Gelation Rheology and Water Absorption Behavior of Semi-Interpenetrating Polymer Networks of Polyacrylamide and Carboxymethyl Cellulose

Gelation rheology and swelling behavior of novel semi-interpenetrating polymer network (semi-IPN) hydrogels based on polyacrylamide are described. These hydrogels were prepared by solution cross-linking of partially hydrolyzed polyacrylamide and carboxymethyl cellulose (CMC), using chromium triacetate. Effects of CMC content on the gelation process and swelling behavior in tap water and different electrolyte solutions were investigated. Study of the gelation behavior using dynamic rheometery showed that the limiting storage modulus of the semi-IPN gels increased with increasing CMC content. Enhancement of storage modulus was more than two times for the semi-IPN gels containing 50 wt% CMC. It was also found that increasing the CMC content decreased the loss factor, indicating that the elastic properties of this gelling system increase more strongly than the viscous properties. The swelling ratio of the semi-IPN gels in tap water, NaCl and CaCl₂ solutions, and synthetic oil reservoir water slightly decreased as the concentration of the CMC increased. The improved storage modulus and slightly decreased swelling capacity in oil reservoir water make these semi-IPN hydrogels potentially good candidates for excess water treatment in oil recovery applications.     

Study on the Dynamic Rheometry and Swelling Properties of the Polyacrylamide/Laponite Nanocomposite Hydrogels in Electrolyte Media

Polyacrylamide/laponite/chromium triacetate nanocomposite (NC) hydrogels were prepared by incorporation of the laponite nanoparticles in partially hydrolyzed polyacrylamide followed by cross-linking of their aqueous solutions with chromium triacetate. Influence of nanoparticle, cross-linker, polymer concentrations, and gelation media (water) temperature, salinity, and rheometer frequency on the viscoelastic behavior of the NC hydrogels were studied by probing the network properties. In addition, swelling behaviors of these NC gels in tap and oil reservoir water were evaluated. According to dynamic rheometry of the gelation process, the limiting storage modulus of the NC gels increased with increasing laponite content. The addition of laponite into the polyacrylamide gelling system increased their viscous properties more strongly than the elastic properties. The ultimate elastic modulus of the NC gels increased with increasing water salinity and temperature. Increasing rheometer frequency during gelation retarded the sol–gel transition and decreased the ultimate elastic modulus. The equilibrium swelling ratio of the NC hydrogels in tap water decreased with increasing laponite content. The salt sensitivity of the NC gels in oil reservoir water slightly decreased with increasing laponite content. These results suggest the superiority of the hydrolyzed polyacrylamide (HPAM)/chromium acetate/laponite NC hydrogels for water shut-off applications in oil reservoirs as compared with unfilled HPAM gels.     

Preparation and Swelling Behavior of Partially Hydrolyzed Polyacrylamide Nanocomposite Hydrogels in Electrolyte Solutions

In this research, nanocomposite hydrogels were prepared by cross‐linking of partially hydrolyzed polyacrylamide/sodium montmorillonite aqueous solutions with chromium triacetate. The gelation process and influence of nanoclay content and salt concentration on swelling behavior were investigated. Study of gelation behavior using dynamic rheometry method showed that increasing the nanoclay content decreases the storage modulus, due to the partial adsorption of polymer chains onto the clay surface and ionic interaction between negative layers of sodium montmorillonite and Cr.³⁺ By increasing the cross‐linker concentration of the gelation system, the viscous energy dissipation properties of the nanocomposite gel decreases. Swelling ratio of the nanocomposite gels in distilled water decreased as the concentration of the nanoclay increased. However, nanocomposite gels showed lower salt sensitivity in electrolyte media compared with unfilled gels.     

Physicochemical properties and microstructure of composite surimi gels: The effects of ultrasonic treatment and olive oil concentration

This study was conducted to evaluate the effects of extra virgin olive (EVO) oil incorporation on the physicochemical properties and microstructure of surimi gels subjected to ultrasound-assisted water-bath heating. As the oil content was increased from 0 to 5 g/100 g, the breaking force and gel strength of the surimi gels significantly decreased, while the whiteness level exhibited the opposite tendency irrespective of the heating method. Compared with the traditional water-bath heating method, the ultrasonic heating promoted the unfolding of the α-helix structure and intensified the formation of β-sheet content and non-covalent bonds (ionic bonds, hydrogen bonds, and disulfide bonds), especially disulfide bonds, which contributed to the further crosslinking of the proteins and to gelation, thereby improving the gels’ strength. In addition, smaller cavities and compact microstructures were observed in the low-oil (≤3 g/100 g) surimi gels under ultrasonic treatment, which effectively prevented water migration in the gel network and resulted in a high water holding capacity and uniform water distribution. However, the ultrasonic treatment barely remedied the poor microstructures of the high-oil (>3 g/100 g) surimi gels owing to oil coalescence, which weakened the protein–protein interaction. In conclusion, ultrasonic treatment combined with water-bath heating significantly improved the gelation properties of the low-oil surimi gels, although it did not remarkably improve those of the high-oil gels. The choice of a suitable oil concentration could be of great importance for the production and functioning of surimi products via ultrasound-assisted treatments.     

Preparation and Properties of Polyvinyl Alcohol/Collagen Hydrogel

Hydrogel scaffolds based on poly(vinyl alcohol) (PVA) collagen films were prepared by a chemical cross-linking method. The effects of the contents of PVA and cross-linker on compressive strength and swelling ratio were studied, and the effect of the pH value of the immersion medium on the swelling ratio was also investigated. The results showed that the introduction of PVA improved the compressive strength of PVA/collagen hydrogel, and the swelling ratio of the hydrogel scaffold increased with increasing PVA content in the blends. With increasing cross-linker content, the swelling ratio decreased; however, the compressive strength increased. The swelling ratio of PVA/collagen scaffold increased when pH was decreased. In conclusion, swelling ratio and compressive strength in PVA/collagen blends can be controlled by variation of their contents, cross-linking agent content, and pH value.     

Solid-state 13C NMR study of poly(vinyl alcohol) gels

Poly(vinyl alcohol) (PVA) with 55% and 61% syndiotacticity, and their related dry and hydrated gels obtained by two different freeze–thawing cycles have been investigated using the solid-state ¹³C CP-MAS NMR technique. From a comparative analysis of the spectra, evidence was obtained that the gelation process largely disrupts the intramolecular hydrogen-bonded network of the PVA. The addition of water to the dry gels favours their swelling, destroying intra-chain hydrogen bonds between hydroxyl groups as a function of the degree of tacticity and the gelation procedure, and promotes the formation of new networks of interchain hydrogen bonds. Information on the dynamics of the polymeric domains in the kilohertz range has been obtained from the analysis of the spin relaxation times T_1ρ(¹H) and T_1ρ(¹³C), indicating that homogeneous arrangements of the amorphous or swollen polymeric chains exist, independent of the preparation method or the tacticity of the PVA chains.     

Swelling and Gelation Time Behavior of Sulfonated Polyacrylamide/Chromium Triacetate Hydrogels

A hydrogel was prepared by crosslinking of aqueous solutions of sulfonated polyacrylamide/chromium triacetate for use in water shut-off operations in oil reservoirs. The effects of pH, salinity, retarder and temperature, as well as co-polymer and crosslinker concentration, on the gelation time were investigated. The results indicated that as temperature increased, gelation occurred more rapidly. The activation energy was measured as about 86 kJ/mol. The effects of initial pH and retarder on the gelation time were also examined. The results showed that addition of retarder and increasing of pH increased and decreased the gelation time, respectively. The increase of co-polymer concentration in solution increased the gel swelling. However, the increase of crosslinker concentration decreased the gel swelling. In the presence of electrolytes, the gel swelling decreased by about 80%. Finally, some usable practical recommendations are offered for the gelling systems in reservoirs.     

Preparation and Characterization of Chemically Crosslinked Polyvinyl Alcohol/Carboxylated Nanocrystalline Cellulose Nanocomposite Hydrogel Films with High Mechanical Strength

Chemically crosslinked polyvinyl alcohol (PVA)/carboxylated nanocry-stalline cellulose (PVA/CNCC) nanocomposite hydrogel films were fabricated by film-casting of PVA/CNCC mixture solutions and subsequent thermal-curing of the PVA with the CNCC. Gel fractions of the hydrogel films were measured to confirm the occurrence of crosslinking. Morphologies of the hydrogel films were characterized by polarized light microscopy and scanning electron microscopy (SEM). Thermal properties, swelling behavior and mechanical properties of the hydrogel films were investigated to evaluate the influence of CNCC content (10～30% of PVA mass). Equilibrium water content of the hydrogel films was in the range of 40～49%. At swelling equilibrium, the hydrogel films could be stretched to 3～3.4 times their original length, and their tensile strength was in the range of 7.9～11.6 MPa. The results show that the PVA/CNCC nanocomposite hydrogel films were both extensible and highly tough.     

Preparation,Characterization and Coreflood Investigation of Polyacrylamide/Clay Nanocomposite Hydrogel System for Enhanced Oil Recovery

The synthesis and characterization of polyacrylamide/clay nanocomposites for the development of hydrogel system used in enhanced oil recovery is described. The synthesized nanocomposite copolymer was crosslinked with Chromium (III) acetate to form the hydrogel which exhibited an acceptable gel strength, gelation time and gel stability. The nanocomposite gels prepared with low crosslinker concentration (2000 ppm chromium acetate) showed higher gel strength and required longer gelation time than the conventional polyacrylamide (PAAm) gel; these are desirable properties for the effective placement of gel during enhanced oil recovery operations. The effects of various parameters, such as polymer and crosslinker concentration, on the gelation time and gel strength were evaluated using the bottle testing method. X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM) revealed the formation of intercalated and exfoliated clay morphologies. The effects of the clay content on the thermal stability and gel strength of the gel network were also investigated by thermogravimetric analysis (TGA) and rheological measurements (oscillatory time sweep profiles), respectively. Also, in-situ gelation and core flooding experiments revealed that a significant permeability reduction of the sand pack cores could be achieved at reservoir conditions when they were treated with the developed nanocomposite gel formulation. Hence, this nanocomposite gel system with low crosslinker concentration (10,000 ppm of nanocomposite polymer concentration containing 2000 ppm of clay with 2000 ppm chromium acetate crosslinker) may be suitable in water shut-off treatments required for enhanced oil recovery from the oil fields.     

Study on the Electrospinnability of Polyvinyl Alcohol Solutions by Using Water/N,N-dimethylacetamide or Water/N,N-dimethylformamide as Solvents

The electrospinning of poly(vinyl alcohol) (PVA) (99% hydrolysis degree) aqueous solution with added organic solvents N, N-dimethylacetamide (DMAc) or N, N-Dimethylformamide (DMF) was investigated. After the addition of the organic solvents to the PVA aqueous solutions, the surface tension and conductivity decreased and the viscosity significantly increased, which caused an improved electrospinnability of the PVA solutions. The micro-structures of the three solutions were investigated by dynamic light scattering (DLS), differential scanning calorimetry (DSC) and dynamic viscoelastic measurements. The DLS data revealed that the swelling of the PVA coils was slightly increased but the overlaps of PVA coils decreased greatly after one of the organic solvents was added to the aqueous solution. The DSC data showed both the water-rich phase and PVA-rich phase were destroyed and the solution became more homogenous after the addition due to the interaction between the organic solvent and water. Viscoelastic data showed there was an obvious difference in the storage modulus behavior between the aqueous solutions and the water/solvents solutions. These changes in the micro-structure and properties were the reason for the improved electrospinnability of the PVA solution. According to scanning electron microscopy (SEM), the average diameter of the electrospun PVA nanofibers was about 308 nm for the DMF/water system, and 255 nm for the DMAC/water system, as compared with uneven diameter nanofibers for the water system.     

Synergistic effects of oat β-glucan combined with ultrasound treatment on gel properties of silver carp surimi

The effect of oat β-glucan (OG) combined with ultrasound treatment on the gelation properties of silver carp surimi with different salt contents was investigated. The results demonstrated that the gelation properties of surimi gels at high salt concentration were superior than those at low salt level. The addition of OG or ultrasound treatment could significantly enhance the texture properties, gel strength and water holding capacity (WHC) of gel samples, regardless of salt contents. The ultrasound treatment improved the whiteness of surimi gels, whereas the OG addition slightly declined the whiteness. Both OG addition and ultrasound treatment markedly reduced the total sulfhydryl content (total SH) and strengthened the hydrophobic interactions, forming the more uniform and denser gel network structures, hence more water was captured in network structures and became immobilized. Moreover, the combined treatment of OG and ultrasound showed synergic action on the gelation properties of surimi, and the gel strength and WHC of low-salt surimi gel treated by the combination of OG and ultrasound were even superior than that of high-salt gel without OG by traditional heating.     

Preparation and Characterization of Poly(Vinyl Alcohol) (PVA)/SiO2, PVA/Sulfosuccinic Acid (SSA) and PVA/SiO2/SSA Membranes: A Comparative Study

Abstract

New organic–inorganic nanocomposites based on PVA, SiO₂ and SSA were prepared in a single step using a solution casting method, with the aim to improve the thermomechanical properties and ionic conductivity of PVA membranes. The structure, morphology, and properties of these membranes were characterized by Raman spectroscopy, small- and wide-angle X-ray scattering (SAXS/WAXS), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), water uptake (Wu) measurements and ionic conductivity measurements. The SAXS/WAXS analysis showed that the silica deposited in the form of small nanoparticles (～ 10?nm) in the PVA composites and it also revealed an appreciable crystallinity of pristine PVA membrane and PVA/SiO₂ membranes (decreasing with increasing silica loading), and an amorphous structure of PVA/SSA and PVA/SSA/SiO₂ membranes with high SSA loadings. The thermal and mechanical stability of the nanocomposite membranes increased with the increasing silica loading, and silica also decreased the water uptake of membranes. As expected, the ionic conductivity increased with increasing content of the SSA crosslinker, which is a donor of the hydrophilic sulfonic groups. Some of the PVA/SSA/SiO₂ membranes had a good balance between stability in aqueous environment (water uptake), thermomechanical stability and ionic conductivity and could be potential candidates for proton exchange membranes (PEM) in fuel cells.     

Polyvinyl Alcohol/Na-Montmorillonite Nanocomposite Hydrogels Prepared by Freezing–Thawing Method: Structural,Mechanical, Thermal,and Swelling Properties

Physically crosslinked nanocomposite hydrogels based on polyvinyl alcohol (PVA) containing Na-montmorillonite were prepared by the cyclic freezing–thawing method. The primarily exfoliated morphology of prepared nanocomposite hydrogels was confirmed by X-ray diffractometry (XRD) and transmission electron microscopy (TEM) as complementary techniques. It is shown that some interactions developed between the hydroxyl groups of PVA chains and Na-montmorillonite silicate layers in the nanocomposite hydrogels. Differential scanning calorimetry (DSC) results indicated some shifting in the glass transition temperature of the PVA hydrogel in the presence of the nanoclay. Swelling measurements showed that the swelling ratios of the nanocomposite hydrogels were increased either by decreasing the Na-montmorillonite content or by increasing the swelling medium temperature. Dynamic mechanical–thermal properties results showed higher storage modulus for nanocomposite hydrogels in temperature ranges both below and above 0°C. It was also found that the hardness of the nanocomposite hydrogels increased by increasing the nanoclay loading level.     

In Vitro Assessment of Poly (Vinyl Alcohol) Film Incorporating Aloe Vera for Potential Application as a Wound Dressing

Since skin tissue acts as a vital protective barrier between the body and the external atmosphere, the repair or regeneration of skin injuries serves as a great challenge in regenerative medicine. Herein, hydrogel films composed of poly vinyl alcohol (PVA) and aloe vera (AV) extracted gel were prepared and characterized for wound dressing application. The physical and morphological properties, water absorption capacity, biodegradation behavior, and water transmission rate were characterized for several variations in the AV content (0–50%). The cytocompatibility of the films, as well as cell morphology in response to different films, was assessed using MTT assay and SEM, respectively. According to the results, AV incorporation improved the surface morphology, water absorption capacity, in vitro degradation rate, and water vapor permeability of the PVA films. However, these properties were affected by the AV content. The mechanical properties of the films were enhanced by introducing AV up to 30%, and then decreased significantly with further AV increase. Evaluation of fibroblast proliferation showed that AV can positively improve the bioactivity of the films without any cytotoxicity. In conclusion, the results demonstrated that PVA/AV optimized hydrogel film can be suggested as promising wound dressings for improving wound treatment.     

Improvement of gelation properties of silver carp surimi through ultrasound-assisted water bath heating

The present work investigated the effects of water bath heating coupled with different ultrasound treatments on the gel properties, protein conformation, microstructures and chemical interactions of silver carp surimi at low/high salt levels. Results showed that the gel strength, hardness, springiness and water holding capacity (WHC) of surimi gels at low salt concentration were inferior to those at high salt content, regardless of the treatments. Compared with the traditional water bath heating, ultrasonic-assisted treatments significantly improved the gelation properties of surimi at the same salt level. In fact, ultrasound treatment also facilitated the unfolding of α-helix structure of the protein, with the resulting exposure of internal groups further enhancing hydrophobic interactions and hydrogen bonds between protein molecules, thereby leading to the formation of denser microstructures with smaller holes. Furthermore, the most noteworthy ultrasonic treatment group was ultrasound-assisted preheating (U + W) group, whose gelation performance under low salt condition, was comparable with that of the traditional two-stage heating (W + W) group with high salt content. Overall, ultrasound-assisted water bath preheating proved to be a feasible approach to improve the gel properties and microstructures of low-salt surimi gels.     

Fractal Dimensions of κ-Carrageenan Gels during Gelation and Swelling

A photon scattering technique for research on the sol-gel and gel-sol transitions in κ-carrageenan-water systems with various carrageenan contents (CC) was utilized for characterizing the fractal dimensions during gelation and swelling. It was observed that the scattered photon intensity, I_sc, increased at all temperatures with an increase in the CC when I_sc was monitored against temperature. Additionally, the sol-gel transition temperatures were found to be much lower than the gel-sol transition temperatures, causing hysteresis of the phase transition loops. I_sc increased with an increase in CC at all test temperatures, which is attributed to the formation of a fractal-like carrageenan gel. After drying, the gels were used in swelling experiments where the gels were immersed in water at room temperature, reswelling to the original structure. It was observed that I_sc from the carrageenan gels increased as the CC was increased. The fractal dimension, d, during gelation was found to increase as the gelation temperature was increased. On the other hand, the d values during swelling decreased as the swelling time was increased.     

Effect of high intensity ultrasound on gelation properties of silver carp surimi with different salt contents

Surimi from silver carp with different salt contents (0–5%) was obtained treated by high intensity ultrasound (HIU, 100 kHz 91 W·cm⁻²). The gelation properties of samples were evaluated by puncture properties, microstructures, water-holding capacity, dynamic rheological properties and intermolecular interactions. As the salt content increased from 0 to 5%, gel properties of surimi without HIU significantly improved. For samples with low-salt (0–2% NaCl) content, HIU induced obvious enhancement in breaking force and deformation. HIU promoted the protein aggregation linked by SS bonds, hydrophobic interactions and non-disulfide covalent bonds in surimi gels with low-salt content. Moreover, microstructures of HIU surimi gels with low-salt content were more compact than those of the corresponding control samples. HIU also improved the gelation properties of surimi with 3% NaCl to an extent. However, for high-salt (4–5% NaCl) samples, HIU decreased the breaking force and deformation of surimi gels due to the degradation of proteins suggested by increased TCA-soluble peptides. In conclusion, HIU effectively improved the gelation properties of surimi with low-salt content (0–2% NaCl), but was harmful for high-salt (4–5% NaCl) surimi. This might provide the theoretical basis for the production of low-salt surimi gels.     

Transglutaminase-induced gelation properties of soy protein isolate and wheat gluten mixtures with high intensity ultrasonic pretreatment

Soy protein isolate (SPI) and wheat gluten (WG) are widely used in commercial food applications in Asia for their nutritional value and functional properties. However, individually each exhibits poor gelation. In this study, we examined the microbial transglutaminase (MTGase)-induced gelation properties of SPI and WG mixtures with high intensity ultrasonic pretreatment. Ultrasonic treatment reduced the particle size of SPI/WG molecules, which led to improvements in surface hydrophobicity (H_o) and free sulfhydryl (SH) group content. However, MTGase crosslinking facilitated the formation of disulfide bonds, markedly decreasing the content of free SH groups. Ultrasonic treatment improved the gel strength, water holding capacity, and storage modulus and resulted in denser and more homogeneous networks of MTGase-induced SPI/WG gels. In addition, ultrasonic treatment changed the secondary structure of the gel samples as determined by Fourier transform infrared spectroscopic analysis, with a reduction in α-helices and β-turns and an increase in β-sheets and random coils. Thus, ultrasound is useful in facilitating the gelation properties of MTGase-induced SPI/WG gels and might expand their utilization in the food protein gelation industry.     

Improving the gel properties of salted egg white/cooked soybean protein isolate composite gels by ultrasound treatment: Study on the gelling properties and structure

In this study, the effects of ultrasound treatment on the texture, physicochemical properties and protein structure of composite gels prepared by salted egg white (SEW) and cooked soybean protein isolate (CSPI) at different ratios were investigated. With the increased SEW addition, the ζ-potential absolute values, soluble protein content, surface hydrophobicity and swelling ratio of composite gels showed overall declining trends (P < 0.05), while the free sulfhydryl (SH) contents and hardness of exhibited overall increasing trends (P < 0.05). Microstructural results revealed that composite gels exhibited denser structure with the increased SEW addition. After ultrasound treatment, the particle size of composite protein solutions significantly decreased (P < 0.05), and the free SH contents of ultrasound-treated composite gels were lower than that of untreated composite gels. Moreover, ultrasound treatment enhanced the hardness of composite gels, and promoted the conversion of free water into non-flowable water. However, when ultrasonic power exceeded 150 W, the hardness of composite gels could not be further enhanced. FTIR results indicated that ultrasound treatment facilitated the composite protein aggregates to form a more stable gel structure. The improvement of ultrasound treatment on the properties of composite gels was mainly by promoting the dissociation of protein aggregates, and the dissociated protein particles further interacted to form denser aggregates through disulfide bond, thus facilitating the crosslinking and reaggregation of protein aggregates to form denser gel structure. Overall, ultrasound treatment is an effective approach to improve the properties of SEW-CSPI composite gels, which can improve the potential utilization of SEW and SPI in food processing.     

Synthesis and Properties of Polyacrylamide-Based Conducting Gels with Enhanced Mechanical Strength

A nanocomposite conducting hydrogel, polyacrylamide/MWNT/clay (abbreviated as PAM/MWNT/clay), prepared through in situ free radical aqueous polymerization and crosslinked by both clay, as a functional physical crosslinker, and N,N′-methylenebisacrylamide (MBA) as a chemical crosslinker, is reported. The morphology of the gels was characterized by scanning electron microscopy (SEM). The mechanical properties and electrical conductivity were also studied. The results show that the prepared hydrogels had the expected chemical components, with a highly porous structure, and the gels also showed high mechanical strength. The mechanical strength and electrical conductivity value increased with increasing content of multi-walled nanotube (MWNT), and decreased with increasing content of water.