Materials Sciences and Applicatio ns, 2011, 2, 270-275
doi:10.4236/msa.2011.24035 Published Online April 2011 (http://www.scirp.org/journal/msa)
Copyright © 2011 SciRes. MSA
Development of Hydrogel Polyelectrolyte
Membranes with Fixed Sulpho-Groups via Radical
Copolymerization of Acrylic Monomers
Ivan A. Stadniy1, Viktoriia V. Konovalova1*, Yuriy M. Samchenko2, Ganna A. Pobigay1,
Anatoliy F. Burban1, Zoya R. Ulberg2
1Department of Chemistry, National University of Kiev-Mohyla Academy, Kiev, Ukraine;
2Institute of Biocolloidal Chemistry NAS of Ukraine, Kiev, Ukraine
E-mail: vita@ukma.kiev.ua
Receives September 24th, 2010; revised December 7th, 2010; accepted February 14th, 2011.
ABSTRACT
Electrolyte hydrogels are perspective materials for applications in electrochemical devices, which work at ambient
temperatures. In this work, hydrogel sulpho-modified membranes were formed by radical co-polymerization of sodium
styrensulphonate and potassium sulphopropyl acrylate with acrylamide and acrylonitrile. The hydrogel membranes
were obtained in the form of thin films. Properties of the membranes were studied by thermogravimetry, mass-spectro-
metry and IR-spectrometry. The prepared membranes were thermally stable up to 70˚C - 90˚C, and showed ion ex-
change capacity and swelling coefficients sufficient for use as ion-exchange or proton-conducting membranes.
Keywords: Copolymer Hydrogels, Polyelectrolyte Membranes, Acrylamide, Swelling, Ion Exchange Capacity,
Thermogravimetry
1. Introduction
Synthetic polymeric membranes continue to be the object
of intensive researches because of their important role in
separation science and technology [1,2]. Fast growing
application of membrane processes in various industrial
sectors demands development of synthetic membranes
with diversified properties and performance characteris-
tics. Membrane separation properties are governed by
methods of membranes preparation and chemicophysical
nature of the membrane materials.
Hydrogel membranes have attracted vast attention be-
cause of their novel properties and high potential for ap-
plications in industry [3], in particular, as electrolyte
membranes [4-6]. One of the advantages of hydrogels is
that they can be easily transformed into thin films. Sev-
eral different polymerization techniques have been de-
veloped for this purpose. Moreover, such techniques al-
low introduction of interacting species with active func-
tional groups into a polymeric network. These advan-
tages, alongside with gels good ionic conductivity and
ionic exchange property [7,8], have led to their interest-
ing applications in many electrochemical solid state ionic
devices, such as high-energy density batteries, fuel cells,
sensors and electrochemical display devices [9,10].
Hydrogels have 3D networks, which contain hydro-
philic functional groups. They are insoluble in water but
rather swell more than 95% after absorbing water. Hy-
drogel membranes can essentially be thought of as sub-
stances which contain solid skeleton and are composed
of polymers or long chain molecules cross-linked to cre-
ate a tangled network, enclosing a continuous liquid
phase. The properties of such gels depend strongly on the
interaction of the skeleton and the liquid phase. The liq-
uid phase prevents polymer network from collapsing into
compact mass while the skeleton prevents liquid from
escaping the network. Macroscopically most gel materi-
als are solid, but exhibit liquid-like characteristics mi-
croscopically due to the presence of a large number of
liquid-filled micro- and nanopores. Such ultraporous
structure of gels provides channels for directed ion mi-
gration [11] and allows to synthesize ion conducting ma-
terials and membranes.
One of the methods for producing polymeric electro-
lyte membranes is entrapping of an aqueous solution of
strong acid with a polymeric matrix [12,13]. It has been
Development of Hydrogel Polyelectrolyte Membranes with Fixed Sulpho-Groups via Radical 271
Copolymerization of Acrylic Monomers
determined, in particular, that polyacrylamide-based hy-
drogels after doping with H3PO4 exhibit ionic conductivi-
ties in the range of 10–3 - 10–2 S·cm–1 at room temperature.
The observed ionic conductivity increases with the in-
crease in water content up to about 30 mass% - 35 mass%,
remaining almost constant for higher water concentra-
tions reaching a conductivity plateau. At this conductiv-
ity plateau, ionic conductivity increases with the increase
of phosphoric acid/acrylamide molar ratio to a maximum
at 1.8 - 2.0 mol of H3PO4 to 1 mol of acrylamide [13].
All characteristics of an electrolyte directly depend
upon the structure and morphology of the host matrixes
of the prepared hydrogels. The electrical conductivity
change is most likely related to changes in local viscosity
due to H3PO4 interactions both with the solvent and the
polymer matrix which stiffens the hydrogel [14]. Polar
organic or inorganic fillers and additives are known to
lower the host polymer’s crystallinity and flexibility [15].
For example, addition of alkali salts to a phosphoric
acid-polyacrylamide proton conducting hydrogel influ-
ences the hydrogel structure and morphology [16]. This
effect was examined by measuring changes polyacryla-
mide chain vibrations, in phosphate group and in intersti-
tial water molecules as a function of concentration and
cationic nature of the additive. After addition of H3PO4 to
the polyacrylamide hydrogel matrix, amide groups be-
come more accessible and the polyacrylamide-phosphor-
ric acid network behaves like structure-maker promoting
larger association with the ‘bulk’ liquid water molecules.
Another way of producing polymeric hydrogel elec-
trolyte membranes is introduction of copolymers based
on highly conducting monomers [17]. In the present
study we have prepared hydrogel electrolyte membranes
containing highly conducting sulpho-group by radical
copolymerization of sodium styrensulphonate or potas-
sium sulphopropyl acrylate with acrylamide and acry-
lonitrile. The most important parameters for the prepared
polyelectrolyte hydrogels (e.g. thermostability, ion ex-
change capacity and swelling) were also studied.
2. Experimental
Hydrogels with potassium sulphopropyl acrylate (SpA)
and sodium styrene sulphonate (SS) were prepared by
radical copolymerization with acrylamide (AA) and
acrylnitrile (AN) in aqueous media at room temperature.
Cross-linking was accomplished by N,N-methylenebisa-
crylamide (BIS). The process of gel formation was in-
duced using potassium persulfate-sodium metabisulphite
oxidation-reduction system. Scheme of polymerization is
presented on Figure 1.
Potassium sulphopropylacrylate concentration varied
from 0 to 25 wt% of the total co-monomers content, sty-
rene sulphonate concentration-from 0 to 14 wt%, limited
by the solubility of the respective monomers. Total con-
tent of monomers was 50 wt% of polymerization mixture.
Concentation of BIS was equal to 0.4 wt%. The ratio of
AA to AN was 1:1 (wt).
All chemicals were purchased from Sigma-Aldrich
and used as received except AN, which was washed from
the polymerization inhibitor and then distilled.
Hydrogel membranes were obtained in the form of
films (plates) with thickness of about 500 µm. In order to
separate non-reacted monomers and initiators after the
synthesis, hydrogel samples were washed with signifi-
cant amount of distilled water at 45˚C. Water was
changed once per day, and the ratio between mass of gel
and mass of water was always 1:4 [18].
FT-IR spectra of the synthesized hydrogel films were
obtained using a FT-IR spectrometer TENSOR 37
BRUKER to a depth of about 2 - 3 µm into the sample.
The dynamic weight loss tests were conducted on
thermogravimetric analyzer (TGA) Derivatograph Q-
1500D for 50 mg samples with heating rate 10 ˚C/min in
the 20˚C - 900˚C temperature range. Measurements were
performed under nitrogen atmosphere. This analysis
produced differential curves of mass loss, allowing for
evaluation of thermal degradation.
Hydrogel mass-spectrometry was carried out at 30 to
800˚C and for molecular weights ranging from 10 to 200
Daltones. Hydrogel samples were placed in a quarts-
molibdenum cell and evacuated at 5 × 10–1 Pa, then sys-
tem was connected to the admission system of MI-1201
mass-spectrometer (Ukraine). Rate of samples heating
was 10˚C per minute.
Water uptake (WU) of the membranes was evaluated
from the mass change of membrane before and after
drying. The dry membrane swelled in de-ionized water
for a day, then the surface water was wiped carefully
with a filter paper, and it was immediately weighed. Af-
ter drying the sample overnight in a vacuum oven at
60˚C, the water uptake (WU), was calculated using the
formula:
WU =100
wet dry
dry
mm
т
where mwet and mdry are mass of fully hydrated membrane,
and that of the dry membrane, respectively.
The ion exchange capacity (IEC, mequiv/g) of the
membranes was found titrimetrically. To determine the
membranes’ IEC, the membranes were placed in 0.1M
hydrochloric acid solution, washed with distilled water
and immersed in 10 ml of 2 M NaCl solution for 1 day to
C
opyright © 2011 SciRes. MSA
Development of Hydrogel Polyelectrolyte Membranes with Fixed Sulpho-Groups via Radical
272
Copolymerization of Acrylic Monomers
CH2
NH2
O
CH2
N
NH
NH
O
CH2
O
CH2
+++
crosslinking
agent
monomers
Initiator
NH2
NR
NH
NH
R
N
O
NH2
O
OO
nm lk
nmlk
CH2
R
crosslink
R:
SO
O
O-
O
OCH3
SOO
O-
or
Figure 1. Scheme of hydrogels formation.
fully replace hydrogen of the sulphogroups with sodium.
After this, Н+ ions in solution were titrated with 0.01 M
NaOH. IEC is defined as mequiv of sulphonic groups per
gram of a dried sample.
3. Results and Discussion
3.1. IR-Spectroscopy
IR spectra (Figure 2) showed absorption bands in 4000 -
1500 cm–1 range corresponding to ν(ХН) (stretching vi-
brations of the X-H bonds), ν(СХ) (stretching vibrations
of the C-X bonds) and δ(ХН) (bending vibrations of the
X-H bonds) vibrations (where Х = О, N, C). 3426 and
3346 cm–1 bands were produced by amino groups with-
out hydrogen bonds, 3200 cm–1 bands—by amino groups
with hydrogen bonds. 2243 cm–1 band corresponded to
the nitrile group. Carbonyl group was characterized by an
intensive band ν(СО) = 1662 cm–1 and a “shoulder” at
1618 cm–1. Bending vibrations δ(ХН), (Х = N, O) were ob-
Figure 2. IR-spectra of copolymer hydrogels: curve 1-
AA-AN hydrogel; curve 2 – AA-AN-SpA hydrogel; curve 3
– AA-AN-SS hydrogel.
served at 1400 cm–1. СС bond vibrations were observed
in the 1400 - 1200 cm–1 range. When sulphogroup was
introduced into hydrogels (curves 2 and 3), 3380 cm–1
band was intensified due to hydrophilisation of hydrogels.
Spectra 2 and 3 showed new bands at 1180 and 1040 cm–1,
which correspond to ν(SO) vibrations.
3.2. Thermal Analysis
To determine membranes’ thermal stability and rate of
water loss upon heating, we conducted thermo-gravim-
etric analysis (TGA). TGA curves (Figure 3) showed
that cross-linked membranes based on acrylamide and
acrylonitrile have 5 stages of mass loss. Percentage of
weight loss for each stage and temperature of maximum
weight loss for each stage are listed for convenience in
Table 1. The first temperature range (25˚C to 120˚C)
with maximum weight loss rate at 90˚C corresponds to
the loss of lightly bound water. DTG curve shape in this
region depended on sample composition. As data in Ta-
ble 1 for the 25˚C - 120˚C temperature range show,
membranes with sulphopropyl acrylate lost the most wa-
ter in this temperature range because of high concentra-
tion of sufogroups with weakly bound water. Acrlyam-
ide-acrylonitrile membranes have proven to be thermally
stable up to 90˚C with overall mass loss under 6%.
Membranes with sodium styrene sulphonate were just as
stable. Membranes with sulphopropyl acrylate lost up to
20% of their mass at 70˚C.
Temperature range of 120˚C to 195˚C was the most in-
teresting for our studies. The mass loss at this stage was
attributed to the evaporation of bound water, which
Table 1. Membrane thermooxidative destruction.
Membrane Stage temperature
range,Тstart - Тfinal, ˚C
Temperature of
maximum
destruction rate, ˚C
Mass loss
on the stage, %
AA-AN
25 - 120
120 - 195
195 - 290
290 - 450
450 - 800
90
175
235
375
695
15.3
2.1
7.6
18.7
52
AA-AN-SpA
25 - 120
120 - 195
195 - 295
290 - 450
450 - 800
80
175
250
375
620
43.1
6.8
4.1
10.5
27.8
AA-AN-SS
25 - 120
120 - 195
195 - 290
290 - 450
450 - 800
95
160
250
390
665
14
6.9
8.2
14
46.8
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opyright © 2011 SciRes. MSA
Development of Hydrogel Polyelectrolyte Membranes with Fixed Sulpho-Groups via Radical
Copolymerization of Acrylic Monomers
Copyright © 2011 SciRes. MSA
273
0100 200 300 400 500 600 700 800 900
-10
0
10
20
30
40
50
DTA
TG
DTG
T
o
C
TG,mg AA-AN
0100 200 300 400 500 600 700 800
-10
0
10
20
30
40
50
T
o
C
TG,mg
DTA
TG
DTG
AA-AN-SpA
0100 200 300 400 500 600 700 800 900
0
10
20
30
40
50
DTA
TG
DTG
ToC
TG,mg AA-AN-SS
Figure 3. TGA curves of membranes without sulphogroups (AA-AN), with sulphopropyl acrylate (AA-AN-SpA), with styrene
sulphonate (AA-AN-SS).
greatly influenced proton-conductive properties of
membranes. In this range we have observed a peak in
mass loss rate with the maximum at 175˚C. The pres-
ence of sulphogroups in the polymer matrix increased
bound water content in the hydrogel. This was indicated
by 6.9% loss for membranes with sulphogroups and
2.9% loss for membranes without them, as shown by
the data for 120˚C - 195˚C range in Table 1. The mass
loss was still relatively low, indicating that membranes
will withstand steam sterilization, if necessary. Differ-
ential curves for membranes with styrene sulphonate
showed a band between 180˚C and 350˚C with a peak at
250˚C, when the sulphogroups started to destruct. All
membranes lost high percentage of their mass in 290 to
450˚C range. At this stage the mass lost corresponds to
the destruction of cross-links, followed by dehydration
of functional groups. Further heating to 800˚C leads to
total destruction of membranes, as seen by large loss of
weight in Tab le 1.
3.3. Mass-Spectrometry
As shown by mass-spectrometry, AA-AN copolymers
produce small amount of monomer acrylamide with m/z
(mass to charge ration) 71 and acrylonitrile (m/z 53) in
this temperature range with a maximum around 350˚C.
At temperatures over 440˚C copolymer chain itself starts
to destruct. Hydrogels with sulphogroup additives appear
more stable in this temperature range (see also Table 1).
With the rise of temperature AA-AN hydrogels lose
two kinds of fragments with m/z = 28 as well as water
bound with different strength (Figure 4). The first frag-
ment appears at 350˚C - 400˚C and may be attributed to
СО molecules from amide groups. The second fragment
appears at temperatures over 600˚C and may be attrib-
uted to СН2СН2, which forms with the hydrocarbon
chain destruction.
AA-AN hydrogels also tend to lose numerous small
fragments near 350˚C (Figure 5). M/z = 44, m/z = 43
and m/z = 45 bands may be attributed to amide group and
CONH
and 3
CONH
fragments, m/z = 57 band being
most likely due to the loss of CHCONH2.
1, arb. units
Figure 4. Thermograms for molecular ions m/z 28 ([M]+ for
СО+ and , line 1), m/z 53 ([M]+ for AN, line 2)
and m/z 71 ([M]+ AA, line 3) for the hydrogel АА-АN with
50% AN content.
+
2
CH= CH2
Development of Hydrogel Polyelectrolyte Membranes with Fixed Sulpho-Groups via Radical
274
Copolymerization of Acrylic Monomers
1, arb. units
Figure 5. Mass-spectrum of AA-AN hydrogel with 50% AN
content at 3500˚C.
3.4. Ion Exchange Capacity and Water Swelling
IEC and water swelling are among the most important
parameters for any polyelectrolyte hydrogel. They are
determined by nature of polymer matrix, number of
charged groups, density of cross-links and external pa-
rameters (e.g. temperature, pH). An increase in ionogenic
group content facilitates better proton exchange and
makes a membrane more proton conductive. On the other
hand, an increase in the number of polar groups makes
the membrane more swellable, which also leads to in-
creased mobility of ions in the membrane, but greatly
decreases its mechanical strength.
IEC and swelling were measured as a function of sul-
pho-monomer content in the initial mixture. Both IEC
and water swelling grew almost linearly with the increase
in the number of sulphogroups (Figure 6). Both IEC and
water swelling coefficient were slightly larger for hy-
drogels with sulphopropyl acrylate. This is probably due
to the sulpho-group of styrene sulphonate being attached
directly to a benzene ring, thus increasing hydrophobicity
and decreasing mobility.
The best IEC of 1.4 mg-equiv/g was shown by mem-
branes formed with 21% of sulphopropyl acrylate. Fur-
ther increase in sulphomonomer concentration brought
very little change in the IEC and caused them to swell
excessively (up to 120%). Styrene sulphonate content
was limited by its solubility in the initial mixture, but
membranes formed with 16% of styrene sulphonate
showed high IEC values (0.8 mg-eqiv/g) and low swell-
ing (50%) combined with elasticity and mechanical
strength.
4. Conclusions
Most polymer electrolyte systems are based on linear or
0
20
40
60
80
100
120
140
01020
sul fo-monom er conte nt, wt/ %
WU, %
30
SS
SpA
а
sulfo-monomer content, wt%
(a)
(b)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
01020
Figure 6.Water swelling (a) and IEC (b) vs sulpho-monomer
content in the membrane.
branched polymers having basic sites in the main chain
or side chain and doped with strong acids such as H3PO4
or H2SO4 [13,14]. These electrolytes exhibit high ionic
conductivity but suffer from a propensity for attack of
acid even in the presence of traces of moisture which
limits their application. Therefore, introduction of pro-
ton-conductive groups such as sulpho- and phosphorus
groups in the polymer skeleton is more advantageous
method for polymer electrolyte formation.
In this work hydrogel membranes with fixed high-con-
ducting sulpho-groups have been formed by radical co-
polimerization of various monomers (potassium sulpho-
propyl acrylate and sodium styrene sulphonate with acry-
lamide and acrylonitrile). TGA results indicate that
membranes are capable of holding water at temperatures
3
0
sulfo-monomer conte nt, w t. %
IEC, meqv/g
SS
SpA
b
sulfo-monomer content, wt%
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opyright © 2011 SciRes. MSA
Development of Hydrogel Polyelectrolyte Membranes with Fixed Sulpho-Groups via Radical
Copolymerization of Acrylic Monomers
Copyright © 2011 SciRes. MSA
275
up to 70˚C for membranes with sulphopropyl acrylate
and up to 90˚C for membranes with styrene sulphonate.
Water swelling coefficient was shown to increase line-
arly with the increase of sulphomonomer content. We
were able to obtain membranes with IEC up to 1.4 mg-
equiv/g for membranes with sulphopropyl acrylate and
up to 0.8 mg-equiv/g for membranes with styrene sul-
phonate. This makes the membranes viable for further
research both as ion-exchange membranes or proton-
conducting membranes.
REFERENCES
[1] R. Singh, “Industrial Membrane Separation Processes,”
Chemtech, Vol. 28, No. 4, 1998, pp. 33-44.
[2] M. Mulder, “Basic Principles of Membrane Technology,”
Kluwer Academic Publishers, Dordrecht, 1996.
[3] Y. Osada, J. Gong and Y. Tanaka, “Polymer Gels,” In: K.
Takemoto, R. Ottenbrite and M. Kamachi, Eds., Func-
tional Monomers and Polymers, Marcel Dekker, New
York, 1997, p. 497.
[4] S. Chandra, S. Sekhon, R. Srivastava and N. Arora, “Pro-
ton-Conducting Gel Electrolyte,” Solid State Ionics, Vol.
154-155, No. 2, December 2002, pp. 609-619.
doi:10.1016/S0167-2738(02)00505-2
[5] Y. Wan, K. Creber, B. Peppley and V. Bui “Chito-
san-Based Electrolyte Composite Membranes II. Me-
chanical Properties and Ionic Conductivity,” Journal of
Membrane Science, Vol. 284, No. 2, November 2006, pp.
331-338. doi:10.1016/j.memsci.2006.07.046
[6] N. Choudhury, S. Prashant, S. Pitchumanl, P. Sridhar and
A. Shukla, “Poly (Vinyl Alcohol) Hydrogel Membrane as
Electrolyte for Direct Borohydride Fuel Cells,” Journal of
Chemical Sciences, Vol. 121, No. 5, September 2009, pp.
647-654. doi:10.1007/s12039-009-0078-8
[7] M. G. Kodzwa, M. E. Staben and D. G. Rethwisch,
“Photoresponsive Control of Ion-Exchange in Leucohy-
droxide Containing Hydrogel Membranes,” Journal of
Membrane Science, Vol. 158, No. 1, June 1999, pp. 85-92.
doi:10.1016/S0376-7388(99)00008-3
[8] T. Kasuga, M. Nakano and M. Nogami, “Fast Proton
Conductors Derived from Calcium Phosphate Hy-
drogels,” Advanced Materials, Vol. 14, No. 20, October
2002, pp. 1490-1492.
doi:10.1002/1521-4095(20021016)14:20<1490::AID-AD
MA1490>3.0.CO;2-M
[9] T. Akamatsu, T. Kasuga and M. Nogami, “Formation of
Metaphosphate Hydrogels and Their Proton Conductivi-
ties,” Journal of Non-Crystalline Solids, Vol. 351, No.
8-9, April 2005, pp. 691-696.
doi:10.1016/j.jnoncrysol.2005.01.066
[10] S. Sampath, N. Choudhury and A. Shukla, “Hydrogel
Membrane Electrolyte for Electrochemical Capacitors,”
Journal of Chemical Sciences, Vol. 121, No. 5, Septem-
ber 2009, pp. 727-734.
[11] A. M. Valente, A. Ya. Polischuk, V. M. Lobo and G.
Geukens, “Diffusions Coefficients of Lithium Chlorite
and Potassium Chlorites in Hydrogel Membranes Derived
from Acrylamide,” European Polymer Journal, Vol. 38,
No. 1, January 2002, pp. 13-18.
doi:10.1016/S0014-3057(01)00161-6
[12] W. Wieczorek, Z. Florjanczyk and J. R. Stevens, “Proton
Conducting Polymer Gels Based on a Polyacrylamide
Matrix,” Electrochimica Acta, Vol. 40, No. 13-14, Octo-
ber 1995, pp. 2327-2330.
[13] J. R. Stevens, W. Wieczorek, D. Raducha and K. R. Jef-
frey, “Proton Conducting Gel/H3PO4 Electrolytes,” Solid
State Ionics, Vol. 97, No. 1-4, May 1997, pp. 347-358.
doi:10.1016/S0167-2738(97)00036-2
[14] W. Wieczorek, P. Lipka, G. Żukowska and H. Wyciślik,
“Ionic Interactions in Polymeric Electrolytes Based on
Low Molecular Weight Poly(Ethylene Glycol)s,” Journal
of Physical Chemistry B, Vol. 102, No. 36, September
1998, pp. 6968-6974. doi:10.1021/jp981397k
[15] W. Wieczorek, K. Such, H. Wycilik and J. Pocharski,
“Modifications of Crystalline Structure of Polymer Elec-
trolytes with Ceramic Additives,” Solid State Ionics, Vol.
36, No. 3-4, September 1989, pp. 255-257.
doi:10.1016/0167-2738(89)90185-9
[16] A. M. A. da Costa and A. M. Amado, “Cation Hydration
in Hydrogelic Polyacrylamide-Phosphoric Acid Network:
A Study by Raman Spectroscopy,” Solid State Ionics, Vol.
145, No. 1-4, December 2001, pp. 79-84.
doi:10.1016/S0167-2738(01)00916-X
[17] M. M. Nasef and E.-S. A. Hegazy, “Preparation and Ap-
plications of Ion Exchange Membranes by Radia-
tion-Induced Graft Copolymerization of Polar Monomers
onto Non-Polar Films,” Progress in Polymer Science, Vol.
29, No. 6, June 2004, pp. 499-561.
doi:10.1016/j.progpolymsci.2004.01.003
[18] M. Gertsyuk and Yu. Samchenko, “Separation of Nonre-
acted Acrylamide from Polyacrylamide Gel for Endo-
prothesing,” Ars Separatoria Acta, Vol. 5, 2007, pp.
98-101.