Synthesis, characterization and effect of reaction parameters on swelling properties of...

REACTIVE

Reactive & Functional Polymers 63 (2005) 11–26

&FUNCTIONALPOLYMERS

www.elsevier.com/locate/react

Synthesis, characterization and effect of reaction parameterson swelling properties of acrylamide–sodium

methacrylate superabsorbent copolymers

Y. Murali Mohan, P.S. Keshava Murthy, K. Mohana Raju *

Synthetic Polymer Laboratory, Department of Polymer Science & Technology, Sri Krishnadevaraya University,

Anantapur-515003, AP, India

Received 25 June 2004; received in revised form 31 January 2005; accepted 31 January 2005

Available online 19 March 2005

Abstract

Superabsorbent copolymers (SAPs) based on acrylamide (AAM) and sodium methacrylate (NMA) were prepared

by simultaneous free radical polymerization in aqueous solution using ammonium persulfate (APS) and N,N,N 0,N 0-tet-

ramethylethylenediamine (TMEDA) as initiating systems at room temperature. Eight different copolymers were pre-

pared by varying NMA with a fixed concentration of acrylamide and 1,4-butanediol diacrylate (BDDA) or

ethyleneglycol dimethacrylate (EGDMA) as crosslinkers. The acrylamide–sodium methacrylate (AAM–NMA) SAP

formation was confirmed by IR spectroscopy. The percentage of swelling, swelling and diffusion kinetic parameters

(i.e., initial swelling network constant, swelling rate constant, maximum swelling equilibrium, network structure con-

stant and type of diffusion, etc.) were investigated for all the SAPs at 25 �C. The swelling behaviour was also studied

at different temperatures ranging from 10 to 50 �C. The effect of different reaction parameters, such as, crosslinker, ini-

tiator and activator concentrations on the swelling capacity was investigated. The effect of variation of crosslinker con-

centration on the copolymer network structure was studied by SEM analysis. Further the saline sensitivity of the

copolymers was investigated at different saline concentrations. The effect of pH and de-swelling behaviour for

AAM–NMA SAPs prepared at optimum reaction conditions was studied.

� 2005 Elsevier B.V. All rights reserved.

Keywords: Superabsorbent copolymer; Simultaneous free radical polymerization; Swelling ratio; Activator; Crosslinked network

structure

1381-5148/$ - see front matter � 2005 Elsevier B.V. All rights reserv

doi:10.1016/j.reactfunctpolym.2005.01.005

* Corresponding author. Tel.: +91 08554 255655; fax: +91

28554 255710.

E-mail address: [email protected] (K.M. Raju).

1. Introduction

Superabsorbent polymers are an important

class of polymers which can absorb large amount

ed.

mailto:[email protected]

12 Y. Murali Mohan et al. / Reactive & Functional Polymers 63 (2005) 11–26

of water compared with general absorbing materi-

als and the absorbed water is hardly removable

even under pressure [1,2]. Superabsorbent poly-

mers are known as hydrophilic network structured

polymers having hydrophilic functional groupssuch as, hydroxyl, carboxylic acid, and amines

[1,2]. Due to their excellent properties such as,

hydrophilicity, high swelling capacity, lack of tox-

icity, and biocompatibility, these superabsorbent

polymeric materials are used for many applica-

tions including, soil conditioners for agriculture

and horticulture, disposable diapers, water block-

ing tapes, absorbent pads, gel actuators, drillingfluid additives, polymer cracks blocking materials,

feminine napkins, firefighting, extraction of pre-

cious metals, extraction of solvents, release of

agrochemicals, etc. [1–9]. The first superabsorbent

polymer was reported by the U.S. Department of

Agriculture in 1961. The aqueous gels of water

swellable acrylic polymers were developed for en-

hanced viscosity and resistance to degradation un-der sunlight purpose. The superabsorbents were

also developed for the adsorption of some cationic

dyes, uranyl ions, and serum albumin [10–12]. The

pH and temperature-sensitive hydrogels are being

used for various applications including controlled

drug delivery system and immobilized enzyme sys-

tems [13–19]. In all the above applications, the

amount of water absorption and retention proper-ties are most important. To obtain high water

absorbency and to reduce the cost of production,

these materials are suitably modified or produced

by copolymerization. The grafted kaolinite SAP

was recognized as low cost production material

with improved swelling properties [20–22]. Com-

posite and porous superabsorbents were developed

for higher and sharp swelling capacity as well asfor reducing the cost of the material [20–25]. Wu

et al. [20–22] reported the composite SAPs based

on starch/acrylamide/kaolinite and poly(acrylic

acid)/mica and studied the effect of hydrophilic

groups on water absorbency of starch-g-acrylam-

ide/kaolinite SAPs. The SAPs developed so far

have modified with a view to enhance the proper-

ties such as improved absorbency, gel strength andabsorption rate.

The inverse suspension polymerization method

was employed by Kiatkamjornwong and Phun-

chareon [26] for the preparation of neutralized

poly(acrylic acid-co-acrylamide) superabsorbents

in presence of different suspending agents and also

studied the influence of reaction parameters on

swelling capacity of superabsorbents. Lee et al.[27–30] employed the inverse suspension polymer-

ization for the preparation of crosslinked poly(so-

dium acrylate-co-hydroxyethyl methacrylate),

poly(SA-co-HEMA); poly(sodium acrylate-co-so-

dium,2-acryalamido-2-methyl propanesulfonate),

poly(SA-NaAMPS); poly[sodium acrylate-co-3-di-

methyl (methacryloxyethyl) ammonium propane

sulfonate], poly(SA-co-DMAPS) and poly(sodiumacrylate-co-sulfobetaines) using 4,41-azobis(cyano-

valeric acid) as initiator and N,N1-methylene-

bis-acrylamide (MBA) as crosslinking agent in

presence of sorbitan monostearate (Span 60) as

suspending agent. The spherical beads of poly-

(acrylamide-co-sodium acrylate) hydrogels were

also prepared by inverse suspension polymeriza-

tion. The 4,41-azo-bis-isobutyronitrile (AIBN) ini-tiated terpolymers based on trialkyl-4-vinylbenzyl

phosphonium chlorides (TRVB) were prepared

and employed for the applications of antibacterial

activity and adsorption ability for anionic surfac-

tants of the hydrogels. Sodium methacrylate

(NMA) and lithium methacrylate polymers were

prepared by free radical polymerization in metha-

nol solution at 60 �C using 0.1% (w/v) of AIBN asinitiator [31].

In contrast to the above synthetic methods, the

redox polymerizations are well suitable for the

preparation of SAPs [20–25,32–38]. Zhou et al.

[32,33] studied the synthesis and swelling proper-

ties of copolymers based on acrylamide with anio-

nic monomers. A few series of copolymers were

synthesized by simultaneous polymerization andalso reported their swelling and diffusion charac-

teristics [34–37]. Bajpai and Sonkusley [34,35] re-

ported the synthesis and characterization of

acrylamide/itaconic acid for oral drug delivery of

peptide; and also studied the swelling and de-

swelling behaviour of poly(acrylamide-co-maleic

acid), poly(AAM-co-MA). Karadag and Saraydin

[36] reported the synthesis as well as the swellingbehaviour and diffusion studies of copolymer,

poly(acrylamide-co-crotonic acid). Isik [37] re-

ported the swelling behaviour and diffusion char-

Y. Murali Mohan et al. / Reactive & Functional Polymers 63 (2005) 11–26 13

acteristics of acrylamide-acrylic acid hydrogels. In

the present work, the authors report the synthesis

of acrylamide–sodium methacrylate (AAM–

NMA) crosslinked SAPs using ammonium persul-

fate (APS)/N,N,N 0,N 0-tetramethylethylenediamine(TMEDA) initiating system in presence of BDDA

or EGDMA crosslinkers at room temperature

and also investigated the optimum reaction condi-

tions to obtain higher water absorbency for the

SAPs.

2. Experimental

2.1. Materials

Acrylamide (AAM) and ammonium persulfate

supplied by S.D. Fine Chem. Ltd. (India). Metha-

crylic acid (MA), 1,4-butanediol diacrylate

(BDDA), 1,2-ethyleneglycol dimethacrylate (EGD-

MA) and N,N,N 0,N 0-tetramethylethylenediamine(TMEDA) were purchased from Aldrich (Sigma–

Aldrich Chemicals Private Limited, India). All the

chemicals were used as received. Double distilled

waterwasusedforall thecopolymerizationreactions

as well as for swelling studies. Sodiummethacrylate

CH2 = CH

CONH2

OC

O

OC

OH3C

CH2 CH2 CH

CH2 CH

Acrylamide (AAM)

Ethyleneglyco

Butanediol diac

Scheme 1. Chemical structures of monomers an

was prepared by complete neutralization of metha-

crylic acid with sodium hydroxide.

2.2. Synthesis of acrylamide–sodium methacrylate

superabsorbent copolymer

All the polymerization reactions were carried

out in poly(vinyl chloride) (PVC) straws (3 mm

dia). A weighed quantity of monomers along with

crosslinking agent were taken in 50 ml beaker con-

taining 2 ml of distilled water. After 10 min of stir-

ring of the reaction mixture, APS and TMEDA

were added in a sequence and immediately trans-ferred to PVC straws. The polymerization reac-

tions are exothermic in nature and all the ratios

of the monomers gave gels within 1 h of reaction

time. However, the polymerization reactions were

continued for 24 h. The polymer gels obtained

were cut into pieces of 3–4 mm length. They were

dried in air and then under vacuum, and utilized

for swelling and other studies. The chemical struc-tures of monomers and crosslinkers used in

the present investigation are given in Scheme 1.

The representative polymerization reaction for

the preparation of AAM–NMA superabsorbent

copolymer crosslinked with EGDMA was shown

in Scheme 2.

O C

O

O C

O

CH3

CH2 = C

COONa

CH3

2 CH2

2

Sodium methacrylate (NMA)

l methacrylate (EGDMA)

rylate (BDDA)

d crosslinkers used in the polymerization.

CH2 = CH

CONH2

CH2 = C

COONa

CH3

OO C

O

C

O

CH2 CH2+ +

APS/TMEDA

AAM-NMA Superabsorbent copolymers crosslinked with EGDMA

AAM EGDMA NMA

Scheme 2. Schematic representation of AAM–NMA superabsorbent copolymer crosslinked with EGDMA.


2.3. Swelling measurements

Accurately weighed dry superabsorbent poly-

mers (40–50 mg) were immersed in a 100 ml

beaker containing double distilled water or saline

or pH solution until they reach equilibrium atroom temperature. To obtain equilibrium the

superabsorbents took 24–30 h. After residual

water was removed superficially with filter paper

the swollen superabsorbents were weighed. The

Swelling ratio or swelling capacity (S%) of super-

absorbents was calculated using the following

Eq. (1) [36–39]:

Swelling ratioðg=gÞ ¼ ½ðM s �MdÞ=Md�; ð1Þwhere Ms and Md denote the weight of the swollen

superabsorbent at equilibrium and the weight of

the dry superabsorbent at time 0, respectively.

2.4. IR and SEM analysis

The IR spectra of AAM–NMA superabsorbent

copolymers were carried out on a Perkin–Elmer

Spectrophotometer ASCII (Perkin–Elmer CetusInstruments, Norwalk, CT). The SAPs were

coated with a thin layer of palladium gold alloy.

The morphological variations of the SAPs were

observed by using a JEOL JSM 840A (Japan)

scanning electron microscope (SEM).

3. Results and discussion

3.1. IR Spectra

The IR spectra of the copolymers showed peaks

corresponding to the functional groups of themonomeric units present in the polymeric chains.

The peaks observed at 1735 cm�1 corresponds to

the mC@O of the acrylate units of NMA and cross-

linker (BDDA or EGDMA); and 1658 cm�1 corre-

sponding to the mC@O group of acrylamide unit;

1239 and 1172 cm�1 corresponding to C–O–C

stretching coupling interactions of ester groups.

From the IR analysis, it is confirmed that all themonomeric units and crosslinker units, i.e., acryl-

amide, NMA and BDDA or EGDMA are present

in the copolymer chain. The representative IR

spectra of the copolymers are presented in Fig. 1.

3.2. Influence of parameters on swelling behaviour

The diffusion process of the SAPs can be esti-mated by studying the swelling behaviour of the

polymers. The diffusion process generally repre-

sents the affinity between the polymer networks

and external solution. The absorption of external

solutions can be balanced by three main forces:

(1) free energy between chain networks of the poly-

mers and external solvent; (2) electrostatic repul-

3200 2800 2400 2000 1600 1200 800-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02 BDDA Crosslinked Copolymer EGDMA Crosslinked Copolymer

T)%(

Wavenumber (cm-1)

Fig. 1. IR spectra of AAM–NMA superabsorbent copolymers.


sion (domain effect); (3) elastic retractile response

of the networks [40]. Among these three factors

the first two forces promote the swelling behaviour

and the third one suppresses the swelling phenom-

ena of the superabsorbent polymer. The absorp-

tion of superabsorbent polymers depends on the

strength of the hydrophilic groups, crosslinkingdensity, polymer network behaviour, and elasticity

of the polymer networks, type of solvent and

strength of the external solution as well as the

characteristics of the external solution, etc.

[3,4,27–30]. The key properties of superabsorbent

polymers are swelling capacity and the elastic

modulus of the swollen crosslinked gel [1]. Both

of these properties are related to the cross-linkdensity of the net works of the gel. To improve

the swelling capacity of the AAM–NMA superab-

sorbent copolymers, various reaction parameters

are employed. The complete details of the influ-

ence of the reaction parameters, such as, mono-

mer, crosslinker, initiator and activators, are

given below.

3.2.1. Effect of monomer concentration

The hydrophilic monomer concentration is one

of the influencing factors that affect the swelling

properties of the SAP [3,4,27–30]. Table 1 illus-

trates the equilibrium swelling, swelling kinetic

parameters and diffusion characteristics of copoly-

mers at various temperatures as a function of

monomeric units in the copolymer network. The

SAPs swelled slowly and reached the equilibrium

in about 20–30 h. The swelling ratio ranged from

25.88 to 168.21 g/g for SAPs crosslinked by

BDDA and 84.69 to 192.69 g/g for SAPs cross-linked with EGDMA at swelling medium temper-

ature 25 �C. The swelling capacity of the

copolymers increases with increase in NMA units

(–COONa) in the acrylamide SAP backbone

chain. This is due to the hydrophilic character of

the NMA. It can be explained that the presence

of NMA in the copolymer makes the gel ionic

character thereby increasing the ionic concentra-tion inside the copolymer net works to create

ion-swelling pressure. With an increase of NMA

content in the copolymer, the ionic swelling pres-

sure increases to result in increase of swelling ratio.

Further, it was also established that the introduc-

tion of ionic groups improves the flexibility in

the chain and hence its extensibility thereby

increasing the swelling ratio. Our previous reportsof absorption studies of SAPs or hydrogels also

indicated the similar swelling behaviour [3,4].

It is observed that the SAPs crosslinked with

EGDMA have higher equilibrium swelling ratio

than the BDDA crosslinked SAPs. Our earlier

studies also revealed similar behaviour on water

absorbency with increment in the hydrophilic con-

tent [3,4,41,42]. In the present investigation, it isidentified that the crosslinked SAPs have maxi-

mum swelling ratio values at 1.40 · 10�2 of

AAM and 9.25 · 10�3 mol of NMA feed in the

polymerization. This swelling value is in close

agreement with swelling value of pure NMA SAPs.

The swelling kinetics of the SAPs on tempera-

ture dependency in distilled water were studied at

a temperature range from 10 to 45 �C and the re-sults were plotted in Figs. 2 and 3 for BDDA

and EGDMA series, respectively. The SAPs have

swelled rapidly to a maximum extent at higher

temperatures and reached the equilibrium in less

time when compared at lower temperatures. For

both series of SAPs, the highest swelling ratios

are observed at 50 �C and the lowest at 10 �C.The reason to increase in the swelling ratio at high-er temperatures is due to fast diffusion process

because diffusion process is a function of tempera-

ture and increases with increasing temperature. All

Table 1

Influence of NMA content on the swelling behaviour of AAM–NMA superabsorbent copolymera

Polymer code Monomers in the feed (mol) Swelling ratio

(g/g)

Swelling kinetics Diffusion

characteristic

Acrylamide Sodium acrylate Initial swelling rate,

ki (g water/g gel)/min

Equilibrium swelling, Seq

(g gal/g water)/min

Swelling rate constant,

ks (g water/g gel)

Swelling exponent, n

BDDA1 1.40 · 10�2 4.63 · 10�4 07.44 5.07 763.35 8.70 · 10�06 0.486

BDDA2 1.40 · 10�2 9.25 · 10�4 25.88 0.08 28.97 1.02 · 10�05 0.547

BDDA3 1.40 · 10�2 1.38 · 10�3 50.34 0.12 58.20 3.67 · 10�05 0.697

BDDA4 1.40 · 10�2 1.85 · 10�3 68.52 0.15 77.27 2.63 · 10�05 0.698

BDDA5 1.40 · 10�2 2.31 · 10�3 67.61 0.25 76.39 4.32 · 10�05 0.723

BDDA6 1.40 · 10�2 4.62 · 10�3 80.94 0.33 91.32 3.98 · 10�05 0.692

BDDA7 1.40 · 10�2 9.25 · 10�3 102.41 0.54 112.35 4.33 · 10�05 0.730

BDDA8 – 9.25 · 10�3 168.21 0.69 189.03 1.95 · 10�05 0.765

EGDMA1 1.40 · 10�2 4.63 · 10�4 15.2 0.03 17.63 1.25 · 10�04 Not Determined

EGDMA2 1.40 · 10�2 9.25 · 10�4 84.69 0.14 108.34 1.27 · 10�05 0.919

EGDMA3 1.40 · 10�2 1.38 · 10�3 85.79 0.19 105.15 1.79 · 10�05 0.942

EGDMA4 1.40 · 10�2 1.85 · 10�3 132.24 0.30 162.07 1.15 · 10�05 0.835

EGDMA5 1.40 · 10�2 2.31 · 10�3 176.86 0.34 223.71 6.89 · 10�06 0.758

EGDMA6 1.40 · 10�2 2.77 · 10�3 189.03 0.39 241.54 6.70 · 10�06 0.897

EGDMA7 1.40 · 10�2 4.62 · 10�3 168.76 0.45 202.42 1.12 · 10�05 0.828

EGDMA8 1.40 · 10�2 9.25 · 10�3 191.11 0.72 216.91 1.54 · 10�05 0.835

EGDMA9 – 9.25 · 10�3 192.69 0.76 218.34 1.60 · 10�05 0.874

a Reaction conditions: BDDA, 7.567 · 10�5 mol; EGDMA, 7.567 · 10�5 mol; APS, 4.38 · 10�5 mol; TMEDA, 8.60 · 10�5 mol; temperature = 25 �C.

16

Y.MuraliMohanet

al./Reactive

&Functio

nalPolymers

63(2005)11–26

0 500 1000 1500 2000 25000

50

100

150

200

250

Temperature 10˚C

lewS

iloitar gn

/g(g)

Time (min)

EGDMA2 EGDMA4 EGDMA5 EGDMA6 EGDMA7 EGDMA8

0 500 1000 1500 2000 25000

50

100

150

200

250Temperature 20˚C

lewS

iloitar gn

/g(g)

Time (min)


0 500 1000 1500 2000 2500 3000 35000

50

100

150

200

250

300

350


wSle

ilar gn

)g/g(oit

Time (min)

EGDMA1 EGDMA2 EGDMA3 EGDMA4 EGDMA5 EGDMA6 EGDMA7 EGDMA8 EGDMA9

0 500 1000 1500 2000 25000

100

200

300

400

500

600

700

800

Temperature 50˚C

Swe

illgn

r at

io( g

g/)

Time (min)


ig. 3. Influence of temperature on swelling kinetics of AAM–

MA superabsorbent copolymers crosslinked with EGDMA.

0 500 1000 1500 2000 25000

50

100

150

Temperature 10˚C

Swe

illgn

r at

io(g

g/)

Time (min)

BDDA2 BDDA3 BDDA4 BDDA5 BDDA6 BDDA7

0 500 1000 1500 2000 25000

50

100


wSleli

gnar

ito

(g/g

)

Time (min)

BDDA3 BDDA4 BDDA5 BDDA6 BDDA7

0 500 1000 1500 2000 2500 3000 35000

50

100

150

200

250


wS)g/g( oi tar gnille

Time (min)

BDDA1 BDDA2 BDDA3 BDDA4 BDDA5 BDDA6 BDDA7 BDDA8

0 500 1000 1500 2000 25000

100

200

300

400


ewS

lliar gn

ito

)g/g(

Time (min)

BDDA2 BDDA3 BDDA4 BDDA5 BDDA6 BDDA7 BDDA8

Fig. 2. Influence of temperature on swelling kinetics of AAM–

NMA superabsorbent copolymers crosslinked with BDDA.


F

N

0 500 1000 1500 2000 2500 3000

0

1

2

3

4

S/t

Time (min)


0 500 1000 1500 2000 2500 3000

0.00

0.08

0.16

0.24

0.32

0.40

/tS

Time (min)

EGDMA1 EGDMA2 EGDMA3 EGDMA4 EGDMA5 EGDMA6 EGDMA7 EGDMA8

(a)

(b)

Fig. 4. Swelling rate curves of AAM–NMA superabsorbent

copolymers crosslinked with (a) BDDA and (b) EGDMA.


the SAPs have exhibited temperature dependency

in their swelling behaviour due to their associa-

tion/dissociation of the hydrogen bonding by the

hydrophilic groups in the copolymer.

3.2.1.1. Swelling and diffusion characteristics. The

swelling and diffusion characteristics of the

AAM–NMA superabsorbent copolymers were

found at 25 �C. To investigate the mechanism of

swelling processes or extensive swelling, a simple

kinetic analysis with a second order equation was

followed as shown below [43–45]:

dS=dt ¼ kSðSeq � SÞ2; ð2Þwhere S, Seq and kS denotes the degree of swelling

at any time, degree of swelling at equilibrium, and

swelling rate constant, respectively. The integra-

tion of the above equation over the limits S = S0

at time t = t0 and S = S at equilibrium at time

t = t, gives the following equation:

t=S ¼ Aþ Bt; ð3Þwhere B = 1/Seq is the inverse of the maximum or

equilibrium swelling, A ¼ ð1=kSS2eqÞ is the recipro-

cal of the initial swelling rate of the SAP, and ksis the swelling rate constant. Fig. 4 shows the

swelling isotherms of AAM–NMA copolymers

crosslinked with BDDA and EGDMA. To exam-

ine the above kinetic model, graphs were plotted

against t/S and t. The initial rate of swelling (ri),

swelling rate constant (kS), and the theoretical

equilibrium swelling (Seq) values of hydrogels

were calculated from the slope and the intersec-tion of the lines and the results are tabulated in

Table 1. It is found in the BDDA series of

copolymers the initial swelling rate was very high

for the first two copolymers, and the other

copolymers showed gradual increment in their

swelling rate from 0.085 to 0.69. The swelling rate

constant decreases as the NMA content increases.

Generally, the swelling characteristics of thecopolymers varies as the nature of chemical

groups (hydroxyl, carboxyl, carbonyl, amide,

and amine, etc.) changes in the AAM copolymer

chains and this is due to many types of polymer–

solvent interactions [3,4,36,37,41,42,45]. From the

Table 1, it was identified that there is significant

variation in their initial swelling rate, equilibrium

swelling and swelling rate constants due to the

variation in the structure as well as the mono-

meric units. From the swelling studies, it is con-

cluded that the swelling phenomena of the

copolymeric hydrogels are directly related to the

chemical repeating units present in the copoly-mers [1,3,4,36,37] as well as the density of cross-

linking networks [1,3,4,36,37].

When a solid polymer is brought into contact

with a penetrating liquid, the liquid diffuses into

the polymer, which causes to swell the polymer.

The concentration gradient-controlled diffusion

and relaxation controlled swelling contributes the

rate and extent of penetrant sorption into the poly-

3 4 5 6 74

6

8

10

12

nl( F

wsp)

ln t


3.0 3.5 4.0 4.5 5.0 5.5 6.00

2

4

6

8

F( nL

swe

illng

)

Ln (t)

EGDMA1 EGDMA2 EGDMA3 EGDMA4 EGDMA5 EGDMA6 EGDMA7 EGDMA8 EGDMA9

(a)

(b)

Fig. 5. Diffusion kinetic curves of AAM–NMA superabsorbent



mer. To analyze the sorption mechanism, the diffu-

sion phenomena of the copolymers analyzed using

the equation given below

F swp ¼ M s �Md=Md ¼ ktn; ð4Þwhere Fswp, Ms and Md represents the swellingpower or swelling capacity of gel, weight of swol-

len hydrogel at equilibrium and weight of dried

gel at time t = 0, respectively; k is the characteristic

constant of the SAP related to the structure of the

polymer network; and n is the swelling characteris-

tic exponent, which suggest the kind of transport

of the penetrant. From this equation, the swelling

characteristic exponent (n) can be used to deter-mine the mechanism of sorption. For n = 0.5, Fic-

kian diffusion will dominate; for n = 0.5–1.0, the

system mechanism will be non-Fickian diffusion.

To estimate the swelling characteristic exponents

(n) and swelling constant (k), Eq. (1) can be used

upto 60% of the swelling data of the copolymer.

The graphs were plotted between lnFswp and ln t

and obtained as straight lines. Fig. 5 shows thegraphs of AAM–NMA copolymers crosslinked

by BDDA and EGDMA. The swelling exponents

were calculated from the slope of the lines of

lnFswp � ln t plots [42,46,47]. The calculated swell-

ing exponent values (n) are in between 0.489–0.765

and 0.919–0.874 for BBDA and EGDMA cross-

linked AAM–NMA copolymers, respectively.

These swelling exponent values indicate that thetransport of all SAPs was of non-Fickian

character.

3.2.1.2. Salinity effect on swelling behaviour. Thecharacteristics of external solution such as charge

valencies and salt concentration greatly influence

the swelling behaviour of the superabsorbent poly-

mers [3,4,27–30]. The effect of concentration of so-

dium chloride solution on swelling behaviour of

AAM–NMA superabsorbent copolymers was

investigated. The effect of salinity on the swelling

behaviour of various copolymers, namely, poly(SA-NaAMPS), poly(SA-HEMA), poly(SA-

DMAPS) copolymers [27,30] was studied in detail.

Fig. 6 shows the swelling ratio of AAM–NMA

superabsorbent copolymers as a function of con-

centration of sodium chloride solution. The studies

indicates that the swelling ratio of SAPs decreased

in salt solution as the ionic concentration of the

salt solution increases. This is due to the decre-

ment in expansion of the gel network due to repul-

sive forces of counter ions on the polymeric chain

shielded by the bound ionic charges. Therefore, theosmotic pressure difference between the gel net-

work and the external solution decreased with in-

crease in the ionic strength of the saline

concentration.

The dimensionless factor (a) is the ratio of

absorption at a given salinity to salt free water

[48]. This factor is a measure for the salt sensitivity

of the superabsorbent polymers. The a values fordifferent saline concentrations are given in Table

2 for copolymers having different contents of

NMA. The values indicates that the sensitivity of

absorbance to changes in salinity is decreased as

the NMA concentration varies.

Table 2

Dependency of dimensionless swelling factor (a), on the monomer content at various saline concentrations

Polymer Code Dimensionless factor (a) Polymer Code Dimensionless factor (a)

(a)0.0171 (a)0.0855 (a)0.171 (a)0.0171 (a)0.0855 (a)0.171

BDDA2 0.507 0.459 0.444 EGDMA2 0.278 0.278 0.211

BDDA3 0.357 0.279 0.246 EGDMA3 0.396 0.280 0.209

BDDA4 0.262 0.255 0.198 EGDMA4 0.331 0.204 0.153

BDDA5 0.676 0.245 0.192 EGDMA5 0.253 0.164 0.110

BDDA6 0.423 0.216 0.174 EGDMA6 0.258 0.114 0.113

BDDA7 0.302 0.212 0.169 EGDMA7 0.284 0.162 0.123

BDDA8 0.680 0.257 0.217 EGDMA8 0.517 0.314 0.235

0.00 0.04 0.08 0.12 0.160

20

40

60

80

100

120

140

160

180

200

llewS

initar g

)g/g( o

NaCl concentration (mole)

BDDA2 BDDA3 BDDA4 BDDA5 BDDA6 BDDA7 BDDA8

0.00 0.04 0.08 0.12 0.16 0.200

40

80

120

160

200

240

llewS

initar g

)g/g( o

NaCl concentration (mole)

EGDMA2 EGDMA3 EGDMA4 EGDMA5 EGDMA6 EGDMA7 EGDMA8

(a)

(b)

Fig. 6. Influence of saline concentration on swelling ratio of

AAM–NMA superabsorbent copolymers crosslinked with

(a) BDDA and (b) EGDMA.


3.2.1.3. Water retention capacity. In order to deter-

mine the water retention capacity of the AAM–

NMA copolymers, de-swelling experiments were

performed at 25 �C. 2–3 g of swollen gels were ta-

ken on aluminum foil and the weight loss of water

in swollen gels was estimated gravimetrically at

different time intervals. A large variation in their

de-swelling behaviour was noticed in the two

crosslinked copolymer series. De-swelling studies

indicates that the EGDMA crosslinked copoly-

mers have higher water retention capacity thanthe BDDA crosslinked copolymers. The water

retention curves of the AAM–NMA copolymers

are shown in Fig. 7. The figure indicates that the

BDDA crosslinked copolymers have less water

retention capacity of 2.18–6.28% compared to

EGDMA crosslinked copolymers having high

water retention capacity of 8.43–25.09%. How-

ever, in the BDDA series also one SAP shows highwater retention capacity (24.15%) due to its higher

swelling behaviour than other copolymers in this

series. The water retention capacity of the AAM–

NMA superabsorbent copolymers indicates that

these copolymers may find an application in agri-

culture and horticultural fields due to their high

water retention capacity [3,4,41].

3.2.2. Effect of crosslinking agent type and

concentration

The crosslinkers plays an important role in the

formation of three dimensional network structurespermanently to the SAPs in the polymerization

process. This is also a promising factor directly

affecting the swelling ratio of the superabsorbent

polymers. Fig. 8 shows the swelling ratio of the

AAM–NMA superabsorbent copolymer as a func-

tion of crosslinker concentration. It is concluded

from the graph that the swelling ratio increases

as BDDA concentration increases from 5.04 ·10�5 to 3.02 · 10�5 mol and decreases slightly with

0.0000 0.0002 0.0004 0.0006 0.00080

100

200

300

400

500

600

Swel

ling

rati

o (g

/g)

Crosslinker concentration (mole)

BDDA 11 EGDMA 11

Fig. 8. Influence of crosslinker concentration on swelling ratio

of AAM–NMA superabsorbent copolymers.

0 1000 2000 3000 4000 5000

0

20

40

60

80

100

Wa

itneter reto

%( n)

Time (min)

BDDA2 BDDA3 BDDA4 BDDA5 BDDA7 BDDA8

0 1000 2000 3000 4000 5000

0

20

40

60

80

100

Wa

itneter reto

%( n)

Time (min)

EGDMA2 EGDMA3 EGDMA4 EGDMA5 EGDMA6 EGDMA7 EGDMA8 EGDMA9

(a)

(b)

Fig. 7. Water retention curves of AAM–NMA superabsorbent



further increase of BDDA concentration. The

swelling ratio also increases as EGDMA concen-

tration proceeds from 5.04 · 10�5 to

7.05 · 10�4 mol and decreases considerably whenthe concentration of EGDMA is greater than

7.05 · 10�4 mol. As the crosslinker concentration

increases the swelling ratio of the SAPs increases

up to a certain level and then decreases with fur-

ther increase in the concentration of the cross-

linker. This behaviour in both the crosslinked

SAPs is due to decrease in space between the

copolymer chains as the crosslinker concentrationincreases. However, it is noticed that maximum

swelling ratio was found when EGDMA was used

as crosslinker.

The SEM studies of SAPs in presence of water

is difficult, therefore the studies were carried outon dried SAPs. The samples were prepared with

out destroying the microstructure of the SAPs.

The gold-coated surfaces of the SAPs were ana-

lyzed using Hitachi-520 Scanning Electron

Microscope, Japan. The SEM photographs of

the SAPs crosslinked with different concentra-

tions of BDDA and EGDMA are presented in

Figs. 9 and 10, respectively. Fig. 9(b) shows awell defined crosslinks in the copolymers having

higher swelling ratio behaviour instead of irregu-

lar structures observed in other copolymers in

BDDA series. Similar results were also noticed

in the case of EGDMA crosslinked copolymers.

Fig. 10(b) and (c) shows distinguished structures

possessing higher swelling ratio values. But the

copolymer having irregular structural arrange-ment as shown in Fig. 10(a) shows lower swelling

ratio.

3.2.3. Effect of initiator and activator

In free radical addition polymerizations, the ini-

tiator have great influence on polymerization rate

as well as on the molecular weight of the resulted

polymer. In the process of crosslinking polymeri-zation reactions also, the initiator affects both the

Fig. 9. SEM micrographs of AAM–NMA superabsorbent copolymers crosslinked with BDDA.


Fig. 10. SEM micrographs of AAM–NMA superabsorbent copolymers crosslinked with EGDMA.


degree of crosslinking and molecular weight be-

tween two crosslinking points and contributes for

the inhomogeneity in the polymer system. The

low concentrations of initiator results in loweringof the crosslinking density as well as conversion.

In the present investigation, the polymerizations

of AAM, NMA starts with the reaction between

APS and TMEDA to form free radicals to initiate

the polymerization of monomers as well as the

crosslinking reactions between the chains simulta-

neously [36,37,42].As shown in Fig. 11, the low concentrations of

APS shows higher swelling ratios. At 2.19 · 10�5

mol of APS the BDDA crosslinked AAM–NMA

0.00000 0.00006 0.00012 0.00018 0.00024 0.000300

100

200

300

400

500

llewS

initar g

)g/g( o

APS concentration (mole)

BDDA 12 EGDMA 12

Fig. 11. Influence of APS concentration on swelling ratio of

AAM–NMA superabsorbent copolymers.

0.00000 0.00004 0.00008 0.00012 0.000160

100

200

300

400

500

600

llewS

initar g

)g /g( o

TMEDA concentration (mole)

BDDA EGDMA

Fig. 12. Influence of TMEDA concentration on swelling ratio

of AAM–NMA superabsorbent copolymers.


copolymer shows higher swelling ratio (392.99 g/g)

than with other concentrations of APS. The EGD-

MA crosslinked copolymers, shows higher swell-

Table 3

AAM–NMA superabsorbent copolymer reaction parameters variatio

SAP code AAM (mol) NMA (mol) BDDA (mol

BDDA11 1.40 · 10�2 9.25 · 10�3 –

BDDA12 1.40 · 10�2 9.25 · 10�3 5.04 · 10�4

BDDA13 1.40 · 10�2 9.25 · 10�3 5.04 · 10�4

EGDMA11 1.40 · 10�2 2.31 · 10�3 –

EGDMA12 1.40 · 10�2 2.31 · 10�3 –

EGDMA13 1.40 · 10�2 2.31 · 10�3 –

ing ratio (191.07 g/g) at 8.76 · 10�5 mol of

concentration of APS.

The effect of activator TMEDA concentration

on the swelling ratio of SAPs was studied. The re-

sults are presented in Fig. 12 for AAM–NMAcrosslinked copolymers. The figure indicates that

the swelling ratio of the SAPs was influenced to

a great extent with the variation of TMEDA con-

centration. It is found that the swelling ratio in-

creases with increase of TMEDA concentration.

When the concentration of activator increased

from 8.60 · 10�6 to 6.88 · 10�5 mol, the swelling

ratio of the BDDA crosslinked copolymers in-creased from 121.88 to 296.93 g/g. In the case of

EGDMA crosslinked copolymers, the swelling

ratio increased from 167.50 to 552.19 g/g as

TMEDA concentration increases from 1.72 ·10�5 to 1.20 · 10�4 mol and further increase in

the concentration of TMEDA decreases the swell-

ing ratio.

Table 3 demonstrates the reaction parametersof the AAM–NMA superabsorbent copolymers

prepared at different concentration of crosslinker,

initiator, and activator.

3.3. Effect of pH on swelling ratio of SAPs

Buffer solution 1 was prepared by mixing 12.3 g

of anhydrous boric acid (0.20 M) and 10.51 g ofcitric acid (0.05 M) in 1000 ml of distilled water

and buffer solution 2 was prepared with 38.01 g

of tri-sodium phosphate in 1000 ml of distilled

water. In order to prepare a specific buffer solu-

tion, pH solutions 1 and 2 were mixed at different

volumes based on Shugar and Dean [49]. Table 4

gives the preparative details of buffer solutions

with their ionic strengths along with the influenceof pH solution (2–12) on the swelling ratio of the

n

) EGDMA (mol) APS (mol) TMEDA (mol)

– 2.19 · 10�4 8.60 · 10�5

– – 8.60 · 10�5

– 1.09 · 10�4 –

– 2.19 · 10�4 8.60 · 10�5

1.26 · 10�5 – 8.60 · 10�5

1.26 · 10�5 2.19 · 10�4 –

Table 4

Influence of pH solution (2–12) on the swelling ratio of the copolymers

Desired pH Solution 1 (ml) Solution 2 (ml) Ionic strength

(mol ion dm�3)

Swelling ratio of BDDA

copolymer (g/g)

Swelling ratio of EGDMA

copolymer (g/g)

2 97.50 2.50 0.1866 02.05 19.20

3 88.00 12.00 0.1762 05.95 25.08

5 67.00 33.00 0.1521 32.60 38.17

7 49.50 50.50 0.1243 47.23 62.44

11 22.00 78.00 0.0886 Viscous liquid Viscous liquid

12 8.50 91.50 0.0711 Viscous liquid Viscous liquid


copolymers. From the table, it is clear that the

swelling behaviour of the copolymers is dependent

on the pH of solution.

4. Conclusion

The AAM–NMA superabsorbent copolymers

were successfully synthesized in high concentrated

aqueous solutions under normal conditions of

atmosphere at room temperature. The effect of

NMA content on the swelling behaviour of the

AAM–NMA copolymers was studied in detail at

different temperatures and the swelling/diffusioncharacteristic of the SAPs were evaluated at

25 �C. The effect of reaction parameters, such as,

concentrations of comonomer (NMA), cross-

linker, initiator and activator on the swelling

behaviour of the AAM–NMA copolymers was

investigated. The main conclusions of the present

study are summarized as follows:

� The swelling ratio of the copolymers increased

with increase of temperature in all the cases.

The monomer composition at 1.40 · 10�2

(AAM), 9.25 · 10�3 mol (NMA) shows the

maximum swelling ratio and the value is in close

agreement with the pure NMA SAP. The effect

of saline solution on the swelling ratio of the

copolymers with different NMA compositionswas evaluated and the results shows the swelling

ratio decreases when the saline concentration

increases. The de-swelling behaviour of the

copolymers was also investigated.

� The effect of crosslinker concentration on the

swelling ratio was studied in detail. The opti-

mized concentration for higher swelling ratio

values was found to be 4.032 · 10�4 and

7.05 · 10�4 mol, for BDDA and EGDMA

crosslinkers, respectively. The morphological

changes in the SAP structure with different con-

centrations of crosslinkers are in good agree-

ment with the swelling behaviour of thecopolymers.

� The optimized condition values of initiator and

activator for getting high swelling ratio were

found to be 2.19 · 10�4 and 6.88 · 10�5 mol

for BDDA crosslinked copolymer, 8.76 · 10�5

and 1.20 · 10�4 mol for EGDMA crosslinked

copolymer, respectively.

� The higher swelling ratios were observed at pH7 for both crosslinked copolymers.

References

[1] F.L. Buchholz, Preparation methods of superabsorbent

polyacrylates, in: F.L. Buchholz, N.A. Peppass (Eds.),

Superabsorbent Polymers: Science and Technology, ACS

Symposium Series, vol. 573, American Chemical Society,

Washington, DC, 1994, p. 27.

[2] O. Wichterle, in: H.F. Mark, N.G. Gaylord, N.M. Bikales

(Eds.), Encyclopedia of Polymer Science & Technology,

vol. 15, Interscience, New York, 1971, p. 273.

[3] K. Mohana Raju, M. Padmanabha Raju, Y. Murali

Mohan, J. Appl. Polym. Sci. 85 (2002) 1795.


Mohan, Polym. Int. 52 (2003) 768.

[5] K. Hogari, F. Ashiya, Advances in Superabsorbent Poly-

mers, American Chemical Society, Washington, DC, 1994.

[6] T. Sakiyama, C.H. Chu, T. Fujii, T. Yano, J. Appl.

Polym. Sci. 50 (1993) 2021.

[7] T. Shiga, Y. Hirose, A. Okada, T. Kurauchi, J. Appl.

Polym. Sci. 44 (1992) 249.

[8] T.K. Kobayashi, J. Appl. Polym. Sci. 36 (1987) 1312.


[9] M. Yoshida, M. Asano, M. Kumarakura, Eur. Polym.

J. 25 (1989) 1197.

[10] E. Karadag, D. Saraydin, H.N. Oztop, O. Guven, Polym.

Adv. Technol. 5 (1994) 664.

[11] E. Karadag, D. Saraydin, O. Guven, J. Appl. Polym. Sci.

61 (1996) 2367.

[12] S. Oren, T. Caykara, O. Kantoglu, O. Guven, J. Appl.

Polym. Sci. 78 (2000) 2219.

[13] K.S. Kazanskii, S.A. Dubrouskii, Adv. Polym. Sci. 104

(1992) 97.

[14] D. Saraydin, E. Karadag, O. Guven, Polym. Adv. Tech. 6

(1994) 719.

[15] E. Karadag, D. Saraydin, S. Cetinkaya, O. Guven,

Bimaterials 17 (1996) 67.

[16] S.H. Kim, C.Y. Won, C.C. Chu, J. Biomad. Mater. Res.

46 (1999) 160.

[17] X. Li, Y. Huang, J. Xiao, C. Yan, J. Appl. Polym. Sci. 55

(1995) 1779.

[18] B. Yildiz, B. Isik, M. Kis, Polymer 42 (2001) 2521.

[19] B. Yildiz, B. Isik, M. Kis, React. Funct. Polym. 52 (2002)

3.

[20] J.-H. Wu, J.-M. Lin, G.-Q. Li, Polym. Int. 50 (2001) 1050.

[21] J.-M. Lin, J.-H. Wu, Z.-F. Yang, M.-L. Pu, Macromol.

Rapid. Commun. 22 (2001) 422.

[22] J. Wu, Y. Wui, J.-M. Lin, S. Lin, Polym. Int. 52 (2003)

1909.

[23] K. Kabiri, H. Omidian, M.J. Zohuriaan-Mehr, Polym. Int.

52 (2003) 1158.

[24] K. Kabiri, H. Omidian, S.A. Hashemi, M.J. Zohuriaan-

Mehr, Eur. Polym. J. 39 (2003) 1341.

[25] J.-T. Zhang, S.-X. Cheng, R.-X. Zhuo, J. Polym. Sci.

Polym. Chem. 41 (2003) 2390.

[26] S. Kiatkamjorwong, P. Phunchareon, J. Appl. Polym. Sci.

72 (1999) 1349.

[27] W.F. Lee, R.J. Wu, J. Appl. Polym. Sci. 62 (1996)

1099.

[28] W.F. Lee, R.J. Wu, J. Appl. Polym. Sci. 64 (1997) 1702.

[29] W.F. Lee, P.L. Yeh, J. Appl. Polym. Sci. 64 (1997)

2371.

[30] W.F. Lee, C.H. Hue, J. Appl. Polym. Sci. 69 (1998)

229.

[31] E.S. Rufino, E.E.C. Monterio, Polymer 41 (2000) 4213.

[32] W.J. Zhou, K.J. Yao, M.J. Kurth, J. Appl. Polym. Sci. 62

(1996) 911.

[33] W.J. Zhou, K.J. Yao, M.J. Kurth, J. Appl. Polym. Sci. 64

(1997) 1001.

[34] S.K. Bajpai, J. Appl. Polym. Sci. 80 (2001) 2782.

[35] S.K. Bajpai, J. Sonkusley, J. Appl. Polym. Sci. 83 (2002)

1717.

[36] E. Karadag, D. Saraydin, Polym. Bull. 48 (2002) 299.

[37] B. Isik, J. Appl. Polym. Sci. 91 (2004) 1289.

[38] B.J. Tighe, Brit. Polym. J. 18 (1986) 8.

[39] E. Karadag, D. Saraydin, O. Guven, Macromol. Mater.

Eng. 286 (2001) 42.

[40] R.A. Siegel, B.A. Firestone, Macromolecules 21 (1988)

3254.


Mohan, Int. J. Polym. Mater. 53 (2004) 419.

[42] Y. Murali Mohan, P.S. Keshava Murthy, K. Madhusudh-

ana Rao, J. Sreeramulu, K. Mohana Raju, Int. J. Polym.

Mater. (in press).

[43] C. Peniche, H.N. Cohen, B. Vazquez, J.S. Romen, Polymer

38 (1997) 5977.

[44] D. Saraydin, H.N. Oztop, E. Karadag, Y. Caldiran,

O. Guven, Appl. Biochem. Biotechnol. 82 (1999) 115.

[45] K.J. Yao, W.J. Zhou, J. Appl. Polym. Sci. 53 (1994) 1533.

[46] N.A. Peppas, N.M. Franson, J. Polym. Sci. Part B: Polym.

Phys. 21 (1983) 983.

[47] C.R.R. Robert, P.A. Buri, N.A. Peppas, J. Appl. Polym.

Sci. 30 (1985) 301.

[48] H. Omidian, S.A. Hashemi, P.G. Sammes, I.G. Meldrum,

Polymer 40 (1999) 1753.

[49] G.J. Shugar, J.A. Dean, The Chemist�s Ready Handbook,

McGraw-Hill, New York, 1990.

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