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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 is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Journal of Agricultural and Food Chemistry

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Synthesis, Characterization and Swelling Behaviors of Salt-sensitivity Maize

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bran-Poly(acrylic acid) Superabsorbent Hydrogel

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

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Junfeng Li†*

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China

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130118, People’s Republic of China

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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]

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Meijuan Hu: E-mail: [email protected]

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ABSTRACT

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A novel composite hydrogel was prepared via UV-irradiation copolymerization

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

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

22

(N,N’–methylenebisacrylamide).

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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

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

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

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

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

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

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excellent water retention capability of the hydrogels make this intelligentized polymer

31

had wide potential applications.

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KEYWORDS: superabsorbent, swelling behaviors, maize bran, hydrophilic

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polymers, water retention, salt-sensitivity

Under

the

optimized

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maize

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INTRODUCTION

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Superabsorbent polymers (SAP) are three-dimensional crosslinked hydrophilic

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

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

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as agriculture

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

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

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market evoke a strong desire for researchers.

1-3

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

10

. Needs of the

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

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

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water absorbent capacity, excellent biocompatibility and biodegradability, research on

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polysaccharide-based natural materials superabsorbent is in growing interest

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

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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

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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

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

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,

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

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

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synthesis.

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

16, 17

18

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and suspension polymerization

. However, these conventional synthetic methods

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

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new synthesis technologies, such as ultrasound

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irradiation 22, electron-beam irradiation

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

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time and improve the efficiency.

23-25

19

, microwave irradiation

and UV irradiation

26

20, 21

, γ

come out. Among

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

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(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)

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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

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

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

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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

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

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All the agents were analytical grade.

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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

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

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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

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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

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

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

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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

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

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

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

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

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that no MB.

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Water absorbency measurement

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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

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

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

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Qt =

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

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

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

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

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were averaged.

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Water retention measurement

mt − m0 m0

(1)

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

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

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

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

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

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Mt × 100 M0

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WR (%) =

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Characterization

(2)

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

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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

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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

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

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

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

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RESULTS AND DISCUSSION

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Hydrogel preparation

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

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

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

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

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

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and thereafter themselves became free radical donor to neighboring molecules which

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

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

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crosslinked structure combined with network structure was formed gradually 27.

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Characterization of materials

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Elemental analysis

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

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

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

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

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

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

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

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

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Morphology

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

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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

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

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

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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

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facilitated the permeation of water into the polymeric network 28.

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FITR spectroscopy 8

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

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

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

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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

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

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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

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

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

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

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

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

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

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

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the polymerization.

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Thermal Properties

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

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

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

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

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corresponded to the first three stages which represented the maximum decomposition

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

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

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

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

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

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28

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

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

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

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

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

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

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polymer, and destruction of crosslinked network structure

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

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decomposition or degradation of residual organic matter at high temperatures.

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

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

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

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

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MB–PAA.

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Water Retention

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

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. The last weight loss

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The water retention capacity was determined under the condition of centrifugal

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

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

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

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bonding interaction between the superabsorbent and water molecules directly affected

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

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

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

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

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

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

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

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

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

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

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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.

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Swelling behavior

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Effect of the maize bran content

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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Effect of the reaction time

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

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

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

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

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

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

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

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

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

256

resulted in a low water absorbency.

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Effect of various cations

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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

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

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calculated by the following equation 29:

263

f = 1−

Qs Qd

(3)

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

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

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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

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

270

crosslinking density.

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(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

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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

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

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36

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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

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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

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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

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

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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

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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.

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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):

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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

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< 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

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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

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

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carboxymethylcellulose/clay hydrogel prepared by electron beam irradiation.

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

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

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guar gum-based superabsorbent composite by the simultaneous introduction of

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

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

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

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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

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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

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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

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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

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Q e, cal (g g–1) 2720 2629 2588

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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

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

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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)

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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)

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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. 53x18mm (600 x 600 DPI)

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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. 53x18mm (600 x 600 DPI)

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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)

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