Synthesis, Characterization, and Swelling Behaviors of Salt-Sensitive

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Synthesis, Characterization and Swelling Behaviors of Saltsensitivity Maize bran-Poly(acrylic acid) Superabsorbent Hydrogel Ming yue Zhang, Zhiqiang Cheng, Tian qi Zhao, Meng zhu Liu, Mei juan Hu, and Junfeng Li J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/jf5021279 • Publication Date (Web): 18 Aug 2014 Downloaded from http://pubs.acs.org on August 21, 2014

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Journal of Agricultural and Food Chemistry

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Synthesis, Characterization and Swelling Behaviors of Salt-sensitivity Maize

2

bran-Poly(acrylic acid) Superabsorbent Hydrogel

3

Mingyue Zhang†, Zhiqiang Cheng ‡ , Tianqi Zhao†, Mengzhu Liu†, Meijuan Hu†,

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Junfeng Li†*

5

†

6

China

7

‡

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130118, People’s Republic of China

9

Correspondence to: Junfeng Li (E–mail: [email protected])

College of Chemistry, Jilin University, Changchun 130012, People’s Republic of

College of Resources and Environment, Jilin Agriculture University, Changchun

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Tel.: 0431–85168470–4

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Fax: 0431–85168470–4

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Mingyue Zhang: E-mail: [email protected]

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Zhiqiang Cheng: E-mail: [email protected]

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Tianqi Zhao: E-mail: [email protected]

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Mengzhu Liu: E-mail: [email protected]

16

Meijuan Hu: E-mail: [email protected]

17

1

ACS Paragon Plus Environment


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ABSTRACT

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A novel composite hydrogel was prepared via UV-irradiation copolymerization

20

of acrylic acid and maize bran (MB) in the presence of composite initiator

21

(2,2–dimethoxy–2–phenylacetophenone and ammonium persulfate) and crosslinker

22

(N,N’–methylenebisacrylamide).

23

bran–poly(acrylic acid) was obtained (2507 g g–1 in distilled water and 658 g g–1 in 0.9

24

wt.% NaCl solution). Effect of granularity, salt concentration, various cations and

25

anions on water absorbency was investigated. It was found that the swelling was

26

extremely sensitive to the ionic strength, cationic and anion type. Swelling kinetic and

27

water diffusion mechanism in distilled water were also discussed. Moreover, the

28

product showed an excellent water retention capability under the condition of high

29

temperature or high pressure. The salt-sensitivity, good water absorbency and

30

excellent water retention capability of the hydrogels make this intelligentized polymer

31

had wide potential applications.

32

KEYWORDS: superabsorbent, swelling behaviors, maize bran, hydrophilic

33

polymers, water retention, salt-sensitivity

Under

the

optimized

34

2


conditions,

maize

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35

INTRODUCTION

36

Superabsorbent polymers (SAP) are three-dimensional crosslinked hydrophilic

37

materials which can absorb and retain a large quantities of water or aqueous solutions.

38

With these superior performances, SAP are widely applied in many applications, such

39

as agriculture

40

dewatering 7, wastewater treatment 8 and metal-ion removal 9. The global demand for

41

SAP is increasing and will reach 1.9 million metric tons in 2015

42

market evoke a strong desire for researchers.

1-3

, architecture 4, drug-delivery systems 5, hygienic products 6, coal

10

. Needs of the

43

Although a variety of monomers can synthesize SAP, polyacrylic acid dominates

44

the market for its low price and simple synthesis process 11. As the low toxicity, good

45

water absorbent capacity, excellent biocompatibility and biodegradability, research on

46

polysaccharide-based natural materials superabsorbent is in growing interest

47

especially the study of starch–based superabsorbent. Wheat, corn, cassava and potato

48

starch have been used in the synthesis of superabsorbent resin

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reports for the use of maize bran in this field. Maize is one of the most widely

50

distributed food crops in the world. According to US Dept. of Agriculture, in

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2013-2014 year (September to August), the global maize production is expected to

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reach 966.6 million tons. As a byproduct of maize, maize bran (MB) is an abundant

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bioresource, which has low utilization at present, mainly for animal feed. However,

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the effective use of existing resources is significant in contemporary society.

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Therefore, the study of making its effective use is particular importance. The main

56

ingredients of MB we used are natural cellulose and hemicelluloses (11.7%), crude 3


10, 12-14

,

15, 16

, but almost no


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protein (12.0%), moisture (8.75%), crude fat (3.09%), crude ash (1.28%), nitrogen

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element (1.9%) and starch. In short, the advantages of abundant production, high

59

active ingredients and low prices make MB an ideal raw material for superabsorbent

60

synthesis.

61

There are many methods to prepare SAP, such as solution polymerization

16, 17

18

62

and suspension polymerization

. However, these conventional synthetic methods

63

often require exorbitant specialized equipment and are time-consuming. Recently,

64

new synthesis technologies, such as ultrasound

65

irradiation 22, electron-beam irradiation

66

those methods, UV irradiation can simplify the synthetic route, shorten the reaction

67

time and improve the efficiency.

23-25

19

, microwave irradiation

and UV irradiation

26

20, 21

, γ

come out. Among

68

In this work, a novel superabsorbent polymer maize bran–poly(acrylic acid)

69

(MB–PAA) was synthesized by UV irradiation. A new way was provided to extend

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the utilization of MB. What’s more, cheap of MB made the production cost reduced.

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MB–PAA was characterized by Fourier transform infrared radiation (FTIR)

72

spectroscopy, scanning electron microscopy (SEM) and thermogravimetry (TG/DTG).

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In addition, the water retain capacity, effect of particle size and swelling medium

74

(such as the ionic concentration, cations, anions valence and ionic strength ) on water

75

absorbency were discussed. Furthermore, the swelling kinetics and water diffusion

76

kinetics at different granularity were systematically investigated.

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MATERIALS AND METHODS

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Materials 4


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MB (particle size = 120 mesh) was obtained from Jilin Agricultural University

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(Jilin, China) and used without further purification. AA and Ammonium persulfate

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(APS) were supplied by Tianjin Fuchen Chemical Reagent Co., Ltd. (China). AA was

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distilled under reduced pressure before use. N, N’–methylenebisacrylamide (MBA;

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purity > 99.9%), 2,2-dimethoxy-2-phenylacetophenone (BDK), methanol, sodium

84

hydroxide and sodium chloride were from Beijing Chemical Works (Beijing, China).

85

All the agents were analytical grade.

86

Preparation of MB–PAA and PAA Superabsorbent Composite

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A certain amount of AA was neutralized in an ice bath with 20 wt.% NaOH

88

solution in a 500-mL glass beaker with constant stirring until obtained 85%

89

neutralization degrees of AA solution. Certain amount of MB, MBA (0.02 g mL–1),

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APS and BDK (0.05 g mL–1) (the mass ratio of AA, MB, MBA, APS and BDK was

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100: 40: 0.12: 0.40: 0.25) were added to 2.0 mL prepared AA solutions in 50-mL glass

92

beakers and the mixtures were treated by ultrasonic for 1 min. Then, exposed the

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homogenous mixtures under a UV lamp (1000 W) for 5 minutes (The self-regulated

94

UV irradiation system consisted of an iron box containing an UV lamp with a

95

wavelength of 365nm.). The distance between light source and the reaction mixtures

96

was 37 cm. After complete polymerization, the resulting products were immersed in

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methanol to remove water soluble oligomer, uncrosslinked polymer and unreacted

98

monomer. Finally, the gels were dried in an oven to a constant weight (at 70 °C) and

99

pulverized through 80, 60 and 20 mesh steel screen. The particles obtained were used

100

to determine the effect of grain diameter on water absorbency, and 80-mesh particles 5



101

were used in the rest experiments. PAA was prepared using the above method, except

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that no MB.

103

Water absorbency measurement

104

The swelling characteristics of the prepared hydrogel were measured via

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gravimetric analysis. Approximately 0.10 g of hydrogel was immersed in 500 mL

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distilled water at room temperature (25°C) at different intervals (t; 5, 10, 15, 20, 25,

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30, 40, 50, 60, 120, 150, 200, 240, 300 min). The swollen sample was filtered through

108

a 100-mesh nylon bag to separate unabsorbed water and then weighted. The water

109

absorbency of hydrogel (Qt; g g−1) was calculated by the following equation:

110

Qt =

111

Where m0 (g) and mt (g) are the weights of the samples in the dry state and the

112

swollen state at a certain time, respectively. Water absorbency in salt solutions was

113

tested in the same way, and Qe is defined as the water absorbency at the equilibrium

114

swelling state. All of the experiments were performed in triplicate, and the results

115

were averaged.

116

Water retention measurement

mt − m0 m0

(1)

117

The percentage of water retention (WR) was determined under high temperature

118

and high pressure. A certain quality of water-swollen gel (M0) was centrifuged at 6000

119

rpm (centrifugal radius is 8.6 cm) or heated at 60°C in an air oven for different

120

intervals (t; min), respectively. Then the quality of remaining gel was weighed and

121

denoted by Mt. The WR of the sample was calculated according to Eq. (2): 6


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Mt × 100 M0

122

WR (%) =

123

Characterization

(2)

124

Scanning electron microscope (SEM) (SHIMDZU SSX–550, Japan) was used to

125

analyze the morphology of the samples. Before tested, all samples were sprayed with

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gold by ETD–2000 auto sputter coater (Beijing, China) for 2 min (with a current of 4

127

mA). A fourier transform infrared spectrometer (SHIMDZU, 1.50SU1, Japan) was

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used to record the vibration in functional groups of the samples (spectral range from

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4000 to 400 cm–1 and data averaged over 20 scans). Thermal gravimetric analysis

130

(TGA) was carried out by using PerkinElmer Pyris 1 TGA (United States) from 25°C

131

to 850°C at a heating rate of 10°C min–1 under a flowing nitrogen atmosphere. A

132

Techcomp CT14D centrifuge (Shanghai, China) was used to calculate WR.

133

RESULTS AND DISCUSSION

134

Hydrogel preparation

135

The proposed mechanism for chemically crosslinking reactions of MB–PAA are

136

outlined in Figure1. Initially, initiators decomposed into sulfate and benzoyl radicals

137

under UV-irradiation. Secondly, the radicals extracted hydrogen from the hydroxyl

138

group of the active ingredients in MB to form alkoxy radicals on the substrate. Then

139

the monomer molecules became acceptor of the radicals, resulting in chain initiation

140

and thereafter themselves became free radical donor to neighboring molecules which

141

led to the growth of polymer chain. Finally, the end vinyl groups of the crosslinker

142

(MBA) reacted with the polymer chains during chain propagation. In this way a 7



143

crosslinked structure combined with network structure was formed gradually 27.

144

Characterization of materials

145

Elemental analysis

146

Elemental analysis of MB, PAA and MB–PAA are shown in Table 1. Compared

147

with PAA, C content in MB–PAA increased from 34.16 to 35.32%, N content

148

increased from 0.13 to 0.33%, and H content increased from 4.859 to 5.126%. This

149

was because that C content in MB (45.91%) was approximately 10% higher than PAA,

150

and the polysaccharide molecules with relatively high carbon contents in MB reacted

151

with the monomer molecules and changed the polymer chains. Meanwhile, part of the

152

nitrogen containing compound such as protein in MB was added to polymer chains;

153

thus, attributed to the above reasons, the contents of C, N and H are increased.

154

Morphology

155

The SEM images revealed the morphology of the employed MB, PAA and

156

MB–PAA [Figure 2 (a)-(c)]. As can be seen, MB showed a lamellar and loose

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morphology. PAA displayed a tight, smooth and pores surface. However, MB–PAA

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exhibited a comparatively coarse, loose, and porous surface, which combined the

159

characteristic of both MB and PAA. The morphology of MB–PAA illustrates that the

160

starch particles were added to polymer chains and many irregular aggregates were

161

formed during the copolymerization reaction. The larger specific surface area of

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MB–PAA led to a larger contact area with the solution than that of PAA, which

163

facilitated the permeation of water into the polymeric network 28.

164

FITR spectroscopy 8


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165

The FTIR spectra of the MB, PAA and MB–PAA are shown in Figure 2 (d). In

166

the spectrum of MB, the absorption bands at 3418 cm−1 and 2924 cm−1 were owing to

167

the stretching vibrations of the hydroxyl group (OH) and C–H, respectively. The band

168

at 1036 cm−1 was related to –CH–O–CH stretching vibration 29. The three bands were

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all characteristic absorptions of starch structures. In the spectrum of PAA, the bands at

170

3558, 2935, 1698 and 1405 cm−1 were assigned to the stretching vibration of O–H and

171

N-H, C–H, CO-NH and symmetric stretching of –COO−, respectively. The band at

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1553 cm−1 was due to the C=O asymmetric stretching of –COO– and amide II

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stretching 30. Compared with the spectrum of PAA, the absorption peaks of MB–PAA

174

revealed some changes. Absorption band at 3439 cm−1 which corresponded to

175

stretching vibration of O–H and N-H group showed a major shift and the peaks at

176

1693 cm−1 (CO-NH), 1558 cm−1 (C=O asymmetric stretching of –COO– and amide II

177

stretching), 1410 cm−1 (symmetric stretching of –COO−) showed a slight shift. They

178

were due to the chemical interaction of MB with the copolymer. What’s more, an

179

additional absorption peak at 1038 cm−1 (–CH–O–CH stretching vibration) was

180

observed 31, providing an evidence of functional groups of MB grafting to PAA during

181

the polymerization.

182

Thermal Properties

183

The TG/DTG curves of MB, PAA and MB–PAA were presented in Figure 2 (e)

184

and (f). The TG curve of MB exhibited four steps: 25–138°C, 138–333°C, 333–530°C

185

and 530–850°C, with weight losses of 4.83%, 43.60%, 29.20% and 3.92%,

186

respectively. Meanwhile, three peaks in DTG curve at about 90°C, 298°C, and 358°C 9



187

corresponded to the first three stages which represented the maximum decomposition

188

speeds. The PAA had a four-step thermogram: 25–150°C, 150–415°C, 415–558°C and

189

558–850°C, with weight losses of 11.11%, 11.84%, 27.82% and 4.69%, respectively.

190

Compared with PAA, the TG curves of MB–PAA also had four stages, but the various

191

stages of weightlessness showed large difference. Similar to PAA, the first stage

192

occurring before 159°C (11.97% weight losses) was due to loss of absorbed moisture

193

28

194

this was assigned to splitting of the starch structure, chain scission eliminating CO

195

and CO2 and forming carbonaceous residues 31. At this stage, the decomposition rate

196

was much faster than PAA, which was due to the rapid degradation of MB. The

197

21.97% weight loss occurring between 379 °C to 530°C was the contribution of the

198

elimination of the water molecule from the two neighboring carboxylic groups of the

199

polymer chains with the formation of anhydride, main-chain decomposition of the

200

polymer, and destruction of crosslinked network structure

201

(11.27%) region occurring from 530 °C to 850°C was associated with the further

202

decomposition or degradation of residual organic matter at high temperatures.

203

Ultimately, the weight of the residual sample was 31.67% [less than PAA (44.57%),

204

more than MB (18.45%)]; this was attributed to part of decomposition products and

205

inorganic salts. The TG/DTG results confirmed that chemical reaction between MB

206

and AA occurred, and the incorporation of MB reduced the thermal stability of

207

MB–PAA.

208

Water Retention

. The major weight loss 23.12% occurred in the second stage from 159°C to 379°C;

10


32

. The last weight loss

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209

The water retention capacity was determined under the condition of centrifugal

210

separation (6000 rpm) and high temperature drying (60 °C), the results are shown in

211

Figure 3 (a). It indicates that the WR maintained 39 % and 0.5 % after centrifugation

212

for 60 min and drying for 70 h, respectively. Van der Waal’s forces and the hydrogen

213

bonding interaction between the superabsorbent and water molecules directly affected

214

the water retention performance of the absorbent resin 33. The substantial carboxylate

215

groups containing in MB–PAA made the chemical interaction stronger, which could

216

improve the WR. From the centrifugation curve, it can be seen that in the first 20

217

minutes, the curve showed a sharp decline, while 20 minutes later, the water retention

218

went down slowly. The reasons for this phenomenon was that, initially, more

219

weak-absorption water containing in the gel was easier to break free, and with the

220

passage of the centrifugation time, the relative proportion of strong-absorption water

221

increased, leading to a relatively slow dehydration. The drying curve showed similar

222

properties, but the changes in the rate of water loss was not so obvious. This indicates

223

that the water-absorbing resin was more sensitive to the pressure than the temperature.

224

The obtained good water absorbency and retention properties would allow for a

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further development of the material, such as in the agricultural, forestry and

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horticulture application as a water-retaining agent in arid soils.

227

Swelling behavior

228

Effect of the maize bran content

229

The effect of the weight ratio of MB to AA on the water absorbency was studied.

230

It is observed from Figure 3 (b) that with increase in content of MB, water absorbency 11



231

increased to 2286 g g−1 in distilled water and 545 g g−1 in 0.9 wt.% NaCl solution and

232

then decreased (reaction conditions, AA: MBA: APS: BDK = 100: 0.2: 0.3: 0.75;

233

reaction time, 20 min; neutralization degree of AA, 85%). A number of hydrophilic

234

functional groups (–OH, –NH2 and –COOH groups) contained in MB, which could

235

react with the monomer; and enhanced the water absorbency. The ratio of MB from 0

236

to 40%, because of the introduction of these hydrophilic groups, the polymeric

237

network was enhanced and the water absorption increased. When the amount of MB

238

exceeded the best quantity, the water absorbency reduced. This phenomenon may be

239

due to the conformation of more crosslinking points in the polymeric network; this

240

would lead to an excessive increase in the crosslinking density of the superabsorbent

241

hydrogel. Moreover, too much MB may have resulted in a relative ratio reduction in

242

hydrophilic groups, such as –OH, –COOH, –NH2 and –COO–, in the superabsorbent

243

composite. The above two causes decreased the water absorbency 34.

244

Effect of the reaction time

245

As shown in Figure 3 (c), the water absorbency in distilled water increased from

246

2 to 5 min and then decreased from 5 to 20 min (reaction conditions, AA: MBA: APS:

247

BDK: MB = 100: 0.2: 0.3: 0.75: 40; neutralization degree of AA, 85%). The water

248

absorbency in 0.9 wt.% NaCl solution exhibited the similar trend. The maximum

249

absorbency (2507 g g−1 in distilled water and 658 g g−1 in 0.9 wt.% NaCl solution)

250

was observed at 5 min. On the one hand, a short reaction time resulted in incomplete

251

polymerization and a short polymer chain, which was not conducive to water

252

absorption; on the other hand, too long irradiation time caused an increase in polymer 12


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253

crosslinking degree, and many branched chains were formed in the network structure,

254

which tangled with each other and produced small holes in three-dimensional

255

networks, which obstructed the expansion of the polymer 35. So, both of these respects

256

resulted in a low water absorbency.

257

Effect of various cations

258

The characteristics of external solution such as salt concentration and ionic

259

valence greatly influence the swelling behavior of the superabsorbent polymers. To

260

achieve a comparative measure of sensitivity of the hydrogels to the kind of aqueous

261

fluid, a dimensionless salt sensitivity factor (f) for 150 mmol L–1 salt solution was

262

calculated by the following equation 29:

263

f = 1−

Qs Qd

(3)

264

Where Qs and Qd are the swelling in salt solution and in deionized water, respectively.

265

The effect of the ionic strength on the water absorbency can be expressed by

266

Flory’s equation 32:

267

Q 5/3 ≈

268

Where Q is water absorbency, i/Vu is the charge density of polymer, I is the ionic

269

strength of solution, (1/2−x1)/V1 is the polymer-solvent affinity and ve/V0 is the

270

crosslinking density.

271

(i / 2Vu I 1/2 ) 2 + (1/ 2 − x1 ) / V1 ve / V0

(4)

The effect of various ions on water absorbency can be concluded from Figure 3 (d)

272

and Table 2. Three conclusions were obtained from Figure 3 (d), firstly, the water

273

absorbency was appreciably decreased as the salt concentration of the solutions 13



274

increased from 0 to 50 mmol L−1, and gentle decreased in the concentration above 50

275

mmol L−1. This was because, for ionic hydrogels, the additional cations causing a

276

anion–anion electrostatic repulsion, leading to a reduced osmotic pressure difference

277

between the external solution and the polymer network, resulting swelling decreased

278

36

279

solutions was KCl > NaCl > NH4Cl > AlCl3 > FeCl3 > MgCl2 > CaCl2 when the

280

solution concentrations were under 50 mmol L–1. We found that hydrogels were hard

281

and very rubbery after swelling in MgCl2 and CaCl2 solution. In contrast, it exhibited

282

lower strength in other solutions. This may be due to the ionic crosslinking of the

283

polymer particles; different ions affected the crosslinking density differently, which

284

influenced the gel strength in different degrees. The effect of seven cationic on the

285

swelling had the following order: K+ > Na+ > NH4+ > Al3+ > Fe3+ > Mg2+ > Ca2+; but

286

Al3+ > NH4+ when the concentration was above 50 mmol L–1. The curves of water

287

absorbency for monovalent metal cationic salt solution were gentler than those for

288

divalent and trivalent cationic salt solutions. This was because that polyvalent metal

289

ion would form complexes with the carboxylate group and the solutions had higher

290

ionic strength. As shown in Table 2, the ionic strength of 150 mmol L–1 salt solution

291

was trivalent cationic > divalent cationic > monovalent cationic. According to Eq. (4),

292

when the ionic strength of saline solution increased, the water absorbency decreased.

293

Interestingly, the water absorbency in AlCl3 and FeCl3 solution were better than in

294

MgCl2 and CaCl2 solution, while their ionic strength was higher (Table 2). This may

295

be due to divalent and trivalent cationic impacted the charge density of polymer in

. Secondly, the order of water absorbency of the hydrogel in the chloride salt

14


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296

different degrees, being greater of trivalent cationic, leading to stronger water

297

absorbency. Thirdly, we found that the larger the radius of monovalent metal cationic,

298

the more water absorption capacity was (K+ > Na+), and the less the radius of the same

299

polyvalent monatomic cation, the larger water absorbency was (Mg2+ > Ca2+ and

300

Al3+ > Fe3+). Moreover, the water absorbency decreased more in polyatomic

301

monovalent cations (NH4+) solution than in single atom monovalent cations (K+ and

302

Na+) solutions. The sensitivity of the gel to various cations was K+ < Na+ < NH4+
SO42– > PO43–. This may be due to the

308

ionic strength Na3PO4 > Na2SO4 > NaCl. These results indicates that trivalent anion

309

and divalent anion salt solution showed larger influence on water absorbency. The

310

sensitivity factor of Na3PO4, Na2SO4 and NaCl was 0.9869, 0.9769 and 0.7375,

311

respectively [Figure 3 (f)]. The sensitivity of the gel to multivalent anion was much

312

larger than monovalent anion at same concentration.

313

Effect of hydrogel particle size

314

To study the effect of particle size of the hydrogel on water absorbency,

315

composite hydrogel particles with different granularity were examined. Figure 4. (a)

316

showed the influence of the hydrogel particle sizes on water absorbency at different

317

times. It is clear that the hydrogel particle size had a significant influence on water 15



318

absorbency before equilibrium, and almost had no impact on the equilibrium water

319

absorbency. Smaller particles had a larger water absorption rate before equilibrium,

320

particularly at the first 20 minutes. This was because that smaller particles had a

321

greater surface area, leading to a larger contact area and a higher water absorbency 8.

322

The process of water absorption (80 mesh) consisted of three steps: (1) 0–10 min, a

323

rapid increase phase, in which the water absorption rate was fastest and up to 39 % of

324

equilibrium absorption capacity; (2) 10–120 min, a slower increase phase, whose

325

contribution to equilibrium absorption was approximately 59 %; (3) 120–300 min, an

326

equilibrium phase, where absorption remained almost constant. The other two (60

327

mesh and 20 mesh) had the similar swelling process.

328

Kinetic analysis

329

Swelling Kinetic in Distilled Water

330

To investigate the kinetic mechanism, the pseudo-first order and pseudo-second

331

order models were adopted to fit the experimental data. The pseudo-first order

332

swelling kinetic model is based on the approximation that the absorption rate relates

333

to the quantity of the unoccupied. The model 9, 37 can be expressed in Eq. (3):

334

ln(Qe − Qt ) = ln Qe − K1t

(3)

335

The pseudo-second order swelling kinetic model is deduced based on the concept

336

that the absorption relates to the squared product of the difference between the

337

number of the available equilibrium absorptive sites and that of the occupied sites.

338

The model 38, 39 was expressed in Eq. (4):

16


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339

t 1 t = + Qt K 2Qe 2 Qe

340

Where Qe (g g–1) is the equilibrium water absorbency, Qt (g g–1) is water absorbency

341

at contact time t (min), K1 (min–1) and K2 (g g–1 min–1) are the rate constants.

(4)

342

Figure 4. (b, c) were the pseudo-first order plot and pseudo-second order plot.

343

The values of K1, K2, and Qe,cal along with those of the correlation coefficient (R2) are

344

summarized in Table 3. Based on the values of the correlation coefficient (R2), it fitted

345

the experimental data better with the pseudo-second order swelling kinetic than the

346

pseudo-first order swelling kinetic. In addition, the Qe,cal values as obtained from the

347

pseudo-first order swelling kinetic model appeared to be large different to the

348

experimental data, and the values from the pseudo-second order swelling kinetic

349

model were very close to the experimentally observed values. Therefore, the swelling

350

process in distilled water followed the pseudo-second order swelling kinetic model

351

better.

352

Diffusion Kinetic in Distilled Water

353

The diffusion behavior of water into polymers networks were analyzed using an

354

empirical equation as follows 10:

355

log(

356

Where Mt and Me are the mass of water absorbency at time t (min) and at equilibrium,

357

respectively. k is a characteristic constant of the polymer and n is a diffusional

358

exponent. From the slope and intercept of the plot of log(Mt/Me) versus log(t), the

359

kinetic parameters n and k were calculated. The results are presented in Figure 4 (d)

360

and Table 4. According to the classification of the diffusion mechanism, at value of n

Mt

Me

) = log(k ) + n log(t )

(5)

17



Page 18 of 37

361

< 0.5, the rate of water moving into the polymer is dominated by a Fickian diffusion

362

mechanism. At n > 0.5, it conforms to the non-Fickian diffusion mechanism, in which

363

the relaxation rate was greater than or equal to water diffusion rate. Therefore, it

364

complied with non-Fickian diffusion mechanism at the particle size of 20 mesh, and

365

corresponded to Fickian diffusion mechanism at 60 and 80 mesh. We believed that the

366

smaller particles had a relatively larger contact area to the liquid, making easier the

367

macromolecular chain relaxation, the relaxation rate greater than water diffusion

368

rate10.

369

ABBREVIATIONS USED

370

SAP, superabsorbent polymers; MB, maize bran; AA, acrylic acid; BDK,

371

2,2–dimethoxy–2–phenylacetophenone; APS, ammonium persulfate; MBA, N,

372

N’–methylenebisacrylamide;

373

poly(acrylic acid; FTIR, Fourier transform infrared radiation; SEM, scanning electron

374

microscopy; TG/DTG, thermogravimetry.

375

ASSOCIATED CONTENT

376

Acknowledgment

377

MB–PAA,

maize

bran–poly(acrylic

acid;

PAA,

The authors thank Jilin Provincial Science and Technology Department and

378

Changchun Science and Technology Department.

379

Supporting Information

380

Figure. S1. Effect of (a) neutralization degree of AA, (b) crosslinker content, (c)

381

water-soluble initiator content, (d) photoinitiator content on water absorbency of the

382

superabsorbent polymer. The factors of neutralization degree of AA, weight ratio of 18


Page 19 of 37


383

crosslinker, initiator content to AA and reaction time on water absorbency had been

384

studied. The optimized synthetic conditions were identified. This information is

385

available free of charge via the Internet at http: //pubs.acs.org.

386

AUTHOR INFORMATION

387

Corresponding Author

388

*

(J.F.L.) Mail: College of Chemistry, Jilin University, Changchun, China. E-mail:

389

[email protected]. Phone: 0431–85168470–4. Fax: 0431–85168470–4.

390

Funding

391

This work was financially supported by Basic Research Program of Jilin

392

Provincial Science and Technology Department (20130102040JC) and Changchun

393

Science and Technology Project (13NK01).

394

Notes

395

The authors declare no competing financial interest.

396

19



397

REFERENCES

398

1. Jin, S.; Yue, G.; Feng, L.; Han, Y.; Yu, X.; Zhang, Z., Preparation and Properties of

399

a Coated Slow-Release and Water-Retention Biuret Phosphoramide Fertilizer with

400

Superabsorbent. J. Agric. Food Chem. 2010, 59, 322-327.

401

2. Bansiwal, A. K.; Rayalu, S. S.; Labhasetwar, N. K.; Juwarkar, A. A.; Devotta, S.,

402

Surfactant-Modified Zeolite as a Slow Release Fertilizer for Phosphorus. J. Agric.

403

Food Chem. 2006, 54, 4773-4779.

404

3. Teodorescu, M.; Lungu, A.; Stanescu, P. O.; Neamţu, C., Preparation and

405

Properties of Novel Slow-Release NPK Agrochemical Formulations Based on

406

Poly(acrylic acid) Hydrogels and Liquid Fertilizers. Ind. Eng. Chem. Res. 2009,

407

48, 6527-6534.

408 409

4. Esteves, L. P., Superabsorbent polymers: On their interaction with water and pore fluid. Cement & Concrete Composites 2011, 33, 717-724.

410

5. Singh, B.; Sharma, V., Influence of polymer network parameters of tragacanth

411

gum-based pH responsive hydrogels on drug delivery. Carbohydr. Polym. 2014,

412

101, 928-940.

413

6. Loo, S.-L.; Fane, A. G.; Lim, T.-T.; Krantz, W. B.; Liang, Y.-N.; Liu, X.; Hu, X.,

414

Superabsorbent Cryogels Decorated with Silver Nanoparticles as a Novel Water

415

Technology for Point-of-Use Disinfection. Environ. Sci. Technol. 2013, 47,

416

9363-9371.

417

7. Kabiri, K.; Omidian, H.; Zohuriaan-Mehr, M. J.; Doroudiani, S., Superabsorbent

418

Hydrogel Composites and Nanocomposites: A Review. Polymer Composites 2011, 20


Page 20 of 37

Page 21 of 37


419

32, 277-289.

420

8. Li, D.; Zhang, X.; Simon, G. P.; Wang, H., Forward osmosis desalination using

421

polymer hydrogels as a draw agent: Influence of draw agent, feed solution and

422

membrane on process performance. Water Res. 2013, 47, 209-215.

423

9. Liu, D.; Li, Z.; Li, W.; Zhong, Z.; Xu, J.; Ren, J.; Ma, Z., Adsorption Behavior of

424

Heavy Metal Ions from Aqueous Solution by Soy Protein Hollow Microspheres.

425

Ind. Eng. Chem. Res. 2013, 52, 11036-11044.

426

10. Witono, J. R.; Noordergraaf, I. W.; Heeres, H. J.; Janssen, L. P. B. M., Water

427

absorption, retention and the swelling characteristics of cassava starch grafted

428

with polyacrylic acid. Carbohydr. Polym. 2014, 103, 325-332.

429

11. Zhang, J.; Liu, R.; Li, A.; Wang, A., Preparation, Swelling Behaviors, and

430

Slow-Release Properties of a Poly(acrylic acid-co-acrylamide)/Sodium Humate

431

Superabsorbent Composite. Ind. Eng. Chem. Res. 2005, 45, 48-53.

432

12. Wang, Y.; Liu, M.; Ni, B.; Xie, L., κ-Carrageenan–Sodium Alginate Beads and

433

Superabsorbent Coated Nitrogen Fertilizer with Slow-Release, Water-Retention,

434

and Anticompaction Properties. Ind. Eng. Chem. Res. 2011, 51, 1413-1422.

435

13. Demitri, C.; Scalera, F.; Madaghiele, M.; Sannino, A.; Maffezzoli, A., Potential of

436

Cellulose-Based Superabsorbent Hydrogels as Water Reservoir in Agriculture.

437

International Journal of Polymer Science 2013.

438

14. Sawut, A.; Yimit, M.; Sun, W.; Nurulla, I., Photopolymerisation and

439

characterization of maleylatedcellulose-g-poly(acrylic acid) superabsorbent

440

polymer. Carbohydr. Polym. 2014, 101, 231-9. 21



441

15. Xie, L.; Lü, S.; Liu, M.; Gao, C.; Wang, X.; Wu, L., Recovery of Ammonium onto

442

Wheat Straw To Be Reused as a Slow-Release Fertilizer. J. Agric. Food Chem.

443

2013, 61, 3382-3388.

444

16. Yang, Y.; Tong, Z.; Geng, Y.; Li, Y.; Zhang, M., Biobased Polymer Composites

445

Derived from Corn Stover and Feather Meals as Double-Coating Materials for

446

Controlled-Release and Water-Retention Urea Fertilizers. J. Agric. Food Chem.

447

2013, 61, 8166-8174.

448

17. Xie, L.; Liu, M.; Ni, B.; Wang, Y., Utilization of Wheat Straw for the Preparation

449

of Coated Controlled-Release Fertilizer with the Function of Water Retention. J.

450

Agric. Food Chem. 2012, 60, 6921-6928.

451

18. Hussain, Y. A.; Liu, T.; Roberts, G. W., Synthesis of Cross-Linked, Partially

452

Neutralized Poly(Acrylic Acid) by Suspension Polymerization in Supercritical

453

Carbon Dioxide. Ind. Eng. Chem. Res. 2012, 51, 11401-11408.

454

19. Wan, T.; Huang, R.; Zhao, Q.; Xiong, L.; Qin, L.; Tan, X.; Cai, G., Synthesis of

455

wheat straw composite superabsorbent. J. Appl. Polym. Sci. 2013, 130,

456

3404-3410.

457

20. Ge, H.; Pang, W.; Luo, D., Graft copolymerization of chitosan with acrylic acid

458

under microwave irradiation and its water absorbency. Carbohydr. Polym. 2006,

459

66, 372-378.

460

21. Lin, C.-x.; Zhan, H.-y.; Liu, M.-h.; Fu, S.-y.; Huang, L.-h., Rapid Homogeneous

461

Preparation of Cellulose Graft Copolymer in BMIMCL Under Microwave

462

Irradiation. J. Appl. Polym. Sci. 2010, 118, 399-404. 22


Page 22 of 37

Page 23 of 37


463

22. Bardajee, G. R.; Hooshyar, Z.; Zehtabi, F.; Pourjavadi, A., A superabsorbent

464

hydrogel network based on poly ((2-dimethylaminoethyl) methacrylate) and

465

sodium alginate obtained by gamma-radiation: synthesis and characterization.

466

Iranian Polymer Journal 2012, 21, 829-836.

467

23. Zhang, S.; Wang, W.; Wang, H.; Qi, W.; Yue, L.; Ye, Q., Synthesis and

468

characterisation of starch grafted superabsorbent via 10MeV electron-beam

469

irradiation. Carbohydr. Polym. 2014, 101, 798-803.

470

24. El Salmawi, K. M.; Ibrahim, S. M., Characterization of superabsorbent

471

carboxymethylcellulose/clay hydrogel prepared by electron beam irradiation.

472

Macromolecular Res. 2011, 19, 1029-1034.

473

25. El-Mohdy, H. L. A.; Safrany, A., Preparation of fast response superabsorbent

474

hydrogels by radiation polymerization and crosslinking of N-isopropylacrylamide

475

in solution. Radiation Physics and Chemistry 2008, 77, 273-279.

476

26. Shi, X.; Wang, W.; Wang, A., Synthesis and enhanced swelling properties of a

477

guar gum-based superabsorbent composite by the simultaneous introduction of

478

styrene and attapulgite. J. Polym. Res. 2011, 18, 1705-1713.

479

27. Rashidzadeh, A.; Olad, A.; Salari, D.; Reyhanitabar, A., On the preparation and

480

swelling properties of hydrogel nanocomposite based on Sodium alginate-g-Poly

481

(acrylic acid-co-acrylamide)/Clinoptilolite and its application as slow release

482

fertilizer. J. Polym. Res. 2014, 21, 1-15.

483

28. Yadav, M.; Singh, S. K.; Rhee, K. Y., Synthesis of partially hydrolyzed graft

484

copolymer (H-Ipomoea hederacea seed gum-g-polyacrylonitrile). Carbohydr. 23



485

Page 24 of 37

Polym. 2013, 95, 471-478.

486

29. Akar, E.; Altinisik, A.; Seki, Y., Preparation of pH- and ionic-strength responsive

487

biodegradable fumaric acid crosslinked carboxymethyl cellulose. Carbohydr.

488

Polym. 2012, 90, 1634-41.

489

30. Wan, T.; Huang, R.; Zhao, Q.; Xiong, L.; Luo, L.; Tan, X.; Cai, G., Synthesis and

490

swelling properties of corn stalk-composite superabsorbent. J. Appl. Polym. Sci.

491

2013, 130, 698-703.

492

31. Anirudhan, T. S.; Rejeena, S. R.; Tharun, A. R., Investigation of the Extraction of

493

Hemoglobin by Adsorption onto Nanocellulose-Based Superabsorbent Composite

494

Having Carboxylate Functional Groups from Aqueous Solutions: Kinetic,

495

Equilibrium, and Thermodynamic Profiles. Ind. Eng. Chem. Res. 2013, 52,

496

11016-11028.

497

32. Bao, Y.; Ma, J.; Li, N., Synthesis and swelling behaviors of sodium

498

carboxymethyl

499

hydrogel. Carbohydr. Polym. 2011, 84, 76-82.

500

33. Patra,

T.;

Pal,

cellulose-g-poly(AA-co-AM-co-AMPS)/MMT

A.;

Dey,

J.,

A

smart

supramolecular

superabsorbent

hydrogel

of

501

N(alpha)-(4-n-alkyloxybenzoyl)-L-histidine exhibiting pH-modulated properties.

502

Langmuir : the ACS journal of surfaces and colloids 2010, 26, 7761-7.

503

34. Zhang, M.; Cheng, Z.; Liu, M.; Zhang, Y.; Hu, M.; Li, J., Synthesis and properties

504

of a superabsorbent from an ultraviolet‐ irradiated waste nameko mushroom

505

substrate

506

http://onlinelibrary.wiley.com/doi/10.1002/app.40471/full.

and

poly

(acrylic

acid).

J.

Appl.

24


Polym.

Sci.

2014,

Page 25 of 37


507

35. Liu, J.; Li, Q.; Su, Y.; Yue, Q.; Gao, B.; Wang, R., Synthesis of wheat straw

508

cellulose-g-poly (potassium acrylate)/PVA semi-IPNs superabsorbent resin.

509

Carbohydr Polym. 2013, 94, 539-46.

510

36. Zhao, Y.; Su, H.; Fang, L.; Tan, T., Superabsorbent hydrogels from poly(aspartic

511

acid) with salt-, temperature- and pH-responsiveness properties. Polymer 2005, 46,

512

5368-5376.

513

37. Evren, M.; Acar, I.; Guclu, K.; Guclu, G., Removal of Cu2+ and Pb2+ ions by

514

N-vinyl 2-pyrrolidone/itaconic acid hydrogels from aqueous solutions. Canadian

515

Journal of Chemical Engineering 2014, 92, 52-59.

516 517

38. Ho, Y. S.; McKay, G., Pseudo-second order model for sorption processes. Process Biochemistry 1999, 34, 451-465.

518

39. Singh, T.; Singhal, R., Reuse of a waste adsorbent poly(AAc/AM/SH)-Cu

519

superabsorbent hydrogel, for the potential phosphate ion removal from waste

520

water: Matrix effects, adsorption kinetics, and thermodynamic studies. J. Appl.

521

Polym. Sci. 2013, 129, 3126-3139.

522

25



523

Figure captions

524

Figure 1. Proposed reaction mechanism for the synthesis of MB–PAA.

525

Figure 2. SEM micrographs of (a) MB, (b) PAA, and (c) MB–PAA. (d) FTIR spectra

526

of MB, PAA and MB–PAA. (e) TG curves of MB, PAA and MB–PAA. (f) DTG

527

curves of MB, PAA and MB–PAA.

528

Figure 3. (a) Water retaining capacity of MB–PAA centrifuged at 6000 rpm and dried

529

at 60 °C. Effect of (b) maize bran content to AA, (c) reaction time on water

530

absorbency. Water absorbency of superabsorbent hydrogel in (d) KCl, NaCl, NH4Cl,

531

CaCl2, MgCl2, FeCl3 and AlCl3, (e) NaCl, Na2SO4 and Na3PO4 aqueous solutions. (f)

532

Salt sensitivity factor of various salt at concentration of 150 mmol L–1.

533

Figure 4. (a) The swelling behavior of different hydrogel particle sizes in distilled

534

water absorbency. (b) pseudo-first order kinetic model, (c) pseudo-second order

535

kinetic model and (d) the water diffusion behavior for different hydrogel particle

536

sizes.

537

26


Page 26 of 37

Page 27 of 37


538 539

Tables Table 1. Elemental analysis of MB, PAA and MB–PAA. Sample

C (%)

N (%)

H (%)

MB

45.91

1.98

7.084

PAA

34.16

0.13

4.859

MB–PAA

35.32

0.33

5.126

540

27



541

Table 2. Effect of salt solutions on water absorption. Solution (150 mmol L–1) KCl NaCl NH4Cl CaCl2 MgCl2 FeCl3 AlCl3 Na2SO4 Na3PO4

Ionic strengtha (mol-ion dm–3) 0.15 0.15 0.15 0.45 0.45 0.90 0.90 0.45 0.90

Qe (g g–1) 702.0 658.0 36.2 22.6 28.8 34.2 38.9 58.0 32.8

542

a

543

and charge on each individual ion, respectively.

I = 0.5∑ (Ci Zi 2 ) , where I, Ci and Zi are the ionic strength, the ionic concentration,

544

28


Page 28 of 37

Page 29 of 37


545

Table 3. Kinetic parameters for the water absorbency of MB–PAA in distilled water. Sample Qe,exp Pseudo-first order swelling kinetic (mesh) g g–1 R2 K1 Q e, cal –1 (min ) (g g–1) 20 2507 0.9516 0.0228 1618 60 2507 0.9659 0.0239 1416 80 2507 0.9554 0.0233 1188

Pseudo-second order swelling kinetic R2

K2 (10 g g–1 min–1) 2.63 3.96 5.29 –5

0.9974 0.9994 0.9994

546 547

29


Q e, cal (g g–1) 2720 2629 2588


548

Table 4. Diffusion parameters and the correlation coefficients for the water diffusion

549

of MB–PAA in distilled water. Sample (mesh) 20 60 80

R2 0.9648 0.9446 0.9624

n 0.5301 0.3320 0.2287

k 0.1114 0.2408 0.3630

550 551

30


Page 30 of 37

Page 31 of 37


Figure 1. Proposed reaction mechanism for the synthesis of MB–PAA. 91x100mm (600 x 600 DPI)



Figure 2. SEM micrographs of (a) MB, (b) PAA, and (c) MB–PAA. (d) FTIR spectra of MB, PAA and MB–PAA. (e) TG curves of MB, PAA and MB–PAA. (f) DTG curves of MB, PAA and MB–PAA. 80x40mm (300 x 300 DPI)


Page 32 of 37

Page 33 of 37


Figure 3. (a) Water retaining capacity of MB–PAA centrifuged at 6000 rpm and dried at 60 °C. Effect of (b) maize bran content to AA, (c) reaction time on water absorbency. Water absorbency of superabsorbent hydrogel in (d) KCl, NaCl, NH4Cl, CaCl2, MgCl2, FeCl3 and AlCl3, (e) NaCl, Na2SO4 and Na3PO4 aqueous solutions. (f) Salt sensitivity factor of various salt at concentration of 150 mmol L–1. 57x21mm (600 x 600 DPI)





Page 34 of 37

Page 35 of 37





Figure 4. (a) The swelling behavior of different hydrogel particle sizes in distilled water absorbency. (b) pseudo-first order kinetic model, (c) pseudo-second order kinetic model and (d) the water diffusion behavior for different hydrogel particle sizes. 114x86mm (600 x 600 DPI)


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Table of Contents Graphic 45x24mm (300 x 300 DPI)


Synthesis, Characterization, and Swelling Behaviors of Salt-Sensitive

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