preparation and characterization of proton-exchange hybrid membranes

Polymer International Polym Int 54:972–979 (2005)DOI: 10.1002/pi.1796

Preparation and characterization ofproton-exchange hybrid membranesAmelia Linares∗ and Jose Luis AcostaInstituto de Ciencia y Tecnologıa de Polımeros (CSIC), c/. Juan de la Cierva 3, 28006 Madrid, Spain

Abstract: Two types of composite were prepared, based on a thermoplastic polymer, polyvinylidenefluoride (PVDF), and an elastomer, ethylene-propylene-diene terpolymer (EPDM), respectively. Weobtained both series by addition of an inorganic proton-conducting antimonic acid derivative (HSb) andpolystyrene crosslinked with a small percentage of divinylbenzene (PS-co-DVB). From these composites,membranes were obtained and subjected to a heterogeneous-phase sulfonation reaction with chlorosulfonicacid. All experimental materials were characterized from a morphological and electrical point of view,by means of techniques such as differential scanning calorimetry (DSC), dynamic mechanical analysis(DMA), non-isothermal crystallization and complex impedance analysis. 2005 Society of Chemical Industry

Keywords: polyblends; sulfonation; ionomers; proton conductors

INTRODUCTIONThe definition of the word ionomer has been subjectto some uncertainly. Historically, the term was appliedto olefin-based polymers containing a relatively smallpercentage of ionic groups and in which the strongionic interaction forces play the dominant role inconducting properties.

As a general rule, unless specified otherwise, we shallconsider the term ionomer to mean polymers contain-ing less than 15–20 mol% ionic groups. Generally,cationic, anionic or both ionic groups may be chemi-cally attached to a polymer backbone, and the respec-tive ionomers are then called cationomers, anionomersor zwitterionomers.1–4 A number of different back-bones and pendant acid groups have been preparedand described. The backbones include polybuta-diene, polystyrene, polyethylene, polyoxymethylene,and polypentenamer. Although acid groups such ascarboxylic, thioglycolic and phosphonic have beenincluded, the major interest focuses on sulfonic acidgroups.

Ionic groups, chemically combined with a non-polar polymer backbone, have a strong influence onpolymer properties. This effect is not observed withconventional homopolymers or with copolymers basedon non-ionic units. Specific properties such as meltviscosity, glass transition temperature and dynamicmechanical behaviour can be modified by even smallamounts of ionic groups.5–8 It is well known that theimprovement of material properties is directly relatedto the aggregation of ionic groups into microphase-separated regions. This phenomenon occurs in wholly

amorphous or semi-crystalline thermoplastic polymersas well as in elastomers.6,9,10

The application of solid ionic polymers as elec-trolyte membranes spans a variety of electrochemi-cal technologies: low-temperature fuel cells, batter-ies, electrodialyzers, chloro-alkali cells, sensors, elec-trochromic devices and supercapacitors.11,12 Nafion

is a perfluorinated ionomer that has been researchedextensively13,14 and is widely utilized as a membranematerial, particularly in fuel cells and in the chloro-alkali process. This membrane material exhibits goodchemical stability and proton conductivity but it isvery expensive, it is a pollutant, and it presents seriousproblems when it is dehydrated. It also has the seriousdisadvantage of methanol crossover. Much effort isthus directed at replacing it. The objective is to findan alternative membrane to Nafion that has the sameconductivity and cell performance, but is cheaper,does not have problems of crossover and dehydrationand, in the case of using methanol as a fuel (such as indirect methanol fuel cells), can work at temperaturesof about 150 C. Operation at elevated temperaturereduces the adsorption of CO onto the platinum elec-trocatalyst and improves the kinetics of the methanoloxidation reaction at the anode. In addition, it isexpected to minimize problems due to electrode flood-ing and to lower the methanol crossover rates becauseof lower gas permeabilities of the polymer electrolyteat elevated temperatures. For these reasons, intenseresearch efforts are attempting to achieve low-cost,non-perfluorinated ionomer membranes to be used inseveral electrochemical systems.

∗ Correspondence to: Amelia Linares, Instituto de Ciencia y Tecnologıa de Polımeros (CSIC), c/. Juan de la Cierva 3, 28006 Madrid, SpainE-mail: [email protected](Received 20 August 2004; revised version received 26 November 2004; accepted 6 January 2004)Published online 2 March 2005

2005 Society of Chemical Industry. Polym Int 0959–8103/2005/$30.00 972

Proton-exchange hybrid membranes

Within this scope, the main objective of this workwas to undertake a preliminary study focused on thesynthesis of new membranes with good conductivitiesand low cost. We thus chose commercial and cheappolymers as starting materials, and, for the preparationof the membranes, we used conventional techniquesas used in industrial processes of polymeric materials.We prepared two types of proton-conducting compos-ite, based on poly(styrene-co-divinylbenzene) (PS-co-DVB), an insulating polymeric matrix (polyvinylidenefluoride [PVDF]), an elastomer (ethylene-propylene-diene terpolymer [EPDM]), and an inorganic proton-conducting compound (antimonic acid [HSb]). Theincorporation of the latter component improves thephysical and electrical properties, forming a contin-uous conducting phase in the material and, takingadvantage of its good hydrophilicity, promotes thehydration properties of membranes. Subsequently,membranes from these composites were preparedand characterized from a morphological and electricalpoint of view.

EXPERIMENTALMaterialsThe polymers were commercial products and wereused as received. PS-co-DVB with a crosslink densityof 2 % was supplied by Aldrich Chemical Company,PVDF under the trade name Solef 6010 was suppliedby Solvay, EPDM (Vistalon 9500) was suppliedby Exxon Chemical, and 1,2-dichloroethane andchlorosulfonic acid were from Scharlau and Fluka,respectively.

The HSb was synthesized by oxidation of Sb2O3

(Merck) with an excess of 35 % H2O2 (Merck). Thesuspension was vigorously stirred and heated at 65 Cfor 30 h. Once the HSb was obtained, it was washedwith deionized water and then centrifuged. Finally,the product was dried at 40 C under low pressure.15

Milli-Q deionized water was used.The commercial Dupont membrane Nafion 117

was used as a reference.

ProceduresThe blends were prepared in a thermoplastic mixingchamber. The experimental conditions established toobtain membranes from each type of composite arelisted in Table 1.

Membranes of 2 cm × 4 cm and suitable thickness(0.2–0.3 mm) were manufactured by compression ina Collins hydraulic press. They were then sulfonatedwith a 0.3 M chlorosulfonic acid/1,2-dichloroethane

Table 1. Experimental conditions

Blending Moulding

Composite type T (C) rpm t (min) T (C) p (psi) t (min)

Based on PVDF 180 70 10 180 200 10Based on EPDM 160 70 10 100 200 15

solution at 50 C for 3 h and 24 h for the PVDF-basedcomposites, and at room temperature for 2 h and 4 hfor those based on EPDM. The samples were thenwashed several times with acetone and Milli-Q wateruntil neutral pH was obtained.

Calorimetric measurements and non-isothermalcrystallization were carried out on a Mettler TA4000differential scanning calorimeter (DSC) operating inan N2 atmosphere. The following procedure wasemployed to study non-isothermal crystallization. Thesamples were first held for 10 min at 220 C to destroytheir thermal history. They were then cooled at arate of 8, 10 and 12 C min−1 to total crystallization.The fraction of polymer crystallized at a certaintemperature and time, the onset temperature andthe peak temperature were determined from the non-isothermal plot. Finally, the crystallized samples wereheated at a rate of 10 C min−1 and the endothermwas recorded.

The dynamic mechanical analysis (DMA) wasperformed with a DMTA Polymer Laboratory in thetensile mode. The frequencies chosen were 5, 10, 20and 30 Hz and the temperature range was between−150 and +150 C.

Before recording DSC thermograms at a heatingrate of 10 C min−1, the samples were quenched atlow temperature from the melt. The midpoint of theslope change of the heat capacity was taken as the glasstransition temperature Tg.

A computer-assisted Hewlett Packard 4192A ana-lyzer was used for impedance spectroscopy of thesamples. Complex impedance measurements were car-ried out in the two-electrode AC impedance mode,at ambient temperature, over the frequency range0.01–10 000 kHz, and 0.1 V amplitude of the signalapplied. To measure the conductivity, we designeda special cell composed of two silver electrodes with0.07 cm2 surface area each. The electrode surfaceswere kept clean to avoid any contact resistance duringmeasurements and the membrane sample was sand-wiched in between these two flat circular electrodes.

Three measurements, from different samples ofeach blend were carried out to determine the Tg byDSC and complex impedance analysis. The Tg andconductivity data are the average values. In all cases,the linear regression correlation coefficient, r2, was0.97 or higher.

RESULTS AND DISCUSSIONComposites based on PVDFGlass transition temperatures from DSC and DMAanalysisTable 2 lists the Tg and the specific heat Cp valuesreached by the samples and obtained according to theprocedure detailed in the Experimental section above.In all cases, only one Tg, attributable to PVDF, wasfound.

Although none of the values differed significantlyfrom those of the pure PVDF homopolymer, some

Polym Int 54:972–979 (2005) 973

A Linares, JL Acosta

Table 2. Glass transition temperature and heat capacity obtained from DSC for PVDF-based composites

Non-sulfonated Sulfonated for 3 h Sulfonated for 24 hPVDF/PS-co-DVB/HSb

composition (wt%) Sample Tg (C) Cp (J g−1 K−1) Sample Tg (C) Cp (J g−1 K−1) Sample Tg (C) Cp (J g−1 K−1)

100/0/0 PVDF −38.2 0.15 — — — — — —70/30/0 PF-03 −41.4 0.19 PF-33 −39.0 0.16 PF-43 −37.8 0.1770/20/10 PF-02 −40.8 0.17 PF-32 −39.1 0.17 PF-42 −37.1 0.1870/10/20 PF-01 −38.9 0.16 PF-31 −38.7 0.18 PF-41 −36.3 0.16

Table 3. Glass transition temperature at various frequencies obtained from DMA for PVDF-based composites


composition (wt%) Frequency (Hz) Sample Tg (C) Sample Tg (C) Sample Tg (C)

100/0/0 5 PVDF −40.1 — — — —10 −38.220 −36.530 −34.7

70/30/0 5 PF-03 −43.1 PF-33 −41.5 PF-43 −39.610 −42.4 −40.9 −39.020 −40.7 −39.1 −37.130 −38.9 −37.2 −35.3

70/20/10 5 PF-02 −43.0 PF-32 −42.5 PF-42 −38.610 −42.3 −40.7 −37.420 −40.5 −39.6 −36.130 −39.0 −38.8 −34.9

70/10/20 5 PF-01 −40.4 PF-31 −40.2 PF-41 −37.710 −39.1 −38.5 −36.920 −37.9 −37.2 −35.130 −36.8 −36.2 −34.6

differences were recorded in composite behaviourdepending on whether or not the samples hadundergone the sulfonation reaction. In fact, all thenon-sulfonated samples had a lower Tg than thatof the PVDF matrix, with the differential becominggreater with increasing crosslinked copolymer contentand associated lower HSb content. The sulfonationprocess caused the Tg values to be shifted towardshigher temperatures, showing similar values to those ofpure PVDF for samples sulfonated for 3 h but highervalues when the sulfonation reaction extended for24 h. Tg values those rose in proportion to ionic groupcontent. The variation in Cp in the glass transitionrange was not significant.

Table 3 lists the Tg data obtained from DMA forthe various samples, tested at the four frequenciesemployed. By way of illustration, Fig 1 shows thevariation of the Tg (at 10 Hz) plotted against HSbcontent for non-sulfonated and sulfonated samples,and Fig 2 shows the graphs of the variation of lossmodulus E′′ with temperature from which Tg wascalculated—it has been shown that maximum E′′relates much better to the value obtained by DSCthan tan δ.16

As expected, Tg increased as a function of frequency,and the effects exerted by the different componentsof the composites and the sulfonation conditionson Tg are similar to what was observed by thermalanalysis, ie the Tg increased as a function of sulfonation

0 5 10 15 20

HSb content (wt%)

-44

-42

-40

-38

-36

Tg

(˚C

)

24 h

3 h

0 h

10 Hz

Figure 1. Variation of the glass transition temperature with HSbcontent for non-sulfonated and sulfonated membranes, obtainedfrom DMA analysis at 10 Hz.

time and inorganic acid content. This means thatthe interactions produced between the crosslinkedpolymer and PVDF are stronger and more prevalentthan those activated by the HSb.

After sulfonating the composites, ionic interactionsappear which may form multiplets or even clusters10

974 Polym Int 54:972–979 (2005)


−200 −100 0 100 200

0

50

100

150

200

250 PF-02

T (°C)

E″

(MP

a)

0

52

104

156

208

260 PF-01

−200 −100 0 100 200

T (°C)

E″

(MP

a)

0

48

96

144

192

240 PF-32

−200 −100 0 100 200

T (°C)

E″

(MP

a)

−200 −100 0 100 200

0

40

80

120

160

200 PF-33

T (°C)

E″

(MP

a)

0

44

88

132

176

220 PF-31

−200 −100 0 100 200

T (°C)

E″

(MP

a)

0

36

72

108

144

180 PF-43

−200 −100 0 100 200

T (°C)

E″

(MP

a)

0

48

96

144

192

240 PF-41

−200 −100 0 100 200

T (°C)

E″

(MP

a)

0

40

80

120

160

200 PF-42

−200 −100 0 100 200

T (°C)

E″

(MP

a)

0

50

100

150

200

250 PF-03

−200 −100 0 100 200

T (°C)

E″

(MP

a)

−200 −100 0 100 200

T (°C)

0

48

96

144

192

240E

″ (M

Pa)

PVDF

Figure 2. Loss modulus versus temperature obtained forPVDF-based composites.

at high sulfonation times, inhibiting macromolecularmovement and, as a result, causing the Tg to increase.

Non-isothermal crystallizationNon-isothermal crystallization can be followed andanalyzed by applying the Ziabicki equation,17 whichis an extension of the Avrami equation and is used todescribe the morphological variation as a functionof cooling rate. The kinetics of non-isothermalcrystallization can be characterized by determining

the constants Zt and n′′ occurring in the equation:

log[− ln(1 − X)] = n′′ log t + log Zt

where the rate constant Zt is equivalent to the Avramirate constant in isothermal crystallization kinetics,n′′ is the morphological exponent, and X is thecrystalline fraction at time t. The value of n′′ at aspecific cooling rate is obtained from the slope of aplot of log[− ln(1 − X)] versus log t. In the Ziabickianalysis, the effects of the cooling rate on the crystallinemorphology are indicated by the dependence of theexponent n′′ on the cooling rate. The intercept in thelogarithm plot yields the parameter log Zt. The finalform of Zt has to be corrected for the cooling rate β:

log Zc = log Zt

β

When applying the Ziabicki equation (up to amaximum of 60 % conversion), the data adjust almostperfectly to a straight line for small rate values. Forhigher rates, the data deviate from strict linearity,although the correlation coefficient r2 remained at0.97 or higher.

Table 4 lists the n′′ and log Zt values obtainedunder the non-isothermal conditions described above.Although no clear correlation between n′′ and com-position or treatment could be established, in generalterms n′′ ranged from 3 to 4. So, it can be inferred thatPVDF adopts the same growth geometry in all sys-tems, and it corresponds to a thermal heterogeneousnucleation followed by spherulitic growth.

The rate constant Zt logically increases in all caseswith increasing cooling rate. As a representative exam-ple, Fig 3 illustrates the behaviour of the rate constantas a function of crosslinked polymer content and sul-fonation time, at the cooling rate of 12 C min−1.

For any of the cooling rates, the PVDF crystalliza-tion rate was observed to increase as the HSb portionwas raised or, conversely, as the PS-co-DVB contentdecreased in the system. This phenomenon can beinterpreted, on the one hand, as HSb exerting a nucle-ating effect on PVDF crystallization and, on the otherhand, as a certain degree of similarity between thetwo polymers, which hampers the movement of thesegments towards the surface for crystal formationto take place. This behaviour is indicative of a certaincompatibility between the polymers. At longer sulfona-tion times, this effect became much more prominent,when there existed great numbers of aromatic ringsfor the sulfonic groups to incorporate into and, as aconsequence, ionic pair association was so frequentthat it may even have given rise to cluster formation,which then acted as genuine barriers to inhibit polymerbackbone movement.

Complex impedance spectroscopyThe electrical properties were determined by meansof complex impedance spectroscopy. Although the

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Table 4. Kinetic studies: parameters obtained from non-isothermal crystallization for PVDF-based composites


composition (wt%) β (C min−1) Sample n′′ log Zt Sample n′′ log Zt Sample n′′ log Zt

100/0/0 8 PVDF 3.9 0.391 — — — — — —10 3.8 0.52812 3.1 0.575

70/30/0 8 PF-03 4.3 0.184 PF-33 3.2 −0.265 PF-43 2.8 −1.28110 4.5 0.427 2.9 0.188 2.5 −0.69112 4.1 0.837 2.4 0.54 2.4 −0.492

70/20/10 8 PF-02 4 0.787 PF-32 3.2 0.302 PF-42 4 −0.51410 3 0.968 3.2 0.709 3.5 −0.20912 3.9 1.172 3.2 1.015 3.2 0.356

70/10/20 8 PF-01 3.9 1.079 PF-31 2.9 0.162 PF-41 3.9 −0.00610 3.7 1.244 3.5 0.658 3.3 0.3912 3.6 1.493 3.3 1.022 3.6 0.415

10 15 20 25 30

PS-co-DVB content (wt%)

−2

−1

1

2

3

log

Zt

0 h

3 h

24 h

b = 12 °C min−1

Figure 3. Variation of the rate constant with crosslinked polymercontent and sulfonation time, at a cooling rate of 12 C min−1.

measurements were taken at room temperature, priorto measuring the samples were hydrated by immersionin deionized water at 50 C for various periods of time.For purposes of comparison, we also analyzed a Nafion117 sample at the same experimental conditions. Theresults are shown in Table 5.

Imaginary versus real impedance plots similar to theone shown in Fig 4 (sample PF-31) were obtainedfor all membranes. By using suitable software thatlet us fit the data to a circuit model, we inferred theresistance of the sample from the intercept of the low-frequency part of the arc on the real impedance axis,Z ′. The overall conductivity of the membranes wasthen calculated as usual.

The ion conductivity of the two polymeric matrices,PVDF and PS-co-DVB, are both below 10−10 S cm−1.Hence they may both be considered to be totally insu-lating materials. The conductivity determined for theinorganic conductor was in the order of magnitude of10−6 S cm−1.

0.0E+0 1.0E+6 2.0E+6 3.0E+6

0.0E+0

2.0E+5

4.0E+5

6.0E+5 PF-31

60 min

120 min

180 minZ

″ (Ω

)

Z′ (Ω)

Figure 4. Complex plane diagram for the PF-31 blend at varioushydration times.

In general terms, the best values were obtainedfor membranes containing only the polymeric PVDFmatrix and the crosslinked polymer. With increasingHSb derivative content, these values would thengradually deteriorate. In principle, and as would seemlogical, high values were likewise obtained when thesulfonation reaction was extended to 24 h.

In all cases, the most significant conductivityincrement was reached through sample hydration,even at very short hydration times. Hydration broughtto light a difference in conductivity for the sampleseries sulfonated for 3 and 24 h. Whereas for theformer the conductivity diminished as a function ofhydration time, for the latter conductivity improved.

Composites based on EPDMGlass transition temperatures from DSC and DMAanalysisAs was the case with composites based on PVDF,only a single Tg, attributable to EPDM, was foundin the DSC analyses. The results obtained (Table 6)show remarkable differences in composite behaviour,depending on whether the samples had or had not

976 Polym Int 54:972–979 (2005)


Table 5. Conductivity data obtained from complex impedance spectroscopy analyses for PVDF-based composites

Non-sulfonated Sulfonated for 3 h Sulfonated for 24 h

PVDF/PS-co-DVB/HSbcomposition (wt%)

Hydration time(min) Sample

Conductivity(S cm−1) Sample


Conductivity(S cm−1)

100/0/0 0 PVDF <10−10 — — — —— 0 Nafion 7.2 × 10−5 — — — —

60 6.9 × 10−4

120 1.4 × 10−3

180 2.1 × 10−3

70/30/0 0 PF-03 <10−10 PF-33 <10−8 PF-43 <10−8

60 1.3 × 10−5 3.6 × 10−4

120 3.8 × 10−6 3.7 × 10−4

180 6.8 × 10−6 4.4 × 10−4

70/20/10 0 PF-02 <10−10 PF-32 <10−8 PF-42 <10−8

60 6.9 × 10−6 1.8 × 10−5

120 3.4 × 10−6 1.9 × 10−5

180 4.5 × 10−6 2.8 × 10−5

70/10/20 0 PF-01 <10−10 PF-31 <10−8 PF-41 <10−8

60 1.8 × 10−6 1.3 × 10−6

120 1.0 × 10−6 6.9 × 10−6

180 5.1 × 10−7 1.1 × 10−6

Table 6. Glass transition temperature and heat capacity obtained from DSC for EPDM-based composites

Non-sulfonated Sulfonated for 2 h Sulfonated for 4 hEPDM/PS-co-DVB/HSb

composition (wt%) Sample Tg (C) Cp (J g−1 K−1) Sample Tg (C) Cp (J g−1 K−1) Sample Tg (C) Cp (J g−1 K−1)

100/0/0 EPDM −42.5 0.37 — — — — — —70/30/0 E-03 −41.7 0.35 E-23 −33.1 0.39 E-43 −32.5 0.4070/20/10 E-02 −41.4 0.39 E-22 −35.1 0.40 E-42 −34.1 0.3870/10/20 E-01 −42.2 0.40 E-21 −38.6 0.38 E-41 −38.2 0.38

Table 7. Glass transition temperature at various frequencies obtained from DMA for EPDM-based composites


composition (wt%) Frequency (Hz) Sample Tg (C) Sample Tg (C) Sample Tg (C)

100/0/0 5 EPDM −40.1 — — — —10 −39.920 −38.030 −36.5

70/30/0 5 E-03 −39.6 E-23 −32.0 E-43 −31.510 −39.2 −31.6 −31.120 −37.8 −31.2 −30.630 −36.0 −30.5 −30.2

70/20/10 5 E-02 −39.9 E-22 −33.9 E-42 −33.010 −39.6 −33.1 −32.620 −37.4 −32.7 −32.130 −35.3 −32.4 −31.8

70/10/20 5 E-01 −40.0 E-21 −35.9 E-41 −35.010 −39.9 −35.1 −34.720 −37.8 −34.6 −34.330 −36.0 −34.3 −34.0

undergone the sulfonation reaction. All the non-sulfonated blends had Tg values fairly similar to that ofthe unblended EPDM matrix. Analogously to PVDF-based composites, the sulfonation process causesthe Tg values to shift towards higher temperatures,

showing, in these cases, no significant differencesbetween 2 and 4 h of sulfonation time. The variationin Cp in the glass transition range was not significant.

Table 7 lists the Tg data obtained from DMA forthe samples at four experimental frequencies. The

Polym Int 54:972–979 (2005) 977


effects exerted on the Tg by the various componentsin the composites, and by the sulfonation conditions,were similar to those observed by thermal analysis.Furthermore, Tg increased as a function of frequency.

As can be deduced from both techniques, the mostsignificant increase in Tg occurred as a consequenceof the sulfonation process and not as a result of theduration of sulfonation nor of the composition.

With regard to the effect of sulfonation in preventingmacromolecular movement, we can consider that theexplanations in the case of PVDF are also valid for thistype of composite.

Complex impedance spectroscopyThe measurements were carried out under the sameexperimental conditions established for the compositesbased on PVDF. Table 8 lists the data obtainedand Fig 5, as an example, shows the complex planediagram for the E-22 blend at various hydration times.

1.5E+3 2.9E+3 4.3E+3 5.7E+3

0.0E+0

5.0E+2

1.0E+3

1.5E+3 E-22

180 min120 min

60 min

Z″ (

Ω)

Z′ (Ω)

Figure 5. Complex plane diagram for the E-22 blend at varioushydration times.

Two important consequences can be extracted byanalyzing these results. First, for this type of compositeshort sulfonation and hydration times are enough toreach quite good values of conductivity and, second,there are no significant differences between the valuesof conductivity found for the two studied times ofsulfonation.

It is also remarkable that, as the hydration timeincreased, the conductivity diminished, in such a waythat we tested samples at sulfonation times greater(8 and 24 h) than the established ones for this workand these blends had practically lost their conductingcharacter. This fact can be explained by taking intoaccount that the samples suffered a serious physicaldeterioration (directly observed). Thus, with the aimto confirm this aspect, all these membranes weresoaked for 6 h in deionized water at 60 C. Thesurface of the membranes were then quickly wipedwith an absorbent paper to remove the excess wateradhering to it and the samples were then weighed todetermine the uptake. The weight of the membraneswere then measured again after drying them overnightunder vacuum at 80 C. The water uptake content wascalculated by:

uptake content (%) =(

Wwet − Wdry

Wdry

)× 100

and Fig 6 shows its variation with PS-co-DVB content.It is known that the presence of water is

very important for the ionic conductivity of solidelectrolytes, since the diffusion of protons through thepolymer electrolyte is directly related to the extent ofthe hydration membrane. However, at high contents,the water is involved not only in the solvation of theprotons and sulfonated ions of the membranes butalso in filling the pores and swelling the polymer. Thisinvolves condensation of water on the hydrophobicsurfaces of the polymer, which is unfavourable forconducting purposes.

Table 8. Conductivity data obtained from complex impedance spectroscopy analyses for EPDM-based composites

Non-sulfonated Sulfonated for 2 h Sulfonated for 4 h

EPDM/PS-co-DVB/HSbcomposition (wt%)

Hydration time(min) Sample



Conductivity(S cm−1)

100/0/0 0 EPDM <10−10 — — — —70/30/0 0 E-03 <10−10 E-23 1.7 × 10−6 E-43 2.0 × 10−5

60 1.8 × 10−2 4.2 × 10−2

120 8.1 × 10−3 3.0 × 10−3

180 4.8 × 10−3 7.2 × 10−4

70/20/10 0 E-02 <10−10 E-22 1.3 × 10−5 E-42 1.7 × 10−5

60 2.3 × 10−3 7.3 × 10−3

120 8.7 × 10−4 2.1 × 10−3

180 5.3 × 10−4 1.9 × 10−3

70/10/20 0 E-01 <10−10 E-21 1.9 × 10−5 E-41 2.4 × 10−5

60 1.1 × 10−3 2.8 × 10−3

120 6.7 × 10−4 2.5 × 10−3

180 1.0 × 10−4 8.1 × 10−5

978 Polym Int 54:972–979 (2005)


0 20 40

PS-co-DVB content (wt%)

210

230

250W

ater

upt

ake

(%)

4 h

2 h

Figure 6. Variation of water uptake with crosslinked polymer contentfor sulfonated EPDM-based composites.

Considerable swelling thus reduces conductivityand contributes significantly to the physical anddimensional deterioration of these membranes.

CONCLUSIONSWe prepared two series of materials based on PVDF,a thermally stable, insulating crystalline polymer,and EPDM, an elastomer thermoplastic. From thesehomopolymers, composites with various compositionswere prepared, by addition of an inorganic protonconductor, ie an HSb derivative, and a styrenecopolymer crosslinked with a small amount (2 %)of divinylbenzene. The composites were moulded intomembranes and then sulfonated with chlorosulfonicacid for various periods of time depending on the typeof composite. All the membranes were characterizedmorphologically and electrically.

With respect to the PVDF composites, the non-sulfonated membranes consisting exclusively of PVDFand PS-co-DVB had the lowest Tg values andcrystallization rates. This is indicative of the fact thatmacromolecular interactions emerge between the twopolymer matrices, which, on the one hand, favourthe joint and localized movements of the PVDF, asif the PS-co-DVB exerted a plasticizing effect, thuslowering the temperature at which the glass transitionstrain occurs. On the other hand, it seems as ifthe same copolymer prevented the transfer of themacromolecular backbone towards the domain of thegrowing crystals, thus diminishing the crystallizationrate of the blend as a consequence.

Treating the membranes with chlorosulfonic acidwas observed to increase Tg and decrease Zt, as afunction of sulfonation time. This behaviour can beexplained by considering that, when increasing thenumber of ionic groups integrated in the polymericnetwork, ionic pair association is favoured. This gives

rise to multiplets or even cluster formation, which inturn enhance backbone stiffness (Tg rises) and act asgenuine blockers of macromolecular movements (Zt

reduces).From an electrical point of view, the best

conductivities (of the order of 10−4 S cm−1) wereachieved for blends containing both the two polymermatrices and sulfonated for 24 h. This latter factcould be explained by assuming the existence ofthe aforementioned clusters, which would build up acontinuous phase of an ionic nature and hence favourelectric transfer.

With respect to the EPDM composites, we couldconclude, firstly, that the increases in Tg, observedby both DSC and DMA, were a consequence ofcarrying out the sulfonation process and were notaffected significantly by the composition or by thetime of reaction. Secondly, even at short sulfonationand hydration times, these hybrid membranes hadexcellent values of conductivity and, as is known,they will probably be higher at elevated temperatures.However, it is also necessary to consider thesignificant deterioration in their physical appearanceand dimensional stability. These two aspects are,undoubtedly, a serious obstacle in the way of thistype of membrane being employed as an electrolytein fuel cells. Thus, we are undertaking a newresearch consisting of preparing hybrid membranes,similar to these based on EPDM, but incorporatinga new constituent, polyethylene (PE), that for itstechnological qualities it will promote the physicaland dimensional stability that these membranes lack.

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