Development of Hydrogel Polyelectrolyte Membranes with Fixed Sulpho-Groups via Radical Copolymerization of Acrylic Monomers

doi:10.4236/msa.2011.24035

Paper Menu >>

Journal Menu >>

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)

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: [email protected]

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

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

Development of Hydrogel Polyelectrolyte Membranes with Fixed Sulpho-Groups via Radical

272

Copolymerization of Acrylic Monomers

CH2

NH2

CH2

+++

crosslinking

agent

monomers

Initiator

NH2

nm lk

nmlk

CH2

crosslink

O-

OCH3

SOO

O-

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. С–С 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

175

235

375

695

15.3

2.1

7.6

18.7

AA-AN-SpA

25 - 120

120 - 195

195 - 295

290 - 450

450 - 800

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

160

250

390

665

6.9

8.2

46.8

Development of Hydrogel Polyelectrolyte Membranes with Fixed Sulpho-Groups via Radical

Copolymerization of Acrylic Monomers

273

0100 200 300 400 500 600 700 800 900

-10

DTA

DTG

TG,mg AA-AN

0100 200 300 400 500 600 700 800

-10

TG,mg

DTA

DTG

AA-AN-SpA

0100 200 300 400 500 600 700 800 900

DTA

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 СН2＝СН2, 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.

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

100

120

140

01020

sul fo-monom er conte nt, wt/ %

WU, %

SpA

sulfo-monomer content, wt%

(a)

(b)

0.2

0.4

0.6

0.8

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

sulfo-monomer conte nt, w t. %

IEC, meqv/g

SpA

sulfo-monomer content, wt%

Development of Hydrogel Polyelectrolyte Membranes with Fixed Sulpho-Groups via Radical

Copolymerization of Acrylic Monomers

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.