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Solid State Ionics 176

Proton exchange membranes based on sulfonated

poly(phthalazinone ether ketone)s/aminated polymer blendsB

Yan Gaoa,b, Gilles P. Robertsona, Michael D. Guiver a,*, Xigao Jianb,

Serguei D. Mikhailenkoc, Serge Kaliaguinec

aInstitute for Chemical Process and Environmental Technology, National Research Council, Ottawa, Ontario, Canada K1A 0R6bDepartment of Polymer Science and Materials, Dalian University of Technology, Zhongshan Road 158-42#, Dalian 116012, P.R. China

cChemical Engineering Department, Laval University, Quebec, Canada G1K 7P4

Received 18 March 2004; received in revised form 26 July 2004; accepted 11 August 2004

Abstract

Acid–base blend proton exchange membrane films were prepared from sulfonated poly(phthalazinone ether ketone)s (SPPEK)s, either

polymerized from sulfonated monomer (M-SPPEK) or post-sulfonated (P-SPPEK), and three kinds of aminated polysulfones, namely: UdelRPSf aminated ortho to ether linkage (ortho-O-APSf), PSf phenylmethylene aminated ortho to the sulfone linkage (ortho-S-PMAPSf), or 6F

polysulfone aminated ortho to the sulfone linkage (ortho-S-6F-APSf). Films were prepared by casting mixed N,N-dimethylacetamide

(DMAc) solutions of (SPPEK)s in ammonium or sodium salt form and aminated polysulfones, followed by treatment of the blend membrane

films in aqueous sulfuric acid. The blend membrane films showed slightly lower thermal stabilities than SPPEKs. M-SPPEK-based blend

membrane films showed an obvious decrease in proton conductivity compared with their parent sulfonated polymer, whereas the P-SPPEK-

based blend membrane films showed similar proton conductivities to their parent sulfonated polymer from room temperature to about 80 8C.All blend membrane films showed increased swelling stability.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Sulfonated poly(arylene ether ketone); Proton exchange membrane; Acid-base blend; Sulfonated polyphthalazinone; Aminated polysulfone;

Poly(phthalazinone ether ketone)s; Blend membrane; Aminated polymer; Ionomer; Fuel cell

1. Introduction

Proton exchange membrane fuel cells (PEMFC)s have

gained international attention as candidates for alternative

automotive, mobile and stationary power sources due to

their advantageous characteristics such as scalability, energy

conversion efficiency, low operation temperatures, short

start-up time and environmental safety. The proton

exchange membrane (PEM) is a vital part of the fuel cell,

which performs two basic functions: separator preventing

mixing of the fuel (i.e. hydrogen or methanol) and the

0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.ssi.2004.08.009

B NRCC No. 46476.

* Corresponding author. Tel.: +1 613 9939753; fax: +1 613 9912384.

E-mail address: michael.guiver@nrc-cnrc.gc.ca (M.D. Guiver).

oxidant (i.e. pure oxygen or air), and an electrolyte for

transporting protons from the anode to cathode. For

applicability in fuel cells, PEMs must have not only high

proton conductivity but also high electronic resistivity; low

reactant permeation; mechanical strength under both dry and

humidified circumstances and it must be thermally and

chemically stable under fuel cell operation conditions.

Perfluorinated ionomers, such as NafionR, are the most

broadly used as PEMs today. Due to the high price of

NafionR membranes and other drawbacks such as high

methanol permeation and dehydration above 808C, much

research is now focused on the development of alternatives

to perfluorinated membranes [1–17].

Most alternative PEM materials are based on various

sulfonated derivatives of non-fluorinated aromatic high-

performance polymers, such as PSf and poly(ether ether

(2005) 409–415

Y. Gao et al. / Solid State Ionics 176 (2005) 409–415410

ketone) (PEEK). In order to improve selected PEM proper-

ties such as mechanical strength, water-swelling behavior,

and methanol permeability, acid–base polymer blends have

been investigated [18–24]. Kosmala and Schauer [18]

reported that sulfonated poly(2,6-dimethyl-1,4-phenylene

oxide) with degree of sulfonation (DS) of 0.42 blended with

20 wt.% polybenzimidazole was insoluble in DMAc and

showed polarization curves similar to Nafion 117 in a H2/O2

fuel cell at room temperature. Manea and Mulder [19]

reported that a blend of sulfonated polysulfone having a DS

of 0.75 with 12.5 wt.% polybenzimidazole showed

decreased swelling in methanol–water mixture compared

with Nafion 117 and had a methanol permeability value of

about one order of magnitude lower than Nafion 117 and

two times lower than that of sulfonated polysulfone

homopolymer membrane. Kerres et al. [20–24] investigated

several proton conductive acid–base blend membranes

systems. They prepared acid–base blend membranes mainly

from sulfonated poly(ether ether sulfone) or poly(ether

sulfone) and different kinds of basic polymers, such as

polybenzimidazole, aminated polysulfone, poly(4-vinylpyr-

idine), and polyethylenimine. They observed high decom-

position temperatures, low swelling ratio, reduced methanol

permeability, and a performance similar to Nafion 112 in a

H2/O2 fuel cell.

Poly(phthalazinones) such as poly(phthalazinone ether

ketone) (PPEK), poly(phthalazinone ether sulfone) (PPES)

and the copolymer poly(phthalazinone ether sulfone ketone)

(PPESK) have been recently commercialized in China

(Dalian Polymer New material). These high-performance

polymers have high glass transition temperatures, excellent

thermo-oxidative stability, membrane-forming ability, and

many other good properties [25–30]. Recently, we reported

the preparation of sulfonated PPEK derivatives via methods

of both post-sulfonation reaction of PPEK and direct

polymerization reaction of sulfonated monomers. Both P-

SPPEKs and M-SPPEKs with DS of 1.0–1.2 have high

proton conductivities of z10�2 S cm�1 at room temper-

ature, indicating that they are promising candidates for PEM

materials [31–33]. However, the highly sulfonated SPPEKs,

particularly P-SPPEKs, swelled excessively or even became

water-soluble after about 2 h at elevated temperatures and

lost good mechanical properties.

The present work reports acid–base blend membrane

films based on M-SPPEK and P-SPPEK and three kinds of

aminated PSf with the goal of preparing new thermally and

chemically stable PEM films with improved water-swelling

properties.

2. Experimental

2.1. Materials

4-(4-Hydroxyphenyl)-1(2H)-phthalazinone (DHPZ) was

synthesized from phenolphthalein and PPEK was synthe-

sized from DHPZ and 4,4V-difluorobenzophenone accord-

ing to the procedure described in the Chinese Patents

[25,26]. PPEK was supplied by the Dalian New Polymer

Material- P-SPPEK was prepared from the sulfonation

reaction of PPEK in dilute fuming sulfuric acid at room

temperature [31]. M-SPPEK was synthesized by direct

polymerization of DHPZ, 4,4V-difluorobenzophenone and

disodium 3,3V-disulfonate-4,4V-difluorobenzophenone in

N-methyl-2-pyrrolidone at 160–170 8C [32]. The metal-

ation route was used to prepare ortho-O-APSf [34], ortho-

S-PMAPSf [35] and ortho-S-6F-APSf [36]. The acidic and

basic polymers investigated are listed in Fig. 1. All other

chemicals obtained commercially were reagent grade and

used as received.

2.2. Thermal analysis

A TA Instruments thermogravimetric analyser (TGA)

instrument model 2950 was used for measuring the

degradation temperatures (Td). Polymer samples for TGA

analysis were preheated to 120 8C at 10 8C/min under

nitrogen atmosphere and held isothermally for 30 min for

moisture removal. Samples were then heated from 80 to 750

8C at 10 8C/min for Td measurement.

2.3. Preparation of membrane films

P-SPPEK was converted into the ammonium salt form by

reaction with aqueous NH4OH at room temperature over 24

h, followed by evaporation of the aqueous phase at 80 8C in

vacuum oven for 2 days.

DMAc solutions (5%) of M-SPPEK in the sodium form or

P-SPPEK in the ammonium form mixed with 10 mol% of

aminated PSf were filtered and cast onto a glass plate and the

solvent was slowly evaporated at 40 8C for about 2 days.

Residual solvent was evaporated at 80 8C under vacuum for 1

day, resulting in pale yellow membrane films. The resulting

membrane films were immersed in 2 N aqueous H2SO4 at

room temperature for 1 day, washed several times with

deionized water and immersed in deionized water for 1 day.

2.4. Water uptake content measurement and swelling ratio

The sample films were soaked in deionized water for

24 h at determined temperatures. Weights of dry and wet

membranes were measured. The water uptake content was

calculated by

Uptake content ð%Þ ¼ xwet � xdry

xdry

� 100%

where xdry and xwet are the masses of dried and wet

samples, respectively. The swelling ratio was calculated

from films 5–10 cm long by

Swelling ratio ð%Þ ¼ lwet � ldry

ldry� 100%

Fig. 1. Investigated polymers.

Y. Gao et al. / Solid State Ionics 176 (2005) 409–415 411

where ldry and lwet are the lengths of dry and wet samples,

respectively.

2.5. Proton conductivity

The proton conductivity measurements were performed

by AC impedance spectroscopy over a frequency range of

1–107 Hz with oscillating voltage 50–500 mV, using a

system based on a Solatron 1260 gain phase analyzer. A 13-

mm2 disc sample was placed in an open, temperature

controlled cell, where it was clamped between two blocking

stainless steel electrodes with a permanent pressure of ~3

kg/cm2. Specimens were soaked in deionized water for 24 to

48 h prior to the test. The conductivity (r) of the samples in

the transverse direction was calculated from the impedance

data, using the relation r=d/RS where d and S are the

thickness and face area of the sample respectively and R

was derived from the low intersect of the high frequency

semi-circle on a complex impedance plane with the Re (Z)

axis.

3. Results and discussion

All the acidic and basic polymers used in this work were

investigated previously by our group. P-SPPEK was

obtained by the sulfonation reaction of PPEK in fuming

sulfuric acid and the 1H-NMR analysis showed that the DS

value was 1.2 [31]. M-SPPEK was obtained by copoly-

merization reaction of sulfonated and non-sulfonated

monomers with a feed ratio to give a DS of 1.2 (actual

DS of 1.12 determined by 1H-NMR) [32]. ortho-O-APSf,

ortho-S-6F-APSf, and ortho-S-PMAPSf had a degree of

amination of 2.0, 1.3 and 2.0, respectively. P-SPPEK was

converted into its ammonium salt form before blending and

M-SPPEK was used directly as the polymerization product

in the sodium salt form. The blend membranes, composed of

aminated polymers containing 0.01 mol of amine groups

with sulfonated polymers containing 0.1 mol sulfonic acid

groups, were prepared by casting the DMAc solutions of the

polymer mixtures evaporating into dry films, followed by

immersion in 2 N aqueous H2SO4 for conversion into the

Y. Gao et al. / Solid State Ionics 176 (2005) 409–415412

acid form. Fig. 2 shows the formulation of the blend

membrane film under study and their TGA curves. Among

these eight tested membrane films, only Blend-1 and Blend-

2 were somewhat opalescent, while SPPEKs as well as other

blend membrane films were completely transparent. FT-IR

has previously been used to investigate ionic interactions

between the components in blend membranes [18–24].

However, FT-IR studies of the blend materials in the current

study show no obvious differences between SPPEK and

blend membrane films. The high 10:1 feed ratio of sulfonic

acid in SPPEKs to amine groups in aminated polymers

could be the reason why absorption bands of aminated

polymers were masked by the absorption bands of uncon-

verted SPPEKs.

3.1. Thermal stability

Fig. 2 displays the TGA curves of all blend membranes

compared with those of their individual component basic

and acidic polymers. All the blend membranes had high

thermal stabilities and had an onset degradation temperature

ranging from 287 to 305 8C and 5% weight loss temper-

Fig. 2. TGA traces of blen

atures from 320 to 336 8C, respectively. Both starting acidic

polymers had similar thermal stabilities. M-SPPEK had an

onset degradation temperature of 304 8C and a 5% weight

loss temperature of 339 8C whereas P-SPPEK had an onset

degradation temperature of 307 8C and a 5% weight loss

temperature of 346 8C. The three types of aminated

polymers showed a large difference in their thermal

stabilities. Ortho-S-6F-APSf showed the highest onset

degradation and 5% weight loss temperatures of 397 and

376 8C, respectively, ortho-O-APSf showed an onset

degradation and 5% weight loss temperatures of 363 and

360 8C, respectively, and ortho-S-PMAPSf had the lowest

onset degradation and 5% weight loss temperatures of 283

and 299 8C, respectively. The thermal stability data indicate

that the polymer with amine groups attached ortho to the

ether linkage is less thermally stable than the one attached

ortho to the sulfone linkage, but more thermally stable than

aliphatic amine. Although the stabilities of the aminated

polymers were quite different, the thermal stabilities of the

blend membranes were not affected to a great extent by their

components. In general, the blend membrane films had

slightly decreased temperatures of degradation onset and 5%

d membrane films.

Y. Gao et al. / Solid State Ionics 176 (2005) 409–415 413

weight loss in comparison with their parent acidic polymers.

The M-SPPEK-based membrane films had slightly lower

degradation temperatures. Since the expected temperature

limit of PEMs is below 150 8C, all the resulting membranes

should be regarded as sufficiently thermally stable for PEM

application.

3.2. Water uptake and swelling ratio

Adequate hydration of membranes is critical to fuel cell

application. Water assists the transportation of protons from

the anode to the cathode. If the electrolyte membrane is

insufficiently hydrated, its conductivity falls. On the other

hand, a water excess results in electrode flooding and

morphological instability of the membrane. Both P-SPPEK

and M-SPPEK are highly hydrophilic. P-SPPEK and M-

SPPEK absorb a moderate amount of water and do not swell

excessively at room temperature. However, at 80 8C, P-SPPEK is almost completely dissolved in water and M-

SPPEK shows water absorption of 2300% and dimensional

swelling of 150%. In contrast, all aminated polymers are

only moderately hydrophilic and absorb little water. The

water uptake and swelling ratio of resulting blend mem-

brane films are listed in Table 1. The results show that the

blending of aminated polymers with M-SPPEK caused

almost no change in water uptake and swelling ratio at room

temperature. Blend membrane films based on P-SPPEK

even show slightly increased swelling at room temperature.

However, the ionic interactions have an obvious effect on

the water uptake and swelling ratios of membrane films at

80 8C. Although the blends were composed of only 10% of

aminated polymers, water uptakes of M-SPPEK blend

membranes decreased four to five times, and swelling ratios

also decreased substantially. The three aminated polymers

did not display big differences in their influence on water

uptake and swelling ratio of M-SPPEK-based blend

membrane films. However, P-SPPEK-based blend films

displayed big differences in water uptake and swelling ratios

depending on which aminated polymer was added. Pure P-

SPPEK film was almost completely dissolved in water at 80

8C after about 2 h. Blend-5, composed of P-SPPEK and

ortho-S-6F-APSf, showed decreased water uptake. How-

ever, it still swelled significantly and lost its mechanical

Table 1

Water uptake and swelling ratio

Room temperature

Water uptake (%) Swelli

M-SPPEK 60 20

Blend-1 M-SPPEK/ortho-O-APSf 49 19

Blend-2 M-SPPEK/ortho-S-6F-APSf 62 19

Blend-3 M-SPPEK/ortho-S-PMAPSf 61 24

P-SPPEK 28 6.8

Blend-4 P-SPPEK/ortho-O-APSf 46 16

Blend-5 P-SPPEK/ortho-S-6F-APSf 53 17

Blend-6 P-SPPEK/ortho-S-PMAPSf 35 13

strength in 80 8C water. Blend-4, composed of P-SPPEK

and ortho-O-APSf, showed moderately decreased water

uptake and swelling ratio values compared with P-SPPEK.

Blend-6, composed of P-SPPEK and ortho-S-PMAPSf,

displayed significantly decreased water uptake and swelling

ratio in comparison with parent P-SPPEK. The effects of

aminated polymers on the decrease in swelling of P-SPPEK-

based blend membrane films are consistent with the basicity

of these aminated polymers. Aliphatic ortho-S-PMAPSf

with the strongest basicity showed the strongest effect on

decrease in water uptake and swelling ratio, moderately

basic aromatic ortho-O-APSf showed a moderate effect on

membrane swelling and the weakest base ortho-S-6F-APSf

showed the weakest effect on decreasing of the water uptake

and swelling ratio.

3.3. Proton conductivity

Proton conductivities of M-SPPEK-based and P-SPPEK-

based blend membrane films as a function of temperature

are displayed in Fig. 3. For M-SPPEK-based blend

membrane films, it is obvious that the proton conductivities

decreased significantly compared with their parent polymer,

just like their water uptake and swelling ratio values. The

decrease in proton conductivity can be explained by the

decrease in available sulfonic acid groups for transferring

protons due to ionic cross-linking between amine groups in

basic polymers and sulfonic acid groups in M-SPPEK.

However, the room temperature proton conductivities of all

three blend membrane films are still higher than 10�2 S/cm,

which makes them interesting for use as PEMs in fuel cells.

The proton conductivities of Blend-1, Blend-2 and Blend-3

increase with temperature up to different values that are all

lower than that of M-SPPEK. At about 95 8C, proton

conductivities of Blend-1, Blend-2 and Blend-3 membrane

films begin to decrease, presumably due to dehydration of

membrane films at elevated temperatures. The decreases in

proton conductivities of Blend-1, Blend-2 and Blend-3

membrane films at elevated temperatures compared to M-

SPPEK, have some relationship with the basicity of the

aminated polymers. As discussed before, the aliphatic

ortho-S-PMAPSf has the strongest basicity, and the

corresponding Blend-3 membrane film showed the most

80 8C

ng ratio (%) Water uptake (%) Swelling ratio (%)

2300 150

461 99

564 101

594 115

Almost completely dissolved.

676 110

Swelled and lost mechanical strength.

170 38

Fig. 3. Proton conductivities.

Y. Gao et al. / Solid State Ionics 176 (2005) 409–415414

obvious decrease in proton conductivity at elevated temper-

atures; moderate basic aromatic ortho-O-APSf showed a

moderate effect on decrease in proton conductivity; and the

weakest base ortho-S-6F-APSf showed the weakest effect

on decrease in the proton conductivity. For P-SPPEK-based

blend films, Fig. 3 shows that the conductivities of Blend-4,

Blend-5 and Blend-6 display different dependences on the

basic polymers compared with M-SPPEK blend films.

Unlike M-SPPEK blend films, P-SPPEK blend films show

similar proton conductivities to their parent acidic P-SPPEK

from room temperature to about 80 8C, and thereafter show

differences. Blend-5 and Blend-6 show earlier decrease in

proton conductivities than P-SPPEK. Blend-4 shows a rapid

increase in proton conductivities compared with P-SPPEK

from about 70 to 80 8C, and then the proton conductivity

decreases as sharply as P-SPPEK.

The differences in proton conductivities of M-SPPEK-

and P-SPPEK-based blend membrane films could be

explained by the differences in acidity of the parent acidic

polymers. The sulfonic acid groups in M-SPPEK polymer

are located on the benzophenone unit, and are activated by

the electron withdrawing ketone, resulting in a stronger

acidity. In contrast, P-SPPEK has the sulfonic acid groups

located on the ortho positions of electron donating ether

linkage, and is therefore of a weaker acidity. Since M-

SPPEK has stronger acidity compared with P-SPPEK, it

shows a higher conductivity. However, when a portion of

the sulfonic acid groups were consumed by crosslinking

between amine and sulfonic acid groups, the decrease in

proton conductivities of M-SPPEK blend films are also

more obvious than P-SPPEK blend films. It was reported

[37,38] that in sulfonated polymer membrane films, the

hydrophobic backbone and the hydrophilic sulfonic acid

groups nanophase separate into two domains in the presence

of water. The hydrophobic domain provides the polymers

with morphological stability and the hydrophilic domain is

responsible for transporting protons and water. In addition,

the structure and distribution of ionic pathway are related to

the structure of polymer chains. When the basic polymer

was added, the interaction between acidic and basic groups,

which are both restricted to polymer main chains, may result

in some change in the distribution of sulfonic acid groups in

membrane films. Consequently, there may be some changes

the distribution of ionic pathways and the proton conduc-

tivity. Perhaps, this is part of the reason that some

phenomena appeared. For example, higher conductivity

was observed for Blend-4 than for P-SPPEK and the blend

membrane films show highest proton conductivity at

temperatures different from their parent acidic polymers. It

is also likely that steric hindrance also produces some effect

that has not been taken into consideration here.

4. Conclusion

In this paper, acid–base blend membrane films were

prepared from two kinds of acidic polymers M-SPPEKP, P-

SPPEK with three kinds of aminated polymers, ortho-O-

APSf, ortho-S-PMAPSf, and ortho-S-6F-APSf, by casting

mixed DMAc solutions of P-SPPEK in the ammonium or

M-SPPEK in the sodium salt form with aminated polymers,

followed by treatment of the blend membrane films in

aqueous sulfuric acid. The blend membrane films showed

only slightly lower thermal stabilities. M-SPPEK-based

blend membrane films showed obvious decrease in proton

conductivity in comparison with their parent acidic polymer,

whereas P-SPPEK blend films showed proton conductivity

similar to that of their parent acidic P-SPPEK in the

temperature range from room temperature to about 80 8C.All blend membrane films showed increased swelling

stability, especially the blend membrane composed of P-

SPPEK with aliphatic amine polymer ortho-O-APSf.

Acknowledgements

This work was supported by the National Research

Council of Canada. Partial support was also provided by the

National Natural Science Foundation of China (Contract

grant number: 20104001).

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