Shimaa Mohamed Elsaeed1,2, Elsayed Gamal Zaki1, Ahmed Abdelhafes3, Ayman S Al-Hussaini3. 1. Egyptian Petroleum Research Institute, P.O. Box 11727, Nasr City, Cairo 11727, Egypt. 2. National Committee of Women in Science (NCWS), ASRT, Cairo 11334, Egypt. 3. Chemistry Department, Faculty of Science, Port Said University, P.O. Box 42521, Port Said 42521, Egypt.
Abstract
Water shortages have become a serious issue, so the importance of developing innovative cellulose-based superabsorbent polymer (SAP) was experimentally assessed as an environmentally friendly alternative to acrylate-based SAPs for the optimization of water consumption. The development of a biodegradable superabsorbent hydrogel composite based on a graft copolymer of carboxymethyl cellulose (CMC) and mixtures of different comonomers such as an acrylamide-co-acrylic acid-co-2-acrylamido-2-methylpropanesulfonic acid (Am-co-AA-co-AMPS) CMC-g-TerPoly interpenetrating network was characterized by infrared spectroscopy (FT-IR), scan electron microscopy (SEM), atomic force microscope (AFM), thermal gravimetric analysis (TGA), and swelling capacity in different aqueous media. The optimized CMC-g-(TerPoly) composite showing outstanding superabsorbance with high water retention, the ratio of constituents, temperature, and pH effect on equilibrium swelling have been optimized by using multistage response surface methodology (RSM). In distilled water (d-water), the equilibrium water absorption capacity (EW) of the synthesized composite hydrogels (SAP-IPN1) is 1200 g (d-water/1g hydrogel) which is superior to any other commercial polyacrylate SAP hydrogels, while in saline water the EW is 650 g (s-water/1g hydrogel). The performance of the SAP-IPN for water retention was evaluated by studying several swelling/deswelling cycle measurements. The prepared SAP-IPN hydrogels were found to show pH and salt solution dependence on the swelling behavior. The new SAP-IPN can work with commercial SAP, which is recommended for application as a water reservoir in arid land for irrigation for agriculture purposes.
Water shortages have become a serious issue, so the importance of developing innovative cellulose-based superabsorbent polymer (SAP) was experimentally assessed as an environmentally friendly alternative to acrylate-based SAPs for the optimization of water consumption. The development of a biodegradable superabsorbent hydrogel composite based on a graft copolymer of carboxymethyl cellulose (CMC) and mixtures of different comonomers such as an acrylamide-co-acrylic acid-co-2-acrylamido-2-methylpropanesulfonic acid (Am-co-AA-co-AMPS) CMC-g-TerPoly interpenetrating network was characterized by infrared spectroscopy (FT-IR), scan electron microscopy (SEM), atomic force microscope (AFM), thermal gravimetric analysis (TGA), and swelling capacity in different aqueous media. The optimized CMC-g-(TerPoly) composite showing outstanding superabsorbance with high water retention, the ratio of constituents, temperature, and pH effect on equilibrium swelling have been optimized by using multistage response surface methodology (RSM). In distilled water (d-water), the equilibrium water absorption capacity (EW) of the synthesized composite hydrogels (SAP-IPN1) is 1200 g (d-water/1g hydrogel) which is superior to any other commercial polyacrylate SAP hydrogels, while in saline water the EW is 650 g (s-water/1g hydrogel). The performance of the SAP-IPN for water retention was evaluated by studying several swelling/deswelling cycle measurements. The prepared SAP-IPN hydrogels were found to show pH and salt solution dependence on the swelling behavior. The new SAP-IPN can work with commercial SAP, which is recommended for application as a water reservoir in arid land for irrigation for agriculture purposes.
The deficiency of water and desertification are serious problems
in numerous regions of the world, and a serious problem is the concession
of the development of agriculture. Egypt suffers from a deficiency
of water, and we must find a tool to save water in order to save lives
and to achieve sustainable development of agricultural goals. Recently,
the production of eco-friendly, low cost, and biodegradable materials
based on natural raw material alternating with petroleum synthetic
sources that affect the environment has been studied.Superabsorbent
hydrogels were well-defined as three-dimensional
networks of hydrophilic polymers fabricated by physical and/or chemical
cross-linking. SAPs can absorb more than a thousand times their original
weight, and they are able to retain and hold water or aqueous solution
when compared with other normal absorbents even under difficult conditions,[1] which reduces the need for irrigation and consumption
of water.Use in agriculture,[2,3] wastewater
treatment,[4−6] biosensors, drug release,[7] self-healing,[8] tissue engineering,[9] adsorbents for organic[10] and inorganic
pollutants,[11,12] and enhanced oil recovery[13,14] are just a few of the applications for superabsorbent hydrogels.
Generally, superabsorbent hydrogel based natural polymers such as
chitosan,[15] cellulose,[16] starch,[17] collagen,[9] and sodium alginate[18] and their substitutes are more interesting because of their eco-friendliness,
low cost, nontoxicity, biodegradability, and high hydrophilic network.[19] However, most of the SAPs existing on the market
are mostly based on acrylate polymer from petroleum materials. The
use of SAPs for agricultural applications has revealed encouraging
results toward reduction of irrigation water consumption and improvement
of water retention in soil.Many research efforts have been
concerned with the incorporation
of inorganic clay for the preparation of superabsorbent hydrogel composites[2] such as kaolin,[3] bentonite,
attapulgite,[4] montmorillonite (MMT),[5] and sodium silicate,[6] which enhance the swelling absorbance, thermal and mechanical stability,
and other properties of SAP hydrogels and generate a new class of
SAP for unique applications. Bentonite (BN)[7] is composed of aluminum silicate layered with exchangeable cations
and −OH groups on the surface. BN has excellent absorption
properties, which lead to improvement in the swelling and absorption
of a water molecule into an SAP hydrogel.[8]The objective of our work was to prepare an eco-friendly renewable
superabsorbent based on a CMC-grafted-AA-co-AMPS-co-Am (CMC-g-TerPol) interpenetrating terpolymer
network (IPN) hydrogel containing bentonite (BN) with better swelling,
high water capacity, and reusability abilities that could compete
with commercial SAPs for water storage and be used to save water and
the environment. The swelling properties of the hydrogels in different
solutions (water, NaCl) and different pH were thoroughly studied.
The SAP-IPN hydrogels synthesized were characterized through infrared
spectroscopy, scanning electron microscopy, atomic force spectroscopy,
and thermal analysis of these hydrogel systems and evaluated. Water
retention of prepared SAP-IPN hydrogels was evaluated.
Experimental Section
Materials
Carboxy
methyl cellulose
(CMC), acrylamide (AM), acrylic acid (AA), potassium persulfate (KPS),
acrylamide-co-2-acrylamido-2-methylpropanesulfonic
acid (AMPS), bentonite (Al2H2Na2O13Si4)BN, N,N′
methylene bis(acrylamide) (MBA), NaOH, ethanol, and methanol were
acquired from Sigma-Aldrich.
CMC-g-TerPols
Superabsorbent
Hydrogel Preparation
The CMC-g-TerPols superabsorbent
hydrogel was synthesized using a free-radical polymerization process.
A particular amount of d-water (100 mL) was required to dissolve 5
g of CMC. After that, an appropriate amount of bentonite BN (0, 5,10,
15 wt percent) was added, and the mixture was sonicated for 20 min
to generate a homogeneous dispersed solution. AA, Am, and AMPS were
consistently neutralized by adding NaOH with a concentration of 4
mol/L solution. KPS (0.8 g) in d-water was added to the mixture when
the temperature reached 70 °C. MBA (0.06 g) in d-water was added
to the entire mixture with a purge of N2 after mixing for
15 min and lowering the temperature to 40 °C. To obtain CMC-g-(TerPols)/bentonite, (SAP-IPN1), the water bath was held
at 70 °C to complete the polymerization reaction. To eliminate
the unreacted monomers and other contaminants, this gel was broken
into small pieces and washed with a sufficient ratio of distilled
water and ethanol (60:40 mL). The SAP-IPN1 was produced and dried
in a 60 °C oven overnight. As previously indicated, a comparable
approach to make CMC-g-(TerPols) (SAP-IPN2) superabsorbent
hydrogel without adding bentonite (BN) was needed. Equation was used to obtain the grafting
yield or percentage grafting (G):W0 and W1 represent the weights of pure
CMC and grafting CMC-terpolymer, respectively. Table shows the chemical composition of CMC superabsorbent
hydrogels (1).
Table 1
Chemical Composition of CMC-g-(TerPol)
Superabsorbent Hydrogel Composite
samples
CMC (g)
Ac (ml)
Am (g)
AMPS (g)
BN (%)
KPS (g)
MBA (g)
SAP-IPN1
5
2
2
2
5, 10, 15
0.8
0.06
SAP-IPN2
5
2
2
2
0
0.8
0.06
Characterization of SAP-IPN Hydrogels
The chemical
structures were validated using Fourier transform infrared
spectroscopy (ATI Matson Genesis Series FTIR). The thermal characteristics
of the produced hydrogels were determined by thermogravimetric analysis
of samples (TGA 55, Meslo, USA). Weight loss was plotted versus temperature
after film samples were placed in a pan of platinum and heated in
the range of 30–800 °C under N2 atm at a heating
rate of 10 °C per minute.SEM was used to examine the morphology
of the gel (which had been freeze-dried for 24 h) (SEM). Quanta FEG
250 scanning electron microscopes (FEI Company, USA) were used to
capture surface images of hydrogels at the EDRC, DRC, and Cairo. SEM
stubs were used to mount the samples. The SEM settings used were a
10.1 mm working distance and a 20 kV excitation voltage in the in-lens
detector. AFM is the best-fitted instrument to identify the superficial
structure of topography because it is capable of giving three-dimensional
(3D) images. AFM is performed using a cantilever in static mode. Moreover,
response surface methodology (RSM), an all graphic-based analysis,
was carried out using the ANOVA software program.
Optimization Parameters by Response Surface
Methodology (RSM)
In addition to the individual effects of
amounts of KPS, total monomers, MBA, and CMC on G (%), the simultaneous
effects of these parameters on G (%) should be studied. However, the
one-factor-at-a-time based study of these effects is very tedious,
and thus, RSM, a statistical approach, has been adopted to optimize
the synthesis parameters by performing the minimum number of experimental
runs.
(CMC-TerPols) Superabsorbent Hydrogels Swelling
and Equilibrium Water Absorbance Behavior
Dried SAP-IPN hydrogel
(0.5 g) was immerged over d-water (200 mL) to reach the swelling equilibrium
state (Qeq) to assess equilibrium water
absorbance and swelling of manufactured hydrogel. The solution was
filtered out of the swelled gel, which was then dried. The swelled
gel was then weighed, and eq was used to calculate the equilibrium water absorbancewhere Qeq is the equilibrium water absorption (g/g) or maximum water
absorption capacity (EW) at time t (min), Ws is the swollen hydrogel weight (g), and Wd is the drying hydrogel weight (g).In
saline water (s-water), 0.9 wt % NaCl solution, and different buffer
mediums, the swelling behavior of the CMC-g-(TerPols) superabsorbent
was measured as follows: 100 mL of saline or buffer solution was put
into 500 mL beakers containing 0.5 g of gel. After that, the swollen
gels were filtered, and the swelling behavior of SAP-IPN was estimated
using eq .
Water Retention Capacity of (CMC-TerPols)
Superabsorbent Hydrogels
Water retention capacity (WRC) and
water loss are the key features used to examine SAP-IPN hydrogels’
ability to store water within their network structure.[9] It is very interesting to study the performance of the
SAP-IPN hydrogels through deswelling and how the SAP-IPN hydrogels
peform after some swelling/deswelling cycles. The water retention
for porous SAP-IPN hydrogels prepared was measured by taking about
0.5 g of the dried SAP-IPN hydrogels and then adding in d-water until
maximum swelling was reached.[10] Next, the
swollen hydrogels were set into an oven at 60 °C for different
times. The water retention ratio (WR) of the prepared SAP-IPN hydrogels
was calculated by eq (9)while Wt is the weight of the sample at a deswelling time (t) and Ws is the weight of the
swollen sample.
Result and Discussion
Synthesis of CMC-g-TerPols and Spectral Analysis
The
expected process for graft polymerization and cross-linking
of AA, AMPS, and Am onto CMC chains is shown in Figure . This development was carried out using
APS as the radical initiator and MBA used as the cross-linking agent.
Heating causes APS to disintegrate at 70 °C under an N2 purge to produce sulfate radicals, which grab hydrogen atoms from
the CMC matrix’s −OH groups to form macroradicals. In
this stage, AA, Am, and AMPS monomers can be grafted onto these active
macroradicals and MBA to form a three-dimensional interpenetrating
network-IPN structure by cross-linking monomers with radicals on the
CMC matrix. BN clay was distributed and bonded in a superabsorbent
hydrogel network by forming a 3D-IPN network. By comparison, the FTIR
spectra of CMC, BN, and SAP-IPN/BN, which found BN incorporation into
the superabsorbent IPN, was confirmed.
Figure 1
Scheme of synthesis of
CMC-g-(TerPoly)/BN superabsorbent hydrogel.
The photo was taken by Prof. Shimaa Elsaeed.
Scheme of synthesis of
CMC-g-(TerPoly)/BN superabsorbent hydrogel.
The photo was taken by Prof. Shimaa Elsaeed.Parts a–c of Figure show the FTIR spectra of the CMC-g-(TerPolys)/BN superabsorbent
composite, CMC, and BN clay, respectively. The −OH stretching
of CMC is related to the movement of the peak at 3437.7 cm–1, and the band of CMC at 1000–1166 cm–1 is
weakened due to the contribution of −OH of CMC in reaction.
The peaks lose the −OH stretching of BN at 3400–3700
cm–1, and the intensities of the peaks due to Si–O
and Na–O of BN are weakened as revealed in Figure at 860–920 cm–1. The characteristic bands at 1548, 1670, and 1040 cm–1 are due to the carboxylate anion, carboxamide, and sulfonate group,
respectively. Thus, SAP-IPN1 includes a cross-linked structure of
CMC/BN with carrying carboxylate, carboxamide, and sulfonate groups
in the network.[11]
Figure 2
FTIR spectra of (a) CMC-g-(TerPoly)/BN
hydrogel, (b) CMC, (c) BN.
FTIR spectra of (a) CMC-g-(TerPoly)/BN
hydrogel, (b) CMC, (c) BN.
Response Surface Methodology (RSM)
Optimization of the Synthesis Parameters
The effects
of amounts of KPS, total monomers, MBA, and CMC on
the grafting ratio, G (%), and the simultaneous effects
of these parameters on G (%) should be studied. RSM,
a statistical approach, has been adopted to optimize the synthesis
parameters. Face-centered central composite design (FCCCD), a standard
RSM technique, was adopted to optimize the amounts of KPS (X1, g), total monomers (X2, g), MBA (X3, g), and CMC (X4, g) within 0.4–1.2, 3–9, 0.02–0.1,
and 1–3 g, respectively (Table ), to obtain the hydrogel presenting the maximum G (%) by using an empirical second-order polynomial (eq ) to generate eq .Here, Y, α0, α, α, and α are predicted response, constant,
linear, quadratic, and interaction coefficients, respectively. Moreover,
the input variables, experimental, and software generated responses
are given in Table . The acceptability of the quadratic model was confirmed from the
sequential model sum of squares, model summary statistics tests, lack
of fit tests, and ANOVA (Tables –6). The response surface plots presenting the interactive
effects of X1–X2, X1–X3, X1–X4, X2–X3, X2–X4, and X3–X4 with G (%) are given in Figure a–f. Importantly, the
optimum G (%) = 89.22% was obtained at 0.89, 6.01,
0.05, and 2 g for X1, X2, X3, and X4, respectively.
Table 2
Design
Matrix for Optimization of G (%)
coded
values
uncoded
values
G (%)
runs
X1
X2
X3
X4
KPS (g)
total monomers
(g)
MBA (g)
CMC (g)
experimental
predicted
1
–1
–1
–1
–1
0.40
3.00
0.02
1.00
51.60
40.95
2
1
–1
–1
–1
1.20
3.00
0.02
1.00
35.60
34.35
3
–1
1
–1
–1
0.40
9.00
0.02
1.00
11.00
7.52
4
1
1
–1
–1
1.20
9.00
0.02
1.00
4.40
5.66
5
–1
–1
1
–1
0.40
3.00
0.10
1.00
5.00
17.14
6
1
–1
1
–1
1.20
3.00
0.10
1.00
34.00
23.08
7
–1
1
1
–1
0.40
9.00
0.10
1.00
10.00
–3.45
8
1
1
1
–1
1.20
9.00
0.10
1.00
6.00
7.25
9
–1
–1
–1
1
0.40
3.00
0.02
3.00
11.00
12.58
10
1
–1
–1
1
1.20
3.00
0.02
3.00
4.40
10.73
11
–1
1
–1
1
0.40
9.00
0.02
3.00
28.40
32.19
12
1
1
–1
1
1.20
9.00
0.02
3.00
44.40
35.09
13
–1
–1
1
1
0.40
3.00
0.10
3.00
10.00
1.62
14
1
–1
1
1
1.20
3.00
0.10
3.00
6.00
12.32
15
–1
1
1
1
0.40
9.00
0.10
3.00
30.00
34.08
16
1
1
1
1
1.20
9.00
0.10
3.00
46.00
49.53
17
–1
0
0
0
0.40
6.00
0.06
2.00
69.20
83.57
18
1
0
0
0
1.20
6.00
0.06
2.00
85.20
87.99
19
0
–1
0
0
0.80
3.00
0.06
2.00
22.80
27.63
20
0
1
0
0
0.80
9.00
0.06
2.00
17.20
29.52
21
0
0
–1
0
0.80
6.00
0.02
2.00
76.40
88.12
22
0
0
1
0
0.80
6.00
0.10
2.00
78.00
83.43
23
0
0
0
–1
0.80
6.00
0.06
1.00
57.20
82.29
24
0
0
0
1
0.80
6.00
0.06
3.00
97.20
89.25
25
0
0
0
0
0.80
6.00
0.06
2.00
97.41
89.25
26
0
0
0
0
0.80
6.00
0.06
2.00
97.63
89.25
27
0
0
0
0
0.80
6.00
0.06
2.00
97.87
89.25
28
0
0
0
0
0.80
6.00
0.06
2.00
97.01
89.25
29
0
0
0
0
0.80
6.00
0.06
2.00
96.87
89.25
30
0
0
0
0
0.80
6.00
0.06
2.00
97.15
89.25
Table 3
Sequential Model
Sum of Squares
models
sum of squares
dfb
mean square
F-value
p-value
mean vs total
67611.52
1
67611.52
linear vs mean
420.70
4
105.18
0.0699
0.9905
2FI vs linear
3347.18
6
557.86
0.3093
0.9243
quadratic vs 2FI
31718.01
4
7929.50
46.6224
< 0.0001a
cubic vs quadratic
1403.84
8
175.48
1.0706
0.4707
residual
1147.35
7
163.91
total
105648.60
30
3521.62
Significance.
Degrees of freedom.
Table 6
ANOVA
source
sum of squares
dfb
mean square
F-value
p-value
model
35485.89
14
2534.71
14.90
< 0.0001a
X1
88.00
1
88.00
0.52
0.4830
X2
16.06
1
16.06
0.09
0.7629
X3
98.94
1
98.94
0.58
0.4575
X4
217.71
1
217.71
1.28
0.2757
X1X2
22.56
1
22.56
0.13
0.7208
X1X3
157.50
1
157.50
0.93
0.3511
X1X4
22.56
1
22.56
0.13
0.7208
X2X3
165.12
1
165.12
0.97
0.3401
X2X4
2814.30
1
2814.30
16.55
0.0010a
X3X4
165.12
1
165.12
0.97
0.3401
X12
20.98
1
20.98
0.12
0.7303
X22
9341.46
1
9341.46
54.92
<0.0001a
X32
20.98
1
20.98
0.12
0.7303
X42
20.98
1
20.98
0.12
0.7303
residual
2551.19
15
170.08
std dev
13.04
lack of fit
2551.19
10
255.12
mean
47.47
pure error
0
5
0.00
CV (%)
27.47
cor total
38037.08
29
press
12641.55
Degrees of freedom.
Significance.
Figure 3
3D response surface plots of G (%) vs (a) total
monomers (g)/KPS (g), (b) MBA (g)/KPS (g), (c) CMC (g)/KPS (g), (d)
MBA (g)/total monomers (g), (e) CMC (g)/total monomers (g), and (f)
CMC (g)/MBA (g).
3D response surface plots of G (%) vs (a) total
monomers (g)/KPS (g), (b) MBA (g)/KPS (g), (c) CMC (g)/KPS (g), (d)
MBA (g)/total monomers (g), (e) CMC (g)/total monomers (g), and (f)
CMC (g)/MBA (g).Significance.Degrees of freedom.Standard deviation.Degrees of freedom.Degrees of freedom.Significance.
RSM Optimization on EW
The face-centered
central composite design (FCCCD), a standard RSM technique, was adopted
to optimize the individual and interactive effects of BN (X5, g), pH (X6, −),
and temperature (X7, K) on EW (g g–1) by performing the minimum number of experimental
runs. The input variables, i.e., X5, X6, and X7, were
varied within 5–15 g, 4–7, and 288–318 K, respectively
(Table ), for obtaining
the maximum EW (g g–1) by using the empirical second-order
polynomial eq to generate eq .The input variables, software generated, and
experimental responses are listed in Table . Here, the acceptability of quadratic model
was confirmed from sequential model sum of squares, model summary
statistics tests, lack of fit tests, and ANOVA (Table –11). The response surface plots envisaging
interactive effects of X5–X6, X5–X7, and X6–X7 with EW (g g–1) were given
in Figure a–c.
Importantly, the optimum EW of 1186.88 g g–1 was
obtained at 10.10 g, 7.06, and 302.60 K for X5, X6, and X7, respectively.
Table 7
Design
Matrix for Optimization of
ESR (g g–1)
coded
values
uncoded
values
ESR (g g–1)
runs
X5
X6
X7
BN (g)
pH (−)
T (K)
experimental
predicted
1
–1
–1
–1
5
4
288
910
425.53
2
1
–1
–1
15
4
288
870
440.49
3
–1
1
–1
5
10
288
800
471.49
4
1
1
–1
15
10
288
760
498.36
5
–1
–1
1
5
4
318
930
404.63
6
1
–1
1
15
4
318
830
411.03
7
–1
1
1
5
10
318
910
429.43
8
1
1
1
15
10
318
820
447.73
9
–1
–1
–1
5
7
303
1000
962.71
10
1
–1
–1
15
7
303
900
979.33
11
–1
1
–1
10
4
303
1100
975.35
12
1
1
–1
10
10
303
850
1016.69
13
–1
–1
1
10
7
288
750
863.90
14
1
–1
1
10
7
318
900
828.14
15
–1
1
1
10
7
303
1200
1185.99
16
1
1
1
10
7
303
1198
1185.99
17
–1
0
0
10
7
303
1191
1185.99
18
1
0
0
10
7
303
1196
1185.99
19
0
–1
0
10
7
303
1200
1185.99
20
0
1
0
10
7
303
1197
1185.99
Table 8
Sequential Model Sum of Squares
models
sum of squares
dfb
mean square
F-value
p-value
mean vs total
1.32 × 107
1
1.32 × 107
linear vs mean
8159.83
3
2719.94
0.02
0.9959
2FI vs linear
331.53
3
110.51
6.69 × 10–4
1.0000
quadratic vs 2FI
2.06 × 106
3
6.87 × 105
78.79
< 0.0001a
cubic vs quadratic
83116.43
4
20779.11
30.78
0.0004
residual
4050.27
6
675.05
total
1.54 × 107
20
7.70 × 105
Significance.
Degrees
of freedom.
Table 11
ANOVA
source
sum of squares
dfa
mean square
F-value
p-value
model
2068865.10
9
229873.90
26.37
<0.0001b
X5
691.23
1
691.23
0.08
0.7840
X6
4271.66
1
4271.66
0.49
0.4999
X7
3196.94
1
3196.94
0.37
0.5583
X5X6
70.81
1
70.81
0.17
0.0930
X5X7
36.64
1
36.64
0.01
0.9496
X6X7
224.08
1
224.08
0.03
0.8758
X52
127078.98
1
127078.98
14.58
0.0034
X62
99239.85
1
99239.85
11.39
0.0071
X72
317837.10
1
317837.10
36.46
0.0001
residual
87166.70
10
8716.67
std. dev.
lack
of fit
87166.70
5
17433.34
mean
pure
error
0.00
5
0.00
CV (%)
cor total
2156031.80
19
press
Degrees of freedom.
Significance.
Figure 4
3D response surface plots of EW (g g–1) vs (a)
pH (−)/BN (g), (b) temperature (K)/BN (g), and (c) temperature
(K)/pH (−).
3D response surface plots of EW (g g–1) vs (a)
pH (−)/BN (g), (b) temperature (K)/BN (g), and (c) temperature
(K)/pH (−).Significance.Degrees
of freedom.Standard deviation.Degrees of freedom.Degrees of freedom.Significance.
Analysis
of Morphology
SEM
SEM is one
of the most important
aspects of superabsorbent hydrogels and can be used to examine the
porosity and pore size of the hydrogel. SEM morphologies of the CMC-g-(AA-Am-AMPS)/BN, SAP-IPN1, and CMC-g-(AA-AMPS-Am),
SAP-IPN2 were measured in the dry state. Parts a and b of Figure show the SEM of
the synthesized SAP hydrogel. It can be shown that for SAP-IPN1 (Figure a) the pores are
smaller when they are connected to form a cross-linked network, which
was beneficial for the diffusion of water into an SAP hydrogel and
to the swelling rate. However, SAP-IPN2 Figure b), a porous structure with several big pores,
which detached in a sheetlike manner, penetrate the water molecule
but with a low rate of swelling. These SEM data showed that the BN
was finely distributed throughout the composite, resulting in homogeneous
composition.
Figure 5
SEM morphology of (a) SAP-IPN1, (b) SAP-IPN2.
SEM morphology of (a) SAP-IPN1, (b) SAP-IPN2.
AFM
An AFM image of SAP-IPN1 superabsorbent
is shown in Figure b. AFM displays are in agreement with the SEM result that the CMC-grafted
composite was rough and porous, which aided in the diffusion of water.
The rougher the porous surface of the composite, the more water penetrates,
resulting in more water absorbency by the composite. The root-mean-square
(RMS) roughness of the composite is 191.2 nm, and the average roughness
(Sa) is 141.0 nm. Figure a shows that th eRMS of SAP-IPN2 is 170.1 nm, and Sa is 111.2
nm, which means lower water absorbance than SAP-IPN1. This result
agrees with SEM and the swelling rate behavior.[12]
Figure 6
AFM of (a) SAP-IPN2, (b) SAP-IPN1.
AFM of (a) SAP-IPN2, (b) SAP-IPN1.
Thermogravimetric (TGA) Analysis
The effect of grafting and cross-linking of monomers chains onto
CMC and interpenetration of bentonite within the CMC on the thermodynamic
stability was investigated. The thermal property of the SAP-IPN1 and
SAP-IPN2 hydrogels was evaluated by comparing the weight loss (%)
in the temperature range of 100–600 °C as shown in Figure S1. Three-stage decomposition mechanism
was found, and maximum weight loss was observed after the first stage
decomposition of cross-linked samples; this indicates the chemical
modification of CMC. Table S1 summarizes
the weight loss of two hydrogels; in every case the hydrogel’s
initial decomposition temperature (IDT) was 200–300 °C,
which is due to the loss of moisture or volatile compounds. The second
stage was observed in the temperature range of 400–500 °C
due to the elimination of side chains. Finally, the third stage of
decomposition was observed at 600 °C due to the breakdown of
the cross-linked structure. In comparison, the rate of decomposition
was lower in case of SAP-IPN2 than SAP-IPN1; this implies a more compact
and stable cross-linked polymeric network which may be because the
incorporation of bentonite into the polymeric network provides a protective
barrier to both mass and energy transport in hydrogel composites interpenetrating
network.
Measurement of Equilibrium Water Absorbency
and Swelling Behavior of SAP-IPN
Effect
of BN on Water Absorption Capacity
The incorporation of BN
clay into SAP-IPN hydrogels shows an effect
on ES. The highest ES value was found to be at 10% BN. It is shown
in Table S2 that SAP-IPN1 is about 1200
g/g. In contrast, in SAP-IPN2 it is 950 g/g. This may be due to the
presence of −OH groups on the surface of BN interacting with
hydroxyl, carboxylate, carboxamide, and sulfonate groups in the network,
which increases and enhances the affinity of the network structure
to water molecule.[13]As a result,
as clay concentration rises, it may act as an extra cross-linking
point in polymer networks, increasing the cross-linking density and
thus decreasing the network spaces for water storage.
Water Absorption Capacity in Saline Water
The water
absorbance of both SAP-IPN hydrogels was measured in
s-water 0.9 wt % NaCl solution and d-water. Figure S2a,b depicts a decrease in swelling behavior when compared
to the data in d-water. Ionic hydrogels have a charge screening effect,
which produces a nonperfect anion–anion repulsion, which results
in a decreased ionic pressure between the SAP hydrogels and water
molecules, resulting in a decrease in water absorption capacity. d-Water
displays good results as it has lower ionic concentration, which aids
in better water intake by osmotic pressure.
Swelling
Behavior in Various Buffer Media
Table S3 reveals the equilibrium swelling
(EW) of both SAP-IPN hydrogels at different pH values (the pH values
are adjusted by using a pH-meter AD1030 Professional pH-ORP-Temp Bench
Meter ADWA). In acidic media, both SAP-IPN hydrogels had a low EW
and a maximum value in neutral media. The increase in pH causes carboxyl
groups in the CMC to be changed to carboxylate ions, causing repulsion
and the formation of space between chains, allowing water particles
to enter the SAP-IPN hydrogels and increasing water absorption. However, as the pH rises
over pH 7, the ability for absorption decreases. This could be due
to the presence of Na+ cations in basic solutions, which
prevent anion–anion repulsion and decrease spaces between chains,
thereby decreasing water absorption capacity.[14,15]Table S3 summarizes the values of EW
in different media.
Water Retention Capability
of SAP-IPN hydrogels
To study the water absorbency capability
to estimate the reusability
of the SAP, the SAP-IPN hydrogels were put in d-water to reach the
maximum swelling ratio. The were dried at a temperature of 30 °C
several times. The swelling and deswelling cycles of the SAP-IPN1
and SAP-IPN2 were repeated four times. As shown in Figure S3, the equilibrium swelling capacity of SAP-IPN2 decreased
from 950 to 220 g/g after four sequential swelling/deswelling cycles.
In contrast, the equilibrium swelling capacity of SAP-IPN1 was 1200
g/g in the first swelling/deswelling cycle and 799.11 g/g in the fourth
swelling/deswelling cycle, respectively. The slight loss of swelling
capacity for SAP-IPN1 is due to the stable 3D-network, and the incorporation
of rigid BN-clay prevented linking of grafted polymeric chains and
weakened the hydrogen-bonding interaction between COOH groups. This
decreased the degree of physical cross-linking and improved water
absorption. Therefore, this structure of the SAP-IPN1 hydrogel network
avoids the collapse during the swelling/deswelling cycles, which makes
reduced damage of swelling capacity and enable to resist high temperature.[16] As a result, the SAP-IPN/BN composite has the
ability to retain and hold water more than four cycles with the same
efficiency. In addition, SAP-IPN/BN could be used as a water storage
source in the arid region and desert land[17] for agriculture application.[18]
Comparison of the Results
Several
biopolymer-based superabsorbent hydrogels, gums, oxide/co/biopolymers,
and IPN hydrogels for water retention of varying components, temperatures,
and effect of pH values have been provided (Table S4). In this work, it is important to note that the addition
of bentonite to CMC increases the structure of hydrogel and, therefore,
allows networks to absorb more water. From Table S4, it could be observed that the terpolymer interpenetrating
network/bentonite of CMC was either closer to or much better than
previously reported super absorbent hydrogels within the specified
working range.Hydrogels can be used in real life as soil conditioners,
and now there are prototypes and real samples send to farmers for
use in soil. These trial are underway.
Mechanism
of Swelling Behavior of the SAP
Hydrogel
The promising property of hydrogels is the capability
to swell when interacting with an aqueous solution (Figure S4). The water molecules penetrate the polymeric network.
Frequently, the meshes of the 3D network structure in the rubbery
stage will start expanding, allowing more penetration of aqueous molecules
within the hydrogel 3D structure network. The rate of swelling is
a very important feature of hydrogel swelling behavior.[19] It is measured by numerous physico-chemical
issues such as the particle size, porosity, and the types of porous
structure such as nonporous, micro-, macro-, and superporous (SAP).[20−22] Superabsorbent hydrogels (SAP) are connected to form the open canal
and act as a tube system for fast uptake of water into the porous
structure until an equilibrium state is reached. The fast swelling
of SAP hydrogel is attributed to the absorption of water through a
capillary force rather than by normal absorption.[23,24] Conventional SAP hydrogels are characterized by fast swelling, high
swelling ratio, and weak mechanical properties. To overcome their
weak mechanical properties, a superabsorbent hydrogel/composite has
been prepared to improve mechanical properties.[25] Osmotic pressure, electrostatic, and viscoelastic forces
are the three focal forces controlling the swelling behavior of hydrogels.
The different models describe and study the effect of these forces
on swelling behavior. Achilleos et al. have developed a system for
the actual time of dynamic distortion through swelling processes.[26] The swelling is not a repeated process. When
compared to the osmotic force, there is an opposite elasticity force,
which balances the expanding of the network and avoids shrinking.
At the equilibrium state, there is no additional swelling due to elasticity,
and osmotic forces are equal.
Conclusions
An innovative CMC-g-(TerPolymer) interpenetrating
superabsorbent network (SAP-IPN) was evaluated as an eco-friendly
alternate to acrylate-based SAPs for the optimization of water consumption.The main findings are summarized in the following points:CMC-g-TerPols superabsorbent
hydrogels
were prepared successfully by a free-radical polymerization technique
using acrylic acid, acrylamide, and AMPS and cross-linked using MBA.
The proposed mechanism for the grafting was a free-radical mechanism
using potassium persulfate as the initiator.A composite (SAP-IPN1 and SAP-IPN2) based on the newly
synthesized hydrogel was also prepared by incorporating bentonite
in the polymer matrix.The newly synthesized
hydrogels were characterized by
FTIR. They confirm the successful grafting and incorporation of the
bentonite within the polymeric matrix.The thermal stability of SAP-IPN1 and SAP-IPN2 was investigated
by TGA, a more condensed and stable cross-linked polymeric network
in SAP-IPN1 that may be due to the incorporation of bentonite into
the polymeric network to provide a protective barrier for both mass
and energy transport.The equilibrium
swelling properties is optimized by
RSM (response surface methodology).The
incorporation of BN clay into SAP-IPN hydrogels
empowers effects on EW.AFM displays
in agreement with the SEM result that the
CMC-grafted composite was a rough and porous structure.The newly developed SAP hydrogel composite may be able
to compete with synthetic SAP hydrogels on the market today and are
promising for agricultural applications for saving water in the irrigation
of arid and desert land and other novel applications
Authors: F A Dorkoosh; J C Verhoef; G Borchard; M Rafiee-Tehrani; J H M Verheijden; H E Junginger Journal: Int J Pharm Date: 2002-10-24 Impact factor: 5.875
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Figure 6