Swelling–deswelling behavior of poly(acrylamide‐co‐maleic acid) hydrogels

In this study the poly(acrylamide‐co‐maleic acid) hydrogels containing small amounts of maleic acid have been synthesized, and the effect of pH, ionic strength, and nature of counterions on the equilibrium water uptake has been investigated. The incorporation of small amount of maleic acid results in the transition of swelling mechanism from Fickian to non‐Fickian. The equilibrium mass swelling has been found to increase with pH of the swelling medium while increase in ionic strength causes a decrease in the swelling. The amount of maleic acid present in the hydrogel affects the swelling behavior in rather an unusual way. With lower acid contents, the equilibrium mass swelling increases while higher concentrations of maleic acid cause a decrease in the degree of swelling. The hydrogels have been found to undergo a number of swelling–deswelling cycles when pH of the swelling medium changes from 8.0 to 2.0. Hydrogels require more time to deswell compared to the time required for swelling, which has been explained on the basis of the fact that gels follow different mechanisms for the two processes. Various swelling parameters such as equilibrium mass swelling, diffusion coefficient, intrinsic diffusion coefficient, swelling exponent, etc., have been evaluated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2782–2789, 2001     

Swelling and mechanical properties of cellulose hydrogels. I. Preparation,characterization, and swelling behavior

Cellulose hydrogels have been synthesized by reacting solutions of cellulose xanthate with different amounts of epichlorohydrin (0–24% w/w on cellulose) after which the cellulose was regenerated. The weight fraction of crystalline cellulose, determined by density measurements, decreases with the extent of chemical crosslinking and was estimated to vary between 30 and 42% for dry gels. The degree of equilibrium swelling in water of the prepared hydrogels varied between 3.05 and 6.33 g water/g dry gel (g/g). The degree of swelling decreases with increasing chemical crosslinking. As a result of the irreversible changes occurring during drying, the degree of swelling in water can be reduced down to 0.74 g/g. According to density gradient column measurements, the partial specific volume of water is 0.865 cm³/g at water contents below 0.13 g/g. It is suggested that water having this partial specific volume is the specific hydration water. At higher water contents, the partial specific volume of gel water equals the specific volume of bulk water. It is implicit in the interpretation of the density data in terms of a two state model of gel water that the crystallinity of cellulose is independent of the water content. Depending on the degree of swelling, heat treatment resulted in either an irreversible increase or decrease of the degree of swelling.     

Humic acid embedded chitosan/poly (vinyl alcohol) pH-sensitive hydrogel: Synthesis,characterization, swelling kinetic and diffusion coefficient

In the present research, humic acid (HA) embedded chitosan (CHT)-poly (vinyl alcohol) (PVA) pH-sensitive hydrogels have been synthesized using glutaraldehyde as a crosslinker. The hydrogels were characterized by Fourier-transform infrared (FT-IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) analysis. The effect of HA content, pH, and temperature on the equilibrium solvent uptake has been investigated. With higher HA contents, the equilibrium swelling capacity increases while lower concentrations of acid cause a decrease in the degree of the swelling. The hydrogels have been found to undergo a number of swelling/deswelling cycles when pH of the swelling medium changes from 1.5 to 11.0. Fick’s law, first-order, and second-order kinetic models were applied to the swelling ratio of hydrogels to explain the mass transfer mechanism and specify swelling kinetic parameters. The swelling parameters such as equilibrium swelling capacity, swelling rate constant, swelling exponent, initial swelling rate, diffusion coefficient, etc., have been evaluated.     

DSC studies on states of water in crosslinked poly(methyl methacrylate-co-n-vinyl-2-pyrrolidone) hydrogels

Mixtures of methyl methacrylate (MMA) with N-vinyl-2-pyrrolidone (VP) of various composition in the presence of fixed concentration of ethylene glycol dimethacrylate (EDMA) have been copolymerized to 100% conversion by γ-irradiation. The resultant solid xerogels were swollen in water to yield hydrogels of equilibrium water contents ranging from 15 to 76wt%. The state of water in the poly(MMA-co-VP) hydrogels has been studied by differential scanning calorimetry (DSC) and the percentages of free freezing, freezable bound and nonfreezing water were found to vary with the xerogel composition, i.e. the degree of hydration of the hydrogels. At low equilibrium swelling, most of the water exists as a non-freezing type, whereas at higher equilibrium swelling the majority of water exists as free freezing water. The maximum number of non-freezing water molecules per VP unit in the hydrogel is about 7.5.     

Swelling and mechanical properties of cellulose hydrogels. IV. Kinetics of swelling in liquid water

The swelling kinetics of cellulose hydrogels have been studied in experiments where partially dried gels were reswollen to equilibrium in liquid water. The swelling interval studied was from the dry state to 3.6 g water/g dry gel, and the temperature range was from 15 to 65°C. The experimental arrangement and the theoretical analysis was based on the unidimensional, unsteady sorption of water into a swelling sheet. The effects of sheet thickness, temperature and the initial degree of swelling were investigated. The integral sorption curves obtained were sigmoid in shape when plotted as the sorbed amount of water versus the square root of time. These anomalous, non-Fickian sorption curves were analyzed under the assumption that the anomalous behavior is due to a slow establishment of concentration equilibrium at the boundary surfaces. Each sorption curve can then be described by a combination of a mutual diffusion coefficient and a surface relaxation rate. The sigmoidicity of the sorption curves is accordingly determined by the ratio of a characteristic diffusion time to a characteristic surface relaxation time. More classical, Fickian-type behavior is then explained by an increase in this ratio. The experimental sorption curves were found to become more nearly classical as the sheet thickness, the temperature and/or the initial degree of swelling was increased. The relaxation process was found to be associated with a higher apparent activation energy than the diffusion process.     

高强度水凝胶P(HEMA-co-NEVER)溶胀特性的研究

该文研究了酚醛环氧型乙烯基酯树脂(NEVER)/甲基丙烯酸-β-羟乙酯(HEMA)共聚水凝胶的溶胀性质。NEVER质量分数的增大,加强了水凝胶内部疏水基团间的相互作用,使平衡含水量EWC从39.7%下降到15.9%,聚合物体积分数φ2从0.5440上升到0.8085。溶胀介质离子强度增大,极性增强,形成了更加紧密的低溶胀水凝胶,导致产生盐出效应,水凝胶初始溶胀速率r0从0.189g水·g-1干胶·h-1下降到0.047g水·g-1干胶·h-1,理论最大吸水量S∞也从0.395g水·g-1干胶降到0.364g水·g-1干胶。水凝胶溶胀初期表现为Fickian行为,扩展溶胀符合Schott二级动力学方程。当温度由278K升至323K,水凝胶样品H/N3的r0从0.060g水·g-1干胶·h-1升高到0.287g水·g-1干胶·h-1,但S∞却从0.559g水·g-1干胶降到0.351g水·g-1干胶。热力学研究表明,水凝胶在水中的溶胀过程是放热过程。     

Radiation Induced Superabsorbent Hydrogels. Acrylamide/Itaconic Acid Copolymers

Acrylamide/itaconic acid hydrogels in the form of rods are prepared by γ‐irradiation of ternary mixtures of acrylamide/itaconic acid/water with 2.00–5.71 kGy γ‐rays. Spectroscopic and thermal characterization of the copolymers were examined. Dynamic and equilibrium swelling behavior of superabsorbent acrylamide/itaconic acid hydrogels was investigated in water. The equilibrium swelling degree, the swelling rate coefficient, the diffusion constant, the diffusion exponent, the diffusion coefficient, and the intrinsic diffusion coefficient of acrylamide/itaconic acid hydrogels were calculated. The superabsorbent hydrogels have shown mass swelling capabilities in the range 650–700% (for polyacrylamide hydrogels) and 900–2100% (for acrylamide‐itaconic acid hydrogels). Diffusion type of the hydrogels is non‐Fickian diffusion. The equilibrium swelling studies were used to determine important parameters of the crosslinked structure of the hydrogels, including the average molar mass between crosslinks and crosslink density.     

Superabsorbent composite XXII: Effects of modified sepiolite on water absorbency and swelling behavior of chitosan‐g‐poly(acrylic acid)/sepiolite superabsorbent composite

In this work, a series of chitosan‐g‐poly(acrylic acid)/sepiolite (CTS‐g‐PAA/ST) superabsorbent composites containing raw sepiolite, acid‐activated sepiolite, and cation‐exchanged sepiolite were synthesized by free‐radical graft polymerization in aqueous solution, using N,N′‐methylenebisacrylamide as a crosslinker and ammonium persulfate as an initiator. The effects of raw sepiolite, acid‐activated sepiolite, and cation‐exchanged sepoilite on equilibrium water absorbency, swelling rate, and swelling behavior in different pH value solution of superabsorbent composites were systematically investigated. The results from FTIR spectra showed that chitosan and sepiolite participated in graft polymerization reaction with acrylic acid. The introduction of acid‐activated and cation‐exchanged sepiolite into chitosan‐g‐poly(acrylic acid) polymeric network could improve water absorbency and swelling rate compared with that of the raw sepiolite. All prepared samples have similar swelling behavior in different pH solutions and the equilibrium water absorbencies of samples keep roughly constant in the pH range from 4 to 12. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Swelling and mechanical properties of cellulose hydrogels. III. Temperature effects on the swelling and compliance levels studied by dilatometry and 1H-NMR spectroscopy

The temperature dependence of the swelling and the creep compliance has been investigated for swollen isotropic cellulose hydrogels. The measurements were performed in a high precision type of dilatometer between 5 and 65°C. The thermal expansion of the gels in silicone oil (closed system) and the temperature dependence of the equilibrium swelling in water (open system) were studied. The influence of compressive stress in these experiments was also evaluated. The swelling level in equilibrium with water diminishes slightly with increasing temperature due to migration of water from the gel phase to the surrounding water phase. A secondary transition was found at 35°C where the temperature dependence of the swelling level is changed. When measured at constant gel composition the creep compliance of a highly swollen gel decreases with increasing temperature. The decrease is not, however, large enough for entropy elasticity to dominate over energetic elasticity. The energetic contribution f_U/f was determined to be 0.61 for a gel swollen to 3.9 g water/g dry gel (g/g) and 1.24 for a gel swollen to 1.05 g/g. The swelling and compliance data have also been analyzed in terms of a model where the gels are assumed to behave as a filler-reinforced rubbery network. The amorphous parts of the hydrogels are thus assumed to be described by the statistical theory for polymeric networks. In proton magnetic resonance studies of a gel swollen to 4.4 g/g the spin-lattice relaxation time T₁ was determined to be considerably longer than the spin-spin relaxation time T₂. T₂ has a maximum at 30°C. This maximum marks the onset (on the NMR time scale) of an exchange process between two types of proton species. These species are suggested to be specific hydration water and free gel water, respectively.     

Swelling properties of copolymeric hydrogels prepared by gamma irradiation

Crosslinked xerogels in the form of rods have been prepared to > 99.9% conversion by the γ-ray initiated copolymerization in vacuo ob n-butyl acrylate (BA) with N-vinyl-2-pyrrolidone (VP) in the presence and absence of a crosslinking agent. Thin (about 0.5 mm) disks were swollen in water at 21°C, the final equilibrium swelling being the true water content only for copolymers of high BA content. At low-medium BA content, swelling was accompanied by the release of a small water-soluble fraction, leading to a slightly reduced apparent value for the water content. True water contents could be obtained more rapidly by prior Soxhlet extraction with boiling water. For xerogels containing > about 75% VP by weight, the resultant hydrogels had water contents > about 80%. The influences of dose, dose rate copolymer composition, and concentration of crosslinker on the water content, sol fraction, and extension ratio were investigated. A high level of reproducibility in properties of the hydrogels was obtained by numerous replicate experiments. The feasibility of scaling up the preparation to produce long (60 cm) xerogel rods of uniform properties has been demonstrated. For these copolymers, swelling in several common organic solvents generally exceeds that in water.     

Swelling behaviour of rubber vulcanizates: 2. Effects of tensile strain on swelling

The effects of tensile strain on the swelling behaviour of acrylonitrile-butadiene copolymer rubber vulcanizates were studied by real-time pulsed nuclear magnetic resonance (n.m.r.) measurements and volume swelling measurements at equilibrium. It was shown that tensile strain causes an increase of the initial swelling rate evaluated by n.m.r. measurements and of the swelling ratio of the rubber matrix at equilibrium. This behaviour was discussed qualitatively in terms of the molecular mobility of the network chains on the basis of Treloar's theory for swelling under deformation. It was suggested that the presence of reinforcing fillers in the rubber matrix exerts two noticeable effects on swelling under deformation: (1) a transient effect through some oriented structure induced by stretching, which restricts the increase of the swelling rate; (2) a strain amplification effect, which causes the increase of the average local strain in the rubber matrix and enhances the swelling ratio more remarkably proportional to the filler concentration as compared with that of the unfilled system.     

Adsorption of anionic dyes on a reversibly swelling cationic superabsorbent polymer

A cationic monomer [2‐(methacryloyloxy)ethyl]trimethylammonium chloride was polymerized using N,N′‐methylenebisacrylamide as the crosslinker to obtain a cationic superabsorbent polymer (SAP). This SAP was characterized by Fourier transform‐infrared spectroscopy, and the equilibrium swelling capacity was determined by swelling in water. The SAP was subjected to cyclic swelling/deswelling in water and NaCl solution. The conductivity of the swelling medium was monitored during the swelling/deswelling and was related to the swelling/deswelling characteristics of the SAP. The adsorption of five anionic dyes of different classes on the SAP was carried out and was found to follow the first‐order kinetics. The Langmuir adsorption isotherms were found to fit the equilibrium adsorption data. The dye adsorption capacity of the SAP synthesized in this study was higher than that obtained for other hydrogels reported in the literature. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Hydroxyalkyl methacrylates: hydrogel formation based on the radical copolymerization of 2-hydroxyethylmethacrylate and 2,3-dihydroxypropylmethacrylate

The synthesis of hydrogels by radical copolymerization of mixtures of 2,3-dihydroxypropylmethacrylate (DHPM) and 2-hydroxyethylmethacrylate (HEMA) has been performed using ethylene dimethacrylate (DME) as crosslinking agent, in the presence of 2,2′-azobisisobutyronitrile (AIBN) or of a redox system as initiator. The hydrophilicity of the obtained materials increases as the proportion of 2,3-dihydroxypropylmethacrylate increases. The maximum swelling degree of poly (HEMA) hydrogels is limited by thermodynamic effects whereas the equilibrium swelling degree of materials with high DHPM unit content is structure dependent. The gels obtained have been characterized by their equilibrium swelling degrees in water or in organic solvents and by the determination of their elastic modulus by uniaxial compression measurements. All the materials are of great interest as potential biomaterials especially for soft contact lens manufacture.     

Optimization of hybrid oil/water-swelling ethylene/propylene/diene monomer compounds

Hybrid swell packers are rubber products capable of swelling in (saline) water and hydrocarbon oils, by exploiting the hydrocarbon diffusion properties of an nonpolar elastomer with the osmotic swelling of a water absorbent (WA). In this study, a variety of modified natural and synthetic WAs and mixtures thereof have been screened in a rubber compound with respect to swelling in decalin, demineralized, and saline water, respectively. We aimed at achieving high and fast swelling in all used media. Mixing of the various WAs in an ethylene/propylene/diene monomer (EPDM)/carbon black masterbatch at an addition level of 75 phr did not pose any problems, although it was found that the WAs did affect the sulfur vulcanization. Decalin swelling of the vulcanizates is very fast, reaching high equilibrium swelling within 1.5 days. Swelling in demineralized or saline water is much slower, with up to 300% swelling in water and up to 100% swelling in saline water. Significant leaching of the organic WA from the EPDM vulcanizates to the water phase occurs. Some combinations of WAs show synergetic effects for swelling in water. To our knowledge, such a systematic study of hybrid swelling elastomers constitutes a novelty in the open literature.     

Synthesis and characterization of novel poly(acrylamide‐co‐acrylic acid‐co‐2‐hydroxy ethyl actylate) hydrogels: Mathematical approach in investigation of swelling kinetics and diffusion models

In this study, a series of poly(acrylamide‐co‐acrylic acid‐co‐2‐hydroxy ethyl actylate) [AM‐co‐AA‐co‐HEA] hydrogels have been synthesized by varying the acrylic acid (AA)content over eightfold in feed in the range of 33.34–93.76% by keeping other monomer constant. These hydrogels were characterized by FTIR, SEM analysis, elemental analysis, residual acrylic acid analysis, network parameters, and dynamic swelling behavior. The swelling study showed that equilibrium swelling ratio was nonlinearly increased with increasing AA content. Interestingly, the equilibrium swelling ratio decreased from 53.42 to 48.52 for 75–80% AA content hydrogel. The swelling data were found to satisfactorily fit Fick's second law, demonstrating that diffusion rate of water uptake was primarily Fickian. From model fitting, it was observed that early model was applicable for first 30% water absorption, and late model was applicable for latter 70% water absorption for increasing AA content from 33.34–90.90%. For 93.76% AA, early‐time model was extended up to first 50% of water absorption and late model was contracted for latter 50% water absorption, indicating that excessive AA content affects the applicability range of early‐time and late‐time diffusion models for water absorption. Etters model was best applicable to all type of hydrogels and followed over all swelling range. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Poly(hydroxamic acid) hydrogels from poly(acrylamide): preparation and characterization

Summary Poly(hydroxamic acid) (PHA) hydrogels from crosslinked poly(acrylamide) (PAAm) hydrogels containing different type crosslinkers such as such as N, N'-methylene bisacrylamide (N) and ethyleneglycol dimethacrylate (E). The effect of crosslinkers on some physical properties was investigated. The structure of hydrogels was characterized spectroscopically and thermally. The swelling parameters of PHA in distilled water, such as percent equilibrium swelling (S_eq%), initial swelling rate (r_o), maximum equilibrium swelling (S_max), diffusional exponent (n), network parameter (k) and diffusion coefficient (D) were calculated. It has been found that the ionogenity of PHA and the structure of the used crosslinkers during the synthesis have been effective on the swelling behaviour of the hydrogels. Received: 12 February 2001 / Revised version: 30 June 2001 / Accepted: 11 July 2001     

Swelling and mechanical properties of cellulose hydrogels. II. The relation between the degree of swelling and the creep compliance

The mechanical properties of swollen cellulose hydrogels have been studied. The degree of swelling of the gels was varied between 0.75 and 6.3 g water/g dry gel (g/g) by partial drying followed by reswelling in water. Creep rate was measured in uniaxial compression in the time interval 15–900 s for gels in equilibrium with water. Isochronous relations between stress and reversible strain were found to be linear, and creep compliance was calculated from the slopes. Both the creep compliance and the creep rate increase with an increased degree of swelling. General observations, such as the high strain limit of linearity in the stress–strain curves and the magnitude of the creep compliance, indicate similarities between swollen cellulose gels and rubbery networks. It is therefore assumed that the statistical theories for swollen networks can describe the amorphous matrix of the gels. In order to obtain creep compliance values representative of the amorphous matrix, the experimental values were corrected for the presence of crystalline regions. It is also suggested that non-load-bearing microvoids are present at high swelling levels. According to calculations based on the theory, the network chains of the amorphous regions in a gel swollen to 2.4 g/g contain about 11 monomer units and the Flory-Huggins interaction parameter χ equals 0.2.     

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.     

Swelling behavior and structural characteristics of polyvinyl alcohol/montmorillonite nanocomposite hydrogels

A series of nanocomposite hydrogels based on polyvinyl alcohol containing 0–10 wt % of the organically modified montmorillonite clay were prepared by freezing‐thawing cyclic method. The morphology of the nanocomposite hydrogels was observed by the scanning electron microscopy technique. The structural properties were determined by measuring the network mesh size, crosslinking density, and average molecular weight of polymer chains between crosslinks. The swelling behavior and the effect of swelling medium temperature on the swelling kinetics and characteristics of the nanocomposite hydrogels were also investigated. The results showed that two structural characteristics i.e., network mesh size and average molecular weight of polymer chains between crosslinks have inverse dependence on the clay loading level in the nanocomposite hydrogel, while crosslinking density shows completely direct dependence. Swelling measurements demonstrated a linear relation between the degree of swelling and the square root of immersion time at all swelling medium temperatures. The results indicated that the swelling characteristics of the nanocomposite hydrogels including the equilibrium degree of weight and volume swelling and the equilibrium water content were decreased by increasing the quantity of the clay incorporated into the hydrogel as well as by decreasing the temperature of swelling medium. While, the time required to reach to the equilibrium condition, as another swelling characteristic of the hydrogels, exhibited a completely opposite behavior. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Characterization and determination of swelling and diffusion characteristics of poly(n‐vinyl‐2‐pyrrolidone) hydrogels in water

Hydrogels in the form of rods with varying crosslink densities and three‐dimensional network structures were prepared from Poly(N‐vinyl‐2‐pyrrolidone) (PVP)/water and PVP/water/persulfate systems by irradiation with γ rays at ambient temperature. Average molecular weights between crosslinks, percent swelling, swelling equilibrium values, diffusion/swelling characteristics (i.e., the structure of network constant, the type of diffusion, the initial swelling rate, swelling rate constant), and equilibrium water content were evaluated for both hydrogel systems. Water diffusion to the hydrogel is a non‐Fickian type diffusion and diffusion coefficients vary from 6.56 × 10⁻⁷ to 2.51 × 10⁻⁷cm²min⁻¹ for PVP and 6.09 × 10⁻⁷ to 2.14 × 10⁻⁷ cm²min⁻¹ for PVP/persulfate hydrogel systems. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 994–1000, 2000