Preparation and swelling kinetic behavior of salt and pH-sensitive semi-IPN hydrogel nanocomposite based on chitosan and montmorillonite Ali Olad a

Preparation and swelling kinetic behavior of salt and pH-sensitive semi-IPN hydrogel nanocomposite based on chitosan and montmorillonite
Ali Olad a,*, Hamed Gharekhani a, Fatemeh Doustdar a, Mahyar Pourkhiyabi a
a Polymer Composite Research Laboratory, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran.

* Corresponding Author
Tel: +98 413 3393164
Fax: +98 413 3340191
E-mail: [email protected]
Semi-interpenetrating polymer network (semi-IPN) superabsorbent nanocomposite based on montmorillonite (MMT) and chitosan (CS)-g-poly(acrylic acid (AA)-co-acrylamide (AAm))/polyvinypyrrolidone (PVP) was synthesized in this work. Successful grafting of acrylate monomers onto CS backbone as well as interpenetration of PVP chains through hydrogel network in the presence of MMT was ascertained from FTIR results. Addition of MMT into polymeric matrix not only improved thermal stability but also made an interlinked porous structure, as depicted in SEM images. The effect of factors such as MMT content, solution pH, salt solution type and concentration, and temperature was assessed on the swelling behavior of hydrogels. Incorporation of MMT into hydrogel network up to 13 wt% provided higher equilibrium swelling capacity of 1568 g/g as well as lower swelling rate. Superabsorbent nanocomposite indicated better salt and pH-sensitive swelling behavior, and good water retention capability, suggesting that it can be a potential candidate for irrigation management in agricultural applications.

Keywords: Semi-interpenetrating polymer network; Chitosan; Montmorillonite; Polyvinylpyrrolidone; Superabsorbent nanocomposite.

1. Introduction
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Till now, many kinds of natural polymers such as starch ADDIN EN.CITE <EndNote><Cite><Author>Ma</Author><Year>2016</Year><RecNum>22</RecNum><DisplayText>(Ma, Zhu, Cao, Wang, &amp; Zhang, 2016)</DisplayText><record><rec-number>22</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501271798″>22</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Ma, Dongzhuo</author><author>Zhu, Baodong</author><author>Cao, Bo</author><author>Wang, Jian</author><author>Zhang, Jianwei</author></authors></contributors><titles><title>The Microstructure and Swelling Properties of Poly Acrylic Acid-Acrylamide Grafted Starch Hydrogels</title><secondary-title>Journal of Macromolecular Science, Part B</secondary-title></titles><periodical><full-title>Journal of Macromolecular Science, Part B</full-title></periodical><pages>1124-1133</pages><volume>55</volume><number>11</number><dates><year>2016</year></dates><isbn>0022-2348</isbn><urls></urls></record></Cite></EndNote>(Ma, Zhu, Cao, Wang, & Zhang, 2016), cellulose ADDIN EN.CITE <EndNote><Cite><Author>Kono</Author><Year>2012</Year><RecNum>23</RecNum><DisplayText>(Kono &amp; Fujita, 2012)</DisplayText><record><rec-number>23</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501272287″>23</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Kono, Hiroyuki</author><author>Fujita, Sayaka</author></authors></contributors><titles><title>Biodegradable superabsorbent hydrogels derived from cellulose by esterification crosslinking with 1, 2, 3, 4-butanetetracarboxylic dianhydride</title><secondary-title>Carbohydrate Polymers</secondary-title></titles><periodical><full-title>Carbohydrate Polymers</full-title></periodical><pages>2582-2588</pages><volume>87</volume><number>4</number><dates><year>2012</year></dates><isbn>0144-8617</isbn><urls></urls></record></Cite></EndNote>(Kono & Fujita, 2012), sodium alginate (NaAlg) ADDIN EN.CITE <EndNote><Cite><Author>Rashidzadeh</Author><Year>2014</Year><RecNum>24</RecNum><DisplayText>(Rashidzadeh, Olad, Salari, &amp; Reyhanitabar, 2014)</DisplayText><record><rec-number>24</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501272397″>24</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Rashidzadeh, Azam</author><author>Olad, Ali</author><author>Salari, Dariush</author><author>Reyhanitabar, Adel</author></authors></contributors><titles><title>On the preparation and swelling properties of hydrogel nanocomposite based on sodium alginate-g-poly (acrylic acid-co-acrylamide)/clinoptilolite and its application as slow release fertilizer</title><secondary-title>Journal of Polymer Research</secondary-title></titles><periodical><full-title>Journal of Polymer Research</full-title></periodical><pages>344</pages><volume>21</volume><number>2</number><dates><year>2014</year></dates><isbn>1022-9760</isbn><urls></urls></record></Cite></EndNote>(Rashidzadeh, Olad, Salari, & Reyhanitabar, 2014), and chitosan (CS) ADDIN EN.CITE <EndNote><Cite><Author>Wang</Author><Year>2010</Year><RecNum>25</RecNum><DisplayText>(Spagnol et al., 2012; Q. Wang, Xie, Zhang, Zhang, &amp; Wang, 2010)</DisplayText><record><rec-number>25</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501273323″>25</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Wang, Qin</author><author>Xie, Xiaoling</author><author>Zhang, Xiaowei</author><author>Zhang, Junping</author><author>Wang, Aiqin</author></authors></contributors><titles><title>Preparation and swelling properties of pH-sensitive composite hydrogel beads based on chitosan-g-poly (acrylic acid)/vermiculite and sodium alginate for diclofenac controlled release</title><secondary-title>International Journal of Biological Macromolecules</secondary-title></titles><periodical><full-title>International Journal of Biological Macromolecules</full-title></periodical><pages>356-362</pages><volume>46</volume><number>3</number><dates><year>2010</year></dates><isbn>0141-8130</isbn><urls></urls></record></Cite><Cite><Author>Spagnol</Author><Year>2012</Year><RecNum>26</RecNum><record><rec-number>26</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501273353″>26</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Spagnol, Cristiane</author><author>Rodrigues, Francisco HA</author><author>Pereira, Antonio GB</author><author>Fajardo, André R</author><author>Rubira, Adley F</author><author>Muniz, Edvani C</author></authors></contributors><titles><title>Superabsorbent hydrogel composite made of cellulose nanofibrils and chitosan-graft-poly (acrylic acid)</title><secondary-title>Carbohydrate Polymers</secondary-title></titles><periodical><full-title>Carbohydrate Polymers</full-title></periodical><pages>2038-2045</pages><volume>87</volume><number>3</number><dates><year>2012</year></dates><isbn>0144-8617</isbn><urls></urls></record></Cite></EndNote>(Spagnol et al., 2012; Q. Wang, Xie, Zhang, Zhang, & Wang, 2010) have been used to prepare multi-component superabsorbent hydrogels. CS, an N-deacetylated polysaccharide derived from chitin, is one of the most abundant biopolymers in the world, which possesses more interesting features such as biocompatibility, biodegradability, non-toxicity, and bioadsorbability. The reactive –NH2 and –OH groups of CS make it as a desirable backbone for grafting reactions with acrylate-based monomers to generate a multi-component hydrogel with novel or improved properties ADDIN EN.CITE <EndNote><Cite><Author>Liu</Author><Year>2008</Year><RecNum>28</RecNum><DisplayText>(Jianghua Liu &amp; Wang, 2008; Jianghua Liu, Wang, &amp; Wang, 2007)</DisplayText><record><rec-number>28</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501274269″>28</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Liu, Jianghua</author><author>Wang, Aiqin</author></authors></contributors><titles><title>Study on superabsorbent composites. XXI. Synthesis, characterization and swelling behaviors of chitosan?g?poly (acrylic acid)/organo?rectorite nanocomposite superabsorbents</title><secondary-title>Journal of applied polymer science</secondary-title></titles><periodical><full-title>Journal of applied polymer science</full-title></periodical><pages>678-686</pages><volume>110</volume><number>2</number><dates><year>2008</year></dates><isbn>1097-4628</isbn><urls></urls></record></Cite><Cite><Author>Liu</Author><Year>2007</Year><RecNum>29</RecNum><record><rec-number>29</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501274369″>29</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Liu, Jianghua</author><author>Wang, Qin</author><author>Wang, Aiqin</author></authors></contributors><titles><title>Synthesis and characterization of chitosan-g-poly (acrylic acid)/sodium humate superabsorbent</title><secondary-title>Carbohydrate Polymers</secondary-title></titles><periodical><full-title>Carbohydrate Polymers</full-title></periodical><pages>166-173</pages><volume>70</volume><number>2</number><dates><year>2007</year></dates><isbn>0144-8617</isbn><urls></urls></record></Cite></EndNote>(Jianghua Liu & Wang, 2008; Jianghua Liu, Wang, & Wang, 2007). Polyvinylpyrrolidone (PVP) is a non-ionic water-soluble linear polymer. The good solubility, non-toxicity, excellent affinity to various polymers and resins, biodegradability, and compatibility are the most fascinating features of PVP, making it as a suitable candidate for use in medicine, pharmaceuticals, cosmetics, foods, printing inks, textiles, and many other fields ADDIN EN.CITE <EndNote><Cite><Author>Wang</Author><Year>2011</Year><RecNum>30</RecNum><DisplayText>(W. Wang, Wang, &amp; Wang, 2011)</DisplayText><record><rec-number>30</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501274492″>30</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Wang, Wenbo</author><author>Wang, Qin</author><author>Wang, Aiqin</author></authors></contributors><titles><title>pH-responsive carboxymethylcellulose-g-poly (sodium acrylate)/polyvinylpyrrolidone semi-IPN hydrogels with enhanced responsive and swelling properties</title><secondary-title>Macromolecular Research</secondary-title></titles><periodical><full-title>Macromolecular Research</full-title></periodical><pages>57-65</pages><volume>19</volume><number>1</number><dates><year>2011</year></dates><isbn>1598-5032</isbn><urls></urls></record></Cite></EndNote>(W. Wang, Wang, & Wang, 2011). The good compatibility of PVP enables it to be used as an appropriate component along with CS and other acrylate-based monomers to prepare a high performance multi-component hydrogel system through graft copolymerization and semi-IPN techniques. However, most of these multi-component hydrogel systems cannot be exploited to the practical use because of their high production cost. To cope with this limitation, low cost inorganic compounds can be introduced into polymer matrix in large extents to produce multi-component superabsorbent nanocomposite. Among the inorganic compounds, clay minerals due to the exceptional features, i.e. small particle size and intercalation properties have gained great interest. The strong interfacial interactions between the dispersed layers of clay materials and polymeric matrix cause a substantial improvement in mechanical, thermal, and barrier characteristics of the prepared SH. Montmorillonite (MMT) is a layered aluminosilicate with exchangeable cations and reactive hydroxyl groups on the surface. The special features of MMT including high in plane strength, stiffness, and high aspect ratio have led to its broad utilization as filler to prepare superabsorbent nanocomposites ADDIN EN.CITE <EndNote><Cite><Author>Yadav</Author><Year>2012</Year><RecNum>31</RecNum><DisplayText>(Yadav &amp; Rhee, 2012)</DisplayText><record><rec-number>31</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501274579″>31</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Yadav, Mithilesh</author><author>Rhee, Kyong Yop</author></authors></contributors><titles><title>Superabsorbent nanocomposite (alginate-g-PAMPS/MMT): synthesis, characterization and swelling behavior</title><secondary-title>Carbohydrate polymers</secondary-title></titles><periodical><full-title>Carbohydrate Polymers</full-title></periodical><pages>165-173</pages><volume>90</volume><number>1</number><dates><year>2012</year></dates><isbn>0144-8617</isbn><urls></urls></record></Cite></EndNote>(Yadav & Rhee, 2012).
Based on the above description, in the recent work, a novel pH-sensitive multi-component chitosan (CS)-g-poly(acrylic acid (AA)-co-acrylamide (AAm))/PVP/MMT (CS-g-poly(AA-co-AAm)/PVP/MMT) semi-IPN superabsorbent nanocomposite was synthesized by the free-radical graft copolymerization and semi-IPN techniques. The effect of introduced MMT on the equilibrium swelling capacity of the prepared hydrogels was studied. The swelling kinetics and swelling behavior of the hydrogel samples in various pH values and different saline solutions were also evaluated. Moreover, water retention capability of the hydrogels under constant temperatures was assessed. Additionally, to examine the swelling behavior of the hydrogels under different loads, water absorbency under load (AUL) studies were performed.

2. Experimental
2.1. Materials
Chitosan (medium molecular weight, degree of deacetylation 85%) and sodium montmorillonite (Na-MMT, specific surface area = 20-40 m2/g, cation exchange capacity = 30 meq/100 g) were purchased from Sigma-Aldrich (USA). Acrylic acid (AA), acrylamide (AAm), polyvinylpyrrolidone (PVP, the average molecular weight Mr=25000), N,N?-methylene bisacrylamide (MBA), ammonium persulfate (APS), and N,N,N?,N?-tetramethylethylenediamine (TEMED) were all obtained from Merck company (Germany). Sodium chloride, calcium chloride, and ferric chloride hexahydrate (FeCl3.6H2O) were procured from Merck company (Germany). All other chemicals possessed analytical grade and were used without any further purification. Distilled water was used to prepare all solutions.

2.2. Preparation of semi-IPN CS-g-poly(AA-co-AAm)/PVP/MMT superabsorbent nanocomposite
A certain amount of CS powder (0.6 g) was first solubilized in 20 mL acetic acid aqueous solution (1% v/v) under continuous stirring in a 250 mL four-necked round bottom flask equipped with a reflux condenser, a mechanical stirrer, a thermometer, and a nitrogen line. An appropriate amount of MMT (3-18 wt%, with respect to CS) was dispersed in 10 mL acetic acid aqueous solution (1% v/v) by sonication at 50 W power for 5 min. The resultant suspension was then poured into the reaction flask, while stirring. After heating the solution to 40 °C by a water bath, 0.045 g PVP and 0.8 g AAm were added gradually into the prepared mixture and allowed to stir until a homogeneous sticky solution was formed. The prepared solution was bubbled with nitrogen gas for 30 min to remove the dissolved oxygen. Thereafter, a mixed solution of partly neutralized (50%) AA (3.2 mL) and MBA (0.83 wt%, with respect to CS) was charged into the reaction mixture. Afterwards, a 5 mL of aqueous solution of APS (6.6 wt%, with respect to CS) was introduced under continuous mechanical stirring. After 5 min, 100 µL of TEMED aqueous solution (20% v/v) was added into the flask. The system temperature was maintained at 40 °C for 1 h under stirring and continuous purging of nitrogen gas to complete the polymerization reaction. The as prepared gel product was cut into small pieces, submerged in ethanol for 24 h to remove unreacted species, and finally it was dried in a vacuum oven at 60 °C for 24 h. The dried gels were ground and passed through 40-80 mesh sieves for further experiments. As comparison, a semi-IPN CS-g-poly(AA-co-AAm)/PVP hydrogel was also prepared similar to the above mentioned procedure without addition of MMT.

2.3. Characterization
To characterize chemical structure of the materials, a blend compound of KBr powder and dried materials was first pressed into tablets, and then FTIR spectrum was acquired using a Bruker Tensor 27 FTIR spectrophotometer operating in the wavenumber range of 400-4000 cm-1. Thermogravimetric analysis (TGA) of the hydrogel samples was performed using a thermal gravimetric analyzer (TGA/DSC-1, Mettler Toledo) under nitrogen atmosphere from 47 °C to 610 °C at a heating rate of 10 °C/min. To study the surface morphology of the hydrogels, the gold-coated surface of the prepared samples was scanned using a field emission scanning electron microscope (FE-SEM) system (MIRA3 FEG-SEM, Tescan, Czech).

2.4. Study of swelling properties
2.4.1. Measurement of equilibrium water absorbency and swelling kinetics
To evaluate swelling kinetics of the hydrogel samples, pre-weighted dry hydrogel sample (Wd) was initially placed in 100 mL distilled water and allowed to swell. At pre-determined time intervals, the swollen hydrogel sample was withdrawn from distilled water, and after wiping off the excess surface water with filter paper, it was weighed, accurately (Wt). When hydrogel sample reached to own equilibrium swelling capacity, its swollen weight remained at a constant value of Weq. Three replicates were done for each hydrogel sample and finally average values were reported. Eventually, the swelling capacity (St) and equilibrium swelling capacity (Seq) were determined using following equations:
St (g/g) = Wt – WdWd (1)
Seq (g/g) = Weq – WdWd (2)
2.4.2. Evaluation of pH-sensitive swelling behavior
To study the effect of various pH values on the swelling behavior of the hydrogel samples, different pH solutions ranging from 2-12 were prepared by diluting aqueous solutions of NaOH (0.1 M) and HCl (0.1 M). The equilibrium swelling capacity of the hydrogel samples at each pH value was determined according to the Eq. (2).

2.4.3. Swelling behavior in different saline solutions
The swelling behavior of the hydrogel samples was assessed in various saline solutions (NaCl, CaCl2, and FeCl3) at different concentrations (0.1, 0.3, 0.5, 0.7, 0.9, and 1.1 wt%), and finally equilibrium water absorption capacity of the hydrogels was calculated using the previously described method in section 2.4.1.

2.4.4. Evaluation of water absorbency under load (AUL)
To examine water absorbency under load properties of the hydrogels, measurement of the swelling kinetics of the hydrogels was conducted in 0.9 wt% NaCl aqueous solution as swelling medium under different pressures (0.3, 0.6, and 0.9 psi). Initially, certain amount of granular hydrogel sample (0.1 g, 40-80 mesh) was put onto the round shaped piece of a wire cloth (d = 80 mm, h = 200 µm, 100 mesh), and then it was placed onto a porous sintered glass filter plate (d = 80 mm, h = 7 mm) in a petri dish (d = 118 mm, h = 12 mm). Thereafter, 0.9 wt% NaCl aqueous solution was poured slowly into petri dish up to the upper surface of the porous sintered glass filter plate. Afterwards, the surface of the round shaped piece of wire cloth was surrounded by a glass cylinder (d = 60 mm, h = 50 mm), and then different cylindrical solid loads (Teflon, d = 60 mm, variable height) were passed through it freely to exert pressure. To avoid any changes in saline solution concentration by surface evaporation, the prepared system was covered properly. At given time intervals, the swollen hydrogel samples were taken out of the system and after weighing, swelling capacity was determined using the similar procedure mentioned earlier in section 2.4.1.

2.4.5. Water retention capability under constant temperature
Water retention capability of the hydrogel samples was assessed through examining their deswelling behavior under constant temperatures of 40 °C and 80 °C. For this purpose, given amount of dry hydrogel sample was first soaked in 100 mL distilled water and allowed to reach equilibrium swelling capacity at room temperature. Then, the swollen hydrogel sample was separated from the swelling medium by a 200-mesh cloth, and after weighing, it was transferred into a beaker and placed in a vacuum oven at 40 °C or 80 °C. Every 1 h, the swollen hydrogel sample was taken out of oven and weighed accurately. These measurements were continued for 10 h and finally swelling capacity of the hydrogel samples was determined using the previously described method in section 2.4.1.

3. Results and discussion
3.1. Synthesis mechanism of semi-IPN superabsorbent nanocomposite
Semi-IPN superabsorbent nanocomposite was synthesized by simultaneous graft copolymerization of AA and AAm monomers onto CS backbone and interpenetration of PVP chains through hydrogel network in an aqueous solution containing APS (initiator) and MBA (crosslinking agent) (Scheme 1). Initially, dissociation of APS molecules in the presence of TEMED as catalyst generates sulfate anion-radicals. These active radicals are able to extract hydrogen from the hydroxyl or amine groups of CS, resulting in the formation of macro-radicals. These macro-radicals react with the nearest AA or AAm monomers, causing propagation of the grafted polymeric chains. During the chain propagation reactions, the end vinyl groups of MBA may react synchronously with the active centers onto the copolymer chains and so crosslinked structure is formed. The linear PVP chains are also combined and interpenetrated with hydrogel network through hydrogen bonding interactions to build semi-IPN structure. The function of MMT particles in the graft copolymerization reaction can be considered as a crosslinking agent or a physical inhibitor to prevent growing of polymer chains by a chain transfer mechanism ADDIN EN.CITE <EndNote><Cite><Author>Uthirakumar</Author><Year>2004</Year><RecNum>33</RecNum><DisplayText>(Uthirakumar, Nahm, Hahn, &amp; Lee, 2004)</DisplayText><record><rec-number>33</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501509886″>33</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Uthirakumar, Periyayya</author><author>Nahm, Kee Suk</author><author>Hahn, Yoon Bong</author><author>Lee, Youn-Sik</author></authors></contributors><titles><title>Preparation of polystyrene/montmorillonite nanocomposites using a new radical initiator-montmorillonite hybrid via in situ intercalative polymerization</title><secondary-title>European Polymer Journal</secondary-title></titles><periodical><full-title>European Polymer Journal</full-title></periodical><pages>2437-2444</pages><volume>40</volume><number>11</number><dates><year>2004</year></dates><isbn>0014-3057</isbn><urls></urls></record></Cite></EndNote>(Uthirakumar, Nahm, Hahn, & Lee, 2004).

Scheme 1. Proposed graft copolymerization mechanism for synthesis of semi-IPN CS-g-poly (AA-co-AAm)/PVP/MMT superabsorbent nanocomposite.

3.2. FTIR spectra analysis
The FTIR spectra of CS, MMT, PVP, CS-g-poly(AA-co-AAm)/PVP hydrogel, and CS-g-poly(AA-co-AAm)/PVP/MMT superabsorbent nanocomposite have been shown in Fig. 1(a). As shown in Fig. 1(a) for CS, the relatively broad band emerged at 3441 cm-1 is attributed to the O-H stretching vibration, N-H stretching vibration, and the intermolecular H-bonds of the polysaccharide moieties ADDIN EN.CITE <EndNote><Cite><Author>Azhar</Author><Year>2014</Year><RecNum>34</RecNum><DisplayText>(Azhar &amp; Olad, 2014)</DisplayText><record><rec-number>34</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501514999″>34</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Azhar, Fahimeh Farshi</author><author>Olad, Ali</author></authors></contributors><titles><title>A study on sustained release formulations for oral delivery of 5-fluorouracil based on alginate–chitosan/montmorillonite nanocomposite systems</title><secondary-title>Applied Clay Science</secondary-title></titles><periodical><full-title>Applied Clay Science</full-title></periodical><pages>288-296</pages><volume>101</volume><dates><year>2014</year></dates><isbn>0169-1317</isbn><urls></urls></record></Cite></EndNote>(Azhar & Olad, 2014). Also, the characteristic absorption bands at 1649 cm-1, 1556 cm-1, 1388 cm-1, 1085 cm-1, and 1024 cm-1 are related to the stretching vibration of carbonyl group (C=O) of amide I, N-H bond, -NHCO of amide III, C3-OH, and C6-OH of CS, respectively ADDIN EN.CITE <EndNote><Cite><Author>Zhang</Author><Year>2007</Year><RecNum>35</RecNum><DisplayText>(J. Zhang, L. Wang, &amp; A. Wang, 2007a)</DisplayText><record><rec-number>35</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501515390″>35</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Zhang, Junping</author><author>Wang, Li</author><author>Wang, Aiqin</author></authors></contributors><titles><title>Preparation and properties of chitosan-g-poly (acrylic acid)/montmorillonite superabsorbent nanocomposite via in situ intercalative polymerization</title><secondary-title>Industrial &amp; engineering chemistry research</secondary-title></titles><periodical><full-title>Industrial &amp; engineering chemistry research</full-title></periodical><pages>2497-2502</pages><volume>46</volume><number>8</number><dates><year>2007</year></dates><isbn>0888-5885</isbn><urls></urls></record></Cite></EndNote>(J. Zhang, L. Wang, & A. Wang, 2007a). In FTIR spectrum of MMT (Fig. 1(a)), the characteristic absorption bands for stretching vibration modes of Si-O-Al and Si-O-Si groups were appeared at 794 cm-1 and 1026 cm-1, respectively, while their bending modes were observed at 522 cm-1 and 460 cm-1, respectively. Also, the peak at 1630 cm-1 is due to the bending mode of –OH group of the adsorbed water. Moreover, the broad bands at 3400 cm-1 and 3625 cm-1 are assigned to the stretching mode of –OH groups of water and –OH groups in the inner structure of MMT ADDIN EN.CITE <EndNote><Cite><Author>Azhar</Author><Year>2014</Year><RecNum>34</RecNum><DisplayText>(Azhar &amp; Olad, 2014; Patel, Somani, Bajaj, &amp; Jasra, 2007)</DisplayText><record><rec-number>34</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501514999″>34</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Azhar, Fahimeh Farshi</author><author>Olad, Ali</author></authors></contributors><titles><title>A study on sustained release formulations for oral delivery of 5-fluorouracil based on alginate–chitosan/montmorillonite nanocomposite systems</title><secondary-title>Applied Clay Science</secondary-title></titles><periodical><full-title>Applied Clay Science</full-title></periodical><pages>288-296</pages><volume>101</volume><dates><year>2014</year></dates><isbn>0169-1317</isbn><urls></urls></record></Cite><Cite><Author>Patel</Author><Year>2007</Year><RecNum>36</RecNum><record><rec-number>36</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501518862″>36</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Patel, Hasmukh A</author><author>Somani, Rajesh S</author><author>Bajaj, Hari C</author><author>Jasra, Raksh V</author></authors></contributors><titles><title>Preparation and characterization of phosphonium montmorillonite with enhanced thermal stability</title><secondary-title>Applied Clay Science</secondary-title></titles><periodical><full-title>Applied Clay Science</full-title></periodical><pages>194-200</pages><volume>35</volume><number>3</number><dates><year>2007</year></dates><isbn>0169-1317</isbn><urls></urls></record></Cite></EndNote>(Azhar & Olad, 2014; Patel, Somani, Bajaj, & Jasra, 2007). 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ADDIN EN.CITE.DATA (Lü, Liu, Ni, & Gao, 2010; W. Wang & Wang, 2010; X. Wang et al., 2016). As can be seen from FTIR spectra of CS-g-poly(AA-co-AAm)/PVP hydrogel and CS-g-poly(AA-co-AAm)/PVP/MMT superabsorbent nanocomposite (Fig. 1(a)), the absorption bands observed respectively at 1689 cm-1 and 1692 cm-1 are attributed to the overlapped stretching vibration of carboxamide and bending vibration of N-H in amide group. The asymmetric stretching vibrations of the carboxylate groups of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT overlapped with the vibration mode of carbonyl group of amide I of CS were appeared at 1651 cm-1 and 1648 cm-1, respectively. The symmetric stretching modes of the carboxylate groups in FTIR spectra of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT were also emerged at 1532 cm-1, 1399 cm-1, and 1546 cm-1 and 1370 cm-1, respectively. The peaks in the region of 1150-1350 cm-1 are related to the stretching modes of C-N and C-O groups as well as bending mode of O-H bond. Moreover, the combined stretching vibration of CH2 groups in both AA and AAm for CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT was appeared as two peaks between 2850 cm-1 and 2980 cm-1. Additionally, the broad and intense peaks, observed between 3400 cm-1 and 3600 cm-1 are related to the overlapped absorption bands of O-H and N-H groups ADDIN EN.CITE <EndNote><Cite><Author>Marandi</Author><Year>2011</Year><RecNum>39</RecNum><DisplayText>(Marandi, Mahdavinia, &amp; Ghafary, 2011; J. Zhang et al., 2007a)</DisplayText><record><rec-number>39</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501578996″>39</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Marandi, Gholam Bagheri</author><author>Mahdavinia, Gholam Reza</author><author>Ghafary, Shahrzad</author></authors></contributors><titles><title>Collagen-g-poly (Sodium Acrylate-co-Acrylamide)/sodium montmorillonite superabsorbent nanocomposites: synthesis and swelling behavior</title><secondary-title>Journal of Polymer Research</secondary-title></titles><periodical><full-title>Journal of Polymer Research</full-title></periodical><pages>1487-1499</pages><volume>18</volume><number>6</number><dates><year>2011</year></dates><isbn>1022-9760</isbn><urls></urls></record></Cite><Cite><Author>Zhang</Author><Year>2007</Year><RecNum>41</RecNum><record><rec-number>41</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501579316″>41</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Zhang, Junping</author><author>Wang, Li</author><author>Wang, Aiqin</author></authors></contributors><titles><title>Preparation and properties of chitosan-g-poly (acrylic acid)/montmorillonite superabsorbent nanocomposite via in situ intercalative polymerization</title><secondary-title>Industrial &amp; engineering chemistry research</secondary-title></titles><periodical><full-title>Industrial &amp; engineering chemistry research</full-title></periodical><pages>2497-2502</pages><volume>46</volume><number>8</number><dates><year>2007</year></dates><isbn>0888-5885</isbn><urls></urls></record></Cite></EndNote>(Marandi, Mahdavinia, & Ghafary, 2011; J. Zhang et al., 2007a). According to the FTIR spectra of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT, it can be clearly seen that the characteristic absorption bands of N-H (1556 cm-1 and 1388 cm-1) and C3-OH (1085 cm-1) groups of CS have been disappeared after reaction. These findings revealed that –NH2, -NHCO, and –OH groups of CS have been effectively participated in grafting reaction with AA and AAm monomers ADDIN EN.CITE <EndNote><Cite><Author>Zhang</Author><Year>2007</Year><RecNum>32</RecNum><DisplayText>(J. Zhang, Q. Wang, &amp; A. Wang, 2007b)</DisplayText><record><rec-number>32</rec-number><foreign-keys><key app=”EN” db-id=”00020taxmsr2aaexwr6pr0pf2z9xwzd0ewzs” timestamp=”1499239511″>32</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Zhang, Junping</author><author>Wang, Qin</author><author>Wang, Aiqin</author></authors></contributors><titles><title>Synthesis and characterization of chitosan-g-poly (acrylic acid)/attapulgite superabsorbent composites</title><secondary-title>Carbohydrate Polymers</secondary-title></titles><periodical><full-title>Carbohydrate Polymers</full-title></periodical><pages>367-374</pages><volume>68</volume><number>2</number><dates><year>2007</year></dates><isbn>0144-8617</isbn><urls></urls></record></Cite></EndNote>(J. Zhang, Q. Wang, & A. Wang, 2007b). The characteristic absorption band of carbonyl group of PVP (1667 cm-1) shifted to the higher wavenumbers of 1689 cm-1 and 1692 cm-1 in respectively FTIR spectra of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT, and overlapped with the corresponding peaks of carboxamide and N-H groups. The strong hydrogen-bonding interactions between carboxamide and carbonyl groups are responsible for this phenomenon. In addition, the peaks related to the C-N stretching vibration of PVP have been appeared with slight shift in FTIR spectra of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT. These results imply that PVP chains have interpenetrated through hydrogel network by hydrogen-bonding interactions ADDIN EN.CITE <EndNote><Cite><Author>Wang</Author><Year>2010</Year><RecNum>21</RecNum><DisplayText>(W. Wang &amp; Wang, 2010)</DisplayText><record><rec-number>21</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501271680″>21</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Wang, Wenbo</author><author>Wang, Aiqin</author></authors></contributors><titles><title>Synthesis and swelling properties of pH-sensitive semi-IPN superabsorbent hydrogels based on sodium alginate-g-poly (sodium acrylate) and polyvinylpyrrolidone</title><secondary-title>Carbohydrate Polymers</secondary-title></titles><periodical><full-title>Carbohydrate Polymers</full-title></periodical><pages>1028-1036</pages><volume>80</volume><number>4</number><dates><year>2010</year></dates><isbn>0144-8617</isbn><urls></urls></record></Cite></EndNote>(W. Wang & Wang, 2010). Comparing with the FTIR spectrum of CS-g-poly(AA-co-AAm)/PVP, the appearance of the characteristic absorption band of MMT (1026 cm-1) with slight shift in FTIR spectrum of CS-g-poly(AA-co-AAm)/PVP/MMT confirms its successful incorporation into hydrogel network. All these results indicated that synthesis of semi-IPN CS-g-poly(AA-co-AAm)/PVP and semi-IPN CS-g-poly(AA-co-AAm)/PVP/MMT has been accomplished, successfully.

3.3. Thermogravimetric analysis (TGA)
TGA and differential TGA (DTG) curves of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT (with 13 wt% MMT) have been shown in Fig. 1(b) and (c), respectively. As shown in Fig. 1(b) and (c), CS-g-poly(AA-co-AAm)/PVP exhibited four distinct decomposition stages from 47 °C to 610 °C, while in the case of CS-g-poly(AA-co-AAm)/PVP/MMT, decomposition process accomplished within five stages. As the temperature increased to 200 °C, CS-g-poly(AA-co-AAm)/PVP showed a sharp weight loss of 23.07%, implying the loss of moisture present in the sample. A minor weight loss (3.52%) was observed for CS-g-poly(AA-co-AAm)/PVP/MMT when the temperature increased from 47 °C to 100 °C, which was due to the dehydration of adsorbed water, interlayer water, and coordinated water to exchangeable cations of MMT ADDIN EN.CITE <EndNote><Cite><Author>Bao</Author><Year>2011</Year><RecNum>32</RecNum><DisplayText>(Bao, Ma, &amp; Li, 2011)</DisplayText><record><rec-number>32</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501274654″>32</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Bao, Yan</author><author>Ma, Jianzhong</author><author>Li, Na</author></authors></contributors><titles><title>Synthesis and swelling behaviors of sodium carboxymethyl cellulose-g-poly (AA-co-AM-co-AMPS)/MMT superabsorbent hydrogel</title><secondary-title>Carbohydrate Polymers</secondary-title></titles><periodical><full-title>Carbohydrate Polymers</full-title></periodical><pages>76-82</pages><volume>84</volume><number>1</number><dates><year>2011</year></dates><isbn>0144-8617</isbn><urls></urls></record></Cite></EndNote>(Bao, Ma, & Li, 2011). With increasing temperature to 180 °C, the moisture present in CS-g-poly(AA-co-AAm)/PVP/MMT was evaporated gradually, which in turn induced a weight loss of 8.97%. The weight losses within the temperature ranges of 200-328 °C and 180-337 °C for respectively CS-g-poly(AA-co-AAm)/PVP (22.02%) and CS-g-poly(AA-co-AAm)/PVP/MMT (18.56%) are associated with the complex processes, including dehydration of saccharide rings and breaking of glycosidic C-O-C bonds in the main chain of CS ADDIN EN.CITE <EndNote><Cite><Author>Zhang</Author><Year>2007</Year><RecNum>32</RecNum><DisplayText>(J. Zhang et al., 2007b)</DisplayText><record><rec-number>32</rec-number><foreign-keys><key app=”EN” db-id=”00020taxmsr2aaexwr6pr0pf2z9xwzd0ewzs” timestamp=”1499239511″>32</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Zhang, Junping</author><author>Wang, Qin</author><author>Wang, Aiqin</author></authors></contributors><titles><title>Synthesis and characterization of chitosan-g-poly (acrylic acid)/attapulgite superabsorbent composites</title><secondary-title>Carbohydrate Polymers</secondary-title></titles><periodical><full-title>Carbohydrate Polymers</full-title></periodical><pages>367-374</pages><volume>68</volume><number>2</number><dates><year>2007</year></dates><isbn>0144-8617</isbn><urls></urls></record></Cite></EndNote>(J. Zhang et al., 2007b). The major weight losses of 29.74% and 31.41% were found for CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT samples, respectively, which were occurred respectively within the temperature ranges of 328-420 °C and 337-440 °C. This phenomenon is attributed to the thermal decomposition of carboxyl and amide groups of copolymer chains as well as scission of copolymer chains, which is associated with emission of ammonia and CO2 gases ADDIN EN.CITE <EndNote><Cite><Author>Zhang</Author><Year>2007</Year><RecNum>44</RecNum><DisplayText>(J. Zhang &amp; Wang, 2007)</DisplayText><record><rec-number>44</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501616415″>44</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Zhang, Junping</author><author>Wang, Aiqin</author></authors></contributors><titles><title>Study on superabsorbent composites. IX: synthesis, characterization and swelling behaviors of polyacrylamide/clay composites based on various clays</title><secondary-title>Reactive and Functional Polymers</secondary-title></titles><periodical><full-title>Reactive and Functional Polymers</full-title></periodical><pages>737-745</pages><volume>67</volume><number>8</number><dates><year>2007</year></dates><isbn>1381-5148</isbn><urls></urls></record></Cite></EndNote>(J. Zhang & Wang, 2007). The last decomposition stages for CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT started at 420 °C and 440 °C, respectively, and extended to about 610 °C. The weight losses of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT within the corresponding temperature ranges were obtained as 32.87% and 30.33%, respectively. These weight losses are interpreted as elimination of water molecules as a result of association of two neighboring carboxylic groups of the polymer chains through formation of anhydride, decomposition of the copolymer chains, and destruction of the crosslinked network structure ADDIN EN.CITE <EndNote><Cite><Author>Bao</Author><Year>2011</Year><RecNum>32</RecNum><DisplayText>(Bao et al., 2011)</DisplayText><record><rec-number>32</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501274654″>32</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Bao, Yan</author><author>Ma, Jianzhong</author><author>Li, Na</author></authors></contributors><titles><title>Synthesis and swelling behaviors of sodium carboxymethyl cellulose-g-poly (AA-co-AM-co-AMPS)/MMT superabsorbent hydrogel</title><secondary-title>Carbohydrate Polymers</secondary-title></titles><periodical><full-title>Carbohydrate Polymers</full-title></periodical><pages>76-82</pages><volume>84</volume><number>1</number><dates><year>2011</year></dates><isbn>0144-8617</isbn><urls></urls></record></Cite></EndNote>(Bao et al., 2011). The results indicated that incorporation of MMT into polymeric matrix of hydrogel induces a lower thermal decomposition rate as well as less total weight loss over the temperature range of 47-610 °C. The heat barrier effect of the MMT layers is the main reason for this phenomenon, which hinders diffusion of oxygen and volatile thermo-oxidation products throughout the hydrogel composite network ADDIN EN.CITE <EndNote><Cite><Author>Qiu</Author><Year>2006</Year><RecNum>45</RecNum><DisplayText>(Qiu, Chen, &amp; Qu, 2006)</DisplayText><record><rec-number>45</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501616661″>45</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Qiu, Longzhen</author><author>Chen, Wei</author><author>Qu, Baojun</author></authors></contributors><titles><title>Morphology and thermal stabilization mechanism of LLDPE/MMT and LLDPE/LDH nanocomposites</title><secondary-title>Polymer</secondary-title></titles><periodical><full-title>Polymer</full-title></periodical><pages>922-930</pages><volume>47</volume><number>3</number><dates><year>2006</year></dates><isbn>0032-3861</isbn><urls></urls></record></Cite></EndNote>(Qiu, Chen, & Qu, 2006). Besides, the additional physical crosslinkages within hydrogel network made by introduced MMT build a firm three-dimensional hydrogel structure with good thermal stability.

Fig. 1. The FT-IR spectra of CS, MMT, PVP, CS-g-poly(AA-co-AAm)/PVP, and CS-g-poly(AA-co-AAm)/PVP/MMT (a), and TGA (b) and DTG (C) curves of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT (with 13 wt% MMT).

3.4. Surface morphology analysis
The changes on the surface morphology of CS-g-poly(AA-co-AAm)/PVP hydrogel made by incorporated MMT were investigated using SEM images (Fig. 2(a-d)). The surface morphology of CS-g-poly(AA-co-AAm)/PVP (Fig. 2(a) and (b)) shows an undulant and coarse surface with low porosity, which have been generated by interpenetration of PVP chains through hydrogel network. These structures make a convenient pathway for penetration of water molecules into hydrogel network, but, due to the lower porosity, cannot be of benefit to improve water absorption capacity. Compared with the CS-g-poly(AA-co-AAm)/PVP surface morphology, CS-g-poly(AA-co-AAm)/PVP/MMT (Fig. 2(c) and (d)) exhibits interlaced and highly interlinked porous structure because of the formation of additional physical crosslinkages within hydrogel network in the presence of MMT. These interlinked porous channels not only increase the contact surface area but also provide large amounts of available voids, which can be occupied by water molecules. Therefore, it can be expected that CS-g-poly(AA-co-AAm)/PVP/MMT will be capable of absorbing great amount of water compared with neat hydrogel.

Fig. 2. SEM images of CS-g-poly(AA-co-AAm)/PVP (a) and (b), and CS-g-poly(AA-co-AAm)/PVP/MMT (with 13 wt% MMT) (c) and (d).

3.5. Swelling kinetic studies
Fig. 3(a) shows the equilibrium swelling capacity of semi-IPN CS-g-poly(AA-co-AAm)/PVP/MMT superabsorbent nanocomposites at different MMT contents. As depicted in Fig. 3(a), the equilibrium swelling capacity of CS-g-poly(AA-co-AAm)/PVP/MMT is increased with increasing MMT content until the highest value of 1568 g/g was achieved at 13 wt% MMT content. Further increase in MMT content up to 18 wt% resulted in a decreased water absorption capacity. These observations may be attributed to the following facts. At MMT contents lower than 13 wt%, the strong electrostatic repulsive forces between carboxylate groups of the polymeric matrix and negative surface charges of MMT cause more expansion in hydrogel network and consequently swelling capacity is increased. Besides, hydrophilic effect of the hydroxyl groups of MMT induces an enhancement in the osmotic pressure difference between swelling medium and hydrogel network, and hence an increase in swelling capacity. However, at MMT contents higher than 13 wt%, the additional physical crosslinkages will be formed within hydrogel network, leading to the increased crosslinking density and reduced swelling capacity. Additionally, polymeric matrix of the hydrogel, due to the high hydrophilicity compared to MMT, is most liable for its water absorbency. Therefore, at higher MMT contents, the proportion of the polymeric matrix in hydrogel composition decreases, resulting in reduced swelling capacity PEVuZE5vdGU+PENpdGU+PEF1dGhvcj5NYXJhbmRpPC9BdXRob3I+PFllYXI+MjAxMTwvWWVhcj48
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ADDIN EN.CITE.DATA (Marandi et al., 2011). According to the swelling kinetic curves of CS-g-poly(AA-co-AAm)/PVP/MMT (with 13 wt% MMT) and CS-g-poly(AA-co-AAm)/PVP (Fig. 3(b)), swelling capacity increased quickly during the initial time periods after immersion in distilled water. In continue, the growth rate of swelling capacity followed by a slower process until an equilibrium swelling capacity was achieved. As can be seen from Fig. 3(b), CS-g-poly(AA-co-AAm)/PVP sample reached to own equilibrium swelling capacity (1270 g/g) within 780 min, while maximum water absorption capacity of 1568 g/g was attained for CS-g-poly(AA-co-AAm)/PVP/MMT after 1200 min. These results revealed that incorporation of MMT into hydrogel network greatly improves the equilibrium water absorption capacity and increases the time required to reach equilibrium condition. This was due to the interlinked porous channels within CS-g-poly(AA-co-AAm)/PVP/MMT network, which hinder diffusion of water molecules, and hence extend the necessary time to acquire an equilibrium swelling capacity.

The dynamic swelling characteristics of the hydrogels were assessed by Voigt-based equation, which can be expressed as follow ADDIN EN.CITE <EndNote><Cite><Author>Pourjavadi</Author><Year>2010</Year><RecNum>46</RecNum><DisplayText>(Pourjavadi, Jahromi, Seidi, &amp; Salimi, 2010)</DisplayText><record><rec-number>46</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501685150″>46</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Pourjavadi, A</author><author>Jahromi, P Eftekhar</author><author>Seidi, F</author><author>Salimi, H</author></authors></contributors><titles><title>Synthesis and swelling behavior of acrylatedstarch-g-poly (acrylic acid) and acrylatedstarch-g-poly (acrylamide) hydrogels</title><secondary-title>Carbohydrate Polymers</secondary-title></titles><periodical><full-title>Carbohydrate Polymers</full-title></periodical><pages>933-940</pages><volume>79</volume><number>4</number><dates><year>2010</year></dates><isbn>0144-8617</isbn><urls></urls></record></Cite></EndNote>(Pourjavadi, Jahromi, Seidi, & Salimi, 2010):
St=Se1- e-t? (3)
St (g/g) is the swelling capacity at time t (min); Se (g/g) is the equilibrium swelling capacity; t (min) is the swelling time; and ? (min) is the rate parameter. The rate parameter is a measure of the swelling rate so that the lower ? value reflects a hydrogel sample with higher swelling rate. The plot of -Ln (1-StSe) versus t for CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT samples gives straight lines with good linear correlation coefficients (Fig. 3(c)). The rate parameter values were determined from the slope of the plotted lines, and were collected in Table 1. According to the results, the rate parameter values for CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT were found to be 196.07 and 270.27 min, respectively. The lower rate parameter of CS-g-poly(AA-co-AAm)/PVP sample denotes that it swells faster than that of CS-g-poly(AA-co-AAm)/PVP/MMT. This mainly originates from the interlinked porous structure of CS-g-poly(AA-co-AAm)/PVP/MMT. On the other hand, despite large specific surface area of CS-g-poly(AA-co-AAm)/PVP/MMT, which facilitates water diffusion process initially, presence of more tortuous interlinked pathways within hydrogel network, restricts penetration of water molecules. Therefore, the essential time to attain an equilibrium swelling state is prolonged, and thus swelling rate is decreased.
In order to determine the water diffusion mechanism into hydrogel network, the first 60% of the fractional swelling data were fitted with the following equation ADDIN EN.CITE <EndNote><Cite><Author>Rashidzadeh</Author><Year>2014</Year><RecNum>24</RecNum><DisplayText>(Rashidzadeh et al., 2014)</DisplayText><record><rec-number>24</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501272397″>24</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Rashidzadeh, Azam</author><author>Olad, Ali</author><author>Salari, Dariush</author><author>Reyhanitabar, Adel</author></authors></contributors><titles><title>On the preparation and swelling properties of hydrogel nanocomposite based on sodium alginate-g-poly (acrylic acid-co-acrylamide)/clinoptilolite and its application as slow release fertilizer</title><secondary-title>Journal of Polymer Research</secondary-title></titles><periodical><full-title>Journal of Polymer Research</full-title></periodical><pages>344</pages><volume>21</volume><number>2</number><dates><year>2014</year></dates><isbn>1022-9760</isbn><urls></urls></record></Cite></EndNote>(Rashidzadeh et al., 2014):
WtW?=ktn (4)
Where Wt (g/g) and W? (g/g) are the swelling capacities at time t (min) and at equilibrium, respectively. The k is a proportionality constant and the diffusional exponent (n) identifies the type of diffusion mechanism. The value of n = 0.5 implies the Fickian diffusion mechanism; 0.5<n<1.0 indicates the non-Fickian or anomalous transport behavior; n = 1.0 designates case-II diffusion (relaxation-controlled transport), and n>1.0 defines supercase-II diffusion process. The plots of Ln (WtW?) versus Ln (t) for CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT samples have been depicted in Fig. 3(d). The values of n and k were determined from the slope and intercept of the plotted lines, respectively, and were provided in Table 1. According to Table 1, the n values for CS-g-poly(AA-co-AAm)/PVP (0.4795) and CS-g-poly(AA-co-AAm)/PVP/MMT (0.3687) were lower than 0.5, demonstrating that water transport behavior complies with the Fickian diffusion mechanism.

Table 1. The swelling and diffusion parameters for CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT samples.

Sample ?nkCS-g-poly(AA-co-AAm)/PVP 196.07 0.4795 0.059
CS-g-poly(AA-co-AAm)/PVP/MMT 270.27 0.3687 0.091

Fig. 3. The equilibrium swelling capacity of CS-g-poly(AA-co-AAm)/PVP/MMT samples at different MMT contents (a), swelling kinetic diagrams of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT (with 13 wt% MMT) samples (b), Plots of -Ln (1-StSe) versus t (c), and plots of Ln (WtW?) versus Ln (t) for CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT samples (d).

3.6. Water absorbency under load (AUL)
Water absorption capacity of the hydrogels under load is an important factor, which can be considered as a measure of swollen gel strength. Fig. 4(a) and (b) show swelling kinetics of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT samples in NaCl aqueous solution (0.9 wt%) as swelling medium under different pressures (0.3, 0.6, and 0.9 psi). As shown in Fig. 4(a) and (b), swelling kinetics of both hydrogel samples follow a similar trend in which swelling capacity increased dramatically at the initial time periods, and then the swelling degree rose slowly until equilibrium swelling capacity was achieved. In the case of CS-g-poly(AA-co-AAm)/PVP (Fig. 4(a)), the minimum time needed to reach maximum value of AUL at each pressure was 170 min, while for CS-g-poly(AA-co-AAm)/PVP/MMT (Fig. 4(b)) the maximum value of AUL at each pressure was obtained within 240 min. These results were in good compliance with the previous findings about the lower swelling rate of CS-g-poly(AA-co-AAm)/PVP/MMT. According to Fig. 4(a) and (b), the maximum AUL for CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT at the applied pressures of 0.3, 0.6, and 0.9 psi was 74.6, 63.2, 45.7 g/g and 92.8, 83.5, and 68.4 g/g, respectively. From these results it can be inferred that CS-g-poly(AA-co-AAm)/PVP/MMT possesses greater AUL at all pressures compared with CS-g-poly(AA-co-AAm)/PVP. The reason is that, the incorporated MMT made strong electrostatic repulsive forces with carboxylate groups of polymeric matrix, leading to the expanded hydrogel network with enhanced swelling capacity. Additionally, according to the fact that the greater applied pressure can induce lower AUL, the maximum AUL for both hydrogel samples (Fig. 4(a) and (b)) decreases with increasing the amount of loading.

Fig. 4. Water absorption capacity under load (AUL) for CS-g-poly(AA-co-AAm)/PVP (a) and CS-g-poly(AA-co-AAm)/PVP/MMT (b).

3.7. Swelling behavior at various pH solutions
To evaluate the effect of solution pH on the swelling behavior of hydrogels, equilibrium swelling capacity of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT samples was examined in various pHs ranging from 2-12 (Fig. 5(a)). According to Fig. 5(a), both hydrogel samples showed more sensitive swelling behavior relative to pH in solutions of pH<5 and pH>9. The high electrostatic repulsions among protonated ammonium groups (-NH3+) in acidic solutions (pH<5) cause an expansion in hydrogel network and thus enable hydrogel to swell more. However, in highly acidic solutions (pH?3) the excess Cl- counterions in the swelling medium shield the ammonium charges and prevent effective cation-cation repulsions. At pHs 5-7, most of amine and carboxylic acid groups are in own non-ionized state, which hydrogen-bonding interactions between them lead to increased physical crosslinking density. Therefore, hydrogel network tends to shrink and hence swelling capacity decreases. Since the pKa value of the weak polyacid is about 6.4, at pH values higher than 6.4 dissociation of the carboxylic acid groups (-COOH ? -COO- + H+) is occurred. At this condition, the hydrogen-bonding interactions are weakened because of the decreased effect of H+. Besides, the reinforcement of anion-anion repulsions among carboxylate groups makes an expansion in hydrogel network, resulting in increased swelling capacity. With increasing pH values from 9-12, due to the enhanced shielding effect of Na+ counterions in the swelling medium, the effective anion-anion repulsions are reduced, and hence water absorption capacity is decreased ADDIN EN.CITE <EndNote><Cite><Author>Mahdavinia</Author><Year>2004</Year><RecNum>47</RecNum><DisplayText>(Mahdavinia, Pourjavadi, Hosseinzadeh, &amp; Zohuriaan, 2004; Zhou, Fu, Zhang, &amp; Zhan, 2013)</DisplayText><record><rec-number>47</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501753270″>47</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Mahdavinia, GR</author><author>Pourjavadi, A</author><author>Hosseinzadeh, H</author><author>Zohuriaan, MJ</author></authors></contributors><titles><title>Modified chitosan 4. Superabsorbent hydrogels from poly (acrylic acid-co-acrylamide) grafted chitosan with salt-and pH-responsiveness properties</title><secondary-title>European Polymer Journal</secondary-title></titles><periodical><full-title>European Polymer Journal</full-title></periodical><pages>1399-1407</pages><volume>40</volume><number>7</number><dates><year>2004</year></dates><isbn>0014-3057</isbn><urls></urls></record></Cite><Cite><Author>Zhou</Author><Year>2013</Year><RecNum>48</RecNum><record><rec-number>48</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501753365″>48</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Zhou, Yiming</author><author>Fu, Shiyu</author><author>Zhang, Liangliang</author><author>Zhan, Huaiyu</author></authors></contributors><titles><title>Superabsorbent nanocomposite hydrogels made of carboxylated cellulose nanofibrils and CMC-gp (AA-co-AM)</title><secondary-title>Carbohydrate polymers</secondary-title></titles><periodical><full-title>Carbohydrate Polymers</full-title></periodical><pages>429-435</pages><volume>97</volume><number>2</number><dates><year>2013</year></dates><isbn>0144-8617</isbn><urls></urls></record></Cite></EndNote>(Mahdavinia, Pourjavadi, Hosseinzadeh, & Zohuriaan, 2004; Zhou, Fu, Zhang, & Zhan, 2013).

3.8. Effect of salt solution on swelling behavior
Fig. 5(b) and (c) exhibit the equilibrium swelling capacity of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT in various saline solutions (NaCl, CaCl2, and FeCl3) with different concentrations, respectively. As shown in Fig. 5(b) and (c), the equilibrium water absorption capacity of the hydrogels decreased drastically as the saline solution concentration increased. This well-known phenomenon, commonly observed in swelling of ionic hydrogels, is mainly attributed to the charge-screening effect of the counterions in the swelling medium. On the other hand, shielding of the carboxylate anions by excess cations in the swelling medium leads to a non-perfect anion-anion electrostatic repulsion, resulting in the shrinked hydrogel network and decreased swelling capacity. Besides, osmotic pressure difference between hydrogel network and swelling medium decreases with increasing saline solution concentration, which causes a reduction in water absorbency. From Fig. 5(b) and (c) it is evident that water absorbency of the hydrogel samples is strongly dependent on the type of the salt added to the swelling medium and its value for both hydrogels is in the order of NaCl > CaCl2 > FeCl3. The reason can be elucidated as follows. The coordination of the multivalent cations (Ca2+ and Fe3+) with the carboxylate or carboxamide groups generates more ionic crosslinking points within hydrogel network, leading to the increased crosslinking density and decreased water absorption capacity. Additionally, cations with greater charge valencies, due to the higher electrostatic attraction with carboxylate ions, form a more ionically-crosslinked hydrogel network with lower water absorbency ADDIN EN.CITE <EndNote><Cite><Author>Bao</Author><Year>2011</Year><RecNum>32</RecNum><DisplayText>(Bao et al., 2011; Spagnol et al., 2012)</DisplayText><record><rec-number>32</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501274654″>32</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Bao, Yan</author><author>Ma, Jianzhong</author><author>Li, Na</author></authors></contributors><titles><title>Synthesis and swelling behaviors of sodium carboxymethyl cellulose-g-poly (AA-co-AM-co-AMPS)/MMT superabsorbent hydrogel</title><secondary-title>Carbohydrate Polymers</secondary-title></titles><periodical><full-title>Carbohydrate Polymers</full-title></periodical><pages>76-82</pages><volume>84</volume><number>1</number><dates><year>2011</year></dates><isbn>0144-8617</isbn><urls></urls></record></Cite><Cite><Author>Spagnol</Author><Year>2012</Year><RecNum>26</RecNum><record><rec-number>26</rec-number><foreign-keys><key app=”EN” db-id=”pfz0sppw3dr5frepszc5vxz3xfztx59zvw90″ timestamp=”1501273353″>26</key></foreign-keys><ref-type name=”Journal Article”>17</ref-type><contributors><authors><author>Spagnol, Cristiane</author><author>Rodrigues, Francisco HA</author><author>Pereira, Antonio GB</author><author>Fajardo, André R</author><author>Rubira, Adley F</author><author>Muniz, Edvani C</author></authors></contributors><titles><title>Superabsorbent hydrogel composite made of cellulose nanofibrils and chitosan-graft-poly (acrylic acid)</title><secondary-title>Carbohydrate Polymers</secondary-title></titles><periodical><full-title>Carbohydrate Polymers</full-title></periodical><pages>2038-2045</pages><volume>87</volume><number>3</number><dates><year>2012</year></dates><isbn>0144-8617</isbn><urls></urls></record></Cite></EndNote>(Bao et al., 2011; Spagnol et al., 2012). Swelling behavior assessments in different saline solutions indicated that CS-g-poly(AA-co-AAm)/PVP/MMT superabsorbent nanocomposite possessed greater swelling capacity in all saline solutions compared with CS-g-poly(AA-co-AAm)/PVP sample. This mainly arises from the electrostatic repulsive forces between carboxylate groups of polymeric matrix and hydroxyl groups of MMT, which made an expanded hydrogel network with higher water uptake capacity.

Fig. 5. The effect of pH on the equilibrium swelling capacity of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT (a) and the effect of salt solution type and concentration on the equilibrium water absorption capacity of CS-g-poly(AA-co-AAm)/PVP (b) and CS-g-poly(AA-co-AAm)/PVP/MMT (c).

3.9. Water retention studies at various temperatures
Water retention capability of the hydrogels is one of the most important factors, which greatly influences feasibility of their use in agricultural applications. Water retention capacity of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT samples at two temperatures of 40 °C and 80 °C has been depicted in Fig. 6. As can be seen from Fig. 6, water releasing rate of CS-g-poly(AA-co-AAm)/PVP/MMT during all heating times was slower than that of CS-g-poly(AA-co-AAm)/PVP sample. According to Fig. 6, water retention capacity of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT at 40 °C and 80 °C was respectively 10.9%, 0.42% and 18.9% and 2.6%. These results revealed that CS-g-poly(AA-co-AAm)/PVP/MMT possesses higher water retention capability compared with CS-g-poly(AA-co-AAm)/PVP sample. These findings may be justified as follows. The strong hydrogen-bonding interactions between hydroxyl groups of incorporated MMT and absorbed water molecules hinder water evaporation rate, and thus improve water retention capacity.

Fig. 6. Water retention capacity of CS-g-poly(AA-co-AAm)/PVP and CS-g-poly(AA-co-AAm)/PVP/MMT samples under constant temperatures (40 °C and 80 °C).

4. Conclusion
Semi-IPN CS-g-poly(AA-co-AAm)/PVP/MMT superabsorbent nanocomposite was prepared by simultaneous graft copolymerization of AA and AAm monomers onto CS backbone and interpenetration of linear PVP chains through hydrogel network in the presence MMT (filler), APS (initiator), and MBA (crosslinking agent). Chemical structure and thermal stability of the synthesized materials were characterized by FTIR and TGA analyses techniques, respectively. The addition of MMT up to 13 wt% into the polymeric matrix substantially improved thermal stability of the hydrogel. In addition, SEM images indicated that hydrogel morphology changes from loosely coarse surface to a highly porous structure with interlinked channels as MMT is introduced into hydrogel network. The equilibrium water absorption capacity of CS-g-poly(AA-co-AAm)/PVP/MMT (1568 g/g) was greater than that of CS-g-poly(AA-co-AAm)/PVP (1270 g/g) sample. Swelling kinetic studies revealed that CS-g-poly(AA-co-AAm)/PVP had higher swelling rate compared with CS-g-poly(AA-co-AAm)/PVP/MMT. This was due to the interlinked porous channels within CS-g-poly(AA-co-AAm)/PVP/MMT network, which restrict penetration of water molecules, prolong the time needed to reach an equilibrium state, and hence decrease the swelling rate. Also, water transport behavior in both hydrogel samples was Fickian diffusion type. The swelling behavior of the hydrogels was considerably influenced by the saline solution type and concentration, solution pH, applied pressure, and temperature. Semi-IPN CS-g-poly(AA-co-AAm)/PVP/MMT superabsorbent nanocomposite presented better salt and pH-sensitive swelling behavior. Also, CS-g-poly(AA-co-AAm)/PVP/MMT had higher AUL for all applied pressures and good water retention capability compared with neat hydrogel. By virtue of these good properties, it can be concluded that developed superabsorbent nanocomposite can be utilized as an efficient water management system in agricultural and horticultural applications.

Acknowledgements
The financial support of this work by the University of Tabriz is gratefully acknowledged.

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