Studies on poly(acrylic acid)/attapulgite superabsorbent composites. II. Swelling behaviors of superabsorbent composites in saline solutions and hydrophilic solvent–water mixtures

The effect of the attapulgite content on the swelling for a series of poly(acrylic acid)/attapulgite superabsorbent composites in water was studied. The effects of the temperature and pH values on the water absorbency of the superabsorbent composites were investigated. The swelling behavior of the superabsorbent composites in various saline solutions was also investigated. The water absorbency in various salt solutions decreased with an increase in the ionic strength of the solutions. At a high ionic strength (>1 × 10^?3M), the water absorbency in monovalent cationic solutions was higher than that in multivalent cation solutions. This dramatic reduction of the water absorbency in multivalent cationic solutions of high ionic strength may have been due to the complexing ability of the carboxylate groups inducing the formation of intramolecular and intermolecular complexes, which resulted in an increased crosslink density of the network. The swelling behavior of the superabsorbent composites in mixtures of water and hydrophilic solvents, including methanol, acetone, ethanol, and dimethyl sulfoxide (DMSO), was also investigated. The water absorbency decreased with an increase in the concentration of any of the four organic solvents, and two transitions were observed in the superabsorbent composite/hydrophilic solvent–water mixture systems. The main transition for the four hydrophilic solvent–water mixtures was a collapse of the swollen gel (at 50–80% methanol, 30–80% acetone, 50–80% ethanol, and 50–80% DMSO). For the methanol–water system, the magnitudes of the first and second transitions for the poly(acrylic acid)/attapulgite superabsorbent composites containing lower proportions of attapulgite were larger than those for the superabsorbent composites with higher attapulgite contents. The effect of the mixture temperature on the water absorbency of the superabsorbent composites in 10 min was also reported. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1869–1876, 2004     

Study on superabsorbent composite. XI. Effect of thermal treatment and acid activation of attapulgite on water absorbency of poly(acrylic acid)/attapulgite superabsorbent composite

The effect of acid activation and thermal treatment of attapulgite on water absorbency of superabsorbent composite were investigated. Under the same preparation conditions, superabsorbent composite prepared with natural attapulgite exhibited a water absorbency of 639 g/g and it merely kept 71% of its initial water absorbency after 5 times of swelling–deswelling–reswelling test. However, superabsorbent composites prepared with 2–10 M hydrochloric acid acidified attapulgite and 100–400°C thermal treated attapulgite respectively exhibited the water absorbency of 884–1,241 g/g and 701–1,515 g/g. Also, those superabsorbent composites can keep 87% and 85% of their initial water absorbency after 5 times of swelling–deswelling–reswelling test, respectively. These results showed that, compared with superabsorbent composite prepared with natural attapulgite, the comprehensive water‐absorbing properties of poly(acrylic acid)/ attapulgite superabsorbent composites were improved effectively by acid activation and thermal treatment of attapulgite. This improvement of water absorbencies and gel strength of superabsorbent composite may be due to synthetical factors such as changes in the crystalline structure and the specific surface area and improvement of the number and the activity of hydroxyl groups of attapulgite, which in turn influence the grafting efficiency of monomer, crosslinking density, and the structure of superabsorbent composite network. POLYM. COMPOS., 28:397–404, 2007. © 2007 Society of Plastics Engineers     

瓜尔胶基高吸水性树脂的制备、溶胀行为与保水性能

以天然瓜尔胶（GG）和丙烯酸（AA）为原料,过硫酸铵（APS）为引发剂,N,N′-亚甲基双丙烯酰胺（MBA）为交联剂,采用水溶液聚合法制备了瓜尔胶接枝聚丙烯酸（GG-g-PAA）高吸水性树脂。考察了MBA浓度对树脂溶胀动力学和溶胀能力的影响,研究了树脂在不同亲水有机溶剂/水混合溶液、不同阳离子盐（NaCl、CaCl₂和FeCl₃）和阴离子盐（KNO₃、K₂SO₄和K₃PO₄）溶液中在各浓度和离子强度下的溶胀行为,测定了高吸水性树脂在室温和高温下的保水性能。结果表明,该树脂对亲水有机溶剂较为敏感,吸水倍率随着亲水有机溶剂浓度的增加迅速减小;在各种盐溶液中的吸水倍率随着离子强度的增加而下降。     

Study on superabsorbent composites. IX: Synthesis,characterization and swelling behaviors of polyacrylamide/clay composites based on various clays

A series of clay-based superabsorbent composite from acrylamide (AM) and various clays, such as attapulgite, kaolinite, mica, vermiculate and Na⁺-montmorillonite, was prepared by free-radical aqueous polymerization, using N,N′-methylenebisacrylamide (MBA) as a crosslinker and ammonium persulfate (APS) as an initiator, and then saponified with sodium hydroxide solution. In this paper, the reaction mechanism and thermal stability of the superabsorbent composites incorporated with various clays were characterized by FTIR, XRD and TGA, respectively. The effects of clay kind and clay content on equilibrium water absorbency of these composites were also investigated and compared. In addition, the influences of clay kind on comprehensive swelling behaviors of the PAM/clay superabsorbent composites were studied. The results indicated that the introduced clays could influence physicochemical properties of obtained superabsorbent composites. Mica could improve thermal stability of corresponding superabsorbent composites to the highest degree comparing with the other clays. The PAM/clay superabsorbent composites incorporated with 10 wt% clay of various kinds were all endowed with equilibrium water absorbency of more than 1300 g g⁻¹. The equilibrium water absorbency decreases with increasing clay content and correlates with the kind of clay. Attapulgite-based superabsorbent composite was endowed with higher water absorbency in univalent cationic saline solution, however, the vermiculite- and the kaolinite-based ones acquired the highest water absorbency in CaCl₂ and FeCl₃ aqueous solution, respectively. Moreover, the superabsorbent composites incorporated with Na⁺-montmorillonite have higher swelling rate and that of doped with mica was endowed with higher reswelling capability.     

Preparation and swelling behaviors of a high temperature resistant superabsorbent using tetraallylammonium chloride as crosslinking agent

A new high temperature resistant superabsorbent was synthesized through solution polymerization of acrylamide (AM) and partially neutralized acrylic acid (AA), using tetraallylammonium chloride (TAAC) as crosslinker, ammonium persulfate (APS) as initiator. Parameters that influence water absorbency of the superabsorbent at 25 °C and 200 °C such as molar ratios of AM to AA, TAAC to AA, APS to AA and neutralization degree, were investigated. Swelling behaviors of superabsorbent prepared at the optimum conditions in different pH and saline solutions were studied. The swelling ratios of superabsorbent in distilled water and 1 wt% NaCl solution at 250 °C reach 287 g/g and 69 g/g, respectively.     

Gum arabic–acrylic superabsorbing hydrogel hybrids: Studies on swelling rate and environmental responsiveness

The swelling rate and the environmental sensitivity of novel superabsorbent gum arabic–acrylic hydrogel hybrids were investigated. The swelling kinetics of the hydrogel hybrids was studied by means of a Voigt‐based viscoelastic model. The effects of concentration of the initiator, crosslinker, and the monomer ratio on the swelling rate were studied. The superswelling properties of the hydrogel hybrids were evaluated in various environmental pH, salinity and solvent–water mixtures. The optimally prepared hydrogel, MR5, showed a reproducible on–off switching behavior when the swelling medium was alternatively changed between distilled water and alkaline solutions. The hydrogel hybrid MR5 was also tested to be swollen and deswollen alternatively in distilled water and sodium chloride solution. The sorption–desorption behavior was found to be quite repeatable. A similar capability was interestingly observed when a calcium chloride solution with the same molar concentration was used. The swelling changes of the hydrogel hybrid were examined in various water–solvent systems including the aqueous solutions of methanol, ethanol, acetone, ethylene glycol, glycerol, and dimethylsulfoxide. One and/or two volume‐phase transitions were induced by the nonsolvents. The transitions were explained according to the solubility parameters of the solvents and water–solvent mixtures. The swelling–deswelling capability of the hydrogel in alternatively changed solvent–water mixtures was also studied. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5667–5674, 2006     

Synthesis and water absorbency of the copolymer of acrylamide with anionic monomers

A series of novel copolymer superabsorbents based on acrylamide, sodium allylsulfonate, sodium acrylate, and N,N′-methylenebisacrylamide were prepared by copolymerization. The resulting superabsorbents have a fast swelling rate. The experimental results show that absorbency increases to a maximum as the cross-linking increases, but an excess of cross-linking leads to a swelling decrease. Their water retention was observed at pressures of 1–10 kg/cm² and temperatures of 60 and 100°C, respectively. The water retention of soil has been enhanced by using the poly(acrylamide–sodium allysulfonate–sodium acrylate) superabsorbent; its use for bean growth was also investigated. © 1994 John Wiley & Sons, Inc.     

Water absorption and mechanical properties of electrospun structured hydrogels

Electrospun nanofibers are made when electrostatic forces overcome the surface tension of a polymer solution, causing an electrically charged jet to be ejected; as the jet travels through the air, the solvent evaporates, leaving behind an electrically charged fiber, which can be collected in the form of a nonwoven sheet. A superabsorbent was added to a polymer solution containing an elastomer (concentrations = 0–85%). The mixture was electrospun, producing nanofibers in which the superabsorbent particles were held in place with nanoscale elastic fibers. The nanofibers were tested for absorbency in water and synthetic urine. Fluid absorption by the nanofibers led to the formation of structured hydrogels. Increases in the weight gain from water absorption ranged from 400 to 5000%. The linear dimensions of samples cut from the nonwoven sheet were measured; wetting the superabsorbent increased the thickness dimension of the sheet dramatically and produced a smaller change in the plane of the sheet. The rate of water absorption was calculated; the samples containing 0–70% superabsorbent reached essentially their maximum absorbency within 5 s. The excellent strength and elasticity of the wet samples make these structured hydrogels ideal for many uses, including wound care, drug delivery, and sanitary goods. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 427–434, 2005     

Synthesis and characterization of superabsorbent hydrogels composites based on polysuccinimide

A kind of novel superabsorbent hydrogel with high swelling ratio property that could be used for the development of water absorbing resin, soil water retention agent, and chemical sand‐fixing material was synthesized in this study. The hydrogels were prepared by the crosslinking reaction of polysuccinimide (PSI). The relationships between swelling ratio and volume of solvent as well as the concentration of crosslinking agent were investigated in detail. Several composites, such as starch, carrageenan, and polyacrylamide, were added into hydrogels to enhance the swelling ratio. It was found that the swelling ratio was significantly increased, which the maximum water absorbency was enhanced 2.46 times when the composite polyacrylamide (PAM) was added compared to the control. The effects of ionic strength and sensitivity of pH on hydrogels were also studied. The modified hydrogels products with swelling ratio less sensitivity to the salinity as well as relative high swelling ration in salinity system were also obtained by adding PAM. Through the Fourier transform infrared spectroscopy (FTIR) characterizations, the crosslinking reaction mechanism and the structure of composite were proposed. In addition, the transmission electron microscopy (TEM) examinations showed that some composite materials elevated the physical crosslinked and connected channels density substantially. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 550–557, 2006     

Synthesis of Poly (Acrylamide-co-Acrylic Acid) based Superabsorbent Hydrogels: Study of Network Parameters and Swelling Behaviour

This article investigates the synthesis of superabsorbent hydrogels (SAHs) based on acrylamide and acrylic acid by solution polymerization in the presence of N,N′-methylenebisacrylamide (NMBA) as a cross-linker and potassium per sulphate as initiator. In this work the acrylamide content was varied in a large range of 30–70% (mole of total monomer) in feed, in order to study its effect on swelling behavior. The results indicate that when acrylamide content present in the hydrogels was increased from 17.23–35.85% (mole of the total monomer); then equilibrium water absorbency also increased from 276–573 (g water/g sample). But when acrylamide content was further increased from 35.85–50.24%, then equilibrium water absorbency abruptly decreased. The equilibrium water absorbency of various copolymeric hydrogels was also investigated in different concentration of saline solutions. The effect of SAHs on growth of seeds of lentil was also studied. The hydrogels were also characterized by fourier-transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM); and network parameters such as average molecular weight between cross-links (Mc) and cross-link density (q) were determined to explain the swelling behavior.     

The effects of solvent polarity and pKa on the absorption of solvents into poly(glutaric acid‐glycerol) films

In this study, solvent absorption into the matrices of poly(glutaric acid‐glycerol) films has been evaluated. It was determined that the combined effects of polarity and the size and shape of the solvent molecule, rather than pKa, have the most significant influence on absorption into the films. Polar aprotic solvents (with solvent polarity index values >4) such as 1,4‐dioxane (absorbed 163.8% ± 0.3% [w/w] of the original weight of the polymer), pyridine (200.4% ± 3.5%), and dimethyl sulfoxide (186.0% ± 11.4%) were among the highest absorbed solvents into the polymer matrix. Solvents with polarity index values ≤4.0 were absorbed poorly (≤5.3% ± 1.5%). The polymer films only absorbed ≤26.5% ± 2.1% of their weight of most protic solvents (water and mono‐alcohols) but absorbed 72.6% ± 6.5% of ethylene glycol, a diol. The only high absorbing polar protic solvent was acetic acid (131% ± 13.1%). Except for chloroform, ethyl acetate, and ethanol, all of the solvents examined displayed small increases in absorption (7.8%, on average) when the films were desorbed and used again to absorb solvent. Erosion of the films ranged from 0.0% ± 0.0% to 22.0% ± 3.2% after 2–10 h absorption cycles. Miscible (7.7% ± 2.3% to 15.1% ± 2.2%) and immiscible (12.3% ± 6.4% to 80.0% ± 1.9%) solvents were preferentially absorbed from aqueous solutions. However, up to approximately 5% of those absorption values could be from water absorption. © 2014 Wiley Periodicals, Inc.^? J. Appl. Polym. Sci. 2014 , 131, 40434.     

Study on superabsorbent composite XXV. Synthesis,characterization, and swelling behaviors of poly(acrylic acid‐co‐N‐acryloylmorpholine)/attapulgite superabsorbent composites

In this work, a novel poly(acrylic acid‐co‐N‐acryloylmorpholine)/attapulgite superabsorbent composite was prepared by graft copolymerization among acrylic acid, N‐acryloylmorpholine and attapulgite in aqueous solution, using N,N′‐methylenebisacrylamide as a crosslinker and ammonium persulfate as an initiator. The result from FTIR spectra showed that  OH of attapulgite participated in graft copolymerization with acrylic acid and N‐acryloylmorpholine. Proper monomer ratio and atapulgite content could form a loose surface, and improve reswelling ability and initial swelling rate. The buffer action of the  COOH and  COO⁻ groups in the superabsorbent composite keeps the water absorbency a rough constant in the pH range of 4.4–9.6. Both polarity and structure of an organic solvent are responsible for the phase transition point of the superabsorbent composite. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Preparation of superabsorbent polymers by crosslinking acrylic acid and acrylamide copolymers

High water-absorbent copolymers comprising acrylic acid (AA) and acrylamide (AM) were prepared in the presence of a crosslinking agent, monofunctional aldehyde, by a solution polymerization technique using a redox initiation system. Such copolymers have very high water absorbency and absorbing kinetics to the distilled water. The copolymer formed which absorbed about 900 g water/g dry copolymer was used to study the influence of sodium chloride on the absorption capacity at 24°C. The swelling of this copolymer was studied in alcohol/water mixtures of increasing alcohol content at 294, 304, and 314 K. The main transition for ethanol/water and methanol/water mixtures is a rapid decrease of the retention capacity of the copolymer at 50–60 vol % ethanol and 55–65 vol % methanol, respectively. Swelling in distilled water at different temperatures (T) and the effect of solvent composition were also studied. Among the variables examined were initiator concentration, polymerization temperature, and amount of AM in the copolymer. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1345–1353, 1997     

A Study of the Synthesis and Properties of AM/AMPS Copolymer as Superabsorbent

Summary: A new superabsorbent polymer, PAMA, has been prepared in an aqueous solution using acrylamide (AM) and 2‐acrylamido‐2‐methyl‐propanesulfonic acid (AMPS) as monomers, potassium persulfate (PPS) as initiator, and N,N′‐methylenebisacrylamide (NMBA) as cross‐linker. The absorbing properties and water retention of PAMA have been investigated. It is found that the absorbency of PAMA can reach 2 451 and 119 g · g^?1 in distilled water and in 0.9 wt.‐% NaCl solution, respectively. This copolymer also can absorb a large amount of pure methanol (277 g · g^?1), a property that has not been reported for the other superabsorbent polymers in the literature. The swelling behavior of PAMA in some water/organic solvent mixtures and water retention of PAMA in sand have been investigated.

Water retention of the PAMA in sand at 80 °C. 1) Sample containing PAMA; 2) Sample without PAMA.     

Poly(acrylamide‐co‐maleic acid) hydrogels for removal of Cr(VI) from aqueous solutions,Part 1: Synthesis and swelling characterization

The swelling behavior of poly (acrylamide‐co‐maleic acid) hydrogels has been investigated in distilled water at 30°C. The gels were characterized with respect to structural parameters, Fourier transform infrared, and thermogravimetric analysis. The gels showed fair pH‐dependent swelling and exhibited double “s”‐shaped curve between equilibrium water uptake and pH of the swelling media. The two pK_a values, as determined from the curve, were found to be 2.46 and 6.58. The activation energy of the water uptake process for plain and acid containing gels was found to be 7.93 and 3.26 kJ mol^?1 respectively. Similarly, the enthalpy of mixing between dry polymer and solvent showed positive values, thus indicting endothermic nature of the process, and the values increased from 10.06 to 16.29 kJ mol^?1 with increase in acid content from 2.1 × 10^?1 to 4.7 × 10^?1 mM respectively. There was an optimum initiator concentration 24.0 × 10^?2 mM and reaction temperature 60°C at which gels synthesized showed maximum absorbency. The dilution of the reaction mixture resulted in the formation of hydrogels with enhanced absorbency. Finally, the gels with varying content of monomer acid in the feed mixture showed different swelling behavior when studied in the medium of pH 1.0 and 7.0. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2759–2769, 2006     

Theoretical and experimental investigation of the swelling behavior of sodium polyacrylate superabsorbent particles

Sodium polyacrylate superabsorbent polymers were prepared in particle forms by the inverse suspension technique. The surfactant concentration effect on the polymer particle size was analyzed with molecular sieves and an optical microscope. Water absorption and desorption characteristics were analyzed by the gravimetric method. The equilibrium water uptake in sodium polyacrylate particles was strongly dependent on both the salt concentration of the aqueous media and the crosslinking density of the polymers. The polymer crosslinking density was determined from the measurement of Young's modulus and the polymer–solvent interaction parameter from the equilibrium swelling experiment. The degree of inonization was predicted from experimental measurements, and theoretical analysis was performed on the effects of the salt concentration and polymer crosslinking density on the equilibrium water swelling ratio. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 252–257, 2003     

Superabsorbent composite. XIII. Effects of Al3+‐attapulgite on hydrogel strength and swelling behaviors of poly(acrylic acid)/Al3+‐attapulgite superabsorbent composites

Al³⁺‐attapulgite (Al³⁺‐APT) was prepared by treating attapulgite (APT) with AlCl₃ aqueous solution of various concentrations. The poly(acrylic acid)/Al³⁺‐attapulgite (PAA/Al³⁺‐APT) superabsorbent composite was prepared by reaction of partly neutralized acrylic acid, and Al³⁺‐APT in aqueous solution using N, N′‐methylenebisacrylamide as a crosslinker and ammonium persulfate as an initiator. The surface morphology of the composite was investigated by SEM, and the Al³⁺‐APT composite generated a relatively planar surface comparing the nature APT. The effects of Al³⁺‐APT on hydrogel strength and swelling behaviors, such as equilibrium water absorbency, swelling rate, and reswelling capability, of the superabsorbent composites were also studied. The hydrogel strength and reswelling capability were improved, however, the equilibrium water absorbency and swelling rate decreased with increasing AlCl₃ solution concentration. The equilibrium water absorbency firstly increased, and then decreased with increasing Al³⁺‐APT content. The results indicate that Al³⁺‐APT acts as an assistant crosslinker in the polymeric network, which has great influences on hydrogel strength and swelling behaviors of the PAA/Al³⁺‐APT superabsorbent composites. POLYM. ENG. SCI., 47:619–624, 2007. © 2007 Society of Plastics Engineers.     

Novel biodegradable superabsorbent hydrogels derived from cotton cellulose and succinic anhydride: Synthesis and characterization

The synthesis of novel superabsorbent hydrogels was investigated with the reaction of cotton cellulose and succinic anhydride (SA) in the presence of 4‐dimethylaminopyridine as an esterification catalyst in a mixture of lithium chloride (LiCl) and N‐methyl‐2‐pyrrolidinone (NMP) or in a mixture of tetrabutylammonium fluoride (TBAF) and dimethyl sulfoxide (DMSO), followed by NaOH neutralization. Interestingly, a hydrogel was obtained without any crosslinking agent, and this indicated the partial formation of a diester between the cellulosic hydroxyl group and SA. The products obtained in LiCl/NMP exhibited superior absorbency to these obtained in TBAF/DMSO. The former absorbed an amount of water about 400 times its dry weight, and this was comparable to a conventional sodium polyacrylate superabsorbent hydrogel. Furthermore, in an aqueous NaCl solution, the absorbency of the product hydrogels was higher than that of the sodium polyacrylate superabsorbent hydrogel. The formed hydrogels biologically degraded almost completely after 25 days, and this showed their excellent biodegradability. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3251–3256, 2006     

High‐temperature resistant superabsorbent based on poly(acrylic acid) with triallylammonium chloride as crosslinking agent

A novel high‐temperature resistant superabsorbent was prepared by solution polymerization of partially neutralized acrylic acid (AA), using triallylammonium chloride as crosslinker, potassium persulfate as initiator. The factors that influence the water‐absorbing capacity at 25 and 200°C such as mass concentration of monomer, mass ratio of crosslinker to AA, mass ratio of initiator to AA, and neutralization degree were investigated. The structure of the superabsorbent was characterized by Fourier transform infrared, thermogravimetric analysis, and scanning electron microscopy. The optimum conditions were obtained and the swelling ratios in distilled water and 1 wt % of NaCl solution could reach 841 and 74 g/g at 300°C, respectively. The superabsorbent also showed high swelling rate and good salt resistance. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41243.     

Multiporous microstructure for enhancing the water absorption and swelling rate in poly(sodium acrylic acid) superabsorbent hydrogels based on a novel physical and chemical composite foaming system

In this article, we report a novel physical and chemical composite foaming system, which was used to successfully prepare high‐performance and low‐cost composite superabsorbent [poly(sodium acrylic acid) (PAA–Na)] hydrogels based on acrylic acid by free‐radical polymerization in a water bath under a nitrogen atmosphere without the use of any organic solvents. The prepared hydrogels showed superabsorbent properties, high water‐absorption abilities and swelling rates, a lighter packing density, and a multiporous microstructure. Fourier transform infrared spectroscopy and scanning electron microscopy revealed that the sodium dodecyl sulfate surfactant, sodium bicarbonate chemical foaming agent, and 1,1,2‐trifluorotrichloroethane physical foaming agent were evenly distributed and grafted onto the PAA–Na matrix. Water‐absorption, swelling rate, and packing density testing confirmed that the superabsorbent had a high water‐absorption ability and swelling rate and a lighter packing density. Furthermore, we investigated the effects of different foaming agents, including chemical and physical foaming agents, on the swelling and water‐retention capabilities of the superabsorbent hydrogels (SAHs).The results show that the combination of these foaming agents significantly improved the water‐absorbing capacity. With the help of these foaming agents, we obtained PAA–Na hydrogels without any organic solvents for posttreatment or special porogens; this is an environmentally beneficial way to prepare SAHs for hygiene and biomedical products. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44149.