Preparation and characterization of sulfonated polyethersulfone for cation-exchange membranes

Preparation and Characterization of Sulfonated Polyacrylamide from Polyacrylonitrilefor Proton Conductive Membranes

Azra Touheed and Husnul Maab*Department of Chemistry, Abdul Wali Khan University, Mardan Khyber Pakhtoonkhawa, Pakistan

(Received: Jan. 7, 2012; Accepted: May 28, 2012; Published Online: ??; DOI: 10.1002/jccs.201200009)

Polyacrylonitrile (PAN) was sulfonated and the membranes prepared were then characterized by the

FTIR-ATR, Elemental Analyzer EA, TGA, DSC, SEM, Tensile, Water uptake and Impedance tests. FTIR-

ATR spectra show the substitution of the sulfonic group (SO3) to the main stem of the chain and also the

hydrolysis of the cyanide group to amide group confirm the conversion of polyacrylonitrile to sulfonated

polyacrylamide. Increase in water uptake property as compared to pure PAN also confirms the sulfonation

process has occurred. Thermal properties also confirm the enhancement of the materials after sulfonation

reaction.

Keywords: PAN; SPAAm; Membrane; Fuel cell.

INTRODUCTION

Polyacrylonitrie (PAN) is amongst the versatile poly-

mers to be used in copolymers; fibers, plastics, rubbers and

also as a membrane due to its good solvent resistant prop-

erty. PAN has a wide range of applicability to be used as a

nice substrate for nanofiltration and reverse osmoses (RO).

The thermosetting property of PAN has made it a suitable

precursor for carbon membranes and carbon fibers where

light weight and high strength are the primary require-

ments. PAN is also one of the good precursors for making

nanofibers by the modern electrospinning technology with

a diameter down to hundreds of nanometers with desirable

properties like high strength, abrasion resistance, high sur-

face area and etc making it quite compatible in different ap-

plication fields.1-4 Due to the high polarity and unusual sol-

ubility properties of polyacrylonitrile, the compound added

great potential interest to block graft copolymers system

with others membranes materials and different applications

have been reported in the literature.5,6 All over the word,

the researchers are investigating new types of technologies

that produce power without causing any harm to the envi-

ronment. Fuel cells are the most compromising device for

energy conversion technology. Many research groups all

over the world tend to find new materials for proton ex-

change membranes (also termed as polymer electrolyte

membrane, PEM) to be used in fuel cell for transportation

of proton and also as separator between the two electrodes

(Cathode and Anode). The most common method to mod-

ify membranes materials for proton conduction; is to em-

ploy electrophilic substitution of (SO3) group by sulfona-

tion process. Aromatic polymers can be easily sulfonated

while aliphatic polymers are some what difficult because

during the sulfonation of aliphatic polymer one, two or

three (-SO3H) take part at the same time. Usually concen-

trated sulphuric acid, fuming sulphuric acid, chlorosul-

fonic acid and sulphur trioxide are reported for the sulfona-

tion of polystyrene, polyether ether ketone (PEEK), poly

(arylene ether sulfone), phenylted poly sulfone, poly-

imides, poly (4-phenoxybenzoyl-1,4-phenylene), poly-

benzimidazole (PBI), polyphosphazenes, low density poly-

ethylene (LDPE), high density polyethylene (HDPE),

polypropylene (PP) and etc.7,8,9,10,11 While direct sulfuriza-

tion of PAN and its determination by the thermal analysis

(SC & TGA) were also reported.17 The process of sulfona-

tion may take place at room temperature or at some high

temperature depending on the nature of polymer under

study and also on the degree of sulfonation required.

In this paper the sulfonation of polyacrylonitrile is

discussed and the idea was taken from the sulfonation of

low density polyethylene (LDPE). The main idea was to

sulfonate PAN for proton conductive membrane as quite

new materials and to observe its behaviour during imped-

ance tests. After the sulfonation it was observed through

the FTIR-ATR analysis that PAN has been hydrolysed to

sulfonated polyacrylamide (SPAAm). The acid and base

treatment of PAN has been already investigated12,16 for dif-

ferent desired properties. But in this research the main ob-

jective was to sulfonate the PAN with concentrated sul-

phuric acid and to testify the new materials for proton con-

ductivity.

J. Chin. Chem. Soc. 2012, 59, 000-000 © 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1

JOURNAL OF THE CHINESE

CHEMICAL SOCIETYArticle

* Corresponding author. E-mail: [email protected]

EXPERIMENTAL

Materials

Polyacrylonitrile (PAN; Mw 200,000) 99.5% pur-

chased from Dolan GmbH; H2SO4 97% and DMSO 99.9%

were purchased from Merck and used as received. For the

impedance tests, electrodes and diffusion layers were pur-

chased from E-Tek (BASF).

Sulfonation of polyacrylonitrile (PAN)

Polyacrylonitrile (PAN) was dried at 100 oC under

vacuum overnight and thereafter 3 g of PAN was dissolved

in 300 g of concentrated sulphuric acid (95%-97%) and

vigorously stirred at 100 °C for 48 hours using silicon oil as

bath. Then the solution was cooled down to room tempera-

ture and then precipitated in ice cold water under constant

mechanical agitation. The reddish suspension was allowed

to settle down in few hours. After filtration, the polymer

was washed with distilled water until the pH was neutral.

The precipitated polymer was dried in vacuum oven at 100

°C overnight. An 11% (Degree of sulfonation, DS) re-

quired 48 hour reaction time at 100 °C under strong me-

chanical stirring. The sulfonation degree was determined

by elemental analysis. The sulfonation reaction was carried

out according to the procedure described in refs. 13, 14 and

15.

Preparation of the membranes

For the preparation of membranes Sulfonated poly-

acrylamide SPAAm) (degree of sulfonation DS 11%) was

dissolved in DMSO solvent and mixing was carried out at

temperature 80 oC with constant stirring for 24 hours. After

the mixing step, the solution was filtered and SPAAm films

were prepared by casting the polymer solution on clean

glass plates which were heated at 80 oC for 24 hours, fol-

lowed by an additional 24 hours at 100 oC in vacuum oven

in order to eliminate any rest of solvent. The films were

easily detached from the glass and were immersed in de-

ionised water.

Morphology

The morphology of the pure polyacrylonitrile (PAN)

and sulfonated polyacrylamide (SPAAm) membranes cross-

section were studied by scanning electron microscopy

(SEM) using a LEO 1550 VP field emission microscope.

The samples were prepared by fracturing the films in liquid

nitrogen and coating it by Au/Pd sputtering.

FTIR Spectroscopy and Elemental analysis

FTIR spectra were obtained on a Bruker EQUINOX

55FTIR spectrometer equipped with attenuated total re-

flectance (ATR) accessory. All spectra were acquired at

room temperature from 4000 to 550 cm-1 in N2 atmosphere.

The number of scans taken was 128 with spectral resolu-

tion of 2 cm-1. Degree of sulfonation was determined by the

estimation of Sulfur/carbon ratio by elemental analysis.

The analysis was performed with a Carlo Erba CHNS-O

analyzer Model EA 1110.

Thermal properties

Differential scanning calorimetry (DSC) for mem-

brane samples were characterized in the temperature range

from 25 to 400 °C on a Netzsch DSC 204 calorimeter

equipped with a refrigerated cooling system. Measure-

ments, including baseline determinations were performed

at the scan rate of 10 K/min. The experiments were con-

ducted in a nitrogen purge gas stream, and the glass transi-

tion (Tg) temperature values were obtained from the first

scan thermograms. Thermogravimetric analyses (TGA)

were performed from 25 °C to 700 �C, in an argon stream

with a Netzsch 209 instrument and a heating rate 10 K/min.

Tensile test

Mechanical properties like elastic modulus and yield

strength of the membranes were evaluated in a stress–strain

universal testing machine Zwick-Roell 20 kN, calibrated

according to standard procedures (EN ISO 527-3) and

equipped with a load cell and an integrated digital display

that provided force determination. A load of 0.5 N and a

strain rate of 5 mm/min were used. The dimension (width �

length) of all membrane samples taken were (10 mm � 50

mm) with an approximate thickness around 35 ± 5 µm. The

distance between the gauges was 20 mm.

Water uptake

Water uptake was measured in de-ionised water first

at room temperature and then at 60 oC respectively. Before

the experiments the pure PAN and sulfonated PAAm mem-

branes were dried in a vacuum oven at 120 oC for 24 hours.

4.0 cm � 4.0 cm films were weighed and then immersed in

de-ionised water for 24 hours. The membranes were wiped

by removing the excess of water with tissue paper. The

measurements were repeated three times, the results re-

ported being the average values. The water uptake was cal-

culated according to the following equation 1.

Uptake %mass wet mass dry

mass dry

( ) ( )

( )

�� 100 (1)

where mass (wet) and mass (dry) are the masses of the fully

hydrated and the dry membrane respectively.

Impedance measurement

The proton conductivities of the SPAAm membranes

2 www.jccs.wiley-vch.de © 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim J. Chin. Chem. Soc. 2012, 59, 000-000

Article Touheed and Maab

were measured by impedance spectroscopy by using a

Zahner IM6 Electrochemical workstation. Impedance spec-

tra were scanned in a frequency range from 106 to 10 Hz

and with a.c. signal amplitude of 5 mV. Before the measure-

ment the membranes were conditioned in de-ionised water

for 24 hours at room temperature. Five pieces of mem-

branes (with total thickness around 400 �m) were stacked

in between the two diffusion layers (carbon cloth) in a

through-plane conductivity cell. Measurements were car-

ried out at 100% relatively humidity (RH) and at tempera-

ture varying from 40 °C to 100 °C. The proton conductivity

of the membranes materials were calculated by impedance

values at phase angle zero according to following equation

2.

proton conductivity =Tm

Rm Am*(2)

where Tm denotes the thickness of the membranes, Rm is

for membranes resistance and Am shows the area of the

membranes materials.

RESULTS AND DISCUSSION

Membranes morphology (SEM Images)

Figure 1(a,b) presents some of the SEM pictures of

the cross-section of pure polyacryloniotrile (PAN) at dif-

ferent scanning scale for a closer view of inside morphol-

ogy. The SEM images show large budding structures re-

sulting from fracture of frozen sample prepared in liquid

nitrogen. These structures show the ductile fractured fea-

tures of the pure PAN membranes while the SEM images in

Figure 1 (c,d) show the brittle fractured features of the

sulfonated polyacrylamide (SPAAm) membrane after sul-

fonation; and also indicating that sulfonation has increased

the stiffness of the membranes materials as compared to

pure polyacrylonitrile membrane. After sulfonation the

surface looks smoother due to progressive changes as

compared to pure PAN.

FTIR and EA study

ATR-FTIR spectra for pure polyacrylonitrile (PAN)

and sulfonated polyacrylamide (SPAAm) were obtained

and can be seen in Figure 2. From the peaks it is evident

that significant changes took place after sulfonation of

polyacrylonitrile. Also the intensity of the absorption peaks

after sulfonation dramatically increased. The absorption

peak at 2243 cm-1 that can only be assigned to CN stretch-

ing in pure polyacrylonitrile has been oxidized completely

and now can be determined as amide group (NH2-C=O) by

its characteristics peak at region (1676 cm-1) (stretching)

during the hydrolysis reaction which in turn confirms the

synthesis of polyacrylamide. There are other absorption

peaks that can be related to sulfonic group (O=S=O)

stretching in the region (1197 cm-1) and (1040 cm-1) and it

shows the possibility of substitution of (O=S=O) group to

the main stem during the sulfonation process. While the

peak at (738 cm-1) due to (Sulphur-Carbon) stretching also

confirms the sulfonation process. Also the increase in ab-

sorption intensity at peak region of 3211 cm-1 (OH stretch-

ing) shows the presence of strong hydrogen bonding in the

system after sulfonation of poylacrylonitrile. The degree of

sulfonation obtained by elemental analysis was 11% and

this is in good agreement with FTIR analysis discussed

above. The sulfonation mechanism of polyacrylonitrile

may be expectedly the same in comparasion along with

good agreements with the sulfonation mechanism of low

density polyethylene and poly propylene (PP) carried out

J. Chin. Chem. Soc. 2012, 59, 000-000 © 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.jccs.wiley-vch.de 3


Polyacrylonitrile for Proton Conductive Membranes CHEMICAL SOCIETY

Fig. 1. SEM images of the membranes; pure polyacryl-

oniotrile (a, b) and sulfonated polyacrylamide

(c, d).

Fig. 2. FTIR-ATR spectra of polyacryloniotrile and

sulfonated polyacryloniotrile.

by different groups before.19,20,21-22 The possible reaction

mechanisms for the hydrolysis and sulfonation reactions of

the polyacrylonitrile can be observed in Scheme 1(a, b); but

the formation of sultone mechanism was not described

here.

Thermal properties

The thermal gravimetric-differential thermal analysis

(TGA-DTG) results for pure polyacrylonitrile (PAN) and

sulphonated polyacrylamide (SPAAm) can be seen in the

Figure 3 (a). The exothermic peaks of DTG for pure PAN at

300 °C and 450 °C are due to cyclization and aromatization

of PAN with evolution of acetonitrile, acrylonitrile, benzo-

nitrile, methane, acetylene, ethylene, ethane, propene, pro-

pane, 1, 3 butadiene, ethyl nitrile, vinyl acetonitrile, croto-

nitrile, benzene, pyridine, dicyanobutene, adiponitrile, di-

cyanobenzene, naphthalene, HCN and Ammonia like prod-

ucts leaving behind black orlon. Even at high temperature

than this (450 °C) hydrogen and nitrogen are also lost with

the formation of carbon fibres as the end product.16 The

same Figure 3 (a) also represents the TGA-DTG curves for

the sulfonated PAN. The peaks at 100 °C represent the

mass loss that corresponds to water solvent. But the mass

loss at 300 °C as observed in the TGA-DTG curve for the

SPAAm membranes is inversely proportional to the degree

of sulfonation in this work (DS 11%). That confirms the

desulfonation process occurs at this temperature and in turn

it also confirm that PAN was successfully sulfonated and

converted to SPAAm. The TGA-DTG curve at 400 °C rep-

resents maximum mass loss of SPAAm membrane materi-

als along with simultaneous mass loss of some other prod-

ucts during cyclization and aromatization as mentioned

above. This also confirms the enhanced thermogravimetric

property of the sulfonated PAAm as compared to pure

PAN. These results are some what in good agreements with

the work done before18 where the desulfonation occurred at

wide range from 300 °C – 390 °C.

Figure 3 (b) shows a sharp DSC endothermic peak at

300 °C which is mainly due to the same cyclization and

aromatization of the PAN materials degradation with the

evolution of some of the products discussed above from the

TGA-DTG analysis. But after sulfonation of PAN and its

conversion to sulfonated polyacrylamide, the absence of

crystalline nature of the polymer with no exo or endo

thermic peaks appeared up to 400 °C is prominent because



Scheme I Reaction mechanism for (a) hydrolysis of (�CN) group and (b) sulfonation of polyacryloniotrile

of the enhanced amorphous property of the membrane

materials.

Stress-Strain relationship

The stress-strain behaviour of the pure PAN and sul-

fonated polyacrylamide (SPAAm) can be observed from

the Figure 4. For the pure PAN the stress applied to the

sample is linearly proportional to the strain until it breaks.

However the sulfonated polyacrylamide doesn’t show this

proportionality behaviour confirming that PAN mem-

branes have stronger strain and stress along with more flex-

ible elasticity than the sulfonated polyacrylamide. This

property of the pure PAN and sulfonated polyacrylamide

membranes materials can also be seen from the SEM

pictures in Figure 1.

Water uptake

Swellings of membranes in the relevant solution di-

rectly affect the proton conductivity and mechanical prop-

erties of membranes. Figure 5 represents the percent water

uptake of membranes in de-ionised water at different tem-

peratures 25 °C and 60 °C. Also the results were compared

with the water uptake for pure SPEEK and Nafion 117.

Pure polyacrylonitrile being hydrophobic does not show

any tendency to absorb water at room temperature and also

at high temperature 60 °C. But after sulfonation the water

absorption capacity increased significantly up to a value of

6.6% at room temperature and even more up to 10.3%

when the temperature was raised to (60 °C). Usually hydra-

tion increases with the raising in temperature but it also de-

pends on the type of membranes materials and its hydro-

phobicity. The water uptake by SPEEK and Nafion 117 is

much higher than the target materials because the degree of

sulfonation (DS) of these materials is quite high.

Proton conductivity

The comparisons of proton conductivities for the

sulfonated PAAm with pure SPEEK23 and Nafion 11724

membranes at 100% RH were plotted as a function of the

temperature in Figure 6 (a). The results show that the pro-

ton conductivity of the SPAAm is lower than Nafion 117

and pure SPEEK membranes and surely the reason is the

low sulfonation degree of polyacrylamide. Figure 6(b)




Fig. 3. Thermal properties; (a) TGA-DTG curves for

the pure sulfonated polyacrylamide and (b)

DCS curves for the pure and sulfonated poly-

acryloniotrile membranes.

Fig. 4. Tensile strength determination tests for the

membranes materials.

Fig. 5. Water uptake properties in comparasion with

sulfonated poly (etheretherketone) (SPEEK)

and Nafion 117 membranes.

shows the nyquist plot for sulfonted polyacrylamide at

100% RH and at 30 °C and 50 °C temperature. The sizes of

the impedance nyquist plots at these temperatures are quite

large and these correspond to poor contact between the

electrodes and membrane surface. But as the temperature

increases (75 °C and 100 °C) then the sizes of nyquist plots

reduce many folds as compared to previous ones and the

proton conductivities at these higher temperatures also in-

creased to 5 mS cm-1 at 75 °C and 10 mS cm-1 at 100 °C re-

spectively. The nyquist plots at higher temperatures can be

observed in Figure 6 (c). The proton conductivity of the

sulfonated polyacrylamide even at high temperature 100

°C (10 mS cm-1) is quite lower as compared to both pure

SPEEK and Nafion 117. An improvement in the conductiv-

ity can be made by increasing the sulfonation degree of the

polyacrylamide with sulphuric acid at higher temperature

than 100 °C or sulfonation with fuming sulphuric acid at

low temperature or at high temperature but without any

compromise on the mechanical and chemical properties of

the membranes materials. The nyquist plots for the sul-

fonated polyacrylamide shows a semicircle indicating typi-

cal characteristics of diffusion impedance which in turn in-

dicates a diffusion of charge carriers (protons) in mem-

brane material under the applied condition of temperature

and RH.

CONCLUSIONS

The sulfonation of Polyacrylonitrile at high tempera-

ture (100 °C) leads to formation of sulfonated polyacryl-

amide (SPAAm) that can be used as conducting membranes

in fuel cells. It was observed from the degree of sulfonation

and time required for the reaction that the sulfonation of

polyacrylamide is difficult as compared to other aliphatic

compounds like low density polyethylene (LDPE) and

polypropylene (PP). The main reason might be the hydroly-

sis of (-CN) group on the PAN stem. However sulfonation

can be improved by carrying out the reaction at even high

temperature than 100 °C with pure concentrated sulphuric

acid or at room temperature with fuming sulphuric acid or

mixture of sulphuric acid with fuming agent in some pro-

portionate at room temperature or at some high temperature

but without any damage to mechanical and chemical prop-

erties of new membrane materials.

ACKNOWLEDGMENT

The authors would like to thank Karen M. Prause for

the scanning electron microscopy, Silvio for the TG and

DSC analysis and H. Böttcher for tensile test, Prof Dr.

Hossein Mahdavi for his guidance about the reaction mech-

anism.

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Preparation and characterization of sulfonated polyethersulfone for cation-exchange membranes

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Transcript of Preparation and characterization of sulfonated polyethersulfone for cation-exchange membranes