synthesis, characterization, and proton-conducting properties of organic–inorganic hybrid...

Synthesis, Characterization, and Proton-ConductingProperties of Organic–Inorganic Hybrid MembranesBased on Polysiloxane Zwitterionomer

WUU-JYH LIANG,1 CHIEN-PANG WU,2 CHANG-YU HSU,2 PING-LIN KUO2

1Fire Protection and Safety Research Center, National Cheng Kung University, Tainan, Taiwan 70101 Republic of China

2Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 70101 Republic of China

Received 23 September 2005; accepted 11 March 2006DOI: 10.1002/pola.21455Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The synthesis and characterization of a series of zwitterionic hybrid mem-branes based on a zwitterionic siloxane precursor (ZS) are described. Flexible, trans-parent, optically homogeneous films were prepared. With the further incorporation ofpoly(ethylene glycol) (PEG), the hybrid films became more flexible but translucent.The structure of the inorganic sides was probed with solid-state 29Si NMR spectros-copy, and the organic sides and the chemical process involved were characterizedwith solid-state 13C cross-polarization/magic-angle spinning NMR. A higher contentof ZS led to higher proton conductivity of the hybrid electrolytes. Moreover, the pro-ton conductivity was enhanced by the addition of the plasticizing component of PEGto the hybrid matrix; this was ascribed to the increased water uptake and free vol-ume of the hybrid matrix and the dissociation of sulfonic acid groups. The proton con-ductivity of these hybrid membranes could be increased up to 3.5 � 10�2 S/cm by thetemperature and relative humidity being increased to 85 8C and 95%, respectively.The proton-conduction behavior of these hybrid membranes is also briefly discussed.VVC 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3444–3453, 2006

Keywords: interpenetrating networks (IPN); ionomers; microstructure; organic–inorganic hybrids; proton conduction

INTRODUCTION

Composite materials consisting of both inorganicand organic phases have the potential to lead toa wide range of new technologies because theycan simultaneously combine the best features ofoxides and polymers.1–3 Among the many vari-eties of organic–inorganic materials explored upto now, charged ones have recently received par-ticular attention for such applications as bat-teries, capacitors, sensors, electrochromic win-dows, displays, and low- and medium-tempera-

ture fuel cells4–10 because this kind of hybridmaterial can associate the mechanical and ther-mal resistance of the silica backbone with theflexibility of the organic components. Organic–inorganic composites can be formed with a vari-ety of synthetic approaches. Sol–gel processeshave been used to produce hybrids with mor-phologies ranging from isolated nanoparticles ofone component in a matrix of the other compo-nent to interpenetrating networks of inorganicand polymeric materials.2,11,12

An important class of organic–inorganic mate-rials is represented by 3-glycidoxypropyltrime-thoxysilane (GPTMS)-derived hybrids.13–16 Inthese materials, organic and inorganic units arelinked together via covalent bonds, whereas the

Correspondence to: P.-L. Kuo (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 3444–3453 (2006)VVC 2006 Wiley Periodicals, Inc.

3444

presence in GPTMS of an epoxide, which can bea former of organic polymers, allows the buildupof a polymeric network. GPTMS may react ei-ther with the coreactant via oxirane ring cleav-age leading to a polymeric network or via hydro-lysis of the Si(OR)3 groups leading to speciescontaining the silica fragments. Low-molecular-weight poly(ethylene glycol)s (PEGs) are typi-cally viscous liquids or soft, waxy solids that areknown for their ability to dissolve and conductsmall cations. In this work, a novel series of or-ganic–inorganic hybrid composites derived fromGPTMS and/or PEG has been developed andcharacterized. The incorporation of the otherinorganic species, the zwitterionic siloxane pre-cursor (ZS), into these hybrid materials hasbeen used to control the chemical and physicalproperties and makes it possible to expand theapplication field. These organic–inorganic hybridsare some of the most promising materials becausethey possess both organic and inorganic func-tionalities. Fourier transform infrared (FTIR)and multinuclear solid-state NMR spectroscopyare used to provide information about the micro-scopic structure and molecular behavior, ther-mogravimetric analysis (TGA) is used to charac-terize the thermal stability, differential scanningcalorimetry (DSC) provides evidence for phasetransitions in these hybrid composites, and com-plex impedance measurements are used to de-scribe the proton conductivity. These results arefurther correlated and discussed.

EXPERIMENTAL

Materials

c-Aminopropyltriethoxysilane (APTES) and GPTMSwere obtained from Dow Corning Corp. and used inthe reactions without further purification. c-Pro-panesultone (Fluka) was distilled in vacuo. Tet-rahydrofuran (THF; Tedia) was distilled andkept dry over molecular sieves. PEG (weight-av-erage molecular weight ¼ 400 g/mol; Fluka) wasused as received. All solvents were reagent-grade or were purified by standard methods.

Preparation of ZS

APTES was stirred in THF, and an equivalentamount of a solution of c-propanesultone in THFwas added dropwise in a nitrogen atmosphere.The reaction was exothermic, and the rate ofaddition of c-propanesultone was adjusted so that

the temperature would not exceed 60 8C. After-ward, the solution was heated and stirred at50 8C for 1 h. After the solvent was evaporated,the light-yellow product was dried in vacuo at40 8C. The obtained precursor dissolved inwater, yielding acidic liquors (pH � 2). The syn-thetic route of ZS is shown in Scheme 1.

The IR spectrum displays the characteristic peaksof sulfonate groups17 (920 cm�1), sulfonic acids(hydrated;18 RSO3

�H3Oþ, 1204 and 1163 cm�1),

substituted quaternary ammonium (1615 cm�1

and a broad absorbance in the region of 2900–2600 cm�1), and Si��O��R (1100–1000 cm�1).The peaks at 528 and 604 cm�1 have been as-signed to this type of salt.

1H NMR (400 MHz, D2O, d): 0.8 (��Si ��CH2

��CH2��), 1.2 (��Si��OCH2CH3), 1.8 (��Si��CH2

��CH2��), 2.2 (��NH2þ��CH2CH2CH2 ��SO3

�),2.8–3.2 (��CH2��NH2

þ��CH2��), 3.4 (��NH2þ

��CH2CH2CH2��SO3�), 3.6 (��Si��OCH2 CH3).

13C NMR (400 MHz, D2O, d): 9.2 (��Si��CH2��CH2��), 17.3 (��Si��OCH2CH3), 17.7 (��NH2

þ��CH2CH2CH2��SO3

�), 24.8 (��Si ��CH2��CH2��),42.2 (��NH2

þ��CH2CH2CH2 ��SO3�), 46.6 (��NH2

þ

��CH2CH2CH2��SO3�), 48.5 (��Si��CH2��CH2

��NH2þ��CH2��), 57.9 (��Si��OCH2CH3).

Preparation of the Organic–InorganicZwitterionic Membranes

The obtained ZS and GPTMS were mixed in adesired weight ratio (ZS/GPTMS ¼ 0.2, 1.0, or 2.0)in an appropriate amount of MeOH/H2O (1.0 v/v)and stirred at room temperature for 2 h. Thesehomogeneous solutions were then poured onto analuminum plate, and this was followed by the slowremoval of the solvent at room temperature andcuring at 100 8C for 24 h and at 155 8C for 2 h toform a hybrid charged zwitterionic membrane. Thethickness of these specimens was controlled to bein the range of 150–200 lm. The PEG-doped hybridfilms were also made in the same way with theaddition of 10 wt % PEG, which was based on theweight of the siloxane precursors. The schematicstructure of the organic–inorganic zwitterionichybrids is shown in Scheme 1. The samples arelabeled ZSG-x or ZSG-x–PEG, where x refers to theweight ratio of ZS to GPTMS.

Characterization

FTIR

FTIR spectra were measured with a Nicolet5700 system with a wave-number resolution of

POLYSILOXANE ZWITTERIONOMER 3445

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

2 cm�1, and a minimum of 64 scans were signal-averaged at room temperature. Each samplewas prepared via mixing with a potassiumbromide (KBr) pellet, and the films were vac-uum-dried to reduce the absorbed water in thesamples.

Solid-State NMR Measurements

Solid-state NMR experiments were carried outon a Bruker Avance 400 spectrometer equippedwith a 7-mm double-resonance probe. The Lar-mor frequencies for 1H and 13C nuclei were400.17 and 100.58 MHz, respectively. Magic-angle spinning (MAS) of the samples in therange of 3–5 kHz was employed to obtain NMRspectra. The Hartmann–Hahn condition for1H?13C cross-polarization (CP) experimentswas determined with adamantane, and protondecoupling was applied during acquisition to

enhance the spectra sensitivity. 29Si MAS NMRspectra were acquired at 79.46 MHz with a p/2pulse width of 4.6 ls and a recycle delay of200 s. The p/2 pulse length for 1H was 4 ls. Typ-ically, a repetition time of 5 s was used in allthe NMR experiments. The 29Si and 13C chemi-cal shifts were externally referenced to tetrame-thylsilane at 0.0 ppm.

Thermal Analysis

Before the measurements, the samples wereplaced in vacuo at 80 8C for several days toremove the absorbed water in the samples. Thethermal stability was determined with TGA(TGA 7, PerkinElmer) over a temperature rangeof 100–900 8C at a heating rate of 20 8C/minunder a nitrogen atmosphere. DSC (TA2010,DuPont) measurements were conducted over thetemperature ranges of �120 to 120 8C at a heat-

Scheme 1. Synthetic route and schematic representation for the organic–inorganichybrids based on ZS.

3446 LIANG ET AL.


ing rate of 10 8C/min under dry nitrogen atmo-sphere. The samples, sealed into aluminumpans, were first heated at 120 8C for 10 min toremove the thermal history, cooled to �120 8Cand then scanned. All the thermograms werebaseline-corrected and calibrated against indiummetal. Glass-transition temperatures (Tg’s) werereported as the onset of the curve.

Water-Uptake Measurements

Properly weighted dry membranes were placedinside an oven to reach equilibrium at variousrelative humidities (RHs). The membranes werethen weighed on a microbalance immediately af-ter the surface water was wiped off, and thewater uptake was calculated.

Proton-Conductivity Measurements

Proton-conductivity measurements were con-ducted with polished stainless steel as blockingelectrodes. Before the measurements, the cellswith the membranes were placed inside thehumidifying chamber for at least 3 h to reachthe equilibrium water uptake. Proton conductiv-ity was measured by an alternating-current im-pedance technique with an electrochemical im-pedance analyzer (model 604A, CH Instrument,United States) under an oscillation potential of10 mV from 100 kHz to 10 Hz. The impedancewas measured in the temperature range of 60–95 8C and RH range of 30–95%.

RESULTS AND DISCUSSION

Preparation of the Hybrid Zwitterionic Membranes

In general, c-propanesultone is highly reactivetoward amine compounds.19 In this study, thereaction of APTES and c-propanesultone wasexothermic and ran to completion with no sidereaction. A series of crosslinked zwitterionichybrid networks were prepared via a sol–gelapproach on the basis of ZS and GPTMS withvarious weight ratios, as sketched in Scheme 1.Alkoxysilane groups in ZS and GPTMS canreact through a two-stage mechanism:20 (1) thefirst stage is hydrolysis of alkoxysilanes to formsilanols and (2) the second stage is water- and/or alcohol-producing condensation to form inor-ganic siloxane chains. Meanwhile, epoxidegroups on GPTMS can proceed by self-polymeri-

zation to form pseudo-poly(ethylene oxide) chainsor react with water and methanol to form diolsand methyl ether terminal groups, respec-tively.21,22 In this study, a clear and homogene-ous solution was formed when the silica precur-sors were hydrolyzed and PEG was furtheradded as well. The apparent mechanical strengthof the obtained hybrid zwitterionic films withoutPEG depends on the weight ratio of ZS toGPTMS used. Flexible, transparent, opticallyhomogeneous, and slight yellow films were ob-tained with greater amounts of GPTMS incorpo-rated into the hybrid matrix (ZSG-0.2 and ZSG-1.0), whereas a slightly brittle one was formedfor ZSG-2.0. With the introduction of PEG intothe hybrid matrix, the composite films becamemore flexible but translucent. This resulted fromthe phase separation and was further confirmedby 13C CP–MAS NMR and DSC measurements.

FTIR Studies

Figure 1 illustrates the FTIR spectra of theZSG-x hybrids. All the samples show a charac-teristic band at about 800 cm�1 (characteristicof the symmetric Si��O��Si stretching) and at1039 cm�1 (characteristic of the asymmetricSi��O��Si stretching). Weak peaks associatedwith noncondensed Si��OH groups in the rangeof 960–940 cm�1 are also present. The absorp-tion peak of epoxide at 912 cm�1 disappears,and this indicates the complete reaction of epox-

Figure 1. FTIR spectra of (a) ZSG-0.2, (b) ZSG-1.0,(c) ZSG-2.0, and (d) ZSG-2.0–PEG.



ide on GPTMS. The peaks corresponding to theC��H stretching and bending vibrations appearin the range of 3000–2850 and 1460 cm�1, re-spectively. The broad peak absorption around3400 cm�1 in these hybrids indicates that thereis a significant number of ��OH groups, which aredue to the noncondensed SiOH, the hydroxyl-containing derivatives formed from a ring-open-ing reaction of the epoxide group and/or someamount of the absorbed moisture in the sample.The presence of the absorption bands at 1204,604, and 528 cm�1 indicates the existence of theSO3

� group. Additionally, the contribution of newlyformed C��O��C groups cannot be well identi-fied because of the overlapping of the stretchingvibration of C��O��C and sulfonic acid groupsin the range of 1200–1100 cm�1.

As shown in Figure 1 for the ZSG-x hybrids,the presence of substituted quaternary ammo-nium in the zwitterionic hybrids (1615 cm�1 anda broad absorbance in the region of 2900–2600cm�1) can be obviously observed with anincrease in the ZS content. The gradual shift ofthe OH band toward lower wave numbers (from3416 to 3400 cm�1) as the ZS concentrationincreases is probably due to the increase in theinteractions among the quaternary ammoniumgroups, the sulfonate groups, and the OHgroups through hydrogen bonding. This behav-ior is also responsible for the progressive shift ofthe stretching band at 1200–1100 cm�1 (from1121 cm�1 for ZSG-0.2 to 1153 cm�1 for ZSG-

2.0). Figure 1 also shows a representative FTIRspectrum of the zwitterionic hybrid doped withPEG (ZSG-2.0–PEG). A new absorption peakaround 1113 cm�1 can be observed due to theC��O stretching mode of PEG, in comparisonwith that of ZSG-2.0. Moreover, the peak at1153 cm�1 for ZSG-2.0 is shifted to 1145 cm�1

for ZSG-2.0–PEG, and this indicates the disso-ciation of sulfonic acid groups by the polyoxy-ethylene segments.

13C CP–MAS and 29Si MAS NMR

13C CP–MAS and 29Si MAS NMR spectra wereintroduced to investigate the structure of thehybrids on the organic and inorganic sides andto study the chemical process involved. Figure 2shows the 13C CP–MAS NMR spectra of the pre-pared samples with different ZS contents, to-gether with the assignments of the peaks. Forcomparison, the spectrum of a sample preparedwithout GPTMS is also presented. As shown inFigure 2(a) for the ZS-only sample, one peak atabout 10 ppm is assigned to the methylene car-bons in the a position to the silicon atom. Theoverlapping signal located around 20 ppm isattributed to the carbons in the b position to thesilicon atom and the sulfonate group. The peakat 50 ppm is due to the carbons next to the sul-fonate group, whereas the peak at 70 ppm isassigned to those carbons adjacent to the substi-tuted quaternary ammonium. Upon the addition

Figure 2. (a) 13C CP–MAS NMR spectra of the ZSG-based hybrids and (b) assign-ments of the peaks. The asterisks denote spinning sidebands.

3448 LIANG ET AL.


of GPTMS, a significant increase in the inten-sity of the two peaks at 64 and 59 ppm can beobserved. These peaks are associated with theformation of the diol function (64 ppm) andmethyl ether terminal group (59 ppm).22 Besides,a peak around 72–74 ppm can also be observed,which is characteristic of carbon atoms in oligo-or poly(ethylene oxide) derivatives formed froma ring-opening reaction of the epoxide group onGPTMS. Another notable observation is that anincrease in the relative intensity of the peak at50 ppm can be observed with an increase in theZS concentration.

Additionally, as shown in Figure 2(a), the 13CCP–MAS NMR spectrum of ZSG-2.0–PEG is

similar to what is observed for that of ZSG-2.0.However, a sharp peak at 70 ppm, which origi-nated from polyoxyethylene segments, is exam-ined. This implies that the doped PEG moleculesare not fully interpenetrated into the hybrid ma-trix, and this leads to the formation of phaseseparation.

Solid-state 29Si NMR spectroscopy was employedto provide further information about the levelsof condensation and the structure of the inor-ganic network of the hybrid materials. It hasbeen well established that the chemical moietiesassociated with siloxane subunits can be identi-fied on the basis of their chemical shifts.23 Fig-ure 3 shows the 29Si MAS NMR spectra of theZSG-x and ZSG-2.0–PEG samples. They have acommon feature of two peaks being observed:the main one lying at d ¼ �68 ppm and the minorone at d ¼ �59 ppm. These signals are attrib-uted to homocondensed siloxane fragments in T3

[RSi(OSi)3] and T2 [RSi(OR)(OSi)2] arrange-ments, respectively. Furthermore, the spectra donot show evidence of the peak associated withT0 species, namely uncondensed materials trappedin these hybrids. Figure 3 also shows that theintensity of the T3 resonance is much higherthan that of T2 in all the samples, providing apredominating content of the three-dimensionalsilsesquioxane network on the inorganic side.

The condensation degree of alkoxysilane (per-centage of siloxane bonds formed) has been cal-culated from the signal intensity of the deconvo-luted 29Si NMR spectra according to the follow-ing formula: condensation degree (%) ¼ 0.66 [0.5� (T1 area) þ 1.0 � (T2 area) þ 1.5 � (T3

area)].24 Practically, the resultant condensationdegrees of all the samples are similar, about96%, and also reveal that the addition of PEG hasno influence on the condensation of silanol groups.As a result, the zwitterionic content in thehybrids can be further estimated and is listed inTable 1.

Thermal Behavior

The thermal stability of the resultant hybridmembranes is illustrated by TGA. The TGAcurves measured under flowing nitrogen areshown in Figure 4 for the ZS-based hybrid mem-branes. Obviously, these curves exhibit threemain degradation stages arising from the pro-cesses of thermal desolvation, the cleavage ofrelatively unstable groups, and the thermal oxi-dation of the hybrid matrix. The first weight

Figure 3. 29Si MAS NMR spectra of the ZSG-basedhybrids.



loss, less than 5 wt % before 250 8C, is due tothe evaporation of physically absorbed watermolecules in the hybrid matrix. The secondweight loss occurs around 340 8C and is attrib-uted to the loss of ��SO3

�. In the third weight-loss region (600–750 8C), the hybrid residues arefurther degraded, and this corresponds to thedecomposition of the organic fragments on thesehybrids, as previously shown in 13C CP–MASNMR spectra. Moreover, the residual SiO2 con-tent of these hybrids increases with an increasein the GPTMS uptake.

DSC was carried out to investigate the ther-mal transitions of these ZS-based hybrid mem-branes, and the resulting curves are presented

in Figure 5. As evidenced in Figure 5, one sec-ond-order transition temperature (Tg) can beobserved for all the ZSG-x samples in the rangeof analyzed temperatures. Also, this Tg valueincreases with an increase in the ZS concentra-tion (from �12 8C for ZSG-0.2 to 51 8C for ZSG-2.0). This observation could be due to the tight-ness of the hybrid membrane. With a highercontent of ZS in the hybrid, the membrane maybe tighter because of the higher level of cross-linking and the interactions among the chargedgroups within the hybrid matrix.

Moreover, as shown in Figure 5, the thermalevents significantly change with the addition ofPEG to the hybrid matrix. The DSC trace forZSG-2.0–PEG shows two second-order transi-tions at �56 and 40 8C and one first-order tran-sition around 3 8C. These two Tg’s could be asso-ciated with the biphasic character of the hybridnetwork. The lower Tg is predominantly attrib-uted to the motion of the polyoxyethylene seg-ments trapped in the hybrid network, higher

Table 1. Deconvoluted Results of 29Si MAS NMR and Zwitterionic Contentsof the Hybrid Samples

Sample T2 (%)a T3 (%)a cd (%)b

SpecificZwitterionic

Content (mequiv/g)

ZSG-0.2 12.3 87.7 94.9 0.63ZSG-1.0 6.9 93.1 96.7 1.85ZSG-2.0 8.0 92.0 96.4 2.51ZSG-2.0–PEG 7.7 92.3 96.5 2.22

a The estimated accuracy of the measurements is 65%.b Condensation degree of alkoxysilane.

Figure 4. TGA thermograms for the ZS-basedhybrid membranes under a nitrogen atmosphere.

Figure 5. DSC curves for (a) ZSG-0.2, (b) ZSG-1.0,(c) ZSG-2.0, and (d) ZSG-2.0–PEG.

3450 LIANG ET AL.


than that of PEG only (�70 8C). The other onecorresponds to Tg of the hybrid network, abovewhich mainly chain motion takes place, and islower than that observed in the PEG-free sam-ple (ZSG-2.0). Another, the endotherm appearsand is ascribed to the melting temperature ofthe crystalline phase of PEG. These results indi-cate that some of the PEG molecules are inter-penetrated into the hybrid host, but not fully, tosoften the host matrix. Therefore, the incorpora-tion of PEG into the hybrid matrix can lead tothe decrease in the interactions among chargedgroups and the increase in the free volume ofthe host.

Proton-Conductivity Measurements

The proton conductivity of all the resultanthybrid zwitterionic membranes was determinedby means of impedance spectroscopy. The fre-quency dependence of the impedance obtainedat a lower temperature and RH for the sampleswith a lower ZS content shows two well-definedregions: an arc in the high-frequency region thatis attributable to the bulk properties of the sam-ples and a linear spur increasing with decreas-ing frequency in the low-frequency limit that isassociated with the diffusion process of themetal electrode/hybrid zwitterionic membraneinterface. The frequency dependence, approxi-mating a semicircle that indicates an equivalentcircuit corresponding to ion-blocking Pt electro-des, gradually disappears with an increase inthe RH and ZS content. From the complex im-pedance plot (Cole–Cole plot), the proton con-ductivity (r) was calculated from the intercept(R) of the semicircular arc with the Z0 axisunder various conditions: r ¼ d/RA, where d isthe thickness and A is the contact area of thesample membranes.

Figure 6(a) shows the variation of the protonconductivity with the temperature for the ZS-based hybrid membranes at 50% RH. As shownin Figure 6(a), the proton conductivity increaseswith an increasing content of ZS for the ZSG-xhybrid membranes, and the temperature de-pendence is changed from a linear relation to amore curvature-like one. In addition, the appa-rent activation energy for proton conduction,which is obtained from the slope of the plot,increases with an increase in the ZS content.These suggest that the proton-conduction mech-anism changes and the proton transport be-

comes more coupled with the segmental motionsof polymer chains as the content of ZS is in-creased. That is, for the ZSG-2.0 sample, a Grot-thus-type mechanism dominates the ionic con-duction, whereas a Vehicle-type mechanism ap-pears to be the primary route of conduction forthe ZSG-0.2 and ZSG-1.0 samples.25 Addition-ally, as the ZS content increases, the changes inthe free volume, electrostatic interactions, andhydrogen bonding will affect proton conduction.The increase in the proton conductivity with theZS content may be due to the increased numberof the charge carriers and the increased wateruptake in the hybrid, as shown in Figure 6(b).The increases in the charge carrier and wateruptake result from the fact that the zwitterionicsites promote the water dissociation to provideprotons (2H2O $ H3O

þ þ OH�) and from theincrease in the hydrophilicity of the hybrid, re-spectively.

Figure 6. Variations of (a) the proton conductivity(r) and (b) the water uptake with the temperature (T)for the ZS-based hybrid membranes at 50% RH.



Furthermore, as evidenced in Figure 6(a), theproton conductivity is enhanced by the additionof the plasticizing component of PEG in theZSG-2.0 sample, and the improvement is about10-fold. The proton-conduction behavior becomesless coupled with the segmental motions of poly-mer chains as PEG is incorporated. These obser-vations can be rationalized by the fact that thewater uptake and the free volume of the hybridmatrix are increased as well as the dissociationof sulfonic acid, as shown in Figure 6(b) and dis-cussed in DSC and FTIR studies. In addition,the proton conduction would proceed throughthe molecular coordination of proton with ethyl-ene oxide units in the host matrix.26

Figure 7 shows the proton conductivity andwater uptake of the ZSG-2.0–PEG compositemembrane under different RHs at 85 8C. The con-ductivity of this hybrid membrane is increasedfrom 3.9 � 10�4 to 3.5 � 10�2 S/cm as RHincreases from 30 to 95%. This is probably due tothe fact that water acts as the active site of theproton transport via hydrogen bonding. Also, thepresence of water makes the polymeric chain flex-ible, and consequently the carrier ions are easy totransport in the hybrid complex. From this, itsuggests that the absorbed water in the ZSG-2.0–PEG composite contributes to a large increase inthe proton conductivity. Furthermore, in Figure7, a significant increase in the proton conductivitycan be observed with increasing RH in the rangeof 80–95%. Under lower humidity, the water isengaged in strong interactions with ionic compo-nents of the hybrid composite. Hydronium (H3O

þ)

can release a proton to a neighboring water mole-cule, which accepts it while releasing another pro-ton, and consequently, charge transportation oc-curs. At a higher humidity, a water molecule canform a physically absorbed water channel for ionconduction, which is the major medium governingthe proton conductivity.27

To evaluate the stability of the conductivity,time-course measurements were also carried outunder 95% RH at 85 8C (not shown here). Theconductivity rapidly decreases within 10 h downto the order of 10�4 S cm�1. This behavior is asso-ciated with the dissociation of PEG molecules bywater as a result of the weak physical bonds(hydrogen bonding or van der Waals bonds)between the zwitterionic hybrid matrix and PEG.Hence, further studies regarding the preparationof the chemically covalent polyether-charged zwit-terionic siloxane hybrids are now in progress andwill be published in a forthcoming article.

CONCLUSIONS

A series of zwitterionic hybrid membranes basedon ZS and GPTMS were successfully preparedand characterized. These composite membranesshowed good thermal stability up to 250 8C. Withthe addition of PEG, the hybrid films becamemore flexible but translucent as a result of phaseseparation. The FTIR spectra proved that therewere hydrogen-bonding and protonic interactionsbetween the hybrid host and polyoxyethylene seg-ments. PEG played a role in increasing the freevolume and in dissociating the sulfonic acidgroups, which contributed to higher proton con-ductivity. The absorbed water in the ZSG-2.0–PEG composite membrane contributed to a largeincrease in the proton conductivity, which in-creased from 3.9 � 10�4 to 3.5 � 10�2 S/cm withRH increasing from 30 to 95% at 85 8C.

The authors thank the National Science Council (Tai-pei, Taiwan, Republic of China) for its generous finan-cial support of this research.

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Figure 7. Variations in (l) the proton conductivity(r) and (u) the water uptake with RH for the ZSG-2.0–PEG composite membrane at 85 8C.

3452 LIANG ET AL.


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