Synthesis, characterization and effect of reaction parameters on swelling properties of...
Transcript of 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.
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.
14 Y. Murali Mohan et al. / Reactive & Functional Polymers 63 (2005) 11–26
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.
Y. Murali Mohan et al. / Reactive & Functional Polymers 63 (2005) 11–26 15
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)
EGDMA2 EGDMA4 EGDMA5 EGDMA6 EGDMA7 EGDMA8
0 500 1000 1500 2000 2500 3000 35000
50
100
150
200
250
300
350
400Temperature 25˚C
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)
EGDMA2 EGDMA3 EGDMA4 EGDMA5 EGDMA6 EGDMA8
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
150Temperature 20˚C
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
300Temperature 25˚C
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
500Temperature 50˚C
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.
Y. Murali Mohan et al. / Reactive & Functional Polymers 63 (2005) 11–26 17
F
N
0 500 1000 1500 2000 2500 3000
0
1
2
3
4
S/t
Time (min)
BDDA1 BDDA2 BDDA3 BDDA4 BDDA5 BDDA6 BDDA7 BDDA8
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.
18 Y. Murali Mohan et al. / Reactive & Functional Polymers 63 (2005) 11–26
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
BDDA1 BDDA2 BDDA3 BDDA4 BDDA5 BDDA6 BDDA7 BDDA8
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
copolymers crosslinked with (a) BDDA and (b) EGDMA.
Y. Murali Mohan et al. / Reactive & Functional Polymers 63 (2005) 11–26 19
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.
20 Y. Murali Mohan et al. / Reactive & Functional Polymers 63 (2005) 11–26
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
copolymers crosslinked with (a) BDDA and (b) EGDMA.
Y. Murali Mohan et al. / Reactive & Functional Polymers 63 (2005) 11–26 21
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.
22 Y. Murali Mohan et al. / Reactive & Functional Polymers 63 (2005) 11–26
Fig. 10. SEM micrographs of AAM–NMA superabsorbent copolymers crosslinked with EGDMA.
Y. Murali Mohan et al. / Reactive & Functional Polymers 63 (2005) 11–26 23
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.
24 Y. Murali Mohan et al. / Reactive & Functional Polymers 63 (2005) 11–26
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
Y. Murali Mohan et al. / Reactive & Functional Polymers 63 (2005) 11–26 25
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.
[4] K. Mohana Raju, M. Padmanabha Raju, Y. Murali
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.
26 Y. Murali Mohan et al. / Reactive & Functional Polymers 63 (2005) 11–26
[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.
[41] K. Mohana Raju, M. Padmanabha Raju, Y. Murali
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.