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Study on swelling behavior of poly(sodium acrylate-co- 2-acryloylamino-2-methyl-1-propanesulfonic acid)/Attapulgite macroporous superabsorbent composite.

INTRODUCTION

As functional polymeric materials, superabsorbents have received much attention for their ability to absorb and retain large amounts of aqueous solution, since it was reported by the U.S. Department of Agriculture [1-4], They have been widely used in many fields, such as disposable diapers, agriculture, food packaging, artificial snow, biomedical applications, and healthcare and agriculture applications [5, 6], In such applications, the water absorbency is essential. In general, superabsorbents have the greatest absorbency in distilled water. When the external condition, such as, pH value, ionic type, and ionic strength of swelling medium changes, the swelling behavior of superabsorbent changes correspondingly. And, the different water absorbency will be observed. Moreover, the influence of each factor on swelling behavior is different [7-10], Therefore, it is important to know the swelling behavior of superabsorbent in different external conditions. The results will be useful for practical applications.

The swelling behaviors of superabsorbents in various solutions have been investigated by many researchers. Rosa et al. have studied the swelling properties of copolymers of acrylamide and 2-acrylamido-2-methyl-propanosulfonic acid, and the swelling kinetic was interpreted by the diffusion-relaxation model [11], The swelling behaviors of a series of superabsorbent composites, such as, polyfacrylic acid)/attapulgite, poly(acrylic acid)/[Al.sup.3+]-attapulgite, polyacrylamide/attapulgite, polyacrylamide/organo-attapulgite, poly(sodium acrylate)/vermiculite, chitosan-g-poly(acrylic acid)/sepiolite, poly(acrylic acid-co-N-acryloylmorpholine)/attapulgite, and starch phosphate-gra//'acrylamide/attapulgite have been investigated by Wang and coworkers [5, 12-18]. Chen et al. have reported that the swelling behaviors of poly(sodium acrylate), poly(sodium acrylate-co-2-hydroxypropyl acrylate), and poly(sodium acrylate-cosodium 1-(acryloyloxy) propan-2-yl phosphate) were remarkably affected by cations, especially multivalent ones [19]. Diez-Pena et al. studied the swelling kinetics of poly(N-isopropylacrylamide-co-methacrylic acid), and an overshooting effect was observed under acidic medium [20]. As a continuous work on superabsorbents [19, 21-23], a fast swelling macroporous superabsorbent composite was prepared by copolymerization of partially neutralized acrylic acid (AANa), 2-acryloylamino-2-methyl-1-propanesulfonic acid (AMPS), and attapulgite (APT) using sodium bicarbonate (NaHC[O.sub.3]) as a foaming agent. And then, our attention focused on the swelling behaviors of this kind of sulfonic acid macroporous superabsorbent composite. The effects of pH value, ionic type, and ionic strength of swelling medium on swelling behaviors were investigated.

EXPERIMENTAL SECTION

Materials

Acrylic acid (AA, Beijing Eastern Chemical Works, China, CP) and AMPS (Shandong Linyi Viscochem Co., China) were used directly. Attapulgite (APT) by acid activation with the average diameter of 325 mesh was provided by Gansu APT Co., China. N, /V'-methylenebisacrylamide [NNMBA, China Medicine (Group) Shanghai Chemical Reagent Corporation, China, CP] and trihydroxymethyl propane glycidol ether (6360, Wuxi Wells Synthetic Material Co., China) were used as received. Polyethylene glycol (PEG, [[bar.M].sub.n]=35,000, Fluka) and sodium carboxymethylcellulose (CMC, Tianjin Guangfu Fine Chemical Research institute, China, CP) were used directly. Peroxide hydrogen ([H.sub.2][O.sub.2]) and L-ascorbic acid (Vc) were all analytical grade and purchased from Xi'an Chemical Reagent Plant, China. Sodium bicarbonate (NaHC[O.sub.3]) and sodium carbonate (Na2C[O.sub.3]) were all analytical grades and purchased from Tianjin No. 6 Chemical Reagent Factory, China. Sodium chloride (NaCl) was analytical grade and purchased from Tianjin Guangfu Fine Chemical Research institute, China. Potassium chloride (KCl), ammonium chloride (NH4C1), magnesium chloride (Mg[Cl.sub.2]), calcium chloride (Ca[Cl.sub.2]), barium chloride (Ba[Cl.sub.2]), sodium acetate (NaAc), aluminium chloride (Al[Cl.sub.3]), sodium bromide (NaBr), sodium nitrate (NaN[O.sub.3]), sodium iodide (NaI), and sodium sulfate ([Na.sub.2]S[O.sub.4]) were all analytical grades and purchased from Tianjin No. 3 Chemical Reagent Factory, China.

Preparation of Poly(sodium aerylate-co-2-aeryloylamino-2-methyl-1-propanesulfonic acid)/Attapulgite Macroporous Superabsorbent Composite

The sulfonic acid macroporous superabsorbent composite was prepared based on the radical copolymerization according to following procedure:

PEG (0.124 g), CMC (0.03 g), and [Na.sub.2]C[O.sub.3] (3.3 g) were added to 22 g of distilled water, and stirred until they were dissolved successively. Then, 0.9 g of APT was dispersed into them with electromagnetic stirring for 5 min and ultrasonic agitation for 25 min. After that, the following components were added to the mixture sequentially: 0.011 g of NNMBA, 0.93 g of AMPS, and 8.4 g of AA mixed with 6360. After electromagnetic stirring for 5 min, 0.30 g of NaHC[O.sub.3] was added. Finally, the mixture of [H.sub.2][O.sub.2] (0.75 mg) and Vc (0.53 mg) were added. The reactant was then controlled at 60 [+ or -] 1[degrees]C for 3 h. The resulting copolymer was dried in an air oven for 11 h at 155[degrees]C, ground into a narrow size fraction, milled through 40-110 mesh screens, and then used for measurement of swelling kinetics.

Measurement of Swelling Kinetics

The sample (0.2 g) was enclosed in a sealed tea bag (40 X 50 mm), and then placed into a beaker containing 100 mL of swelling medium. The sample was removed, and weighed in set intervals. The water absorbency ([Q.sub.t]) at corresponding time was calculated as Eq. 1. Each measurement was repeated three times.

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

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

RESULTS AND DISCUSSION

Effect of pH Value on Swelling Behavior of Sulfonic Acid Macroporous Superahsorbent Composite

Effect of pH Value on Equilibrium Water Absorbency ([Q.sub.[infinity]]). The relationship between equilibrium water absorbency of sample and pH value in 0.9 wt% NaCl aqueous solutions is investigated as shown in Fig. 1. The pH value was adjusted by 11.5 mol [L.sup.-1] aqueous solution of HCl or NaOH to eliminate the influence of the ionic strength on equilibrium water absorbency of the sample. As we can see, the equilibrium water absorbency of sample increases when pH value increases from 2.2 to 6.1, and decreases in the pH range of 6.1-7.0. After that, the equilibrium water absorbency increases slightly with increasing pH value again. But, a dramatic decrease of equilibrium water absorbency is observed when the pH values of solutions are higher than 11.2.

At very acidic condition, anionic groups on the polymer chains are protonated. It gives rise to the formation of hydrogen bonds between acid groups, which act as physical crosslinking points. In addition, the repulsive force among anionic groups is low. Meanwhile, the chelation between [Na.sup.+] and anionic groups is weak and can be considered to be ignored. Therefore, the low equilibrium water absorbency is observed. With increase of pH value, the quantity of anionic groups on the polymer chains increases. The repulsive force among anionic groups increases. Hydrogen bonds between acid groups decrease. Both of them lead to more relaxing polymer network. Although chelation increases, the influence of chelation on equilibrium water absorbency is minor. Thus, the equilibrium water absorbency increases in the pH range of 2.2-6.1. With further increase of pH value, large amount of [Na.sup.+] bond to polymer network and decrease the osmotic pressure and repulsion force [19]. At the same time, the chelation becomes strong. The chelation plays an important role in the equilibrium water absorbency, so a decrease of equilibrium water absorbency is observed when pH value is larger than 6.1. With large amount of [Na.sup.+] diffusing into polymer network, the ratio of oxygen and sodium decreases. The coordination methods (interchains and intrachains) and coordination numbers change correspondingly [19], which makes the hydrogel network stretch. Therefore, the equilibrium water absorbency increases again with increasing pH value from 7.0 to 11.2. When pH value is higher than 11.2, the equilibrium water absorbency decreased sharply. The concentration of NaOH aqueous solution is near to or more than that of NaCl aqueous solution gradually, and the osmotic pressure decreases dramatically. Thus, the equilibrium water absorbency decreases sharply.

Effect of pH Value on Swelling Kinetic. The swelling kinetic curves of sample in 0.9 wt% NaCl aqueous solutions with different pH value are presented in Fig. 2. It is obvious that macroporous superabsorbent composite swell to a measurable extent during the 1-min time period regardless of pH value of solution. It indicates that the sample has the fast swelling rate. In general, when superabsorbent swells, it takes times to swell from the surface to the core. For macroporous superabsorbent composite, open channels are formed in sample particle according to our previous studies. When it is placed in an aqueous solution, water flows through the open channels by capillary effect, instead of diffusion of water through the glassy layer. This allows fast swelling [24].

The classical expression of Fick's law has been usually used to provide a general description of polymer-fluid mass transport empirically. In this article, we also used it to determine the diffusion mechanism. The initial swelling data are fitted to the exponential equation [25]:

F = [Q.sub.r]/[Q.sub.[infinity]] = [kt.sup.n] for ([Q.sub.t]/[Q.sub.[infinity]] [less than or equal to] 0.6) (2)

where F is the fractional uptake, [Q.sub.t] denotes the water absorbency at time t, and [Q.sub.[infinity]] is the equilibrium water absorbency, k is a constant incorporating characteristic of the polymer network and the penetrant, n is a kinetic exponent of the model of solute transport. If n = 0.5, corresponding to a Fickian diffusion, the rate of diffusion is lower than the rate of polymer relaxation. If 0.5 < n < 1, corresponding to a non-Fickian diffusion, the rate of diffusion and relaxation is nearly equivalent to each other. n = 1.0 indicates case II diffusion, and n > 1.0 is for supercase [PI] diffusion, which show that water transport is controlled by polymer relaxation process, and the diffusion is very fast.

As can be seen from Table 1, supercase [PI] diffusion is observed in all solutions with different pH value. The same result has been reported by others [25, 26], It can be concluded that water transport is controlled by polymer relaxation process. Open channels are formed in sample particles. They lead to fast swelling rate, and the mechanism is more polymer relaxation-controlled.

Effect of Cation on Swelling Behavior of Sulfonic Acid Macroporous Superabsorbent Composite

Effect of Cation on Equilibrium Water Absorbency. To investigate the influence of cation on swelling behavior of sample, some cations are chosen. Table 2 shows the effect of cations existed in swelling medium on equilibrium water absorbency of sample. All experiments were performed with 4.2 of pH value, and the ionic strength of solutions was kept at 0.1539 mol [L.sup.-1]. It can be found that the equilibrium water absorbency of sample in solutions with different cation decreases as the following order: [K.sup.+] [approximately equal to] N[H.sub.4.sup.+] [approximately equal to] [Na.sup.+] > [Al.sup.3+] > [Mg.sup.2+] > [Ca.sup.2+] > [Ba.sup.2+]. This phenomenon may be attributed to the complexing ability of the cations to anionic groups on the polymer chains and the hydration of cation. As we have reported previously [19], the theory of hard and soft acids and bases is used to determine the stability of complex compounds. For cations, the hardness order is listed as follows: [Al.sup.3+] > [Mg.sup.2+] > [Ca.sup.2+] > [Ba.sup.2+] > [Na.sup.+] [greater than or equal to] N[H.sub.4.sup.+] [greater than or equal to] [K.sup.+] [27]. For anions on the polymer network, according to the calculating method of the group's electronegativity, --S[O.sub.3.sup.-] and --CO[O.sup.-] are all hard bases. So, for RCO[O.sup.-], the stability order of complex compounds should be RCOO--[Al.sup.3+], RCOO--[Mg.sup.2+], RCOO--[Ca.sup.2+], RCOO-[Ba.sup.2+], RCOO--[Na.sup.+], RCOO--N[H.sub.4.sup.+], RCOO--[K.sup.+]. Considering the chelation [28], the stability order is shown below, RCOO--[Al.sup.3+], RCOO--[Mg.sup.2+], RCOO--[Ca.sup.2+], RCOO--[Ba.sup.2+], RCOO-[Na.sup.+], RCOO-N[H.sub.4.sup.+], RCOO--[K.sup.+]. However, because R denotes the hydrogel network and it is very large, RCOO-- will shift toward softness. The stability order of the complex compound may be RCOO--[Ba.sup.2+], RCOO--[Ca.sup.2+], RCOO--[Mg.sup.2+], RCOO--[Al.sup.3+], RCOO--[K.sup.+], RCOO--N[H.sub.4.sup.+], RCOO--[Na.sup.+]. For RS[O.sub.3.sup.-], the same method is applied to determine the stability of corresponding compounds, and the same stability order as RCOO is obtained. Considering the composition of functional groups on the polymer chains, the stability order of complex compounds should be RCOO--[Ba.sup.2+], RCOO--[Ca.sup.2+], RCOO--[Mg.sup.2+], RCOO-[Al.sup.3+], RCOO--[K.sup.+], RCOO--N[H.sub.4.sup.+], RCOO--[Na.sup.+]. Conversely, the hydration radius of various cations increase as follows: [K.sup.+], N[H.sub.4.sup.+], [Na.sup.+], [Ba.sup.2+], [Ca.sup.2+], [Mg.sup.2+], [Al.sup.3+]. The cation with small hydration radius has high charge density, which gives an evident screening effect on the anionic groups of polymer chains, and leads to low equilibrium water absorbency [29]. In addition, for eliminating the effect of ionic strength on equilibrium water absorbency, all solutions were adjusted to the same ionic strength. So, the concentration of [Al.sup.3+] is lower than that of other cations. For [Al.sup.3+], the chelation varies correspondingly. Therefore, the equilibrium water absorbency of sample in solutions containing different cation changes as shown in Table 2.

Effect of Cation on Swelling Kinetics. The swelling kinetics curves of macroporous superabsorbent composite in NaCl, Ca[Cl.sub.2], and Al[Cl.sub.3] aqueous solutions are shown in Figs. 3-5, respectively. It is observed that the swelling curves in NaCl aqueous solutions increase sharply at the initial swelling stage, and then increases slowly, until an equilibrium swelling state arrives in Fig. 3. But the swelling curves in Ca[Cl.sub.2] and Al[Cl.sub.3] aqueous solutions exhibit overshooting effect, that is, show a maximum followed by a decrease in water absorbency and then a gradual attainment of the swelling equilibrium as shown in Figs. 4 and 5. Moreover, it is observed that before the maximum reaches, the hydrogel is almost transparent, and after the overshooting, the opacity increases for sample in Ca[Cl.sub.2] and Al[Cl.sub.3] aqueous solutions.

The overshooting effect has been reported in other literatures [20, 30-32]. According to Diez-Pena et al., the overshooting effect relates to changes of crosslinking degree, geometry of the sample, temperature, pH value, concentration of fixed charges, and approaching of chains to allow interactions between neighboring charged moieties. In this work, we propose that appearance of overshooting effect may be attributed to the chelation between cations and the anionic groups on the polymer chains, which leads to the variation of crosslinking degree and the reorganization of the gel structure. In Fig. 6, a scheme representing the structural reorganization inside the gel in the swelling process of sample in salt solution containing multivalent cation is shown. It may be interpreted as two consecutive swelling-deswelling processes. During the swelling process, with the entrance of water into hydrogel network, the cations in swelling medium enter. Then, competition of the two opposite processes appears: on the one hand, due to the osmotic pressure between hydrogel and the external solution, the solution enters gradually. The repulsive force among the polyanion makes the polymer network stretch. Both of them lead to swelling of superabsorbent composite. On the other hand, the chelation between anionic groups on the polymer network and cations in solution take place. More crosslinking points form and it gives rise to the contraction of network with expelling solution. When the swelling rate is equal to the deswelling rate, the water absorbency reaches a maximum. After that, with the continual entrance of cations in the polymer network, the crosslinking degree of polymer network increases further. The deswelling rate exceeds the swelling rate, the water absorbency begins to decrease, and an overshooting effect is observed till a new equilibrium reaches. For multivalent cations, both [Ca.sup.2+] and [Al.sup.3+], overshooting effect appear [19]. But for the monovalent cations, the obvious overshooting effect is not observed as shown in Fig. 3. It may be caused by the low stability of complex compound.

It can be seen from the Fig. 3 that the water absorbency decreases with increasing ionic strength of NaCl solution as reported by other works [29, 33]. It is caused by the reduction of the osmotic pressure between sample and external swelling medium. It is found that the maximum and equilibrium water absorbency decreases with increasing ionic strength of solution containing multivalent cation as shown in Figs. 4 and 5. It is attributed to the increase of the chelation ability and the decrease of the osmotic pressure, which prevent the polymer chains from spreading.

Although the overshooting effect has been reported early in the 1980s, only a quantitative kinetics model was given by Diez-Pena et al. for swelling of poly(N-Isopropylacrylamide-co-methacrylic). We also use this model to interpret the overshooting effect appearing in swelling of macroporous superabsorbent composite in saline solution containing multivalent cations. There are four species of water existing in the swelling process: at the beginning of the swelling process, the polyanion and the cations from external solution are not chelate, and possess a capability of water uptake that is called [A.sub.1]; the absorbed water at each time for this hydrogel is called [A.sub.2]; the entrance of cations from external swelling medium promotes the rearrangement of the hydrogel structure through the chelation, resulting in a more compact structure with a lower water absorption. The water content of this second structure is called [A.sub.3]. Because the water content of this structure is higher than its equilibrium value, water release occurs. The deswelled water is called [A.sub.4]. The concentrations of these four species of water are regulated by the six rate constants as shown in Eq. 3.

If the concentrations of the four species of water are [A.sub.1], [A.sub.2], [A.sub.3], and [A.sub.4] and the six rate constants may be denoted by [k.sub.1], [k.sub.2], [k.sub.3], [k.sub.4], [k.sub.5], and [k.sub.6], the process may be expressed by means of the following equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

That is, three consecutive reversible first-order reactions.

If the first and second stages of the reactions are nonreversible, that is, [k.sub.2] = [k.sub.4] = 0. Then, the rate may be expressed by the following series of fundamental equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (4)

The overall content of water imbibed by the gel at time t is then given by [Q.sub.t]:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

where [alpha] represents the concentrations of the dried gel at t = 0, that is, [A.sub.1] = [alpha].

The equilibrium value [Q.sub.[infinity]] is the concentration of [A.sub.3] species in the equilibrium. It is a function of the rate constants [k.sub.5] and [k.sub.6]:

[Q.sub.[infinity]] = [A.sub.3[infinity]] = [[k.sub.6]/[k.sub.6] + [k.sub.5]] [alpha]. (6)

The values of the four rate constants calculated by the fitting of the experimental data for sample in different concentration of Ca[Cl.sub.2] and Al[Cl.sub.3] aqueous solutions are gathered in Table 3. As it can be seen, the values of the determination coefficients are good, in most of the cases [R.sup.2] > 0.90. The value of [k.sub.3] is related to the form of crosslinking points. The high [k.sub.3] rate constant means that the overshooting maximum appears early in the swelling process. As we can see, with the increase of concentration, [k.sub.3] increases as shown in Table 3. And, the time to reach the maximum value of water absorbency decreases in Ca[Cl.sub.2] and AICI3 aqueous solutions, as shown in Figs. 4 and 5. It is attributed that the cations with high concentration in swelling medium can enter into hydrogel easily, and form large amount of crosslinking points in short time.

Effect of Anion on Swelling Behavior of Sulfonic Acid Macroporous Superabsorbent Composite

Effect of Anion on Equilibrium Water Absorbency. Table 4 shows the effects of monovalent and divalent anions on the equilibrium water absorbency of sample. All experiments were performed under the conditions of pH = 11.2 and I = 0.154 mol [L.sup.-1]. As we can see, the equilibrium water absorbency is hardly affected by monovalent anions. It is attributed to that the chelate takes place only between polyanion and cations in hydrogel. However, compared with divalent anions, the equilibrium water absorbency of sample in swelling medium with monovalent anions and a common cation ([Na.sup.+]) decreases a little. The same results are also reported by Zhang et al. [29]. When the ionic strength of swelling medium containing monovalent anion is equal to that of divalent anion, the concentration of Na+ in swelling medium containing monovalent anion is more than that with divalent anion. It means that an evident screening effect on the anionic groups of polymer chains occurs in solution with monovalent anion. Thus, the less equilibrium water absorbency is obtained.

Effect of Anion on Swelling Kinetic. The swelling curves of sample in NaCl, NaAc, [Na.sub.2]S[O.sub.4], and [Na.sub.2]C[O.sub.3] solution are shown in Fig. 7. It reveals that sample possess similar swelling kinetics no matter which anion exists in swelling medium. We also use Eq. 2 to determine the diffusion mechanism. As shown in Table 5, the supercase n diffusion appears in all solutions due to the pores in sample.

CONCLUSIONS

Considering the importance of swelling behavior on practical application, the effects of pH value, anionic type, and their concentration on swelling behavior of a sulfonic acid macroporous superabsorbent composite were investigated systemically. The results indicated that the swelling behavior of sample was affected by pH value. And, the sample showed supercase [PI] diffusion in NaCl solution regardless of pH value.

Cations had significant effect on swelling behavior of sample. A remarked overshooting effect was observed in the saline solutions containing multivalent cations. It is attributed to two consecutive swelling-deswelling processes. The deswelling process may be caused by the reorganization of the gel structure due to the formation of complex compound between cation and the anionic groups on the polymer chains, which diminishes the swelling capacity, and therefore, leads to water expelling during the swelling of sample. But in the saline solutions containing monovalent cations, the overshoot phenomenon hardly appeared. It results from the low stability of complex compound between monovalent cation and polymer network.

A quantitative kinetics model proposed by Diez-Pena was used to interpret the overshooting effect appearing in swelling of macroporous superabsorbent composite in saline solution containing multivalent cations, and the theoretical curves were in good agreement with the experimental data.

Anions showed little effect on swelling process and normal swelling curves. The supercase n diffusion was also observed no matter which anions existed in swelling medium.

NOMENCLATURE

Abbreviations

AA                       Acrylic acid

AMPS                     2-Acryloy lamino-2-methy 1-1 -propanesul-
                         fonic acid

APT                      Attapulgite

CMC                      Sodium carboxymethylcellulose

NNMBA                    N, A'-Methylenebisacrylamide

PEG                      polyethylene glycol

[V.sub.c]                L-ascorbic acid

6360                     Trihydroxymethyl propane glycidol ether

[Q.sub.t]                Water absorbency

M                        The weight of the swollen hydrogel

[M.sub.0]                The weight of dried sample

[Q.sub.[infinity]]       Equilibrium water absorbency

F                        The fractional uptake in Fick's law

k                        The characteristic constant of the hydrogel
                         in Fick's law

n                        The kinetic exponent of the model of solute
                         transport in Fick's law

[A.sub.1], [A.sub.2],    The concentrations of the four species of
[A.sub.3], and           water in the swelling process of sample
[A.sub.4]                containing multivalent cations

[k.sub.1], [k.sub.2],    The six rate constants in quantitative
[k.sub.3], [k.sub.4],    kinetics model about overshooting effect
[k.sub.5], and
[k.sub.6]

[alpha]                  The concentrations of the dried gel

[R.sup.2]                The determination coefficients


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

(1) Department of Chemistry, School of Sciences, Chongqing Jiaotong University, Chongqing 400074, People's Republic of China

(2) Department of Chemistry, State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, People's Republic of China

(3) College of Materials Science and Engineering, State Key Laboratory of Gansu Advanced Non-Ferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, People's Republic of China

Correspondence to: Xiaohua Qi; e-mail: light_shadow@163.com

DOI 10.1002/pen.23938

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Swelling kinetics parameters fitted to experimental data from
Eq. 2: swelling exponent, n, and swelling constant, k, in 0.9% NaCl
solution with different pH value.

Experimental
number         pH value      k       n     [R.sup.2]

1                2.22     0.00314   2.41     0.999
2                6.61     0.04835   1.47       1
3                13.0     0.06357   1.32       1

TABLE 2. Effect of cations in swelling medium on equilibrium
water absorbency of sample.

Cation types     [Na.sup.+]   [K.sup.+]   N[H.sub.4.sup.+]

Equilibrium          47          47              47
water
absorbency
(g [g.sup.-1])

Cation types     [Mg.sup.2+]   [Ca.sup.2+]   [Ba.sup.2+]   [Al.sup.3+]

Equilibrium          18            13             9            21
water
absorbency
(g [g.sup.-1])

TABLE 3. Rate constants [k.sub.1], [k.sub.3], [k.sub.5], and
[k.sub.6], [A.sub.2[infinity]], and determination coefficients
[R.sup.2] calculated according to Eqs. 5 and 6 for sample in
different concentration of Ca[Cl.sub.2] and Al[Cl.sub.3],
aqueous solutions.

Cation          Ionic strength     [k.sub.1]      [k.sub.3]
types          (mol [L.sup.-1])   ([s.sup.-1])   ([s.sup.-1])

[Ca.sup.2+]         0.0513           0.0321         0.1411
[Ca.sup.2+]         0.1539           0.0878         0.1644
[Ca.sup.2+]         0.3078           0.1292         0.2242
[Al.sup.3+]         0.0513           0.0981         0.0005
[Al.sup.3+]         0.1539           0.1000         0.0803
[Al.sup.3+]         0.3078           0.0704         0.0927

                                                [A.sub.3
Cation          [k.sub.5]      [k.sub.6]       [infinity]
types          ([s.sup.-1])   ([s.sup.-1])   (g [g.sup.-1])   [R.sup.2]

[Ca.sup.2+]       0.1256         0.0154            14           0.983
[Ca.sup.2+]       0.1190         0.0456            12           0.929
[Ca.sup.2+]       0.1387         0.0858            10           0.976
[Al.sup.3+]       0.0803         0.0963            43           0.936
[Al.sup.3+]       0.0311         0.0222            21           0.957
[Al.sup.3+]       0.0551         0.0374            19           0.862

TABLE 4. Effect of anions in swelling medium on equilibrium water
absorbency of sample.

Anion types     [Ac.sup.-]   [Cl.sup.-]   [Br.sup.-]   N[O.sub.3.sup.-]

Equilibrium         50           49           50              48
water
absorbency (g
[g.sup.-1])

Anion types     [I.sup.-]   S[O.sub.4.sup.2-]   C[O.sub.3.sup.2-]

Equilibrium        48              54                  57
water
absorbency (g
[g.sup.-1])

TABLE 5. Swelling kinetics parameters fitted to experimental data
from Eq. 2: swelling exponent, n, and swelling constant, k, in
NaCl, NaAc, [Na.sub.2]S[O.sub.4], and [Na.sub.2]C[O.sub.3] solution
with 0.1539 mol [L.sub.-1] of ionic strength and 11.2 of pH value.

Experimental
number            Anion type          k         n      [R.sup.2]

1                 [Cl.sup.-]       0.07443   1.11014       1
2                 [Ac.sup.-]       0.06037   1.05058    0.98672
3              S[O.sub.4.sup.2-]   0.06013   1.00559    0.99047
4              C[O.sub.3.sup.2-]   0.04306   1.18934    0.98551
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Author:Qi, Xiaohua; Liu, Mingzhu; Chen, Zhenbin
Publication:Polymer Engineering and Science
Article Type:Report
Date:Mar 1, 2015
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