Density Of Covalent Cross-links Research Articles

Design of hybrid multi-crosslinked hydrogel with a combination of high stiffness, elastic applied in high temperature and high salinity reservoirs

The mechanical strength of conventional hydrogels is inadequate for preventing leakage in high-temperature and high-salinity reservoirs, primarily due to the increased mobility of polymer chains and the degradation of crosslinked networks. Herein, we employed the strategy of ’network nesting and layer reinforcement’ to design a hybrid multi-site crosslinked hydrogel. This approach enhances the interconnections between polymer chains and fortifies the crosslinked network. The shear rheological behavior and temperature sensitive characteristics of the profile control working fluids were evaluated based on rheological dynamics. In the high temperature (140 ℃) and high salinity (21.81 × 104 mg/L) preparation environment, the effects of different components to the mechanical strength of the hydrogel were investigated under dynamic shear and uniaxial compression conditions. The incorporation of N-(hydroxymethyl)acrylamide (NMA) in the copolymerization with acrylamide (AM) resulted in an increase in the covalent cross-linking density, leading to a rise in the shear elastic modulus from 105 Pa to 238 Pa. The use of pre-gelatinized cassava starch (PGCM) as the grafting framework enhanced the bonding between the polymer chains, as evidenced by the increase in uniaxial compressive strength from 0.018 MPa to 0.081 MPa and the rise in peak strain from 55.0 % to 72.9 %. Furthermore, the synergistic effect of fumed silica (AEROSIL), which exploits mechanical coupling between inorganic nano-fillers and organic polymers, increased the compressive strength from 0.081 MPa to 0.418 MPa and raised the peak strain from 72.9 % to 81.8 %. It provides an effective approach to enhance the plugging performance of hydrogels in high temperature and high salinity reservoirs.

Crack-Resistant and Tissue-Like Artificial Muscles with Low Temperature Activation and High Power Density.

Constructing soft robotics with safe human-machine interactions requires low-modulus, high-power-density artificial muscles that are sensitive to gentle stimuli. In addition, the ability to resist crack propagation during long-term actuation cycles is essential for a long service life. Herein, a material design is proposed to combine all these desirable attributes in a single artificial muscle platform. The design involves the molecular engineering of a liquid crystalline network with crystallizable segments and an ethylene glycol flexible spacer. A high degree of crystallinity can be afforded by utilizing aza-Michael chemistry to produce a low covalent crosslinking density, resulting in crack-insensitivity with a high fracture energy of 33720 J m-2 and a high fatigue threshold of 2250 J m-2. Such crack-resistant artificial muscle with tissue-matched modulus of 0.7MPa can generate a high power density of 450W kg-1 at a low temperature of 40°C. Notably, because of the presence of crystalline domains in the actuated state, no crack propagation is observed after 500 heating-cooling actuation cycles under a static load of 220kPa. This study points to a pathway for the creation of artificial muscles merging seemingly disparate, but desirable properties, broadening their application potential in smart devices.

Engineering the viscoelasticity of gelatin methacryloyl (GelMA) hydrogels via small "dynamic bridges" to regulate BMSC behaviors for osteochondral regeneration.

The dynamic extracellular matrix (ECM) constantly affects the behaviors of cells. To mimic the dynamics of ECM with controllable stiffness and energy dissipation, this study proposes a strategy in which a small molecule, 3,4-dihydroxybenzaldehyde (DB), was used as fast "dynamic bridges'' to construct viscoelastic gelatin methacryloyl (GelMA)-based hydrogels. The storage modulus and loss modulus of hydrogels were independently adjusted by the covalent crosslinking density and by the number of dynamic bonds. The hydrogels exhibited self-healing property, injectability, excellent adhesion and mechanical properties. Moreover, the in vitro results revealed that the viscous dissipation of hydrogels favored the spreading, proliferation, osteogenesis and chondrogenesis of bone marrow mesenchymal stem cells (BMSCs), but suppressed their adipogenesis. RNA-sequencing and immunofluorescence suggested that the viscous dissipation of hydrogels activated Yes-associated protein (YAP) by stabilizing integrin β1, and further promoted nuclear translocation of smad2/3 and β-catenin to enhance chondrogenesis and osteogenesis. As a result, the viscoelastic GelMA hydrogels with highest loss modulus showed best effect in cartilage and subchondral bone repair. Taken together, findings from this study reveal an effective strategy to fabricate viscoelastic hydrogels for modulating the interactions between cells and dynamic ECM to promote tissue regeneration.

Tyramine-Functionalized Alginate-Collagen Hybrid Hydrogel Inks for 3D-Bioprinting.

Extrusion-based 3D-bioprinting using hydrogels has exhibited potential in precision medicine; however, researchers are beset with several challenges. A major challenge of this technique is the production of constructs with sufficient height and fidelity to support cellular behavior in vivo. In this study, we present the 3D-bioprinting of cylindrical constructs with tunable gelation kinetics by controlling the covalent crosslinking density and gelation time of a tyramine-functionalized alginate hydrogel (ALG-TYR) via enzymatic reaction by horseradish peroxidase (HRP) and hydrogen peroxide (H2O2). The extruded filament was crosslinked for a second time on a support bath containing H2O2 to increase fidelity after printing. The resulting tubular construct, with a height of 6 mm and a wall thickness of 2 mm, retained its mechanical properties and had a maximum 2-fold swelling after 2 d. Furthermore, collagen (COL) was introduced into the ALG-TYR hydrogel network to increase the mechanical modulus and cell cytocompatibility, as the encapsulated fibroblast cells exhibited a higher cell viability in the ALG-TYR/COL construct (92.13 ± 0.70%) than in ALG-TYR alone (68.18 ± 3.73%). In summary, a vascular ECM-mimicking scaffold was 3D-bioprinted with the ALG-TYR/COL hybrid hydrogel, and this scaffold can support tissue growth for clinical translation in regenerative and personalized medicine.

A molecular design approach towards elastic and multifunctional polymer electronics

Next-generation wearable electronics require enhanced mechanical robustness and device complexity. Besides previously reported softness and stretchability, desired merits for practical use include elasticity, solvent resistance, facile patternability and high charge carrier mobility. Here, we show a molecular design concept that simultaneously achieves all these targeted properties in both polymeric semiconductors and dielectrics, without compromising electrical performance. This is enabled by covalently-embedded in-situ rubber matrix (iRUM) formation through good mixing of iRUM precursors with polymer electronic materials, and finely-controlled composite film morphology built on azide crosslinking chemistry which leverages different reactivities with C–H and C=C bonds. The high covalent crosslinking density results in both superior elasticity and solvent resistance. When applied in stretchable transistors, the iRUM-semiconductor film retained its mobility after stretching to 100% strain, and exhibited record-high mobility retention of 1 cm2 V−1 s−1 after 1000 stretching-releasing cycles at 50% strain. The cycling life was stably extended to 5000 cycles, five times longer than all reported semiconductors. Furthermore, we fabricated elastic transistors via consecutively photo-patterning of the dielectric and semiconducting layers, demonstrating the potential of solution-processed multilayer device manufacturing. The iRUM represents a molecule-level design approach towards robust skin-inspired electronics.

High-Temperature-Aging Induced Sequential Recovery of Shape Memory Nitrile Butadiene Rubber Composites.

The sacrificial bonds in natural materials have inspired the preparation of shape memory polymer (SMP), which can be prepared through the construction of dual cross-linking networks in a polymer matrix. With the rise of 4D printing technology, fine control over the shape recovery of SMPs, especially control over the recovery time, is urgently needed. In this study, the high-temperature aging method is adopted to tune the shape recovery time of dual cross-linked SMPs. Shape memory acrylonitrile butadiene rubber composite (i.e., NBR-C) is prepared by introducing Zn2+-C≡N coordination bonding and sulfur covalent cross-linking networks into the rubber matrix and then thermal aging at 180 °C for various time frames. Aging increases the covalent cross-linking density, ruptures rubber chains, and generates imine structures. Moreover, the composition of the coordination bonding network becomes diversified because of the formation of coordination bonds between imines and Zn2+ ions. The mechanical "tough-brittle" transition of aged NBR-C is observed, and its glassy temperature increases with aging time, which in turn changes the shape recovery time at the same recovery temperature. On the basis of these findings, the special shape memory rubber components with sequential recovery are fabricated by partially aging the NBR-C strings. This methodology provides novel solutions for the preparation of sequential SMP products without programming heating design or using redundant chemical materials. We believe that this work will be able to help promote comprehensive research of SMPs and widen applications of SMPs in the industry.

Thermal-oxidative aging behaviors of shape memory nitrile butadiene rubber composite with dual crosslinking networks

Inspired by natural materials, sacrificial bonds have been successfully applied in the design of flexible shape memory polymers (SMPs). However, thermal oxidative stability of these SMPs is worth to be concerned because it is inevitable for the applications. Herein, model shape memory rubber composite was prepared by adding ZnCl2 and sulfur into nitrile butadiene rubber (NBR). Dual crosslinking networks with sacrificial bonds, composed of Zn2+-CN coordination bonds, was constructed in NBR composite, which endowed itself a good shape memory property in air, water and even oil. Through 20-day thermal oxidation at 100 °C, Zn2+-CN coordination bonds were partially dissociated and ZnCl2 agglomerations were gradually dissolved into rubber matrix. The growth of covalent crosslinking density accompanied with chains scission during aging. Excessive growth in covalent crosslinking density and the loss of coordination bonds induced a rapid ductile-brittle transition of NBR composite in mechanical tests. Ruptured chains developed an additional relaxation with Zn2+ ions to broaden glassy transition of rubber matrix through aging, which increased the shape fixing ratio of NBR composite and descended its recovery ratio. Moreover, the shape recovery processes were obviously slowed down owing to aging. These novel findings reveal that thermal-oxidative aging processes change the architecture of dual crosslinking networks and affect shape memory behaviors of modeled rubbers which are practically useful for the lifetime of their products.

Tuning the structure and mechanical properties of double-network elastomer: Molecular dynamics simulation

Double-network polymers constructed by physical interaction network and covalent network, have aroused many concerns for their improved mechanical properties. Even though many experiments have revealed how to combine the physical interaction network with the covalent network, we still need a understanding of the structure-property relationship in the double-network polymers. Through coarse-grained molecular dynamics (MD) simulation, we have successfully designed the double-network system consisting of repairable physical interaction network and covalent cross-linking network. Firstly, we examined the kinetic of the covalent cross-linking reaction under high temperature ( T *>2.0 k B T / e ), which is consistence with the experiment. With the model above, we try to tune the mechanical properties by adjusting the covalent cross-linking density and the strength of non-bonding physical interaction. By systematically analyzing the radial distribution function, the virial coefficient of the beads constructing the physical interaction network under equilibration state and the stress-strain curves of all systems under unixial deformation, we found that the strength of non- bonding interaction finally dominate the contribution from physical network on the mechanical properties. Typically, only when the strength of non-bonding interaction of the physical network is higher than 5.0 e can the physical network finally form via the aggregation of physical cross-linking sites and then enhance the mechanical properties of the system. Meanwhile, the covalent network with high cross-linking density evidently perform an improvement of the tensile stress, which also better promote the contribution from physical network on the total non-bonding interaction energy. Moreover, the stress relaxation curves of double-network systems with different covalent cross-linking density also validate that the coarse-grained cross-linking process is reasonable. Finally, to investigate the self-healing property of the physical interaction network, we defined the efficiency of self-healing and study the effect of temperature. The physical interaction network could be fully reversible at high temperature ( T *>1.0 k B T / e ), though it cannot fully recover to its initial state after long enough self-healing process at the normal temperature T *>1.0 k B T / e . In general, we found the threshold of the physical interaction network where the physical network can effectively strength the mechanical properties. Constructing double-network with proper strength of physical interaction (above 12.5 kJ mol-1) may achieve the formation and good dispersion of physical cross-linking sites. Recently, the double-network polymer consisting of the physical and chemical cross-linking network has become the new trend, which may combine good mechanical properties and self-healing properties. Our work above may provide some guidances for the design of high-performance elastomer by studying the structure double-network, and it also meets the demand of tuning the structure and property of the double-network polymer.

Freestanding ultrathin films for separation of small molecules in an aqueous environment

Alternative separation methods operating in an aqueous environment are of increasing importance for further progress of molecular separation in life sciences and other industrial sectors working towards a biobased economy. By spincoating, membranes with thicknesses under 100 nm and 20 cm2 surface area were prepared from an epoxy based resin. For the first time such ultrathin epoxy films were used for the selective separation of small molecules and metabolites within an aqueous environment. Initially, selectivity is demonstrated by the separation of two dyes of similar size (0.7 and 1.4 nm diameter). By variation of the precursor concentrations, both mechanical stability and selectivity for molecular transport are shown to be tunable. The observed transport properties of the different membranes correlated with their biaxial moduli and ultimate tensile strengths which were in the range of 0.3–3.5 GPa and 10–44 MPa, respectively. These observations agreed with the conclusion drawn from FTIR analysis that variations in the covalent crosslinking density determine the emergent properties. Finally, permeation rates for small molecules of industrial relevance were assessed to confirm a size based diffusion cutoff for compounds with hydrodynamic diameters below 2 nm.

Gallium-Loaded Dissolvable Microfilm Constructs that Provide Sustained Release of Ga(3+) for Management of Biofilms.

The persistence of bacterial biofilms in chronic wounds delays wound healing. Although Ga(3+) can inhibit or kill biofilms, precipitation as Ga(OH)3 has prevented its use as a topical wound treatment. The design of a microfilm construct comprising a polyelectrolyte film that releases noncytotoxic concentrations of Ga(3+) over 20 d and a dissolvable micrometer-thick film of polyvinylalcohol that enables facile transfer onto biomedically important surfaces is reported. By using infrared spectroscopy, it is shown that the density of free carboxylate/carboxylic acid and amine groups within the polyelectrolyte film regulates the capacity of the construct to be loaded with Ga(3+) and that the density of covalent cross-links introduced into the polyelectrolyte film (amide-bonds) controls the release rate of Ga(3+) . Following transfer onto the wound-contact surface of a biologic wound dressing, an optimized construct is demonstrated to release ≈0.7 μg cm(-2) d(-1) of Ga(3+) over 3 weeks, thus continuously replacing Ga(3+) lost to precipitation. The optimized construct inhibits formation of P. aeruginosa (two strains; ATCC 27853 and PA01) biofilms for up to 4 d and causes pre-existing biofilms to disperse. Overall, this study provides designs of polymeric constructs that permit facile modification of the wound-contacting surfaces of dressings and biomaterials to manage biofilms.

Synthesis and characterization of a d-glucose sensitive hydrogel based on CM-dextran and concanavalin A

A facile method is described for the preparation of d-glucose-sensitive hydrogel membranes based on cross-linking carboxymethyl dextran (CM-dextran) with the glucose binding lectin concanavalin A (ConA) using carbodiimide chemistry. The mesh size of these hydrogels is determined by both covalent cross-links and affinity interactions between ConA and terminal glucose moieties on the dextran chains. Competitive displacement of the affinity interactions by d-glucose leads to changes in both morphology and permeability. The carbodiimide coupling chemistry has the advantage that it introduces no potentially cytotoxic groups into the gels formed rendering them more suitable for potential in vivo applications. Infrared spectra of the hydrogels suggest that, in addition to direct cross-linking of CM-dextran chains, gels are also stabilized by formation of amide bonds between amine groups in the ConA and carboxyl groups in CM-dextran. This suggests that the mechanical properties of the gels can be modified by the inclusion of inert proteins to increase the density of covalent cross-links. The permeability of the hydrogels, as shown by protein diffusion, increases in response to changes in the d-glucose concentration of the external medium, causing competitive displacement of the affinity cross-links. Sequential addition and removal of external glucose in a stepwise manner showed that these permeability changes are reversible. Results obtained using isothermal titration calorimetry confirmed the competitive binding between ConA-dextran and d-glucose this was confirmed to be a biospecific effect by the observation that l-glucose was not bound by free or dextran coupled ConA. Scanning electron microscopy of samples prepared using cryofixation and cryofracturing techniques showed that observed changes in permeability correlate well with glucose dependent structural changes in the gel. The effects of varying pH and ionic strength show that ion exchange effects also influence diffusion rates. However, an advantage of the synthetic route described is that the intrinsic charge density of the hydrogels produced can be tailored to the specific interaction by varying the degree of carboxylic substitution on the original dextran. The ease of preparation and control of final gel properties allowed by this approach offers possibilities for a range of biotechnological applications including drug delivery and biosensors.

Biodegradability of wheat gluten based bioplastics

A large variety of wheat gluten based bioplastics, which were plasticized with glycerol, were subjected to biodegradation. The materials covered the total range available for the biochemical control parameter F i , which expresses the percentage of aggregated proteins. This quantity can be related to the density of covalent crosslinks in the wheat gluten network, which are induced by technological treatments. The biodegradability tests were performed in liquid medium (modified Sturm test) and in farmland soil. All gluten materials were fully degraded after 36 days in aerobic fermentation and within 50 days in farmland soil. No significant differences were observed between the samples. The mineralization half-life time of 3.8 days in the modified Sturm test situated gluten materials among fast degrading polymers. The tests of microbial inhibition experiments revealed no toxic effects of the modified gluten or of its metabolites. Thus, the protein bulk of wheat gluten materials is non-toxic and fully biodegradable, whatever the technological process applied.

Independent Control of Rigidity and Toughness of Polymeric Hydrogels

Refined control over the mechanical properties of hydrogel-based materials has increased as these materials have found broader application. We investigated various aspects of gel cross-linking to independently regulate the elastic modulus (E) and toughness (W). Alginate hydrogels were chosen as a model system, since alginate can be gelled via ionic or covalent cross-linking, and its block structure dictates the structure of ionic cross-links. Increasing the density of covalent cross-links increased E but led to more brittle gels. In contrast, increasing the density of ionic cross-links and length of the blocks responsible for the cross-linking increased both E and W. Oscillatory shear measurements suggested that ionic cross-links and their length were important in dissipating the energy of deformation due to a partial and stepwise de-cross-linking. In contrast, covalently cross-linked gels underwent energy accumulation. This study demonstrates a novel approach to independently control different mechanical properties of gels.

Thermal and devitrification behavior of CaO–Ga 2O 3–SiO 2 glasses

A study of the influence of the substitution of Ga 2O 3 for CaO, at constant O/Si ratio, on thermal properties and non-isothermal devitrification of 2.5CaO.2SiO 2 is reported. Differential thermal analysis (DTA) and X-ray diffraction analysis were used. The X-ray diffraction pattern of the crystallized Ga 2O 3 base glass shows that the αCaO SiO 2, that should be stable only above 1125°C, forms in the temperature range 900–1000°C. A new ternary crystalline phase, whose reflections are not reported in the JCPDS cards, was found to form during crystallization of the glass. The glass transformation temperature, T g, and softening, T s, temperature decrease as Ga 2O 3 is substituted for CaO. This is the result of the substitution of the network modifying cation Ca 2+, of higher coordination number, by a network forming cation Ga 3+ in fourfold coordination, in a composition range of relative insensitivity to changes of covalent cross-linking density. The crystal growth activation energy, E c, decreases with substitution; this is the consequence of the decrease of the structural rigidity and of the shift of crystallization to a higher temperature range. Devitrification involves a mechanism of surface nucleation; surface nuclei behaving as bulk nuclei in samples that soften and sinter before devitryfing.

Effects of solvent steric properties on the equilibrium swelling and kinetics of solvent swelling of coal

The effects of solvent molecular size on the equilibrium swelling ratio and kinetics of the swelling of a high-volatile bituminous coal in straight-chain amines were examined. The swelling ratio increases with amine size up to a critical level, above which the swelling ratio decreases. This decrease is attributed to reduced access of the solvent to the coal structure with increasing molecular size. The maximum swelling is evidently limited by the density of covalent cross-links, which prevents the coal structure from swelling further to accommodate larger molecules. Rate constants for solvent swelling decrease with increase in molecular size of the amine. Values of the apparent activation energy show that the energy barrier for the swelling process increases with amine size. Clearly, molecular size is a factor which must be considered when choosing solvents for solvent swelling studies on coal.