ARTICLE pubs.acs.org/IECR
Reversible Swelling/Deswelling Characteristics of Ethylene Glycol Dimethacrylate Cross-Linked Poly(acrylic acid-co-sodium acrylateco-acrylamide) Superabsorbents Neelesh Bharti Shukla and Giridhar Madras* Department of Chemical Engineering, Indian Institute of Science, Bangalore, 560012, India ABSTRACT: Poly(acrylic acid-co-sodium acrylate-co-acrylamide) superabsorbent polymers (SAPs) cross-linked with ethylene glycol dimethacrylate (EGDMA) were synthesized by inverse suspension polymerization. The SAPs were swollen in DI water, and it was found that the equilibrium swelling capacities varied with the acrylamide content. The SAPs were subjected to reversible swelling/deswelling cycles in DI water and aqueous NaCl solution, respectively. The effect of the addition of an electrolyte on the swelling of the SAP was explored. The equilibrium swelling capacity of the SAPs was found to decrease with increasing concentration of added electrolyte in the swelling medium. The effect of the particle size of the dry SAPs on the swelling properties was also investigated. A first order model was used to describe the kinetics of swelling/deswelling, and the equilibrium swelling capacity, limiting swelling capacity, and swelling/deswelling rate coefficients were determined.
1. INTRODUCTION The superabsorbent polymers (SAPs) are cross-linked threedimensional network structures of hydrophilic polymers with an ability to absorb and retain more than 100 times of their own weight.1 The superabsorbent nature of these polymers is due to the presence of ionic groups in the cross-linked polymer network. The presence of these ionic groups results in a high osmotic pressure difference between the polymer and surrounding medium causing the inflow of a large amount of water and swelling of the polymer network.2 The extremely high liquid adsorption and retention properties of the SAPs make them suitable for personal hygiene products such as diapers, sanitary napkins, and adult incontinence products.3 The other important applications are as drug delivery devices,4 contact lenses,5 food packaging,6 water retention of soil79 and water purification. SAPs for a particular application are chosen based on swelling capacity, rate of swelling, swelling/deswelling capacities, and adsorption capacity. The superabsorbent polymers exhibit stimuli responsive behavior depending on the swelling medium.10 Variation in pH, temperature, ionic composition, and solvent composition results in the transition in gel volume.11 Various studies have been carried out to understand the deswelling of the SAPs. The effect of the temperature of the medium on the shrinking of polyampholyte gels based on vinyl 2-aminoethyl ether and sodium acrylate12 and N, N0 -methylenebisacrylamide cross-linked poly(N,N-diethylacrylamideco-acrylic acid) gels13 has been investigated. The effect of cycling of the pH of the swelling medium on the swelling/deswelling of ethylene glycol dimethacrylate cross-linked 2-hydroxyethyl methacrylate, acrylic acid, and sodium acrylate-based hydrogels has been explored.14 The pH of the swelling medium was changed from 8.0 to 4.0, and the hydrogel was found to exhibit reversible swelling/deswelling behavior. Dual, that is, temperature and pH, sensitive poly(acrylic acid-co-Niospropylacrylamide) hydrogels based on N,N0 -methylenebisacrylamide and melamine triacrylamide have also been synthesized.15 Thermo and pH sensitive poly(N-isopropylacrylamide-co-acrylamide) and poly(N-isopropylacrylamide-co-acrylic acid) copolymeric and r 2011 American Chemical Society
composite hydrogels have been synthesized.16 Temperature and pH responsive N-isopropylacrylamide and acrylic acid-based hydrogels were adsorbed on silicon wafers precoated with polyethylene imine, and their reversible swelling and deswelling characteristics were investigated.17 pH sensitive polyampholyte hydrogels based on acrylic acid, (N,N-diethylamino) ethyl methacrylate, and acrylamide without any cross-linker have also been studied.18 Semi-interpenetrating networks hydrogels of poly(acrylic acid-co-acrylamide-co-methacrylate) and amolyse have been synthesized, and their pH response has been explored.19 pH sensitivity of poly(acrylic acid-co-acrylamide) hydrogels has been studied, and the upper critical solution temperature (UCST) was found to decrease with increasing pH of the medium.20 Deswelling kinetics of 4,40 -di(methacryloylmino) azobenzene cross-linked n-alkyl methacrylate esters, acrylic acid, and acrylamide based hydrogels has also been reported.21 Reversible swelling/deswelling of poly(methacrylic acid-coacrylamide) hydrogels cross-linked with N,N0 -methylenebisacrylamide has been investigated in water and sodium chloride (NaCl) solutions.22 Deswelling of γ-irradiation cross-linked poly(acrylic acid) based hydrogels has been carried out in univalent, divalent, and trivalent salt solutions.23 All univalent salts (independent of the size of the cation or the type of the anion) of the same concentration had a similar effect on the water absorbency of the hydrogel. Deswelling of N-isopropylacrylamide/[[3-(methacryloylamino) propyl] dimethy(3-sulfopropyl) ammoniumhydroxide] (NIPAAm/MPSA) copolymer hydrogels has also been carried out in aqueous NaCl and CaCl2 solutions.24 The swelling/deswelling behavior is of great importance in the applications of hydrogels in intelligent drug delivery systems such as biological onoff switch,25 chemical separation systems,26 and Received: April 5, 2011 Accepted: August 21, 2011 Revised: August 11, 2011 Published: August 22, 2011 10918
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Table 1. Recipe for the Synthesis of the SAPs NaOH solution SAP AA/SA-1
AA/SA (mol %)
AM (mol %)
AA (VAA (mL))
water (mL)
NaOH (g)
AM solution AM (g)
water (mL)
KPS (g)
100
0
12.0
8.0
5.2456
0
10
0.2363
AA/SA/AM-1
80
20
9.6
10.4
4.1965
2.4857
10
0.2363
AA/SA/AM-2
60
40
7.2
12.8
3.1474
4.9715
10
0.2363
AA/SA/AM-3
40
60
4.8
15.2
2.0983
7.4572
10
0.2363
AA/SA/AM-4
20
80
2.4
17.6
1.0491
9.9429
10
0.2363
0
100
0
20.0
0
12.4286
10
0.2363
60
40
7.2
12.8
4.1965
4.9715
10
0.2363
AM-1 SA/AM-1
physiologically sensitive drug delivery devices.27 Gemeinhart et al.28 have reported poly(acrylic acid-co-acrylamide) smart hydrogels exhibiting fast swelling and deswelling kinetics in simulated intestinal fluid and simulated gastric fluid, respectively. Ceylan et al.29 have stated, “In many gel applications, the swelling and shrinking kinetics are very important” and investigated the swelling and deswelling of hydrogels and cryogels of ionic poly(acrylamide) in DI water and acetone, respectively. The slow response rate of normal poly(Niospropylacrylamide) restricts its applications, and, therefore, hydrogels with fast deswelling rates have also been synthesized.30 The reversible phase transition of poly(N-isopropylacrylamide) allowed it to be used as a thermoreversible support for the immobilization of β-galactosidase31 and lipase32 and separation of antibodies.33 Owing to the stimuli responsive swelling and deswelling characteristics, the hydrogels are used in contact lens-based ophthalmic drug delivery systems.34 Poly(acrylamide-co-acrylic acid)/chitosan biodegradable nanostructured hydrogels have been synthesized, and the drug delivery mechanism has been studied.35 Periodic microgel arrays based on N-isopropylacrylamide, acrylic acid, and acrylamide have been proven to be an efficient way of preparing perforated gold films.36 The swelling of the SAPs is usually carried out in DI water and the amount of DI water absorbed is claimed to be maximum swelling capacity of the SAP. But in practical applications, most of the SAPs are used in the personal hygiene products, where the swelling capacity in salt solutions and saline resistance are of utmost importance. Although, we find many studies on the deswelling of the SAPs in salt solutions are available, only a few studies report the reversible swelling/deswelling behavior of the SAPs. The investigation of the reversible swelling/deswelling behavior helps us answer many important questions, such as (i) What happens when a SAP swollen to equilibrium in DI water is transferred to a salt solution? (ii) Does the SAP absorb more water or lose already absorbed water? (iii) Is the cyclic swelling/ deswelling behavior completely reversible i.e., does the SAP achieve the same equilibrium swelling capacity or limiting swelling capacity in each cycle? The present study is an attempt to answer these questions. It involves the investigation of the kinetics of swelling and deswelling of poly(acrylic acid-co-sodium acrylate-co-acrylamide) superabsorbents. The ethylene glycol dimethacrylate cross-linked SAPs were synthesized by inverse suspension polymerization. The swelling and deswelling of the SAPs were carried out in DI water and aqueous NaCl solutions, respectively, and the conductivity of the medium was monitored to understand the response behavior of the SAPs. The effect of the acrylamide content on the swelling and deswelling of the SAPs was also investigated. To study the effect of added electrolyte in the
swelling medium on the swelling capacity of the SAP, an acrylic acid/sodium acrylate-based SAP was swollen in aqueous NaCl solutions of various concentrations. The effect of size of the SAP particles on the swelling properties was also investigated.
2. EXPERIMENTAL SECTION 2.1. Materials. Monomers, acrylic acid, and acrylamide were obtained from Merck Limited, India, and S.D. Fine-Chem Ltd., India, respectively. The cross-linking agent, ethylene glycol dimethacrylate (EGDMA), was purchased from Aldrich. Sodium hydroxide and potassium persulfate (KPS) were procured from S.D. Fine-Chem Ltd., India. Sodium chloride (NaCl) and methanol were purchased form Merck Limited, India. Span-80, used as surfactant, was obtained from Rolex Chemical Industries (Mumbai). Milli-Q water was used for all the experiments. 2.2. Synthesis of the Superabsorbent Polymers. The superabsorbent polymers were synthesized by inverse suspension polymerization.37 The polymerization was carried out in a four-necked round-bottom flask having a reflux condenser, an inlet for N2, and a temperature sensor. The fourth inlet was used to add the reactants. The reactants were mixed using a magnetic stirrer during the polymerization. The flask was maintained at the desired temperature using a temperature controller. The dispersed phase contained the monomers, namely acrylic acid (AA), sodium acrylate (SA), acrylamide (AM), and the initiator potassium persulfate (KPS) (Table 1). The volume of the dispersed phase was maintained at 30 mL (1/3 of the continuous phase) for all the polymerizations. The required amount of AM was dissolved in 10 mL of DI water. The required volume of AA, VAA (in milliliters), was taken in a beaker and neutralized by NaOH soluiton. NaOH required to neutralize the AA to a partial neutralization degree of 75% was dissolved in 20 (VAA) mL of DI water. NaOH solution was dropwise added to AA under constant stirring and partially neutralized acrylic acid/ sodium acrylate, AA/SA, was obtained. AM solution was added to the AA/SA mixture under stirring. Finally, KPS (0.5 mol % of total monomers) was added to the monomer mixture and dissolved under nitrogen bubbling for 15 min. The required amounts of the reactants for the preparation of dispersed phase are given in Table 1. A 90 mL portion of toluene (continuous phase) was taken in the four-necked round-bottom flask. A water-in-oil surfactant Span-80 (0.33 vol % of toluene) was added to the flask, and it was heated to 50 °C under N2 bubbling and stirring. The crosslinking agent ethylene glycol dimethacrylate (EGDMA), 0.5 mol % of the total monomers, was added to the flask and dissolved by raising the temperature to 80 °C. 10919
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The dispersed phase was dropwise added to the continuous phase and polymerization was carried out at 80 °C for 2 h under nitrogen bubbling. After the polymerization, the polymer was taken out of the flask and added to an excess of methanol to remove unreacted monomers and water. The polymer was further dried in a hot-air oven maintained at 80 °C for 72 h. Poly(acrylic acid-co-sodium acrylate-co-acrylamide) superabsorbents with 0, 20, 40, 60, 80, and 100 mol % AM content were termed as AA/SA-1, AA/SA/AM-1, AA/SA/AM-2, AA/SA/ AM-3, AA/SA/AM-4, and AM-1, respectively. Poly(sodium acrylate-co-acrylamide), with SA/AM ratio of 60/40, was obtained by complete neutralization of AA monomer feed and termed as SA/AM-1. 2.3. Fourier Transform Infrared Spectroscopy. Fourier transform infrared spectroscopy for the synthesized polymers was carried out with a Perkin-Elmer Spectrum RX-I spectrometer. The spectra were recorded in transmission mode at a resolution of 4 cm1 in the range of 4000500 cm1. 2.4. Determination of Swelling Capacity of the Superabsorbent Polymers. The swelling capacity of the SAPs was determined gravimetrically. A known weight of the dry polymer was kept in a plastic basket and immersed in beakers containing 500 mL of DI water. The baskets were taken out at different times and excess water was removed by wiping with tissue papers. The swollen samples were weighed and then returned to the respective beakers. The swelling capacity, S (g of water/g of SAP), is defined with respect to the weight of dry (Wd) and swollen (Ws) polymers as S¼
Ws Wd Wd
ð1Þ
2.5. Gel Content. To determine the gel content of the SAPs, a known weight of the dry polymer was allowed to swell to its equilibrium swelling capacity. The swelled gel was transferred to a perforated aluminum foil cup where the excess water and the water-soluble fraction of the SAP were allowed to drain and the swelled gel was retained. The retained gel was dried for 72 h and then weighed. The gel content of the SAP was calculated based on the initial (Wi) and final (Wf) weight of the dry SAP.
gel content ¼
Wi Wf
ð2Þ
2.6. Reversible Swelling/Deswelling Characteristics. 2.6.1. Swelling Cycle 1. Known weights of the dry SAPs were taken in
plastic baskets and swelled to their equilibrium swelling capacity by immersing them in beakers containing 500 mL of DI water. The samples were taken out at various times, excess water was removed by soaking with tissue paper, and then the samples were weighed to determine the amount of water absorbed. The equilibrium swelling capacity, Seq, was achieved when the polymer did not absorb further water. 2.6.2. Deswelling Cycle 1. The equilibrium swollen SAPs were transferred to beakers containing 400 mL of 0.2 M NaCl solution. The deswollen samples were removed at different times and weighed after the removal of excess water. The deswelling was carried out until there was no more reduction in the swelling capacity. The swelling capacity in the deswelling medium when SAP does not release water further is defined as its limiting swelling capacity, Slim.
Figure 1. FTIR spectra of AA/SA-1, AA/SA/AM-1, and AM-1.
2.6.3. Swelling Cycle 2 and Deswelling Cycle 2. After the polymers had deswelled at the end of first deswelling cycle, they were subjected to the second cycle of swelling and deswelling, as described previously. 2.6.4. Swelling Cycle 3 and Deswelling Cycle 3. After the completion of the second swelling and deswelling cycle, the superabsorbent polymer SA/AM-1 (60/40) was subjected to the third cycle of swelling and deswelling, as described previously. 2.7. Swelling of the SAP in Electrolyte Solution. To investigate the effect of added electrolyte, the swelling of the homopolymeric superabsorbent AA/SA-1 was carried out in aqueous NaCl solutions (0.0025 to 0.15 M). A 0.1 g portion of the dry SAP was swollen in 500 mL of the aqueous NaCl solution, and the swelling capacity was determined gravimetrically, as described in previous section. 2.8. Effect of the Size of the Superabsorbent Particles on the Equilibrium Swelling Capacities. To investigate the effect of size of the superabsorbent polymers on the equilibrium swelling capacity, single particles of SA/AM-1 (60/40) superabsorbent of different weights (0.00540.3384 g) was swelled to its equilibrium swelling capacity in DI water and the swelling capacity and kinetics were determined.
3. RESULTS AND DISCUSSION 3. 1. Fourier Transform Infrared Spectroscopy. FTIR spectra for AA/SA-1, AA/SA/AM-1, and AM-1 are shown in Figure 1. The characteristic peaks of CdO of acrylamide and NH stretching of the acrylamide group were observed at 1667 and 3436.5 cm1, respectively. The peaks at 1721 and 1561 cm1 correspond to CdO of acrylate and (CO)O stretching of acrylate group, respectively, and are absent in AM-1. The peak at 1561 cm1 is absent in AM-1 and occurs in AA/SA/AM-1 and AA/SA-1 indicating the presence of COONa+ groups. 3.2. Equilibrium Swelling Capacity of the Superabsorbents. The superabsorbent nature of the partially neutralized acrylic acid-based SAPs results from the electrostatic repulsion between the negatively charged carboxyl groups of the polymer backbone, which tends to expand the three-dimensional network.2 Donnan membrane equilibria exist between the ionic polymer and its surrounding water. The gel acting as its own membrane prevents the mobile charges (Na+) from diffusing into the surroundings. Owing to the attracting power of fixed 10920
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Figure 2. Schematic of a swollen cross-linked three-dimensional ionic network in (a) distilled water and (b) NaCl solution.
Figure 3. Variation of equilibrium swelling capacity with the AM content.
charges, the concentration of mobile ions inside the swollen polymer will always be greater than that in the surroundings. Thus the osmotic pressure inside the polymer exceeds that of the swelling medium and the network expands by the intake of large quantity of water. The polymer chains assume elongated configurations and an elastic retractive force develops opposing the swelling process. The polymer attains its equilibrium swelling capacity when these two opposing forces balance each other, as shown in Figure 2a. SAPs swell to their equilibrium swelling capacity when immersed in distilled water due to the higher osmotic pressure inside the polymer than that of the surrounding water, as described in the previous section. But when the SAPs are immersed in an electrolytic medium, such as NaCl, the mobile ions from the electrolyte have a screening effect, which reduces the repulsion forces between the fixed charges, reducing the swelling capacity.2 If the surrounding electrolyte is sufficiently concentrated so that the osmotic pressure of the medium is higher than that of the gel, the already swollen polymer deswells (Figure 2b). Figure 3 shows the equilibrium swelling capacity, Seq, of various SAPs. AA/SA/AM-1 (20 mol % AM) exhibits the highest swelling capacity of 621.7 g of water/g of SAP, while AM-1 (100 mol % AM) exhibits the lowest swelling capacity of 30.1 g of water/g of SAP. Interestingly, the addition of AM to AA/SA results in an increase in the swelling capacity of the resulting
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Figure 4. Variation of swelling capacity of SAPs during reversible swelling/deswelling cycles.
copolymeric hydrogels up to a certain extent. These results are in accordance with investigation of AA/AM SAPs cross-linked with N,N-methylene bisacrylamide where the SAP with 15 wt % AM content showed the highest swelling capacity.38 Copolymeric superabsorbents with 20 and 40 mol % AM content have higher swelling capacities than the homopolymeric superabsorbent, AA/SA-1. Beyond 40 mol % of AM, the addition of AM results in the reduction of swelling capacity. Among the copolymeric superabsorbents, the reduction in the equilibrium swelling capacity with increase in mol % of AM is due to reduced number of anionic repeat units in the polymer backbone. The superabsorbent polymer SA/AM-1 (60/40) showed higher swelling capacity (713.8 g of water/g of SAP) than the AA/SA/AM superabsorbents. This can be attributed to the higher degree of neutralization of SA/AM (100%) as compared to that of AA/SA/ AM superabsorbents (75%). 3.3. Gel Content. The gel content is the fraction of insoluble three-dimensional network structures. The gel contents of AA/ SA-1, AA/SA/AM-1, AA/SA/AM-2, AA/SA/AM-3, AA/SA/ AM-4, and AM-1 were calculated by eq 2 and were found to be 0.81, 0.93, 0.81, 0.83, 0.86, and 0.69, respectively. The gel content is the fraction of the superabsorbent polymer that is cross-linked and thus insoluble in water. It is this fraction of the SAPs that swells and holds the water in the three-dimension cross-linked network. The trend of the gel content exhibited by the SAPs could be due to the variation in extent and efficiency of cross-linking agent with the varying monomer composition. 3.4. Swelling/Deswelling Kinetics of the Superabsorbent Polymers. Figure 4 exhibits the variation of the swelling capacity of the SAPs over two swelling/deswelling cycles. The swelling of the SAPs to their equilibrium swelling capacity was carried out in DI water and then they were transferred to 0.2 M NaCl solution to undergo deswelling. All the SAPs in this study followed the first order swelling/deswelling kinetics described in the following sections. In some earlier investigations of the kinetics of swelling of acrylamide with anionic monomers,6 carboxymethyl celluloseg- poly(acrylamide-co-2-acrylamido-2-methylpropan sulfonic acid)39 and anionic and cationic starch-based superabsorbents,40 a first order model has been used to describe the swelling characteristics. 3.4.1. 1st Order Model for Swelling Kinetics. dS ¼ ks ðSeq SÞ dt 10921
ð3Þ
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Figure 5. Experimentally determined swelling capacities and their model fit for (a) swelling cycle 1, (b) deswelling cycle 1, (c) swelling cycle 2, and (d) deswelling cycle 2.
where S and Seq are swelling capacity at any time t and equilibrium, respectively, and ks is the swelling rate constant. For first swelling cycle, at t = 0, S = S0 = 0, S ¼ Seq ½1 expð ks tÞ
ð4Þ
As tf∞, S = Seq For a second swelling cycle, at t = 0, S = S0 6¼ 0, then S ¼ Seq ðSeq S0 Þ expð ks tÞ
ð5Þ
3.4.2. 1st Order Model for Deswelling Kinetics.
dS ¼ kd ðS Slim Þ dt
ð6Þ
where Slim is the limiting swelling capacity and kd is the deswelling rate constant. At t = 0, S = S0, thus, S ¼ Slim þ ðS0 Slim Þ expð kd tÞ
ð7Þ
as tf∞, S = Slim. Equation 4 describes the kinetics of swelling for the swelling cycle 1. To the best of our knowledge, this is the first time when the first order model has been modified, that is, eq 5, to fit the swelling cycles after deswelling of the polymer. Moreover, the deswelling kinetics has also been explained by the first order model (eq 7). At the start of the swelling cycle 1, the dry SAPs are used so that they achieve their equilibrium swelling capacity, Seq, by the end of swelling cycle 1. These equilibrium swollen SAPs lose
water rapidly when subjected to deswelling cycle 1, and achieve the limiting swelling capacity, Slim, beyond which there is no more loss of water. The SAPs do not lose water completely upon deswelling and have a limiting swelling capacity, Slim. Thus, for the swelling cycle 2, the initial swelling capacity, S0, is equal to the limiting swelling capacity of the deswelling cycle 1. Equations 4 and 5 describe the kinetics of swelling cycle 1 and 2, respectively, while the kinetics of deswelling cycles 1 and 2 are given by eq 7. Figure 5 panels ad show the experimental data and the model fits for the different swelling/deswelling cycles of the SAPs. The data was fitted using the nonlinear curve fit option of the Origin software, and the regression coefficient was found to be greater than 0.99 for all the fits. The kinetic parameters obtained are given in Table 2. The equilibrium and the limiting swelling capacities obtained from the model closely match those obtained experimentally. AM-1 does not have any ionic groups in its backbone and has very low swelling capacity, so it will not be discussed anymore. The rates of swelling for the swelling cycle 1 of remaining SAPs are in order of AA/SA-1 > AA/SA/AM-3 > AA/SA/AM-4 > AA/ SA/AM-2 > AA/SA/AM-1 and the equilibrium swelling capacities follow the order AA/SA/AM-1 > AA/SA/AM-2 > AA/SA1 > AA/SA/AM-3 > AA/SA/AM-4. The highest rate of swelling of AA/SA-1 is due to its highest content of anionic repeat units, resulting in higher osmotic pressure difference than other SAPs. When the AM content is lower than the AA/SA content, that is, 20 and 40 mol % AM, SAPs with higher swelling capacities but lower rates of swelling are observed. When the AM content exceeds AA/SA content, that is, 60 and 80 mol % AM, lower swelling capacities but higher rates of swelling are obtained. 10922
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Table 2. Kinetic Parameters for Swelling/Deswelling Cycles swelling cycle 1 SAP
Seq (g/g)
ks (h1)
deswelling cycle 1 Slim (g/g)
swelling cycle 2
kd (h1)
Seq (g/g)
ks (h1)
deswelling cycle 2 Slim (g/g)
kd (h1)
(Seq)2/(Seq)1
AA/SA-1 (0% AM)
306.8
1.098
48.3
8.585
202.3
1.677
40.7
8.950
0.66
AA/SA/AM-1 (20% AM)
621.7
0.408
71.9
6.953
325.1
0.797
55.9
6.877
0.52
AA/SA/AM-2 (40% AM)
496.1
0.454
59.2
8.110
292.6
0.967
50.8
8.052
0.58
AA/SA/AM-3 (60% AM)
285.0
0.892
42.1
8.842
208.9
1.189
38.8
9.984
0.73
AA/SA/AM-4 (80% AM)
215.0
0.530
36.9
5.639
168.1
0.747
38.1
7.614
0.78
30.1
0.902
22.9
4.413
713.8
0.332
80.2
4.585
360.4
0.502
73.0
3.297
0.50
AM-1 (100% AM) SA/AM-1 (40% AM)
Figure 6. Variation of swelling capacity of SA/AM-1 during the three reversible swelling/deswelling cycles.
Generally materials that exhibit high equilibrium swelling capacities show a lower rate of swelling and vice versa. The rate of swelling and equilibrium swelling capacities are determined by the kinetics and thermodynamics of absorption and cannot be directly compared. Thus, by choosing an appropriate AA/SA/ AM composition, we can obtain a polymer having a desired rate of swelling and equilibrium swelling capacity for the required application. The gel content of the polymer has an effect on the swelling capacity and the rate of swelling of the SAP of a particular composition. But in the case of SAPs with varying composition, other factors such as fraction of ionic repeat units and molecular weight between cross-links also vary. This influences the swelling capacity and the rate of swelling but this is not directly proportional to the gel content of various SAPs. The limiting swelling capacities also follow the same order as that of the equilibrium swelling capacities. AA/SA/AM-1 has the highest equilibrium swelling capacity as well as highest limiting swelling capacity in both the swelling and deswelling cycles, respectively, but has lowest rate of swelling. Among the copolymeric SAPs, AA/SA/AM-3 has the second highest rate of swelling in both the swelling cycles and the highest rate of deswelling in both the deswelling cycle. As AA/SA/AM-3 has an equilibrium swelling capacity almost equal to that of AA/SA-1, their swelling/deswelling behavior can be compared. AA/SA-1 swells at a faster rate than AA/SA/AM-3 in both swelling cycles, but deswells at a lower rate. This can be attributed to higher amount of anionic repeat units in the AA/SA-1 than that in AA/ SA/AM-3, which causes higher osmotic pressure difference when
immersed in DI water but lower osmotic pressure when immersed in NaCl solution. It was found that the equilibrium swelling capacity of the superabsorbents was lower in the second swelling cycle than that in the first swelling cycle. To investigate this reduction in the swelling capacity, SA/AM-1 superabsorbent was subjected to three swelling/deswelling cycles (Figure 6) and the conductivity of the swelling/deswelling medium was measured by a conductivity meter (ECTestr 11+, Eutech Instruments). SA/AM-1 swells to its equilibrium swelling capacity in the swelling cycle 1 but to a lower extent in the second and third swelling cycles. Moreover, the equilibrium swelling capacity achieved by the SAP is almost equal in the second and third swelling cycles. Figure 7 panels af show the change in the conductivity of the medium with time. The conductivity of the medium increases in the first swelling cycle indicating the release of mobile ions from the swelling gel and decreases during the first deswelling cycle, indicating the reduction in the NaCl concentration due to the diffusion of NaCl into the shrinking gel. After the completion of the first deswelling cycle, the gel is transferred to DI water for the second swelling cycle, an increase in the conductivity of the medium is observed. However, the increment in conductivity is significantly higher than that in the first swelling cycle. The increase in the conductivity is caused by the release of NaCl (diffused into the gel during the first deswelling cycle) into the DI water. Similar observations were made for the second and third swelling/deswelling cycles. Hence, the swelling medium, which was initially DI water turned into NaCl solution, thus the SAP swells to a lower extent. This was verified by swelling the dry SAP in NaCl solution (0.003 M) of equivalent conductivity and it was found that the SAP swelled to the same extent as that in the second and third swelling cycles (Figure 6). As the conductivity (thus the effective NaCl concentration) of the medium during the second and third swelling cycles is the same, the SAP also swells to the same extent in both the swelling cycles. 3.5. Swelling of the SAP in the Aqueous NaCl Solution. Figure 8 shows the variation in the equilibrium swelling capacity of the AA/SA-1 in DI water and aqueous NaCl solutions (0.00250.15 M). The swelling of AA/SA-1 in aqueous NaCl solutions also followed first order kinetics given by eq 4 and the equilibrium swelling capacities and the swelling rate constants are listed in Table 3. The equilibrium swelling capacity of the SAP decreased with an increase in the concentration of the NaCl solution. The addition of the NaCl to the DI water increases the concentration of mobile ions (Na+) in the swelling medium. It results in lower osmotic pressure difference between the polymer and the swelling medium causing reduction in the swelling capacity.2 10923
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Figure 7. Variation in the conductivity of the medium during the three reversible swelling/deswelling cycles of SA/AM-1: (a) swelling cycle 1, (b) deswelling cycle 1, (c) swelling cycle 2, (d) deswelling cycle 2, (e) swelling cycle 3, and (f) deswelling cycle 3.
The mobile ions from the added electrolyte also have a screening effect that reduces the electrostatic repulsion force between the fixed anionic repeat units in the SAPs, which further reduces the swelling capacity (Figure 2 b).2 The relationship between the equilibrium swelling capacity (Seq) of the SAP and the concentration (C*) of the added electrolyte in the medium is2 A S5=3 eq ¼ þ B C
ð8Þ
where A is a function of concentration of fixed charges in the unswollen polymer network and B contains the polymersolvent interaction parameter, molecular volume of the solvent, volume of unswollen polymer network, and effective number of chains in a real network.2
The variation in the equilibrium swelling capacity of AA/SA-1 with the concentration of NaCl solution is shown in Figure 9. The inset figure shows that the swelling of AA/SA-1 in NaCl solutions follows the relationship given by Flory (eq 8). The experimental data was fitted using linear fit option of Origin software, and the values of constants A and B were found to be 1077 (g/g)5/3 and 32.03 (g/g)5/3 mol/L, respectively. 3.6. Effect of the Particle Size on the Equilibrium Swelling Capacity. To investigate the effect of the particle size of the superabsorbent polymer on the equilibrium swelling capacity and the rate of swelling, SA/AM-1 (60/40) superabsorbent of various initial dry weights (0.0054, 0.0104, 0.0298, 0.1008, 0.1978, 0.2493, and 0.3384 g) was swelled to equilibrium in DI water. Figure 10a shows the variation in swelling capacity with swelling time for SA/AM-1 particles of different weights. 10924
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Figure 8. Variation in swelling capacity of AA/SA-1 with time in aqueous NaCl solutions of various concentrations.
Table 3. Kinetic Parameters for Swelling of AA/SA-1 in Aqueous NaCl Solutions of Various Concentrations C* (M) 0 0.0025 0.0050 0.0075 0.01 0.05 Seq (g/g) 306.0 303.9 216.0 182.4 136.5 91.8 ks (h1)
1.099 0.315
0.477
0.588
0.10 80.4
0.15 65.1
0.313 0.603 0.519 1.175
Figure 10. (a) Effect of size of the SA/AM-1 particles on the swelling capacity; (b) variation in the equilibrium swelling capacity of SA/AM-1 with the initial weight of the superabsorbent.
Table 4. Kinetic Parameters for the Effect of Size of SA/AM-1 Particles
Figure 9. Variation in the equilibrium swelling capacity of AA/SA-1 with the concentration of NaCl solution. Inset figure shows (equilibrium swelling capacity)5/3 versus (concentration)1 plot.
The equilibrium swelling capacity and the rate of swelling increased with a decrease in the weight of the superabsorbent particles (Table 4). Similar results have been obtained by Omidian et al.41 for modified acrylic-based superabsorbents, and this has been attributed to the increment in the surface area with decrease in the weight or size of the particles. The dependence of equilibrium swelling capacity on the weight or size (Figure 10b) can be expressed as Seq = KWn0, where Seq is the equilibrium swelling capacity determined from the first order swelling model and W0 is the initial dry weight of the superabsorbent. As W0 = FV0, where F is the density, V0 is the initial dry volume of the superabsorbent polymer, and r is the radius of a sphere of . volume V0, then if, Seq is proportional to 1/r then, Seq = KW(1/3) 0 This expression fits the experimental data well (Figure 10 b) with K = 275 g1/3, and thus the equilibrium swelling capacity of
W0 (g)
0.0054
0.0104
0.0298
0.1008
0.1978
0.2493
0.3384
Seq (g/g) ks (h1)
1619.2 0.580
1281.3 0.441
731.5 0.420
558.8 0.391
509.6 0.283
443.3 0.201
428.6 0.154
the superabsorbent polymer is inversely proportional to the radius of the particles.
4. CONCLUSIONS In this study, poly(acrylic acid-co-sodium acrylate-co-acrylamide) superabsorbents cross-linked with ethylene glycol dimethacrylate (EGDMA) were synthesized by an inverse suspension polymerization technique. The SAPs had a wide range of equilibrium swelling capacities and rate of swelling. The SAPs were subjected to reversible swelling and deswelling cycles in DI water and aqueous NaCl solutions, respectively. The equilibrium swelling capacities in the second swelling cycles were lower than that in the first swelling cycle. This was attributed to the release of NaCl (diffused into the shrinking gel during the first deswelling cycle) into the DI water; thus the swelling medium effectively turned into NaCl solution, resulting in the reduction of the equilibrium swelling capacities. The SAP with 0 mol % AM (AA/ SA-1) was swollen in aqueous NaCl solutions of various concentrations. With increasing concentration of NaCl, the osmotic pressure difference between the polymer and swelling medium decreased resulting in the lowering of equilibrium swelling 10925
dx.doi.org/10.1021/ie200713y |Ind. Eng. Chem. Res. 2011, 50, 10918–10927
Industrial & Engineering Chemistry Research capacity of the SAP. The swelling capacity of AA/SA-1 decreases with increasing concentration of NaCl solution as the osmotic pressure difference between the polymer and the swelling medium decreased. The equilibrium swelling capacities and the rate of swelling increased with the decrease in the particle size of the SAPs. The kinetics of swelling/deswelling of the SAPs was modeled by the first order models, and the rate parameters were determined.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: 091-80-22932321. Fax: 091-80-23600683. E-mail: giridhar@ chemeng.iisc.ernet.in.
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