Synthesis and characterization of chitosan-g-poly(acrylic acid)/attapulgite superabsorbent composites
Junping Zhang a,b, Qin Wang a,b, Aiqin Wang a,*
a Center of Eco-material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, PR China
b Graduate University of the Chinese Academy of Sciences, Beijing, 100049, PR China
Received 8 July 2006; received in revised form 15 November 2006; accepted 29 November 2006
Available online 17 January 2007
Abstract
A novel chitosan-g-poly(acrylic acid)/attapulgite superabsorbent composite with water absorbency of 159.6 g g1 in distilled water
and 42.3 g g1 in 0.9 wt% NaCl solution was prepared by graft polymerization with chitosan, acrylic acid and attapulgite in aqueous
solution, using N,N0-methylenebisacrylamide as a crosslinker and ammonium persulfate as an initiator. Factors influencing water absorbency
of the superabsorbent composite were investigated, such as average molecular weight of chitosan, weight ratio of acrylic acid to
chitosan, dewatering method, the amount of crosslinker and attapulgite. The result from FTIR spectra showed that –OH of attapulgite,
–OH, –NHCO and –NH2 of chitosan participated in graft polymerization with acrylic acid. The introduced attapulgite enhanced thermal
stability of the chitosan-g-poly(acrylic acid) superabsorbent and formed a loose and more porous surface. Introducing a small amount of
attapulgite also enhanced water absorbency of the chitosan-g-poly(acrylic acid) superabsorbent.
2006 Elsevier Ltd. All rights reserved.
Keywords: Superabsorbent composite; Water absorbency; Attapulgite; Chitosan
1. Introduction
Superabsorbents are crosslinked networks of hydrophilic
polymers that can absorb and retain a lot of aqueous fluids,
with the absorbed water hardly removable even under
some pressure. Due to their excellent properties relative to
traditional water absorbing materials (such as sponge, cotton
and pulp, etc.), superabsorbents are widely used in
many fields, such as hygienic products, horticulture, gel
actuators, drug-delivery systems and coal dewatering
(Buchholz & Graham, 1998; Dorkoosh et al., 2000; Ende,
Hariharan, & Peppas, 1995; Raju, Raju, & Mohan, 2003;
Shiga, Hirose, Okada, & Kurauchi, 1992). However, most
of these examples are synthetic polymers, which are poor in
degradability, with potential for inherent environmental
issues.
Extensive attention has been directed toward superabsorbent
polymers prepared through graft copolymerization
of vinyl monomers onto the chain of such natural polymers
as starch (Kiatkamjornwong, Mongkolsawat, & Sonsuk,
2002), cellulose (Farag & Al-Afaleq, 2002) and chitosan
(Mahdavinia, Pourjavadi, Hosseinzadeh, & Zohuriaan,
2004). Chitosan, a high molecular weight polysaccharide
from chitin, is one of the most abundant biomass sources
in the world. Reactive –NH2 and –OH of chitosan are convenient
for graft polymerization of hydrophilic vinyl
monomers, making this is an efficient way to acquire
hydrogels with novel properties. It has been reported previously
that superabsorbent from chitosan has antibacterial
activities and is thus suitable in infant diapers, feminine
hygiene products and other special fields (Dutkiewicz,
2002; No, Park, Lee, Hwang, & Meyers, 2002). Chitosan
has been widely used in the fabrication of biomedical materials
owing to its biocompatibility and antibacterial properties
(Chen & Tan, 2006). Thus, a novel superabsorbent
polymer prepared through graft polymerization of acrylic
0144-8617/$ - see front matter 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbpol.2006.11.018
* Corresponding author. Tel.: +86 931 4968118; fax: +86 931 8277088.
E-mail address: aqwang@lzb.ac.cn (A. Wang).
www.elsevier.com/locate/carbpol
Carbohydrate Polymers 68 (2007) 367–374
acid onto the chitosan chain, not only improves biodegradability
of its corresponding superabsorbent materials, but
also reduces dependence on petrochemical-derived
monomers.
Recently, clay has become the focus for the preparation
of superabsorbent composite in order to improve swelling
properties, enhance gel strength and reduce production
cost of corresponding superabsorbents. Clays, including
montmorillonite (Kabiri & Zohuriaan-Mehr, 2004), kaolin
(Wu, Wei, & Lin, 2003), mica (Lee & Chen, 2005; Lin, Wu,
Yang, & Pu, 2001), attapulgite (Li & Wang, 2005) and sericite
(Wu, Lin, Zhou, & Wei, 2000), have already been
incorporated into poly(acrylic acid) and polyacrylamide
polymeric network. Attapulgite, a kind of hydrated octahedral
layered magnesium aluminum silicate absorbent mineral
(schematic structure was shown in Fig. 1(a)), has
exchangeable cations and reactive –OH groups on its surface
(Neaman & Singer, 2004). On the basis of our previous
work with superabsorbent composite (Li, Liu, & Wang,
2005; Li & Wang, 2005; Li, Wang, & Chen, 2004; Zhang,
Chen, & Wang, 2005; Zhang, Li, & Wang, 2006) and chitosan
(Sun & Wang, 2006; Sun & Wang, 2006), a novel chitosan-
g-poly(acrylic acid)/attapulgite superabsorbent
composite was synthesized. The graft reaction mechanism,
thermal stability, surface morphology and factors influencing
water absorbency of the composite were investigated in
this paper.
2. Experimental
2.1. Materials
Acrylic acid (AA, distilled under reduced pressure
before use), ammonium persulfate (APS, recrystallized
from distilled water before use) and N,N0-methylenebisacrylamide
(MBA, used as received) were supplied by
Shanghai Reagent Corp. (Shanghai, China). Chitosan
(CTS, degree of deacetylation is 0.85, average molecular
weight is 90 · 104) was supplied by Zhejiang Yuhuan
Ocean Biology Co. (Zhejiang, China). Attapulgite (APT,
supplied by Linze Colloidal Co., Gansu, China) was milled
through a 320-mesh screen and treated with 37% hydrochloric
acid for 72 h, followed by washing with distilled water
until pH = 6 was achieved, and then dried at 105 C for
8 h before use. Other agents used were all of analytical
grade and all solutions were prepared with distilled
water.
2.2. Preparation of CTS with different average molecular
weight
CTS (2.0 g) was dispersed in 40 ml distilled water, and
then specific volumes of 30% H2O2 solution (0.3, 1.0, 3.7,
9.4 ml, respectively) were added to the suspension. Each
suspension was stirred and kept at 50 C for 2 h. After
reaction, the solution was filtrated. The collected solid
was washed with distilled water to pH = 7, and then dried
under vacuum at 50 C to a constant weight. Average
molecular weight of CTS was determined by viscometry
measurement (Kubota, Tatsumoto, Sano, & Toya, 2000).
2.3. Preparation of chitosan-g-poly(acrylic acid)/attapulgite
(CTS-g-PAA/APT) superabsorbent composites
A series of superabsorbent composites from CTS, AA,
and APT were synthesized according to the following procedure;
an appropriate amount of CTS was dissolved in
30 ml acetic acid solution (1%) in a 250 ml four-neck flask,
equipped with a mechanical stirrer, a reflux condenser, a
funnel and a nitrogen line. After being purged with nitrogen
for 30 min to remove the oxygen dissolved from the
system, the solution was heated to 60 C, and then 0.10 g
APS was introduced to initiate CTS to generate radicals.
Ten minutes later, the mixed solution of 3.55 g AA, specific
amounts of MBA and APT (as noted) were added. The
water bath was kept at 60 C for 3 h. The resulting granular
product (as shown in Fig. 1(b)) was transferred into
sodium hydroxide aqueous solution (1 M) to be neutralized
to pH = 7, and then dehydrated through oven drying or
with various dewatering agents including methanol, ethanol
and acetone. After wiping off excessive dewatering
agents on the surface using filter paper, the samples were
spread on a dish to dry overnight at room temperature.
Fig. 1. Schematic structure of APT (a) and digital photo of granular CTS-g-PAA/APT superabsorbent composite (b).
368 J. Zhang et al. / Carbohydrate Polymers 68 (2007) 367–374
The product was milled and all samples used for test had a
particle size in the range of 40–80 mesh.
CTS-g-PAA (without APT) was prepared according to
the same procedure to study the effect of introduced APT
on properties of the superabsorbent composite. Uncrosslinked
CTS-g-PAA/APT (without MBA) was exhaustively
extracted with distilled water and ethanol to be free from
homopolymer in order to investigate the graft polymerization
mechanism.
2.4. Measurement of water absorbency
Sample (0.05 g) was immersed in excess distilled water
(500 ml) at room temperature for 8 h to reach swelling
equilibrium. Swollen samples were then separated from
unabsorbed water by filtering through a 100-mesh screen
under gravity for 30 min with no blotting of samples.
Water absorbency in distilled water of the superabsorbent
composite, Qeq, was calculated using the following
equation:
Qeq ¼
m2 m1
m1
ð1Þ
where m1 and m2 are the weights of the dry sample and the
swollen sample, respectively. Qeq is calculated as grams of
water per gram of sample. Water absorbency of the sample
in 0.9 wt% NaCl solution, Qeq’, was tested according to the
same procedure.
2.5. Characterization
IR spectra of samples as KBr pellets were taken using a
Thermo Nicolet NEXUS TM spectrophotometer. The
micrographs of samples were obtained using Scanning
Electron Microscopy (SEM), (JSM-5600LV, JEOL, Ltd.).
Before SEM observation, all samples were fixed on aluminum
stubs and coated with gold. Thermal stability of
samples was studied on a Perkin–Elmer TGA-7 thermogravimetric
analyzer (Perkin–Elmer Cetus Instruments,
Norwalk, CT), with a temperature range of 25–700 C at
a heating rate of 10 C min1 using dry nitrogen purge at
a flow rate of 50 ml min1.
3. Results and discussion
3.1. IR spectra
IR spectra of APT, uncrosslinked CTS-g-PAA/APT,
CTS, CTS-g-PAA and CTS-g-PAA/APT are shown in
Fig. 2. As can be seen, intensity of the absorption bands
at 3616 cm1 and 3544 cm1 ascribed to –OH of APT
was decreased in spectrum of uncrosslinked CTS-g-PAA/
APT comparing with Fig. 2(a). A series of new absorption
bands at 2928 cm1, 1712 cm1, 1456 cm1 and 1404 cm1
ascribed to C–H stretching, –COOH stretching, symmetric
–COO stretching and C–H bending appeared in Fig. 2(b).
The information from Fig. 2(a) and (b) indicates the participation
of –OH group of APT in the graft reaction between
APT and AA. As can be seen from Fig. 2(c), the absorption
bands at 1647 cm1, 1598 cm1, 1380 cm1, 1094 cm1 and
1037 cm1 are ascribed to C@O of amide I, –NH2, –NHCO
of amide III, C3–OH and C6–OH of CTS, respectively.
However, the absorption bands of N-H (1598 cm1 and
1380 cm1) and C3–OH (1094 cm1) disappeared after the
reaction with AA as shown in Fig. 2(d). This information
reveals that –NH2, –NHCO and –OH of CTS took part
in graft reaction with AA. The absorption band at
1647 cm1 (C@O of amide I) was overlapped by asymmetric
–COO stretching and resulted in a broad absorption
band in the range of 1550 cm1 1650 cm1. The new
Fig. 2. IR spectra of (a) APT, (b) uncrosslinked CTS-g-PAA/APT, (c) CTS, (d) CTS-g-PAA and (e) CTS-g-PAA/APT. Weight ratio of AA to CTS is 7.2;
average molecular weight of CTS is 22.9 · 104; MBA content is 2.94 wt%; APT content is 10 wt%; dewatered with methanol.
J. Zhang et al. / Carbohydrate Polymers 68 (2007) 367–374 369
absorption bands at 1456 cm1 (C–H), 1405 cm1 (symmetric
–COO stretching), 1169 cm1 and 1074 cm1 indicate
the existence of PAA chains. After incorporating APT
into the polymeric network, intensity of absorption band at
1576 cm1 ascribed to asymmetric –COO stretching
increased, which indicates that the chemical environment
of –COO has changed, which may have some influence
on the absorbing ability of the corresponding superabsorbent
composite. Absorption bands of APT at 1030 cm1
and 988 cm1 ascribed to Si–OH also appeared in
Fig. 2(e), which shows the existence of APT in the composite.
It can be concluded from Fig. 2 that graft reaction has
taken place among AA, APT and CTS.
3.2. Thermal stability
The effect of introduced APT on thermal stability of
CTS-g-PAA was investigated by TGA in this section.
TGA curves of CTS-g-PAA and CTS-g-PAA/APT incorporated
with 10 wt% APT were shown in Fig. 3. As can
be seen from Fig. 3, both CTS-g-PAA and CTS-g-PAA/
APT exhibit a three-stage thermal decomposition process.
As the temperature increased to 381.7 C, the weight of
samples decreased gradually implying a loss of moisture,
dehydration of saccharide rings and breaking of C–O–C
glycosidic bonds in the main chain of CTS (Douglas &
Sergio, 2004). There is a sharp weight loss with increasing
temperature from 381.7 to 391.9 C and 21% of sample
was lost in this temperature range. There was no obvious
difference between CTS-g-PAA and CTS-g-PAA/APT as
the temperature was below 391.9 C. With further increasing
temperature to 501.6 C, CTS-g-PAA exhibits a second
step decomposition implying the decomposition of carboxyl
groups of PAA chains. Similar thermal behavior has
been reported by Chen et al. for carboxymethylchitosang-
poly(acrylic acid) (Chen & Tan, 2006). During this period,
the onset of CTS-g-PAA was at 470.9 C, however, the
onset of CTS-g-PAA/APT was not obvious. The sharp
weight losses of CTS-g-PAA and CTS-g-PAA/APT at
578.4 and 604.3 C, respectively, are suggested to be due
to the thermal decomposition of the PAA chain backbone.
As can be seen, CTS-g-PAA/APT showed a lower
weight loss rate and smaller total weight loss within the
temperature of 391.9–700 C comparing with CTS-g-
PAA. This result indicates that the incorporation of APT
is helpful for the improvement of thermal stability of
CTS-g-PAA. The role of APT in the polymeric network
may be the main reason for the difference in TGA curves.
APT acts as heat barrier, thus delaying the diffusion of volatile
thermo-oxidation products to gas, and gas to the composite.
This enhances thermal stability of the system.
Similar effect of clay on thermal stability of composite
materials has been reported previously (Ray & Okamoto,
2003).
3.3. Morphological analysis
SEM micrographs of APT, CTS-g-PAA and CTS-g-
PAA/APT superabsorbent composite were observed and
are shown in Fig. 4. As can be seen, APT shows a fibrous
surface. CTS-g-PAA shows a tight surface, however, the
introduction of APT forms a relatively loose and fibrous
surface. This surface morphology change by introducing
APT may influence the penetration of water into the polymeric
network, and then may has some influence on swelling
ability of corresponding superabsorbent composites.
3.4. Effect of average molecular weight of CTS on water
absorbency
Many previously reports from the literature focused on
the effects of external factors, such as initiator, monomer
concentration and ratio of CTS to monomer, on water
absorbency and graft polymerization between CTS and
monomers (Chen & Tan, 2006; Ge, Pang, & Luo, 2006;
Huang, Jin, Li, & Fang, 2006). No information about the
effect of average molecular weight of CTS on water absorbency
can be seen, to the best of our knowledge. The effect
of this factor was investigated in this section and shown in
Fig. 5. Water absorbency of CTS-g-PAA/APT in distilled
water and in 0.9 wt% NaCl solution increased evidently
with decreasing average molecular weight of CTS. This
may be attributed to the fact that CTS solution of smaller
average molecular weight has lower viscosity, which would
facilitate the penetration of AA to CTS and enhance graft
efficiency, and then the improvement of water absorbency.
CTS of higher average molecular weight could restrict the
graft reaction by factors such as steric hindrance, and then
the decrease of water absorbency.
3.5. Effect of MBA content on water absorbency
According to Flory’s network theory (Flory, 1953),
crosslinking density is a key factor influencing water absorbency
of superabsorbents and water absorbency is in inverse
proportion to crosslinking density. Water absorbency can
150 300 450 600 750
20
40
60
80
100
CTS-g-PAA/APT
CTS-g-PAA
Weight ( )
Temperature (oC)
Fig. 3. TGA curves of CTS-g-PAA and CTS-g-PAA/APT. Weight ratio
of AA to CTS is 7.2; average molecular weight of CTS is 22.9 · 104; MBA
content is 2.94 wt%; APT content is 10 wt%; dewatered with methanol.
370 J. Zhang et al. / Carbohydrate Polymers 68 (2007) 367–374
