Higher Crosslinker Content Research Articles

Effect of particle stiffness on microgel self-assembly and suspension phase behavior over a broad temperature range

We synthesized thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) colloidal microgel particles of different stiffnesses by controlling the concentration of a polar crosslinker in a precipitation polymerization synthesis method. When suspended in an aqueous medium, the particles collapsed by expelling water as the temperature was raised toward the volume phase transition temperature (VPTT) of ≈ 34 °C. We noted that the sizes of the stiffer particles, synthesized with higher crosslinker concentration, collapsed less abruptly. Using Fourier transform infrared spectroscopy, we observed enhanced particle dehydration with increasing temperature and decreasing particle stiffness. Oscillatory rheology experiments on dense aqueous PNIPAM suspensions, prepared at a fixed particle effective volume fraction ϕeff = 1.5 at 25°C, revealed that suspensions constituted by the stiffest particles are the most elastic over a broad temperature range. Above the VPTT, suspensions of particles of intermediate stiffnesses exhibited two-step yielding, a typical signature of fragile gel formation. Zeta potential measurements showed that PNIPAM particles of lower stiffnesses are rendered electrostatically unstable in aqueous suspension. Combining cryogenic scanning electron microscopy and rheology, we noted a glass–glass transition when the temperature of a dense suspension of stiff PNIPAM particles was raised across the VPTT. In contrast, suspensions of particles of the lowest stiffnesses showed a gel-viscoelastic liquid–gel transition during an identical temperature ramp experiment. Our study reveals that temperature-induced phase transformations in dense PNIPAM suspensions depend sensitively on the stiffness of the constituent particles and can be explained by considering amphiphilicity-driven morphological changes in the suspension microstructures.

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.

Probing the influence of crosslinkers on the properties, response, and degradation of enzymatic hydrogels for electrochemical glucose biosensing through fluorescence analysis.

Drop-cast crosslinked hydrogels are a common platform for enzymatic electrochemical biosensors. Despite the widespread use of these complex systems, there are still several questions about how their physicochemical properties affect their performance, stability, and reproducibility. In this work, first-generation faradaic biosensors composed of glucose oxidase and branched polyethyleneimine (BPEI) are prepared using either glutaraldehyde (GA) or ethylene glycol diglycidyl ether (EGDGE) as crosslinkers. While EGDGE gels present an increasing electrochemical response with increasing crosslinker concentration, the current of GA gels decreases at high crosslinker concentration probably due to the hampered diffusion on tightly networked gels. We compared different strategies to use fluorescence microscopy to gain insight into the gel structure either by labeling the gel components with fluorophores or taking advantage of the intrinsic fluorescence of the imines formed upon crosslinking with GA. By monitoring the fluorescence of the crosslinking bonds and the electrochemical response, we demonstrate that hydrolysis, a common hydrogel degradation mechanism, is not responsible for the loss of electrical current over time in gels prepared with glutaraldehyde. Most hydrogel-based electrochemical biosensor studies do not perform specific experiments to determine the cause of the degradation and instead just infer it from the dependence of the current on the preparation conditions (most commonly concentrations). We show that, by taking advantage of several analytical techniques, it is possible to gain more knowledge about the degradation mechanisms and design better enzymatic biosensors.

Cofilin-mediated actin filament network flexibility facilitates 2D to 3D actomyosin shape change

The organization of actin filaments (F-actin) into crosslinked networks determines the transmission of mechanical stresses within the cytoskeleton and subsequent changes in cell and tissue shape. Principally mediated by proteins such as α-actinin, F-actin crosslinking increases both network connectivity and rigidity, thereby facilitating stress transmission at low crosslinking yet attenuating transmission at high crosslinker concentration. Here, we engineer a two-dimensional model of the actomyosin cytoskeleton, in which myosin-induced mechanical stresses are controlled by light. We alter the extent of F-actin crosslinking by the introduction of oligomerized cofilin. At pH 6.5, F-actin severing by cofilin is weak, but cofilin bundles and crosslinks filaments. Given its effect of lowering the F-actin bending stiffness, cofilin- crosslinked networks are significantly more flexible and softer in bending than networks crosslinked by α-actinin. Thus, upon local activation of myosin-induced contractile stress, the network bends out-of-plane in contrast to the in-plane compression as observed with networks crosslinked by α-actinin. Here, we demonstrate that local effects on filament mechanics by cofilin introduces novel large-scale network material properties that enable the sculpting of complex shapes in the cell cytoskeleton.

Impact of composition and salinity on swelling and gel strength of poly (acrylamide-co-acrylic acid) preformed particle gel

The effects of various material compositions and reservoir environments on the ultimate strength and swelling kinetics of a commercial preformed particle gel (PPG) have been investigated. This study used different ratios of acrylamide and acrylic acid copolymers with a specific crosslinker concentration. Results have indicated that increasing the acrylic acid proportion enhances the PPGs’ ability to swell but weakens their network structure. In contrast, increasing the crosslinker content decreases the swelling ratio and increases the gel strength. The highest equilibrium swelling capacity among the six preformed particle gel samples was obtained for PPG2, which has the highest acrylic acid amount and the lowest crosslinker content, with a swelling ratio of 2400 g/g in deionized water and 59.8 g/g in brine 1 (67535.8 mg/l). On the contrary, PPG5, with the lowest acrylic acid and highest crosslinker content, has a swelling capacity of 239 g/g and more than 17 g/g in distilled and brine 1, respectively. Yet, PPG5 has the highest swollen gel strength of 615.5 Pa in deionized water and 3344 Pa in brine 1. The PPGs’ swelling ratios showed stepwise improvements along with increasing temperature, notably after 50 °C, yet, the storage modulus (G′) was negatively affected. The PPGs revealed the highest swelling behavior in pH 6–8, decreasing dramatically in more acidic and basic conditions. The swelling ratios of the PPGs in brine 1 at 50 °C were between 12 and 32 g/g, having strengths in the range of 566–5508 Pa, depending on the crosslinker ratio. The PPGs also demonstrated the ability to compete with other commercial PPGs as they have shown physical and thermal stability when aging at 50 °C, specifically those with high crosslinker content (PPG5).

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.

Measuring dn/dc for Polysaccharide Microgels of Varying Crosslinking Density

Static light scattering (SLS) is a powerful, noninvasive experimental method that yields the molecular weight (average molecular weight, Mw), the size (radius of gyration, Rg), and the interactions between the scatters (the second virial coefficient, A2). However, proper SLS measurements require determination of the specific refractive index increment (dn/dc) for the samples studied. While tables of dn/dc values are available for various substances, they are not generally available for microgel particles, which in our case are crosslinked chains of an amphiphilic polymer. This paper is focused on measuring dn/dc for microgel samples of varying crosslinking concentrations. Microgel dn/dc values were found to be different from the value of the parent polymer given in literature and found to have a temperature dependence as well as a crosslinker concentration dependence at higher crosslinker concentrations. Using the measured values of microgel dn/dc instead of tabulated parent polymer values on estimates of microgel Mw, Rg, and A2 highlights the importance of direct dn/dc measurements for samples studied by SLS.

Electrospun PVA–Dextran Nanofibrous Scaffolds for Acceleration of Topical Wound Healing: Nanofiber Optimization, Characterization and In Vitro Assessment

Electrospun polyvinyl alcohol–dextran (PVA–Dex)-based nanofibers (NFs) are explored as a novel class of bioactive injury dressing materials, which have an essential role for topical injury mending. Sodium ampicillin-loaded citric acid-cross-linked PVA–Dex NFs were fabricated by electrospinner for wound recuperating purposes. Results revealed that PVA (10%)–dextran (10%) cross-linked with 5% citric acid (CA) was chosen as an optimized condition for obtaining non-beaded and morphological accepted nanofibers. Altered concentrations of CA as cross-linker progressively enhanced significantly the mechanical/thermal stability and wettability-proof of NFs scaffolds, compared to un-cross-linked (PVA–Dex) scaffolds. Meanwhile, swelling (%), protein adsorption and released ampicillin of NFs decreased dramatically with the increase in the CA concentration, and conversely enhanced with increasing dextran concentrations. Interestingly, resultant PVA–Dex NFs with high concentrations of dextran promoted the proliferation of HFB-4 cells in a high concentration-dependent manner and high antimicrobial activity behavior, compared to NFs containing high concentrations of CA cross-linker after 24 and 48 h of cell exposure. Notably, all fabricated NFs have remarked ability to accelerate the rate of in vitro wound gap closure (%) after treatment for 24 and 48 h, compared to control sample. However, reducing CA concentration in NFs showed the highest percentages of wound healing for scratched HFB-4 cells with clear observed healing process.

Glutaraldehyde: Introducing Optimum Condition for Cross-linking the Chitosan/Gelatin Scaffolds for Bone Tissue Engineering

Large bone defects caused by trauma or disease needs extra intervention. Gelatin/chitosan complex is one of the most valuable compositions for bone healing, but fast degradation in aqueous solution and low mechanical properties increase the need for cross-linking agent. The cross-linker concentration and cross-linking method had a significant effect on the properties of fabricated scaffolds. Here, three different cross-linking methods of Glutaraldehyde (GA), including addition to the solution, vapor exposure, and immersion, were studied by different in-vitro analyses to find the best GA cross-linker concentration and cross-linking method. Scanning electron microscopy showed homogeneous microstructures in all samples. Fourier transform infrared spectrophotometry revealed cross-linking reactions in all samples. Swelling ratio and biodegradation ratio was reduced by increasing cross-linking concentration and exposure time. Nonetheless, higher cross-linker concentration and exposure time improved mechanical properties, while it seems the cross-linking exposure time had more effect than concentration. Accordingly, GA (1 wt%) cross-linked scaffold with solution addition method showed suitable performance with 39.3° contact angle, 1.45±0.05 MPa compressive strength, 22.31±1.3 (%) swelling ratio, and 26.33±4.47 (%) biodegradation ratio. In-vitro experiments indicated cells were spread all over the scaffolds with higher than 80 (%) cell viability in all time points. The expression of alkaline phosphatase (ALP) and osteo-related genes (osteocalcin and related transcription factor 2) were improved during 14 days of cell incubation and showed the high capacity of the scaffold support in mineralization and osto-differentiation. Therefore GA (1 wt% ) cross-linked scaffold with solution addition was introduced as the best candidate for bone repair and further studies.

The effect of crosslinking on ion transport in nanocellulose-based membranes

Ion selective membranes are at the heart of energy conversion and harvesting, water treatment, and biotechnologies. The currently available membranes are mostly based on expensive and non-biodegradable polymers. Here, we report a cation-selective and low-cost membrane prepared from renewable nanocellulose and 1,2,3,4-butanetetracarboxylic acid which simultaneously serves as crosslinker and source of anionic surface groups. Charge density and structure of the membranes are studied. By using different degrees of crosslinking, simultaneous control over both the nanochannel structure and surface charge concentration is achieved, which in turn determines the resulting ion transport properties. Increasing negative charge concentration via higher crosslinker content, the obtained ion conductivity reaches up to 8 mS/cm (0.1 M KCl). Optimal ion selectivity, also influenced by the solution pH, is achieved at 20 wt% crosslinker addition (with ion conductivity of 1.6 mS/cm). As regular ~1.4 nm nanochannels were formed at this composition, nanofluidic contribution to ion transport is likely.

Glyceraldehyde as an Efficient Chemical Crosslinker Agent for the Formation of Chitosan Hydrogels.

The rheological changes that occur during the chemical gelation of semidilute solutions of chitosan in the presence of the low-toxicity agent glyceraldehyde (GCA) are presented and discussed in detail. The entanglement concentration for chitosan solutions was found to be approximately 0.2 wt.% and the rheological experiments were carried out on 1 wt.% chitosan solutions with various amounts of GCA at different temperatures (25 °C and 40 °C) and pH values (4.8 and 5.8). High crosslinker concentration, as well as elevated temperature and pH close to the pKa value (pH ≈ 6.3–7) of chitosan are three parameters that all accelerate the gelation process. These conditions also promote a faster solid-like response of the gel-network in the post-gel region after long curing times. The mesh size of the gel-network after a very long (18 h) curing time was found to contract with increasing level of crosslinker addition and elevated temperature. The gelation of chitosan in the presence of other chemical crosslinker agents (glutaraldehyde and genipin) is discussed and a comparison with GCA is made. Small angle neutron scattering (SANS) results reveal structural changes between chitosan solutions, incipient gels, and mature gels.

Formation and Performance Evaluation of Colloidal Dispersion Gels Prepared Using Sulfonated Polyacrylamides and Chromium (III) Acetate

Using a sulfonated polyacrylamide (SPAM) and Cr3+, a new colloidal dispersion gel (CDG) was prepared. The viscosity of the CDG samples in different crosslinker concentrations and brine compositions was measured. The results showed that CDGs approach a Newtonian-like behavior in high crosslinker concentrations and salinities, signifying that they possess more rigid, less flexible particles that can be used to block some of the pore throats of the high-permeability layers. Therefore, three core flood tests were performed and the retention of the polymers and the final RRF values (residual resistance factor) were determined. Although CDGs showed a lower tendency to be adsorbed onto the rocks, they caused drastically higher RRF values (caused higher permeability reductions). Thus, it can be concluded that CDGs are superior compared to normal polymer solutions in modifying the permeability. Moreover, changing the post-flood fluid from brine to distilled water caused the RRF to decrease, hence a weaker effect on the permeability.

Liquid crystal polymer networks directed by scanning wave photopolymerization of oxetane monomer and crosslinker

In this study, we fabricated liquid crystal polymer networks (LCNs) containing a large amount of crosslinker by scanning wave photopolymerization, and examined the effect of crosslinker concentration on molecular alignment behavior. Samples with different crosslinker concentrations were photopolymerized by unidirectionally scanning ultraviolet light, resulting in LCN films with uniaxial molecular alignment. The LCN films obtained from the compositions of only monomer or monomer and crosslinker had mesogens aligned in parallel to the light scanning direction. Moreover, the LCN films with high crosslinker concentrations were able to maintain their initial molecular alignment after repeated heating cycles.

A universal method to easily design tough and stretchable hydrogels

Hydrogels are flexible materials that have high potential for use in various applications due to their unique properties. However, their applications are greatly restricted by the low mechanical performance caused by high water content and inhomogeneous networks. This paper reports a universal strategy for easily preparing hydrogels that are tough and stretchable without any special structures or complicated processes. Our strategy involves tuning the polymerization conditions to form networks with many polymer chain entanglements to achieve energy dissipation. Tough and stretchable hydrogels can be prepared by free radical polymerization with a high monomer concentration and low cross-linker content to optimize the balance between physical and chemical cross-links by entanglements and covalent bonds, respectively. The strategy of using polymer chain entanglements for energy dissipation allows us to overcome the limitation of low mechanical performance, which leads to the wide practical use of hydrogels.

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.

A comparison of the network structure and inner dynamics of homogeneously and heterogeneously crosslinked PNIPAM microgels with high crosslinker content.

Poly(N-isopropylacrylamide) microgel particles were prepared via a "classical" surfactant-free precipitation polymerization and a continuous monomer feeding approach. It is anticipated that this yields microgel particles with different internal structures, namely a dense core with a fluffy shell for the classical approach and a more even crosslink distribution in the case of the continuous monomer feeding approach. A thorough structural investigation of the resulting microgels with dynamic light scattering, atomic force microscopy and small angle neutron scattering was conducted and related to neutron spin echo spectroscopy data. In this way a link between structural and dynamic features of the internal polymer network was made.

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.

Thermomechanical characterisation of cross-linked β-cyclodextrin polyether binders

Cyclodextrins are promising building blocks for the synthesis of industrial binders. A new binder was prepared by cross-linking β-cyclodextrin with variable amounts of polyethylene glycol diglycidyl ether (40–60% w/w) to produce a soft polyether network that was soluble in water and alcohol, and the thermomechanical properties of the binder were determined. Increasing the amount of cross-linker reduced the glass transition temperature of the binder, as determined by differential scanning calorimetry and dynamic mechanical analysis. Cooling experiments revealed sudden stress relief below the glass transition temperature, reflecting the de-bonding of the polymer from the metallic supports. This was prevented by contact with polytetrafluoroethylene tape. Optical microscopy confirmed the stress relief in the form of cracking, and revealed self-healing by reptation, promoted by a higher cross-linker content and temperature. The information gained on the influence of the support medium on the thermomechanical properties of the cross-linked β-cyclodextrins can be used by industry for optimising manufacture and storage methods for new binders.