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aromatic polymers such as polybenzimidazole [6,7], poly(etherether ketone)s [813], poly(ether sulfone) [14,15] , polyimides[16,17] and poly(phenylene sulde)s [18], which have excel-lent chemical resistance, high thermo-oxidative stability, goodmechanical properties and very low cost.

A series of poly(phthalazinone ether sulfone ketone)(PPESK) copolymers, containing different ratios of diphenyl-sulfone and diphenylketone units, were previously synthe-sized [1921] . The PPESK family of polymers has highglass transition temperature ( T g = 275 C) and excellent phys-ical properties and thermostability. The sulfonation reactionsof PPESK, and the nanoltration and ultraltration mem-brane properties have been studied before [22,23] . RecentlySPPESK used in PEM was reported by Gao et al. [24]. Belong-ing to derivates of poly(phthazinones) (PP) family, sulfonatedpoly(phthalazinone ether sulfone ketones) (SPPEKs) and sul-fonated poly(phthalazinone ether ketones) (SPPESs) were stud-ied for using in PEM eld [2527] . Those materials show theattractive properties such as high conductivity, high thermal

stability, etc. In this paper, synthesis, characteristics, thermalanalysis, water uptake and swelling rate, proton conductivityand methanol permeability of SPPESK were studied.

2. Experimental

2.1. Materials and chemicals

Poly(phthalazinone ether sulfone ketone) with a sulfone/ ketone ratio of 1:1 (PPESK S/K = 1/1) were synthesizedaccording to the procedure reported previously [19,20] . N , N -dimethylacetamide (DMAc), N -methypyrrolideone (NMP),

methanol, ether, acetone, fuming sulfuric acid (2023% SO 3),sodium hydroxide, sodium chloride, sulfuric acid (9598%),hydrogen peroxide, phenolphthalein and other chemicals wereobtained commercially and used without further purication.All chemicals used in the experiments were analytical grade.

2.2. Synthesis and separation of SPPESK

Previous dried for 24 h at 120 C in a vacuum oven, a mountof PPESK powder was gradually put into a reaction vessel lledwith thequantitativemixture of sulfuric acid (9698%) andfum-ing sulfuric acid (2023% SO 3) stirring with magnetic sitter andxedat 60 C for 2 h. SPPESKwith different degrees of sulfona-tion (DSs) would be obtained according to different ratios of thesulfuric acid and fuming acid.

After a determined reaction time, the reaction was terminatedby adding ether; the polymer would precipitate in ether, andthen was ltered under reduced pressure to remove the resid-ual ether. Then the polymer was re-dissolved in quantitativeDMAc. The polymer re-precipitated when the DMAc solutionwas added dropwise into quantitative acetone, then the polymerwas washed to pH 7 by acetone. Through the above procedures,the SPPESK was separated and puried satised to use in thePEM experiment. At last, the SPPESK was dried for 24 h at120 C in a vacuum oven to remove the moisture and residual

acetone.

2.3. Determination of degree of sulfonation of SPPESK

SPPESK can be divided to two sorts: L-SPPESK, whichhas lower DS and cannot solve in water, and H-SPPESK,which has higher DS and can solve in water. The DS of L-SPPESK was determined by ion exchange method. Preciselyweighted L-SPPESK sample ( 0.5g) was soaked in excesssaturated NaCl solution for 12 h, and then ltered. The l-trate was titrated by 0.1 M NaOH with phenolphthalein as anindicator. The DS of H-SPPESK was determined using an elec-trical conductivity instrument. Precisely measured quantitiesof H-SPPESK sample ( 0.5g) was dissolved in water andtitrated directly with 0.1 M NaOH, and then electrical conduc-tivity was measured to calculate the DS. The DS of SPPESKwere determined by these two methods correspondingly in thispaper.

2.4. Characteristics of SPPESK

2.4.1. Fourier transform infrared spectroscopy (FTIR)FTIR spectra of PPESK or SPPESK were recorded on a

EQUINOX 55 Fourier transform infrared spectrometer (BrukerOptics) with powder samples by using KBr pellets com-posed of 200mg of IR spectroscopic grade and 1 mg polymersamples.

2.4.2. Nuclear magnetic resonance (NMR)The 1H NMR spectra were recorded on a Varian Unity

Inova DLG400 spectrometer at a resonance frequency of 399.961 MHz. 15 wt.% SPPESK solutions were prepared indeuterated dimethyl sulfoxide (DMSO-d6), and the PPESK was

dissolved in CDCl 3 to proceed to the analysis, and tetramethyl-silane (TMS) was used as the internal standard.

2.5. Thermal analysis

2.5.1. Thermogravimetric analysis (TGA)Thermal stability of SPPESKs and PPESK was ana-

lyzed using thermogravimetric analysis (Mettler ToledoTGA/SDTA851 e) from 100 to 800 C at a rate of 10 C/minunder a nitrogenatmosphere (N 2 ow rate:80 ml/min).Thesam-ples were dried for 2 h at 120 C in vacuum to remove moistureprior to the study. DTG curve is the rst order differential of TGA curve on temperature.

2.5.2. Differential scanning calorimetry (DSC)The glass transition temperature ( T g) was determined from

the DSC curves. DSC (Mettler Toledo DSC821 e) data wereobtained from 150 to 450 C ata heating rateof 10 C/minundera nitrogen atmosphere (N 2 ow rate: 50 ml/min).

2.6. Membrane preparation

Membranes were prepared by dissolving the SPPESK inNMP (30 wt.%), casting the solutions on a glass plate, and cur-ing and drying the membranes at 50 C for 48h, 80 C for 23

days, respectively. Then the membranes (3060 m thick) were


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S. Gu et al. / Journal of Membrane Science 281 (2006) 121129 123

peeled off from the glass plate, and immersed in a 0.5M H 2SO4solution for 1 h at 80 C, washed by deionized water, immersedin4wt.%H 2O2 solutionfor 1 h at80 C, washed again by deion-ized water. Finally the membranes were rinsed with deionizedwater.

2.7. Water uptake and swelling

Weighted membranes (4 cm 6 cm) were immersed indeionized water at different temperature for more than 10 h toinsure the membranes were saturated with water. The liquidwater on the surface of wet membranes was removed quicklywith lterpaper, andthen theweights anddimensionallengthsof wet membranes were measured. The weights and lengths of drymembranes were obtained after the wet membranes were driedfor 24h at 120 C in vacuum. The water uptake and swellingrate can be calculated by relationships as follows:

Wateruptake (%)=

W wet W dryW dry

100

Swelling rate (%) =lwet ldry

ldry 100

where W wet and W dry are the masses of wet and dry mem-brane samples, respectively, lwet and ldry are the average length[lwet = (lwet1 lwet2 )1/2 , ldry = (ldry1 ldry2 )1/2 ] of wet and drysamples, respectively. Where, lwet1 , lwet2 , and ldry1 , ldry2 arethe lengths and widths of wet membranes and dry membranes,respectively.

2.8. Conductivity measurement

The proton conductivity of the SPPESK membrane sampleswas measured by two electrode AC impedance spectroscopymethod (over a frequency range 100 Hz100 kHz) and the appa-ratus is designed by Direct Alcohol Fuel Cells Laboratory of Dalian Institute of Chemical Physics, Chinese Academy of Science. The measurements were carried out on the Potentio-stat/Galvanostat 273A workstation (EG&G Princeton AppliedResearch) with a lock in amplier 5210 (0.5 Hz120kHz)(Perkin-Elmer instruments) and the software Powersuite wasused to collect data and plot gures. The measurements weremade in transversal direction across the membranes. A sampleof membrane 20 mm 20 mm was placed in the temperature-controlled cell, where it was clamped between two blockingCu electrodes (with 16 mm diameter and plated a thin lmAu) with a permanent pressure of about 0.2 MPa. Impedancespectra of membrane samples gave Nyquist plots [28,29] andthe proton conductivity of membrane samples, , can be cal-culated from the plots, using the relation = L /[( R r ) A],where L and A are the thickness of the membrane samplesand electrodes area, respectively [30], and R was derived fromthe low intersect of high frequency semi-circle on a compleximpedanceplane with theRe( Z ) axis, and r is the systeminternal

resistance.

2.9. Methanol permeability

The methanol permeability of the membranes was deter-mined using a glass diffusion cell. This cell consisted of tworeservoirs and separated by a vertical membrane. One com-partment of the cell ( V A = 50 ml) was lled with a solution of methanol (1 M) and 1-butanol (40 g/l) (used as an internal stan-dard) in deionized water. The other ( V B = 20 ml) was lled witha 1-butanol (40 g/l) solution in deionized water. The membrane(area, 2 cm 2) was clamped between the two reservoirs, whichwere both stirred during the experiments. Prior to the test, themembraneswereequilibratedin deionizedwater forat least12 h.The increase in concentration of methanol in the B reservoir wasmeasuredacrosstimeusinggaschromatography. In thegas chro-matography measurements, 3 l samples were analyzed using aShimadzu GC-14B gas chromatograph. In the permeation tests,the temperature was controlled by a thermostatic water bath.Methanol concentration in the receiving cell detected as a func-tion of time has the following relationship:

CB(t ) =AV B

DKL

CA(t t 0)

where C A and C B are the concentration of methanol on cell Aand cell B, V B is the volume of cell B, A and L are area andthickness of membrane samples, and D and K are the methanoldiffusionand partition coefcient, respectively. The product DK is the membrane permeability P (P = DK ) [31,32] . The value of C B was measured several times during the experiments, and thepermeability was calculated from the gradient of the straightlines obtained from plots of the data.

3. Results and discussion

3.1. Synthesis of SPPESK

Sulfonation is commonly applied to modify polymers toincrease hydrophilicity, water ux and proton conductivity.Using the fuming sulfuric acid agent, PPESK can be sulfonatedeasily. Their chemical structures are shown in Fig. 1. It is notdifcult to control theDS of SPPESKby varying the sulfonationreaction conditions such as temperature, reaction time and con-centration of reactants, etc. Here, we take the method of varyingintensity of sulfonating agent (mixture of fuming sulfuric acidand sulfuric acid), and the results were shown in Fig. 2. Theintensity of sulfonating agent was denoted by equivalent con-centration of H 2SO4(CH2SO4 ), which can be calculated by thefollow relationship:

CH2SO4 =V f f (1 + 18C f / 80) + V s sC s

V f f + V s s

where V f and V s areis thevolume of fumingsulfuricacidandsul-furic acid; f and s are the densities of fuming sulfuric acid andsulfuric acid,respectively; C f istheSO 3 concentrationof fumingsulfuric acid (here C f = 21.5%) and C s is the H2SO4 concentra-tion of sulfuric acid (here C s = 96.5%). When CH2SO4 100%,the sulfonating agent belongs to sulfuric acid category, while

when CH2SO4 > 100%, that is a kind of fuming sulfuric acid.


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Fig. 1. Sulfonation reaction of PPESK (DS: 100%).

Through the purication procedure of the precipitating inether andreprecipitating in acetone the residual sulfuric acid canbe removed mostly from the SPPESKs. From Fig. 2, it is clearthat the DS of SPPESK increases with the intensity of sulfonat-ing agent initially slowly and then rapidly, nally, it levels off.The DS of SPPESKincreases slowlyowing to lower intensity of sulfonating agent. It will increase rapidly when the intensity islarge enough. When DS> 100%, the existence of SO 3H on thepolymer will hinder the further sulfonation,making DS increaseslightly. Anyway we can obtain the SPPESK with wished DS(here 0132%) from this method conveniently.

3.2. Characteristics of SPPESK

The FTIR study and 1H NMR analysis were used to conrmthe SPPESK has been synthesized successfully.

Fig. 2. DS of SPPESK along with different sulfonating agent intensity

(PPESK/sulfonating agent= 1 g/10ml, 60

C, 2h).

There are someobviousdifferences between theFTIRspectraof theSPPESK andthatofPPESK.The FTIR spectra ofSPPESKwith higher DS (e.g. DS= 132%) have larger absorption at 1084and 1026 cm 1 which attributes to the asymmetric and sym-metric stretching vibration of O S O in the aromatic SO 3H,respectively. The absorption at 617 cm 1 can be related to the C(aromatic)S. At 3443cm 1 the abroad and strong absorptionattributes to the OH in the SO 3H. All the above absorptionsincrease with DS and are not found in the spectrum of PPESK.All conrm thesulfonic acid group exists on theSPPESK. How-ever, there are a lot of similarities among them (PPESK andSPPESK). Both PPESK and SPPESK appear sulfone asymmet-ric and symmetric stretching vibration absorption of O S Oin main chain at 13041312cm 1 and at 11501165cm 1 , aro-matic C O C absorptionat12381258 cm 1, and C N absorp-tion at 15851589cm 1, respectively. There is a little differenceabout above absorptions among the PPESK and SPPESK withdifferent DS. The possible reason of that is the introduction of SO3H on the polymer make the exact wave numbers of thoseabsorptions increase slightly.

1H NMR was employed in order to characterize SPPESKsand PPESK. However, the spectra of them were complex anddifcult to assigndueto therandomconnection andnatureof therepeat units [22]. Thespectrum ofPPESKcanbroadlybedividedinto three groups: 8.728.50, 8.157.60, and 7.207.00. In thedowneld 8.728.50 can be assigned to H 8 and in the upeld7.207.00 to H 1 , H4, H14 , and H15 (the numbered H atoms wereshown in Fig.1 .). Thechemicalshift 8.157.60canbe assignedotherHs.A higherchemicalshiftupeld wouldindicatethatpro-tons on this site would be most susceptible to sulfonation sincethe sulfonation of PPESK is an electrophilic substitution reac-

tion. H1, H

4, H

14, and H

15being upeld should attribute to the


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Fig. 3. TGA curves of PPESK and SPPESKs.

activated function by ether linkage, which makes the aromaticring electron rich. So the H 1 /H4 /H14 /H15 is the right site wheresulfonation is most likely to occur. In the spectrum of SPPESK

140%, thegroupof chemical shift at upeld( 7.207.00)dimin-ishes compared with thespectrum of PPESK (relative integralof 7.207.00 is 4.01, 3.41, and 2.66 of DS= 0%, 66%, and 140%,

respectively). The other characteristic of SPPESK spectrum isa series of downeld signals appear at the range: 8.408.20but not appear in PPESK spectrum. The reasonable explanationis the presence of SO 3H group perturbs the chemical shifts of downeld. Both upeld and downeld changes in those spec-tra indicate that the sulfonation substitution reaction has beenoccurred.

3.3. Thermal analysis of SPPESK

3.3.1. TGAThe thermal stabilities of SPPESKs and PPESK are studied

by TGA analysis, as shown in Fig. 3 and the rst derivativeof TGA data on temperature (i.e. DTG curves) were shown inFig. 4 . There is only one weight loss step for the PPESK curve at487 C, attributing to the degradation of the polymer chain. Thecurve of SPPESKs has obvious three weight loss steps. The ini-tial weight loss around 200230 C attributes to residual solvent(DMAc) release from the SPPESK (DMAc was largely usedin purication procedure). However the release temperature ismuch higher than the boiling point of DMAc (166 C). The pos-sible reasonof that includestwo aspects: ononehand,the heating

rate of TGA isnt low enough. On the other hand, the interac-tion is likely to be formed between the DMAc and SPPESKs.

Fig. 4. DTG curves of PPESK and SPPESKs.

The second weight loss ( T >300 C) was assigned to the splitof SO3H groups (corresponding temperature dened as T d1).It is obviously that this weight loss was enhanced with increas-

ing DS from DTG curves in Fig. 4. The introduction of sulfonicgroups weakens dramatically the thermal stability of polymerdue to collapse of sulfonic groups. The last weight loss step(T >480 C) was assigned to the degradation of main polymerchain (corresponding temperature dened as T d2). All the ther-mal decomposed temperatures ( T ds) of PPESK and SPPESKswere shown in Table 1 . Interestingly, both the decomposed tem-peratures ( T d2: 488482 C) and the corresponding max lossrate temperature ( T d2-max : 513523 C) of polymer main chainof SPPESKs are slightly higher than those of PPESK ( T d2 :478 C; T d2-max : 505 C), respectively. This indicates the mainchain of SPPESK is slightly higher thermal stability than that of PPESK, and then it is speculated that the main chain structure of SPPESKs (after SO 3H groups were split off) are not identicalwith that of PPESK. The exact difference of that is not clear sofar. Though the decomposed temperature ( T d1) of SPPESK ismuch lower than that of PPESK, it is high enough to apply thismaterial in the PEM eld.

3.3.2. DSC Glass transition temperatures ( T g onset here) were deter-

mined by DSC curves. Fig. 5 shows the DSC thermograms of PPESK and SPPESKs as a function of DS. From Fig. 5, the heatabsorption of glass transition of PPESK is not very big. Theincreased heat absorption is observed depending on DS. It is

mainly due to the split off SO 3H groups from SPPESKs, whichcan be proved that the maximum heat absorption temperature

Table 1T d1 , T d2 , T d1-max , T d2-max , T g , and T DSC-max of PPESK and SPPESKs

Sample TGA DTG DSC

T d1 ( C) lossonset

T d2 ( C) lossonset

T d1-max ( C)max loss rate

T d2-max ( C)max loss rate

T g ( C) onset T DSC-max ( C) maxheat absorption

PPESK 478 505 275 SPPESK 28% 300 488 320 513 287 322SPPESK 66% 300 484 321 519 287 321SPPESK 91% 303 483 325 520 295 323SPPESK 132% 305 482 327 523 301 330


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Fig. 8. Swelling rate of SPPESKs depending on temperature.

Fig. 9. Conductivity of SPPESKs and Naon115 at 30 C.

of membranes is the function of DS. As expected, increas-ing the DS increases the conductivity of SPPESK membranes.For example, SPPESK membranes with DS of 66%, 81%and 91% exhibit conductivity of 4.2, 6.7 and 7.7 10 3 S/cm,respectively. However, those conductivities are less than that

Fig. 10. Conductivity of SPPESKs and Naon115 depending on temperature.

Fig. 11. Methanol permeability of SPPESKs and Naon115.

of Naon115 (1.56 10 2 S/cm) at the same condition at30 C.

Fig. 10 shows the temperature dependence of proton conduc-tivity for SPPESK and Naon115 membranes. Similar to theNaon115 membrane, the conductivity of the SPPESK mem-branes increases with temperature. The conductivity of mem-brane with DS 91% (3.0 10 2 S/cm) is almost the same withthat of Naon115 (3.1 10 2 S/cm) at 80 C. Unfortunately themembrane with DS 91% cant sustain too much time at highertemperature due to too much swelling. While the membranewith DS 81% exhibits 1.3 10 2 S/cm at 80 C. The apparentactivation energies of the conductivity E a of membranes withDS 81% and DS 91% are 20.5 and 22.2 kJ/mol, respectively.However, the activation energies of both DS 81% and DS 91%are much higher than that (10.8 kJ/mol) of Naon115. It is pos-sibly because of weak acidity of SPPESK and different microstructure in SPPESK membranes. The ionization of SO 3H inSPPESKmay be affected largely by temperature. When temper-ature increases, the protonation will be enhanced and then makeconductivity increase largely. However, the exact reason is notclear so far.

3.6. Methanol permeability

SPPESK membranes exhibit methanol permeability at 15 C1.3 10 79.7 10 8 cm2 /s depending on DS from 0 to 99%,as shown in Fig. 11. All the values are reduced by a factor of 3242 for that of Naon115 membrane (4.2 10 6 cm2 /s) at thesame conditions. Particularly, the membranes with DS 70% andDS 81% present the methanol permeability of 1.1 10 7 and9.8 1 8 cm2 /s, respectively.This indicates that there are differ-ent micro structures between Naon and SPPESK membranes.Compared with Naon, SPPESK membrane has possibly muchless continuous on hydrophilic domains (due to weak acidity of SPPESKand limited wateruptakeat lowertemperature(15 C)),and more exuose transfer alleyway (due to less enough ex-ibility of the polymer backbone, since the SPPESK polymerbackbonesare composed of too much phenyls and heterocyclicswhich make thebackbone tough and rigidity). Hence these char-

acteristics reduce thehydrodynamic solvent transport (water and


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methanol), which may help reduce theproblems associated withwater andmethanol crossover fordirectmethanol fuel cells [33].This property of SPPESK gives a very promising application of PEM in DMFC, since they have excellent methanol resistanceand not much lower conductivity. Very interestingly, methanolpermeability of SPPESK decreases slightly depending on DSat 15 C. With increased DS of SPPESK, there are two factorsaffecting the micro structure: one is to improve the continuity of hydrophilic domains of SPPESKby increasing water uptake, theother is to make transfer alleyway more exuose by increasingembranchmentof SPPESKchain due to increasedSO 3H groupon that. The former factor will enhance the methanol transferthrough SPPESK, but the latter will weaken that. Both factorsaffect the permeability of methanol through SPPESK. At lowertemperature,effect by thelatter maybe slightly stronger than thatof the former since water uptake of SPPESK is limited at lowertemperature, which possibly explains the decrease of methanolpermeability of SPPESK depending on DS.

4. Conclusions

The SPPESK were synthesized by sulfonation using fum-ing sulfuric and sulfuric acid. The sulfonation level was eas-ily controlled by varying intensity of sulfonating agent. TheseSPPESKs are inexpensive and easier to process than Naon andDow membrane. SPPESKs (DS = 0132%) have high thermalproperty as T d , 300305 C and T g , 275301 C. The mem-branes with DS 81% have appropriate water uptake ( 78%)and swelling rate ( 37%) even at 95 C. At even higher DS,SPPESK membranes water uptake rapidly increases with tem-

perature, and then loss the solid morphology due to too muchswelling at 50 C, while thoroughly dissolved when temper-ature 65 C. As expected, increasing the DS increases theconductivity of SPPESK membranes. For example, SPPESK81% and SPPESK 91% membranes exhibit conductivity of 6.7 10 3 and 7.7 10 3 S/cm, respectively. However, thoseconductivity are less than that of Naon115 (1.56 10 2 S/cm)at the same condition at 30 C. Similar to the Naon115 mem-brane, the conductivity of the SPPESK membranes increaseswith temperature. The membrane with DS 91% exhibits con-ductivity of 3.0 10 2 S/cm, which is very close to thatof Naon115 (3.1 10 2 S/cm) at 80 C. The apparent acti-vation energies E a of both SPPESK 81% and SPPESK91% (20.5 and 22.2kJ/mol) are much higher than that of Naon115 (10.8 kJ/mol). It is possibly because of weak acidityof SPPESK and different micro structure in SPPESK mem-branes. SPPESK membranes exhibit much lower methanol per-meability at 15 C 1.3 10 79.7 10 8 cm2 /s depending onDS from 0 to 99%. All the values are reduced by a factorof 3242 for that of Naon115 membrane (4.2 10 6 cm2 /s)at the same conditions. Especially, the membranes with DS70% and DS 81% present the methanol permeability of 1.1 10 7 and 9.8 10 8 cm2 /s, respectively. The SPPESKmembranes with DS 6080% look promising for use in pro-ton exchange membrane, especially in direct methanol fuel

cells.

Acknowledgements

The authors thank in earnest Prof. Gongquan Sun for provid-ing the opportunity of conductivity and methanol permeabilitymeasurements. They also acknowledge the support of the Sci-entic Research Foundation for the Returned Overseas ChineseScholars (State Education Ministry of China), and the supportof National Natural Science Foundation of China (Grant no.50273005).

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