Semi-IPN superabsorbent nanocomposite based on sodium alginate and montmorillonite: Reaction parameters and swelling characteristics

Carbohydrate Polymers 190 (2018) 295–306

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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol

Semi-IPN superabsorbent nanocomposite based on sodium alginate and montmorillonite: Reaction parameters and swelling characteristics

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Ali Olad , Mahyar Pourkhiyabi, Hamed Gharekhani, Fatemeh Doustdar Polymer Composite Research Laboratory, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran

A R T I C LE I N FO

A B S T R A C T

Keywords: Semi-interpenetrating polymer network Sodium alginate Montmorillonite Polyvinylpyrrolidone Superabsorbent nanocomposite

Semi-interpenetrating polymer network (semi-IPN) superabsorbent nanocomposite based on montmorillonite (MMT) and sodium alginate (NaAlg)-g-poly(acrylic acid(AA))/polyvinylpyrrolidone (PVP) was synthesized. Chemical structure and surface morphology of the hydrogels were characterized by FTIR, XRD, thermogravimetric analysis (TGA), SEM, and TEM techniques. FTIR results revealed that graft polymerization, PVP interpenetration through hydrogel network, and nanocomposite formation have occurred. The coarse surface of the hydrogels was changed into interlinked porous structures in the presence of MMT. The effect of polymerization variables on water absorbency of the hydrogels was assessed and optimized. Semi-IPN superabsorbent nanocomposite presented higher equilibrium swelling capacity (618.92 g/g) compared with neat hydrogel (521.17 g/ g). Swelling behavior of the hydrogels strongly depended on pH values of the solution as well as the type and concentration of saline solution. Semi-IPN superabsorbent nanocomposite possessed good reswelling capability, making it as an efficient water reservoir to supply required water to plants in agricultural applications.

1. Introduction Superabsorbent hydrogels are defined as crosslinked three-dimensional polymeric networks, that can absorb and conserve great amounts of aqueous solutions without dissolving or losing own structural integrity (Bao, Ma, & Li, 2011; Rashidzadeh & Olad, 2014; Rodrigues et al., 2012). The high swelling capacity, high swelling rate, good water retention capability, and biodegradability (Kabiri, Omidian, Hashemi, & Zohuriaan-Mehr, 2003) are superior properties of superabsorbent hydrogels, which enable them to have potential applications in various fields such as agriculture and horticulture (Ibrahim, El Salmawi, & Zahran, 2007), drug delivery (Gupta, Vermani, & Garg, 2002), wastewater treatment (Kaşgöz & Durmus, 2008), self-healing concrete (Snoeck, Van Tittelboom, Steuperaert, Dubruel, & De Belie, 2014; Van Tittelboom & De Belie, 2013), hygienic products (Kosemund et al., 2009), and tissue engineering (Wang, Strand, Du, & Vårum, 2010). Superabsorbent hydrogels are mainly divided into two classes; i.e. natural and synthetic-based polymers (Gharekhani, Olad, Mirmohseni, & Bybordi, 2017; Hoffman, 2012). Due to the biodegradability, ease of availability, non-toxicity, and relatively low cost compared with synthetic-based polymers, natural-based polymers such as chitosan (Zhang, Wang, & Wang, 2007), cellulose (Chang, Duan, Cai, & Zhang, 2010), starch (Spagnol et al., 2012), and alginate (Yadav & Rhee, 2012) have gained great attention as hydrogel materials (Li, Xu, Wang, Chen, &

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Feng, 2009). Sodium alginate (NaAlg), a linear anionic polysaccharide composed of (1–4)-linked β-D-mannuronic acid and α-L-guluronic acid units with various proportions, is a water soluble salt of alginic acid, which is derived mainly from brown algae (Hua & Wang, 2009). The most fascinating features of NaAlg such as nontoxicity, biocompatibility, biodegradability, and relatively low cost have led to its broad utilization in pharmaceutical, biomedical, and agricultural fields (Wang & Wang, 2010; Yadav & Rhee, 2012). However, NaAlg-based hydrogels, like most natural-based hydrogels, suffer from poor mechanical strength, which restricts their broad utilization (Mandal & Ray, 2013). To cope with this issue, different chemical or physical methods such as grafting polymerization with synthetic acrylate-based monomers (Olad, Gharekhani, Mirmohseni, & Bybordi, 2017b), polymer blending (Sæther, Holme, Maurstad, Smidsrød, & Stokke, 2008), and compounding with other functional components (Hua & Wang, 2009) have been developed recently. Semi-interpenetrating polymerization is one of the efficient techniques in which two polymers are blended together so that only one of them is crosslinked in the presence of another to make an additional non-covalent interaction (Liu, Li, Su, Yue, & Gao, 2014). On the other hand, semi-interpenetrating polymer network (semi-IPN) provides more facile and convenient pathway to prepare multi-component polymeric materials, which exhibit surprising properties compared with each of their polymeric constituents (Kozhunova, Makhaeva, & Khokhlov, 2012). Polyvinylpyrrolidone (PVP) is a non-

Corresponding author. E-mail address: [email protected] (A. Olad).

https://doi.org/10.1016/j.carbpol.2018.02.088 Received 20 December 2017; Received in revised form 22 February 2018; Accepted 28 February 2018 0144-8617/ © 2018 Elsevier Ltd. All rights reserved.

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Table 1 Various levels of each factor and their amounts. Factor

Symbol

Level 1

Level 2

Level 3

Level 4

NaAlg/PVP weight ratio Acrylic acid monomer Initiator Crosslinking agent Neutralization percent

NaAlg/PVP AA APS MBA NU%

0.3 g/0.3 g 3150 mg (43.7 mmol) 50 mg (0.21 mmol) 10 mg (0.06 mmol) 30

0.4 g/0.2 g 4200 mg (58.2 mmol) 60 mg (0.26 mmol) 20 mg (0.12 mmol) 50

0.45 g/0.15 g 5250 mg (72.8 mmol) 70 mg (0.3 mmol) 30 mg (0.19 mmol) 70

0.48 g/0.12 g 6300 mg (87.4 mmol) 80 mg (0.35 mmol) 40 mg (0.25 mmol) 90

ionic water-soluble linear synthetic polymer. Its unique properties such as good solubility, non-toxicity, excellent affinity to different polymers, biocompatibility, and biodegradability have made it applicable in cosmetics, foods, textiles, printing inks, medicine, and pharmaceuticals fields (Wang & Wang, 2010). By virtue of these features, it is expected that combination of PVP with NaAlg through semi-IPN technique can produce a composite hydrogel with improved structure and high performance. Besides that, graft polymerization of hydrophilic vinyl monomers onto NaAlg backbone is another approach, which can substantially improve swelling and mechanical characteristics of the final hydrogel product. Despite special advantages of superabsorbent hydrogels, their application domain is severely limited by the high production cost. To overcome this shortcoming, incorporation of low cost inorganic nanofillers into hydrogel matrix can be best strategy. Among various inorganic compounds, clay minerals have gained great interest due to their small particle size and intercalation properties. These layered materials generate strong interactions with the polymeric matrix, and hence can cause substantial improvement in mechanical, thermal, and barrier properties as well as swelling and adsorption behavior of the neat polymer (Mansoori, Atghia, Zamanloo, Imanzadeh, & Sirousazar, 2010; Shi, Wang, Kang, & Wang, 2012). Montmorillonite (MMT), a layered aluminum silicate with exchangeable cations, having abundant reactive hydroxyl groups on the surface, high in plane strength, stiffness, and high aspect ratio is broadly used to prepare superabsorbent nanocomposites (Mansoori et al., 2010; Pereira, Minussi, da Cruz, Bernardi, & Ribeiro, 2012). In this study, the highly swollen pH-sensitive semi-IPN superabsorbent nanocomposite based on MMT and NaAlg-g-poly(AA)/PVP was synthesized by the free-radical graft polymerization and semi-IPN techniques. The factors affecting swelling capacity of the hydrogel, NaAlg/PVP weight ratio, the amounts of AA, APS, and MBA and neutralization percent were studied and optimized using Taguchi method. Also, the effects caused by the incorporation of MMT into semi-IPN hydrogel network on the equilibrium water absorption capacity of the hydrogel were also evaluated. Moreover, the swelling kinetic and swelling behavior of semi-IPN NaAlg-g-poly(AA)/PVP hydrogel and semi-IPN NaAlg-g-poly(AA)/PVP/MMT superabsorbent nanocomposite in various pHs and saline solutions were studied. Additionally, utilization potential of the superabsorbent nanocomposite in agricultural and horticultural applications was investigated through water absorbency under load and reswelling capability assays.

(4.824%), CaO (2.339%), Na2O (0.18%), MgO (3.714%), K2O (0.849%), TiO2 (0.314%), MnO (0.041%), P2O5 (0.038%), and SO3 (10%), which was determined using X-ray fluorescence (XRF) analysis. Acrylic acid (AA, Merck), polyvinylpyrrolidone (PVP, the average molecular weight of Mr = 25000, Merck), N,N′-methylene bisacrylamide (MBA, Merck), ammonium persulfate (APS, Merck), sodium hydroxide (Merck), hydrochloric acid (Merck), calcium chloride (Merck), sodium chloride (Merck), and ferric chloride hexahydrate (FeCl3·6H2O, Merck) were used as received. Ethanol was purchased from Mojallali reagent chemicals Co. (Iran, Tabriz). All other chemicals were of analytical grade and all solutions were prepared using distilled water. 2.2. Experimental design 2.2.1. Determination of factors and their levels Among the variety of factors that can severely affect equilibrium swelling capacity of the hydrogels, five important factors including the amounts of AA, MBA, and APS, NaAlg/PVP weight ratio, and neutralization percent (NU%) were chosen. The values of each factor were adopted according to the trial experiments and also previously reported research works. Eventually, four different values were assigned for each factor, which have been specified by level 1, level 2, level 3, and level 4 in Table 1. 2.2.2. Assignment of factors using an orthogonal array To design experimental condition using Taguchi method, a standard orthogonal array (L16) was pursued. The sixteen experiments arranged in a L16 orthogonal array have been depicted in Table 2. The equilibrium swelling capacity (Qeq) and standard deviation (STDEV) for each experiment were calculated, and the obtained values were collected in Table 2. Finally, the results of the experiments designed in an orthogonal array were analyzed using the statistical method of analysis of variance (ANOVA) in Minitab software. 2.3. Synthesis of semi-IPN NaAlg-g-poly(AA)/PVP/MMT superabsorbent nanocomposite A pre-determined amount of MMT (15 wt%, with respect to NaAlg) was first dispersed in 20 mL distilled water using an ultrasonic probe at 50 W power for 5 min. Then, it was poured into a beaker and stirred by a magnetic stirrer for 24 h until a homogeneous MMT suspension was achieved. Thereafter, the resultant suspension was transferred into a 250 mL four necked flask equipped with a mechanical stirrer, a reflux condenser, a thermometer, and a nitrogen line. After heating the solution to 40 °C using a water bath, given amounts of NaAlg (0.4 g) and PVP (0.2 g) were gradually added into the prepared suspension, while stirring continuously by mechanical stirrer. After stirring turned the reaction mixture into a sticky pasty-like solution, the dissolved oxygen was removed by purging N2 gas for 30 min. In continue, certain amount of APS (80 mg) was dissolved in 4 mL distilled water, and then it was added into reaction mixture. Next, the temperature of the reaction mixture was raised to 60 °C and was kept at this condition to generate radicals. Afterwards, a mixed solution of partially neutralized (70%) AA (5 mL) and MBA (10 mg) was decanted into the flask. The reaction mixture with the total volume of 30 mL was maintained at 60 °C while

2. Experimental 2.1. Materials Sodium alginate (NaAlg, chemical grade with 396 kDa MW, Mannuronic acid (M) to Guluronic acid (G) ratio of 1.49 and viscosity of 5.0–40.0 cps for a 1% solution at 25 °C) was procured from SigmaAldrich (USA). Sodium montmorillonite (Na-MMT with chemical formula of Na0.7Al3.3Mg0.7Si8O20OH4·nH2O, specific surface area = 20–40 m2/g, cation exchange capacity = 30 meq/100 g) was purchased from Sigma-Aldrich (USA). The chemical composition of NaMMT as oxides consists of SiO2 (55.209%), Al2O3 (14.749%), Fe2O3 296

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Table 2 Experimental design (L16 orthogonal array) and the amounts of equilibrium swelling capacity (Qeq) and standard deviation (STDEV) for each experiment. Test number

NaAlg/PVP

AA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Level Level Level Level Level Level Level Level Level Level Level Level Level Level Level Level

Level Level Level Level Level Level Level Level Level Level Level Level Level Level Level Level

1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4

APS 1 2 3 4 4 3 2 1 2 1 4 3 3 4 1 2

Level Level Level Level Level Level Level Level Level Level Level Level Level Level Level Level

MBA 1 2 3 4 2 1 4 3 3 4 1 2 4 3 2 1

Level Level Level Level Level Level Level Level Level Level Level Level Level Level Level Level

Weq − Wd , Weq is the weight of swollen hydrogel sample at an equilibrium state, Wd is Wd 1 n Σi = 1 (xi − x)2 , xi is the equilibrium swelling capacity values, x is the average of the set N

NU% 1 2 3 4 3 4 1 2 4 3 2 1 2 1 4 3

Level Level Level Level Level Level Level Level Level Level Level Level Level Level Level Level

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

Qeq (g/g)a

STDEVb

163.4 269.6 363.1 200.6 287.7 198.2 510.6 346.3 261.8 443.8 317.1 429.6 461.1 480.7 134.1 154.2

6.46 6.09 5.12 5.49 5.00 7.22 5.58 4.25 6.38 4.85 4.19 5.01 6.25 4.30 7.31 6.32

a

Qeq (g/g) =

the weight of dry hydrogel sample.

b

STDEV =

of equilibrium swelling capacity values, and N is the number of set of equilibrium

swelling capacity values.

(40–80 mesh) was placed in a cylindrical steel screen (100 mesh) with a given weight. Then, the prepared system was immersed in 100 mL distilled water. At specified time intervals, the system was taken out from the swelling medium and after removing excess surface water by a filter paper, the swollen weight of the hydrogel sample (Wt) in cylindrical steel screen was calculated. These measurements were continued until the constant value of equilibrium swollen weight of the hydrogel sample (Weq) was achieved. Finally, the swelling capacity (Qt) and equilibrium swelling capacity (Qeq) of the hydrogel samples were calculated using Eqs. (1) and (2), respectively:

stirring till it was gelled. After gelation, polymerization process was allowed to proceed for 1 h. The obtained gel product was washed with fresh ethanol to remove unreacted species, and then extracted gel was dried in a vacuum oven at 70 °C for 24 h. The dried gels were milled and sifted through 40−80 mesh sieves for further experiments. All semi-IPN NaAlg-g-poly(AA)/PVP hydrogel samples (test numbers from 1 to 16) were synthesized similar to the above mentioned procedure without addition of MMT. Total volume of the reaction mixture during synthesis of hydrogel samples was also maintained at 30 mL by addition of distilled water if necessary. The yield of the hydrogel samples obtained in all sixteen experiments was almost equal to 91%. Finally, an optimized semi-IPN NaAlg-g-poly(AA)/PVP hydrogel was adopted as a neat hydrogel sample for comparison purposes.

Qt (g/g) =

Qeq (g/g) =

2.4. Methods of characterization The chemical structure of NaAlg, PVP, MMT, NaAlg-g-poly(AA)/ PVP, and NaAlg-g-poly(AA)/PVP/MMT superabsorbent nanocomposite was characterized using a Bruker Tensor 27 FTIR spectrophotometer with KBr pellets operating in the wavenumber range of 400–4000 cm−1. X-ray diffraction (XRD) patterns of MMT, NaAlg-gpoly(AA)/PVP, and NaAlg-g-poly(AA)/PVP/MMT superabsorbent nanocomposite were also obtained using a diffractometer (Siemens AG, Karlsruhe, Germany) equipped with a Cu Kα radiation source in scattering angle range from 2° to 70°. The chemical composition of Na-MMT was determined using Philips PW 1410 X-ray fluorescence (XRF) spectrometer. To evaluate the thermal stability of the prepared hydrogels, thermogravimetric analysis (TGA) was performed using a thermal gravimetric analyzer (TGA/DSC-1, Mettler Toledo) at temperature range of 30 °C–610 °C with a heating rate of 10 °C/min under nitrogen atmosphere. The surface morphology of the NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT superabsorbent nanocomposite was also investigated through a field emission scanning electron microscope (FESEM) system (MIRA3 FEG-SEM, Tescan, Czech) coupled with an Energy Dispersive X-ray analyzer (EDX) (Tescan, Czech, model SAMX). To further characterize the morphology of NaAlg-g-poly(AA)/PVP/MMT superabsorbent nanocomposite, transmission electron microscope (TEM) analysis was performed using a LEO 906 (Zeiss, Germany) TEM instrument.

Wt − Wd Wd

(1)

Weq − Wd Wd

(2)

2.6. Measurement of water absorbency under load (AUL) To investigate water absorbency of the hydrogel samples under load, swelling kinetic measurements were executed in 0.9 wt% NaCl aqueous solution as swelling medium under different pressures (0.3, 0.6, and 0.9 psi). First, 0.9 wt% NaCl aqueous solution was charged into a petri dish (d = 118 mm, h = 12 mm) containing porous sintered glass filter plate (d = 80 mm, h = 7 mm) till the solution level reached the surface of the sintered glass filter plate. In continue, the round-shaped piece of a wire cloth (100 mesh) was placed onto the sintered glass filter plate, and then given amount of dry hydrogel sample (0.1 g, 40–80 mesh) was added onto it. Thereafter, its surface was surrounded by a glass cylinder (d = 60 mm, h = 50 mm). Afterwards, to exert pressure onto the hydrogel sample, a cylindrical solid load (Teflon, d = 60 mm, variable height) was passed through the glass cylinder and put onto the hydrogel particles. Since surface evaporation may change saline solution concentration, the prepared system was covered properly. The swollen hydrogel samples were withdrawn at specified time intervals and after weighing, AUL was determined using the equation described in Section 2.5. 2.7. Swelling behavior in pH solutions To evaluate swelling behavior of the hydrogel samples in various pHs, different pH solutions ranging from 2 to 12 were prepared by diluting aqueous solutions of HCl (0.1 M) and NaOH (0.1 M). To

2.5. Measurement of swelling kinetic A certain amount of dried hydrogel sample (Wd) in powder form 297

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content has an intense effect on the equilibrium swelling capacity of the hydrogel samples. Since the larger signal-to-noise value designates hydrogel sample with higher equilibrium swelling capacity, the optimum value of MBA was found at level 1 (10 mg). At MBA contents higher than 10 mg within levels 2–4, crosslinking density increases, causing a reduction in the equilibrium swelling capacity (Wu, Zhou, Ye, Sun, & Zhao, 2010). From Fig. 1, it can be inferred that equilibrium water absorption capacity of the hydrogel samples is also influenced by NaAlg/PVP weight ratio. The optimum value of NaAlg/PVP weight ratio was achieved at level 2 (0.4 g/0.2 g). At level 1 (NaAlg/PVP weight ratio of 0.3 g/0.3 g), due to the lower content of NaAlg compared with level 2, free radical generation on the NaAlg chains significantly increases. At this condition, after grafting of some AA monomers onto NaAlg backbone, grafted chains generate steric hindrance and prevent grafting of other monomers on the active radical sites. This phenomenon promotes free-radical polymerization of excess AA monomers without grafting reaction, resulting in the formation of a hydrogel sample with undesired properties. With increasing NaAlg content from 0.4 g in level 2–0.48 g in level 4, the viscosity of NaAlg solution severely increases, leading to the decreased initiation efficiency and subsequently inadequate formation of active radical sites onto NaAlg backbone. Therefore, grafting efficiency and molecular weight of grafted chains decrease, which undermine performance of the final product (Hua & Wang, 2009; Wang & Wang, 2010). According to Fig. 1, the optimum values of AA, APS, and NU% factors were found at level 3 (5 mL), level 4 (80 mg), and level 3 (70%), respectively.

determine equilibrium water absorption capacity of the hydrogel samples in each pH solution, a similar method described in Section 2.5, was also used. 2.8. Evaluation of swelling behavior in saline solutions Swelling behavior of the hydrogel samples was investigated in various saline solutions (NaCl, CaCl2, and FeCl3) with different concentrations (0.1, 0.3, 0.5, 0.7, 0.9, and 1.1 wt%). The equilibrium swelling capacity of the hydrogels was calculated using the method mentioned earlier (see Section 2.5). 2.9. Investigation of reswelling capability This experimental assessment was designed to evaluate the equilibrium swelling capacity loss of the hydrogel samples during consecutive swelling/drying cycles. In order to perform this analysis, preweighted dry hydrogel sample was first immersed entirely in excess distilled water, and then it was allowed to reach an equilibrium water absorption capacity at room temperature. Then, the swollen hydrogel sample was removed from the swelling medium and after weighing, it was dried to a constant weight in a vacuum oven at 70 ° C. The dried hydrogel sample was weighed, and then it was again equilibrated in distilled water. To make a comparative measure of the equilibrium swelling capacity loss of the hydrogel samples, swelling/drying cycles were continued for five times. Also, to assess reproducibility of this analysis, every five series of swelling/drying cycles were replicated in triplicate for hydrogel samples. Finally, the precision of the three replicated experiments was determined by error bars.

3.3. FTIR spectra analysis FTIR spectra of NaAlg, PVP, MMT, NaAlg-g-poly(AA)/PVP, and NaAlg-g-poly(AA)/PVP/MMT have been shown in Fig. 2(a). In FTIR spectrum of NaAlg (Fig. 2(a)), the peak appeared at 3314 cm−1 as a broad absorption band is related to the stretching vibration of hydroxyl (O−H) groups. The absorption bands at 892 cm−1 and 943 cm−1 are corresponded to the stretching vibration of etheric (CeOeC) bonds. Also, the absorption bands at 1651 cm−1 and 1415 cm−1 are ascribed to asymmetric and symmetric stretching vibrations of carboxylate (eCOOe) groups, respectively. Moreover, the absorption bands observed at 1031 cm−1 and 1095 cm−1 are attributed to the stretching vibration of alcoholic (eCeOH) groups of polysaccharide structure of NaAlg (Kulkarni, Sreedhar, Mutalik, Setty, & Sa, 2010; Rashidzadeh, Olad, & Salari, 2015; Samanta & Ray, 2014). According to the FTIR spectrum of MMT (Fig. 2(a)), the peak emerged at 3400 cm−1 is due to the presence of hydroxyl groups. The characteristic absorption bands appeared at 794 cm−1 and 1026 cm−1 are assigned to the stretching vibration modes of SieOeAl and SieOeSi groups, respectively. The bending vibration modes of SieOeAl and SieOeSi groups were also observed at 522 cm−1 and 460 cm−1, respectively. Moreover, the peak at 1630 cm−1 is related to the hydroxyl (OeH) bending vibration mode of the adsorbed water molecules (Irani, Ismail, & Ahmad, 2013). For PVP (Fig. 2(a)), the absorption band emerged at 2958 cm−1 is related to the stretching vibration of eCH2 groups. Also, the stretching vibration modes of the C]O and CeN bonds were observed at 1667 cm−1 and 1281 cm−1, respectively (Demeter et al., 2017; Wang et al., 2016). In FTIR spectra of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/ MMT (Fig. 2(a)), the peaks emerged at respectively 1679 cm−1 and 1672 cm−1 are related to the asymmetric stretching vibration of carboxylic acid (eCOOH) groups. The asymmetric stretching modes of carboxylate (eCOOe) groups in FTIR spectra of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT were appeared at 1643 cm−1 and 1651 cm−1, respectively, while their corresponding symmetric stretching modes were observed at 1553 cm−1, 1390 cm−1, and 1541 cm−1, and 1368 cm−1, respectively (Wang & Wang, 2010). Moreover, the peaks emerged between 2850 cm−1 and 2980 cm−1 are attributed to the stretching vibration modes of CeH groups. Additionally, the broad and intense peaks between 3400 cm−1 and

3. Results and discussion 3.1. Formation mechanism of semi-IPN superabsorbent nanocomposite Semi-IPN superabsorbent nanocomposite was synthesized by simultaneous integration of chemical and physical processes including graft polymerization of AA monomers onto NaAlg backbone, the crosslinking reaction in the presence of MBA, and the interpenetration of linear PVP chains through hydrogel network (Scheme 1). First, sulfate anion-radicals were produced by thermal decomposition of APS molecules. These radicals extract hydrogen atoms from the hydroxyl groups of the NaAlg chains, resulting in the formation of alkoxy macroradicals. These macro-radicals then take part in propagation reactions of a new grafted polymeric chains through donating own active radicals onto nearest AA monomers. As the chain propagation reactions proceed, the polymeric chains may react synchronously with the end vinyl groups of MBA to form crosslinked structure. During this process, PVP chains can also interpenetrate with the hydrogel network through hydrogen-bonding interactions. Eventually, MMT, which may act as physical crosslinking agent, builds final semi-IPN superabsorbent nanocomposite network. 3.2. Optimization of reaction parameters Hydrogel samples were synthesized according to the L16 orthogonal array, which has been shown in Table 2. The equilibrium swelling capacity of the hydrogels was measured for three times and finally average values were recorded. In Taguchi design method, control factors are parameters that can be controlled, while noise factors cannot be controlled during production process or product use. Signal-to-noise ratio measures the response changes relative to the target value under different noise conditions. Higher values of the signal-to-noise ratio (S/ N) identify control factor settings that minimize the effect of the noise factors. To determine the optimum value of each factor and the effect of each factor on the equilibrium swelling capacity of the hydrogels, signal-to-noise curves were plotted (Fig. 1). As depicted in Fig. 1, MBA 298

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Scheme 1. Proposed reaction mechanism for synthesis of semi-IPN superabsorbent nanocomposite.

3600 cm−1 are related to the stretching vibration of hydroxyl (OeH) groups (Gharekhani et al., 2017). According to the FTIR spectra of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT (Fig. 2(a)), it can be inferred that the characteristic absorption bands of etheric groups of NaAlg have been disappeared after reaction. Also, the absorption band of the hydroxyl group of NaAlg (3314 cm−1) has shifted to the higher wavenumbers after reaction and eventually it was appeared at 3439 cm−1 and 3618 cm−1 in respectively FTIR spectra of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT. Moreover, the characteristic absorption bands of NaAlg at 1651 cm−1 (asymmetric

stretching vibration of carboxylate groups) and 1415 cm−1 (symmetric stretching vibration of carboxylate groups) have overlapped respectively with asymmetric and symmetric stretching vibration modes of carboxylate groups in FTIR spectra of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT. Additionally, as can be obviously seen from FTIR spectra of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/ PVP/MMT, the characteristic absorption bands of alcoholic (−C−OH) groups of NaAlg have been disappeared after reaction. This is due to the participation of alcoholic groups of NaAlg chains in grafting reaction with AA monomers, which results in the formation of new etheric bonds 299

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Fig. 1. The main effect of each factor reflected by signal-to-noise ratio curves.

with stretching vibration modes at 1178 cm−1 and 1154 cm−1 in respectively FTIR spectra of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly (AA)/PVP/MMT. From these results, it can be concluded that NaAlg chains have been successfully incorporated into hydrogel network and also their alcoholic groups have effectively participated in grafting reaction with AA monomers. As shown in the FTIR spectra of NaAlg-gpoly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT, the absorption band of carbonyl (C]O) group of PVP (1667 cm−1) has emerged at lower wavenumbers. This phenomenon, which is attributed to the strong hydrogen-bonding interaction between carbonyl groups of PVP chains and carboxylic acid (eCOOH) groups of polymeric matrix, confirms successful combination of PVP chains with the hydrogel network (Mandal & Ray, 2013; Shi et al., 2012; Tally & Atassi, 2015; Wang & Wang, 2010). Moreover, in comparison with FTIR spectrum of MMT, NaAlg-g-poly(AA)/PVP/MMT exhibited characteristic absorption band of MMT with slight shift in wavenumber (1011 cm−1), which is an important evidence for successful nanocomposite formation.

Therefore, crystalline reflections associated with the introduced MMT cannot be clearly seen in XRD pattern of NaAlg-g-poly(AA)/PVP/MMT. 3.5. Thermogravimetric analysis (TGA) Fig. 2(c) and (d) depict TGA and differential TGA (DTG) curves of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT (with 15 wt % MMT), respectively. As shown in Fig. 2(d), both hydrogel samples exhibit four-stage decomposition process. The first stage accompanied by a minor weight loss was occurred between 30 °C and 180 °C, which is attributed to the evaporation of water present in the samples. In the second degradation stage, dehydration of saccharide rings and breaking of CeOeC bonds in the NaAlg and grafted polymeric chains were occurred within the temperature range of 180−334 °C. Third stage was found at 334–435 °C, which is related to the decomposition of PVP chains (Huang, Lu, & Xiao, 2007). According to Fig. 2(d), the major weight loss for NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/ MMT was occurred at 475 °C and 482 °C, respectively (fourth stage). This can be attributed to the formation of anhydride groups by association of two neighboring carboxylic groups on the polymer chains, elimination of the water molecule, breaking of polymer chains, and destruction of the crosslinked network structure (Yadav & Rhee, 2012). According to the TGA curves of NaAlg-g-poly(AA)/PVP and NaAlg-gpoly(AA)/PVP/MMT samples (Fig. 2(c)), NaAlg-g-poly(AA)/PVP/MMT showed lower weight loss rate and less total weight loss within the temperature range of 30−610 °C compared with the NaAlg-g-poly (AA)/PVP. These results indicated that incorporation of MMT into polymeric matrix of hydrogel network can significantly improve thermal stability of the hydrogel sample.

3.4. XRD patterns analysis XRD patterns of pristine MMT, NaAlg-g-poly(AA)/PVP, and NaAlgg-poly(AA)/PVP/MMT superabsorbent nanocomposite have been shown in Fig. 2(b). According to the XRD pattern of MMT (Fig. 2(b)), its characteristic peaks emerged at 2θ = 7.48°, 19.87°, 28.56°, 36.08°, 43.9°, and 62.05°. An intense peak at 2θ = 7.48° refers to the basal plane and is ascribed to the interlamellar distance d001 = 12.23 A°, which represents the crystalline phase of MMT (Bortolin, Aouada, Mattoso, & Ribeiro, 2013; Olad, Gharekhani, Mirmohseni, & Bybordi, 2017a; Rashidzadeh & Olad, 2014). This peak has emerged at lower angle of 2θ = 7.26° with low intensity in the XRD pattern of NaAlg-gpoly(AA)/PVP/MMT superabsorbent nanocomposite (Fig. 2(b)). This phenomenon implies that polymeric chains of the hydrogel have intercalated between MMT layers. Thus, NaAlg-g-poly(AA)/PVP/MMT superabsorbent nanocomposite has an intercalated structure and MMT has been successfully incorporated into hydrogel network. In XRD patterns of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT (Fig. 2(b)), two weak broad peaks were also observed at 2θ = 22° and 2θ = 38°, which indicate their amorphous structure with low crystallinity (Olad et al., 2017a). In the case of NaAlg-g-poly(AA)/PVP/MMT, amorphous nature of the polymeric matrix is a dominant phase.

3.6. Morphological analysis Surface morphology of the hydrogel samples was investigated using FE-SEM and TEM techniques (Fig. 3). As shown in Fig. 3(a) and (b), NaAlg-g-poly(AA)/PVP has loose and coarse surface with low porosity, which can be associated with the lower equilibrium swelling capacity. These structures, which result from the physical crosslinking points between PVP molecules and grafted polymeric chains, make convenient pathways for penetration of water into hydrogel network, and thus improve swelling capacity of the NaAlg-g-poly(AA)/PVP sample. In 300

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Fig. 2. FT-IR spectra of NaAlg, NaAlg-g-poly(AA)/PVP, NaAlg-g-poly(AA)/PVP/MMT, PVP, and MMT (a), XRD patterns of MMT, NaAlg-g-poly(AA)/PVP, and NaAlg-g-poly(AA)/PVP/ MMT (b), and TGA (c) and DTG (d) curves of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT samples.

network. However, inhomogeneous distribution of MMT layers in the polymeric matrix of hydrogel causes formation of regions with different physical crosslinking densities, resulting in the generation of irregular pores on the surface of the hydrogel sample (as depicted in Fig. 3(c) and (d)). To further characterize the morphology of NaAlg-g-poly(AA)/ PVP/MMT superabsorbent nanocomposite, TEM analysis was also carried out (Fig. 3(e) and (f)). According to Fig. 3(e) and (f), the dark and translucent regions show respectively MMT layers and polymeric matrix of the hydrogel. As it is evident, MMT layers have been well-dispersed within polymeric matrix of the hydrogel. Nevertheless, A few stack layers of MMT can be observed in Fig. 3(f). As shown in Fig. 3(e)

contrast, SEM images of NaAlg-g-poly(AA)/PVP/MMT (Fig. 3(c) and (d)) present more rough and highly porous structure, arising from the physical crosslinking effect of the introduced MMT. Fig. 3(g) exhibits element mapping results of C elements (red dots), N elements (light blue dots), O elements (dark green dots), Na elements (dark blue dots), Al elements (yellow dots), Mg elements (light green dots), Si elements (orange dots), and Fe elements (purple dots) on the surface of NaAlg-gpoly(AA)/PVP/MMT superabsorbent nanocomposite. According to the element mapping analysis results, presence of elements such as Si, Al, Mg, and Fe in superabsorbent nanocomposite composition confirmed that MMT layers have been successfully dispersed within hydrogel 301

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Fig. 3. SEM images of NaAlg-g-poly(AA)/PVP ((a) and (b)), NaAlg-g-poly(AA)/PVP/MMT ((c) and (d)), TEM images of NaAlg-g-poly(AA)/PVP/MMT (e) and (f), and element mapping results of C elements (red dots), N elements (light blue dots), O elements (dark green dots), Na elements (dark blue dots), Al elements (yellow dots), Mg elements (light green dots), Si elements (orange dots), and Fe elements (purple dots) on the surface of NaAlg-g-poly(AA)/PVP/MMT (g). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

initial time periods after immersion of the hydrogel samples in distilled water, the swelling capacity increased dramatically, but in continue its growth rate decreased gradually until the equilibrium swelling capacity was achieved. Swelling kinetics during initial time periods (the inset in Fig. 4(a)) exhibit that NaAlg-g-poly(AA)/PVP/MMT swells more quickly than that of NaAlg-g-poly(AA)/PVP. This is due to the highly porous structure of NaAlg-g-poly(AA)/PVP/MMT, which provides more contact surface area, facilitates penetration of water molecules into hydrogel network, and therefore induces higher swelling rate. In continue, NaAlg-g-poly(AA)/PVP/MMT absorbs water molecules very slowly compared with NaAlg-g-poly(AA)/PVP. This behavior can be attributed to the interlinked porous channels within NaAlg-g-poly(AA)/ PVP/MMT hydrogel network, which delay penetration of water molecules into hydrogel, prolong required time to reach equilibrium swelling capacity, and thus decrease swelling rate. From Fig. 4(a) it can be obviously seen that NaAlg-g-poly(AA)/PVP/MMT took longer time

and (f), the light interconnected regions in polymeric matrix of hydrogel sample are related to the three-dimensional interlinked porous structure of NaAlg-g-poly(AA)/PVP/MMT superabsorbent nanocomposite, which have been identified by yellow arrow signs. These interlinked porous channels enable superabsorbent nanocomposite to absorb great amounts of water molecules. Therefore, it can be expected that NaAlg-g-poly(AA)/PVP/MMT superabsorbent nanocomposite will have higher equilibrium swelling capacity compared with the NaAlg-g-poly (AA)/PVP sample.

3.7. Swelling kinetic studies To evaluate the effect of MMT on the swelling behavior of the hydrogel sample, swelling kinetic measurements were performed in distilled water as swelling medium (Fig. 4(a)). As shown in Fig. 4(a), the swelling kinetic of both hydrogel samples follows a similar trend. At 302

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Fig. 4. Swelling kinetic curves of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT samples (a), plots of t/W versus t for NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/ MMT samples (b), water absorbency under load (AUL) for NaAlg-g-poly(AA)/PVP (c) and NaAlg-g-poly(AA)/PVP/MMT (d) versus time in aqueous saline solution (0.9 wt% NaCl) at different pressures (0.3 psi, 0.6 psi, and 0.9 psi).

structure of superabsorbent nanocomposite made by physical crosslinking effect of the introduced MMT, provides more vacant spaces to absorb water molecules and so induces a substantial improvement in water uptake capacity. Besides that, special crystalline structure of MMT with negative surface charges generates high electrostatic repulsive forces with eCOOe groups of hydrogel network, which result in an expanded hydrogel network and higher water absorbency (W. Marandi, Mahdavinia, & Ghafary, 2011). These results revealed that PVP and MMT have synergistic effect on improving swelling capacity so that superabsorbent nanocomposite showed a substantial increase in the equilibrium swelling capacity compared with neat hydrogel. To gain more insight about the swelling rate of the hydrogel samples, experimental swelling kinetic data were assessed using the second order swelling kinetic model (Karadağ, Üzüm, & Saraydin, 2005; Liu, Wang, & Wang, 2011):

(550 min) to reach own equilibrium swelling capacity, while equilibrium water absorption capacity of NaAlg-g-poly(AA)/PVP was achieved within 370 min. According to the swelling kinetic results, the equilibrium water absorption capacity of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/ PVP/MMT samples was 521.17 g/g and 618.92 g/g, respectively, and the standard deviation of each swelling capacity was 5.36 and 4.7, respectively. In comparison with the research works reported in the literature (Feng, Ma, Wu, Wang, & Lei, 2014; Mahdavinia & Asgari, 2013; Noppakundilograt, Sonjaipanich, Thongchul, & Kiatkamjornwong, 2013; Pourjavadi, Ghasemzadeh, & Soleyman, 2007; Wang & Wang, 2009; Xie & Wang, 2010; Xie & Wang, 2009; Zhang et al., 2007), semiIPN NaAlg-g-poly(AA)/PVP/MMT superabsorbent nanocomposite developed in the present study possessed higher water absorption capacity. The dispersant effect of non-ionic PVP chains during polymerization reaction of the hydrogel samples increases dispersion of the reactants, improves the network structure of the hydrogel, and so enhances the swelling capacity. Also, the collaborative absorption effect of hydrophilic groups (eC]O and eC−N) of PVP and eCOOH and eCOO− groups of NaAlg and AA components is another factor that improves swelling capacity (Wang & Wang, 2010). Moreover, physical crosslinkages within hydrogel network made by entangled PVP chains in the polymeric matrix cause an increase in surface roughness, which in turn due to the facilitate penetration of water molecules into hydrogel, swelling capacity improves considerably. In the presence of MMT, due to the ionization of its particles, osmotic pressure difference between hydrogel network and swelling medium increases, causing an appreciable increase in swelling capacity. Moreover, interlinked porous

t/ W= A+ Bt

(3)

1 2 k sW∞

(4)

B = 1/W∞

(5)

A=

Where W (g/g) is the swelling capacity at time t (min); the A parameter (g.min/g) corresponds to an initial swelling rate [(dW/dt)0] of the hydrogel; ks (g/g.min) is the constant rate of swelling; and W∞ (g/g) is the theoretical equilibrium water absorption capacity (Gharekhani et al., 2017). The plot of t/W versus t (Fig. 4(b)) gives straight lines with good linear correlation coefficients. W∞ and ks were 303

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calculated from the slope and intercept of the plotted straight lines, respectively. The theoretical equilibrium swelling capacity (W∞) for NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT was achieved as 526.3 (g/g) and 625.0 (g/g), respectively, which were close to those experimental values. Moreover, the swelling rate constant (ks) for NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT samples was obtained as 4.99 × 10−5 g/g.min and 2.58 × 10−5 g/g.min, respectively. These results indicated that NaAlg-g-poly(AA)/PVP swells more rapidly than that of NaAlg-g-poly(AA)/PVP/MMT. This is mainly due to the interlinked porous structure of NaAlg-g-poly(AA)/PVP/MMT. On the other hand, the highly porous structure with tortuous pathways within NaAlg-g-poly(AA)/PVP/MMT hydrogel network hinders penetration of water molecules into hydrogel, extends the time to reach equilibrium water absorption capacity, and so decreases the swelling rate.

(5 < pH < 8), due to the dissociation of the carboxylic acid groups (eCOOH → eCOOe + H+), electrostatic repulsions between carboxylate anions are reinforced. This phenomenon causes further expansion in hydrogel network and subsequently higher swelling capacity. At pH values greater than 8, the swelling capacity loss is attributed to the charge screening effect of the excess Na+ counterions in the swelling medium. In other words, the shielding effect of Na+ cations on the carboxylate anions prevents the effective anion-anion repulsions among carboxylate groups. Therefore, hydrogel network shrinks and equilibrium swelling capacity decreases (Pourjavadi et al., 2007; Spagnol et al., 2012). Besides, at higher pHs (pH > 8), due to the accumulation of carboxylate anions in the hydrogel network, hydrogen-bonding interactions between water molecules and carboxylic acid groups decreases, resulting in the increased discharging of water molecules from hydrogel network and reduced swelling capacity.

3.8. Absorbency under load (AUL)

3.10. Swelling studies in various saline solutions

The equilibrium swelling capacity of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT samples at different loads was determined through examining their swelling kinetic in an aqueous solution of NaCl (0.9 wt%) under different pressures (0.3, 0.6, and 0.9 psi) (Fig. 4(c) and (d)). As shown in Fig. 4(c) and (d), swelling kinetic of both hydrogel samples follows a similar trend in which swelling capacity increased quickly at the initial time periods, and in continue its growth rate decreased gradually until an equilibrium swelling capacity was achieved. The maximum value of AUL at each pressure for NaAlg-gpoly(AA)/PVP (Fig. 4(c)) and NaAlg-g-poly(AA)/PVP/MMT (Fig. 4(d)) samples was obtained within 130 min and 210 min, respectively. These results indicated that the time needed to reach an equilibrium swelling capacity in NaAlg-g-poly(AA)/PVP/MMT was greater than that of NaAlg-g-poly(AA)/PVP. This was due to the lower water absorption rate of NaAlg-g-poly(AA)/PVP/MMT compared with NaAlg-g-poly (AA)/PVP. According to Fig. 4(c), the equilibrium water absorption capacity of NaAlg-g-poly(AA)/PVP was 50.1, 41.4, and 34.9 g/g at pressures of 0.3, 0.6, and 0.9 psi, respectively. In contrast, NaAlg-g-poly (AA)/PVP/MMT (Fig. 4(d)) presented higher water absorption capacity of 59.4, 51.8, and 42.1 g/g at pressures of 0.3, 0.6, and 0.9 psi, respectively. This can be attributed to the strong electrostatic repulsive forces between carboxylate groups of the hydrogel and negative surface charges of the incorporated MMT, which induced an expansion in hydrogel network, and hence an increase in the swelling capacity. Also, as can be clearly seen from Fig. 4(c) and (d), the equilibrium swelling capacity of the hydrogel samples decreases with increasing the amount of the applied pressure. This refers to the fact that greater applied pressure discharges larger amount of water molecules from the swollen hydrogel network, and thus decreases swelling capacity.

To assess the effect of cations charge valencies and salt solution concentration on the swelling behavior of the hydrogels, the equilibrium swelling capacity of NaAlg-g-poly(AA)/PVP (Fig. 5(b)) and NaAlg-g-poly(AA)/PVP/MMT (Fig. 5(c)) samples was evaluated in various saline solutions with different concentrations. As shown in Fig. 5(b) and (c), water absorption capacity of the hydrogel samples decreases appreciably as the saline solution concentration is increased. This can be attributed to the charge screening effect of the excess cations in the swelling medium, which induces non-perfect anion-anion electrostatic repulsions among carboxylate groups, shrinks hydrogel network, and so decreases swelling capacity. Moreover, with increasing saline solution concentration, osmotic pressure difference between the hydrogel network and external swelling medium decreases, resulting in the reduced swelling capacity. According to Fig. 5(b) and (c), the equilibrium swelling capacity of the hydrogel samples in different saline solutions was in the order of NaCl > CaCl2 > FeCl3. This phenomenon is mainly attributed to the complexation ability of multivalent cationic solutions. On the other hand, ionic crosslinking points within hydrogel network made by complexation of carboxylate groups with divalent and trivalent cations, cause an increase in crosslinking density, and hence a decrease in equilibrium swelling capacity. Besides, the higher cation charge generates greater degree of ionic crosslinkages within hydrogel network, and thus brings more reduction in the swelling capacity (Hooper, Baker, Blanch, & Prausnitz, 1990; Zheng & Wang, 2008). According to Fig. 5(b), the equilibrium swelling capacity (g/g) of NaAlg-g-poly(AA)/PVP in saline solutions of NaCl, CaCl2, and FeCl3 with concentrations (wt%) of 0.1, 0.3, 0.5, 0.7, 0.9, and 1.1 was 129.56, 81.88, 67.74, 59.63, 58, and 52.9 for NaCl, 18.63, 11.03, 7.25, 4.16, 3.07, and 2.48 for CaCl2, and 12.3, 8.45, 6.18, 3.41, 2.6, and 1.17 for FeCl3, respectively. In the case of NaAlg-g-poly(AA)/PVP/MMT (Fig. 5(c)), the equilibrium swelling capacity (g/g) in saline solutions of NaCl, CaCl2, and FeCl3 with concentrations (wt%) of 0.1, 0.3, 0.5, 0.7, 0.9, and 1.1 were 146.66, 89.65, 72, 63.72, 61.82, and 53.67 for NaCl, 30.18, 14.92, 9.16, 6.78, 5.13, and 4.09 for CaCl2, and 19.56, 12.17, 8.12, 5.74, 4.23, and 2.52 for FeCl3, respectively. These results revealed that equilibrium swelling capacity of NaAlg-g-poly(AA)/PVP/MMT in all saline solutions is higher than that of NaAlg-g-poly(AA)/PVP. The strong electrostatic repulsive forces between negative surface charges of the introduced MMT and carboxylate groups of polymeric matrix are responsible for this phenomenon, which induce more expanded hydrogel network with higher equilibrium swelling capacity.

3.9. pH-dependent swelling behavior The swelling behavior of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly (AA)/PVP/MMT samples was evaluated in various pH values ranging from 2 to 12 at room temperature (Fig. 5(a)). As shown in Fig. 5(a), the equilibrium swelling capacity of both hydrogel samples increases drastically with increasing pH from 2 to 8, and then it decreases severely with further increase in pH values up to 12. The maximum equilibrium swelling capacity of NaAlg-g-poly(AA)/PVP and NaAlg-gpoly(AA)/PVP/MMT samples was achieved at pH 8. The pH-sensitive swelling behavior of the hydrogel samples can be interpreted as follows. In acidic solutions (pH < 5), most of the carboxylate groups are protonated. At this condition, due to the strengthening of hydrogenbonding interactions among carboxylic acid groups, physical crosslinking density in hydrogel network increases, resulting in the reduced equilibrium swelling capacity. Meanwhile, the electrostatic repulsions between carboxylate anions are restricted. Thus, the hydrogel network tends to shrink and swelling capacity decreases. At higher pH values

3.11. Reswelling capability studies To evaluate reusability of the hydrogel samples, reswelling capability studies were performed on the NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT samples through measuring their swelling capacity loss during sequential swelling/drying cycles (Fig. 5(d)). 304

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Fig. 5. Equilibrium swelling capacity of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT samples at different pH values (a), equilibrium water absorbency of NaAlg-g-poly(AA)/ PVP (b) and NaAlg-g-poly(AA)/PVP/MMT (c) in saline solutions of NaCl, CaCl2, and FeCl3 with different concentrations, and the equilibrium swelling capacity loss of NaAlg-g-poly(AA)/ PVP and NaAlg-g-poly(AA)/PVP/MMT samples during consecutive swelling/drying cycles (d).

the superabsorbent nanocomposite network were in intercalated form, which was deduced by comparing XRD patterns of pristine MMT and NaAlg-g-poly(AA)/PVP/MMT. TGA analysis results indicated that incorporation of MMT into hydrogel network induces a substantial improvement in the thermal stability of the hydrogel sample. Also, in the presence of MMT, the loose and coarse surface of NaAlg-g-poly(AA)/ PVP sample were changed into a rough porous structure with interlinked channels, as depicted in SEM and TEM images. Swelling kinetic studies showed that NaAlg-g-poly(AA)/PVP/MMT sample had higher equilibrium swelling capacity (618.92 g/g) compared with neat hydrogel (521.17 g/g). The experimental swelling kinetic data possessed a good agreement with the second order swelling kinetic model so that theoretical equilibrium swelling capacity values (W∞) were very close to those experimental values. Also, the swelling rate constant (ks) for NaAlg-g-poly(AA)/PVP was greater than that of NaAlg-g-poly(AA)/ PVP/MMT, demonstrating higher swelling rate of hydrogel sample compared with superabsorbent nanocomposite. Moreover, equilibrium swelling capacity of the superabsorbent nanocomposite under different loads was higher than that of neat hydrogel. The swelling behavior of the hydrogels was remarkably affected by the solution pH and salt solution type and concentration. In addition, superabsorbent nanocomposite exhibited good reswelling capability compared with hydrogel sample. These superior characteristics of the semi-IPN superabsorbent nanocomposite revealed that it can be most suitable to manage water consumption in agricultural and horticultural applications.

As shown in Fig. 5(d), the equilibrium swelling capacity of both hydrogel samples presented a decreasing trend during consecutive swelling/drying cycles. The equilibrium swelling capacity of NaAlg-g-poly (AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT samples on the 1th and 5th cycles was 520.34 g/g, 298.45 g/g, and 635.64 g/g, and 470.35 g/g, respectively. According to the obtained results, the swelling capacity loss of NaAlg-g-poly(AA)/PVP and NaAlg-g-poly(AA)/PVP/MMT was 42.64% and 25.98%, respectively. The lower swelling capacity loss of NaAlg-g-poly(AA)/PVP/MMT compared with NaAlg-g-poly(AA)/PVP can be attributed to the firm three-dimensional frameworks within superabsorbent nanocomposite network. These structures created by physical crosslinking effect of MMT keep polymeric chains together tightly, and thus build a firm hydrogel network, which can withstand high pressure of the absorbed water molecules without collapses. As a result, the use of NaAlg-g-poly(AA)/PVP/MMT in soil medium can be most profitable from the sight of reduction of the irrigation frequencies, improvement drought resistance of plants, and extension of the utilization periods of the hydrogels. 4. Conclusion Semi-IPN superabsorbent nanocomposite based on NaAlg-g-poly (AA)/PVP/MMT was synthesized by free-radical graft polymerization and semi-interpenetrating techniques in an aqueous solution containing an initiator (APS), a crosslinking agent (MBA), a filler (MMT), and PVP. FTIR results confirmed the grafting of AA monomers onto NaAlg chains, interpenetration of PVP chains through the hydrogel network, and also successful incorporation of MMT into polymeric matrix. MMT layers in 305

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Acknowledgement

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