Dynamic mechanical properties and swelling of UV‐photopolymerized anionic hydrogels

The tensile dynamic mechanical properties and weight degree of swelling for anionic 2‐hydroxyethyl methacrylate‐co‐acrylic acid hydrogels were observed. Fabrication parameters examined were UV‐photopolymerization exposure time, UV‐photopolymerization intensity, and weight percentage crosslinker. The environmental conditions tested were electrolyte compositions of 0.5 and 0.05 M potassium hydroxide under applied frequencies of 0.1, 1, or 10 Hz. The overall maximum and minimum storage modulus was 1.83 ± 0.18 MPa and 68.5 ± 7.2 kPa, respectively, loss modulus was 432 ± 63 and 7.67 ± 3.22 kPa, respectively, and weight degree of swelling was 14.27 ± 1.27 and 1.95 ± 0.33, respectively. The morphology of fabricated hydrogels was examined using scanning electron microscopy showing a range of porous structures over the fabrication and environmental conditions examined, accounting for the variation in mechanical properties. The properties examined are of interest to researchers fabricating, designing, or modeling active hydrogel‐based microfluidic components. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Synthesis and characterization of biodegradable network hydrogels having both hydrophobic and hydrophilic components with controlled swelling behavior

A new class of biodegradable hydrogels, consisting of hydrophobic poly(D ,L )lactic acid (PDLLA) and hydrophilic dextran segments with a polymer network structure, was synthesized with UV photopolymerization. Unsaturated vinyl groups first were introduced onto the PDLLA and dextran polymer backbones, then followed by a crosslinking reaction of diacrylate-terminated PDLLA and dextran acrylate. The chemical crosslinking forced the hydrophobic PDLLA and hydrophilic dextran segments to mix with each other in the network hydrogels. The new polymers were characterized by standard polymer characterization methods such as NMR, Fourier transform infrared spectroscopy, and gel permeation chromatography. The effects of reaction time, temperature, and molar ratio of the reactants on the incorporation of acrylate onto the polymer backbone were examined. A series of hydrogels with different dextran/PDLLA composition ratios was prepared, and their swelling behaviors were studied. These new bicomponent network hydrogels had a wide range of hydrophilicity to hydrophobicity that was difficult to achieve in totally hydrophilic hydrogels. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4554–4569, 1999     

Microstructured polyacrylamide hydrogels made with hydrophobic nanoparticles

Poly(methyl methacrylate) nanosize particles, made by microemulsion polymerization, were dispersed in an acrylamide aqueous solution, which was polymerized in the presence of a cross-linking agent to yield microstructured hydrogels. The kinetics of swelling and the mechanical properties of these hydrogels were investigated as a function of concentration of particles. The microstructured hydrogels exhibit higher equilibrium swelling and larger Young modulus than conventional (that is, without particles) polyacrylamide hydrogel. The morphology of the microstructured hydrogels was examined by transmission electron microscopy.     

Dual cross-linked networks hydrogels with unique swelling behavior and high mechanical strength: based on silica nanoparticle and hydrophobic association

A series of physically cross-linked hydrogels composed poly(acrylic acid) and octylphenol polyoxyethylene acrylate with high mechanical strength are reported here with dual cross-linked networks that formed by silica nanoparticles (SNs) and hydrophobic association micro-domains (HAMDs). Acrylic acid (AA) and octylphenol polyoxyethylene acrylate with 10 ethoxyl units (OP-10-AC) as basic monomers in situ graft from the SNs surface to build poly(acrylic acid) hydrophilic backbone chains with randomly distributed OP-10-AC hydrophobic side chains. The entanglements among grafted backbone polymer chains and hydrophobic branch architecture lead to the SNs and HAMDs play the role of physical cross-links for the hydrogels network structure. The rheological behavior and polymer concentration for gelation process are measured to examine the critical gelation conditions. The correlation of the polymer dual cross-linked networks with hydrogels swelling behavior, gel-to-sol phase transition, and mechanical strength are addressed, and the results imply that the unique dual cross-linking networks contribute the hydrogels distinctive swelling behavior and excellent tensile strength. The effects of SNs content, molecular weight of polymer backbone, and temperature on hydrogels properties are studied, and the results indicate that the physical hydrogel network integrity is depended on the SNs and HAMDs concentration.     

Gamma radiation synthesis of rapid swelling superporous polyacrylamide hydrogels

In this report a simple route for gamma radiation induced synthesis of superporous hydrogel (SPH) is described. Conventional SPH synthesis requires foaming and cross-linking reactions to take place simultaneously. However, in radiation synthesis it is difficult to introduce foaming during the cross-linking reactions. In order to overcome this limitation, the foaming and radiation cross-linking reactions were decoupled and carried out in two stages. The polyacrylamide SPH synthesized by this approach has very fast swelling kinetics compared to the non-porous hydrogel.     

Surfactant-induced healing of tough hydrogels formed via hydrophobic interactions

Different reversible molecular interactions have been used in the past few years to generate self-healing in synthetic hydrogels. However, self-healing hydrogels synthesized so far suffer from low mechanical strength which may limit their use in any stress-bearing applications. Here, we present a simple technique to heal mechanically strong polyacrylamide hydrogels formed via hydrophobic interactions between stearyl groups. A complete healing in the hydrogels was achieved by the treatment of the damaged areas with an aqueous solution of wormlike sodium dodecyl sulfate micelles. The micelles in the healing agent solubilize the hydrophobes in the cut surfaces, so that they easily find their partners in the other cut surface due to the hydrophobic interactions. Surfactant-induced healing produces high toughness (～1 MPa) gels withstanding 150 kPa of stress at a deformation ratio of 1,100 %. The healing technique developed here is generally applicable to the physical gels formed by hydrophobic associations.     

Earthworm inspired locomotive motion from fast swelling hybrid hydrogels

Diverse motion mechanisms encountered in nature serve successfully as a guide for engineering efficient mobile devices used in cargo transport and force generation. We have previously demonstrated earthworm locomotion inspired directed motion and cargo transport using a pNIPA hydrogel‐based device. The motility mechanism involved sequential shrinking and swelling of segments of a long gel in a glass capillary, induced by volume phase transitions, through a simple temperature stimulus using peltier elements. The same effect is generated in the earthworm by flexing and stretching muscles along the body as it moves in its underground burrow. The shrinking segments move the body forward while the swollen segments anchor against the walls to prevent slippage. Here, we show an improved device, using the same working principle, made of super‐porous, mechanically robust organic‐inorganic hybrid hydrogels (also known as nanocomposite hydrogels), which show large volume phase transitions above 32 °C without requiring lengthy hydrolysis times. The gels demonstrate fast swelling kinetics with complete restoration of their initial size in short times, making the gels reusable for multiple cycles. This improved device, with its reusability, fast swelling kinetics, and efficient slip‐free motion, opens a variety of possibilities for applications in microfluidics, nanobiotechnology, small‐scale robotics, and micro electro mechanical systems. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5027–5033, 2009     

Effects of high-concentration salt solutions and pH on swelling behavior of physically and chemically cross-linked hybrid hydrophobic association hydrogels with good mechanical strength

New physically and chemically cross-linked hybrid hydrophobic association hydrogels (Hybrid HA-gels) were prepared by radical copolymerization. The effect of salinity on the swelling behavior of Hybrid HA-gels was studied. Hybrid HA-gels, like ampholytic hydrogels, displayed antipolyelectrolyte behavior in high-concentration NaCl and MgCl₂ solutions. According to the analysis of experimental data, the antipolyeletrolyte behavior of Hybrid HA-gels should be attributed to the rapid hydrolysis of amide groups on polymer, and the hydrolysis reaction can be promoted by high concentrations of salt solutions. Also, the results of swelling tests verified also the reversibility of complexing action of Ca²⁺ with carboxylate groups within the Hybrid HA-gels. In addition, the effect of pH values on the swelling behavior of the Hybrid HA-gels was also studied in detail.     

Voltammetric characterization on the hydrophobic interaction in polysaccharide hydrogels

Cyclic voltammetric (CV) investigations on the properties of microdomains in polysaccharide hydrogels, methyl cellulose (MC) and kappa-carrageenan (CAR), coated on glassy carbon electrodes were reported in which methylene blue (MB), tris(1,10-phenanthroline)cobalt(III) (Co(phen)3(3+/2+)) cations, and ferricyanide/ferrocyanide (Fe(CN)6(3-/4-)) anions were used as electroactive probes. Information on the patterns and strength of intermolecular interactions in these polysaccharide hydrogels can be inferred from the net shift of normal potentials (E degrees'), the change of peak currents (ip), the ratio of binding constants (K(red)/K(ox)) for reduced and oxidized forms of bound species, and the apparent diffusion coefficients (D(app)) of probe in hydrogels. The transition of hydrophobic interaction in MC hydrogel with temperature was manifested by the CV method, which is in agreement with the evolution of the storage modulus (G') during gelation. It was also found that, in addition to inducing the change of E degrees' and ip of these probes used, the hydrophobic-hydrophilic nature of the microenvironment in hydrogels coated on the substrate electrodes greatly influenced the peak-peak separation (DeltaEp) of MB and the redox reversibility of Fe(CN)6(3-/4-) via modulation of both the heterogeneous electron-transfer process at the gel-substrate interface and the charge-transfer process in hydrogels. The results imply that the CV method is of significant benefit to the understanding of the gelation driving forces in the polysaccharide hydrogels at a molecular level.     

Radiation preparation and swelling behavior of sodium carboxymethyl cellulose hydrogels

Sodium carboxymethyl cellulose (CMC) is a kind of degraded polymer under γ-irradiation. However, in this work, it has been found that CMC crosslinks partially to form hydrogel by radiation technique at more than 20% CMC aqueous solution. The gel fraction increases with the dose. The crosslinking reaction of CMC is promoted in the presence of N₂ or N₂O due to the increase of free radicals on CMC backbone, but gel fraction of CMC hydrogel is not high (<40%). Some important values related to this kind of new CMC hydrogel synthesized under different conditions, such as radiation yield of crosslinking G(x), gelation dose R_g, number average molecular weight of network M_c were calculated according to the Charlesby–Pinner equation. The results indicated that although crosslinked CMC hydrogel could be prepared by radiation method, the rate of radiation degradation of CMC was faster than that of radiation crosslinking due to the character of CMC itself. Swelling dynamics of CMC hydrogel and its swelling behavior at different conditions, such as acidic, basic, inorganic salt as well as temperature were also investigated. Strong acidity, strong basicity, small amount of inorganic salts and lower temperature can reduce swelling ratio.     

A molecular thermodynamic model for the swelling of thermo-sensitive hydrogels

A new molecular thermodynamic model for describing the swelling behavior of thermo-sensitive hydrogels was developed. The model consists of two terms. One is the contribution of the mixing of hydrogel network and water, which is dependent on the local polymer concentration and the interaction between polymer segment and solvent. A closed packed lattice model for polymer solution developed by Yang et al. was adopted for this term. The other is the elastic contribution derived from the network elasticity, which is dependent on the cross-linking degree of gel network. The elastic Gibbs energy model based on the Gaussian chain model developed by Flory was adopted. The model equation has two parameters. One is an energy parameter ? reflecting the interaction between water and gel network, the other is a size parameter V^* that represents the cross-linking degree of the hydrogel. When the energy parameter ? is expressed as a quadratic of inverse temperature, this model can describe the swelling equilibrium behavior of neutral thermo-sensitive hydrogels quite well. The influences of model parameters were discussed in details. The experimental swelling curves of two kinds of polyacrylamide-based gels were correlated and good agreement was obtained.     

Correlation of chemical structure and swelling behavior in N-alkylacrylamide hydrogels

The thermoshrinking properties have been studied for the series of N-alkyl-acrylamide hydrogels (alkyl = methyl, ethyl, isopropyl, and n-propyl), which were prepared by free-radical copolymerization of the alkylacrylamide, sodium acrylate, and N,N′-methylenebis(acrylamide) (BIS) in aqueous solution. The reaction mixtures were prepared using the same nominal compositions in an effort to study the effect of the chemical structure of the alkyl substituent on the gel swelling behavior as a function of temperature. The alkyl group was found to have a pronounced effect on the features of gel swelling. Generally, larger alkyl chains produced dramatic decreases in gel transition temperature. In addition, a change in the nature of the swelling behavior from continuous to discontinuous was noted upon changing the alkyl group from ethyl to the two propyl derivatives. Discontinuous transitions were accompanied by hysteresis. The transition temperatures of the isomeric propyl derivatives were found to differ by 12°C, with n-propyl exhibiting the lower value. Additionally, a quantitative correlation was found between the gel transition temperatures and the water/octanol partition coefficients for appropriately chosen small molecule model compounds. The transition temperatures of other gels in the series, including the cyclopropyl derivative and the n-propyl/isopropyl copolymer gels (NIPA/NNPA), also fit this correlation. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2095–2102, 1998     

Low-temperature-induced swelling of a hydrophobic polymer: a lattice approach

The authors investigate equilibrium properties of a simple model of hydrophobic polymer in aqueous solution by means of dynamic Monte Carlo simulations. The solvent is described by a simplified two-dimensional model, defined on a triangular lattice, which has been previously shown to account for most thermodynamic anomalies of pure water and of hydrophobic solvation for monomeric solutes. The polymer is modeled as a self-avoiding walk on the same lattice. In this framework, the degrees of freedom of water are taken into account explicitly, and in principle there is no need to introduce effective self-contact interactions for the polymer in order to mimic the hydrophobic effect. In certain conditions, the authors observe low-temperature-induced swelling, i.e., expansion of the polymer globule upon decreasing temperature. The authors discuss the relationship between this phenomenon and the anomalous properties of the solvent.     

Properties of low temperature swelling hydrogels prepared with partially saponified PVA and drug release using the hydrogels

The hydrogels prepared by the crosslinking of partially saponified poly(vinyl alcohol) (PVA) which has low critical solution temperature (LCST) in water showed characteristics of swelling at low temperature and shrinkage at high temperature. The hydrogels showed repeatable swelling–shrinking behavior. The hydrogels containing release substances such as cyanocobalamin, p‐acetamidophenol, insulin and ovalbumin were prepared by dipping these aqueous solutions and the release substances were studied. Insulin and ovalbumin were not absorbed by the hydrogels when the use of partially PVA with the degrees of polymerization (DPs) of 540 were prepared, but absorbed by the hydrogels with DPs of 1860. The size of the polymer network prepared with a higher DP was suitable for the absorption of insulin and ovalbumin. In accordance with release substances, the release patterns were different. In this way, the polymer network sizes and their swelling behaviors of partially saponified PVAs were estimated. Copyright © 2003 John Wiley & Sons, Ltd.     

Evaluation of surface roughness of hydrogels by fractal texture analysis during swelling

The surface of a biomaterial reacts in contact with biological fluids. Hydrogels are used to prepare biomaterials. The surface roughness of materials can be explored by several techniques. However, when considering hydrogels, the surface examined in the dry state does not reflect the final conformation. How the surface roughness is affected by swelling has been little explored by quantitative methods. We have evaluated the surface roughness of poly(2-hydroxyethyl methacrylate) (i.e., pHEMA) by image analysis. Images of disks, prepared from linear pHEMA, were obtained on a light microscope after various incubation times in saline. Fractal texture analysis was done on images to determine the fractal dimension D. In this study, D exhibited a significant decrease during swelling and was highly correlated with the swelling ratio (r2 = 0.994, p < 0.00001). Water uptake by the surface of the polymer affected the surface roughness. Image analysis using fractal algorithms appears to be the most interesting technique for the quantitative exploration of surfaces of hydrated materials that cannot be measured by conventional methods.     

Patterned dual pH-responsive core-shell hydrogels with controllable swelling kinetics and volumes

Dual pH-responsive core-shell hydrogels containing both a vinyl pyridine component and a 2-dimethylaminoethyl methacrylate component were prepared using an in situ photopolymerization process. Complementary photomasks were utilized to prepare hydrogels with core/shell volume ratios of 2:1, 1:1, and 1:2. Depending on the location of each polymer component, dramatically different swelling profiles were achieved. Selective swelling of the shell followed by the core components allowed the hydrogel to expand with the usual kinetics; however, by switching the location of each polymer component and swelling the core first, swelling rates decreased by over 1 order of magnitude and were dependent on the shell component's volume. The ability to pattern core/shell volumes also provided the ability to fabricate hydrogels that possess a constant maximum diameter but different cutoff points between its first and its second transition volumes. These materials may be of interest for controlled release applications.     

Superabsorbent hydrogels based on cellulose for smart swelling and controllable delivery

Novel superabsorbent hydrogels were prepared successfully from carboxymethylcellulose sodium (CMC) and cellulose in the NaOH/urea aqueous system by using epichlorohydrin (ECH) as cross-linker. The structure and morphology of the hydrogels were characterized by FT-IR spectroscope, thermogravimetric analysis and scanning electron microscope. The results revealed that the CMC contributed to the enhanced size of pore, whereas cellulose as a strong backbone in the hydrogel to support it for keeping its appearance. Their equilibrium swelling ratio in distilled water and different physiological fluids were evaluated, indicating the maximum swelling ratio in water reached an exciting level of 1000 as the hydrogels still keeping a steady appearance. Moreover, the hydrogels exhibited smart swelling and shrinking in NaCl or CaCl₂ aqueous solution, as well as the release behavior of bovine serum albumin (BSA) that could be controlled by changing CMC content. The cellulose-based hydrogels are promising for the applications in the biomaterials area.     

Modeling the swelling pressure of degrading hydroxyethylmethacrylate‐grafted dextran hydrogels

Degrading hydroxyethylmethacrylate‐grafted dextran (dex‐HEMA) hydrogels generate a relatively sudden increase in osmotic pressure upon degradation into dextran solutions. This phenomenon is currently being examined as a possible means of developing a pulsatile drug‐delivery system. Here a mathematical model based on scaling concepts is presented to describe this sudden increase in swelling pressure and to provide a framework for the rational design of pulsatile delivery systems based on this phenomena. The model provides a good fit to the swelling pressures measured for dex‐HEMA gel/free dextran mixtures that simulate degrading dex‐HEMA gels. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3397–3404, 2004     

Effect of crosslinker functionality on swelling and network parameters of copolymeric hydrogels

Crosslinked xerogels comprising N-vinyl-2-pyrrolidone (VP) and butyl acrylate (BA) with different weight fractions of BA ranging between 0.1 and 0.3 were prepared by γ-ray initiation copolymerization using different concentrations of either hexa- or tetrafunctional crosslinking agents. Methylene bisacrylamide (MBA) was selected as tetrafunctional crosslinker, while 1,1,1-trimethylolpropane trimethacrylate (TPT) was used as a hexafunctional crosslinker. The concentration of both crosslinkers ranged between 0.5% and 2%. Thin disks of the prepared copolymers were swollen in deionized water at 294 K. The final equilibrium water content, volume fraction of polymer and swelling capacity were determined. The effective crosslinking density V_e, the average molecular weight between the crosslinks M_c and the polymer–water interaction parameter were determined from stress–strain measurements. For different compositions of VP/BA, the linear relations between theoretical crosslinking densities V_t and V_e were established. The efficiencies of MBA and TPT crosslinking agents towards VP–BA copolymers were determined. © 1998 John Wiley & Sons, Ltd.