Low Cross-linker Concentrations Research Articles

Injectable Dual Fenton/Enzymatically Cross-Linked Double-Network Hydrogels Based on Acrylic/Phenolic Polymers with Highly Reinforced and Tunable Mechanical Properties.

Injectable hydrogels have been extensively used as promising therapeutic scaffolds for a wide range of biomedical applications, such as tissue regeneration and drug delivery. However, their low fracture toughness and brittleness often limit their scope of application. Double-network (DN) hydrogel, which is composed of independently cross-linked rigid and ductile polymer networks, has been proposed as an alternative technique to compensate for the weak mechanical properties of hydrogels. Nevertheless, some challenges still remain, such as the complicated and time-consuming process for DN formation, and the difficulty in controlling the mechanical properties of DN hydrogels. In this study, we introduce a simple, rapid, and controllable method to prepare in situ cross-linkable injectable DN hydrogels composed of acrylamide (AAm) and 4-arm-PPO-PEO-tyramine (TTA) via dual Fenton- and enzyme-mediated reactions. By varying the concentration of Fenton's reagent, the DN hydrogels were rapidly formed with controllable gelation rate. Importantly, the DN hydrogels showed a 13-fold increase in compressive strength and a 14-fold increase in tensile strength, compared to the single network hydrogels. The mechanical properties, elasticity, and plasticity of DN hydrogels could also be modulated by simply varying the preparation conditions, including the cross-linking density and reagent concentrations. At low cross-linker concentration (<0.05 wt %), the plastic DN hydrogel stretched to over 6,500%, whereas high cross-linker concentration (≥0.05 wt %) induced fully elastic hydrogels, without hysteresis. Besides, DN hydrogels were endowed with rapid self-recovery and highly enhanced adhesion, which can be further applied to wearable devices. Moreover, human dermal fibroblasts treated with DN hydrogels retained viability, demonstrating the biocompatibility of the cross-linking system. Therefore, we expect that the dual Fenton-/enzyme-mediated cross-linkable DN hydrogels offer great potential as advanced biomaterials applied for hard tissue regeneration and replacement.

Closure of Long Surgical Incisions with Hemostatic Tissue Adhesive in a Porcine Skin Model.

Skin adhesives offer many advantages over traditional wound-closure devices. Recently, the current research group reported on tissue adhesives composed of natural polymers (gelatin and alginate), which are biocompatible with mechanical properties suitable for tissue adhesion. The objective of the present study was to conduct clinical and histologic assessment of this hemostatic bioadhesive in the healing of long skin incisions (≥4 cm) in comparison with traditional and commercially available methods. Researchers created 24 long incisions on the ventral side of two domestic pigs to compare four different treatment modalities: two topical bioadhesives based on gelatin and alginate combined with the hemostatic agent kaolin, nylon sutures, and commercial tissue adhesive N-butyl-2-cyanoacrylate. The bioadhesive compounds were spread on the incision surface and then mixed either manually or with a double-headed syringe. After 14 days, clinical and histologic measurements were performed to evaluate the healing phase of the wounds. The bioadhesive formulation that contained a relatively low crosslinker concentration demonstrated superior results to the formulation that contained a standard crosslinker concentration. However, no significant statistical differences were observed compared with the control incisions (sutures and commercial adhesive N-butyl-2-cyanoacrylate). This was verified by immunohistochemical analysis for epithelial integrity and scar formation as well as by clinical assessment. This newly developed bioadhesive demonstrated suitable properties for the closure of long incisions in a porcine skin model.

Molecular simulation of polymer gels synthesized by free radical copolymerization: Effects of concentrations and reaction rates on structure and mechanical properties

The effects of polymerization concentrations and elementary reaction rates on the network structure and mechanical properties of gels synthesized by free radical copolymerization of monomers and cross-linkers are studied by a coarse-grained molecular dynamics simulation. The shear modulus, the number of elastically effective chains, and the number of trapped entanglements are calculated with varying the concentrations. The results reveal that the experimentally reported high cross-linking efficiency of the gels formed at high monomer concentrations and low cross-linker concentrations is due to the large number of trapped entanglements. The molecular mechanisms of changes in the network structure due to changes in the concentrations are elucidated by analyzing the detailed structure of network strands connecting adjacent cross-linking points. The results with systematically varying the elementary reaction rates indicate that the decrease in the propagation rate suppresses the formation of cyclic structures and reduces the influence of the trapped entanglements, i.e., improves the structural uniformity of the gels.

Crystal Contact Engineering for Enhanced Cross-Linking Efficiency of HheG Crystals

The generation of cross-linked enzyme crystals is a very attractive method for immobilization of enzymes displaying high crystalizability. However, the commonly used cross-linker glutaraldehyde is not always compatible with enzyme activity. Therefore, we previously reported the engineering of halohydrin dehalogenase HheG from Ilumatobacter coccineus to enable thiol-specific cross-linking during CLEC generation by insertion of cysteine residues in the crystal contact. To broaden the applicability of this approach, herein crystal contact engineering of HheG has been performed to incorporate additional lysine residues as defined cross-linking sites for CLEC generation. Using the primary amine-specific cross-linker dithiobis(succinimidyl propionate) (DSP), CLECs of HheG variant V46K were obtained that displayed a high gain in thermal stability compared to wild-type HheG, while using only a low cross-linker concentration. Moreover, respective V46K CLECs exhibited a 10 K higher reaction temperature optimum as well as significantly improved activity and stability at acidic pH and in the presence of organic co-solvents. Overall, our study demonstrates that lysine-specific cross-linkers can also be used as an alternative to glutaraldehyde for stable CLEC generation of halohydrin dehalogenases, and that cross-linking efficiency is significantly improved upon crystal contact engineering.

Minimising chemical crosslinking for stabilising collagen in acellular bovine pericardium: Mechanistic insights via structural characterisations

Chemically crosslinked acellular bovine pericardium (ABP) has been widely used in clinical practice as bioprostheses. To ensure its consistency and durability, crosslinkers are used in excess, with stability guided by indicators including the hydrothermal denaturation temperature, the enzymatic resistance and the degree of crosslinking. Yet, understanding of the intermolecular structure in collagen fibrils which imparts the intrinsic stability of the ABPs is lacking, and the discrepancies in the stability criteria in varied conditions are poorly explained. In this study, synchrotron small-angle X-ray scattering (SAXS) in combination with thermal and colorimetric methods are employed to investigate the changes in the structure and the stability of ABPs during crosslinking using glutaraldehyde (GA) or 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) at different concentrations. Based on the findings, a mechanism is proposed to explicate the crosslinking effects on collagen structure and the relationship between the structure and each stability indicator. At low crosslinker concentrations, the telopeptidyl-helical linkages are preferred, which cause rearrangements in the intermolecular structure of collagen, and efficiently contribute to its stabilities. Excess crosslinking is revealed by a revert trend in structural changes and the plateauing of the stabilities, assigning to the helical-helical linkages and monovalent bindings. The former would improve thermal stability but not collagenase resistance, whereas the latter have negligible effects. Overall, this study provides a mechanistic understanding of the chemical crosslinking of ABPs, which will contribute to the future development of more efficient and economically viable strategies to produce bioprostheses. Statement of significanceChemical crosslinking imparts suitable properties to acellular bovine pericardium (ABP) for clinical applications, yet the understanding is lacking on the structure-stability relationship especially under different crosslinking conditions. Structural evidence in this study differentiates the binding sites during crosslinking in collagen fibrils at different crosslinker concentrations, highlighting the excess usage in the conventional crosslinking treatments. The mechanism based on the structure of collagen also successfully explains the dissimilarity in hydrothermal and enzymatic stabilities with varied crosslinking conditions. Future researches focusing on developing biomaterials via chemical crosslinking of ABPs would benefit from this study, for its contribution to the better understanding of the relationship of collagen structure and functions.

Impact of aluminium acetate particles size on the gelation kinetics of polyacrylamide‐based gels: Rheological and molecular simulation study

AbstractInorganically crosslinked polymers have been intensively implemented for conformance control treatments in oil and gas wells at low temperatures (&lt;90°C) because of their faster gelation time compared to organically crosslinked gels. Crosslinkers such as chromium acetate are known to be toxic, and aluminium‐based alternatives have been introduced. This study aims to investigate the impact of aluminium acetate (AlAc) particle size on the gelation kinetics of polyacrylamide (PAM)‐based gels at a pH of 5 and temperature of 75°C. Moreover, bentonite is used as an additive to delay the crosslinking of PAM/AlAc gels. Reducing the particle size increases the specific surface area of the particles and provides more crosslinking sites. Therefore, lower PAM concentrations (up to 5 wt.%) could be used without AlAc settling. Using 7 wt.% PAM/1 wt.% AlAc with sizes of 25 and 48 μm revealed a crosslinking time of 17 and 115 min, respectively. The addition of bentonite at low crosslinker concentrations (0.5–1 wt.%) did not decrease the gel strength of 7 wt.% PAM/1 wt.% AlAc significantly. The gelation time was extended after adding 1 wt.% bentonite to the formulation where the delay was attributed to the adsorption of AlAc on the bentonite surface that was illustrated by molecular simulation.

Probing the microenvironment of polyacrylamide hydrogel matrix using turbidity and fluorescence recovery after photobleaching: One versus Two phases

The microstructure of polyacrylamide hydrogel matrices was probed by turbidity measurements, and by fluorescence recovery after photobleaching (FRAP) of fluorescently labeled bovine serum albumin (fBSA), as a function of polymer concentration and crosslink density. Turbidity increased with increasing polymer and/or crosslinker concentration. At low polymer and crosslinker concentrations, FRAP dynamics were well described by diffusion through a single polymer phase. However for higher concentrations of polymer and crosslinker, the diffusion model was inadequate, and a second mode, likely corresponding to release of fBSA from regions of relatively dense polymer, was observed. At low network concentrations, probe diffusivity (inversely related to FRAP relaxation time) decreased with increasing polymer and crosslinker concentrations. Following onset of the second mode, however, diffusivity increased with increasing monomer concentration, suggesting that the network through which fBSA diffused became looser. The optical and FRAP measurements, while showing qualitatively similar trends, were not directly correlated, and they likely reflected different modes of network phase separation.

Impacts of crosslinker concentration on nanogel properties and enhanced oil recovery capability

The use of nanogels, or crosslinked polymeric nanoparticles, has been proposed as a means of improving oil recovery in low permeability reservoirs. Nanogels can transport deep into reservoirs and improve homogeneity due to their small size and deformability. Typically, nanogels are polymerized using monomers and crosslinkers, which transform polymers from linear structures to 3D structures. Nanogel properties — including its swelling ratio and strength — can be fine-tuned by the crosslinker concentration. In this study, we investigated the impacts of crosslinker concentration on partially hydrolyzed polyacrylamide (HPAM)-based nanogels. The effect of the degree of crosslinking on the physicochemical properties of nanogel, the corresponding adsorbing behavior on rock surfaces, and consequently, the oil recovery improvement was studied.The results show the nanogels maintained a lower swelling ratio and less negative charge when synthesized at a higher crosslinker concentration. Higher crosslinker concentrations also resulted in lower dispersion viscosity because of the lower volumetric fraction and weaker interparticle attraction. The relationship between viscosity and dispersion concentration is in an agreement with the Krieger-Dougherty model.Core flooding experiments were conducted under a water-only condition and an oil–water two-phase condition. Nanogels with a lower crosslinker concentration were better able to reduce core permeability. It was discovered that nanogel injection pressure continuously increased in water-saturated porous media, whereas it reached a stable state in a two-phase condition. Core flooding tests in water-saturated cores also indicated that nanogels with a higher degree of crosslinking adsorbed more onto rock surfaces. The adsorption over time of each test fits well with the pseudo-second order equation. In addition, despite similar interfacial reduction capability, nanogels crosslinked to a lesser degree were able to improve oil recovery to a greater extent.

Universal relation between crack-growth dynamics and viscoelasticity in glass-rubber transition for filled elastomers

The characteristic power-law relationship between crack-growth speed (v) and tearing energy (Γ), Γ ~ vα in high speed regime (typically, v/cs > 10−3 where cs is the shear wave speed), is investigated for the filled elastomers with varying systematically the material parameters. The types of rubber, the concentration of filler and cross-linker concentration are widely varied in order to change the viscoelastic spectra of the elastomers. The power-law relationship is correlated with linear viscoelastic spectra, since Γ and the relaxation modulus are similarly dependent on temperature. An universal relation is found between the exponents α and κ of the relaxation modulus G(t) ~ t−κ in the glass-rubber transition regime, independently of the material parameters. The expectations of the existing theories fail to account for the observed relation, in particular for the elastomers with high filler contents or low cross-linker concentrations. The universal α-κ relation found here will contribute to predict the crack speed in high speed regime from the linear viscoelastic spectra of the elastomers, and will give a definite basis for the theoretical development in the crack-growth dynamics of elastomers.

Rheology of lightly-cured polydimethylsiloxane liquid blends

The rheology of liquid blends based on PDMS has been studied by varying the components molecular weight, the temperature and the concentration of the curing agent, at values well below the gelation threshold. The formulated lightly-cured blends showed interesting unusual behaviors. An interpretative model based on the nanocomposite polymer, the relaxation of dangling chains and the free-volume variation has been proposed.At low temperatures both the M/H (Medium-in-High molecular weights) and L/H (Low-in-High) blends confirm the previously observed anomalous rheology weakening at very low crosslinker concentrations. This is explained by means of the nanosized random coils of the grafted prepolymer nucleated in situ. As the concentration of curing agent increases, crosslinked isolated domains will be sufficiently wide to give the expected increase in rheological properties.At the temperature of 70 °C, the M/H blend shows a reverse creep behavior: the most crosslinked material exhibits an increase in compliance and the less one a decrease. This behavior has been explained taking into account the entanglement onset with a free-volume reduction (Free-Volume Change model); in contrast, the shorter chains of the micro-domains due to thermal agitation would expand the free-volume interface, with the drop of rheological performance.Moving from the M/H blend to the L/H one, at low temperatures we observe a strong decrease in compliance due to the trapping of the short chains on the entanglement sites, with the increase of the entanglement stability (Entanglement Swelling Tube model).The creep experiments show an unexpected shrinkage phenomenon for liquid blends at higher crosslinker concentration: the crosslinked domains enough extended should aggregate by non-bonding interactions giving a prestressed Bingham fluid.The rheology in rotational mode are confirmed in oscillating regime by the dissipation tanδ maximum at high frequency and the peak broadening with the curing agent concentration.

Alcohol-assisted self-healing network polymer based on vicinal tricarbonyl chemistry

We developed a network polymer with solvent-assisted self-healing ability based on vicinal tricarbonyl chemistry. A copolymer bearing vicinal tricarbonyl pendant groups was crosslinked with α,ω-alkanediol through the addition reaction between diol and vicinal tricarbonyl. The resulting hemiketal acted as a dynamic crosslink because it could undergo reversible alcohol-alcohol or water-alcohol exchange reaction in an ambient condition. Thermal and mechanical analyses showed that the polymer networks were in the rubbery state at room temperature while they became a viscoelastic liquid at elevated temperature. The mechanical property and the qualitative self-healing ability were found to depend on the crosslinker concentration. The network polymer with lower crosslinker concentration was less stiff and could be autonomously healed at room temperature. Those with higher crosslinker concentration were stiffer and could not be autonomously healed at room temperature. However, addition of the simplest monool, methanol, to the cut surfaces drastically improved the self-healing ability of the latter samples. The self-healing mechanism and the role of added methanol were discussed in terms of both the chemical and physical aspects.

Synthesis, characterization and evaluation of polyacrylamide graft starch/clay nanocomposite hydrogel system for enhanced oil recovery

In this paper the suitability of a graft polymer nanocomposite hydrogel system for enhanced oil recovery was examined using polyacrylamide graft starch/clay nanocomposite (a laboratory synthesized product) and chromium (III) acetate (crosslinker). X-ray diffraction analysis, Fourier transform infrared spectrometry analysis, field-emission scanning electron microscopy and transmission electron microscopy were carried out to reveal the laboratory synthesized product as a nanocomposite. The effects of various parameters like salt concentration, pH, temperature, polymer concentration and crosslinker concentration on the properties of the developed gel system were systematically evaluated. The thermal stability of the nanocomposite gel and the conventional gel system were also determined by thermogravimetric analysis. The graft polymer nanocomposite gel system exhibited acceptable gel strength, gelation time and gel stability compared with the conventional gel system. The nanocomposite gels prepared using a low crosslinker concentration showed higher gel strength and required longer gelation time than the conventional gel which is more desirable properties for the effective placement of gel during enhanced oil recovery operations. In addition, sand pack flooding experiments show that the graft polymer nanocomposite gels had better plugging capacity than the conventional gel systems under reservoir conditions. Hence, this gel system may be suitable in the water shutoff treatments required for enhanced oil recovery from oilfields.

Investigation of the luminescence, mechanical, and thermal properties of ZnO-entrapped poly(N-isopropyl acrylamide) gels

ZnO nanoparticles were synthesized in the pores that are formed inside the PNIPAm (poly(N-isopropyl acrylamide)) gels during polymerization. Different pore regions were obtained with changing the cross-linker concentrations of gels. We concluded from the SEM results that when the cross-linker concentration of the gels is increased, the average size of the pores becomes narrower. Thus, depending on the internal porosity of the gel, the size and distribution of ZnO nanoparticles were changed and accordingly it ranged in the fluorescence spectra. It is observed that fluorescence spectra of ZnO nanoparticles released from PNIPAm gel with low cross-linker concentration have only UV emission, while the nanoparticles released from PNIPAm gel with high cross-linker concentration have both the UV and the green emissions of ZnO. The size distributions of these nanoparticles were measured by Stabino-Nanoflex analyzer and it clearly indicates that the width of the size distribution decreases with increasing cross-linker concentration. To see the effect of ZnO nanoparticles on the properties of the gel, we investigated the luminescence, mechanical, and thermal properties of these ZnO/PNIPAm composites and plain gels of which the internal morphology was set by changing the cross-linker concentration. The nature of interactions between the nanoparticles and polymer strands affects both mechanical and thermal behaviors. Compared to plain gels, entrapping ZnO nanoparticles into the gels cause an increased compressive elastic modulus and strength. We proved that ZnO nanoparticles improve considerably the mechanical properties of PNIPAm gels.

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

ABSTRACTThe 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.

Poly(Dimethylaminoethyl Methacrylate)-Based pH-Responsive Hydrogels Regulate Doxorubicin Release at Acidic Condition

Stimuli-responsive biomaterials that alter their function through sensing local molecular cues may enable technological advances in the fields of drug delivery, gene delivery, actuators, biosensors, and tissue engineering. In this research, pH-responsive hydrogel which is comprised of dimethylaminoethyl methacylate (DMAEMA) and 2-hydroxyethyl methacrylate (HEMA) was synthesized for the effective delivery of doxorubicin (Dox) to breast cancer cells. Cancer and tumor tissues show a lower extracellular pH than normal tissues. DMAEMA/HEMA hydrogels showed significant sensitivity by small pH changes and each formulation of hydrogels was examined by scanning electron microscopy, mechanical test, equilibrium mass swelling, controlled Dox release, and cytotoxicity. High swelling ratios and Dox release were obtained at low pH buffer condition, low cross-linker concentration, and high content of DMAEMA. Dox release was accelerated to 67.3% at pH 5.5 for 6-h incubation at 37oC, while it was limited to 13.8% at pH7.4 at the same time and temperature. Cell toxicity results to breast cancer cells indicate that pH-responsive DMAEMA/HEMA hydrogels may be used as an efficient matrix for anti-cancer drug delivery with various transporting manners. Also, pH-responsive DMAEMA/HEMA hydrogels may be useful in therapeutic treatment which is required a triggered release at low pH range such as gene delivery, ischemia, and diabetic ketoacidosis.

Synthesis of Temperature Responsive Poly(N-isopropylacrylamide) Using Ultrasound Irradiation

Ultrasound was employed to synthesize poly(N-isopropylacrylamide) [poly(NIPAM)] either as aqueous solutions or microgels in the absence of a chemical initiator. Poly(NIPAM) of different microstructures can be readily prepared via ultrasound irradiation by varying the reaction temperature. At a preparation temperature of 20 degrees C, poly(NIPAM) was formed in aqueous solutions, whereas, at a higher preparation temperature (beyond the lower critical solution temperature of approximately 32 degrees C), poly(NIPAM) microgels were formed. In addition, the high shear gradients generated by the acoustic cavitation process aid to control or vary the molecular weights of poly(NIPAM) formed in aqueous solutions. The swelling behavior of poly(NIPAM) at different concentrations of sodium dodecyl sulfate (SDS) was also studied. An increase in the transition temperature and hydrodynamic size of the particles was attributed to SDS binding to the polymer network through hydrophobic interactions. Light scattering data reflected the formation of larger microgels at low cross-linker concentrations. The encapsulation of rhodamine B within the microgels was achieved by sonicating the monomer containing the dye. The subsequent release of the dye was consistent with Fickian diffusion and the diffusion coefficient of the dye was estimated as 4.0 x 10(-12) and 3.6 x 10(-11) m(2) s(-1) at 20 and 40 degrees C, respectively. From the diffusion coefficients, the viscosities of the polymer samples at 20 and 40 degrees C were determined using the Stokes-Einstein equation to be 77 and 10 cP, respectively.

Cross-Linking of Reactive Lyotropic Liquid Crystals for Nanostructure Retention

In applications requiring nanoscale order, control over the structure formation is critical for appropriate function. Reactive surfactants forming reactive lyotropic liquid crystals (LLC) enable the formation of nanotructured polymers that exhibit enhanced physical properties when the original order is retained after polymerization. In this work, the use of a cross-linker to retain the LLC order for a monomethacrylate quaternary ammonium surfactant monomer in water during polymerization was explored. The cross-linker segregates to the polar regions of the surfactant assembly, likely near the quaternary ammonium group of the reactive surfactant. The use of a cross-linker enables the retention of the original LLC order after polymerization for hexagonal LLC phases that are not retained by other means. Increased polymerization rates are observed at low cross-linker concentrations. At higher cross-linker concentrations, the polymerization rates decrease because of a decrease in LLC order. More regular ordered morphologies are observed for systems that retain the hexagonal LLC order during polymerization. The resulting LLC order is also present after drying and swelling the polymer in water, therefore producing a polymer with robust LLC order. Enhancements in water uptake were also observed for polymers that retain the original order through cross-linking compared to systems that exhibit change in order. This behavior is directly related to the retention of the original LLC order.

A novel poly(N‐isopropylacrylamide‐co‐N‐hydroxymethyl acrylamide) gel: preparation in the absence/presence of a pore‐forming agent and characterization

AbstractPoly(N‐isopropylacrylamide) (PNIPAM) homopolymer gel and copolymer gels of N‐isopropylacrylamide (NIPAM) and N‐hydroxymethyl acrylamide (NHMAAM) were prepared in the absence/presence of a pore‐forming agents, poly(ethylene glycol) (PEG) (PEG 400 or PEG 2000) using N,N′‐methylenebisacrylamide (NMBA) as a crosslinker in aqueous solution at 4°C. The chemical structures and microscopic morphologies of PNIPAM and P(NIPAM‐co‐NHMAAM) copolymer gels were investigated by FT‐IR and SEM (scanning electron microscopy), respectively. The effects of initial monomer concentration (0.5 and 1 M), the contents of crosslinker (0.25 and 1%) and comonomer (1–3 mol%), the absence/presence of pore‐forming agent in the feed, and the molecular weight of pore‐forming agent on the compressive elastic moduli and equilibrium swelling values (ESVs) of both polymers were investigated. While the use of a pore‐forming agent during the polymer synthesis favored the formation of porous gels and increased the swelling values, it led to decrease in the compressive elastic moduli of PNIPAM and P(NIPAM‐co‐NHMAAM) gels. The swelling values of gels, which were prepared at high monomer (1 M) and crosslinker (1%) contents, were not dependent on the comonomer NHMAAM incorporation to PNIPAM, the use of pore‐forming agents, and the use of former and latter parameters together. The twice increase in initial monomer concentration at low NMBA (0.25%) content highly enhanced the elastic moduli of PNIPAM. One mol% NHMAAM incorporation into PNIPAM especially at low monomer (0.5 M) and crosslinker (0.25%) contents made the highest increase (4.7 times) in the elastic moduli of gel, and further increase in the NHMAAM content of gel decreased the moduli of gels at all crosslinker and monomer concentrations. It was concluded that the increase in the elastic moduli of copolymer gels with 1 mol% NHMAAM, which were prepared at low crosslinker and monomer concentrations, can be attributed to the increase in crosslinking through the condensation reaction between N‐methylol groups or N‐methylol and NH groups in NHMAAM and NIPAM. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Micro- and Macrorheological Properties of Isotropically Cross-Linked Actin Networks

Cells make use of semiflexible biopolymers such as actin or intermediate filaments to control their local viscoelastic response by dynamically adjusting the concentration and type of cross-linking molecules. The microstructure of the resulting networks mainly determines their mechanical properties. It remains an important challenge to relate structural transitions to both the molecular properties of the cross-linking molecules and the mechanical response of the network. This can be achieved best by well defined in vitro model systems in combination with microscopic techniques. Here, we show that with increasing concentrations of the cross-linker heavy meromyosin, a transition in the mechanical network response occurs. At low cross-linker densities the network elasticity is dominated by the entanglement length l e of the polymer, whereas at high heavy meromyosin densities the cross-linker distance l c determines the elastic behavior. Using microrheology the formation of heterogeneous networks is observed at low cross-linker concentrations. Micro- and macrorheology both report the same transition to a homogeneous cross-linked phase. This transition is set by a constant average cross-linker distance l c ≈ 15 μm. Thus, the micro- and macromechanical properties of isotropically cross-linked in vitro actin networks are determined by only one intrinsic network parameter.

Preparation and characterization of nanocomposite hydrogels based on polyacrylamide for enhanced oil recovery applications

AbstractIn the present work, attempts have been made to prepare nanocomposite type of hydrogels (NC gels) by crosslinking the polyacrylamide/montmorillonite (Na‐MMT) clay aqueous solutions with chromium (III). The X‐ray diffraction patterns of the NC gels exhibited a significant increase in d001 spacing between the clay layers, indicating the formation of intercalated as well as exfoliated type of morphology. Exfoliation of the clay layers through out the gel network was found to be predominated, which evidences the high interaction between the polyacrylamide segments and montmorillonite layers. Gelation time as well as variation of viscoelastic parameters such as storage modulus (G′) of the gel network during gelation process at 75°C was studied and followed by rheomechanical spectroscopy (RMS). The NC gels prepared with lower crosslinker concentration showed higher strength and elastic modulus compared with the similar but unfilled polyacrylamide gel. This distinct characteristic of the NC gels yields a gel network structure with high resistance towards syneresis at high temperature in the presence of the oil reservoir formation water. The effects of the composition, such as clay content, crosslinker concentration, and also water salinity upon the gelation rate, gel strength as well as rate of syneresis have been investigated. To optimize the injectivity of the intercalated polyacrylamide solution before the onset of gelation with the gel strength of the final developed gel, sodium lactate was employed as retarder. This was found to be effective to balance these two characteristics of the NC gels, which are aimed to be used for water shut‐off and as profile modifier in enhanced oil recovery (EOR) process during water flooding process. The nanocomposite gels showed much more elasticity and extensibility at low crosslinker concentration compared with the similar but unfilled gel, which makes the NC gels suitable as an in‐depth profile modifier, and also as an oil displacing agent in the heterogeneous oil reservoir in chemical EOR. Effects of the clay content on the thermal stability of the gel network have also been investigated by thermogravimetric analysis (TGA). Scanning electron microscopy (SEM) has been performed upon the NC‐gel samples. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2096–2103, 2006