sulfonated poly(ether ether ketone)/ethylene glycol/polyhedral oligosilsesquioxane hybrid membranes...

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Sulfonated poly(ether ether ketone)/ethylene glycol/polyhedral oligosilsesquioxane hybrid membranes for fuelcell applications

Deeksha Gupta, Veena Choudhary*

Centre for Polymer Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

a r t i c l e i n f o

Article history:

Received 8 October 2011

Received in revised form

21 December 2011

Accepted 28 December 2011

Available online 24 January 2012

Keywords:

SPEEK

TSP POSS

Proton conductivity

Water uptake

* Corresponding author. Tel.: þ91 011 265914E-mail addresses: [email protected]

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.12.141

a b s t r a c t

The paper describes the preparation of hybrid membranes using a trisilanol phenyl poly-

hedral oligosilsesquioxane (TSP POSS) as filler, ethylene glycol (EG) as cross-linker and

sulfonated poly(ether ether ketone) [SPEEK] (DS w65%) as polymer matrix.

The performance of membranes has been evaluated in terms of thermal stability

[by thermogravimetric analysis (TGA), differential scanning calorimetery (DSC)], water

uptake, proton conductivity and X-ray diffraction (XRD) analysis. Morphology of the

membranes has been investigated using scanning electron microscopy with energy

dispersive X-ray (SEM-EDX) and atomic force microscopy (AFM). The addition of EG has

increased the proton conductivity and dimensional stability of SPEEK matrix whereas

incorporation of a small amount of TSP POSS to SPEEK/EG membranes has resulted in the

decrease in water uptake and proton conductivity of the hybrid membranes.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction standard. Owing to the drawbacks associated with Nafion,

Protonexchangemembrane fuel cells (PEMFCs) are theobvious

choice in the development of highly efficient and environ-

mental friendly energy generation technologies, since they can

be used in transport applications (buses, trucks, cars),

stationary applications (home energy supply, decentralized

power stations) and in mobile applications (cellular phones,

laptops). Protonexchangemembrane (PEM) is a key component

of PEMFC and must possess several characteristics, including

high proton conductivity, low electrical conductivity, low

permeability to fuel, good mechanical and thermal properties,

hydrolytic, oxidative stability, capability to be fabricated into

membrane electrode assembly and low cost [1,2].

Up till now, perfluorosulfonic acid membranes, in partic-

ular, Nafion has been a material of choice and technology

23; fax þ91 011 26591421m (D. Gupta), veenach@h2012, Hydrogen Energy P

engineering thermoplastics such as poly(ether ether ketone)

[PEEK], polysulfone [PSF], polybenzimidazole (PBI) has

emerged as promising alternative to the state of art per-

fluoroionomer membranes [3]. Chemically modified and fully

aromatic thermoplastic polymers have received significant

attention because they appear to meet the operating

requirements for the fuel cell applications. These polymers

can be converted to sulfonated polymers by sulfonation

process and proton conductivity can be enhanced by addition

of sulfonic acid groups [4,5].

SPEEK is a reasonably good choice for high temperature

PEMFC. Apart from being more economical as compared to

Nafion, it possesses adequate mechanical properties, thermal

properties, and high chemical resistance [6e9]. SPEEK shows

less pronounced hydrophobic/hydrophilic phase separation

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Structure of TSP POSS

[Formula weight: 931.34, Specific gravity: 1.05 -1.1, R: Phenyl group]

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 5 9 7 9e5 9 9 15980

compared to Nafion which leads to narrower, less connected

hydrophilic channels and larger separations between

sulfonic acid groups [10]. At high water content, water

permeation and electro-osmotic drag are maintained, and

thus high proton conductivity is achieved [11]. As the degree

of sulfonation (DS) of SPEEK increases, the water uptake

enhances resulting in high proton conductivity at the cost

of mechanical strength and dimensional stability of

membranes. Cross-linking of SPEEK might be an effective

approach to have optimum balance between proton

conductivity and dimensional stability of the membranes.

A variety of cross-linking agents and specific conditions of

their use are reported in the literature [12e17]. Cross-linking

of SPEEK using polyols is reported by Mikhailenko et al.

[12,13]. They have observed that a solvent such as dimethyl

acetamide (DMAc), dimethyl formamide (DMF), N-methyl

pyrrolidene (NMP) inhibits the thermal cross-linking of

SPEEK. Further, they have reported that the polyols (ethylene

glycol and glycerol) initiate the formation of alcohol ether

oligomers which are bonded to eSO3 functions in SPEEK and

form an interpenetrating network (IPN) which interacts with

polymer main chain.

Among the several strategies explored to develop

improved and inexpensive PEMs, the dispersion of hygro-

scopic inorganic fillers such as SiO2 [18], TiO2 [19,20] and ZrO2

[21,22] into a polymer matrix may help in assisting water

retention and improve mechanical strength of the polymer.

The incorporation of nanosized filler into polymer is expected

to alter the organiceinorganic phase interfacial characteris-

tics due to its large specific surface area, influencing original

properties of neat polymer like proton conductivity,

mechanical and thermal stability. The improvement in

hydration properties is a critical issue in the development of

high performance PEMs [23e28].

Polyhedral oligosilsesquioxane (POSS) could be regarded as

a hybrid nanoparticle since it has a well-defined cube-octa-

meric siloxane skeleton (about 1e3 nm in size) with eight

organic vertex groups, one or more of which are reactive or

polymerizable. These particular structural features render

POSS to be a versatile additive for acquiring enhanced thermo-

mechanical properties, better thermal stability, oxidative

resistance and abrasion resistance. Various polymer/POSS

hybrid nanocomposites are reported in the literature with

improved thermal and mechanical properties [29e41].

There are number of factors which govern the behavior of

POSS into polymermatrix. For example, the size of POSS cage,

nature of organic periphery, number and type of reactive

functional groups, amount of POSS incorporated and the

type of polymer matrix used. Owing to these factors,

different types of POSS can behave differently in different

polymer matrix. POSS could be present either as isolated and

uniformly dispersed molecules or unreacted and phase

separated particles, or matrix-bound aggregates. Further, the

size and distribution of filler particles into polymer matrix

affect the organiceinorganic phase interfacial characteristics

to a large extent thereby influencing the properties of

nanocomposites [42].

Thompson et al. [43] have synthesized three different types

of POSS nanofillers functionalized with proton-conducting

sulfonic acid groups, mixed sulfonic acid and alkyl groups

and phosphonic acid groups. The POSS-sulfonated poly(-

phenylsulfone) [POSS-S-PPSU] composite membranes have

exhibited proton conductivity comparable to Nafion in

combination with superior dimensional stability, heat resis-

tance and mechanical strength. When compared with control

S-PPSU membranes, the composite POSS-S-PPSU membranes

have exhibited superior conductivity, comparable dimen-

sional stability and slightly decreased mechanical strength.

The phenyl groups that are attached to the silanol groups

increase the acidity of silsesquioxane.

To the best of our knowledge, no other reports are avail-

able on the incorporation of TSP POSS into XSPEEK/EG.

Though, the cross-linking of SPEEK using EG is reported by

Mikhailenko et al. [12,13] but the novelty of our work is that

we have systematically studied the influence of TSP POSS

content on cross-linked SPEEK (DS w65). We speculated that

the presence of three hydroxyl groups in TSP POSS might

influence the microphase separation in cross-linked SPEEK,

thereby, affecting the microstructure and proton conduc-

tivity of cross-linked SPEEK which could be an interesting

study. In this work, we have prepared the membranes based

on SPEEK, EG and TSP POSS, where EG acts as cross-linker

and TSP POSS as nanofiller. The sulfonic acid groups in

SPEEK can react with the hydroxyl group in EG, reducing the

swelling of SPEEK (DS w65%) and TSP POSS can help in

retaining water at higher temperatures. Further, the influ-

ence of TSP POSS content on the properties of cross-linked

SPEEK/EG membranes is investigated with respect to its

thermal properties, morphology, water uptake and proton

conductivity.

2. Experimental

2.1. Materials

Victrex PEEK (150 XF ICI, USA), sulfuric acid (98% Merck),

dimethyl acetamide (DMAc, Qualigens, India), ethanol (BDH,

India), ethylene glycol (EG) (CDH, India), trisilanol phenyl

polyhedral oligomeric silsesquioxane (TSP POSS,Hybrid Plas-

tics, USA) were used as received without further purification.

The structure of TSP POSS (C42H38O12Si7) is given below.



i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 5 9 7 9e5 9 9 1 5981

2.2. Sulfonation of PEEK

SPEEK was prepared by electrophilic substitution of PEEK

using 98% conc. H2SO4 at 50 �C for 2e3 h (5 g of PEEK in 95 ml

conc. H2SO4). The detailed procedure of sulfonation and

determination of DS are reported in our previous work [9].

2.3. Membrane preparation

Table 1 depicts the details of membrane preparation along

with their designation. Polymer solutions (5 wt %) were

prepared by dissolving SPEEK (DS w65%) into water:ethanol

(50:50) mixture. EG was weighed in the calculated amounts

and added to the polymer solution. TSP POSS in varying

amounts (1e3 wt %) wereweighed separately and dispersed in

the water:ethanol (50:50) mixture. All the three components

(SPEEK, EG and TSP POSS) were mixed together and stirred

vigorously at room temperature followed by ultrasonication

for 30 min to prepare the homogenous solution. The

membranes were prepared by casting homogenized solution

mixture in the petri dish followed by solvent evaporation at

80 �C for 12 h. The membranes thus obtained were kept in

a vacuum oven at different temperatures for definite time

period, i.e. at 60 �C for 2 h, 80 �C for 2 h, 100 �C for 2 h, 120 �Cfor 2 h and 135 �C for 16 h. Neat SPEEK membranes were tried

to prepare by using water:ethanol (50:50) mixture but could

not be investigated because of their fragile nature. Therefore,

the neat SPEEK membranes were prepared, using DMAc as

solvent. The membranes are designated as XSP/EG/TSP POSS

Y, where, X denotes to those membranes which are obtained

after heat treatment, SP refers to SPEEK, EG to ethylene glycol

and TSP POSS is trisilanol phenyl POSS and Y denotes thewt %

of TSP POSS in the membrane samples.

2.4. Characterization methods

2.4.1. Structural characterizationThermo Nicolet IR 200 FTIR spectrophotometer was used to

record FTIR spectra of SP/EG membranes (before and after

heat treatment) in the scanning range of 500e4000 cm�1.

2.4.2. Thermal characterizationPyris 6 TGA, Perkin Elmer was used to record thermogravi-

metric (TG) and derivative thermogravimetric (DTG) traces of

the membrane samples in nitrogen atmosphere. Sample

weight of 6� 2 mg and a heating rate of 20 �C/min were used

Table 1 e Details of membrane preparation.

Sample designation SPEEK (g) EG (g) TSP POSS (g)

SP 0.15 0 0

XSP/EG 0.15 0.15 0

XSP/EG/TSP POSS-1.0 0.15 0.15 0.0015

XSP/EG/TSP POSS-1.5 0.15 0.15 0.0023

XSP/EG/TSP POSS-2.0 0.15 0.15 0.0030

XSP/EG/TSP POSS-2.5 0.15 0.15 0.0038

XSP/EG/TSP POSS-3.0 0.15 0.15 0.0045

to record TG/DTG traces in the temperature range of

50e850 �C.Modulated DSC studies were carried out using TA DSC Q

200 instrument in the temperature range of 40e300 �C in

nitrogen atmosphere and a sample size of 5� 2 mg were used

in each experiment. Reversing and non-reversing transitions

were separated and the reversing transitions were used to

determine Tg.

2.4.3. X-ray diffraction analysisX-ray diffraction analysis of SPEEK (neat), TSP POSS (neat) and

SPEEK/EG/TSP POSS composite membrane was carried out

using a Panalytical X’pert PRO diffractometer (Philips X’pert

PRO) with CuKa radiation source. The XRD patterns were

obtained for 2q varying between 2� and 35�.

2.4.4. Morphological characterizationThe surface morphology of composite membranes containing

TSP POSS was observed using ZEISS EVO-50 scanning electron

microscope (SEM). The samples were coated with gold before

scanning. Elemental profiles of the composite membranes

were recorded using Bruker-AXS (model Quan Tan) energy

dispersive X-ray system (EDX).

Atomic force microscopy was performed using a Nano-

scope IIIA Veeco Metrology group in tapping mode. For

tappingmode silicon nitride tip was used and themembranes

were freshly prepared. A force constant of w50 N/mwas used

at a scan rate of 1 Hz.

2.4.5. Water uptakeThe prepared membranes were evaluated for their hydro-

lytic stability by immersing them in water for 24 h at

different temperatures, i.e. at 30 �C, 80 �C and 100 �C. To

minimize the experimental error, the water uptake experi-

ments were carried out in triplicate. The procedure and

formula used to calculate water uptake are reported in our

previous work [9].

2.4.6. Proton conductivityThrough-plane proton conductivity of the membranes was

measured using impedance spectroscopy and details of

procedure are given in our previous paper [9]. The measure-

mentwas done at 100 �C by varying the relative humidity from

40 to 100%.

3. Results and discussion

3.1. FTIR and schematic structure of membrane

Fig. 1(AeC) depicts the FTIR spectra of neat SPEEK, neat EG and

membranes before and after scheduled heat treatment. (A)

Neat SPEEK shows a broad band 3450 cm�1 arising due to the

eOH vibration present in eSO3H groups. (B) An inter-

molecular hydrogen bonded broad band of eOH groups at

3370 cm�1, characteristic asymmetric and symmetric

stretching vibrations of alkyl (R-CH2) group of EG at 2945 cm�1

and 2875 cm�1 was observed for neat EG. A broad peak due to

hydrogen bonded eOH groups at 3258 cm�1 and phenyl (CeH)

band at 3073 cm�1 has been observed for neat TSP POSS (FTIR



Wavenumber (cm )

1200160020002400280032003600

Tran

sm

ittan

ce (%

)

Neat SPEEK

3450

Wavenumber (cm )

2600280030003200340036003800

% T

ran

sm

ittan

ce

EG neat

2945

2875

3370

Wavenumber (cm-1

)

2600280030003200340036003800

% T

ran

sm

ittan

ce

80

85

90

95

100

(a) SP/EG

(b) XSP/EG

(c) XSP/EG/TSP POSS-2.0

3419

3368

(a)

(b)

(c)

2943

2872

A B

C

Fig. 1 e FTIR spectra of (A) neat SP, (B) neat EG, and (C) membranes before and after heat treatment.


spectrum of neat TSP POSS is not shown here). (C) (a) SP/EG

membrane before heat treatment shows a very broad peak

centered at 3368 cm�1 (due to eOH groups), two more peak

corresponding to (R-CH2) group of EG at 2943 cm�1 and

2872 cm�1. After heat treatment, (b) XSP/EG shows that the

peak corresponding to eOH groups has shifted to 3419 cm�1

indicating the reduction of free hydroxyl groups of EG and

suggests the possibility of self-condensation of diol or reaction

between hydroxyl group of EG and sulfonic acid group of

SPEEK. It is observed by Mikhailenko et al. [13] that EG mole-

cule initially attached to sulfonic acid function will preferably

react with another EG molecule or its derivative, forming EG

dimers, trimers or polyfunctional moieties. This is because,

the linking of EG with a eSO3 groups provoke electron deple-

tion in the CH2 next to hydroxyl in the molecule, generating

more reactive species than free EG. They have found the

formation of polyaddition products most probably terminated

by an eOH group.

Fig. 1(C) (c), XSP/EG/TSP POSS-2.0 membrane (i.e. TSP POSS

containing membrane after heat treatment) shows that the

peak corresponding to eOH groups has almost disappeared

indicating further reduction in number of eOH groups.

Hydroxyl group of TSP POSS can react with either eOH group

of ethylene glycol or eSO3H group of SPEEK. But the possibility

of TSP POSS self-condensation or rearrangement is very low

because it has been reported in literature [44] that TSP POSS

can undergo such kind of reactions only above 230 �C.The schematic of membrane is drawn in Fig. 2 comprising

various reaction possibilities as indicated by FTIR analysis.

The scheme shows the reaction between eOH groups of EG

and eSO3H groups of SPEEK, self-condensation of EG, linking

of EG oligomers to sulfonic acid groups, acid catalyzed poly-

merization of EG and TSP POSS and some free TSP POSS

molecules. Solubility test was performed after heat treatment

of membranes. Heat treated membranes were kept in

water:ethanol (50:50) mixture for 24 h and observed for

change in weight. No dissolution or disintegration was

observed even after 24 h of solvent immersion. On the other

hand, neat SPEEK dissolved completely in the water:ethanol

(50:50) mixture. The change in solubility after heat treatment

indicates the formation of cross-linking network in the

membrane.



Fig. 2 e Schematic structure of membrane after scheduled heat treatment.


3.2. Thermal characterization

3.2.1. Thermogravimetric analysisTables 2 and 3 depict TG/DTG traces of SPEEK, XSP/EG and

XSP/EG having varying amounts of TSP POSS. Thermal

stability of the samples is compared by observing the peak

temperature from the DTG traces. A three step degradation

was observed in all the samples, and the relative thermal

stability was evaluated by comparing the mass loss in

different temperature ranges, i.e. below 200 �C (due to the loss

of physically and chemically bound water), between 200 and

450 �C (due to decomposition of sulfonic acid groups), between

450 and 800 �C (due to main chain degradation of polymers)

and char yield at 800 �C.All the membranes have shown theminormass loss below

200 �C, i.e. in the range of 0.6e3.0%. It is observed that mass

loss below 200 �C increased upon incorporation of TSP POSS,

because TSP POSS has three eOH groups on its surface,

Table 2 e Results of TG/DTG traces of SP, XSP/EG and XSP/EG/T(heating rate 20 �C/min).

Sample designation Mass loss (%) in the te

Below 200 �C 200e450

SP 0.6 16.7

XSP/EG 0.7 20.2

XSP/EG/TSP POSS-1.0 0.8 20.1





TSP POSS 1.6 3.9

causing more water absorption by membrane. Now, if we

compare among composite membranes having varying

amounts of TSP POSS, it was found that mass loss has

increased aswe go from 1 to 2wt%TSP POSS content and then

have decreased marginally in case of samples having higher

amounts of TSP POSS (>2.0 wt %), which suggest that

maximum hydrophilicity is achieved by membrane having 2

wt% TSP POSS. The reason for increased hydrophilicity can be

related to its morphology, which will be discussed in next

section.

Between 200 and 450 �C, the mass loss is mainly due to the

loss ofeSO3H groups. In case of XSP/EG and XSP/EG/TSP POSS-

1.0, the addition of EG has increased the mass loss compared

to neat SPEEK. We can assume that EG oligomers might

escape from membrane in this temperature range, thereby,

increasing themass loss. XSP/EG/TSP POSS (1.5e3.0 wt%) have

shown the lesser mass moss as compared to XSP/EG because

the presence of TSP POSSmight hinder the path of volatiles or

SP POSS membrane samples in nitrogen atmosphere

mperature range % Char yield at 800 �C

�C 450e800 �C

35.5 47.2

36.5 42.5

36.4 42.8

36.1 44.3

35.7 43.8

36.3 44.7

37.3 45.9

22.3 72.2



Table 3 e Results of derivative thermogravimetric traces for SP, XSP/EG and XSP/EG/TSP POSS membranes.

Sample designation Sulfonic acid related peaksT (�C) aTemperature range (200e450 �C)

Main chain related peakT (�C) btemperature range (500e600 �C)

Main peak Secondary peak Main peak

SP 381 e 552

XSP/EG 291 e 558

XSP/EG/TSP POSS-1.0 289 e 558

XSP/EG/TSP POSS-1.5 290 329 559

XSP/EG/TSP POSS-2.0 290 321 554

XSP/EG/TSP POSS-2.5 285 322 557

XSP/EG/TSP POSS-3.0 283 339 546

a Refers to the temperature range corresponding to the mass loss due to SO3H.

b Refers to the temperature range corresponding to the mass loss due to main chain degradation.


can trap them in their cage but this fact is not true for XSP/EG/

TSP POSS-1.0 because the TSP POSS loading might not be

sufficient to exert its effect on volatiles.

Between 450 and 800 �C, the mass loss associated with

main chain has increasedmarginally with incorporation of EG

and TSP POSS into the polymer matrix. The neat SPEEK

membrane has shown higher char yield than cross-linked

composite membrane because the loss of oligomeric diols or

aliphatic linkages in earlier steps accounts for lesser char

yield. However, among the composite membranes, char yield

has increased with increasing amount of TSP POSS because

volatiles are trapped by TSP POSS so they cannot escape from

the polymer matrix. The temperature for the onset of major

mass loss was higher than 250 �C in all themembranes, which

clearly shows that the membranes can find application in the

field of fuel cells.

It can be interpreted from Fig. 3 and Table 4 that the

process of thermal degradation has grown complex after

addition of EG and TSP POSS. The derivative thermogravi-

metric (DTG) traces of neat SPEEK shows mainly two peaks: at

381 �C arising mainly due to loss of eSO3H groups and at

552 �C is associated with the backbone degradation. XSP/EG

shows a broad peak at 291 �C, indicating the loss of EG oligo-

mers along with sulfonic acid groups. The main peak related

Temperature (o

C)

100 200 300 400 500 600 700 800

DT

G/d

T

(a) SP

(b) XSP/EG


(d) XSP/EG/TSP POSS-1.5

(e) XSP/EG/TSP POSS-2.0

(f) XSP/EG/TSP POSS-2.5

(g) XSP/EG/TSP POSS-3.0

f

g

e

d

c

b

a

(b)

Fig. 3 e DTG traces of SP, XSP/EG and XSP/EG/TSP POSS

membranes.

to sulfonic acid group has decreased from 290 �C to 283 �Cas TSP POSS loading increases. Incorporation of TSP POSS

(2e3 wt %) has generated a pronounced secondary peak

related to sulfonic acid group in the range of 329e339 �Cindicating the participation of TSP POSS in cross-linking

reaction which in turn alter the degradation pathways.

3.2.2. Modulated differential scanning calorimeteryDSC experiment was carried out to investigate the influence of

TSP POSS on the thermal transition behavior of membranes.

DSC scans of dry membranes were recorded using modulated

DSC as we could not detect Tg due to overlapping transitions

like, dehydration or decomposition in normal DSC. In modu-

lated DSC, reversing and non-reversing transitions are sepa-

rated from each other and Fig. 4(a) shows the reversing

transitions only. In all the samples, an endothermic shift in

base line is seen and Tg is noted as the midpoint inflexion.

Fig. 4(b) shows the variation of Tg with respect to TSP POSS

content.

The Tg of SPEEK is 142 �C (which is not shown here in the

figure), increased significantly, i.e. by 17 �C after cross-linking

[i.e. in case of XSP/EG] which could be due to the restricted

mobility of polymer chain segments in the presence of EG

oligomers. Incorporation of TSP POSS has profound influence

on Tg of SPEEK and was found to increase with increasing

amount of TSP POSS. This behavior can be understood with

the help of FTIR analysis which shows that the involvement of

eOH groups of TSP with either eOH groups EG or with eSO3H

group of SPEEK to generate polyol network. The formation of

crosslink structure has influenced the Tg after incorporation of

TSP POSS. There could be two more supporting facts in

understanding the increase of Tg: firstly, the incorporation of

inorganic particles into polymer matrix might have imposed

the restriction on segmental motion of polymer chains and

secondly, with increasing amount of TSP POSS into the poly-

mer matrix, the number of hydroxylic groups also increases,

which might enhance the extent of hydrogen bonding among

sulfonic acid groups and hydroxyl groups present in the

membrane.

Knauth et al. [45] has also reported an increase in Tg due

to H-bonding in sulfonated and silylated, cross-linked

poly(ether ether ketone) (designated as SOSi-PEEK/N, where

N represents the molar percentage of silylated monomeric

units).



Table 4 e Proton conductivity of SP, XSP/EG and XSP/EG/TSP POSS membranes at 100 �C, at varying relative humidity.

Sample designation Proton conductivity (mS/cm) at 100 �C with varying relative humidity (%)

40% 50% 60% 70% 80% 90% 100%

SP 0.23 0.65 1.44 2.99 8.00 27.73 60.88

XSP/EG 0.21 0.97 2.94 5.63 11.62 38.35 95.30

XSP/EG/TSP POSS-1.0 0.07 0.64 2.18 4.62 9.00 26.84 58.87

XSP/EG/TSP POSS-1.5 0.25 0.90 2.32 5.43 9.97 23.20 79.05

XSP/EG/TSP POSS-2.0 0.91 2.00 3.22 5.51 10.34 23.60 88.43

XSP/EG/TSP POSS-2.5 0.15 1.21 3.21 6.36 11.10 22.99 72.12

XSP/EG/TSP POSS-3.0 0.40 1.20 2.73 4.81 9.85 22.46 54.89

Nafion-112 10.36 22.65 30.56 41.70 59.39 78.16 139.65


3.3. XRD analysis

The XRD patterns of TSP POSS, neat SPEEK and XSP/EG/TSP

POSS composite membranes are shown in Fig. 5. As a highly

crystalline material, TSP POSS has shown the diffraction

peaks, and no peak is recorded for neat SPEEK in XRD. After

incorporation of TSP POSS into the SPEEK/EG matrix, a broad

peak appears between 15 and 30� which suggests the pres-

ence of both components in the system. It might happen that

Temperature (o

C)

50 100 150 200 250 300

(a) XSP/EG

(b) XSP/EG/TSP POSS-1.5


(d) XSP/EG/TSP POSS-2.5

(e) XSP/EG/TSP POSS-3.0

(a)

(b)

(c)

(d)

(e)

TSP POSS (wt %)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Tg (o

C)

140

160

180

200

220

240

260

a

b

Fig. 4 e (a) Modulated DSC scans (reversing transition) of

XSP/EG and XSP/EG/TSP POSS composite membranes.

(b) The effect of TSP POSS content on glass transition

temperature of composite membranes (from modulated

DSC scans).

SPEEK has disturbed the crystallinity of TSP POSS and further

has restricted the crystallization of TSP POSS molecules. The

reduction in crystallinity generally refers to good miscibility

between two components. The broad peak corresponds to

the amorphous nature of composite membranes and the

presence of one Tg in DSC results further support the XRD

results. If membranes are amorphous, polymer chains

can move with ease, which accounts for faster proton

exchange resulting in higher proton conductivity of

composite membrane. Morgan et al. [46] prepared composite

membranes based on polystyrene (PS) and octaisobutyl (OIB)

POSS/trisilanol phenyl (TSP) POSS. OIB POSS and TSP POSS

are crystalline in nature. In PS/OIB POSS composite

membranes, diffraction peaks corresponding to both, i.e. PS

and OIB POSS were observed while in case of PS/TSP POSS

composite membranes, no crystalline peaks corresponding to

TSP POSS were seen.

3.4. Morphological characterization

3.4.1. Atomic force microscopyThe topographic images of pristine SPEEK and cross-linked

composite membranes were recorded to study the surface

structure and compatibility between the filler and matrix and

are presented in Fig. 6. It is evident by the figure that neat

SPEEK shows comparatively smoother surface than composite

2θ5 10 15 20 25 30 35 40

In

te

ns

ity

SP

XSP/EG/TSP POSS-1.0

XSP/EG/TSP POSS-2.0

XSP/EG/TSP POSS-2.5

XSP/EG/TSP POSS-3.0

TSP POSS Neat

Fig. 5 e XRD patterns for SPEEK, composite membranes

and neat TSP POSS.



Fig. 6 e AFM topographic images of (a) neat SPEEK (b) XSP/EG/TSP POSS-2.0 (c) XSP/EG/TSP POSS-3.0 (two dimensions),

(d) neat SPEEK, (e) XSP/EG/TSP POSS-2.0 and (f) XSP/EG/TSP POSS-3.0 (three dimensions).


membranes which means that surface morphology of the

membranes has changed after incorporation of TSP POSS.

Qualitatively, we observe from AFM topographic images

that surface roughness of composite membranes is higher

than neat SPEEK membrane and it is shown by scale bar,

indicating the height contrast between hydrophilic and

hydrophobic phases [47]. The maximum height variation in z

direction was found for XSP/EG/TSP POSS-2.0 which is

w100 nm. In case of XSP/EG/TSP POSS-3.0, it is observed that

the size of aggregates does not match with the height

mentioned in the topographic image. We presume that

aggregates might be embedded into polymer matrix and in

that situation; the size of aggregatesmight notmatchwith the

height shown in the scale bar of topographic image. If aggre-

gates are totally present on surface (in other words, no part of

aggregate is embedded into polymermatrix) then it is possible




to find correlation between size of aggregates and height

shown in scale bar. As we see only the surface structure of

membrane by AFM topographic images.

3.4.2. SEMeEDX analysisSEMeEDX pictures of the neat SPEEK and its composite

membranes are shown in Fig. 7. SEM images depict the

distribution of filler and EDX pictures show the amount of

silica incorporated into the membranes. To have a good

interface between the filler and the polymer matrix, it is

desirable that the inorganic additive is uniformly distributed

so that the two components behave in a synergistic manner.

In Fig. 7(a), the SPEEK shows smooth surface and no filler

particles are present. In case of XSP/EG/TSP POSS-1.0 and XSP/

EG/TSP POSS-2.0, i.e. Fig. 7(b) and (c), a good distribution of TSP

POSS particles is seen and in case of XSP/EG/TSP POSS-3.0

(Fig. 7(d)), agglomeration of TSP POSS particles is observed.

The reason for agglomeration is well understood as we know

that smaller the size of particle higher would be the surface

area and greater would be cohesive force so in the physical

mixing of the nanoparticles (particularly at higher particle

concentrations, in present study it is 3 wt % TSP POSS) with

polymermatrix, it is very difficult to have smaller particle size.

So, in case of XSP/EG/TSP POSS-3.0, cohesive force of particles

could be very high as compared to those samples having TSP

POSS �2 wt %.

The quantitative analysis of TSP POSS incorporation into

the SPEEK matrix was performed by EDX. The EDX pictures

confirm the presence of various elements in the membranes.

In Fig. 7(a), neat SPEEK has no silica and also seen by EDX

picture as no silica peak is present in EDX profile. The peak

intensity corresponding to silica content increases with

increasing amount of TSP POSS taken in the initial mixture

thus confirms different silica content in various compositions.

3.5. Water uptake and hot water test

Water uptake is one of the most important factors to be

considered before a membrane to be used for fuel cell appli-

cations. The amount of absorbed water influences various

properties like proton conductivity, mechanical properties

and dimensional stability [48]. So, the optimum amount of

water is required within the membrane to have balance

among various properties. The proton conduction requires

water-assisted pathways hence water acts as a carrier for

proton transport. It is believed that proton can travel along

with hydrogen bonded ionic channels and proton conduc-

tivity is dependent on the connectivity of the hydrated

domains [49]. Therefore, it is necessary for the membrane to

absorb the sufficient amount of water. At the same time,

higher water content has adverse effect on mechanical and

dimensional stability of the membranes. Therefore, it is

necessary to estimate the water uptake and dimensional

stability of the membranes.

Fig. 8 shows the water uptake of membranes at three

different temperatures (30 �C, 80 �C and 100 �C) and with

respect to varying TSP POSS content. It is observed that the

water uptake of all the membranes has increased with

temperature. As temperature rises, polymer chains and water

molecules acquire more thermal energy and move faster

hence, polymer chains can arrange themselves in different

fashions, generating large extent of free volume, which

accounts for the absorption of more water molecules. The

membranes show different water uptake behavior at low

(30 �C) and high temperatures (80 �C and 100 �C). At low

temperature (30 �C), XSP/EG membrane has absorbed lesser

water than composite membranes while, at higher tempera-

tures, XSP/EG membrane has absorbed more water than

composite membranes. The difference in water uptake

behavior at different temperatures can be attributed to the

temperature dependent behavior of EG oligomers present in

the membrane. At high temperature, the mobility of EG olig-

omers might have generated enough free volume to accom-

modate more water molecules, whereas in composite

membranes, the presence of TSP POSS particles has hindered

the entry of water molecules inside membrane. At low

temperature, the hydrophilicity of TSP POSS might be

a driving factor to cause higher water absorption in composite

membranes.

The effect of TSP POSS content on water uptake is also

shown in Fig. 8. It is the well-known fact that a nanofiller has

high surface area but this property can only be taken into

account if they are uniformly distributed in the polymer

matrix. Fig. 8 exhibits a trend in water uptake results with

respect to the TSP POSS content, water uptake first increases

from1 to 2wt%of TSP POSS and thendecrease from2 to 3wt%

of TSP POSS loading. TSP POSS has three hydroxyl groups and

has an affinity for water molecules, the reason for an increase

inwater uptake from1 to 2wt%might be the better dispersion

of TSP POSS and hencemore hydroxylic groups are exposed to

water molecules causing more water absorption into the

membrane. A decrease in water uptake was observed in case

of XSP/EG/TSP POSS-2.5 andXSP/EG/TSP POSS-3.0which could

be due to the agglomeration of TSP POSS particles where TSP

POSS is present as an aggregate so hydroxyl groups might not

be available to interact with water molecules.

Hot water stability of the samples was also evaluated by

immersing the XSP/EG/TSP POSS composite membranes and

neat SPEEK membranes (after heat treatment) in water at

100 �C for 24 h. The neat SPEEK membrane disintegrated

completely after 4 h of water immersion whereas XSP/EG/TSP

POSS membranes remained intact even after boiling water

treatment for 24 h. Water uptake experiment has shown that

membranes are stable in hot water for long time therefore;

these membranes can be useful for fuel cell application.

3.6. Proton conductivity

Proton conductivity is considered as the fundamental prop-

erty when evaluating the membrane for fuel cell applications.

There are two principal mechanisms to describe the proton

transport in the hydrated membranes (a) ‘Grotthus mecha-

nism’ or ‘proton hopping’ and (b) vehicular mechanism

[50,51]. In proton hoppingmechanism, during the oxidation of

hydrogen at the anode, the produced protons attach itself to

the water molecule to generate hydronium ion and hops to

another water molecule.

In vehicular mechanism, due to electrochemical differ-

ence, hydrated proton (H3Oþ) diffuses through the aqueous

medium. The hydration level of membrane decides the



Fig. 7 e SEM-EDX images of (a) XSP/EG, (b) XSP/EG/TSP POSS-1.5, (c) XSP/EG/TSP POSS-2.0 and (d) XSP/EG/TSP POSS-3.0.


prevalence of one or the other mechanism operated during

proton transport. The proton transport mechanism in nano-

composite membranes is more complex because it involves

the surface and chemical properties of the inorganic and

organic phases. But, exact mechanism is still not clear for

composite membranes because there are many more

parameters involved, which determine overall transport

phenomena. Recently, Schmidt et al. [52] have proposed

a water channel model which provides a unified view of the

structure of Nafion. The scattering data of unoriented as well



TSP POSS Content (wt %)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Water u

ptake (%

)

10

15

20

25

30

35

30 C

80 C

100 C

Fig. 8 e The effect of TSP POSS content on water uptake at

three different temperatures (30 �C, 80 �C and 100 �C).


as oriented samples or fiberswith their exclusivelymeridonial

intensity for both the ionomer peak and the small upturn is

explained by this model. Many of the outstanding properties

of Nafion, in particular, its high proton conductivity andwater

permeability have been addressed in the water channel

model. The structure of Nafion established by the water

channel model guides not only the molecular dynamics

simulations of proton transport in Nafion, but also the design

of ionic polymers, nanocomposites or supramolecular

assemblies for PEMs with desirable properties like lower cost

or higher operation temperatures than Nafion.

The proton conductivity depends on the DS, pre-treatment

of the membrane, hydration state, ambient relative humidity

and temperature. In this work, we have studied the effect of

RH and TSP POSS content on the proton conductivity of the

membrane at 100 �C. The proton conductivity of all the

membranes has increased with increasing RH as shown in

Table 4 and Fig. 9. The membranes based on SPEEK have

shown lower proton conductivity than Nafion-112 over whole

humidity range. It has been observed that XSP/EG shows

Relative Humidity (%)

40 50 60 70 80 90 100

Pro

to

n co

nd

uctivity (m

S/cm

)

0.1

1

10

100

SP

XSP/EG

XSP/EG/TSP POSS-1.0

XSP/EG/TSP POSS-1.5

XSP/EG/TSP POSS-2.0

XSP/EG/TSP POSS-2.5

XSP/EG/TSP POSS-3.0

Nafion-112

Proton conductivity at 100 C

Fig. 9 e Plot of proton conductivity (on log scale) at 100 �Cvs. relative humidity for membrane samples.

highest proton conductivity after RH 60% onwards out of all

SPEEK based membranes. But at lower RH (40 and 50%), XSP/

EG/TSP POSS-2.0 shows the highest proton conductivity out of

all SPEEK based membranes prepared in this study. We can

correlate this behavior with water uptake results as it was

observed that XSP/EG/TSP POSS-2.0 shows the highest water

uptake at 30 �C and XSP/EG membrane shows the highest

water uptake at higher temperature (80 �C and 100 �C). Protonconductivity is directly related with the amount of water

absorbed in the membranes which depends on the extent of

dispersion of filler into polymer matrix in case of composite

membranes. Proton transport is a water-assisted mechanism

and higher amount of water in the membrane can cause the

formation of more solvated species hence more sites are

generated for proton transport, developing well connected

ionic channels so that proton can travel at faster rate giving

high values of proton conductivity.

Further, out of all the composite membranes, XSP/EG/TSP

POSS-2.0 membrane has shown the highest proton conduc-

tivity (w88 mS/cm) overwhole humidity rangewhichmight be

due to the highest water uptake of XSP/EG/TSP POSS-2.0 at all

measured temperatures (30 �C, 80 �C and 100 �C). The uniform

distribution of nanofiller is one of the key points to tune the

proton conductivity of the membranes. Proton conductivity

increased significantly when TSP POSS was added in SPEEK

(DS¼ 60%) using dimethylacetamide (DMAc) as solvent [53].

Whereas in case of cross-linked SPEEK membranes (using EG

as cross-linker in the present work), the addition of TSP POSS

particles might enhance the network density, hindering the

entry of water molecules in the membrane and thus,

accounting for the reduction in proton conductivity of

composite membranes as compared to XSP/EG.

In the present study, XSP/EG/TSP POSS-2.0, the proton

conductivity is comparable to XSP/EG membrane even at

lower water uptake values. It is because, at 2 wt % TSP POSS,

the filler is evenly distributed, which might have generated

small butwell connectedwater channels, giving higher proton

conductivity as compared to rest of the compositemembranes

prepared in the present study.

4. Conclusion

XSP/EG and XSP/EG/TSP POSS (1e3 wt %) membranes were

successfully prepared by cross-linking of SPEEK in the pres-

ence of EG and by adding varying amount of TSP POSS under

heat treatment. FTIR has indicated the reaction among

hydroxyl group of EG, hydroxyl group TSP POSS and sulfonic

acid group of SPEEKwhichwas further confirmed by solubility

test of membranes in water:ethanol (50:50) mixture. The

thermal studies have shown that all the preparedmembranes

are stable up to 250 �C and cross-linking of SPEEK in presence

of EG and TSP POSS has altered the degradation pathway of

SPEEK. Cross-linked membranes were found to have higher

glass transition temperature than neat SPEEK. TheXRD results

confirm the amorphous nature of cross-linked composite

membranes even after the incorporation of highly crystalline

TSP POSS. XSP/EG/TSP POSS-2.0 sample has shown a uniform

distribution of TSP POSS particles whereas agglomeration is

observed at higher TSP POSS loadings. The morphology of




membranes has influenced the water uptake and conse-

quently to proton conductivity results. The water uptake of

XSP/EG/TSP POSS membranes has decreased as compared to

XSP/EG membrane at higher temperatures (80 �C and 100 �C)therefore, the proton conductivity was the highest for XSP/EG

membrane (w95 mS/cm) at 100 �C and 100% RH. In spite of

lower water uptake as compared to XSP/EG membrane, XSP/

EG/TSP POSS-2.0 has shown the comparable proton conduc-

tivity w88 mS/cm.

XSP/EG/TSP POSS-2.0 membrane is considered the best

membrane out of all SPEEK based membrane prepared in

this study because it has shown high proton conductivity

w88 mS/cm with Tg more than 200 �C and reasonably good

dimensional stability therefore, this membrane can find

application in fuel cell.

Acknowledgments

The authors are thankful to Naval Research Board (NRB)

Ministry of Defence, India, DAAD and IIT Delhi for their

financial support. We are grateful to Dr. Rostislav Vinokur at

DWI, RWTH Aachen, Germany for their kind support for

proton conductivity measurement. Special thanks to Prof.

Martin Moller and Dr. Xiaomin Zhu at DWI, RWTH Aachen,

Germany for giving the permission to work at DWI, RWTH

Aachen, Germany. Dr. Josemon Jacob, CPSE, IIT Delhi for

useful discussions.

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