Superabsorbent hydrogel composites

A new cost‐effective approach to enhance gel strength of superabsorbent hydrogels was invented. Superabsorbent hydrogel composites (SHCs) were prepared through an optimized rapid solution polymerization of concentrated partially neutralized acrylic acid in the presence of a crosslinking agent under normal atmospheric conditions. Kaolin was used as an inorganic component in the polymerization process to strengthen the hydrogel products. FT‐IR spectroscopy was used to confirm grafting of acrylic chains on to the surface of kaolin particles. Morphology of the products was studied by scanning electron microscopy (SEM). Compared with the kaolin‐free hydrogel (control), kaolin caused a reduced equilibrium swelling and swelling rate as low as 17–31% and 19–29%, respectively. Kaolin, however, resulted in enhanced gel strength as high as 21–35% compared to the control. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to study thermal properties of the composites. The SHCs exhibited higher thermal stability than the control. Meanwhile, changes in certain thermal transitions reconfirmed the chemical interaction of the acrylic chain with kaolin. These thermostabilized strengthened superabsorbent hydrogels may be considered as good candidates for agricultural application to retain more water under soil pressure. Copyright © 2003 John Wiley & Sons, Ltd.     

Swelling Kinetics of Microgels Embedded in a Polyacrylamide Hydrogel Matrix

Composite hydrogels—macroscopic hydrogels with embedded microgel particles—are expected to respond to external stimuli quickly because microgels swell much faster than bulky gels. In this work, the kinetics of the pH‐induced swelling of a composite hydrogel are studied using turbidity measurements. The embedded microgel is a pH‐ and thermosensitive poly(N‐isopropylacrylamide‐co‐acrylic acid) microgel and the hydrogel matrix is polyacrylamide. A rapid pH‐induced swelling of the embedded microgel particles is observed, confirming that composite hydrogels respond faster than ordinary hydrogels. However, compared with the free microgels, the swelling of the embedded microgel is much slower. Diffusion of OH^? into the composite hydrogel film is identified as the main reason for the slow swelling of the embedded microgel particles, as the time of the pH‐induced swelling of this film is comparable to that of OH^? diffusion into the film. The composition of the hydrogel matrix does not significantly change the characteristic swelling time of the composite hydrogel film. However, the swelling pattern of the film changes with composition of the hydrogel matrix.     

Radiation Synthesis of Superabsorbent Hydrogels Based on Chitosan and Acrylic Acid for Controlled Drug Release

Superabsorbent hydrogels based on the natural polymer chitosan and acrylic acid (CS/AAc) was prepared using ⁶⁰Co gamma radiation as a source of initiation and crosslinking. The factors, which affect the preparation of CS/AAc hydrogels such as irradiation dose, CS/AAc ratios, and acrylic acid monomer concentrations, to get the best optimum conditions, were investigated. The kinetic studies of the swelling of CS/AAc hydrogel showed that it follows a Fickian type of water diffusion. The Fickian constant value ‘n’ was more than 0.5 with a high swelling capacity of 300 g/g as superabsorbent hydrogel. In addition, the suitability of CS/AAc hydrogel as carrier material for the drug Chlortetracycline-HCl has been investigated by adsorption isotherm studies. The performance of drug release from hydrogel systems, influenced by acrylic acid ratio and the effect of pH of the medium was studied.     

Modified carrageenan. 5. Preparation,swelling behavior,salt‐ and pH‐sensitivity of partially hydrolyzed crosslinked carrageenan‐graft‐polymethacrylamide superabsorbent hydrogel

The polysaccharide, kappa‐carrageenan (κC) was chemically modified to achieve a novel superabsorbent hydrogel via graft copolymerization of methacrylamide (MAM) onto the substrate followed by alkaline hydrolysis. Ammonium persulfate (APS) and N,N′‐methylene bisacrylamide (MBA) were used as a free‐radical initiator and a crosslinker, respectively. The saponification reaction was carried out using sodium hydroxide aqueous solution. Either κC‐g‐PMAM or hydrolyzed κC‐g‐PMAM (PMAM: polymethacrylamide) was characterized by FT‐IR spectroscopy. The effect of grafting variables (i.e. concentration of MBA, MAM, and APS) and alkaline hydrolysis conditions (i.e. NaOH concentration, hydrolysis time and temperature) were systematically optimized to achieve a hydrogel with swelling capacity as high as possible. The swelling capacity of these hydrogels was also measured in various salt solutions. Results indicated that the swelling ratios decreased with an increase in the ionic strength of the salt solutions. This behavior can be attributed to charge screening effect for monovalent cations, as well as ionic crosslinking for multivalent cations. Absorbency of superabsorbing hydrogels was examined in buffer solutions with pH range 1–13. Also, the pH reversibility and on–off switching behavior, at pH values 3.0 and 8.0, makes the synthesized hydrogels good candidates for controlled delivery of bioactive agents. Finally, swelling kinetics in distilled water and various salt solutions was preliminary investigated. Results showed that the swelling in water was faster than in saline solutions. Copyright © 2004 John Wiley & Sons, Ltd.     

新型半纤维素基磁性水凝胶的制备及性能

采用H₂O₂-Vc氧化还原体系引发半纤维素衍生物,以表面修饰的Fe₃O₄粒子作为磁性组分,利用接枝共聚方法制备了新型半纤维素基磁性水凝胶. 分别用傅里叶变换红外(FTIR)光谱、X射线光电子能谱(XPS)和扫描电子显微镜(SEM)对水凝胶的结构及形貌进行了表征,利用X射线衍射(XRD)和振动样品磁强计(VSM)对水凝胶的晶型结构及磁性能进行了分析,发现Fe₃O₄粒子均匀分散在凝胶网络中,半纤维素基磁性水凝胶表现出良好的顺磁性. 考察了丙烯酸/半纤维素比例、Fe₃O₄粒子含量及交联剂用量对水凝胶溶胀性能的影响,并探讨了该水凝胶的溶胀机理,它在pH 8 缓冲溶液中的溶胀较好符合Fickian 和Schott 动力学模型. 通过SEM和溶胀性能分析表明,随着pH值的升高水凝胶的孔径增大,水凝胶的溶胀率逐渐增大. 制备的水凝胶被用于溶菌酶吸附研究,结果表明磁性凝胶的吸附量大于非磁性水凝胶,水凝胶的吸附行为符合Freundlich 和Temkin 等温模型.     

Modified chitosan III,superabsorbency, salt‐ and pH‐sensitivity of smart ampholytic hydrogels from chitosan‐g‐PAN

Polyacrylonitrile (PAN) grafted chitosan was prepared by ceric‐initiated graft polymerization of acrylonitrile onto chitosan in a homogenous medium. The copolymer chitosan‐g‐PAN product was then hydrolyzed to yield a novel smart hydrogel (H‐chitoPAN) with superabsorbing properties. The influence of add‐on values as well as temperature and time of hydrolysis of the initial chitosan‐g‐PAN on swelling behavior of the hydrogel was evaluated in water and various salt solutions. The swelling kinetics of the superabsorbing hydrogel was studied as well. The hydrogels exhibited ampholytic and pH‐sensitivity characteristics. Several sharp swelling changes were observed in lieu of pH variations in a wide range (pH 2–13). The swelling variations were explained according to swelling theory based on the hydrogel chemical structure. Superabsorbency, pH‐ and salt‐sensitivity of the chitosan‐based hydrogel was briefly compared with the classical starch‐based superabsorbent, H‐SPAN. The pH‐reversibility and on–off switching behavior of the intelligent H‐chitoPAN hydrogels makes them good candidates for considering as potential drug carries. Copyright © 2004 John Wiley & Sons, Ltd.     

Synthesis of Carboxymethyl Cellulose Based Drug Carrier Hydrogel Using Ionizing Radiation for Possible Use as Site Specific Delivery System

A unique natural polymer based colon specific drug carrier was prepared from carboxymethyl cellulose (CMC) and acrylic acid (AAc) in aqueous solution employing γ‐radiation induced copolymerization and crosslinking. The effect of preparation conditions such as the natural polymer content and irradiation dose on gelation process was investigated. The swelling behavior of the prepared hydrogels was characterized by investigating the time and pH dependent swelling of the (CMC/AAc) hydrogels of different CMC content. The effects of the hydrogel composition and pH of the swelling medium on the swelling indices were estimated. The results show that the increment in the CMC content in the feed solution enhances the gelation process. The results also show the dependence of the swelling indices on both hydrogel composition and pH value of the swelling medium. To evaluate the ability of the prepared hydrogel to be used as a colon‐specific drug carrier, the release profile of theophylline was studied as a function of time at pH 1 and pH 7.     

pH敏感的聚(L-谷氨酸)-交联聚丙烯酸水凝胶的合成与性能研究

通过在聚L-谷氨酸侧链部分接枝甲基丙烯酸2-羟乙酯得到含有双键的聚(L-谷氨酸),将其与丙烯酸共聚得到由聚(L-谷氨酸)侧链接枝并交联聚丙烯酸的pH敏感水凝胶.研究水凝胶在不同pH的缓冲溶液中的溶胀性、溶胀动力学,并通过SEM观察水凝胶的微观结构.结果表明,水凝胶在低pH环境下的溶胀率明显低于高pH环境中的溶胀率,不同...     

Modified carrageenan 3. Synthesis of a novel polysaccharide-based superabsorbent hydrogel via graft copolymerization of acrylic acid onto kappa-carrageenan in air

A novel biopolymer-based superabsorbent hydrogel was synthesized through chemically crosslinking graft copolymerization of acrylic acid (AA) onto kappa-carrageenan (κC), in the presence of a crosslinking agent and a free radical initiator. A proposed mechanism for κC-g-polyacrylic acid was suggested and the affecting variables onto graft polymerization (i.e. the crosslinker, the monomer and the initiator concentration, the neutralization percent and reaction temperature) were systematically optimized to achieve a hydrogel with swelling capacity as high as possible. Maximum water absorbency of the optimized final product was found to be 789 g/g. The swelling capacity of the synthesized hydrogels was also measured in various salt solutions. The time-temperature profile of the polymerization reaction, in order to investigate the effect of molecular oxygen was conducted in terms of the absence and presence of the atmospheric oxygen. The overall activation energy (E_a) of the graft polymerization reaction was found to be 2.93 KJ/mol.     

Gamma irradiation mediated synthesis of a new superabsorbent hydrogel network based on poly(acrylic acid) grafted onto salep

A novel and smart biopolymer-based hydrogel was synthesized by graft copolymerization of acrylic acid onto the salep backbones. The new superabsorbent hydrogel was synthesized via simultaneous crosslinking and graft copolymerization of acrylic acid (AA) monomer onto salep backbones using radiochemical methods. In radiochemical methods, γ-rays as initiator, energy source and crosslinker are applied. In our experiments, the effects of reaction variables such as relative contents of salep and AA, as well as γ-rays total dose were examined. According to water absorbency of the entitled network, the best synthesis condition is reported. FTIR spectroscopy, SEM photograph and TGA analysis were used for confirming the structure of the final product and a mechanism for superabsorbent hydrogel formation is also suggested. Furthermore, in this research, several factors affecting the swelling behavior of hydrogel including pH of medium, sensitivity to the salt solution and mixture of solvents were studied.     

Synthesis and Characterization of Hydrogels Based on Gamma Radiation Copolymerization of N‐isopropylacrylamide and Ethylene Glycol Dimethacrylate Monomers

Hydrogels based essentially on N‐isopropylacrylamide (NIPAAm) and different ratios of ethylene glycol dimethacrylate (EGDMA) monomer were synthesized by gamma radiation copolymerization. The thermal decomposition behavior of NIPAAm/EGDMA hydrogels was determined by thermogravimetric analysis (TGA). The effect of temperature and pH on the swelling behavior was also studied. The results showed that the ratio of EGDMA in the comonomer feeding solution has a great effect on the yield product, gel fraction and water content in the final hydrogel. In this regard, it was observed that the increase of EGDMA ratio decreased these properties. The TGA study showed that all the compositions of NIPAAm/EGDMA hydrogels displayed higher thermal stability than the hydrogel based on pure PNIPAAm hydrogel. The swelling kinetics in water showed that pure PNIPAAm and NIPAAm/EGDMA hydrogels reached equilibrium after 6 h. However, NIPAAm/EGDMA hydrogels show swelling in water lower than pure PNIPAAm. The results showed that the swelling character of pure PNIPAAm and NIPAAm/EGDMA hydrogels was affected by the change in temperature within the temperature range 25–40°C, and showed a reversible change in swelling in the pH range 4–7 depending on composition.     

Modified chitosan 4. Superabsorbent hydrogels from poly(acrylic acid-co-acrylamide) grafted chitosan with salt- and pH-responsiveness properties

Graft copolymerization of mixtures of acrylic acid (AA) and acrylamide (AAm) onto chitosan was carried out by using potassium persulfate (KPS) as a free radical initiator in the presence of methylenebisacrylamide (MBA) as a crosslinker. The effect of reaction variables, such as MBA concentration and AA/AAm ratio on the water absorbency capacity have been investigated. The polymer structures were confirmed by FTIR spectroscopy. Water absorbencies were compared for the hydrogels before and after alkaline hydrolysis. In the non-hydrolyzed hydrogel, enhanced water absorbency was obtained with increasing AA in monomer feed. However, after saponification, the sample with high AAm ratio exhibited more water absorbency. These behaviors were discussed according to structural parameters. The swelling kinetics of the superabsorbing hydrogels was studied as well. The hydrogels exhibited ampholytic and reversible pH-responsiveness characteristics. The swelling variations were explained according to swelling theory based on the hydrogel chemical structure. The hydrogels exhibited salt-sensitivity and cation exchange properties. The pH-reversibility and on-off switching properties of the hydrogels make the intelligent polymers as good candidates for considering as potential carriers for bioactive agents, e.g. drugs.     

Synthesis of dual responsive cyclotriphosphazene‐based IPN hydrogels for controlled release of chemotherapeutic agent

Dual responsive cyclotriphosphazene (CTP)‐based hydrogels have been synthesized for a controlled release of FU, a hydrophilic drugs. These hydrogels composed of mono (methacryloyl‐2‐ethoxy)‐pentakis(N¹,N¹‐dimethylpropane‐1,3‐diamino)‐cyclotriphosphazene (HEMA (DMPDA)₅CP), acryl amide and pectin were synthesized by free radical polymerization method using methylenebisacrylamide cross linker. The CTP hydrogels were characterized to understand the structure, drug nature in the network and morphology by FTIR, DSC, XRD and SEM, respectively. In this paper, the swelling (dynamic and equilibrium) properties of cyclotriphosphazene hydrogels were investigated, showing dual (pH and thermo) responsiveness and large variation in the swelling capacity. Based on these results the structural parameters of the hydrogel networks such as the average molecular weight between cross‐links (M_c) and polymer–solvent interaction parameter (χ) were determined. The CTP hydrogels has high FU loading efficiency 65 ± 0.5. In‐vitro FU release of these hydrogels was controlled for about 24 hr also hydrogel showed a distinct initial burst. The CTP hydrogels are bearing both hydrophilic groups of pectin and hydrophobic groups of CTP exhibited dual responsive behaviors with pH and temperature. Copyright © 2015 John Wiley & Sons, Ltd.     

Radiation synthesis of superabsorbent poly(acrylic acid)–carrageenan hydrogels

A series of superabsorbent hydrogels were prepared from carrageenan and partially neutralized acrylic acid by gamma irradiation at room temperature. The gel fraction, swelling kinetics and the equilibrium degree of swelling (EDS) of the hydrogels were studied. It was found that the incorporation of even 1% carrageenan (sodium salt) increases the EDS of the hydrogels from 320 to 800 g/g. Thermal analysis were carried out to determine the amount of free water and bound water in the hydrogels. Under optimum conditions, poly(acrylic acid)–carrageenan hydrogels with high gel fraction (80%) and very high EDS (800 g/g) were prepared gamma radiolytically from aqueous solution containing 15% partially neutralized acrylic acid and 1–5% carrageenan. The hydrogels were also found to be sensitive to the pH and the ionic strength of the medium.     

Synthesis and Evaluation of Biocompatible pH‐Sensitive Hydrogels as Colon‐Specific Drug Delivery Systems

Because of the growing importance of pH‐sensitive hydrogels as drug delivery systems, biocompatible copolymeric hydrogels based N‐vinyl‐2‐pyrrolidinone (NVP) and methacrylic acid (MAA) were designed and synthesized. These hydrogels were investigated for oral drug delivery. Radical copolymerizations of N‐vinyl‐2‐pyrrolidinone (NVP) and methacrylic acid (MAA) with the various ratios of cross‐linking agent were carried out at 70 °C. Azabisisobutyronitrile (AIBN) was the free‐radical initiator employed and Cubane‐1,4‐dicarboxylic acid (CDA) linked to two 2‐hydroxyethyl methacrylate (HEMA) group was the crosslinking agent (CA) used for hydrogel preparations. The hydrogels were characterized by differential scanning calorimetry and FT‐IR. Equilibrium swelling studies were carried out in enzyme‐free simulated gastric and intestinal fluids (SGF and SIF, respectively). A model drug, olsalazine [3,3′‐azobis (6‐hydroxy benzoic acid)] (OSZ) as an azo derivative of 5‐aminosalicylic acid (5‐ASA), was entrapped in these gels and the in‐vitro release profiles were established separately in both enzyme‐free SGF and SIF. The drug‐release profiles indicated that the amount of drug released depended on the degree of swelling. The swelling was modulated by the amount of crosslinking of the polymer bonded drug (PBDs) prepared. Based on the great difference in hydrolysis rates at pH 1 and 7.4, these pH‐sensitive hydrogels appear to be good candidates for colon‐specific drug delivery.     

Phase Transition of Acrylamide‐Based Polyampholyte Gels in Water

The swelling behavior of acrylamide (AAm)–based polyampholyte hydrogels in water and in aqueous salt (NaCl) solutions was investigated. [(Methacrylamido)propyl]trimethyl‐ammonium chloride (MAPTAC) and acrylic acid (AAc) were used as the ionic comonomer in the hydrogel preparation. Three sets of hydrogels containing 70 mol% AAm and 30 mol% ionic comonomers of varying mole ratios were prepared. The variations of the hydrogel volume in response to changes in pH, and salt concentration were measured. As pH increases from 1, the hydrogel volume V _eq in water first increases and reaches a maximum value at a certain pH. Then, it decreases again with a further increase in pH and attains a minimum value around the isoelectric point (IEP). After passing the collapsed plateau region, the gel reswells again up to pH=7.1. The reswelling of the collapsed gels containing 10 and 4% MAPTAC occurs as a first‐order phase transition at pH=5.85 and 4.35, respectively, while the hydrogel with 1% MAPTAC reswells continuously beyond its IEP. Depending on pH of the solution, the hydrogels immersed in salt solutions exhibit typical polyelectrolyte or antipolyelectrolye behavior. The experimental swelling data were compared with the predictions of the Flory‐Rehner theory of swelling equilibrium including the ideal Donnan equilibria. It was shown that the equilibrium swelling theory qualitatively predicts the experimental behavior of polyampholyte hydrogels.     

Collagen-based highly porous hydrogel without any porogen: Synthesis and characteristics

In this contribution we have developed a collagen-based highly porous hydrogel by neutralizing the grafted poly(acrylamide-co-acrylic acid) after gel formation. Preparation of the hydrogels involved free radical polymerization of a combination of hydrolyzed collagen, acrylic acid (AA), acrylamide (AAm) and distilled water, in appropriate amounts and contained a crosslinking agent called N,N′-methylene bisacrylamide (MBA). The chemical structure of the hydrogels was characterized by means of FTIR spectroscopy, DSC and TGA thermal methods. Morphology of the samples was examined by scanning electron microscopy (SEM). Systematically, the certain variables of the graft copolymerization were optimized to achieve maximum swelling capacity. The absorbency under load (AUL) and centrifuge retention capacity (CRC) were measured. The swelling ratio in various salt solutions was also determined and additionally, the swelling of hydrogels was measured in solutions with pH ranged 1-13. The synthesized hydrogel exhibited a pH-responsiveness character so that a swelling-collapsing pulsatile behavior was recorded at pH 2 and 8.     

Synthesis and characterization of κ‐carrageenan/poly(N,N‐diethylacrylamide) semi‐interpenetrating polymer network hydrogels with rapid response to temperature

In this study, a novel classical thermo‐ and salt‐sensitive semi‐interpenetrating polymer network (semi‐IPN) hydrogel composed of poly(N,N‐diethylacrylamide) (PDEAm) and κ‐carrageenan (KC) was synthesized by free radical polymerization. The structure of the hydrogels was studied by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). FTIR and SEM revealed that the semi‐IPN hydrogels possessed the structure of H‐bonds and larger number of pores in the network. Compared to the PDEAm hydrogel, the prepared semi‐IPN hydrogels exhibited a much faster response rate to temperature changes and had larger equilibrium swelling ratios at temperatures below the lower critical solution temperature (LCST). The salt‐sensitive behavior of the semi‐IPN hydrogels was dependent on the content of KC. In addition, during the reswelling process, semi‐IPN hydrogels showed a non‐sigmoidal swelling pattern. Copyright © 2008 John Wiley & Sons, Ltd.     

Sucrose diacrylate: A unique chemically and biologically degradable crosslinker for polymeric hydrogels

A series of degradable hydrogels based on different vinyl monomers such as acrylamide, sucrose-1′-acrylate, and acrylic acid were synthesized using sucrose-6,1′-diacrylate (SDA) as a crosslinking agent. SDA was prepared by enzymatic transesterification of vinyl acrylate with sucrose in pyridine. Base catalyzed hydrolysis of SDA in aqueous solution was studied as a function of pH. As expected, hydrolysis of SDA was faster at higher pHs such that poly(acrylamide), poly(sucrose 1′-acrylate), and poly(acrylic acid) hydrogels underwent substantial degradation at and above pH 7, 9, and 13, respectively. The degradation was characterized by changes in the swelling ratios of the hydrogels indicating breakage of the crosslinking agent. Degradation of the hydrogels at their chemically stable pHs was studied in presence of enzymes. Enzymes, including pepsin and a fungal Lipase, were able to degrade the poly(acrylamide) hydrogel at pH 4 and 5, respectively. Poly(acrylic acid) hydrogel was degraded in presence of a fungal protease at pH 7.8. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2221–2229, 1997     

Synthesis,characterization, and swelling behaviors of chitosan‐g‐poly(acrylic acid)/poly(vinyl alcohol) semi‐IPN superabsorbent hydrogels

A series of granulated semi‐interpenetrating polymer network (semi‐IPN) superabsorbent hydrogels composed of chitosan‐g‐poly(acrylic acid) (CTS‐g‐PAA) and poly(vinyl alcohol) (PVA) were prepared by solution polymerization using ammonium persulfate (APS) as an initiator and N,N′‐methylenebisacrylamide (MBA) as a crosslinker. The effects of reaction conditions such as the concentration of MBA, the weight ratio of AA to CTS, and the content of PVA on water absorbency were investigated. Infrared (IR) spectra and differential scanning calorimetry (DSC) analyses confirmed that AA had been grafted onto CTS backbone, and PVA semi‐interpenetrating into CTS‐g‐PAA networks. SEM analyses indicated that CTS‐g‐PAA/PVA has improved porous surface and PVA was uniformly dispersed in CTS‐g‐PAA network. The semi‐IPN hydrogel containing 10 wt% PVA shows the highest water absorbency of 353 and 53 g g^?1 in distilled water and 0.9 wt% NaCl solution, respectively. Swelling behaviors revealed that the introduction of PVA could improve the swelling rate and enhance the pH stability of the superabsorbent hydrogel. Copyright © 2009 John Wiley & Sons, Ltd.