Starch-based semi-IPN hydrogel nanocomposite integrated with clinoptilolite: Preparation and swelling kinetic study

Accepted Manuscript Title: Starch-based semi-IPN hydrogel nanocomposite integrated with clinoptilolite: preparation and swelling kinetic study Authors: Ali Olad, Fatemeh Doustdar, Hamed Gharekhani PII: DOI: Reference:

S0144-8617(18)30912-3 https://doi.org/10.1016/j.carbpol.2018.08.014 CARP 13911

To appear in: Received date: Revised date: Accepted date:

13-5-2018 15-7-2018 4-8-2018

Please cite this article as: Olad A, Doustdar F, Gharekhani H, Starch-based semi-IPN hydrogel nanocomposite integrated with clinoptilolite: preparation and swelling kinetic study, Carbohydrate Polymers (2018), https://doi.org/10.1016/j.carbpol.2018.08.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Starch-based semi-IPN hydrogel nanocomposite integrated with clinoptilolite: preparation and swelling kinetic study

Ali Olad a*, Fatemeh Doustdar a, Hamed Gharekhani a

Polymer Composite Research Laboratory, Department of Applied Chemistry, Faculty of

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Chemistry, University of Tabriz, Tabriz, Iran.

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a

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* Corresponding Author

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Tel: +98 413 3393164

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Fax: +98 413 3340191

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E-mail: [email protected]

Highlights

Addition of clinoptilolite improves swelling capacity of hydrogel nanocomposite.

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Hydrogel nanocomposite shows higher swelling rate due to its porous structure.

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Hydrogel nanocomposite presents improved mechanical strength and acceptable

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

AUL.

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Hydrogel nanocomposite exhibits good salt and pH-sensitive swelling behavior. Hydrogel nanocomposite possesses reasonable water retention capability.

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Abstract Semi-interpenetrating polymer network (semi-IPN) of starch-graft-poly(acrylic acid-coacrylamide)/polyvinyl

alcohol/clinoptilolite

(starch-g-p(AA-co-AAm)/PVA/clino)

superabsorbent nanocomposite was synthesized by free-radical graft co-polymerization technique in an aqueous solution. Taguchi method was used to optimize the synthesis

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reaction condition based on the equilibrium swelling capacity of the hydrogels. FTIR, XRD,

and SEM analyses were used to study the chemical and structural properties of the hydrogel

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samples. The equilibrium swelling capacity of the semi-IPN superabsorbent nanocomposite (364.82 g/g) was higher than that of neat hydrogel (286.21 g/g) and in both of them water

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penetration into hydrogel network occurred through non-Fickian diffusion mechanism.

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Incorporation of clino into the polymeric matrix not only increased the equilibrium swelling

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capacity of the hydrogel, but also induced a substantial enhancement in its mechanical

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strength. Semi-IPN superabsorbent nanocomposite showed reasonable water absorbency under different loads, good salt and pH-sensitive swelling behavior, and better water retention

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capability, which make it potentially useful for hygiene products.

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Keywords: Semi-interpenetrating polymer network; Superabsorbent nanocomposite; Starch;

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Clinoptilolite; Polyvinyl alcohol.

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1. Introduction Superabsorbent hydrogels are cross-linked three-dimensional hydrophilic networks of polymers, which can absorb and retain great deal of water, saline or physiological liquids even under certain pressure and heat (Q. Li et al., 2012; J. Liu et al., 2013). Based on the main mechanism of water absorption, hydrophilic materials are classified into two main

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categories including chemical and physical absorbents. Chemical absorbents uptake water via

chemical reactions, which alter entire nature of absorbents. The physical absorbents absorb

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water via four different mechanisms: a) the reversible change in crystal structure of absorbent

such as absorption mechanism of silica gel; b) the physical entrapment of water in porous

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structure of absorbent by capillary forces such as the absorption mechanism of polyurethane

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sponge; c) the combination of mechanism (b) and hydration of hydrophilic functional groups

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of the material like the absorption mechanism of tissue paper; (d) the combination of

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mechanisms (b) and (c) and expansion of the material chains within crosslinked structure. The water absorption mechanism of hydrogels conforms to mechanism (d). The presence of

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hydrophilic groups such as −COOH, −OH, −NH2, −CONH, −CONH2, and –SO3H in the hydrogel network, as well as capillary forces of pores in the hydrogel, leads to the

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hydrophilicity of the hydrogels, which in turn enables them to swell in aqueous solutions

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(Zohuriaan-Mehr & Kabiri, 2008). Due to the specific properties like high water uptake capability, good mechanical stability, and biocompatibility, superabsorbent hydrogels have become useful in a variety of fields such as hygiene products (Teli & Mallick, 2017),

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agriculture (Abd El‐ Mohdy, Hegazy, El‐ Nesr, & El‐ Wahab, 2011; Radwan, Al-Sweasy, & Elazab, 2017), controlled drug delivery systems (C. Liu, Gan, & Chen, 2011; Yaling Zhang et al., 2017), water treatment (Bhattacharyya & Ray, 2014; Zheng, Hua, & Wang, 2010), tissue engineering (Naahidi et al., 2017; Ngoenkam, Faikrua, Yasothornsrikul, & Viyoch, 2010), and food industry as crosslinked hydrocolloids to increase the stability of O/W emulsions

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(Chen et al., 2018; Y. Liu et al., 2017). Recently, some limitations such as high production cost, low biodegradability, and environmental concerns have made researchers replace synthetic polymers with safe and greener ones (Shalviri, Liu, Abdekhodaie, & Wu, 2010). Starch is one of the most abundant carbohydrate polymers in nature, which amylose and amylopectin are its two major constituents (Kuang, Yuk, & Huh, 2011). Linear chains of

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amylose have been made of α-1,4-linked glucose unites, while amylopectin has branched chains of α-1,4-linked glucose units interlinked by α-1,6-linked bonds (Dragan & Apopei,

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2011). Starch is a non-toxic, biocompatible, biodegradable, and an inexpensive

polysaccharide but some structural and performance obstacles like poor water solubility at

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room temperature, poor processability, gelatinization at high temperature, and high viscosity

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have limited its applications (Guo et al., 2015). Nevertheless, starch has many hydroxyl

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groups, which enhance its hydrophilicity and so make it a suitable choice for preparing

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biodegradable and highly swollen hydrogels (Xiao, 2013). Gelation of starch occurs in a thermally-assisted hydration-plasticization process of its polymeric network. First, swelling

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takes place by adsorption of water in the hydrophilic starch granules. Then, gelatinization occurs after starch is dissolved by heating, which leads to leaching of the amylose molecules,

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irreversible physical changes, and destruction of the granule structure. In final step, the retrogradation step, the starch gel network is formed upon cooling and aging, resulting in the

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partial recrystallization and reorganization of the polysaccharide structure. It is noteworthy that gel formation process can be affected by two main parameters including amylose content

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and gelatinization temperature (García-González, Alnaief, & Smirnova, 2011). Grafting synthetic acrylate monomers such as acrylic acid (AA) and acrylamide (AAm) onto starch backbone is an effective method to prepare superabsorbent hydrogels with improved water absorbency and water retention properties (Hosseinzadeh & Ramin, 2018; Witono, Noordergraaf, Heeres, & Janssen, 2014). However, superabsorbent hydrogels based on starch

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suffer from low mechanical strength, which restricts their broad utilization (X. Li, Xu, Wang, Chen, & Feng, 2009). To overcome this shortcoming, various techniques have been developed so far. Preparation of semi-interpenetrating polymer network (semi-IPN) hydrogels is one of the most common strategies, that can effectively improve the mechanical strength and thermal features of the final product (Dragan & Apopei, 2011). Semi-IPNs are

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blend of two polymers, in which only one is crosslinked in the presence of another to form hydrogel network without any chemical bonds between them (Maleki, Edlund, & Albertsson,

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2016). Polyvinyl alcohol (PVA) is a linear synthetic and water-soluble polymer, which has been widely used in the preparation of semi-IPN hydrogels because of its hydrophilicity and

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biodegradability. Moreover, it can significantly amplify mechanical strength and chemical

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resistance of the polymeric systems (Zhu, Ma, Wang, & Zhang, 2015). Introducing low cost

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inorganic fillers into the hydrogel network to form nanocomposites is another technique that

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not only can improve mechanical strength and swelling characteristics of the hydrogels but also can substantially reduce final production cost. Since production cost of the hydrogel

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samples is a most important factor to exploit them, therefore preparation of the hydrogel nanocomposites by incorporation of mineral additives into the polymeric matrix can be most

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efficient strategy. Clinoptilolite (clino) is one of the most promising minerals, which due to its large surface area, high ion exchange capacity, and low cost has been extensively used in

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preparation pathway of the superabsorbent nanocomposites as filler (Olad, Gharekhani, Mirmohseni, & Bybordi, 2016).

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In the present study, we aimed to synthesize semi-IPN starch-graft-poly(AA-coAAm)/PVA/clino superabsorbent nanocomposite with improved water absorbency, high water retention capability, and good mechanical strength. Taguchi method was used to optimize reaction synthesis condition and prepare hydrogel sample with high swelling capacity. The influence of clino on the equilibrium swelling capacity and swelling kinetic of

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the hydrogel was investigated. Also, water absorbency under load (AUL), water retention capability under constant temperatures, and salt and pH-sensitive swelling behavior of the hydrogel samples were studied.

2. Experimental

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2.1. Materials

Potato starch (as white fine powder with amylose/amylopectin ratio of about 1:3) was

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obtained from Merck Company. Prior to use, the moisture of potato starch was removed by drying in a vacuum oven at 110 °C for 24 h. Acrylic acid (AA), acrylamide (AAm), polyvinyl

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alcohol (PVA, the average molecular weight Mr=72000), N,N′-methylene bisacrylamide

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(MBA), ammonium persulfate (APS), and sodium chloride were purchased from Merck

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Company (Germany). Clinoptilolite (clino) was obtained from the Meianah mine in East

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Azerbaijan, Iran. The chemical composition of clino consists of SiO2 (65%), Al2O3 (12.03%), Fe2O3 (1.5%), CaO (2.3%), MgO (0.72%), Na2O (1.8%), K2O (3%), P2O5 (0.01%), MnO

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(0.1%), and TiO2 (0.03%), which was determined by X-ray fluorescence (XRF) technique. All other reagents used in this study were of analytical grade and were used as received.

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Distilled water was used to prepare all solutions.

2.2. Design of experiment 2.2.1. Designation of factors and their levels

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The equilibrium swelling capacity of the hydrogels can be affected by many factors, which among them six most effective factors were chosen. These factors include starch/clino weight ratio, the amounts of PVA, AA/AAm, MBA, APS, and neutralization percent (NU%) of AA. Results obtained from some primary experiments and also previous research works were

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considered to assign the proper amounts for each factor. Finally, five different amounts, which have been shown by level 1 to level 5 in Table 1, were allocated for each factor.

2.2.2. Factors allocation using an orthogonal array According to the Taguchi experiment design method a standard orthogonal array (L25) was

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achieved to prepare an optimized hydrogel nanocomposite sample. Table 2 shows the twenty five experiments, which have been arranged into L25 orthogonal array. Also, the equilibrium

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swelling capacity (Qeq) of each hydrogel sample as well as standard deviation (STDEV)

values was calculated and collected in Table 2. Eventually, the results of the designed

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experiments were examined by Minitab software using the statistical method of analysis of

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variance (ANOVA).

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Table 1. Different levels of each factor and their values. Symbol

Level 1

Level 2

Level 3

Level 4

Level 5

Starch/clinoptilolite (g/g)

Starch/clino

0.75/0.0075

1.5/0.045

2.25/0.1125

3/0.21

3.75/0.3375

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Factor

AA/AAm

3/0.5

5/1

7/1.5

9/2

11/2.5

Polyvinyl alcohol (mL)

PVA

2

3

4

5

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Initiator (mg)

APS

40

50

60

70

80

Crosslinking agent (mg)

MBA

10

20

30

40

50

Neutralization percent

NU%

0

30

50

70

90

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Acrylic acid/Acrylamide (mL/g)

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Table 2. Design of experiments (L25 orthogonal array) and equilibrium swelling capacity

MBA

APS

NU%

Qeq (g/g)a

STDEVb

1

Level 1

Level 1

Level 1

Level 1

Level 1

Level 1

10.8

4.62

2

Level 1

Level 2

Level 2

Level 2

Level 2

Level 2

252.5

3.87

3

Level 1

Level 3

Level 3

Level 3

Level 3

Level 3

138.6

2.28

4

Level 1

Level 4

Level 4

Level 4

Level 4

Level 4

200.2

7.89

5

Level 1

Level 5

Level 5

Level 5

Level 5

Level 5

165.6

1.66

6

Level 2

Level 1

Level 2

Level 3

Level 4

Level 5

180.3

7.73

7

Level 2

Level 2

Level 3

Level 4

Level 5

Level 1

223.4

7.32

8

Level 2

Level 3

Level 4

Level 5

Level 1

Level 2

99.7

6.34

9

Level 2

Level 4

Level 5

Level 1

Level 2

Level 3

418.8

5.46

10

Level 2

Level 5

Level 1

Level 2

Level 3

Level 4

50.4

4.29

11

Level 3

Level 1

Level 3

Level 5

Level 2

Level 4

170.0

6.61

12

Level 3

Level 2

Level 4

Level 1

Level 3

Level 5

151.1

5.28

13

Level 3

Level 3

Level 5

Level 2

Level 4

249.5

7.51

14

Level 3

Level 4

Level 1

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Level 1

Level 3

Level 5

41.2

7.13

15

Level 3

Level 5

Level 2

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Level 2

Level 4

Level 1

Level 3

114.7

4.43

16

Level 4

Level 1

Level 4

Level 2

Level 5

Level 3

262.4

6.94

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Level 4

Level 2

Level 5

Level 3

Level 1

Level 4

286.8

7.67

18

Level 4

Level 3

Level 1

Level 4

Level 2

Level 5

32.1

3.84

19

Level 4

Level 4

Level 2

Level 5

Level 3

Level 1

143.6

1.93

20

Level 4

Level 5

Level 3

Level 1

Level 4

Level 2

132.1

7.53

21

Level 5

Level 1

Level 5

Level 4

Level 3

Level 2

271.6

6.05

22

Level 5

Level 2

Level 1

Level 5

Level 4

Level 3

19.8

5.38

23

Level 5

Level 3

Level 2

Level 1

Level 5

Level 4

57.7

7.92

24

Level 5

Level 4

Level 3

Level 2

Level 1

Level 5

340.0

2.41

25

Level 5

Level 5

Level 4

Level 3

Level 2

Level 1

239.9

5.24

Qeq (g/g) =

Weq − Wd Wd

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PVA

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AA/AAm

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Starch/clino

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a

Test number

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(Qeq) and standard deviation (STDEV) values for each experiment.

, Weq is the weight of swollen hydrogel at an equilibrium state and Wd is the weight

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of dry hydrogel sample. b

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STDEV= √ ∑ni=1(xi − x̅)2 , xi is the equilibrium swelling capacity values, x̅ is the average of the set of N

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

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2.3. Treatment method of clino The native clino rock was grounded and then it was sieved. In continue, to remove impurities of the clinoptilolite, 0.6 g of the obtained powder was charged into a beaker containing 20 mL HCl aqueous solution (1 M) and stirred for 12 h at room temperature. Thereafter, the resultant suspension was filtered and washed thoroughly with distilled water. The solid

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material collected on the filter paper was transferred into a beaker containing 100 mL distilled water and agitated for 12 h. Afterwards, the suspended solid material was separated

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by filtration process, and then it was rinsed with distilled water repeatedly until pH of wastewater equals to the pH of distilled water. Finally, the obtained powder was completely

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dried in a vacuum oven at 200 °C for 24 h.

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2.4. Preparation of PVA aqueous solution

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To prepare a 2 wt% PVA aqueous solution, 2 g of PVA powder and 100 mL distilled water were poured into a round-bottom flask, and then it was connected to a reflux condenser. The

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prepared mixture in the flask was heated to 85 °C by an oil bath, while stirring by a magnetic stirrer bar. PVA gradually started to dissolve in water and after its dissolution was completed,

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a clear solution was obtained.

2.5. Preparation of semi-IPN starch-g-p(AA-co-AAm)/PVA/clino superabsorbent nanocomposite

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A 250 mL four-neck round-bottom flask, equipped with a mechanical stirrer, a reflux condenser, a nitrogen line, and a thermometer, was charged with 6 mL of distilled water, 1.5 g of starch, and 0.45 g of NaOH pellets. The resultant mixture was heated to 40 °C by a water bath and stirred continuously until it was turned to a transparent sticky pasty-like solution. 0.045 g of clino (3 wt%, with respect to starch) was completely dispersed in 10 mL distilled

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water using an ultrasonic probe at 30 W power for 5 min. Then, the prepared suspension was added to the reaction mixture, while stirring. In continue, 2.5 g of AAm and 5 mL of PVA aqueous solution (2 wt%) was poured into the reaction flask. Thereafter, 11 mL of AA neutralized up to 90% together with 0.02 g of MBA (1.3 wt%, with respect to starch) was decanted into the flask. After bubbling the reaction mixture with nitrogen gas for 30 min to

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remove the dissolved oxygen, 0.05 g of APS was added to the flask under mechanical

agitation. The temperature of the reaction mixture was risen to 60 °C and maintained at this

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condition to proceed polymerization reaction. The polymerization process was permitted to

complete for 1 h. The obtained gel product was cut into small pieces, immersed in fresh

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ethanol for 24 h to remove unreacted species, and then it was dried in vacuum oven at 65 °C

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for 24 h. The dried product was milled and passed through 40-80 mesh sieves for subsequent

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experiments. The procedure used to prepare semi-IPN superabsorbent nanocomposites during

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all twenty five experiments was similar to the above mentioned pathway. Also, during synthesis of hydrogel samples, total volume of the reaction mixture was kept at 50 mL. As

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without addition of clino.

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comparison, semi-IPN starch-g-p(AA-co-AAm)/PVA sample was also synthesized similarly

2.6. Characterization

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To characterize chemical structure of the materials, Fourier transform infrared spectroscopy (FTIR) analysis was done on the blend of properly milled samples and KBr powder using a

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Bruker Tensor 27 FTIR spectrophotometer in the wavenumber range of 400-4000 cm-1. Morphological study of the hydrogel samples and clino was also performed using field emission scanning electron microscope (FE-SEM) system (MIRA3 FEG-SEM, Tescan, Czech). The samples were first milled and then coated by a thin layer of gold prior to SEM imaging. Crystallinity of starch, PVA, clino, starch-g-p(AA-co-AAm)/PVA, and starch-g-

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p(AA-co-AAm)/PVA/clino samples, which had been properly milled, was characterized using a X-ray diffractometer (Siemens AG, Karlsruhe, Germany) equipped with a Cu Kα radiation source in scattering angle range from 2 o to 70o. The detail chemical composition of clinoptilolite was determined using Philips PW 1410 X-ray fluorescence (XRF) spectrometer.

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2.7. Study of properties

2.7.1. Measurement of equilibrium water absorbency and swelling kinetics

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To study swelling kinetic of starch-g-p(AA-co-AAm)/PVA and starch-g-p(AA-coAAm)/PVA/clino samples, 0.02 g of dry hydrogel sample (Wd) was first submerged in 100

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mL distilled water. Then, at determined time intervals, the swollen hydrogel sample was

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taken out from the swelling medium and after removing the extra surface water by a filter

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paper, it was weighed. At equilibrium swelling state the swollen weight of the hydrogel

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sample reached a constant value (Weq), demonstrating completion of swelling kinetic measurements. These measurements were conducted three times for each hydrogel sample

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and finally the average values were reported. Eventually, to determine the swelling capacity

used:

Wt − W d Wd

(1)

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St (g/g) =

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(St) and equilibrium swelling capacity (Seq) of the hydrogel sample Eq. (1) and Eq. (2) were

Weq − Wd

(2)

Wd

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Seq (g/g) =

2.7.2. Evaluation of water absorbency under loud (AUL) Water absorbency under different loads in 0.9 wt% NaCl aqueous solution was examined for starch-g-p(AA-co-AAm)/PVA and starch-g-p(AA-co-AAm)/PVA/clino samples according to the following procedure. First, a petri dish (d = 90 mm, h = 15 mm) containing a macro11

porous sintered glass filter plate (d = 80 mm, h = 5 mm) was put on a flat surface, and then a round-shaped wire cloth (100 mesh) was placed onto the sintered glass filter plate. Then, certain amount of dry hydrogel sample was distributed uniformly on the wire cloth and its surface was encompassed by a glass cylinder (d = 60 mm, h = 50 mm). Thereafter, a cylindrical solid load (Teflon, d = 60 mm, variable height) was passed through the glass

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cylinder to apply various pressures (0.3, 0.6, and 0.9 psi) onto the hydrogel sample. Afterwards, the petri dish was filled with 0.9 wt% NaCl aqueous solution up to the upper

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surface of the sintered glass filter plate. To prevent solvent evaporation and subsequently possible changes in saline solution concentration, the prepared system was covered properly.

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calculated using Eq. (1) mentioned in section 2.5.1.

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At determined time intervals, the swollen hydrogel sample was weighed and AUL was

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2.7.3. Rheological analysis

To study the rheological characteristics of starch-g-p(AA-co-AAm)/PVA and starch-g-p(AA-

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co-AAm)/PVA/clino samples, Anton Paar rheometer (MCR301, Germany) equipped with a plate-plate geometry (plates diameter of 25 mm and gap of 1 mm) was used at 25 ˚C. First, a

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strain sweep test at a constant frequency of 𝜔 = 10 Hz was performed on the hydrogel samples to specify the linear viscoelastic (LVE) zone, where storage modulus (Gʹ) and loss

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modulus (Gʺ) are independent of the strain amplitude. In continue, viscoelastic behavior of the hydrogel samples in the LVE region was evaluated at a constant strain (γ= 0.5) over

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angular frequencies ranging from 0.1 to 100 Hz.

2.7.4. Swelling measurement in pH solutions Aqueous solutions of NaOH (0.1 M) and HCl (0.1 M) were diluted to prepare solutions with different pH values ranging from 2 to 12. Eq. (2) was used to calculate the equilibrium water

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absorbency of starch-g-p(AA-co-AAm)/PVA and

starch-g-p(AA-co-AAm)/PVA/clino

samples in each pH solution.

2.7.5. Investigation of swelling behavior in NaCl solution To study swelling behavior in NaCl aqueous solution, different concentrations of it (0.1, 0.3,

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0.5, 0.7, 0.9, and 1.1 wt%) were used, and finally the equilibrium swelling capacity was

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determined the same way as described in section 2.5.1.

2.7.6. Water retention behavior

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A pre-weighted dry hydrogel sample (Wd) was immersed entirely in distilled water and after

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reaching to the equilibrium water absorbency, it was weighed (Weq) and placed in the vacuum

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oven at 40 °C or 80 °C. At 1 h time intervals, the swollen hydrogel sample was weighed (Wt)

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and the percentage of water retention (WR%) was calculated using the Eq. (3).

eq

d

(3)

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W −W

WR% = W t −Wd

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3. Results and discussion

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3.1. Synthesis mechanism of semi-IPN superabsorbent nanocomposite In synthesis process of semi-IPN superabsorbent nanocomposite, graft copolymerization of AA and AAm monomers onto starch backbone and penetrating of PVA chains into hydrogel

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network were occurred simultaneously in an aqueous medium (Scheme 1). Initially, thermal decomposition of APS molecules leads to generation of sulfate anion-radicals. Active sulfate radicals abstract hydrogen atoms of starch hydroxyl groups, and thus produce the macroradicals. The active sites on the macro-radicals can then donor radicals onto the nearest AA and AAm monomers, resulting in the initiation of grafting reaction. As the graft 13

copolymerization proceed, crosslinking of polymeric chains may happen by end vinyl groups of MBA. At the same time, PVA chains penetrate into the polymeric matrix and form semiIPN hydrogel network through additional hydrogen bonding interactions. The final semi-IPN superabsorbent nanocomposite network structure is made by incorporated clino (red dots in

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A

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scheme 1), which acts as physical cross-linking agent (Olad et al., 2016; Zhu et al., 2015).

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IP T SC R U N A M ED PT CC E A Scheme 1. Proposed reaction mechanism for synthesis of semi-IPN superabsorbent nanocomposite. 15

3.2. Optimization of reaction parameters Based on the L25 orthogonal array, which has been depicted in Table 2, hydrogel samples were synthesized. The equilibrium swelling capacity measurement for each hydrogel sample was replicated for three times and finally average values were reported. Taguchi method contains two main factors. The controllable factors are named control factors, while factors

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which can’t be controlled during the production process or product use, are called noise factors. The response changes with respect to the target value under different noise condition

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are shown by Signal-to-noise ratio (S/N). To identify control factor settings, which decline the effect of noise factors, higher amounts of the signal-to-noise ratio (S/N) are considered.

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Signal-to-noise curves were plotted to determine the optimum amount of each factor as well

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as its effect on the equilibrium swelling capacity (Fig. 1). As shown in Fig. 1, AA/AAm ratio

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has an intense effect on the equilibrium swelling capacity of hydrogel samples. The optimum

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value of AA/AAm was found at level 5 (11 mL/2.5 g) with higher signal-to-noise value, demonstrating a hydrogel sample with higher equilibrium swelling capacity. Presence of

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hydrophilic groups like –OH, −COOH, −COONa, and –CONH2 in the hydrogel network enhances hydrophilicity of the hydrogel, resulting in the increased water absorption capacity

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(J. Liu et al., 2013). According to Fig. 1, the second factor, which considerably affects the

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equilibrium swelling capacity, is the crosslinker amount. The optimum amount of MBA was detected at level 2 (20 mg). At lower amounts of MBA (level 1), due to the insufficient crosslinking density, gel strength of the hydrogel sample decreases, resulting in the loose

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hydrogel framework with low swelling capacity. Also, with increasing MBA amount beyond level 2 crosslinking density increases causing a reduction in the size of apertures within hydrogel network and subsequently a decline in swelling capacity (J. Liu et al., 2013; Teli & Mallick, 2017). As depicted in Fig. 1, the optimum amounts of starch/clino, PVA (2% w/w),

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APS, and NU% factors were found at level 2 (1.5 g/0.045 g), level 4 (5 mL), level 2 (50 mg),

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and level 4 (90%), respectively.

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

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3.3. FTIR spectra analysis

The FTIR spectra of starch, PVA, clino, semi-IPN starch-g-p(AA-co-AAm)/PVA hydrogel

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and semi-IPN starch-g-p(AA-co-AAm)/PVA/clino superabsorbent nanocomposite have been illustrated in Fig. 2(a). In FTIR spectrum of starch the absorption bands at 3433 cm-1 and

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2925cm-1 are attributed to the –OH and –CH stretching vibrations, respectively. Also, a triplet peak at 982, 1090, and 1168 cm-1 exhibits the stretching vibration mode of C−O−C bond (Zou et al., 2012). The FTIR spectrum of PVA shows a sharp peak at 1444 cm-1, which is ascribed to the symmetric bending vibration mode of –CH2 group. In addition, the peak at 843 cm-1 is considered as the characteristic absorption band of isotactic sequence. The peaks

17

appeared at 3367 cm-1, 2929 cm-1, and 1092 cm-1 are related to the stretching vibration of –OH, −CH, and C−O bonds, respectively (Tang, Sun, Li, Wu, & Lin, 2009; Zhu et al., 2015). The FTIR spectrum of clino exhibits a broad band at 3446 cm-1, which is assigned to the stretching vibration of –OH group in Al−OH−Al and Si−OH−Si groups. The peaks at 1635 cm-1 and 1049 cm-1 are ascribed to H−O−H bending mode and Si−O−(Si), (Al)

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stretching vibration, respectively. Also, the appeared peaks at 796 cm-1 and 607 cm-1 are

related to the stretching vibrations of Si−O−(Si) and Si−O−(Al) groups, respectively

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(Castaldi et al., 2005). In FTIR spectra of starch-g-p(AA-co-AAm)/PVA and starch-g-p(AA-

co-AAm)/PVA/clino, the overlapped absorption bands of –OH and –NH groups were

U

appeared as broad and relatively sharp peaks between 3400 cm-1 and 3600 cm-1. Similarly,

N

the stretching vibration of C=O group of AA and AAm has overlapped with the bending

of

starch-g-p(AA-co-AAm)/PVA

and

starch-g-p(AA-co-AAm)/PVA/clino,

M

spectra

A

vibration mode of –NH group, which was emerged at 1639 cm-1 and 1645 cm-1 in FTIR

respectively (Gharekhani, Olad, Mirmohseni, & Bybordi, 2017). The peaks emerged at 2854 in

FTIR

spectra

of

starch-g-p(AA-co-AAm)/PVA

ED

cm-1

and

starch-g-p(AA-co-

AAm)/PVA/clino samples are corresponded to the stretching vibration of −CH2 groups of

PT

AA and AAm monomers (Owens et al., 2007). Also, the absorption bands between 1250 cm-1

CC E

and 1600 cm-1 in FTIR spectra of starch-g-p(AA-co-AAm)/PVA and starch-g-p(AA-coAAm)/PVA/clino samples are related to the stretching vibration of carboxylate (–COO-) group of sodium acrylate (Z. Zhang et al., 2014). In addition, the peaks appeared between

A

1150 cm-1 and 1350 cm-1 in FTIR spectra of starch-g-p(AA-co-AAm)/PVA and starch-gp(AA-co-AAm)/PVA/clino are related to the overlapped absorption bands of C−O and C−N groups coupled with the stretching vibration mode of –OH groups (Rashidzadeh, Olad, Salari, & Reyhanitabar, 2014). As depicted in the FTIR spectra of starch-g-p(AA-coAAm)/PVA and starch-g-p(AA-co-AAm)/PVA/clino, the characteristic absorption bands of 18

C−O−C groups of starch have weakened and shifted to lower wavenumbers after synthesis. Additionally, the absorption bands of starch-g-p(AA-co-AAm)/PVA and starch-g-p(AA-coAAm)/PVA/clino in the vicinity of 500-700 cm-1 differ from those in starch. These results confirm successful grafting reaction of AA and AAm monomers onto starch backbone (Z. Zhang et al., 2014; Zhu et al., 2015). Also, the characteristic absorption bands of clino can be

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observed with slight shift in FTIR spectrum of starch-g-p(AA-co-AAm)/PVA/clino,

demonstrating that incorporation of clino into the hydrogel network has been performed

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successfully.

U

3.4. XRD patterns analysis

N

The crystallinity of starch in its natural solid form, clino, PVA, semi-IPN starch-g-p(AA-co-

A

AAm)/PVA hydrogel and semi-IPN starch-g-p(AA-co-AAm)/PVA/clino superabsorbent nanocomposite was examined using X-ray diffraction (XRD) analysis (Fig. 2(b)). As shown

M

in Fig. 2(b), XRD pattern of starch exhibits a sharp peak at 2θ=16.95 o, which is assigned to

ED

the characteristic peak of B-type crystalline structure of starch. Therefore, starch has a semicrystalline structure (Bursali, Coskun, Kizil, & Yurdakoc, 2011). XRD pattern of clino (Fig.

PT

2(b)) indicates sharp peaks at 2θ= 9.85o, 11.19o, and 22.4o, which are related to the Miller indices of [020], [200], and [400], respectively (Olad et al., 2016; Rashidzadeh et al., 2014).

CC E

The XRD pattern of PVA (2 wt%) (Fig. 2(b)) exhibits a sharp peak at 2θ=19.37o, which confirms its semi-crystalline structure (Bursali et al., 2011; Zhu et al., 2015). The XRD

A

patterns of semi-IPN starch-g-p(AA-co-AAm)/PVA and semi-IPN starch-g-p(AA-coAAm)/PVA/clino show broad peaks at 2θ=22.4o, 2θ=38.59o, and 2θ=22.19o and 2θ=38.54o, respectively, which demonstrate their amorphous nature. The characteristic peaks of clino have been overlapped by the broad amorphous peaks of the superabsorbent nanocomposite. However, appearance of the crystalline character of clino in the XRD pattern of

19

superabsorbent nanocomposite as relatively sharp peak at 2θ=22.19o, is an indicative of its

A

CC E

PT

ED

M

A

N

U

SC R

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successful incorporation into hydrogel network (Olad et al., 2016; Rashidzadeh et al., 2014).

20

IP T SC R U N A M ED PT CC E A

Fig 2. FTIR spectra of starch, starch-g-p(AA-co-AAm)/PVA, starch-g-p(AA-co-

AAm)/PVA/clino, PVA, and clino (a), and XRD patterns of clino, starch-g-p(AA-coAAm)/PVA, starch-g-p(AA-co-AAm)/PVA/clino, PVA, and starch (b).

21

3.5. Morphological analysis FE-SEM technique was used to study the morphological properties of clino, starch-g-p(AAco-AAm)/PVA and starch-g-p(AA-co-AAm)/PVA/clino samples. FE-SEM images of hydrogel samples with various magnifications have been depicted in Fig. 3(a-e). As shown in FE-SEM images of hydrogel samples (Fig. 3(a-e)), there is no aggregation or phase

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separation in their surface morphology. Thus, it can be concluded that PVA has been distributed uniformly throughout the hydrogel network. According to Fig. 3(a) and (b),

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starch-g-p(AA-co-AAm)/PVA sample shows a rough surface with low porosity, which results from the surface crosslinking effect of PVA (Zhu et al., 2015). In contrast, starch-g-

U

p(AA-co-AAm)/PVA/clino sample (Fig. 3(c) and (d)), due to the physical crosslinking effect

N

of the introduced clino, possesses a coarse surface with highly porous structure. These

A

structures provide more contact surface area for starch-g-p(AA-co-AAm)/PVA/clino in the

M

swelling medium, facilitate water diffusion into the hydrogel network, and so make swelling behavior easier and faster compared with starch-g-p(AA-co-AAm)/PVA sample. Therefore, it

ED

can be expected that starch-g-p(AA-co-AAm)/PVA/clino sample will show greater water absorption rate and also higher swelling capacity compared with starch-g-p(AA-co-

PT

AAm)/PVA sample. Fig. 3(e) shows that the starch-g-p(AA-co-AAm)/PVA/clino network is composed of spherical-like polymer structures, that their dimensions are below 100 nm. Fig.

CC E

3(f) exhibits FE-SEM image of clino. As depicted in Fig. 3(f), clino has a layer-like structure. Clino is classified as a zeolite mineral in which (Si,Al)O 4 tetrahedral structures are connected

A

by oxygen atoms and form a two-dimensional layer-like structure (Olad & Naseri, 2010).

22

IP T SC R U N A M ED PT CC E A Fig. 3. SEM images of starch-g-p(AA-co-AAm)/PVA (a) and (b), starch-g-p(AA-coAAm)/PVA/clino superabsorbent nanocomposite (c-e), and clinoptilolite (f). 23

3.6. Swelling kinetic studies Swelling kinetic measurements were carried out in distilled water as swelling medium to evaluate the effect of clino on the swelling behavior of the hydrogel sample. As depicted in Fig. 4(a), both hydrogel samples possess a similar swelling kinetic behavior. Initially, swelling capacity of the hydrogel samples increases considerably and in continue it rises with

IP T

a slow growth rate until the equilibrium swelling capacity was achieved. According to Fig.

4(a), the swelling rate of starch-g-p(AA-co-AAm)/PVA/clino is higher than that of starch-g-

SC R

p(AA-co-AAm)/PVA so that the equilibrium swelling capacity of starch-g-p(AA-coAAm)/PVA was achieved within 365 min, while it took a short time (200 min) for starch-g-

U

p(AA-co-AAm)/PVA/clino to reach own equilibrium swelling capacity. This phenomenon

N

can be attributed to the highly porous structure of starch-g-p(AA-co-AAm)/PVA/clino

A

sample, which enhances contact surface area with the water molecules and so increases water

M

diffusion rate into hydrogel network. Also, the equilibrium swelling capacity of starch-gp(AA-co-AAm)/PVA/clino (364.82 g/g) was greater than that of starch-g-p(AA-co-

ED

AAm)/PVA sample (286.21 g/g). Hydrophilic hydroxyl groups of PVA as well as other hydrophilic groups (−COOH, −COONa, and −NH2) of hydrogel cause a significant

PT

improvement in swelling capacity. Also, surface crosslinking effect of PVA chains, caused

CC E

by the hydrogen-bonding interactions between PVA and functional groups of hydrogel, generates a coarse porous surface, which improves water uptake property of the hydrogel (Hu, Feng, Wei, et al., 2014; Hu, Feng, Xie, et al., 2014; Zhu et al., 2015). In the presence of

A

clino, the electrostatic repulsive forces between negatively charged surface of clino and carboxylate groups of hydrogel are strengthened, resulting in an expanded hydrogel network with higher swelling capacity. Besides, physical crosslinking effect of clino causes to formation of highly porous hydrogel network, which in turn due to its high water uptake capability, swelling capacity improves considerably (Amnuaypanich, Patthana, &

24

Phinyocheep, 2009; Olad et al., 2016; Rashidzadeh et al., 2014). From these findings, it can be concluded that the synergistic effect of PVA and clino induces a substantial improvement in the swelling capacity of starch-g-p(AA-co-AAm)/PVA/clino compared with starch-gp(AA-co-AAm)/PVA.

Semi-IPN

starch-g-p(AA-co-AAm)/PVA/clino

superabsorbent

nanocomposite developed in this study possessed higher water absorption capacity compared

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with the hydrogel nanocomposite prepared by Hosseinzadeh et al. (Hosseinzadeh & Ramin,

2018). Also, in comparison with the starch-g-poly(AA)/organo-mordenite hydrogel

SC R

composite synthesized by Zhang et al. (Yan Zhang, Gao, Zhao, & Chen, 2015), semi-IPN superabsorbent nanocomposite showed reasonably good water absorption capacity and also

U

high swelling rate.

N

Swelling kinetic of starch-g-p(AA-co-AAm)/PVA and starch-g-p(AA-co-AAm)/PVA/clino

A

samples was evaluated by pseudo-second order swelling kinetic model to get further

M

information about the swelling rate of the hydrogels (Gharekhani et al., 2017; Wang, Wang,

ED

Kang, & Wang, 2011):

1 2 s W∞

(5)

CC E

A=k

(4)

PT

t⁄W = A + Bt

B = 1⁄W∞

A

(6)

Where W (g/g) is the swelling capacity at time t (min); the A parameter (g.min/g) relates 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 swelling capacity. Plotting t/W versus t for each hydrogel sample (Fig. 4(b)) gave straight lines with good linear correlation coefficients, 25

which W∞ and ks were calculated from the slope and intercept of the plotted straight lines, respectively. The amount of theoretical equilibrium swelling capacity (W∞ ) of starch-gp(AA-co-AAm)/PVA and starch-g-p(AA-co-AAm)/PVA/clino was 303.03 g/g and 384.61 g/g, respectively, which were very close to their corresponding experimental values. Also, the swelling rate constant (ks) of starch-g-p(AA-co-AAm)/PVA/clino sample (1.83 × 10-4

IP T

g/g.min) was higher than that of starch-g-p(AA-co-AAm)/PVA (1.65 × 10-4 g/g.min), indicating that starch-g-p(AA-co-AAm)/PVA/clino absorbs water molecules faster than that

SC R

of starch-g-p(AA-co-AAm)/PVA. This is due to the physical crosslinking effect of introduced clino, which generates highly porous structures within hydrogel network, increases contact

U

surface area, and thus induces a higher swelling rate.

N

The swelling data achieved from the first 60% of the fractional water uptake were fitted with

A

the following equation to determine water diffusion mechanism of hydrogel samples

M

(Gharekhani et al., 2017; Olad et al., 2016; Rao et al., 2013):

(7)

ED

Wt ⁄W∞ = kt n

PT

Where Wt (g/g) is the swelling capacity at time t (min), and W∞ (g/g) corresponds to the

CC E

equilibrium swelling capacity. The k parameter is proportionality constant and relates to the hydrogel structure, and the exponent n specifies the diffusion mechanism of water molecules. When n < 0.5, the diffusion process conforms to Fickian diffusion mechanism. n values

A

between 0.5 and 1.0 are related to non-Fickian diffusion mechanism. For n=1, the diffusion mechanism is case-II and for n values greater than 1, the diffusion mechanism is supercase-II. The plots of Ln (Wt ⁄W∞ ) versus Ln (t) were plotted for both hydrogel samples (Fig. 4(c)). The values of n and k were calculated using the slopes and intercepts of the plotted lines, respectively. As the values of n for starch-g-p(AA-co-AAm)/PVA (0.7) and starch-g-p(AA-

26

co-AAm)/PVA/clino (0.754) were greater than 0.5, therefore water diffusion mechanism for both hydrogel samples was non-Fickian diffusion type. Also, the amount of k for starch-gp(AA-co-AAm)/PVA and starch-g-p(AA-co-AAm)/PVA/clino was achieved as 0.0544 and 0.0689, respectively.

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3.7. Absorbency under load (AUL) One of the most important characteristics of hydrogels, which can designate gel strength, is

SC R

water absorbency under load. To investigate the equilibrium swelling capacity of starch-g-

p(AA-co-AAm)/PVA and starch-g-p(AA-co-AAm)/PVA/clino in 0.9 wt% NaCl aqueous solution under different pressures (0.3, 0.6, and 0.9 psi), swelling kinetic measurements were

U

performed (Fig. 4(d) and (e)). As shown in Fig. 4(d) and (e), the swelling kinetic trend of

N

both hydrogel samples under different loads was almost similar. At initial time intervals, the

A

swelling capacity increased dramatically, and then the growth rate of swelling capacity

M

decreased until an equilibrium swelling capacity was achieved. According to Fig. 4(d) and

ED

(e), starch-g-p(AA-co-AAm)/PVA sample reached to own equilibrium swelling capacity within 190 min at each pressure, while the corresponding time for starch-g-p(AA-co-

PT

AAm)/PVA/clino at each pressure was 160 min. These results indicated that starch-g-p(AAco-AAm)/PVA/clino swells more quickly than that of starch-g-p(AA-co-AAm)/PVA sample.

CC E

This was due to the highly porous structure of starch-g-p(AA-co-AAm)/PVA/clino, which facilitates penetration of water molecules into hydrogel network, and so enhances swelling

A

rate. As shown in Fig. 4(d), the equilibrium swelling capacity of starch-g-p(AA-coAAm)/PVA at pressures of 0.3, 0.6, and 0.9 psi was 40.79 g/g, 34.23 g/g, and 29.95 g/g, respectively. In contrast, the equilibrium swelling capacity of starch-g-p(AA-coAAm)/PVA/clino at pressures of 0.3, 0.6, and 0.9 psi was achieved as 44.93 g/g, 39.67 g/g, and 36.57 g/g, respectively. These results revealed that starch-g-p(AA-co-AAm)/PVA/clino

27

possesses greater equilibrium swelling capacity compared with starch-g-p(AA-coAAm)/PVA at each applied pressure. This phenomenon can be ascribed to the strong electrostatic repulsive forces between negative surface charges of clino and carboxylate groups of the hydrogel matrix, which lead to expansion of the hydrogel network and subsequently higher swelling capacity. From Fig. 4(d) and (e), it can be also inferred that

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equilibrium swelling capacity decreases with increasing applied pressure. This is due to the fact that swollen hydrogel sample loses higher amount of absorbed water molecules under

A

CC E

PT

ED

M

A

N

U

SC R

greater applied pressure.

28

IP T SC R U N A M ED PT CC E

Fig. 4. Swelling kinetic curves of starch-g-p(AA-co-AAm)/PVA and starch-g-p(AA-co-

A

AAm)/PVA/clino samples (a), plots of t/W versus t (b) and plots of Ln (Wt ⁄W∞ ) versus Ln (t) for starch-g-p(AA-co-AAm)/PVA and starch-g-p(AA-co-AAm)/PVA/clino samples (c),

and water absorbency under loud (AUL) for starch-g-p(AA-co-AAm)/PVA (d) and starch-gp(AA-co-AAm)/PVA/clino (e) in aqueous saline solution (0.9 wt% NaCl) at different pressures (0.3 psi, 0.6 psi, and 0.9 psi). 29

3.8. Rheological studies To examine gel properties of starch-g-p(AA-co-AAm)/PVA and starch-g-p(AA-coAAm)/PVA/clino samples, rheological analysis was performed. For this purpose, mechanical response of hydrogel samples was measured within angular frequency range of 0.1-100 Hz. First, strain sweep tests at constant frequency of ω= 10 Hz (Fig. 5(a)) were executed on

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hydrogels to specify the liner viscoelastic (LVE) zone, where Gʹ (storage modulus) and Gʺ (loss modulus) are independent of the applied strain. As shown in Fig. 5(a), below 0.5%

SC R

deformation, the applied strain does not change Gʹ and Gʺ values of the hydrogel samples,

indicating LVE region. Therefore, the strain amplitude was found to be 0.5%. The plots of Gʹ and Gʺ versus angular frequencies for starch-g-p(AA-co-AAm)/PVA and starch-g-p(AA-co-

U

AAm)/PVA/clino samples have been depicted in Fig. 5(b). According to Fig. 5(b), Gʹ

A

N

amounts of both hydrogel samples were greater than that of Gʺ amounts throughout the whole

viscous

nature.

Therefore,

M

frequency range, implying the predominant elastic nature of hydrogel samples over their starch-g-p(AA-co-AAm)/PVA

and

starch-g-p(AA-co-

ED

AAm)/PVA/clino samples had a stable three-dimensional crosslinked network structure with good mechanical strength. Also, starch-g-p(AA-co-AAm)/PVA/clino sample possessed

PT

higher Gʹ values compared with starch-g-p(AA-co-AAm)/PVA sample in all frequency range, indicating its more rigid gel network. This can be attributed to the physical

CC E

crosslinkages within hydrogel network made by strong hydrogen-bonding interactions between hydroxyl groups of clino and functional groups of polymeric matrix. These

A

additional physical crosslinking points keep polymer chains together tightly, and thus make a firm hydrogel framework with higher mechanical strength.

30

3.9. Swelling behavior at various pH solutions The effect of pH value on the swelling behavior of starch-g-p(AA-co-AAm)/PVA and starchg-p(AA-co-AAm)/PVA/clino samples was assessed by measuring the equilibrium swelling capacity of hydrogels in different pH solutions ranging from 2-12. The plots of equilibrium swelling capacity of hydrogels versus pH values have been shown in Fig. 5(c). As shown in

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Fig. 5(c), the equilibrium swelling capacity of both hydrogel samples increased considerably as pH value rose from 2 to 5. Then, the equilibrium swelling capacity remained almost

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constant within pH range from 5 to 8. Further increase in pH values up to pH=12 caused a

severe reduction in equilibrium water absorption capacity. At pH values below pH=5, due to

U

the presence of H+ cations in swelling medium, carboxylate (−COO–) groups are transformed

N

into carboxylic acid (−COOH) groups. At this condition, hydrogen-bonding interactions

A

between carboxylic acid groups act as physical crosslinking points, leading to enhanced physical crosslinking density, shrinked hydrogel network, and thus reduced swelling

M

capacity. At higher pH values (pH=5-8), dissociation of carboxylic acid groups (−COOH →

ED

−COO- + H+) reinforces electrostatic repulsions between carboxylate anions, which in turn due to the expansion of hydrogel network swelling capacity increases. In highly basic

PT

medium (pH > 8), charge screening effect made by Na+ counterions causes a non-perfect

CC E

anion-anion repulsions between carboxylate groups. Therefore, hydrogel shrinks and swelling capacity decreases (Gharekhani et al., 2017; Olad et al., 2016; Spagnol et al., 2012).

A

3.10. Swelling behavior in NaCl solutions To study the effect of saline solution concentration on the swelling behavior of hydrogels, equilibrium swelling capacity of starch-g-p(AA-co-AAm)/PVA and starch-g-p(AA-coAAm)/PVA/clino samples was measured in NaCl aqueous solution with different concentrations (Fig. 5(d)). As shown in Fig. 5(d), the equilibrium swelling capacity of both

31

hydrogel samples decreases with increasing NaCl solution concentration. The shielding effect of excess Na+ counterions in the swelling medium on the carboxylate anions is responsible for this phenomenon, which undermines anion-anion repulsions between carboxylate groups, and so reduces swelling capacity. Moreover, as the saline solution concentration increases osmotic pressure difference between polymer matrix and swelling medium decreases, causing

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to the reduced swelling capacity (Spagnol et al., 2012). According to Fig. 5(d), the equilibrium water absorption capacity of starch-g-p(AA-co-AAm)/PVA in NaCl solutions

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with concentrations of 0.1 wt%, 0.3 wt%, 0.5 wt%, 0.7 wt%, 0.9 wt%, and 1.1 wt% was

108.71 g/g, 85.37 g/g, 63.92 g/g, 53.83 g/g, 46 g/g, and 42.54 g/g, respectively, while in the

U

case of starch-g-p(AA-co-AAm)/PVA/clino the equilibrium water absorption capacity at the

N

same saline solution concentrations was 120.35 g/g, 98.01 g/g, 73.80 g/g, 61.72 g/g, 54.14

A

g/g, and 49.53 g/g, respectively. From these results it can be inferred that the equilibrium

M

water absorption capacity of starch-g-p(AA-co-AAm)/PVA/clino in all NaCl solution concentrations is greater than that of starch-g-p(AA-co-AAm)/PVA. The reason for this

ED

phenomenon is related to the electrostatic repulsive forces between negative surface charges of clino and carboxylate groups of polymeric matrix, which generate an expanded hydrogel

CC E

PT

network with higher swelling capacity (Olad et al., 2016; Rashidzadeh et al., 2014).

3.11. Water retention studies at various temperatures Fig. 5(e) shows water retention capacity of starch-g-p(AA-co-AAm)/PVA and starch-g-

A

p(AA-co-AAm)/PVA/clino samples at two temperatures of 40 °C and 80 °C. According to Fig. 5(e), at temperature of 40 °C, the whole water content of starch-g-p(AA-co-AAm)/PVA sample was evaporated within 6 h, while starch-g-p(AA-co-AAm)/PVA/clino sample retained 34.35% of own absorbed water after 6 h. Also, at temperature of 80 °C, it took 5 h for starch-g-p(AA-co-AAm)/PVA sample to lose all of own absorbed water, but starch-g-

32

p(AA-co-AAm)/PVA/clino sample could preserve 3.68% of the absorbed water after 5 h. Therefore, starch-g-p(AA-co-AAm)/PVA/clino has lower water evaporation rate and also higher water retention capability compared with starch-g-p(AA-co-AAm)/PVA sample. This can be attributed to the strong hydrogen-bonding interactions between absorbed water molecules and introduced clino, which hinder evaporation of water, and thus improve water

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retention capability. By comparing these results with those reported by Li et al. (A. Li, Zhang, & Wang, 2007) for starch-graft-poly(AA)/attapulgite superabsorbent composite, it

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can be concluded that clino effectively improves water retention capacity of the

superabsorbent nanocomposite compared with attapulgite. Therefore, the use of clino, as an

U

inexpensive low cost mineral additive, is of most benefit to water absorption capacity,

N

swelling rate, and also water retention capability of the semi-IPN superabsorbent

A

nanocomposite. Also, incorporation of clino into hydrogel network, due to its low cost, can

M

substantially alleviate final production cost, and thus it makes practical utilization of superabsorbent nanocomposite possible. Moreover, physical crosslinking effect of the

ED

introduced clino causes to formation of a firm hydrogel network with high mechanical strength. Additionally, in the presence of clino, superabsorbent nanocomposite showed

PT

reasonable water absorbency under different loads and good salt and pH-sensitive swelling behavior. According to these good characteristics, the synthesized semi-IPN superabsorbent

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nanocomposite can be potentially used in different fields especially in hygiene-related

A

products.

33

IP T SC R U N A M ED PT

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Fig. 5. Strain dependence of the storage modulus (Gʹ) and loss modulus (Gʺ) at a constant angular frequency (ω = 10 Hz) (a), frequency dependence of storage modulus (Gʹ) and loss modulus (Gʺ) at a constant strain (0.5%) for starch-g-p(AA-co-AAm)/PVA and starch-g-

A

p(AA-co-AAm)/PVA/clino samples (b), equilibrium swelling capacity of starch-g-p(AA-coAAm)/PVA and starch-g-p(AA-co-AAm)/PVA/clino samples at different pH values (c) and NaCl solutions with various concentrations (d), and water retention behavior of starch-gp(AA-co-AAm)/PVA and starch-g-p(AA-co-AAm)/PVA/clino samples under constant temperatures (40 ˚C and 80 ˚C) (e). 34

4. Conclusion Semi-IPN

starch-g-p(AA-co-AAm)/PVA/clino

superabsorbent

nanocomposite

was

synthesized in an aqueous medium by simultaneous free-radical graft polymerization and interpenetration of PVA chains through hydrogel network. FTIR and XRD techniques were used to peruse the structural properties of the materials. SEM images indicated that the coarse

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and relatively porous surface of hydrogel sample transform into a highly porous structure with introducing clino into polymeric matrix. The equilibrium swelling capacity of the

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superabsorbent nanocomposite (364.82 g/g) was higher than that of neat hydrogel (286.21

g/g). The theoretical water absorption capacity (W∞ ) of the hydrogels was very close to the

U

experimental values, implying that dynamic swelling data of the hydrogels were in good

N

agreement with the pseudo-second order swelling kinetic model. Also, the higher amount of

A

swelling rate constant (ks) for superabsorbent nanocomposite compared with hydrogel

M

indicated that superabsorbent nanocomposite swells faster than that of hydrogel. Moreover, superabsorbent nanocomposite showed higher water absorbency under different loads

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compared with hydrogel. Superabsorbent nanocomposite possessed stiff hydrogel network with good mechanical strength compared with hydrogel, which was confirmed by rheological

PT

analysis. Hydrogel samples had good pH and salt-dependent swelling behavior in different

CC E

pH and saline solutions. Additionally, superabsorbent nanocomposite demonstrated good water retention capability compared with neat hydrogel. These good features of semi-IPN superabsorbent nanocomposite make it most applicable in hygiene-related fields, which

A

require hydrogel samples with high swelling capacity, high swelling rate, and great water retention capability.

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

35

References Abd El‐ Mohdy, H., Hegazy, E., El‐ Nesr, E., & El‐ Wahab, M. A. (2011). Control release of some pesticides from starch/(ethylene glycol‐ co‐ methacrylic acid) copolymers prepared by γ‐ irradiation. Journal of Applied Polymer Science, 122(3), 1500-1509. Amnuaypanich, S., Patthana, J., & Phinyocheep, P. (2009). Mixed matrix membranes

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prepared from natural rubber/poly (vinyl alcohol) semi-interpenetrating polymer network (NR/PVA semi-IPN) incorporating with zeolite 4A for the pervaporation dehydration of

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water–ethanol mixtures. Chemical Engineering Science, 64(23), 4908-4918.

Bhattacharyya, R., & Ray, S. K. (2014). Enhanced adsorption of synthetic dyes from aqueous

U

solution by a semi-interpenetrating network hydrogel based on starch. Journal of Industrial

N

and Engineering Chemistry, 20(5), 3714-3725.

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