined at0 kHz fixed frequency (Fig. 11) is lower than that of pure SPEEKecause of the hydrophobic nature of the composite. The relatively

ow ionic conductivity is due to the low degree of sulfonation ofPEEK used (DS = 0.75) and to the hydrophobic functionalization ofiO2. In contrast, composites with hydrophilic TiO2 show a higheronic conductivity; the decrease of conductivity is observed also atigher temperature. Here, the positive influence of the hydrophilic

unctionalization can be clearly seen.◦

Proton conductivity data at 100 C obtained by impedance anal-

sis for pure SPEEK and for a composite with hydrophobic TiO2an be observed in Fig. 12 as function of relative humidity. Theseeasurements were carried out at increasing RH from 50 to 90%. All

onductivity data of the composite, as well as those of pure SPEEK at

ig. 11. Comparison of ionic conductivity at 10 kHz vs. temperature: SPEEK (—),PEEK/THP–TiO2 (©), SPEEK/Soil–TiO2 (�).

SPEEK/Soil–TiO2 composite (�) and pure SPEEK (�) annealed at 140 ◦C for 64 h inArrhenius representation. Arrows indicate the conductivity change occurred within12 h after the conductivity had reached a maximum.

RH ≤ 80%, were collected after the conductivity had attained steadystate for at least 2 h. In contrast, the conductivity of SPEEK at 90%RH did not reach a steady state: after increasing RH from 80 to 90%,the conductivity passed through a maximum of 0.03 S cm−1 andthen decreased slowly reaching about 5 × 10−3 S cm−1 after 12 h. Asimilar behaviour was observed for Nafion 117 and attributed toan excessive membrane swelling [4], which is expected for SPEEKtoo on the basis of the hydration data of Fig. 6. In contrast, a homo-geneous increase of proton conductivity up to 90% RH is observedfor the composite, without degradation at high humidity. Fig. 13shows an Arrhenius plot of proton conductivity for both materialsunder 90% RH. As previously shown, the degradation of conductiv-ity is observable already at 100 ◦C for SPEEK, whereas it is seen at120 ◦C for the composite. This underlines an important advantageof such composites for higher humidity/higher temperature opera-tion. The activation energy, around 30 kJ/mol, is in good agreementwith previously published results [36].

4. Conclusions

We have studied composites of SPEEK withsurface-functionalized TiO2, one with hydrophilic(tri(hydroxymethyl)-propane) and one with hydrophobicmolecules (silicone oil). Composites with hydrophilic TiO2show large agglomeration of oxide particles and an inhomoge-neous microstructure, but the agglomeration of titania particlestogether with segregation of sulfonic acid groups gives high protonconductivity. The composites with hydrophobic TiO2 present avery homogeneous microstructure with well dispersed oxide

nanoparticles. However, this composite presents relatively lowproton conductivity. It is clear that further fine-tuning of thesurface treatment is worthwhile to get an optimal compromisebetween high conductivity and homogeneous microstructure.

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44 M.L. Di Vona et al. / Journal of M

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