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e-Polymers 2015; 15(4): 271–278

*Corresponding author: Yueqin Yu, State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, P. R. China, Fax: +86 532 84023927, e-mail: qustyu@https://www.doczj.com/doc/1218495552.html,

Jie Wu, Yanmei Zhou, Yang Meng, Jiaxing Zhang, Qingbing Liu, Qimeng Cao: State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, P. R. China

Jie Wu, Yanmei Zhou, Yang Meng, Jiaxing Zhang, Qingbing Liu, Qimeng Cao and Yueqin Yu*

Synthesis and properties of sodium alginate/

poly(acrylic acid) double-network superabsorbent

DOI 10.1515/epoly-2015-0060

Received March 13, 2015; accepted June 3, 2015

Abstract: A tough double-network (DN) superabsorbent was synthesized by a two-step method using N ,N -methyl-enebisacrylamide as a covalent cross-linker for one mono-mer [acrylic acid (AA)], Ca 2+ (CaCl 2) as an ionic cross-linker for the other monomer (sodium alginate [SA]) and ammo-nium peroxodisulfate as the redox initiator. The optimized experimental conditions for the absorbency in deionized water were determined according to orthogonal experi-ments. Unlike conventional chemical cross-linked single-network superabsorbents, SA/poly(acrylic acid) (PAA) DN superabsorbents exhibit superb mechanical properties. Compared with the tensile strength of PAA-only superab-sorbents, that of SA/PAA DN superabsorbents showed an approximately 371.9% increase with increasing amount of 6 wt.% SA. We also investigated the capacity of SA/PAA DN superabsorbents to remove heavy metal ions. It was found that the addition of SA can truly increase the metal ion removal capacity of the PAA superabsorbents and that the affinity order was Pb 2+>Z n 2+.

Keywords: double network; heavy metal ion removal; sodium alginate; superabsorbent; superb mechanical property.

1 Introduction

Superabsorbents can absorb extremely large amounts of water relative to their own mass in a short time. They are usually formed from three-dimensional cross-linked

polymer networks of flexible chains (1). Superabsorbents can retain the absorbed water even under some pres-sure, so they are widely used in health, agriculture and horticulture applications (2, 3) owing to their excellent characteristics since the first superabsorbent polymer was reported by the US Department of Agriculture in 1976 (4). Great efforts have been made to modify their swell-ing ability, swelling strength and swelling rate in all areas of their application over the past decades (5–7). In our previous work (8, 9), we reported the synthesis of natu-ral-based superabsorbent, the grafting polymerization of hydrophilic (acrylic acid and acrylamide) monomers onto chitosan, and a biocompatible and biodegradable superabsorbent, poly(maleic anhydride-co-acrylic acid) (P(MA-co-AA)) with N -maleoyl chitosan as a cross-linker, to try to solve the environmental compatibility issues of superabsorbents. But the application field of superabsor-bents is somewhat strictly limited because of their poor mechanical properties (10). Most superabsorbents made from synthetic polymers, such as polyacrylamide and polyacrylate, have outstanding swelling properties, but have low mechanical properties. Intense efforts have been devoted to synthesizing superabsorbents with improved mechanical properties (11–19). Recently, double-network (DN) superabsorbents with high mechanical strength, in which a neutral polymer network with relatively high molecular mass has been incorporated within a swollen heterogeneous polyelectrolyte network (20–23), have shown major improvements, and a series of natural and synthetic DN superabsorbents with excellent properties have been designed.

Alginate, an anionic polysaccharide, was chosen as a monomer for this study because it has attractive proper-ties, such as non-toxicity, high swelling ability, and sta-bility over a range of pH-values. Alginate is composed of d-mannuronic acid (M unit) and l-guluronic acid (G unit) irregularly assembled in MM, GG and MG blocks along the chain. The proportion of M and G residues and their mac-romolecular conformation determine the physical proper-ties and affinity of the alginate for heavy metals (24). In an aqueous solution, the G blocks in different alginate chains form ionic cross-links through divalent cations (e.g. Ca 2+, Ba 2+, etc.), resulting in a network in water – an alginate hydrogel (18, 25–27).

272 J. Wu et al.: SA/PAA double-network superabsorbents In the present study, we developed a superabsorbent from polymers forming covalently and ionically cross-linked networks. In the superabsorbent, the poly(acrylic acid) chain forms a network by covalent cross-links and the alginate chain forms a network by ionic cross-links of the G unit through Ca 2+. The mechanical properties of the sodium alginate/poly(acrylic acid) (SA/PAA) DN supera-bsorbents were greatly enhanced compared with those of the PAA single-network superabsorbents. The effects of composition, such as SA, ionic cross-linker (Ca 2+) and solid content, on their mechanical properties were inves-tigated. On the basis of the mechanical principle, the mechanical changes in the network were thereby dis-cussed. The Taguchi method (28) was used to optimally synthesize swelling superabsorbents, which provided a simple, efficient and systematic method to obtain the opti-mized conditions. We also described the removal of Zn 2+ and Pb 2+ ions from an aqueous solution by the superab-sorbents. Various parameters influencing the removal of heavy metal ions, such as SA content, treatment time with the solution and initial pH of the solution, were investi-gated in detail.

2 Experimental sections

2.1 Materials

SA powder was bought from Qingdao Bright Moon Seaweed Group (Qingdao, Shandong, China). N ,N ′-Methylenebisacrylamide (BIS), acrylic acid (AA), ammo-nium peroxodisulfate (AP), calcium chloride (CaCl 2), zinc nitrate [Zn(NO 3)2], lead nitrate [Pb(NO 3)2], nitrate acid (HNO 3), ethylenediaminetetraacetic acid disodium salt (disodium-EDTA), ammonia water, zinc oxide benchmark reagents (ZnO), hexamethylene tetramine and xylenol orange indicator were purchased from Shanghai Fine Chemical Co. Ltd. (Shanghai, China) and were of analyti-cal grade and used as received without purification. The deionized water used for the experiment was obtained in our local laboratory.

2.2 Polymerization procedure

The PAA superabsorbent was prepared as shown in Scheme 1, and the three types of superabsorbents are shown in Figure 1. The procedure of the preparation of SA/PAA DN superabsorbents in water was depicted in detail

as an example. Firstly, the BIS cross-linker (0.0184 g) was

Scheme 1:?Synthesis of the poly(acrylic acid) superabsorbents.

totally dissolved in 10 ml of deionized water in a 100-ml beaker by stirring. Then the monomers, SA (0.16 g) and AA (1.75 ml), were added into the beaker, stirred continu-ously and then neutralized with sodium hydroxide solu-tion (5 mol/l, 2.04 ml). Afterwards, the redox initiation system, AP (4%, 0.18 ml), was introduced into the beaker, stirred for 2 min and then the mixture was put in a con-stant temperature water bath (80°C) for 4 h. The product was removed from the bath and then cut into a mass of tiny cubes (1?×1 cm). The cubes were immersed in an aqueous solution of 29.9 ml 1 wt.% CaCl 2 for 12 h, result-ing in a highly homogeneous and transparent SA/PAA DN superabsorbent cross-linked by Ca 2+. The reaction product was washed with plenty of deionized water (3?×500 ml, 24 h) and freeze-dried.

Experiments were performed to determine the mechanical properties of the superabsorbents using a procedure similar to the one described previously. But the amount of every material increased by the same mul-tiple to make the synthesized superabsorbent films large enough for our experiment on the mechanical properties. After adding all the materials to deionized water and stir-ring, the mixture was cast onto a level glass plate and put in a constant temperature water bath (80°C) for 4 h. The superabsorbent film was sprayed with an aqueous solu-tion of CaCl 2, kept alone for 12 h and then dried at room temperature.

2.3 Orthogonal experiment

According to the basic principle of the synthesis of supera-bsorbents, monomer concentration, weight ratio of SA to (SA+AA), neutralization degree and cross-linker concentra-tion were determined as the key synthesis factors affecting the final properties of superabsorbents. These parameters

J. Wu et al.: SA/PAA double-network superabsorbents 273

A B C

Figure 1:?Schematics of the SA/PAA DN superabsorbents [(A) BIS-cross-linked PAA network; (B) SA going through the PAA network in a physical way; (C) SA/PAA DN of SA cross-linked by Ca2+ and PAA)].

Table 1:?Orthogonal experimental table of the water absorbency of the superabsorbents.

Sample Monomer

concentration (%)Weight ratio of SA

to (SA+AA) (%)

Neutralization

degree (%)

Cross-linker

concentration (%)

Q (g/g)RSD (%)

11010503204.9 1.9 210860475.7 2.5 3106301128.9 1.6 4104402176.4 1.8 51510404205.0 2.1 6158303266.0 2.4 715660275.2 1.7 8154501171.8 2.3 92010302280.2 1.3 10208401584.6 1.7 11206504141.8 1.6 12204603216.4 1.5 132510601330.8 1.9 14258502204.2 1.4 152********.1 2.0 1625430480.3 2.2

X 1146.475255.2174.525304.025

X 2179.5282.625180.675184.000

X 3305.75125.000280.025210.35

X 4192.35161.225188.85125.7

R159.275157.625105.5178.325 Q, water absorbency of the superabsorbents in deionized water (g/g).

274 J. Wu et al.: SA/PAA double-network superabsorbents were varied at four levels, as shown in Table 1. An orthogo-nal array was particularly designed with the symbol of L 16. The columns corresponded to the factors specified in this study, and each column contained four levels. Each row in the array represented a trial condition with the factor levels (8), which are indicated by the numbers in the row.

2.4 Water absorbency measurement

The degree of swelling was determined by a gravimetric method. The freeze-dried superabsorbent samples were weighed and immersed for 24 h in water maintained at room temperature. After reaching the swelling equilib-rium, the water on the surfaces of the samples was wiped off using filter papers and then the weight of the swollen samples was calculated again. Relative water absorbency was expressed as follows:

100(-)Q W W W = [1]

where W 0 and W 1 are the weight of the dry sample and the swollen sample, respectively. The Q -value, which is

the water absorbency of the superabsorbents in deion-ized water, was calculated as grams of water per gram of sample.2.5 Tensile strength and elongation at break Tensile strength (TS) and elongation at break (E ) were meas-ured with an electronic universal testing machine (model MZ-4000D, Jiangsu Mingzhu Testing Machine, China). The cross-head speed was kept constant at 500 mm/min. The unit of TS was mPa, and TS was calculated by dividing the maximum load (N) by the initial cross-sectional area (m 2) of the samples. E was measured using the following formula: E =(l max -l )/l , where l max is the change in length at the maximum load and l is the initial length.

2.6 Removal of heavy metal ions

A stock solution containing Zn 2+ or Pb 2+ was prepared by dis-solving metal nitrate salts in deionized water. An SA/PAA DN superabsorbent was added to 20 ml of the stock solution (0.02 mol/l). The amount of residual metal ions in the ali-quots of the withdrawn solution was followed by chemistry titration up to 24 h. The chemistry titration was as follows.

The pH (4.5) was adjusted using drops of 0.1 mol/l NaOH, 0.1 mol/l HNO 3 and a pH paper. Then two drops of 0.2 wt.% xylenol orange indicator and 20 wt.% hexamethylene tetramine was added to make the solution look stable mul-berry in color, then more 5 ml hexamethylene tetramine added to the solution. The aliquot was titrated with the standardized EDTA solution (about 0.02 mol/l) that was prepared previously until reaching the end point when the mulberry color changed to light yellow immediately.

The metal ion removal capacities of the superabsor-bents were calculated as follows:

i t EDTA superabsorbent (-)q V V C M =× [2]

where V i is the volume (ml) of the EDTA used by the initial metal ion solution, V t is the volume (ml) of the EDTA used by the metal ion solution after the removal of the metal ions; C EDTA is the concentration (mol/l) of the EDTA solution, M superabsorbent is the weight (g) of the SA/PAA DN superabsorbent and q is the metal ion removal capacity (mmol/g superabsorbent) of the superabsorbent.

3 Results and discussion

3.1 Orthogonal experiment analysis An orthogonal experiment was carried out by a four- f actor

and four-level test, and we set AP/AA to 0.2 wt.% and CaCl 2/SA to 18.7 wt.% in every sample. The experimen-tal layout after assigning the values of the parameters is

shown in Table 1. As can be seen in the table, 16 experi-ments were carried out and the water absorbency (Q ) of the 16 samples was measured. Q -values were average values. Five samples were tested for each value, and the relative standard deviation (RSD) was calculated. X was the mean value of the water absorbency of the four superabsorbents obtained by fixing one level for one factor and changing four levels for the other three factors. R was the differ-ence between the maximum and the minimum values of

X in each column, which reflects the amplitude of varia-tion of the swelling ratio along with the change of factors. The contribution of each factor to the water absorbency of the superabsorbents was obtained as follows: cross-linker concentration?>m onomer concentration?>m onomer

weight ratio?>n eutralization degree. The optimized values of cross-linker concentration, monomer concentration, weight ratio of SA to (SA+AA), and neutralization degree of AA were 1 wt.%, 20 wt.%, 6% and 40%, respectively. Under optimal conditions, the water absorbency of the

superabsorbents in deionized water was 584.6 g/g. The outstanding water absorbency property of the superab-sorbents can potentially extend their application in the agricultural and horticultural fields.

J. Wu et al.: SA/PAA double-network superabsorbents 275

3.2.2 Effect of SA content

As shown in Table 2, the mechanical properties of the PAA and SA/PAA superabsorbents were measured. Five samples were tested for each TS and E -value, and RSD was calculated. The PAA-only superabsorbents had a TS of 1.96 mPa and an E of 332.2%. With 4 wt.% of SA, the TS of the SA/PAA superabsorbents was 2.84 mPa, which increased by approximately 44.9%. When increasing the content of SA to 6%, the corresponding results were furthered improved and the highest TS obtained was 9.25 mPa. However, TS decreased to 3.41 mPa with a further increase in SA content (8%). It was found that SA played a crucial role in improving the mechanical properties of the material. All the aforementioned results showed that the TS of the SA/PAA superabsorbents increased signifi-cantly with increasing SA content of up to 6 wt.% and then decreased with further increase in SA content.

The PAA superabsorbents were weak and exhibited brittle fracture. In contrast, the SA/PAA DN superab-sorbents were very tough. It is known that a single PAA network lacks mechanical strength. The low density and the small friction between the PAA chains were both responsible for the lower mechanical strength of a single PAA network (20). The controlled addition of SA sig-nificantly enhanced the TS (2–6%), which was probably caused by the strong interaction between the polymer matrices and by the moderate density of the cross-link points and hydrogen bonds. However, the decrease in TS with 8 wt.% SA might be due to the aggregation of cross-link points with high surface energy. Therefore, the controlled amount of SA addition was essential for the synthesis of SA/PAA DN superabsorbents with desirable tensile properties.

3.2.3 Effect of the ionic cross-link density

We prepared superabsorbents with various content of CaCl 2 to study the effect of alginate chains. Five samples

3.2 M echanical properties of the

superabsorbents

3.2.1 Effect of solid content

The solid content of the superabsorbents has an important effect on the mechanical properties. Putting the supera-bsorbent films at room temperature changed the solid content as time went on. The solid content increased at a rapid rate during the first 90 h, rising from 20% to 80%. Then it tended towards stability, and the superabsor-bents exhibited brittleness because during that period the superabsorbent films were rigid, brittle and wrinkled on the surface. So the following experiments were performed when the solid content was <80%; that is, the polymers were far removed from the conditions corresponding to equilibrium swelling. In contrast, at a low level of solid content, it was difficult to perform normal mechanical testing on the PAA gels because they were too brittle and weak to be held tightly enough between the two clamps of the machine.

Figure 2 shows that the mechanical properties changed greatly with increasing solid content. As shown in the figure, TS increased with increasing solid content. As the superabsorbents tended to be dried, they dwindled in size and this increased the cross-link density of the network. As a result, TS increased and E decreased. So it should be noted that all the PAA and SA/PAA DN superab-sorbents used for the mechanical tests had the same solid content.

Solid content (%)

T S (M P a )

50100

150200

250300

E (%)Figure 2:?Effect of solid content on mechanical properties (reaction condition of the superabsorbents: monomer concentration,

20 wt.%; SA/(SA+AA), 6 wt.%; neutralization degree, 40%; cross-linker concentration, 2 wt.%; AP/AA, 0.2 wt.%; CaCl 2/SA, 18.7 wt.%; polymerization temperature, 80°C; reaction time, 4 h).

Table 2:?Effect of SA content on the mechanical properties of the superabsorbents.SA/(SA+AA) (wt.%)TS (mPa)RSD (%)E (%)RSD (%)

0 1.960.8332.2 1.72 1.910.6256.2 1.84 2.840.474.5 1.369.250.788.9 1.58

3.410.510

4.6 1.6

276 J. Wu et al.: SA/PAA double-network superabsorbents

were tested for each TS and E -value, and the RSD was cal-culated. As shown in Table 3, the TS reached its maximum

value when CaCl 2/SA content was 18.7 wt.%, about or >9.25 mPa, a 112.2% increase compared to that of SA/

PAA superabsorbents without CaCl 2. When the CaCl 2/SA content was <18.7 wt.%, the TS increased with increas-ing CaCl 2 content; In contrast, when the CaCl 2/SA content was above 18.7 wt.%, the TS decreased with increasing CaCl 2 content. The E -value showed a contrary trend with

increasing CaCl 2 content.

The results showed that the addition of CaCl 2 could

improve the mechanical properties of the material. It can

be illustrated that Ca 2+ cross-linking with the carboxylate groups of the G unit on the SA chains resulted in the high

density of the cross-link points. But TS decreased when the CaCl 2/SA content increased to 37.3 wt% because the

high density of cross-link points resulted in the aggrega-tion of stress points and in the cracking of the superab-sorbent materials.

3.3 Removal of heavy metal ions

3.3.1 Effect of SA content

Figure 3 shows the impact of SA content on metal ion removal capacity of the superabsorbents. As shown in the figure, the adsorption capacity of the superabsor-bents was found to be Pb 2+>Z n 2+. The ionic radius of Pb 2+ is bigger than that of Cu 2+, so Pb 2+ is more electro-positive. Pb 2+ ions have a stronger interaction with the superabsorbent than those of Zn 2+ owing to the stronger electrostatic interaction between Pb 2+ and the carboxy-late group (29). As seen from Figure 3, the addition of SA significantly increased the metal ion removal capacity of the superabsorbents. The carboxylate groups of the G residues of SA going through the PAA network exist in the form of -COO -, which can chelate with divalent metal ions to produce a steady complex (known as the “egg box” model), so high SA content helps increase the metal ion removal capacity.

Table 3:?Effect of CaCl 2 content on the mechanical properties of the superabsorbents.CaCl 2/SA (wt.%)TS (mPa)RSD (%)E (%)RSD (%)

0 4.360.6115.5 1.318.79.250.888.9 1.137.3 2.420.5288.5 1.956.0

3.310.7272.9 1.7

2.10

2.15

2.20

2.25

A

B 2.30

q (m m o l /

l )

SA/(SA+AA) (%)

05

101520

q (m m o l /g )

SA/(SA+AA) (%)

Figure 3:?Effect of SA content on metal ion removal capacity (reac-tion condition of the superabsorbents: monomer concentration, 15 wt.%; neutralization degree, 40%; cross-linker concentration, 1 wt.%; polymerization temperature, 80°C; reaction time, 4 h).

3.3.2 Effect of treatment time with the solution

Figure 4 shows the effect of treatment time on the removal of Zn 2+ and Pb 2+ ions by the SA/PAA DN superabsorbents. As seen from the figure, the metal ion removal capacity of the superabsorbents increased rapidly during the first 2 h and then slowed down with the increase in treatment time. The removal capacities reached a saturation value within 3 h and almost no longer increased.

3.3.3 Effect of initial pH of the solution

SA/PAA superabsorbents were added to the stock solution with a pH-value varying from 2 to 6, and the effect of pH on the removal of Zn 2+, Pb 2+ ions was investigated. Figure 5 shows that the higher the pH in the solution the greater

J. Wu et al.: SA/PAA double-network superabsorbents 277

carboxylate groups of the G unit occurred with increas-ing pH and the interaction between the metal ions and the c arboxylate groups in the solution was strengthened (29). A suitable pH-value (4 and 4.5) was considered to be optimal for Pb 2+ and Zn 2+, since the precipitates of lead hydroxide and zinc hydroxide were formed under the higher pH of the solution, respectively.

4 Conclusions

Highly strong SA/PAA DN superabsorbents were prepared successfully based on a two-step method. The optimized synthetic conditions of the superabsorbents were deter-mined by an orthogonal experiment as follows: monomer concentration, 15 wt.%; weight ratio of SA to (SA+AA), 8%; neutralization degree of AA, 40%; and cross-linker concentration, 1 wt%. The deionized water absorbency of the superabsorbents under the optimal conditions was 584.6 g/g.

With increasing amount of SA and CaCl 2, the mechan-ical properties of the superabsorbents increased and then decreased. TS attained its highest value when SA content was 6 wt.% and CaCl 2/SA content was 18.7 wt.%, a 371.9% improvement compared to that of the PAA superabsor-bents. The distinctive mechanical property of the SA/PAA DN superabsorbents can be explained in terms of the density of the cross-linkers and by the interaction between the polymer matrices. The outstanding mechani-cal properties of the superabsorbents may extend their application in more fields.

Finally, SA/PAA DN superabsorbents were success-fully utilized for the removal of metal ions and the factors influencing the increased removal capacity were deter-mined as follows: increasing amount of SA, treatment time with the solution and initial pH of the solution. The order of the removal efficiency of the metal ions was Pb 2+>Z n 2+.Acknowledgments: This study was supported by the National Key Technology Support Program of China (no. 2014BAC10B01) and by the Key Scientific and Techno-logical Project of China’s Shanxi Province (no. MN2014-10).

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0.51.01.52.02.53.03.5q (m m o l /g )

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