Synthesis and Swelling Behaviors of Yeast-g-Poly (acrylic acid

Jul 28, 2014 - The yeast-g-PAA water superabsorbent co-polymers are prepared by graft co-polymerization of acrylic acid (AA) onto the surface of yeast...
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Synthesis and Swelling Behaviors of Yeast‑g‑Poly(acrylic acid) Superabsorbent Co-polymer Diejing Feng College of Environmental Science and Engineering, Chang’an University, Xi’an, 710054, People’s Republic of China

Bo Bai,* Chenxu Ding, Honglun Wang, and Yourui Suo Northwest Plateau Institute of Biology, Chinese Academy of Sciences, Xining, 810001, People’s Republic of China S Supporting Information *

ABSTRACT: The native yeasts microbes were used to prepare novelly eco-friendly water superabsorbent co-polymers, because of their unique physicochemical/biological properties, including biodegradability, biocompatibilities, as well as natural abundance. Specially, the hybrid superabsorbents were successfully synthesized by graft co-polymerization of monomers acrylic acid onto the surface of yeasts by using ammonium persulfate as a free-radical initiator and N,N′-methylene-bisacrylamide as a cross-linker in aqueous solution. The structure of obtained products was determined by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). Factors such as the weight ratio of monomers to yeast, the content of initiator, the concentration of cross-linker, and the degree of neutralization were investigated. The results showed that the swelling and deswelling behaviors of the superabsorbent significantly was dependent on temperature, concentration of the salt solution, and reswelling times. Moreover, the yeast-based superabsorbents exhibited excellent water absorption and retention, and six-time consecutive adsorption−desorption cycles, which are promising for the potential applications in agricultural or industrial areas.

1. INTRODUCTION

rate, the excellent biodegradability and biocompatibility, and the recyclability of water absorbents. Yeast, as a ubiquitous aquatic unicellular eukaryotic microorganism, is extensively used in foods, pharmaceuticals, and regenerative medicine, because of its high multiplication capacity and easy artificial cultivation.25−29 In general, the yeast cell presents a unique hollow shape, which contains the cell wall, cell membrane, cytoplasm, nucleus, vacuoles, and mitochondria. The mass of the yeast cell is composed of 30%− 50% cell wall, 10%−15% protein, and other soluble components.30−32 The cell walls of yeasts are composed of ∼90% polysaccharides, mainly glucans and mannans, and the remainder consists of a small portion of proteins, chitins, and lipids,33,34 which have various functional groups such as hydroxyl (−OH), carboxyl (−COOH), amidogen (−NH2), phosphate (−OPO3H2), acylamino (−CONH2), and sulfonyl (−SO3H) groups on them. The cell wall possesses the inherent semipermeable property, which permits the passage of small molecules and small proteins, especially water and carbon dioxide, with size exclusion estimated to be 30−60 kDa. More importantly, the native traits of stable osmotic environment within the yeast cell walls can help the yeast cell retain water by preventing osmotic lysis.35 Hereby, it seems that such coexistence of the hydrophilic functional groups and the semipermeable property in the framework of yeast cell wall

Superabsorbent polymers (SAPs) are a special type of threedimensional cross-linked hydrophilic polymer that can absorb and retain huge volumes of water and solute molecules in a swollen state, because of their carboxyl groups, amino groups, hydroxyl groups and other hydrophilic groups attaching onto the polymeric backbone.1−4 Because of their superior hydrophilic properties and excellent swelling ratio, the superabsorbent polymers have been widely used in many fields, such as agriculture, horticulture, forestry, hygiene, drug delivery, food storage, wastewater treatment, tissue engineering, biosensor, and the like.5−12 On the basis of the origin of superabsorbent polymers, three types of superabsorbent polymers can be produced: full synthetic, natural, and hybrid.13−16 For the full synthetic superabsorbent polymers, the high production cost, weak salt resistance, and poor biodegradability has caused environmental risks and retarded its commercial application.17,18 In order to overcome this application restriction, some significant contributions have been made toward the synthesis of hybrid superabsorbent polymers through exploiting the natural water-absorbent with eco-friendly materials. For instance, attempts have been made to use cellulose,19,20 starch,21 chitosan,22 clay,4 montmorillonite,18 attapulgite,23,24 and other natural materials to partially improve absorbing properties and avoid environmental jeopardy from the full synthetic superabsorbent polymers. Despite the aforementioned works, new water-absorbent systems are still required to improve the properties, particularly the large water absorbency, the relatively fast water absorption © 2014 American Chemical Society

Received: Revised: Accepted: Published: 12760

June 4, 2014 July 22, 2014 July 28, 2014 July 28, 2014 dx.doi.org/10.1021/ie502248n | Ind. Eng. Chem. Res. 2014, 53, 12760−12769

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immersed in excessive distilled water at room temperature for 12 h to reach the swelling equilibrium. The swollen sample then was separated from unabsorbed water by filtering through a 100-mesh sieve and drained under gravity for 30 min until no free water remained on the surface. The equilibrium water absorbency (Qeq) of the superabsorbent composite was determined by weighting the swollen samples and calculated using the following equation: m − m1 Q eq (%) = 2 × 100 m1 (1)

makes a good reason for yeast cell to be employed in the preparation of superabsorbent composite. In addition, the pretreatment of natural yeasts is much simpler and more convenient than that of cellulose, without any modification.36 However, to date, the synthesis of hybrid superabsorbent materials using the yeast cell is rarely reported. Based on the above consideration, in the present study, we first attempt to synthesize novel yeast-g-PAA superabsorbent composites by graft co-polymerization reaction of yeast and acrylic acid using N,N′-methylene-bisacrylamide as cross-linker and ammonium persulfate as initiator in the aqueous solution. The detailed mechanisms for the formation of yeast-g-PAA superabsorbent composites were proposed. The swelling and deswelling behaviors of products in the water and saline solution were investigated.

where Qeq (g/g) is the equilibrium water absorbency, measured in terms of grams of water per gram of dried sample; m1 (g) and m2 (g) are the mass of dried and swollen samples, respectively. Moreover, the water absorbency in a physiological saline solution (0.9 wt % NaCl) was tested according to the same procedures described previously. 2.3.2. Water Absorption Rate. In order to investigate swelling kinetics of a superabsorbent in distilled water and physiological saline solution, the measurements were as follows: a quantity of 0.5 g of dried superabsorbent samples were put into two filter bags and immersed in two 500-mL beakers filled with enough distilled water and physiological saline solution at predetermined time intervals. Afterward, the filter bags were lifted from the distilled water and physiological saline solution and drained for 2 min. Afterward, the samples were immediately taken out, weighed, and the water absorption capacity at a given swelling time was calculated according to eq 1. In all cases above, three parallel samples were used and averages were reported in this paper. A controlled experiment without yeast in the polymer was also carried out. 2.3.3. Water Retention Capacity. For the swelling study, the dried samples were allowed to absorb distilled water to reach the swelling equilibrium. The swollen samples then were transferred to an oven and the water retention of the superabsorbent polymers at various times was calculated using eq 2: m − m1 R (%) = 3 × 100 m2 − m1 (2)

2. MATERIALS AND METHODS 2.1. Materials. Yeast powder was purchased from the Angel Yeast Corp. (Wuhan, China) and was washed before use. Acrylic acid (AA), ammonium persulfate (APS), and N,N′methylene-bisacrylamide (MBA) were supplied by Tianjin Chemical Reagent Factory (Tianjin, China). Sodium hydroxide (NaOH) and sodium chloride (NaCl) were provided by Xi’an Chemical Reagent Factory (Shaanxi, China). Nitrogen was obtained from Kunming Messer Gas Products Co., Ltd. (Yunnan, China). All agents were of analytical grade and used without further purification. Distilled water was used throughout the work, and all solutions were prepared with distilled water. 2.2. Preparation of the PAA-Yeast Superabsorbent Composites. Natural yeasts had been pretreated by washing with distilled water for three times and ethanol for one time. A series of samples with different amounts of yeast, ammonium persulfate (APS), N,N′-methylene-bisacrylamide (MBA), and acrylic acid (AA) with different degrees of neutralization (shown in Table S1 in the Supporting Information) were prepared via the following procedure. An appropriate amount of pulverized yeast was dissolved in 30 mL of distilled water in a 250-mL three-necked flask equipped with a mechanical stirrer, a thermometer, and a nitrogen pipe. The slurry was heated by a water bath at 60 °C under nitrogen atmosphere. After being purged with nitrogen for 15 min to remove the dissolved oxygen from the system, a certain amount of initiator APS was introduced into the mixture to initiate the yeast radical productions, and also a solution of AA partially neutralized by NaOH solution, with a certain amount of cross-linker MBA, was added under continuous stirring. The temperature of the water bath was increased to 80 °C and maintained for 3 h to complete the polymerization reaction. In these procedures, the values of weight ratio of AA to yeast, initiator content, crosslinker concentration and neutralization degree of AA ranged from 6 to 10, 0.6 wt % to 2.2 wt %, 0.002 wt % to 0.010 wt %, and 50% to 90%, respectively. Afterward, the resulting products were dried to a constant weight in an oven at 70 °C. The dried products were soaked in distilled water and washed several times with distilled water and ethanol and then dried to a constant weight at 70 °C. Finally, the obtained gel products were milled, and all of the samples used for the tests had a particle size in the range of 40−80 mesh. The blank control sample without yeast was prepared according to the same procedures described above. 2.3. Measurement of the Properties. 2.3.1. Water Absorbency. The superabsorbent composite (∼0.5 g) was

where R represents the water retention rate, and m3 is the mass of the sample after being heated in the oven at time t. The other parameters are the same as defined in eq 1. In order to identify the effort of the temperature on the water retention capacity, the study was undertaken at 20 and 60 °C. 2.3.4. Water Reabsorbency. The superabsorbent samples were respectively immersed in distilled water and 0.9 wt % NaCl solution to reach swelling equilibrium. The swollen samples were filtered through a 100-mesh sieve and weighed to calculate the water absorbency for the first progress. The swollen samples then were placed into an oven to dry at 80 °C. Afterward, the absolutely dried samples were taken for the second swelling progress. The same procedures were repeated seven times in distilled water and 0.9 wt % NaCl solution. 2.4. Characterization. The chemical structures of samples were characterized by Fourier transform infrared (FTIR) spectroscopy and operated in a range from 4000 cm−1 to 400 cm−1. The dried samples were mixed with dried potassium bromide (KBr, optical grade) powder and pressed into small slices. The surface morphology was examined using scanning electron microscopy (SEM) equipment using an accelerated 12761

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Scheme 1. Proposed Mechanisms for the Formation of a Superabsorbent Composite Network

conditions, the APS initiator in the mixture was first decomposed and sulfate anion radicals were generated. Then, the addition reaction between sulfate anion radical and monomer AA took place to produce the monomer radical. Meanwhile, the sulfate anion radicals extracted hydrogen from the hydroxyl and amino groups, resulting in the formation of some active groups, such as alkoxy and imino radicals.37,38 At the same time, yeasts were changed into yeast macroradicals, because of the initiating function of the APS initiator under heating conditions. These generated radicals could act as the active centers during the chain propagation. The monomer radicals close to the reaction sites became acceptors of yeast macroradicals, which led to propagate a new polymeric chain.36 In this way, the reaction among monomers, groups, and chains have brought growth of the grafted chain on the yeast. During the chain propagation, the polymer chains reacted synchronously with the end of vinyl groups of cross-linker, MBA, which eventually made the graft co-polymer of yeast form a crosslinked network structure.39,40

voltage of 5.0 kV. Before SEM characterization, the dried samples were fixed on aluminum stubs and coated with gold layer. Thermal stability of superabsorbent polymers were studied on a thermogravimetric analyzer at a heating rate of 10 °C/min from 20 °C to 800 °C under nitrogen atmospheres.

3. RESULTS AND DISCUSSIONS 3.1. Strategy for Preparing the Water Absorbent Composites. The yeast-g-PAA water superabsorbent copolymers are prepared by graft co-polymerization of acrylic acid (AA) onto the surface of yeast in the presence of free radical initiator (APS) and cross-linker (MBA). The proposed mechanism for the chemically grafting and cross-linking reactions is schematically represented in Scheme 1. The pretreatment with distilled water and ethanol rinsing was assumed to remove impurities that might adulterate into the surface of yeasts and prevent the polymerization reaction. In the presence of yeasts, much PAA co-monomer and MBA cross-linker could be seized on the surface of yeast, because of the hydrogen bond interaction. Under the subsequent heating 12762

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above peaks were characteristic absorption peaks in yeast structures. However, the characteristic absorption peaks of yeast at 1200−1000 cm−1 were obviously weakened after the grafting co-polymerization reaction with monomers, demonstrating the participation of hydroxyl groups of yeast in the copolymerization. Furthermore, compared with the FTIR spectrum of yeast, yeast-g-PAA (Figure 1c) superabsorbent co-polymers had new absorption peaks at 3545 cm−1 (O−H stretching vibration in COOH and N−H stretching vibration), 2953 cm−1 (CH3 asymmetric stretching vibration), 1730 cm−1 and 1260 cm−1 (CO stretching vibration in acetyl or ester groups), 1564 cm−1 (C−O in COOH and N−H in-plane flexural vibration), 1407 cm−1 (CH2 bending vibration caused by CO), and 1161 cm−1 (C−O−C asymmetric stretching vibration in ester),18,36,37,47 which were similar to the spectrum of PAA as shown in Figure 1c. It indicated the existence of PAA chains in yeast-g-PAA polymers. Moreover, PAA had the absorption peak at 995 cm−1 (vinyl group) while yeast-g-PAA did not, which reconfirmed that the acrylic acid had been grafted onto the surface of the yeast. Overall, it can be concluded that the grafting co-polymerization between yeasts and acrylic acid monomers had taken place through the active hydroxyl and amino groups in the reaction processes of APS as the initiator and MBA as the cross-linker. 3.3. SEM Surface Morphology. Figure 2 shows the SEM micrographs of the following superabsorbent co-polymers: yeast (Figure 2a), PAA (Figure 2b), and yeast-g-PAA (Figures 2c and 2d). Figure 2a displays the primitive yeast cells, which are ordered ellipsoids 4.6 ± 0.2 μm in length and 3.2 ± 0.2 μm in width. Figure 2b shows that the surface of pure PAA was almost compact and flat stretch. In contrast, the surface morphology of yeast-g-PAA superabsorbent co-polymers in Figures2c and 2d is distinctly different from that of PAA. Obviously, the introduced yeasts into PAA matrix had destroyed the tight surface and generated a considerable number of pleats and cavities. Such surface morphology suggests that the composite yeast-g-PAA co-polymers have much more water-absorbing sites than that of bare PAA, because they have ability to offer the conveniences for the penetration of water into the polymeric network and benefits to enhance the water absorbency. Moreover, with an increase of yeast contents, the pleats on the surface and the cavities within the yeast-g-PAA co-polymers appeared to increase, leading to more water-accumulating cavities and looserstructure within the PAA matrix. This unique hybrid structure of the yeast-g-PAA co-polymers could help more water molecules diffuse into the network of polymers facially and, consequently,result in a higher swelling ratio. 3.4. Thermogravimetric Analysis. Figure 3 shows the thermogravimetry/differential thermogravimetry (TG-DTG) curves of the yeast and yeast-g-PAA superabsorbent copolymers. Figure 3 shows that the decomposition rate of yeast-g-PAA is obviously slower than that of primitive yeast. The weight loss process of yeast-g-PAA composites exhibited three main steps as temperature increased from 20 °C to 800 °C. At the initial stage, it showed a weight loss below 170 °C, implying a loss of moisture absorbed in the hydrogels network. The weight loss of ∼12% from ∼170 °C to 330 °C was ascribed to the dehydration of saccharide rings and breaking of C−O−C glycosidic bonds in yeast cell wall.48,49 In comparison with the TG curve of yeast, the weight loss at ∼291 °C resulted from the decomposition of yeast itself. Subsequent decomposition

From the above analysis, it can be supposed that the chemical functional groups inherited from the pristine yeast cell walls were critical to the assembling of the yeast-g-PAA superabsorbent co-polymers. Generally, the yeast cell walls have played two extraordinarily important roles in the formation of hybrid structures. On one hand, the substantial hydroxyl groups on the surface of yeasts have provided the macroradicals through the APS initiator under heating conditions. The amply hydrophilic groups on the cell wall surface have also provided an outstanding ability to capture the AA co-monomer and MBA cross-linker through hydrogen bond interaction. Such capture of co-monomer and cross-linker make the graft co-polymerization reaction of acrylic acid (AA) and N,N′-methylenebisacrylamide (MBA) partially occur on the surface of yeasts. On the other hand, the yeast microspheres in the network have acted as an additional network point, which will contribute to enhance the mechanical stability of the yeast-g-PAA superabsorbent composite, i.e., the serious shrinkage of the manufactured superabsorbents will be alleviated since the yeast cell walls have considerable tensile strength. In addition, the hollow spherical space within the yeast microbes have endowed the yeast-g-PAA superabsorbent with desired retention capacity, since the hydrophilic cavities inside the yeasts contribute to store and accumulate about two times of water weight than that of yeast itself. 3.2. FTIR Spectroscopy. Figure 1 shows the FTIR spectra of several superabsorbent co-polymers: primitive yeast (Figure 1a), PAA (Figure 1b), and yeast-g-PAA (Figure 1c).

Figure 1. FTIR spectra of (a) yeast, (b) PAA, and (c) yeast-g-PAA superabsorbent composites.

In Figure 1a, the broad and strong peak observed at 3300 cm−1 is due to intermolecular and intramolecular hydroxyl stretching vibration.41,42 The asymmetric and symmetric stretching vibration of methylene group was proved by the appearance of absorption peaks at 2926 and 2854 cm−1 in yeast cells.43 The peaks at 1655, 1541, 1400, 1313, and 1242 cm−1, were assigned to CO in amide I, N−H in amide II, C−N in amide III, C−O in carboxylic acid groups, and C−O in ester groups, respectively.44,45 Besides, the bands in the region of 1200−1000 cm−1 and 881 cm−1 were ascribed to the stretching vibration of C−O−C and C−O−P group ring vibrations of carbohydrates and glucosidic bonds.44 The absorption at 661 cm−1 was related to the hydroxyl bending vibration.46 The 12763

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Figure 2. SEM images of (a) yeast, (b) PAA, and (c,d) yeast-g-PAA superabsorbent composites.

water absorbency of yeast-g-PAA superabsorbent composites is shown in Figure 4.

Figure 3. Thermogravimetric analysis of yeast and yeast-g-PAA superabsorbent composites.

caused a sharp weight loss of the hydrogel from 330 °C to 530 °C. The maximum degradation occurred at 439 °C, with a significant weight loss of ∼45%. It could be attributed to the elimination of water molecules from the two neighboring carboxylic groups of polymer chains, because of the formation of anhydride, main-chain scission, and the destruction of the cross-linked network structure.50 The last stage, at 744 °C, might be ascribed to removing SO2 molecules from pendent chain attached onto polymeric backbone.37 The TG results revealed that the network could act as a heat barrier,51 and thus enhance the thermal stability of yeast-g-PAA superabsorbent polymers. 3.5. Effect of Weight Ratio of AA to Yeast on Water Absorbency. The effect of weight ratio of AA to yeast on

Figure 4. Effect of weight ratio of AA to yeast on water absorbency of yeast-g-PAA superabsorbent composites.

As Figure 4 shows, both water absorptions in distilled water and 0.9 wt % NaCl solutions was enhanced as the increase in the mass ratio of AA to yeast reached a maximum at 8 and then decreased. Because of a definite content of APS, the amount of free radicals derived from the decomposition of APS is invariable. Thus, the concentration of monomers directly influenced the reaction rate and hydrophilicity. As the ratio rose from 8 to 14, more monomers were available to participate in graft polymerization in the vicinity of active sites of yeasts, which contributed to forming and strengthening the three12764

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dimensional network and improving water absorbency. However, a high weight ratio of AA in the reaction system could bring about excess free radicals during chain propagation process and accelerate the polymerization rate. Thereby, the network structure became much tighter, but the absorbency decreased. The difference in absorbency between distilled water and 0.9 wt % NaCl solutions can be attributed to osmotic pressure distinctions between polymeric network and external solution, because of the existence of Na+ ions in the NaCl solution. 3.6. Effect of Initiator Content on Water Absorbency. The effect of initiator content on water absorbency is investigated in Figure 5.

Figure 6. Effect of cross-linker concentration on water absorbency.

cross-linking points and an increase of cross-linking density in radical polymerization, which, in turn, caused the formation of excess network and decreased the available free nodes within the superabsorbent composite.40 As described previously,53,54 the power law relationship between equilibrium water absorbency of superabsorbent and MBA cross-linker concentration was expressed by eq 3 and its logarithmic form in eq 4:

Figure 5. Effect of initiator content on water absorbency.

Q eq = kCMBA −n

(3)

⎛ 1 ⎞ ln(Q eq) = ln k + n ln⎜ ⎟ ⎝ CMBA ⎠

(4)

where Qeq (g/g) is the equilibrium water absorbency defined above; CMBA is the concentration of cross-linker MBA; and k and n are power law constants for an individual superabsorbent, which could be obtained from the curve fitted with eq 3. The digital of ln(Qeq) against ln(1/CMBA) gave perfect straight line with good linear correlation coefficient. The k and n could be figured out through the slope and intercept of the straight line by fitting the data. For yeast-g-PAA hydrogels, the relationship between Qeq and CMBA in distilled water and in 0.9 wt % NaCl solution followed the formulas Qeq = 60.0938CMBA−0.2836 and Qeq = 20.05977CMBA−0.1630, respectively. 3.8. Effect of Neutralization Degree of AA on Water Absorbency. The effect of neutralization degree of acrylic acid (AA) on the water absorbency in distilled water and in 0.9 wt % NaCl solutions is illustrated in Figure 7. Apparently, in Figure 7, the water absorbency of yeast-g-PAA superabsorbent hydrogels increased with increasing neutralization degree of AA from 50% to 80%, and then decreased with further increasing neutralization degree to 90%. The optimum absorbency of 354.18 g/g in distilled water and 54.91 g/g in 0.9 wt % NaCl solutions both appeared at an 80% degree of neutralization of AA. The main reason for the above phenomena was as follows. When the degree of neutralization was low, the number of strong hydrophilic groups −COONa increased as the degree of neutralization increased. As a result of the dissociation of −COONa groups, the negatively charged carboxyl groups hung onto the polymer chains and set up electrostatic repulsion, tending to expand the network. Simultaneously, ion concentration on the network structure of polymers became stronger and osmotic pressure became bigger, which was favorable for penetration of water molecules

In Figure 5, the water absorption initially increased as the amount of initiator increased and then decreased. The maximum absorbency in distilled water and 0.9 wt % NaCl solutions were obtained: 357.84 g/g and 43.88 g/g, respectively, at 1.4 wt % of APS initiator. As can be seen, the initiator content had a notable influence on water absorbency of the hydrogels, which was in accordance with the relationship between average chain length and the concentration of initiator in the polymerization. Sicne many free-radical reactive sites were generated by the APS initiator, AA monomers could be grafted onto the yeast backbone. When the APS initiator dosage was small, a three-dimensional structure of yeast-g-PAA superabsorbent composites was formed and the water absorbency was boosted. However, excess APS initiator not only was prone to lead the primary radicals to terminate the chain propagation reaction mutually, but it also improved the odds of chain termination reaction among yeast macroradicals or between yeast macroradicals and primary radicals. In the meantime, the chance of homopolymerization reaction initiated by primary radicals was enhanced with the increase of the amount of APS initiator, resulting in low water absorbency. 3.7. Effect of Cross-Linker Concentration on Water Absorbency. According to Flory’s theory,52 cross-linker content has a notable effect on the swelling characteristic of hydrogels. The effect of MBA cross-linker concentration on water absorbency of yeast-g-PAA superabsorbent composites is shown in Figure 6. As can be seen in Figure 6, the water absorbency of yeast-gPAA superabsorbent composites was inversely proportional to MBA cross-linker content from 0.002 mol/L to 0.10 mol/L. Generally, the higher cross-linker content resulted in more 12765

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⎡ ⎛ t ⎞⎤ St = Se⎢1 − exp⎜ − ⎟⎥ ⎝ τ ⎠⎦ ⎣

(5)

where St is the swelling at time t (g/g), Se is the equilibrium swelling (g/g), t is the time (h) required for swelling St, and τ denotes the “rate parameter” (h). It was the substantial hydrophilic groups in the yeast-g-PAA superabsorbent composites that enhanced the electrostatic repulsion in hydrogel network and, thus, strengthened the osmotic pressure difference. The larger the osmotic pressure difference that existed, the faster the water molecules permeated into the absorbents.60 So, the synthesized hydrogel in the presence of yeasts showed a higher velocity in premier swelling progress. As the swelling continued, more water diffused into the network and gradually weakened the osmotic pressure difference. As a result of continuously overcoming the osmotic pressure inside the superabsorbent, the swelling rate became smaller and the swelling ability finally reached equilibrium. The yeast-g-PAA hydrogels presented an absorption ability, at equilibrium, of 398.94 g/g in distilled water and 52.56 g/g in 0.9 wt % NaCl solutions, as shown in Figure 8. 3.10. Deswelling Kinetics in NaCl Solutions. Figure 9 displays the deswelling kinetics of the swollen yeast-g-PAA hydrogels in NaCl solutions of different concentrations.

Figure 7. Effect of neutralization degree of AA on the water absorbency.

into the network structure and improved the water absorbency. Nevertheless, a degree of neutralization of >80% brought abundant and strong hydrogen bonds between water molecule and ionic polymer. Because of the directional hydrogen bonds, water molecules had particular spatial orientation and adjacent hydrogen bonds interfered with each other. In addition, the electrostatic repulsion of contiguous −COO− groups restrained the free movement of bonds and impaired the storage capacity of micropore structure of polymers. The resulted dense network structure reduced the water absorbency of the hydrogels. 3.9. Swelling kinetics. Figure 8 represents the swelling kinetic properties of yeast-g-PAA superabsorbents in two different aqueous condition, distilled water, and 0.9 wt % NaCl solutions.

Figure 9. Deswelling kinetics of hydrogels in NaCl solutions of different concentrations.

In Figure 9, all of the swollen hydrogels had fast reducing trends, once they were transferred into NaCl solutions, and reached equilibrium within ∼1.0 h. Samples in low NaCl concentration solution had a slower deswelling rate and a higher water retention ability than that in a high NaCl concentration solution. The water retentions at equilibrium were 24.86% (0.01 mol/L NaCl), 13.58% (0.1 mol/L NaCl), and 8.45% (0.5 mol/L NaCl), respectively. The major reason for these phenomena was that the osmotic pressure was strengthened with the increase of salt concentration. In the course of water deswelling, water in the hydrogels tended to diffuse to the external solution, which had a large salt concentration, resulting in acceleration of water release and reduction of water retention. Meanwhile, Na+ ions penetrated the internal structure and combined with hydrophilic groups, resulting in decrease of external osmotic pressure and weakening of hydrophilicity. In addition, the yeast, which

Figure 8. Swelling kinetic curves of superabsorbents in distilled water and 0.9 wt % NaCl solution.

Judging from the curves, the observed trends of both hydrogels were very similar. The swelling capacities increased quickly at the initial stage, and then leveled off until it reached a platform, which were commonly reported in many literatures.55−57 The swelling trend line of hydrogels can be expressed as a Voigt-based equation (eq 5):58,59 12766

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serves as a storing and retaining reservoir, was able to hold moistures inside. Also, the scatters of yeasts with the hydrogels matrix have reinforced the mechanical strength of the hydrogel skeleton and prevented the structure from severe shrinkage. Besides, capillary action of the interstitial volume of adjacent yeasts could also tightly hold water molecules. Therefore, it limited the water-releasing ability. Therefore, the ionic strength of a medium had a strong effect on the water absorbency of the hydrogels. 3.11. Water Retention at Various Temperatures. Figure 10 shows the water retention ability of the superabsorbent polymers, as a function of time, at two temperatures.

Figure 11. Reswelling capability of hydrogels with and without yeasts.

absorbency reswelling 6 times, while the water absorbency of the hydrogel without yeast declined dramatically after reswelling 3 times. As a consequence, the yeast-g-PAA hydrogel was proved to be a reusable and recyclable superabsorbent material, and it can be reused at least 6 times.

4. CONCLUSION In summary, a novel yeast-g-PAA superabsorbent co-polymer was successfully synthesized by graft co-polymerization reaction in aqueous solution using APS as a free radical initiator in the presence of MBA as the cross-linker. The obtained products were characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and thermogravimetric (TG) techniques, and the corresponding formation mechanisms were proposed. The effects of weight ratio of AA to yeast, initiator content, cross-linker concentration, and degree of neutralization were discussed. The tests of reswelling capacity showed that the yeast-g-PAA superabsorbent co-polymer could be reused for at least six times. In the meantime, they showed quite fast water absorption rate, good water absorbency, and excellent water retention, which is beneficial for the practical applications of the co-polymerization as potential water-manageable materials in agricultural or industrial area. The utilization of natural resource yeasts for the preparation of a yeast-g-PAA superabsorbent co-polymer not only reduces the production costs significantly and makes the technique environmentally friendly, but it also improves the water absorbency by employing hollow spherical yeasts as waterstoring and accumulating reservoirs. It is believed that the present technique can find some practical applications of the biodegradable superabsorbent polymers in the near future.

Figure 10. Effect of temperature on water retention capacity of hydrogels (with and without yeasts).

As can be seen from the curves in Figure 10, the swollen samples displayed a decreasing tendency of water retention over time, and reached equilibrium after being heated in oven for 10 h. The yeast-g-PAA hydrogels had a slower water release rate and had a higher equilibrium moisture holding capacity of 82.65% and 0.94% of distilled water at 20 and 60 °C, respectively. The pure PAA co-polymer hydrogels without yeasts retained 77.90% and 0.04% of distilled water at 20 °C and 60 °C, respectively. Water-retaining ability can be determined by the hydrogen-bonding interaction and van der Waals force between water molecules and the superabsorbent.61 The significant hydroxyl, carboxyl, and amidogen groups in the yeast-g-PAA hydrogels worked in a pattern previously mentioned, which made the interaction stronger and therefore enhanced the water retention ability. 3.12. Reswelling Capability. Reswelling capacity is a crucial factor for the application of a superabsorbent composite in practice. The reswelling abilities of yeast-g-PAA hydrogels with and without yeasts are investigated by measuring their equilibrium water absorbency in distilled water, as shown in Figure 11. After dewatering of swollen yeast-based hydrogels, the dry composites still showed a better water absorbency than that without yeast in Figure 11. Since the swollen hydrogels were dried in an oven, accompanied by moisture evaporation, polymers were decomposed and the network structure was changed, which led to a decrease in water absorbency after reuse. Furthermore, it was observed in Figure 11 that the hydrogel with yeast still retained ∼96.01% of the initial water

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ASSOCIATED CONTENT

S Supporting Information *

Recipe for the synthesis of yeast-g-poly(acrylic acid) superabsorbent co-polymers of different parameters, including the weight ratio of monomer AA to yeast, initiator content, crosslinker concentration, and degree of neutralization of AA, and the produced superabsorbents’ water absorbencies both in distilled water and 0.9 wt % NaCl solution (Table S1). This material is available free of charge via the Internet at http:// pubs.acs.org. 12767

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 298 233 0952. Fax: +86 298 233 9961. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by China Postdoctoral Science Special Foundation, Scientific Research Foundation for the Returned Overseas Chinese Scholars and Fundamental Research Funds for the Central Universities (No. 2013G2291015).

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