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Fast-swelling oxidized starch phosphate-poly(acrylate/ acrylamide/2-acryloylamino-2-methyl-1-propanesulfonic acid) superabsorbent composite.

INTRODUCTION

Superabsorbent composites are lightly crosslinked polymers that swell to a high degree in water or biological fluids. They are widely used in many fields, such as disposable diapers, agriculture, food packaging, artificial snow, and biomedical applications [1-3]. In these applications, especially in hygiene products and gastric retention devices, a fast swelling rate is preferred. However, most superabsorbent polymers often require a long time, ranging from hours to days to reach equilibrium. In principle, the initial swelling rate of a superabsorbent material is primarily due to the penetration of water molecules into the polymeric network through diffusion and capillary action [4]. As previously reported, a higher swelling ability, larger surface area, smaller particle size, and lower crosslinking density lead to a high swelling rate [5], However, compared to the studies on the improvement of swelling ability, reports that are focused on enhancement of the swelling rate are limited.

As a vinyl monomer, acrylic acid (AA) plays an important role in preparation of superabsorbents. It endows superabsorbents with large swelling capacity. However, acrylic-based superabsorbents have low resistance to metallic ions. Recently, many works have been made to improve the performance through copolymerization of acrylic acid and other monomers [6, 7]. Acrylamide (AM), as one of monomer containing amide group, it possesses poor absorption capacity and strong resistance to metallic ions after polymerization [8]. 2-acryloylamino-2-methyl-1 -propanesulfonic acid (AMPS) is an anionic vinyl monomer containing a hydrophilic sulfonic acid functional group and a nonionic amide group [9]. The copolymer also has a high tolerance toward cations [10]. Therefore, AA, AM and AMPS are selected as comonomers. And it is expected that the swelling ability and swelling rate of superabsorbent composites could be improved.

Starch and its derivatives are widely used in food, textile, pharmaceutical and biomedical applications due to their high viscosity, biocompatibility, biodegradability, non-toxicity, and low cost. Many attempts have been made to prepare superabsorbent materials by grafting starch with an acrylic monomer, and these materials exhibited a good swelling capacity [11-14]. Oxidized starch phosphate is a starch derivative that is produced by oxidization and phosphorylation of free hydroxyl groups on the anhydroglucose units of starch molecules [15]. These reactions introduce carboxyl groups and phosphate groups into the starch, which possess better hydrophilic abilities compared to that of the hydroxyl groups [16]. Therefore, oxidized starch phosphate was introduced into the preparation process for the superabsorbent composite, and the prepared superabsorbent composite is expected to exhibit a high swelling ability and swelling rate.

In recent years, various methods, such as phase inversion [17], freeze-drying [18], water-soluble porogens [19], and foaming [20-22], have been introduced for the preparation of superabsorbent materials in order to increase the swelling rate by endowing them with a pore structure. The porous superabsorbent materials prepared using a foaming technique can swell to their equilibrium state in a few minutes regardless of their size and shape due to the presence of capillary channels. Therefore, the foaming technique was introduced for the preparation of superabsorbent composites. Sodium bicarbonate, which acts as a foaming agent, was introduced to form a pore structure using a reaction involving acrylic acid (AA) in the polymerization process of superabsorbent composites. To form fine pores with a uniform distribution during the reaction, a foam stabilizer is necessary. Oxidized starch phosphate, which acts as a natural polymer surfactant, can decrease the interfacial tension between the system and gas bubbles [23]. In addition, the oxidized starch phosphate provides the reaction system with sufficient viscosity. Therefore, the oxidized starch phosphate can act as a foam stabilizer for the preparation of superabsorbent composites, and other foam stabilizers do not participate in the preparation process.

However, due to the large amount of water that exists in the polymer network after the polymerization reaction, the formed pores could swell again during the drying process, causing the pore structures to shrink and collapse and decreasing the swelling rate. According to our previous studies [24], a porogens complex, which vaporizes at a different temperature stage, may provide the hydrogel sample with rigid pore structures. Therefore, ethanol, propyl alcohol and butyl alcohol additives were also added to the reaction system. In the polymerization, ethanol (boil point, b.p: 78[degrees]C) evaporates from the reaction system and forms rigid pore structures. At the later drying stage, propyl alcohol (boil point, b.p: 97.1[degrees]C) vaporizes, which forms new pores and reinforces the pores formed by sodium bicarbonate and ethanol. After water evaporates, butyl alcohol (boil point, b.p.: 117.7[degrees]C) vaporizes. More pores will form, and earlier formed pores would be reinforced further. The pore structure is expected to be effectively retained, and a higher swelling rate can be obtained.

To the best of our knowledge, fast swelling superabsorbent composites that were synthesized using partially neutralized acrylic acid, acrylamide (AM), 2-acryloylamino-2-methyl-lpropanesulfonic acid (AMPS), and oxidized starch phosphate have seldom been reported. Therefore, based on our previous studies [25-27], a superabsorbent composite with an improved swelling rate was synthesized using partially neutralized AA, AM, AMPS and oxidized starch phosphate. Sodium bicarbonate, ethanol, propyl alcohol, and butyl alcohol were employed to enhance the swelling rate, and hydrogen peroxide and Lascorbic acid were used as a redox initial system. Meanwhile, both N, N'-methylenebisacrylamide (NNMBA) and trihydroxymethyl propane glycidol ether (6360) were added into the reaction system as crosslinkers for the formation of homogeneous crosslinked polymer network. Besides, the copolymers were surface-crosslinked by glycerine. And it is hoped that the surface-crosslinking method could enhance swelling rate by reducing the "gel blocking" behavior of superabsorbent composites particles [28], It has been reported that the inorganic gel generally has excellent salt resistance [29], and the use of surfactant can improve the permeability of hydrogel [24]. Therefore, the surface-crosslinked products were then blended with mixed powders consisting of aluminum sulfate ([[Al.sub.2][S[O.sub.4]].sub.3]), sodium silicate ([Na.sub.2]Si[O.sub.3]), and sodium 1-octadecanol phosphate. The influences of some reaction conditions, such as the amount of water, AM, AMPS, oxidized starch phosphate, 6360 and initiator, as well as the neutralization degree of AA, on the equilibrium swelling degree and swelling rate in a 0.9% NaCl aqueous solution were investigated.

EXPERIMENTAL

Materials

Acrylic acid (AA, Beijing Eastern Chemical Works, China, CP) was directly used. N,N'-methylenebisacrylamide (NNMBA, China Medicine (Group) Shanghai Chemical Reagent Corporation, China, CP), trihydroxymethyl propane glycidol ether (6360, Wuxi Wells Synthetic Material Co., China), and 2-acryloylamino-2-methyl-l-propanesulfonic acid (AMPS, Shandong Linyi Viscochem Co., China) were used as received. Peroxide hydrogen ([H.sub.2][O.sub.2]), phosphoryl trichloride (PO[Cl.sub.3]), ethoxyethane ([Et.sub.2]O), glycerine, propanol, butanol, sodium chloride (NaCl), sodium hydroxide (NaOH), hydrogen chloride (HC1), and L-ascorbic acid (Vc) were of analytical grade and purchased from the Xi'an Chemical Reagent Plant, China. Sodium bicarbonate (NaHC[O.sub.3]), ethanol, sodium dihydrogen phosphate (Na[H.sub.2]P[O.sub.4]), sodium hydrogen phosphate ([Na.sub.2]HP[O.sub.4]), and sodium carbonate ([Na.sub.2]C[O.sub.3]) were of analytical grade and purchased from the Tianjin No. 6 Chemical Reagent Factory, China. Aluminum sulfate ([[Al.sub.2][S[O.sub.4]].sub.3]) was of analytical grade and purchased from the Tianjin No. 3 Chemical Reagent Factory, China. Sodium silicate ([Na.sub.2]Si[O.sub.3]), sodium chloride (NaCl), and glycerine were of analytical grade and purchased from the Tianjin Guangfu Fine Chemical Research Institute, China. Octadecanol (OT, Tianjin No. 2 Chemical Reagent Factory, China, CP) was used as received. Acrylamide (AM, Beijing Eastern Chemical Works, China, CP) was directly used. Potato starch was supplied by the Tengsheng Agricultural Products Group, Gansu Province, China.

Preparation of the Oxidized Starch Phosphate (P-O-Starch)

Oxidized starch phosphate was prepared according to a previously reported method [30]. First, 11.1 g of sodium dihydrogen phosphate, 22.2 g of sodium hydrogen phosphate and 0.8 g of peroxide hydrogen were dissolved in 160 mL of distilled water, and then, 40.0 g of starch was added to the mixed solution. The slurry was gradually heated to 50[degrees]C and stirred for 30 min. The starch slurry was filtered and oven-dried at 50[degrees]C to a moisture content of approximately 12%. Then, the starch that contained the impregnated orthophosphates was placed in a flask and stirred at 150[degrees]C for 1 h. The product was washed with 95% ethanol three times to eliminate the residual. Finally, the product was dried, ground, and sieved to pass through a 200 mesh sieve.

Preparation of the Superabsorbent Composite from Oxidized Starch Phosphate and Acrylate/Acrylamide/2-Acryloylamino-2-Methyl-1-Propanesulfonic Acid

A series of superabsorbent composites from oxidized starch phosphate, AA, AM, and AMPS were synthesized according to the following procedure. Typically, 1.19 wt% oxidized starch phosphate (w/w, related to AA) and 265.5 wt% distilled water (w/w, related to AA) were added to a flask. The slurry was stirred and heated to 70[degrees]C for 10 min. After the temperature decreased to 40[degrees]C, [Na.sub.2]C[O.sub.3] (3.66g) was added to the mixture and stirred until it was completely dissolved. Then, 26.71 wt% AM (w/w, related to AA), 2.98 wt% AMPS (w/w, related to AA), NNMBA (0.011 g), propanol (1.0 mL), butanol (1.0 mL), ethanol (1.0 mL), and AA (8.0 mL) mixed with 0.095 wt% 6360 (w/w, related to AA) were slowly and sequentially added to the mixture. After electromagnetic stirring for 15 min, NaHC[O.sub.3] (0.25 g), 0.019 wt% [H.sub.2][O.sub.2] (w/w, related to AA), and 0.027 wt% Vc (w/w, related to AA) were added to the mixture. Then, the reactant was maintained at 60 [+ or -] 1[degrees]C for 4.5 h. The resulting polymer was dried for 11 h at 155[degrees]C, ground into a narrow size fraction, and milled through 40-110 mesh screens.

Then, the post treatment processes were as follows: 4.00 g of the product were surface crosslinked with glycerine (0.04 g) and butanol (0.04 g) followed by blending with mixed powders consisting of [Al.sub.2] (S[O.sub.4])3 (0.132 g), [Na.sub.2]Si[O.sub.3] (0.168 g), and sodium 1-octadecanol phosphate [24] (0.02 g) to yield the final sample.

Infrared Measurement

The structures of the starch, oxidized starch phosphate and polymers without post treatment were characterized on a Nicolet-560 FTIR (Nicolet Analytical Instruments, Madison, America) using a KBr pellet. The sample powders were mixed with KBr and pressed into discs for the infrared measurement. Transmittances were recorded between in the range of 4000 and 400 [cm.sup.-1]

Measurement of Equilibrium Swelling Degree and Swelling Kinetics in 0.9% NaCl Aqueous Solution

The accurately weighed sample (0.2 g) was immersed in a beaker containing 100 mL of a 0.9 wt% NaCl aqueous solution. The swollen hydrogel was filtered through an 80 mesh sieve to remove the non-absorbed water and weighed in set intervals, and the swelling degree ([Q.sub.t]) at the corresponding time was calculated using Eq. 1. The equilibrium swelling degree (Q) in the 0.9% NaCl aqueous solution was also obtained when the sample was swelled for 30 min at room temperature using Eq. 1. Each experiment was carried out in triplicates and the averages of the results were reported. The value of standard deviation for this method is [+ or -] 3 g of solution absorbed per g of dry sample particles.

[Q.sub.t] = (M - [M.sub.0])/[M.sub.0], (1)

where M is the weight of the swollen hydrogel and [M.sub.0] is the weight of the dried sample.

Measurement of the Swelling Rate

The accurately weighed sample (1.0 g) was uniformly spread into a 100 mL beaker. Then, 40.0 mL of the 0.9 wt% NaCl aqueous solution were added to the beaker. The time required for the sample to absorb all of the solution was recorded. The swelling rate was calculated using Eq. 2. Each experiment was carried out in triplicates and the averages of the results were reported. The value of standard deviation for this method is [+ or -] 0.05 of solution (mL) absorbed per g of dry sample particles per second.

SR = 40/[t.sub.a], (2)

where [t.sub.a] is the recorded time.

RESULTS AND DISCUSSION

FTIR Analysis

The FTIR spectra of starch (a), oxidized starch phosphate (b), and the superabsorbent composite (c) are shown in Fig. 1. The results indicate that the IR spectra of potato starch in Fig. la contain the O-H stretching absorption at 3,408 [cm.sup.-1]. The band at 2,927 [cm.sup.-1] corresponds to [v.sub.CH], which is confirmed by [[delta].sub.CH] at 1,463 and 1,376 [cm.sup.-1]. The band peak at 1,655 [cm.sup.-1] was assigned to -OH -O-bending of the intramolecular hydrogen bond. The peaks at 1,166, 1,084, and 982 [cm.sup.-1] are due to C-O-C stretching. The absorption bands at 928, 765, and 710 [cm.sup.-1] correspond to pyranose ring vibrations in starch [11], The absorption bands at 574 and 524 [cm.sup.-1] were due to the -OH group in starch. In comparison to the FTIR spectra of starch and oxidized starch phosphate, the absorption peaks at 3,408, 1,655, 574, and 524 [cm.sup.-1] in Fig. lb were less intense after the reaction, which indicates that the -OH groups on starch changed during the reaction. In addition, the absorption bands at 1,166, 1,084, and 982 [cm.sup.-1] became less intense, which indicates that the oxidation of starch has occurred. The intensity of the absorption bands between 1,084 and 982 [cm.sup.-1] increased, and a new absorption peaks at 1,019 [cm.sup.-1] appeared, which may be due to P-O-C stretching, confirming the phosphorylation of starch [16, 31].

The FTIR spectra of the superabsorbent composite are shown in Fig. 1c. The band located at 3,434 [cm.sup.-1] corresponds to [v.sub.OH] of H-O-H or [v.sub.NH] of N-H. The band located at 2,933 [cm.sup.-1] was due to [v.sub.CH], which was confirmed by [[delta].sub.CH] at 1,452 [cm.sup.-1]. The band at 1,638 cm corresponds to [v.sub.C=O] of -CONH. The [v.sub.c=O] of -CO[O.sup.-] was observed at 1,562 and 1,415 [cm.sup.-1]. The absorption bands at 1,226, 1,052, and 839 [cm.sup.-1] show the appearance of -S[O.sub.3]Na [32, 33]. The absorption peaks at 991 [cm.sup.-1] may be due to P-O-C stretching. In addition, by comparing the FTIR spectra of oxidized starch phosphate to that of the superabsorbent composite, the absorption bands at 927, 762, and 709 [cm.sup.-1] disappeared after the reaction, which indicates the graft polymerization of the monomers onto the oxidized starch phosphate [11]. Based on the FTIR analysis, a graft-copolymerization reaction occurred between AA, AM, AMPS, and oxidized starch phosphate.

Influence of the Amount of Water on Swelling

The relationships between the amount of water ([W.sub.water]/[W.sub.AA]) and Q and SR are shown in Fig. 2. As the amount of water increases, the Q and SR exhibit maxima at 250.0 and 154.8 wt% of water, respectively. When the water content is low, the concentrations of monomers, crosslinking agents and initiator were high in the reaction system. Therefore, the rate at which a propagating radical consumes double bonds is high [34], and the distance between two crosslinking points on the polymer chains is short, which leads to a low equilibrium swelling degree. As the amount of water increases, a suitable network forms, which results in an increase in the equilibrium swelling degree. However, when the amount of water is too high, the sample becomes soluble. Therefore, the equilibrium swelling degree decreases.

The crosslinking density of the polymer network decreases with increasing amounts of water. For this superabsorbent composite with its pore structure, the low crosslinking density will lead to facile stretching of the polymer network, which accelerates the swelling. At a low crosslinking density, the generated pore structure cannot be effectively maintained during drying, which decreases the SR. Therefore, the SR changes are shown in Fig. 2.

Influence of the Amount of AM on Swelling

The influence of the AM amount on the swelling of the sample was investigated, and the results are shown in Fig. 3. The Q increases as the weight ratio of AM to AA first increase and then slightly decreases. In addition, the amount of AM does not appear to influence Q until it reaches 160 wt%. However, the SR has a minimum value at an AM content of 40 wt%. AM is a nonionic monomer. The superabsorbent composite prepared using AM exhibits good salt resistance, and the external conditions have little effect on Q. In addition, the collaborative effect among different hydrophilic groups results in higher absorbency [35, 36], However, AM is more reactive than AA [24], which indicates that more heat is given off, resulting in a high crosslinking degree for 6360. Therefore, when AM was higher than 26 wt%, the Q slightly decreased.

The SR remains approximately constant when the AM content was less than 13 wt%. Then, the SR begins to decrease. However, when the AM content is higher than 40 wt%, the SR increases. As the AM content increases, the crosslinking degree of 6360 increases, which results in poor stretching of polymer network and more well-connected and uniformly distributed capillary channels in the sample to accelerate the absorbency of the aqueous solution. In addition, -CON[H.sub.2] can form hydrogen bonds with water, and the diffusion of water into the polymer network easily occurs. Therefore, the SR changes are shown in Fig. 3.

Effect of the Neutralization Degree of AA on Swelling

Figure 4 shows the effects of the neutralization degree of AA on swelling. The Q and SR increase as the neutralization degree of AA increases, with maxima at 60.0 mol%. As the neutralization degree increases, the content of -COONa increases. The electrostatic repulsion of the polyanion on the polymer network increased, and the osmotic pressure between the hydrogel and the external solution also increases. Therefore, the equilibrium swelling degree of the hydrogel is enhanced. However, an AA neutralization degree that is too high is not beneficial for Q. The poor reactivity of sodium acrylate leads to a portion of the polymer becoming soluble. Therefore, Q decreases when the neutralization degree of AA is higher than 60.0 mol%.

As previously mentioned, the low neutralization degree leads to a polymer network with a high crosslinking density. The sample possesses more well-connected and uniformly distributed capillary channels, which accelerates the absorbency of the aqueous solution. However, the swelling ability of the sample is low. Therefore, the SR is low. At a high neutralization degree, the nonhomogeneous network structure and low Q lead to a low SR, as shown in Fig. 4.

Influence of the Amount of AMPS on Swelling

Figure 5 shows the relationship between the amount of AMPS ([m.sub.AMPS]/[m.sub.AA]) and Q and SR. With increasing amounts of AMPS, the Q slightly increases and then remains approximately constant. Finally, Q slightly decreases at an AMPS content of more than 16.07 wt%, which indicates that the effect of the AMPS amount on Q is minor. The SR reaches a maximum at an AMPS content of 10.71 wt%. AMPS contains two types of hydrophilic groups including the acylamide and sulfonate groups. The sulfonate group is more hydrophilic than the carboxyl group [5], The collaborative effect of different hydrophilic groups in the polymer chains will lead to a higher Q [37]. As previously reported [9], acrylic acid is more active than AMPS. When the AMPS content is high, the exothermic rate is low, which results in an ineffective reaction with 6360 [26], A large amount of linear polymers were produced, and the Q was low.

As the AMPS amount increased, more and more linear polymers are produced. An inhomogeneous polymer network formed, and the capillary channels produced during the polymerization will not be effectively retained in the drying process [3]. In addition, the swelling degree of the sample first increases and then decreases. Therefore, the SR reaches a maximum at an AMPS content of 10.71 wt%.

Effect of the Amount of P-O-Starch on Swelling

The influences of the amount of P-O-Starch ([m.sub.P-O-starch]/[m.sub.AA]) on the Q and SR are shown in Fig. 6. The Q slightly decreased with increasing amounts of starch. The SR first increases and then decreases. When the amount of P-O-Starch is 1.19 wt%, the SR reaches a maximum.

P-O-Starch, one of the materials employed in the preparation of the superabsorbent composite, contains hydrophilic carboxyl groups and phosphate groups [16], and the introduction of this material does not substantially influence the swelling ability of sample. Moreover, the addition of P-O-Starch can decrease the interfacial tension between the reaction system and gas bubbles, which is helpful for the formation of small pores in the polymer network. However, the viscosity of the reaction system increases with an increasing amount of P-O-Starch. During the polymerization, the formation of the pore structure and gelation proceed at practically the same time. The increased viscosity is helpful for keeping most of gas bubbles in the reaction system and accelerating the swelling rate of the sample. However, this behavior simultaneously results in poor mixing of the reactants and nonhomogeneous polymerization. Additionally, the long chains in P-O-Starch can physically entangle with other polymer chains, which is benefit for preserving the pore structure in the sample.

As the P-O-Starch content increases, nonhomogeneous polymerization occurs, which decreases the Q of the sample. In addition, the hydrophilicity of P-O-Starch is weaker than that of the copolymer chains containing carboxyl, sulfonic and acylamino groups. Therefore, Q decreases with increasing amounts of P-O-Starch. However, based on the results shown in Fig. 6, the addition of P-O-Starch does not substantially affect the equilibrium swelling degree. As expected, the swelling ability of the sample decreases slightly after introduction of P-O-Starch into the reaction system. Until the P-O-Starch content reaches 11.9 wt%, the superabsorbent composite has an equilibrium swelling degree of 44 g [g.sup.-1] in a 0.9 wt% NaCl aqueous solution.

As P-O-Starch content increases, a sample with more pore structure is obtained, and the SR increases. When the amount of P-O-Starch is higher than approximately 1.19 wt%, the viscosity of the reaction system becomes too high, which results in unreacted materials and nonhomogeneous polymerization. These factors all affect the formation of open channels in the sample, and the SR decreased, as shown in Fig. 6.

Effect of the Amount of Initiator on Swelling

The results in Fig. 7 indicate that the Q increases monotonically with an increasing initiator content ([m.sub.H2O2]/[m.sub.AA]). However, the variation in the SR is different from that in Q. When the amount of initiator is less than 0.024 wt%, SR increases and then decreases. [H.sub.2][O.sub.2]/Vc is a redox initiator couple. During crosslinking polymerization, the initiator has a substantial effect on both the polymerization rate and the molecular weight of the polymer between two crosslinking points. As the amount of initiator increases, the polymerization rate is enhanced, and 6360 is effectively crosslinked. A high quantity of free radicals may also cause a chain transfer to the polymer network, which leads to a high crosslinking density [36]. A suitable network is formed, and Q increases.

As the initiator content increases, the polymerization degree decreases, and the termination rate increases [38]. Simultaneously, the crosslinking density increases. Moreover, the equilibrium swelling degree of the sample increases. These factors vary the SR of the sample, as shown in Fig. 7.

Influence of the Amount of 6360 on Swelling

NNMBA possesses high reactivity compared to other monomers, such as AA, AMPS, and AM. When NNMBA is used as a crosslinking agent, it typically leads to early depletion during polymerization. Polymer chains in a later polymerization stage cannot be efficiently crosslinked [16]. Therefore, 6360, which is an epoxy crosslinking agent, is introduced into the reaction system. Figure 8 shows the influence of the amount of 6360 ([m.sub.6360]/[m.sub.AA])on Q and SR. Q decreased as the 6360 content increases. However, SR reaches a maximum when the mass ratio of the amount of 6360 to AA is less than 0.19 wt%. With an increase in 6360, polymer chains that were formed in a later polymerization stage are efficiently crosslinked. In addition, the distance between each crosslinking point in the polymer network decreases. Therefore, Q decreases with increasing amounts of 6360.

When the amount of 6360 is low, a large amount of linear polymer is produced, which results in an inhomogeneous polymer network, and the pore structure cannot be effectively maintained. Therefore, the SR is low. As the amount of 6360 increases, well-connected capillary channels is formed, which results in a fast SR. However, when the amount of 6360 is too high, the equilibrium swelling degree of the sample is very low. Therefore, a low SR is obtained.

Swelling Kinetics in a 0.9% NaCl Aqueous Solution

The swelling kinetic curves in a 0.9% NaCl aqueous solution were investigated, and the results are shown in Fig. 9. As we can see, the swelling rate sharply increases initially, and then begins to plateau until the swelling reaches an equilibrium state. It also can be found that the superabsorbent composite can swell to a measurable extent during the 5 min time period from Fig. 9. It indicates that this superabsorbent composite possesses fast swelling rate. For comparing the swelling rate of this superabsorbent composite with that of the conventional superabsorbent without foaming agent, the data about absorption characteristics of a superabsorbent based on sodium acrylate and trimethyl methacrylamidopropyl monium iodide reported by Lee [39] and the superabsorbent composite are gathered and listed in Table 1. It can be seen that the superabsorbent composite with foaming agent swell faster and need less time to reach the swelling equilibrium, compared with that of the conventional superabsorbent without foaming agent. This phenomenon is attributed to the formation of open channels in the polymer structure, when the foaming agent is used in the preparing process. This pore structure makes the transfer of water molecules into the polymeric matrix from the external aqueous solution easier.

The Schott's pseudo second-order kinetics model [40] is usually used to evaluate the swelling kinetics behavior. This kinetics equation is expressed as

t/[Q.sub.t] = 1/[K.sub.is] + (1/[Q.sub.[infinity])t, (3)

where [Q.sub.t] is the swelling degree at time t, [Q.sub.[infinity]] is the power parameter, which denotes the theoretical equilibrium swelling degree and [K.sub.is] is the initial swelling rate constant. By plotting t/ [Q.sub.t] against time t, a straight line with a slope of 1/[Q.sub.[infinity]] and an intercept of 1/[K.sub.is] is obtained.

By the application of the swelling data of samples 1 and 2 to Eq. 3, an excellent straight line with good linear correlation coefficients (R > 0.99) from the plots of t/[Q.sub.t] against time t are obtained in Fig. 10. The kinetic parameters (i.e., [K.sub.is] and [Q.sub.[infinity]), are calculated from the slope and intercept of the lines. The [K.sub.is] values are 82.1693 and 3.5425 for samples 1 and 2, respectively. The [Q.sub.[infinity]] values are 56.75369 and 62.46096 for samples 1 and 2, respectively. It indicates that the swelling of the superabsorbent composites obey the Schott's pseudo second-order kinetics model.

CONCLUSIONS

A fast-swelling superabsorbent composite was prepared by solution polymerization of partially neutralized AA, AM, AMPS, and oxidized starch phosphate. The structure of the superabsorbent composite was characterized using FTIR analysis. The influences of some reaction conditions on the properties of the superabsorbent composite were investigated, and the optimum conditions were as follows: for the equilibrium swelling degree, the mass ratios of water, AM, AMPS, oxidized starch phosphate, initiator and 6360 to acrylic acid were 250.0, 26.2, 5.36, 0, 0.042, and 0 wt%, respectively, and the neutralization degree of AA was 60.0 mol%. Moreover, when the mass ratios of water, AM, AMPS, oxidized starch phosphate, initiator and 6360 to acrylic acid were 154.8, 160.3, 10.71, 1.19, 0.024, and 0.19 wt%, respectively, the neutralization degree of AA was 60.0 mol%, and the superabsorbent composite exhibited the highest swelling rate. The equilibrium swelling degree of the composite prepared under optimal conditions in a 0.9 wt% NaCl aqueous solution was 52 g [g.sup.-1], and the swelling rate reached 0.86 mL [g.sup.-1] [s.sup.-1]. Based on its fast swelling rate and higher equilibrium swelling degree in a 0.9% NaCl aqueous solution, this type of superabsorbent composite can be used in hygiene products, such as feminine hygiene products and disposable diapers.

NOMENCLATURE

AA                   Acrylic acid

AM                   Acrylamide

AMPS                 2-acryloylamino-2-methyl-1-propanesulfonic acid

FTIR                 Fourier transform infrared spectroscopy

[K.sub.i]            Initial swelling rate constant in the Schott's
                     pseudo second-order kinetics model
M                    Weight of the swollen hydrogel

[M.sub.0]            Weight of dried sample

NNMBA                N, N'-methylenebisacrylamide

6360                 Trihydroxymethyl propane glycidol ether

P-O-Starch           Oxidized starch phosphate

[Q.sub.t]            Swelling degree of superabsorbent composite at
                     time t in 0.9% NaCl aqueous solution

Q                    Equilibrium swelling degree of superabsorbent
                     composite in 0.9% NaCl aqueous solution

[Q.sub.[infinity]]   Theoretical equilibrium swelling degree in the
                     Schott's pseudo second-order kinetics model

[R.sup.2]            Determination coefficients

SR                   Swelling rate of superabsorbent composite in
                     0.9% NaCl aqueous solution

Vc                   L-ascorbic acid


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Xiaohua Qi, (1,2) Mingzhu Liu, (2) Zhenbin Chen (2,3)

(1) College of Materials Science and Engineering, Chongqing Jiaotong University, Chongqing, People's Republic of China

(2) Department of Chemistry and State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou, People's Republic of China

(3) College of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou, People's Republic of China

Correspondence to: X. Qi; e-mail: qixiaohua820602@163.com

Contract grant sponsor: Scientific Research Projects of Chongqing Jiaotong University; contract grant number: 100853; contract grant sponsor: Gansu Province Project of Science and Technologies; contract grant number: 0804WCGA130.

DOI 10.1002/pen.24360

TABLE 1. Absorption characteristics for poly(sodium
acrylate-co-trimethyl methacrylamido-propyl ammonium iodide)
(SA/TMMAAI) and the superabsorbent composite (sample 1 and sample 2)
in 0.9 wt% NaCl aqueous solution.

                         Initial swelling rate
                       (mL [g.sup.-1] [s.sup.-1]
            Foaming
Samples      agent    0.5 s   0.5-1 min 1   -3 min

SA/TMMAAI      -       0.1                  0.075
Sample 1       +       1.8       0.07         0
Sample 2       +       1.0       0.68       0.066

            [t.sub.eq]         Q
Samples       (min)      (g [g.sup.-1])

SA/TMMAAI       60             56
Sample 1       0.6             56
Sample 2       3.6             61

-: without foaming agent; +: with foaming agent; [t.sub.eq]: the
time to reach the swelling equilibrium.


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Author:Qi, Xiaohua; Liu, Mingzhu; Chen, Zhenbin
Publication:Polymer Engineering and Science
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Date:Nov 1, 2016
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