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.     

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.     

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.     

Swelling and mechanical properties of polyvinylalcohol hydrogels

Summary The swelling and mechanical properties of Poly(vinylalcohol) hydrogels were examined. It was found that the degree of swelling of PVA hydrogels depends on annealing temperature, but is almost independent of the initial polymer concentration. Mechanical properties of the hydrogels were also influenced by the degree of swelling. A shoulder was observed in double-logarithmic plots of stress vs. strain for the hydrogels, and became clearer as annealing temperature increased. This shoulder was closely related to the breakdown of the microcrystalline domains acting as crosslinks. Also, the shape of stress-strain curves plotted double-logarithmically for the hydrogels changed with the extension rate.     

Swelling and mechanical properties of crosslinked hydrogels containing N-vinylpyrrolidone

Copolymers of 2-hydroxyethyl methacrylate/N-vinyl-2-pyrrolidone (HEMA/NVP) and methyl methacrylate (MMA)/NVP were prepared in the presence of varying amounts of ethylene glycol dimethacrylate (EGDMA) and methylene diacrylamide (MDA) as crosslinkers by photopolymerisation. The resultant solid polymers were swollen to equilibrium in water at 293 K to produce hydrogels. These hydrogels were characterised by soluble fraction and equilibrium water content. The gels were also characterised by compression—strain measurements, which enabled the calculation of Young's modulus and effective crosslink density. The differences in these properties of HEMA/NVP and MMA/NVP polymer series and the effects of MDA versus EGDMA as a crosslinker were explained in terms of compositional drift of polymerisation, heterogeneous crosslinking and hydrophilicity/hydrophobicity of the components involved. In comparison with EGDMA, MDA was found to be more effective in reducing the soluble fraction of the polymers studied and to produce less rigid networks when swollen.     

Crosslinking of unsaturated polyester resins studied by low-resolution 1H-NMR spectroscopy

The curing behavior of five unsaturated polyester resins with different molar ratios of styrene to the double bonds in the polyester chain (MR) was investigated. The gel time was measured according to a standard method by the Society of the Plastic Industry (SPI) and by low-resolution pulse nuclear magnetic resonance (LRP-NMR). The gel and curing times decreased and the maximum temperature of the reaction increased when the MR was decreased. The proton mobilities and populations were measured by LRP-NMR. Three components could be seen in the crosslinking reaction: styrene (very mobile, T₂ ≅ 3 s), free polyester polymers (less mobile, T₂ ≅ 150 ms), and cured resin (immobile, T₂ ≅ 0.04 ms). The third component, cured resin, could be detected at the gel point. At this point, the proton mobilities of styrene and free polyester polymers were rapidly decreasing and the proton populations of the cured resin started to increase. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 77–83, 1997     

Effect of polymer solution concentration on the swelling and mechanical properties of glycol chitosan superporous hydrogels

This study examined the effect of the polymer solution concentration on the swelling and mechanical properties of glycol chitosan (GCS) superporous hydrogels (SPHs). GCS SPHs were synthesized using a gas blowing method using glyoxal as the crosslinking agent at different polymer solution concentrations. A small change in the GCS solution concentration resulted in a remarkable change in compression strength and swelling kinetics without any significant loss in equilibrium water imbibing capacity. The increase in mechanical strength accompanied by the decrease in swelling kinetics was caused by the generation of smaller pores during the gelation process of the reactant systems associated with a higher polymer solution viscosity. The apparent diffusion coefficients for a variety of GCS/simulated gastric fluid solution systems were determined from the theoretical fitting of experimental dynamic swelling data, explaining the effects of the solution concentration and crosslinking density on the swelling kinetics. The diffusion coefficients determined in this study are expected to be used as the basic information in estimating the swelling kinetics of samples in different dimension. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Equilibrium swelling and mechanical properties of hydrogels of acrylamide and itaconic acid or its esters

Summary The equilibrium swelling and the plateau elastic modulus of a family of hydrogels made by the polymerization of acrylamide with itaconic acid or some of its esters were investigated as a function of composition and crosslinking degree to find materials with satisfactory swelling and elastic properties. We show that an appropriate selection of the comonomers and the concentration of the crosslinking agent is very important to produce hydrogels with large swelling capacity and good mechanical attributes. Tailoring of mechanical properties and swelling can also be achieved by this method. Received: 7 September 1999/Revised version: 3 December 1999/Accepted: 6 December 1999     

Swelling and mechanical properties of superporous hydrogels of poly(acrylamide-co-acrylic acid)/polyethylenimine interpenetrating polymer networks

Swelling and mechanical properties were investigated for superporous hydrogels (SPHs) of poly(acrylamide-co-acrylic acid)/polyethylenimine (P(AM-co-AA)/PEI) interpenetrating polymer networks (IPNs). Gelation kinetics of SPHs changed significantly according to the acidic condition of reactant. The compressive strength of neutralized SPHs decreased monotonically with AA concentration, while the maximum swelling was observed around the AA weight fraction of 0.4 for all PEI concentrations. The SPH samples composed of high concentrations of AA and PEI were easily cracked in water due to the swelling stress developed during water uptake. The swelling kinetics decreased with increasing PEI and PAA concentrations because of the high molecular entanglement and network density associated with ionic interaction between PAA and PEI molecules. For non-neutralized SPHs, the equilibrium water uptake decreased but the compressive strength increased with PEI and PAA concentrations by simple plasticization effect.     

Silicone‐based hydrogels prepared by interpenetrating polymer network synthesis: Swelling properties and confinements effects on the formation kinetics

In this work, interpenetrating polymer networks (IPNs) of polydimethylsiloxane (PDMS) and poly(acrylic acid) or poly(2‐hydroxyethyl methacrylate) (PHEMA) have been synthesized employing a sequential method. Monomeric AAc or HEMA was introduced into the PDMS network by swelling the polymer in solutions of monomer. The polymerization of monomers was then conducted in the swollen network. The swelling properties of the IPNs were investigated by varying the monomer concentrations in the polymerization and more swelling was observed with low monomer concentrations due to the prevalence of cyclization reactions. Multi‐step polymerization used to achieve IPNs with high hydrogel contents, did not improve their water uptake. The kinetics of acrylic acid polymerization was studied under various conditions. Specifically, in the presence of confinement effects imposed by the PDMS network a considerable drop in the rate of reaction was observed. The cross‐linking density of the PDMS network was also studied how to affect the reaction rate. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Improved mechanical and swelling behavior of the composite hydrogels prepared by ionic monomer and acid-activated Laponite

A composite hydrogel (CH) with much improved mechanical and swelling properties was prepared using an ionic monomer and acid-activated Laponite XLS which was used as a cross-linking agent. Addition of acid-activated clay solved the gelation problem when ionic monomers were added to clay mineral dispersions. Reaction of Laponite XLS with sulfuric acid yielded amorphous silica. A dispersion of the acid-activated Laponite and the monomers was used to synthesize composite hydrogels by in-situ polymerization. The FT-IR spectra and rheological results of the composite hydrogels demonstrated the formation of a network. The equilibrium swelling ratios of composite hydrogels (> 6000 g/g) were more than 18 times larger than traditional organic cross-linked hydrogels. The moduli G′ and G″ in the observed frequency range were about 4 and 10 times larger than those of organic cross-linked hydrogel (OR gel). The improvement in both the equilibrium swelling ratio and mechanical strength was attributed to the homogeneous cross-linked network structure.     

Network structure and compositional effects on tensile mechanical properties of hydrophobic association hydrogels with high mechanical strength

Hydrophobic association hydrogels (HA-gels) were successfully prepared through micellar copolymerization of acrylamide (AM) and a small amount of octylphenol polyoxyethylene acrylate (OP-4-AC) in an aqueous solution containing sodium dodecyl sulfate (SDS). HA-gels exhibited excellent mechanical properties and transparency. Especially, HA-gels possessed the capability of re-forming, such as self-healing and molding. From Fourier transform infrared, swelling behavior and re-forming capability of HA-gels, the network structure was established. On the basis of the micellar copolymerization theory, the statistical molecular theory of rubber elastic, and using uniaxial stretching data, the length of the hydrophobic microblocks, the effective network chain density and the molecular weight of the chain length between cross-linking points were evaluated for all HA-gels; furthermore, they were also evaluated for the region of medium deformation by the Mooney-Rivlin theory. For HA-gels, we investigated in detail the effects of the content of compositions in the initial solution, OP-4-AC, SDS and AM, on their tensile mechanical properties on the basis of the proposed network structure. The results clearly indicate their tensile strength, fracture energy, elastic modulus, and elongation strongly depended on their composition content.     

Gelatin and amylopectin-based phase-separated hydrogels: An in-depth analysis of the swelling,mechanical, electrical and drug release properties

The current study delineates the development of gelatin–amylopectin-based phase-separated hydrogels for drug delivery applications. Gelatin and amylopectin were used as the representative protein and polysaccharide phases, respectively. The hydrogels were prepared by adding different proportions of amylopectin to gelatin solutions and subsequently cross-linking the mixture using glutaraldehyde. Microscopic analysis showed formation of phase-separated hydrogels. Secondary structure of gelatin was conserved within the hydrogels. The presence of amylopectin drastically reduced the rate of water absorption by the hydrogels. Viscoelastic analysis using stress relaxation study suggested an increase in the viscous component of the hydrogels with the increase in the amylopectin content. After incorporating amylopectin within the gelatin hydrogel, even though the bulk resistance remained unaltered, there was a corresponding variation in the capacitive elements of the equivalent electrical models. The release of the drug from the hydrogels was diffusion mediated. Suitable mathematical models were used for the analysis of the swelling (Peleg’s model), viscoelastic (Weichert model), electrical (RQQ model) and drug release (Korsmeyer–Peppas and Peppas–Sahlin models) properties. The drug retained its antimicrobial activity within the hydrogels. An analysis of the results suggested that the developed hydrogels may be explored as matrices for controlled drug delivery applications.     

Hectorite effects on swelling and gel properties of hectorite/poly(AM/IA) nanocomposite hydrogels

A new hectorite/poly(AM/IA) nanocomposite hydrogel was synthesized by inverse microemulsion polymerization. Influences of hectorite content on water absorbency and salt solution absorbency, swellability, pH-sensitivity, gel strength and temperature-resistance of the nanocomposite hydrogel were investigated. Water and salt solution absorbencies, pH-sensitivity and swellability decreased while gel strength and temperature-resistance of the nanocomposite hydrogel increased with increasing hectorite content. The as-synthesized hydrogel particles were regular and spherical-like in shape and had the average particle size of 43 nm with the range of 30–65 nm. Hectorite clay platelets were exfoliated and transformed into amorphous structure within the polymeric networks.     

Swelling kinetics,mechanical properties,and release characteristics of chitosan‐based semi‐IPN hydrogels

Two series of pH‐sensitive semi‐interpenetrating network hydrogels (semi‐IPN) based on chitosan (CS) natural polymer and acrylamide (AAm) and/or N‐hydroxymethyl acrylamide (HMA) monomers by varying the monomer and CS ratios were synthesized by free radical chain polymerization. 5‐Fluorouracil (5‐FU), a model anticancer drug, has been added to the feed composition before the polymerization. The characterization of gels indicated that the drug is molecularly dispersed in the polymer matrix. The swelling kinetics of drug‐loaded gels have decreased with increased HMA content at 37°C in both distilled water and buffer solutions with a pH of 2.1 or 7.4. Elastic modulus of the gels increased with the increase in HMA content and higher CS concentration enhanced the elastic modulus positively. Moreover, cumulative release percentages of the gels for 5‐FU were ca. 10% higher in pH 2.1 than those in pH 7.4 media. It was determined that they can be suitable for the use in both gastric and colon environments. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41886.     

Studies on preparation and swelling properties of the N-isopropylacrylamide/chitosan semi-IPN and IPN hydrogels

Semi-interpenetrating network (semi-IPN) polymer gels and interpenetrating network (IPN) polymer gels with thermosensitivity were prepared by introducing a biodegradable polymer, chitosan, into the N-isopropyacrylamide (PNIPAAm) gel system. The swelling behavior, temperature sensitivity, pH sensitivity, gel strength, and drug-release behavior of PNIPAAm/chitosan semi-IPN and IPN hydrogels were investigated. The results indicated that the NIPAAm/chitosan semi-IPN and IPN hydrogels exhibited pH and temperature-sensitivity behavior and could slow drug release and diffusion from the gels. From the stress–strain curves of the hydrogels, the compression moduli of IPN gels containing crosslinked chitosan were higher than those of semi-IPN gels. This is because IPN gels have a more compact structure. The morphology of PNIPAAm/chitosan hydrogels was also investigated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2487–2496, 2001     

Effect of hydrotalcite on the swelling and mechanical behaviors for the hybrid nanocomposite hydrogels based on gelatin and hydrotalcite

A series of hybrid nanocomposite hydrogels, based on gelatin and intercalated hydrotalcite (IHT), crosslinked with glutaraldehyde, was prepared in this study. The microstructures of the IHT and sample gels were identified by X‐ray diffraction (XRD). Swelling behaviors and physical properties of these hybrid gels were investigated. XRD results indicated that exfoliation of IHT was achieved in the hybrid nanocomposite gels. The results indicated that adding a small amount of IHT could effectively decrease the swelling ratio of the hybrid gels, but adding excess IHT could increase the swelling ratio of the nanocomposite hybrid gels. The crosslinking densities (ρ_x) of the present gels varied with IHT content and swelling ratio of the gels. The drug release behaviors of these gels were also investigated. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 500–507, 2006     

Preparation and swelling properties of glycoside-bearing hydrogels by gamma-ray radiation to glycoside-bearing polymer aqueous solutions

γ-Ray radiation on poly(glucosyloxyethyl methacrylate) [poly(GEMA)] and poly(glucosyloxyethyl acrylate) [poly(GEA)] aqueous solutions without any crosslinkers gives glycoside-bearing hydrogels in a high yield. The degree of the swelling ratios of each obtained hydrogel in water was decreased with an increase in the total radiation dose to each polymer solution. In order to clarify the formation mechanism of obtained poly(GEMA) hydrogel by γ-ray radiation, the swelling properties of each hydrogel were also compared with those of poly(GEA) under various conditions. Poly(GEMA) in an organic solvent, N,N-dimethylformamide and dimethylsulfoxide, was not gelled by γ-ray radiation, although poly(GEA) was gelled under these conditions. These results suggest that the radiation formation mechanism of the poly(GEMA) hydrogels is different from that of the poly(GEA) hydrogels. In addition, the radiolysis of water is necessary in order to form the hydrogels for poly(GEMA). Next, we predicted the radiation formation mechanism of the poly(GEMA) hydrogels by the crosslinking between the glucoside moieties in poly(GEMA). © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 965–972, 1998     

聚苯胺/碳纳米管杂合水凝胶的合成及膨胀性能研究

合成了一种以甲基丙烯酸-丙烯酰胺共聚网络P(AAm-co-MAA)为基体,以聚苯胺为互穿组分(PANI)的半互穿网络杂合水凝胶。研究表明,当将PANI和MWNTs-COOH被引入到P(AAm-co-MAA)网络中后得到的P(AAm-co-MAA)/PANI/MWCNT-COOH半互穿网络杂合水凝胶则能保持较好的膨胀性,同时发现,依靠调节PA-NI/MWNTs-COOH的含量可以使杂合水凝胶膨胀性增加。这为研究杂合水凝胶的力学性能和pH敏感性提供了一个窗口。