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Enhanced swelling and responsive properties of pineapple peel carboxymethyl cellulose-g-poly(acrylic acid-co-acrylamide) superabsorbent hydrogel by the introduction of carclazyte Hongjie Dai, and Huihua Huang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b04899 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017

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Journal of Agricultural and Food Chemistry

[Title Page] • Title. Enhanced swelling and responsive properties of pineapple peel carboxymethyl cellulose-g-poly (acrylic acid-co-acrylamide) superabsorbent hydrogel by the introduction of carclazyte • Author names and affiliations. Hongjie Dai,† and Huihua Huang*,† †

School of Food Science and Engineering, South China University of Technology,

Guangzhou 510641, China • Corresponding author. Huihua Huang (E-mail: [email protected]; Tel: +86 20-87112851) School of Food Science and Engineering, South China University of Technology, Guangzhou 510641, China. • Present/permanent address. No.381, Wushan Road, Tianhe District, Guangzhou City, Guangdong Province, China.

ACS Paragon Plus Environment


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ABSTRACT

2

The superabsorbent hydrogels were synthesized by grafting acrylic acid and

3

acrylamide onto pineapple peel carboxymethyl cellulose and effect of carclazyte

4

introduction was compared. The structure and morphology of the superabsorbents

5

were investigated by Fourier transform infrared spectroscopy, X-ray diffraction and

6

field emission scanning electron microscope. Swelling behaviors of the

7

superabsorbents were investigated in distilled water, 0.9% NaCl solution, various

8

salt and pH solutions as well as surfactant solutions and simulated physiological

9

fluids. The swelling dynamic mechanism of the superabsorbents was explained well

10

by Fickian diffusion and Schott’s pseudo second order models. The introduction of

11

carclazyte effectively improved the swelling capacity of the superabsorbents in

12

various solutions as well as its salt- and pH-sensitivity. The prepared

13

superabsorbents also exhibited excellent sensitivities to various surfactant solutions

14

and simulated physiological fluids, showing potential applications in the

15

biomaterials field.

16

KEYWORDS

17

pineapple

18

peel

carboxymethyl

cellulose;

carclazyte;

characterization; swelling

19 20 21 22


superabsorbent;

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23 24

INTRODUCTION

25

Until the present, most agricultural by-products or wastes, including straw, leaves,

26

corn cob and fruit peels, are still remained unutilized and discarded as wastes, in

27

which 31 ~ 60% cellulose existed yet.1 Among these wastes, pineapple (Ananas

28

comosus L. Merryl) peel is widely produced during the processing of pineapple

29

salads, juice, jam and can as well as bromelain.2-4 As one of the most abundant tropic

30

fruit, approximately 16 ~ 19 million tonnes of pineapple are harvested around the

31

world annually. However, pineapple peel, accounting for 35% of total pineapple

32

weight, is generally peeled off and discarded with little or no economic value, while

33

its disposal is costly and may cause serious environmental issues and bioresource

34

wastes problem.3 Pineapple peel is mainly composed of cellulose, hemi-cellulose,

35

lignin and pectin, in which cellulose occupies 20 ~ 25% of the dry weight.5-6 Based

36

on a 23% yield of pineapple peel cellulose in our previous study,7 the high-value

37

utilization of pineapple peel cellulose is of great significance. Until now, there are

38

only limited information available concerning the use of pineapple peel cellulose.

39

Cellulose is an extensive crystalline homo-polymer of anhydroglucopyranose

40

units (AGUs) via β-(1→4) glycosidic linkage and intra- and inter-molecular

41

hydrogen bonds,7 Due to its limited dissolution in water and common organic or

42

inorganic solvents,8 cellulose is generally converted to various water-soluble derived

43

forms such as methylcellulose (MC), hydroxypropyl methylcellulose (HPMC), ethyl

44

cellulose (EC), hydroxyethyl cellulose (HEC) and carboxymethyl cellulose (CMC),



45

9-10

46

pharmaceutical and paint industries owing to its good water solubility,

47

biocompatibility, biodegrability and sensitivity to pH and ionic strength variations.11

in which CMC exhibits greatest potential use in food, paper, cosmetics, textile,

48

Superabsorbent hydrogels are moderately defined as three-dimensional networks

49

of hydrophilic polymers formed by chemical and/or physical crosslinking, which can

50

absorb and retain considerable water or biological fluids compared with traditional

51

absorbents (e.g. sponge, cotton, wood pulp and colloidal silica, etc.) even under

52

certain pressure.12-14 Due to this particular superiority, superabsorbent hydrogels are

53

extensively applied in various fields such as agriculture, cosmetics, wastewater

54

treatment, drug delivery, tissue engineering, biosensors, and adsorbents for heavy

55

metals and dyes.12, 15-17 Usually, hydrogels with the characteristics responding to

56

external stimuli such as temperature, pH, light, electric, salt concentration and ionic

57

strengths are often referred to “intelligent” or “smart” hydrogels, with great

58

application potential in various fields.18 Compared with superabsorbent hydrogels

59

prepared from synthetic polymers, hydrogels based on natural polymers such as

60

cellulose,10 starch,19 chitosan,20 sodium alginate,21 collagen,22 and their derivatives

61

have inspired great interest recently due to their high hydrophilicity, favorable

62

biocompatibility, less toxicity and better biodegradability.

63

Recently, incorporation of clays into hydrogels has been proved to be an effective

64

approach to enhance the hydrogels properties.12, 23-24 Strong interfacial interactions

65

between the dispersed clay layers and the hydrogels lead to enhanced mechanical,

66

thermal and barrier properties. Different clays, such as montmorillonite,12, 25 kaolin,7


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67

diatomite,24 rectorite,26 palygorskite,27 sepiolite,28 kaolinite,29 vermiculite,30 and

68

medicinal stone31 have been used in hydrogels formation to achieve lower cost and

69

better properties. However, little information is available on the application of

70

carclazyte in hydrogels. Carclazyte [H2Al2(SiO3)4. nH2O], is a layered aluminum

71

silicate with exchangeable cations and reactive -OH groups on the surface. Due to

72

large specific surface area and pore volume, low activity, strong adsorption capacity

73

and ion exchange ability, carclazyte is extensively applied in various fields such as

74

petroleum refining, lipin decoloring, pharmaceuticals, environmental pollution,

75

catalyst, etc.32

76

Based on above-stated background, in this study, the superabsorbents were

77

synthesized by graft copolymerization of acrylic acid (AA) and acrylamide (AM)

78

along the chains of pineapple peel carboxymethyl cellulose and the effect of the

79

introduction of carclazyte was compared. The structure and morphology of the

80

superabsorbents were characterized by Fourier transform infrared spectroscopy

81

(FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). In

82

addition, the swelling kinetics and swelling behaviors in various media were also

83

investigated systematically.

84

MATERIALS AND METHODS

85

Materials and reagents.

86

Pineapple peel was obtained from a local pineapple processing factory

87

(Guangzhou City, China). Carclazyte was supplied by Xuyi Xinyuan Technology Co.,

88

Ltd (Huaian City, China). The chemical compositions of pineapple peel and



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89

carclazyte are given in Table 1. Acrylic acid (AA; purity ≥ 99.0%) was provided by

90

Tianjin Kermel Chemical Reagent Co., Ltd. (Tianjin City, China). Acrylamide (AM;

91

purity ≥ 98.0%) was provided by Rich Joint Chemical Reagents Co., Ltd. (Shanghai

92

City, China). Ammonium persulfate (APS; purity ≥ 98.0%) was supplied by

93

Sinopharm

94

N,Nꞌ-methylenebisacrylamide (MBA; purity ≥ 99.0%) was purchased from Tianjin

95

Xinchun Chemical Reagent Co., Ltd. (Tianjin City, China). All other chemicals and

96

solvents used in this study were of analytical grade and solutions were prepared with

97

distilled water.

98

Preparation of carboxymethyl cellulose from pineapple peel

99

Chemical

Reagent

Co.,

Ltd.

(Shanghai

City,

China).

Pineapple peel carboxymethyl cellulose (PCMC) was prepared according to the

100

method of Liu et al.

101

cellulose (PPC) extracted according to our previously reported method,7 was

102

stir-treated in a mixture solvent composed of 200 mL isopropanol (90%, v/v), 1.2 mL

103

hydrogen peroxide (30%, w/v) and 16 mL sodium hydroxide (50%, w/v) at room

104

temperature for 2 h. Then the activated PPC was etherified with 14 mL

105

diazomethane (50%, w/v) at room temperature for 0.5 h, 45 °C for 0.5 h, 60 °C for

106

0.5 h, and 75 °C for 1.5 h, respectively. Subsequently, the mixture was neutralized

107

with glacial acetic acid (10%, v/v), then filtered and washed thoroughly with

108

anhydrous methanol and gradient ethanol solutions (75%, 85% and 95%, v/v) by

109

turns. After drying and pulverizing through a 100-mesh screen, PCMC was available.

110

The degree of substitution (DS) of the PCMC was determined as 1.05 by the

33

with some modifications. About 10 g of pineapple peel


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111

standard method.34

112

Preparation of superabsorbent hydrogels

113

Initially, 0.8 g of PCMC was stir-treated in 20 mL distilled water at room

114

temperature until to complete dissolution. Then 0.1 g of carclazyte was added to the

115

PCMC solution under continuous stirring to form a homogeneous solution. After

116

heating the solution to 60 °C, 12 mg of initiator APS was added under stirring and

117

kept at 60 °C for 15 min to generate radicals. Subsequently, the mixture containing

118

3.2 g AA (70% neutralization degree; adjusted by 40% sodium hydroxide solution at

119

an ice bath), 0.8 g AM and 3 mg MBA respectively, was added to the solution. Then

120

the solution was heated to 70 °C and maintained for 2 h to complete reaction. After

121

the required time and cooling down to room temperature, the resulting mixture was

122

immersed in distilled water to remove the residual unreacted monomer. Finally, the

123

PCMC-g-poly(AA-co-AM)/carclazyte superabsorbent hydrogel composite was

124

available after oven drying at 50 °C to constant weight. Similarly, the

125

PCMC-g-poly(AA-co-AM) superabsorbent hydrogel was prepared according to the

126

above method in the case of absence of carclazyte.

127

Characterization

128

FTIR spectra of the samples were recorded on a FTIR spectrometer (Vector 33,

129

Bruker, Germany) using the KBr pressed pellet method35 for sample preparation,

130

within the frequency range of 4000-400 cm-1 at a resolution of 4 cm-1. XRD patterns

131

of the samples were collected using an X-ray diffractometer (D8 ADVANCE, Bruker,

132

Germany) with Cu-Ka radiation (λ = 0.15418 nm) at a voltage of 40 kV and a



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133

current of 40 mA. The scanning speed was set at 2°/min in the region of the

134

diffraction angle (2θ) from 4° to 50°. SEM images of the samples were observed

135

using a field emission scanning electron microscope (S-3700N, Hitachi, Japan).

136

Prior to analysis, a thin gold film was sputter-coated on the surface of the samples

137

using a sputter coater (Cressington 108 auto, Watford, UK).

138

Swelling kinetics

139

In this study, the gravimetric method was employed to measure the swelling ratio

140

(SR) of the superabsorbents in distilled water and 0.9% NaCl solution. Prior to

141

swelling study, the superabsorbents were cut and uniformed into small blocks with

142

size about 5 mm × 4 mm × 2 mm. Then, 0.1 g of the superabsorbents was immersed

143

in excess distilled water or 0.9% NaCl solution at room temperature. After preset

144

time intervals, the swollen superabsorbents were filtered using a sieve and drained

145

for 10 min until no free solution remained. After weighing, the SR of the

146

superabsorbents at a specified time was calculated according to eq 1.

147

SR (g/g) =

Wt − Wd Wd

(1)

148

where SR (g/g) is the swelling ratio defined as grams of absorbed water per gram of

149

the dry superabsorbents; Wt (g) and Wd (g) are the weights of swollen

150

superabsorbents at time t (min) and dry superabsorbents, respectively.

151

Swelling in various pH and saline solutions

152

Various pH solutions were prepared and adjusted by 1 mol/L HCl and 1 mol/L

153

NaOH solutions and determined using a pH meter (PHS-25, Shanghai Leici


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154

Instrument Co., Ltd., China). The ionic strengths of all solutions were adjusted to 0.1

155

mol/L by adding appropriate amounted NaCl. After reaching a swelling equilibrium

156

at room temperature, the swelling ratio was recorded. To investigate the influence of

157

salt species and strengths on the swelling ratio of the superabsorbents, the swelling

158

capacities of the hydrogels were evaluated in various salt solutions (NaCl, CaCl2,

159

and FeCl3) with various concentrations ranged from 0.02 to 0.1 mol/L.

160

Swelling in various surfactant solution and physiological fluids

161

The swelling capacities of the superabsorbents were investigated in various

162

surfactant solution (SDS, CTAB and Triton X-100, respectively) with a

163

concentration of 0.1 mol/L and physiological fluids (D-glucose solution: 50 g

164

D-glucose + 1000 mL distilled water; urea solution: 50 g urea + 1000 mL distilled

165

water; physiological saline water: 9 g NaCl + 1000 mL distilled water; and synthetic

166

urine: 8 g NaCl + 1 g MgSO4 + 20 g urea + 0.6 g CaCl2 + 1000 mL distilled water).

167

RESULTS AND DISCUSSION

168

Hydrogel formation mechanism

169

In this study, the PCMC-g-poly(AA-co-AM)/carclazyte superabsorbent hydrogel

170

composite was synthesized by graft polymerization of AA and AM onto PCMC and

171

the introduction of carclazyte. Additionally, APS and MBA were used as a free

172

radical initiator and a hydrophilic crosslinking agent, respectively. The proposed

173

reaction mechanism is depicted as Scheme 1. The initiator APS was firstly

174

decomposed into sulfate anion-radicals under heating. These radicals extracted

175

hydrogen from hydroxyl group of PCMC chains to form alkoxy radicals. After



176

addition of the monomer molecules (AA and AM), the active radical sites on PCMC

177

chains initiated vinyl groups of the monomers to accomplish chains propagation.

178

During the chains propagation, the end vinyl groups of the cross-linker (MBA)

179

reacted with the various polymer chains to form a crosslinked structure. It was

180

notable that carclazyte probably acted as a crosslinking agent to form the polymer

181

network. Similar reaction mechanism was also reported by Bao et al.12 about the

182

preparation

183

acid-co-acrylamide-co-2-acrylamido-2-methyl-1-propanesulfonic)/ montmorillonite

184

hydrogel.

185

FTIR analysis

of

sodium

carboxymethyl

cellulose-g-poly(acrylic

186

The FTIR spectra of PPC, PCMC, PCMC-g-poly(AA-co-AM)/carclazyte and

187

PCMC-g-poly(AA-co-AM) are shown in Figure 1. For PPC and PCMC, the broad

188

absorption band at 3500 ~ 3200 cm-1 was ascribed to the stretching vibration of -OH

189

groups.7 The bands around 2900 and 1051 cm-1 were associated with the stretching

190

vibrations of C-H and C-O-C groups, respectively. The band around 1320 cm-1 was

191

assigned to the bending vibrations of -OH groups.11 The very intense bands at 1653

192

and 1423 cm-1 observed for PCMC confirmed the presence of the stretching

193

vibration of COO- groups and its salt forms, consequently corresponding to the

194

typical adsorption of carboxymethyl cellulose.36-37 It was noticed that the band

195

corresponding to O-H stretching vibration exhibited a slight shift from 3464 cm-1

196

(for PCMC) to 3451 cm-1 (for the hydrogels) with variations in intensity. The

197

characteristic absorption bands of PCMC at 1653, 1423, 1320 and 1051 cm-1 were


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198

obviously weakened after the hydrogels formation. The COO- band at 1657 cm-1 for

199

PCMC-g-poly(AA-co-AM)

200

PCMC-g-poly(AA-co-AM)/carclazyte, implying the incorporation of carclazyte

201

decreased the hydrogel bonding interaction among polymer chains in contrast to

202

PCMC-g-poly(AA-co-AM).

203

PCMC-g-poly(AA-co-AM)/carclazyte, the strong band at 1035 cm-1 was attributed

204

to the stretching vibration of Si-O-Si.12-13

205

XRD analysis

206

was

shifted

As

to

1647

for

cm-1

after

spectrum

forming

of

XRD is intensively applied as an efficient technique for evaluating the

207

crystallinity

208

PCMC-g-poly(AA-co-AM)/carclazyte and PCMC-g-poly(AA-co-AM) are shown in

209

Figure 2. The peaks and the background correspond to the crystalline and

210

amorphous phase, respectively.28 As observed in Figure 2, PPC displayed a strong

211

peak at 2θ = 21.8° with a shoulder at 2θ = 15.3° and a weak peak at 2θ = 34.6°,

212

corresponding to the characteristic crystalline peaks of cellulose I in nature. However,

213

in comparison with PPC, only a broad diffraction peak with decreased intensity at 2θ

214

= 22.1° was observed for PCMC due to carboxymethylation, implying the

215

destruction of the original PPC crystalline structure and the possible shift of

216

crystalline structure from cellulose I to cellulose II. Meanwhile, the good water

217

solubility of PCMC also confirmed its lower crystallinity. Similar phenomenon also

218

observed in the carboxymethyl cellulose from corn husk agro waste37 and

219

carboxymethyl cellulose from banana pseudo stem (Musa cavendishii LAMBERT).38

of

polymer.

The

XRD

patterns


of

PPC,

PCMC,


Page 12 of 38

220

The main diffraction peaks of carclazyte at 2θ were around of 5.7°, 20.8° and 26.5°.

221

After the hydrogels formation, the original crystallinity of PCMC was further

222

destroyed.

223

PCMC-g-poly(AA-co-AM) significantly exhibited a very broad and weak diffraction

224

peak at 2θ = 22.5° ~ 23.8°, corresponding to the shift from crystalline phase to

225

amorphous phase. Additionally, compared with PCMC-g-poly(AA-co-AM), no

226

obvious

227

PCMC-g-poly(AA-co-AM)/carclazyte, except for a very weak peak at 2θ = 26.5°.

228

The

229

PCMC-g-poly(AA-co-AM)/carclazyte indicated uniform dispersion and exfoliation

230

of carclazyte sheets in this composite network.

231

SEM analysis

232

The

characteristic

absence

Figure

PCMC-g-poly(AA-co-AM)/carclazyte

3

of

diffraction

characteristic

shows

the

SEM

peaks

diffraction

images

were

peaks

of

of

PPC,

and

observed

carclazyte

PCMC,

for

for

the

carclazytes,

233

PCMC-g-poly(AA-co-AM)/carclazyte and PCMC-g-poly(AA-co-AM). As shown in

234

Figure 3a, PPC exhibited a fiber-like structure, which was probably due to the

235

strong self-association of the cellulose chains. As observed in Figure 3b, after

236

carboxymethylation, PPC was modified as PCMC that was substituted with more

237

clearly rod-like structure from 60 to 140 µm in diameter and from 5 to 20 µm in

238

length.. Unlike the fiber- or rod-like structure of cellulose, as shown in Figure 3c,

239

carclazyte showed an obvious agglomerated granular morphology composed of

240

small particles from 2 to 5µm in diameter. As depicted in Figure 3e and f,

241

PCMC-g-poly(AA-co-AM) presented a relatively smooth and compact surface,


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242

whereas PCMC-g-poly(AA-co-AM)/carclazyte appeared a more comparatively

243

undulant, rugged and coarse surface after the introduction of carclazyte,

244

corresponding to the uniformed dispersion and incorporation of carclazyte into the

245

superabsorbent hydrogel composite. Generally, this coarser and undulant surface is

246

beneficial for the penetration of water into the polymeric network, consequently lead

247

to an enhanced capacity of water absorbency.12, 16

248

Swelling kinetics

249

The swelling kinetics for the PCMC-g-poly(AA-co-AM)/carclazyte and

250

PCMC-g-poly(AA-co-AM) in distilled water and 0.9% NaCl solution are depicted in

251

Figure 4a and b, respectively. Obviously, these two superabsorbents exhibited a

252

similar tendency of swelling kinetics with a high swelling capacity, implying their

253

similar characteristics of superabsorbent. The swelling ratio increased sharply during

254

the initial 60 min, and then became slower till reaching a plateau. After the

255

incorporation of carclazyte, the corresponding equilibrium swelling ratio of the

256

prepared superabsorbent was increased from 420.17 to 515.24 g/g in distilled water

257

and was increased from 28.03 to 37.89 g/g in 0.9% NaCl solution, indicating the

258

swelling capacity is correlated with the introduction of carclazyte. The enhanced

259

swelling capacity may be ascribed to the following reasons: The introduction of

260

carclazyte can improve the three-dimensional network structure of the hydrogel,

261

leading to easier diffusion of water molecules into the network as well as relaxation

262

of polymer chains. Meanwhile, the -OH groups on the surface of carclazyte also

263

increased the affinity of the polymeric network to water molecules. Similar



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264

observations have been reported by Irani et al.39 in the swelling study of

265

polyethylene-g-poly (acrylic acid)/organo-montmorillonite superabsorbent hydrogel

266

and

267

nanocomposites.

268

Models of swelling dynamic mechanism

Wang

et

al.13

in

carboxymethyl

cellulose/attapulgite

superabsorbent

269

Generally, the swelling process of hydrogels can be divided into two molecular

270

processes (i.e. penetration of the solvent molecules into the void spaces in the

271

network and subsequent relaxation of the polymeric chains), which can be analyzed

272

by the Fickian diffusion model and the Schott’s second-order-kinetic model,

273

respectively.

274 275 276

At the initial swelling stage, the Fickian diffusion model was applied to describe the penetration mechanism and can be expressed as eq 2.

F=

Mt = Kt n Me

(2)

277

where F is the fractional uptake at a given time t (min); Mt (g) and Me (g) are the

278

mass of water absorbed at time t and equilibrium, respectively; K is a characteristic

279

constant of the hydrogel, and n is the diffusional exponent, corresponding to the

280

transport mode of the penetration. At n < 0.5, Fickian diffusion is dominant, in which

281

the water transport is governed by a simple concentration gradient. At 0.5 < n < 1,

282

the transport is anomalous, where the water uptake is controlled collaboratively by

283

water diffusion and relaxation of polymer chains (non-Fickian diffusion). At n > 1,

284

the relaxation of polymer chains would control the diffusion system (anomalous


Page 15 of 38


285

diffusion).40 By plotting ln (F) against ln (t), the diffusion exponent n and

286

characteristic constant K values as well as the corresponding determination

287

coefficients (R2) were obtained, as listed in Table 2. As shown in Figure 4c and d,

288

the plots displayed straight lines with good linear correlation coefficient (R2 > 0.95).

289

Notably, the n values were all below 0.5, indicating the domination of Fickian

290

diffusion

291

PCMC-g-poly(AA-co-AM), PCMC-g-poly(AA-co-AM)/carclazyte showed a slight

292

higher n value, indicating its easier relaxation of polymer chains for the water

293

accommodation. Meanwhile, as listed in Table 1, the n and K values of two

294

superabsorbents in distilled water were obviously higher than in 0.9% NaCl solution,

295

implying the faster polymer relaxation in distilled water.

during

the

initial

swelling

stage.

Compared

with

296

During the entire swelling period, the Schott’s second-order kinetic model was

297

generally supposed to be more suitable, which was applied in this study and can be

298

expressed as eq 3.

299

t 1 1 = 2+ t Qt kQe Qe

(3)

300

where Qe (g/g) and Qt (g/g) are the swelling ratio of hydrogels at theoretic

301

equilibrium and time t (min), respectively; k (g/g·min) is the initial swelling rate

302

constant. Based on the swelling data, the swelling kinetic parameters including the

303

correlation coefficients (R2), k and calculated swelling ratio (Qe,cal, g/g) can be

304

obtained by linear regression as listed in Table 2. The plots of t/Qt versus t revealed

305

perfect straight lines with good linear correlation coefficient (R2 > 0.99; Figure 4e



Page 16 of 38

306

and f), and the Qe,cal values from the swelling kinetic model were close to the

307

experimental values (Qe,exp, g/g), indicating that the Schott’s second-order-kinetic

308

model can be effectively applied for describing the whole swelling process of the

309

prepared superabsorbents.

310

Salt sensitivity analysis

311

The salt-sensitive superabsorbents have been widely applied in many areas

312

recently, especially in agriculture and horticulture. The swelling behaviors of the

313

PCMC-g-poly(AA-co-AM)/carclazyte and PCMC-g-poly(AA-co-AM) in different

314

salt solutions (NaCl, CaCl2 and FeCl3) at various concentrations (0.02 ~ 0.1 mol/L)

315

are depicted in Figure 5a, b and c. As the increase of the salt concentrations, the

316

swelling

317

PCMC-g-poly(AA-co-AM) obviously decreased. This shrinking behavior could be

318

probably ascribed to the reduced osmotic pressure difference between the polymer

319

network and the external solution, corresponding to the ionic interactions between

320

mobile ions and the fixed charges in salt solutions.41 Due to the increased cationic

321

charge of CaCl2 and FeCl3 in comparison with NaCl, the swelling ratio of the

322

hydrogels exhibited more significant decline in CaCl2 and FeCl3 solution, which

323

could be explained by the Donnan equilibrium theory.41-43 According to Donnan

324

osmotic pressure equilibrium, a high amount of the movable counterions in solutions

325

could induce a shrinking behavior due to a low osmotic pressure inside the

326

hydrogel.42 This phenomenon also can be explained by Flory’s equation.44

327

ratios

Q5/ 3 ≈

of


(i / 2Vu S 1/ 2 ) 2 + (1 / 2 − x1 )V1 VE / V0


(4)

and

Page 17 of 38


328

where Q is the swelling degree of the polymer, i/Vu is the charge density of the

329

polymer, S is the ionic strength of solution, (1/2−x1)/V1 is the affinity between

330

polymer and solvent, VE/V0 is the crosslinking density of the polymer. Based on the

331

Flory’s equation, the swelling degree of the polymer in salt solutions is monovalent >

332

divalent > trivalent cations, which is in agreement with the experimental data (Na+ >

333

Ca2+ > Fe3+).

334 335

Referring to the method of Pourjavadi et al.,45 the salt sensitivity of polymers can be expressed using a dimensionless salt sensitivity factor (f), as calculated by eq 5.

336

f = 1− (

Sg Sd

(5)

)

337

where Sg and Sd are the swelling ratio of the polymer in given solutions and distilled

338

water, respectively. The values of f in various salt solutions are given in Figure 5d.

339

The higher f values are, the higher salt sensitivity the superabsorbents have. As

340

shown in Figure 5d, the values of f increased with the increasing of cationic charge

341

(Na+ < Ca2+ < Fe3+), indicating the greatest salt sensitivity of the superabsorbents in

342

FeCl3 solution.

343

pH sensitivity analysis

344

To

analyze

the

sensitivity

to

pH,

the

swelling

behaviors

of

345


346

investigated in various solutions at different pH values ranging from 2 to 12. As

347

depicted in Figure 6, the swelling behaviors of the prepared superabsorbents clearly

348

exhibited pH sensitivity. For PCMC-g-poly(AA-co-AM)/carclazyte, the swelling

349

ratio considerably increased within pH 2-6, 8-9 and 10-12, and decreased within pH

and

PCMC-g-poly(AA-co-AM)


were


350

6-8 and 9-10. For PCMC-g-poly(AA-co-AM), the swelling ratio evidently increased

351

within pH 2-6, 7-9 and 10-11, and decreased within pH 6-7, 9-10 and 10-12. The

352

sharp transitions of swelling behaviors of the two superabsorbents were observed at

353

pH 6 and 9. Compared with PCMC-g-poly(AA-co-AM)/carclazyte, the swelling of

354

PCMC-g-poly(AA-co-AM) showed a slight shift at pH 11.0. The shrinking

355

behaviors were observed at pH 7.0, 10.0 and 12 for PCMC-g-poly(AA-co-AM),

356

while pH 8.0 and 10.0 for PCMC-g-poly(AA-co-AM)/carclazyte. The evident

357

pH-dependent swelling behaviors confirmed the excellent pH-sensitive characteristic

358

of PCMC-g-poly(AA-co-AM)/carclazyte and PCMC-g-poly(AA-co-AM).

359

The intriguing pH-dependent behaviors of the prepared superabsorbents could be

360

attributed to the following reasons. The PCMC-g-poly(AA-co-AM)/carclazyte and

361

PCMC-g-poly(AA-co-AM) belonged to anionic superabsorbents, mainly containing

362

carboxylate (-COOH) and carboxamide (-CONH2) groups. Under the strong acidic

363

conditions (pH ≤ 2), a screening effect of the counter ions, i.e. Cl− in the swelling

364

medium, prevented an efficient repulsion, leading to a remarkable decreasing in

365

swelling ability (hydrogel collapsing). Meanwhile, under acid medium (usually at

366

pH < 5), most of the -COO− groups were protonated from -COO− to -COOH,

367

resulting in an enhanced hydrogen-bonding interaction among -COOH groups and

368

generation of additional physical crosslinking. Meanwhile, the electrostatic repulsion

369

among -COO− groups was restricted and thus the diffusion of water into network

370

structure was remarkably impaired, consequently leading to the shrinking behaviors

371

at low pH values.16 As the external pH increased, the hydrogen-bonding interaction


Page 18 of 38

Page 19 of 38


372

among -COOH groups was decreased due to the ionization of -COOH groups from

373

-COOH to -COO−. Besides, the reinforcement of the electrostatic repulsion between

374

-COO− groups also caused an enhanced swelling ratio. However, within the range of

375

pH 6-8, most of -COOH and -CONH2 groups exist as non-ionized forms, the

376

hydrogen bonding between -COOH and -CONH2 groups might lead to a kind of

377

crosslinking, subsequently resulting in a decrease of swelling ratio.46 Subsequently,

378

water absorbency increased with the further increased pH, which is probably due to

379

the conversion of -COOH into -COO− groups. However, the continuously increased

380

pH would generate the “charge screening effect” of excess Na+ counterions in the

381

swelling medium, consequently restrict anion-anion repulsions and lead to

382

shrinkage.47 Here, it was notable that, unlike PCMC-g-poly(AA-co-AM), the water

383

absorbency of PCMC-g-poly(AA-co-AM)/carclazyte gradually increased at pH > 11

384

yet, which was probably attributed to the -OH− groups of carclazyte in the hydrogel

385

structure and network. Similar observation was also currently found in the study on

386

polyacrylamide/laponite clay nanocomposite hydrogels.48

387

Swelling in various surfactant solutions and simulated physiological fluids

388

To evaluate the suitability of the superabsorbents as biomaterials, the swelling

389

capacities of the superabsorbents were investigated in three surfactant solutions

390

(SDS, CTAB and Triton X-100) and four simulated physiological fluids (D-glucose

391

solution, urea solution, physiological saline water and synthetic urine). As shown in

392

Figure 7, PCMC-g-poly(AA-co-AM)/carclazyte exhibited a higher swelling ratio in

393

the above solutions than PCMC-g-poly(AA-co-AM) and basically displayed the



394

same swelling trend in different solution. The order of the swelling ratio of two

395

superabsorbents in above solutions from high to low was ranked as urea solution >

396

Triton X-100 > D-glucose solution > SDS solution > CTAB solution > physiological

397

saline water > synthetic urine. The swelling ratios of the superabsorbents were

398

increased obviously in D-glucose, urea and Triton X-100 solutions as compared with

399

the values measured in distilled water. However, the superabsorbents exhibited

400

interestingly significant shrinking behaviors in physiological saline water, synthetic

401

urine, SDS and CTAB solution, which was probably the charge screening effect

402

caused by cations (Na+, K+, Mg2+ and Ca2+) resulted in the inhibition of anion-anion

403

electrostatic repulsions and decrease of the osmotic pressure between the hydrogel

404

network and the external solution.15, 43

405

In summary, the PCMC-g-poly(AA-co-AM)/carclazyte superabsorbent hydrogel

406

composite was successfully prepared by grafting AA and AM into pineapple peel

407

carboxymethyl cellulose and introduction of carclazyte. The incorporation of

408

carclazyte and modification of pineapple peel cellulose were confirmed by SEM and

409

XRD. The PCMC-g-poly(AA-co-AM)/carclazyte presented an undulant and coarse

410

surface, resulting in its higher water absorbency than PCMC-g-poly(AA-co-AM).

411

The swelling dynamic mechanism agreed well with the Fickian diffusion model and

412

the Schott’s second-order-kinetic model. The prepared superabsorbents demonstrated

413

excellent sensitivities to pH, salts solutions and surfactant solutions as well as

414

simulated physiological fluids, exhibiting smart swelling and shrinking behaviors.

415

The introduction of carclazyte was beneficial for the enhancements of swelling


Page 20 of 38

Page 21 of 38


416

capacities and swelling sensitivities. Based on these properties, the prepared

417

superabsorbents can be used as potential biomaterials for various applications such

418

as water-manageable materials, slow release fertilizer and controlled drug release.

419

Funding

420

This work was supported by the National Natural Science Foundation of China under

421

grant number 31471673, 31271978, and the Ministry of Education PRC under grant

422

number 20120172110017.

423

Notes

424

The authors declare no competing financial interest.

425

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426

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mellulose/montmorillonite hydrogel: evidence of synergistic effects for the slow release of

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fertilizers. J Agr Food Chem. 2013, 61, 7431-7439.

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nanocomposite hydrogels from bi-functionalized palygorskite nanorods as easily separable

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adsorbent for cationic basic dye. Appl Clay Sci. 2016, 121, 29-35.

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Bionanocomposite foams based on the assembly of starch and alginate with sepiolite fibrous clay.

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polyacrylamide-modified kaolinite containing poly (acrylic acid-co-methylene bisacrylamide)

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pH-sensitive composite hydrogel beads based on chitosan-g-poly (acrylic acid)/vermiculite and

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sodium alginate for diclofenac controlled release. Int J Biol Macromol. 2010, 46, 356-362.

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a new composite hydrogel based on hydroxyethyl cellulose and medicinal stone. Compos Part

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Page 28 of 38

Figure captions Figure 1. FTIR spectra of PPC (a), PCMC (b), PCMC-g-poly(AA-co-AM)/carclazyte (c) and PCMC-g-poly(AA-co-AM) (d). Figure 2. XRD patterns of PPC, PCMC, PCMC-g-poly(AA-co-AM)/carclazyte and PCMC-g-poly(AA-co-AM). Figure

3.

SEM

images

of

PPC

(a),

PCMC

(b),

carclazyte

(c),

PCMC-g-poly(AA-co-AM)/carclazyte (d) and PCMC-g-poly(AA-co-AM) (e). Figure

4.

Swelling

kinetic

curves

of


and

PCMC-g-poly(AA-co-AM) in distilled water (a) and 0.9% NaCl solution (b), and the plots of ln (F) against ln (t) for the hydrogels in distilled water (c) and 0.9% NaCl solution (d) as well as the plots of t/Qt against t for the hydrogels in distilled water (e) and 0.9% NaCl solution (f). Figure

5.

Water

absorbency

for


and

PCMC-g-poly(AA-co-AM) in NaCl (a), CaCl2 (b) and FeCl3 (c) solutions with various concentrations ranging from 0.02 to 0.1 mol/L, and the salt sensitivity factors (d) of these two

superabsorbents. Figure

6.

Water

absorbency

for


and

PCMC-g-poly(AA-co-AM) in various pH solutions. Figure

7.

Water

absorbency

for


PCMC-g-poly(AA-co-AM) in various surfactant solutions and simulated physiological fluids.


and

Page 29 of 38


Dried hydrogel

Swollen hydrogel

Scheme 1. Hydrogel formation mechanism and the photograph of dried/swollen superabsorbents.



Page 30 of 38

Table 1. Chemical compositions of carclazyte and pineapple peel Carclazyte Component

SiO2

Al2O3

Fe2O3

FeO

TiO

CaO

MgO

MnO

K2O

Na2O

P2O5

Content (%)

62.34

17.24

2.73

0.12

0.15

2.09

5.44

0.15

0.72

0.12

0.03

Pineapple peel a Component

Cellulose

Hemicellulose

Lignin

Pectin

Ash

23.67

15.61

6.87

5.71

2.37

Content (%) b a

Obtained after dried and pulverized fresh pineapple peel.

b

On dry basis.

Table 2. Swelling kinetic parameters for the PCMC-g-poly(AA-co-AM)/carclazyte and PCMC-g-poly(AA-co-AM) in distilled water and 0.9% NaCl solution. Fickian diffusion model

Hydrogels R2

K

n

Schott’s second-order kinetic model Qe,exp

R2

(g/g)

Qe,cal

k×105

(g/g)

(g/g·min)

In distilled water PCMC-g-poly(AA-c

0.9895

0.0764

0.4652

515.24

0.9995

526.32

3.27

0.9563

0.0652

0.4513

420.17

0.9965

434.78

3.23

o-AM)/carclazyte PCMC-g-poly(AA-c o-AM) In 0.9% NaCl solution PCMC-g-poly(AA-c

0.9887

0.0050

0.4277

37.89

0.9902

43.67

17.71

0.9587

0.0050

0.4125

28.03

0.9934

29.94

43.22

o-AM)/carclazyte PCMC-g-poly(AA-c o-AM)


Page 31 of 38


Figure 1

a 675

1642

Transmittance (%)

1425 1320

b

2900 3372

1051 702

2902 1653

3464

c

1423

1320 1051

1320 1414 1035

2922 1641

d

3451

1416 1322 1042 1643

2924 3455

4000

3500

3000

2500

2000

1500 -1

Wavenumbers (cm )


1000

500


Page 32 of 38

Intensity (a.u.)

Intensity (a.u.)

Figure 2

carclazyte

5

PPC

10

15 20 25 30 2-Theta (degree)

35

40

PCMC PCMC-g-poly(AA-co-AM)/carclazyte PCMC-g-poly(AA-co-AM)

5

10

15

20

25

30

2-Theta (degree)


35

40

Page 33 of 38


Figure 3



Page 34 of 38

Figure 4

a

600

48

400 300 200 100

32 24 16 8

0

0 0

120

240

360

480

600

720

840

960

0

Time (min)

0.0

c

100

200

-3.0

500

600

700

800

ln(F)

-3.4

-0.9

-3.6

-1.2

-3.8

-1.5

-4.0

-1.8

-4.2 3.0

3.5

4.0

4.5

5.0

3.0

3.5

4.0

4.5

5.0

ln(t)

ln(t)

e

400

PCMC-g-poly(AA-co-AM)/carclazyte PCMC-g-poly(AA-co-AM)

-3.2

-0.6

300

Time (min)

d


-0.3

ln(F)


40

Swelling ratio (%)

500

Swelling ratio (%)

b


2.5

f


30 PCMC-g-poly(AA-co-AM)/carclazyte PCMC-g-poly(AA-co-AM)

25

2.0

20 15

t/Qt

t/Qt

1.5

1.0

10 0.5

5

0.0

0 0

120

240

360

480

600

720

840

960

0

100

200

t (min)

300

400

t (min)


500

600

700

800

Page 35 of 38


Figure 5

a 180


120 90 60 30 0

12 8 4

0.04

0.06

0.08

0.10

0.02

Concentration (mol/L)

3.0

0.04

0.06

0.08

0.10



2.5

d 1.00 0.95

2.0

0.90

1.5

0.85

f

Swelling ratio (g/g)

16

0 0.02

c


20



150

b 24

0.1 mol/L of salt concentration PCMC-g-poly(AA-co-AM)/carclazyte PCMC-g-poly(AA-co-AM) 0.02 mol/L of salt concentration PCMC-g-poly(AA-co-AM)/carclazyte PCMC-g-poly(AA-co-AM)

0.80

1.0

0.75

0.5

0.70 0.0 0.02

0.04

0.06

0.08


0.10

NaCl


CaCl2

FeCl3


Page 36 of 38

Figure 6



80

60

40

20

0 2

4

6

8

10

pH


12

Page 37 of 38


Figure 7


900

700 600 500 400 300 200 100


Tr ito X n -1 00

SD S

CT A B

Sy nt ur heti in c e

U re a

Ph ys i in cal ew at er D -g lu co se

0

sa l

Swelling ratio (%)

800


The TOC graphic


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Enhanced Swelling and Responsive Properties of ... - ACS Publications

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