Effect Of Crosslink Density Research Articles

Thermoresponsive Supramolecular Hydrogels with High Fracture Toughness

Supramolecular hydrogels formed from random copolymers of N-isopropylacrylamide (NIPAM) and 2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate (FOSA) exhibit a volume phase transition due to a lower critical solution temperature (LCST). The LCST can be tuned between 32 and 5 °C by incorporating up to 14 mol % FOSA in the copolymer. The tensile modulus and strength of the hydrogels increased above the LCST as a consequence of an increase in the effective cross-link density due to the phase separation of water from the hydrogel. Below the LCST, the hydrogels exhibited extraordinary fracture toughness and crack blunting capability. Fracture energies of ∼8000 J/m2 were achieved, which is comparable or greater than that of cartilage and what has been previously achieved with synthetic supramolecular hydrogels. These gels exhibited large hysteresis behavior, though unlike tough double network hydrogels, the NIPAM/FOSA hydrogels can fully recover their dimensions (i.e., no permanent set) and properties after te...

In situ crosslinking of poly(vinyl alcohol)/graphene oxide Nano-composite hydrogel: intercalation structure and adsorption mechanism for advanced Pb(II) removal

Based on the industrialized graphene oxide (GO) product, poly(vinyl alcohol) (PVA)/GO nano-composite hydrogels were prepared through in situ crosslinking by incorporation of N-[(trimethoxysilyl)propyl] ethylenediamine-triacetic acid sodium (CSA) as a compatibilizer. Introduction of CSA led to more efficient grafting of PVA molecules onto GO surface with increasing average layer thickness through covalent and hydrogen bonding interaction, while GO was exfoliated and uniformly distributed in PVA matrix. Addition of appropriate content of GO can improve the storage modulus and the effective crosslinking density (υe) of the composite hydrogel, and the network structure with GO as crosslinking point formed, resulting in the remarkable increase of the hydraulic impact resistance, mechanical strength and toughness of the hydrogel. Pb2+ adsorption capacity of the hydrogel increased with GO content, while the adsorption belonged to the second-order kinetic model and fitted Langmuir adsorption isotherm model, indicating the homogeneous nature of monolayer chemical adsorption of Pb2+. A relatively good reusability of the composite hydrogel beads for Pb2+ removal can be achieved.

Effect of Chemistry and Process Parameters on Shape Recovery Behaviour of Shape Memory Epoxies in Flexure

Epoxy based Shape Memory Polymers (SMEPs) of varying crosslinking densities were prepared by reacting an aromatic epoxy resin, Diglycidyl Ether of Bisphenol A (DGEBA) with different aliphatic amines varying in chain lengths, triethylenetetramine (TETA), 2,2′ethylenedioxybisethylamine (EDEA) and 4,7,10-Trioxa-1, 13-Tridecanediamine (TTD). Shape memory tests were conducted for all three SMEPs by deforming the samples at temperatures greater than their Tg in flexure (3 point bending). The effect of crosslinking density (CLD), shape fixing temperature (Tfix) and shape recovery temperature (Trec) on shape memory behaviour of SMEPs were studied systematically. In terms of the chemical structure, the results showed that with the increase in CLD, glass transition temperature (Tg) increased with an associated reduction in the shape recovery behaviour. However, superior shape memory behaviour was observed for lower CLD SMEPs. With respect to the process parameters, as the Tfix was lowered below the Tg, % shape fixity increased and the recovery curves shifted towards higher temperature. When Trec was increased above the Tg, recovery rate increased.

The mechanics of phantom Mikado networks

The elasticity of networks comprised of semi-flexible polymers plays a vital role in regulating the mechanics of both intra- and extra-cellular matrices. The behaviour of this polymer scaffold will depend on the nature and density of cross-linking between constituent fibres. While modelling efforts have investigated the effects of cross-link density in biopolymer networks, this is often accompanied by changes in both the fibre density and the network structure. We investigate the elasticity of a quasi two-dimensional Mikado network of elastic rods, in which cross-link density is allowed to vary while polymer density is held constant. In particular, this model is extended by allowing constituent rods to cross without forming cross-links, while polymer density and network geometry are preserved. In doing so, the competing contributions to the shear modulus from cross-link density, mesh size, geometry and polymer density are decoupled. We find that previous scaling laws fail to capture the well-studied transition from bend- to stretch- dominated elasticity as cross-link density is varied. We identify a length scale which relates cross-link density to the transition between affine and nonaffine regimes, and which provides a collapse of simulation data curves for varying cross-link densities.

UV Photopolymerization of Vinyl Ether Based Polymer Electrolytes for Solid-State Lithium Batteries

The utilization of solid lithium metal in lithium ion batteries increases performance and capacities but introduces safety issues when combined with liquid electrolytes. This prevents its use in practical energy dense electrochemical cells, like those required for electric vehicles or consumer electronics. Safety can be increased by using polymer or gel electrolytes which are less flammable and provide a more formidable barrier to dendrite formation. However, the production of polymer electrolytes can often be complicated and involve several stages, making the processing time-consuming and expensive to apply on the industrial scale. In this work we developed a simple bulk polymerization method in the presence of lithium salt to produce quick-curing vinyl ether based polymer electrolytes in a one-step cationic UV photopolymerization. These electrolytes are composed of ethyl vinyl ether (EVE) monomer and poly(ethylene glycol) divinyl ether (DVEp) crosslinker with triarylsulfonium hexafluorophosphate salt photoinitiator and bis(trifluoromethane) sulfonimide (LiTFSI) lithium salt. Systems with varying crosslinker to monomer (DVEp:EVE) volume ratios and lithium salt concentration ratios were produced to see the effects of crosslinking density and [O]:[Li] ratio on electrolyte properties. Attenuated Total Reflectance Fourier-transform Infrared spectroscopy (ATR-FTIR) was used to quantify polymer composition and verify complete polymerization. Ionic conductivities were measured from 0 to 80 °C using electrochemical impedance spectroscopy. The highest ionic conductivities were seen in samples with the lowest crosslinker concentrations and the highest lithium salt concentrations. The most conductive sample produced had composition 10:1 [O]:[Li] and 15% crosslinker by volume and reached an ionic conductivity of 1.2x10-6 S/cm at 20°C. Glass transition temperatures (Tg) were measured with differential scanning calorimetry (DSC) for various salt-free DVEp:EVE ratios; Tg was determined to be -15°C for composition with 15% crosslinker in the absence of salt. Investigation of the effect of salt on Tg’s and the electrochemical stability of these compositions is currently underway and will be reported.

Rapid Accessible Fabrication and Engineering of Bilayered Hydrogels: Revisiting the Cross-Linking Effect on Superabsorbent Poly(acrylic acid).

Superabsorbent hydrogels are significant not only in materials science but also in industries and daily life, being used in diapers or soil conditioners as typical examples. The main feature of these materials is their capacity to hold considerable amount of water, which is strongly dependent on the cross-linking density. This study focuses on the preparation of hydrogels by reweighing the effect of cross-linking density on physical properties, which provides green fabrication of bilayered hydrogels that consist of homogeneous structural motifs but show programmed responses via sequential radical polymerization. In particular, when two hydrogel layers containing different cross-linking densities are joined together, an integrated linear bilayer shows heterogeneous deformation triggered by water. We monitor the linear hydrogel bilayer bending into a circle and engineer it by incorporating disperse dyes, changing colors as well as physical properties. In addition, we demonstrate an electric circuit switch using a patterned hydrogel. Anisotropic shape change of the polyelectrolyte switch closes an open circuit and lights a light-emitting diode in red. This proposed fabrication and engineering can be expanded to other superabsorbent systems and create smart responses in cross-linked systems for biomedical or environmental applications.

A facile co-solvent-free process for waterborne polyurethane preparation

In this work, a low-melting-point chain extender (LMCE) was prepared using 1,4-butanediol (BDO) and dimethylol propionic acid (DMPA) as raw materials. A series of co-solvent-free waterborne polyurethanes were prepared based on LMCE. The structure and properties of the waterborne polyurethanes were characterized by gel permeation chromatography, infrared spectroscopy, particle size analysis, and tensile tests. It was found that the molecular weights of the WPU based on LMCE were much higher than that of the WPU based on DMPA. The increase of DMPA/BDO molar ratio led to smaller particle size and better micro-phase separation of the WPU. The results of mechanical test showed that the mechanical properties of the WPU decreased with decreasing of DMPA/BDO molar ratio. In addition, the effective cross-linking chain density of the WPU was evaluated based on the theory of rubber elasticity.

In Situ Cross-Linking of Poly(vinyl alcohol)/Graphene Oxide–Polyethylene Glycol Nanocomposite Hydrogels as Artificial Cartilage Replacement: Intercalation Structure, Unconfined Compressive Behavior, and Biotribological Behaviors

Poly(vinyl alcohol) (PVA)/graphene oxide (GO) nanocomposite hydrogel as artificial cartilage replacement was prepared via freezing/thawing method by introducing polyethylene glycol (PEG). Efficient grafting of PVA molecules onto GO surface was realized by formation of hydrogen bonding, resulting in exfoliation and uniform distribution of GO in PVA matrix. By introduction of appropriate content of GO, the increased crystalline regions of PVA and the formation of GO centered second network structure led to the increase of the storage modulus and effective cross-linking density. And therefore the mechanical strength and toughness of the composite hydrogel were improved simultaneously: the tensile strength, elongation at break, and compressive modulus showed approximately 200%, 40%, and 100% increase of the neat PVA hydrogel. Besides, for the sample with 1.5 wt % GO content, the maximum force retention and dynamic stiffness were improved remarkably in the process of sinusoidal cyclic compression, and the comp...

A Molecular Dynamics Study of Crosslinked Phthalonitrile Polymers: The Effect of Crosslink Density on Thermomechanical and Dielectric Properties.

In this work, molecular dynamics (MD) and molecular mechanics (MM) simulations are used to study well-equilibrated models of 4,4′-bis(3,4-dicyanophenoxy)biphenyl (BPh)–1,3-bis(3-aminophenoxy)benzene (m-APB) phthalonitrile (PN) system with a range of crosslink densities. A cross-linking technique is introduced to build a series of systems with different crosslink densities; several key properties of this material, including thermal expansion, mechanical properties and dielectric properties are studied and compared with experimental results. It is found that the coefficient of linear thermal expansion predicted by the model is in good agreement with experimental results and indicative of the good thermal stability of the PN polymeric system. The simulation also shows that this polymer has excellent mechanical property, whose strength increases with increasing crosslink density. Lastly and most importantly, the calculated dielectric constant—which shows that this polymer is an excellent insulating material—indicates that there is an inverse relation between cross-linking density and dielectric constant. The trend gave rise to an empirical quadratic function which can be used to predict the limits of attainable dielectric constant for highly crosslinked polymer systems. The current computational work provides strong evidence that this polymer is a promising material for aerospace applications and offers guidance for experimental studies of the effect of cross-linking density on the thermal, mechanical and dielectric properties of the material.

Effect of Glycerol on the Properties of the Cross‐Linked Polyvinyl Alcohol Hydrogel Beads

AbstractPoly(vinyl alcohol) (PVA)/glycerol hydrogel beads were created, using saturated boric acid and calcium chloride solution as the chemical crosslinking agents. The structure and properties of the PVA/glycerol hydrogel beads were studied. The results indicated that glycerol could improved the water absorption rate and the swelling rate at equilibrium. Also improved were the capillary water absorption capacity and the permeability of the PVA/glycerol hydrogel beads. The rheological tests showed that glycerol was able to improve the effective crosslinking density of the hydrogel beads. The amount of total water of the PVA/glycerol hydrogel beads slightly increased upon the addition of glycerol.

An in-situ photocrosslinking microfluidic technique to generate non-spherical, cytocompatible, degradable, monodisperse alginate microgels for chondrocyte encapsulation.

Alginate microgels are widely generated by ionic crosslinking methods, but this method has limitations in controlling the microgel degradation and generating non-spherical microgels. By employing oxidized methacrylated alginate (OMA) that is degradable and photocrosslinkable, we have successfully photocrosslinked monodisperse OMA microgels and demonstrated the feasibility to generate discoid alginate microgels. However, several technical issues obstructed our opto-microfluidic method from being a useful technique. Here, we further characterized and optimized this method. Monodisperse discoid OMA microgels with good shape consistency were, for the first time, generated. The curability of OMA microgels was characterized as the macromer concentration varied from 2% to 10%, and the minimum required photoinitiator (VA-086) concentrations were determined. The effects of crosslinking density and the presence of ions in the storage solution on swelling of OMA hydrogels were identified to give insights into accurate controlling of the microgel size. A much quicker degradation rate (within three weeks) compared to ionically crosslinked alginate hydrogels was indirectly identified by quantifying the elastic modulus using atomic force microscopy. The viability of encapsulated chondrocytes in OMA microgels formed by this method was higher than those from other existing methods, demonstrating its favorable cytocompatibility. It was found that the oxygen tension played a critical role in both the curability of microgels and the cytocompatibility of this technique. We also summarize common practical issues and provide related solutions and/or operational suggestions. By this method, OMA microgels are expected to be valuable alternatives to traditional ionically crosslinked alginate microgels in drug delivery, tissue engineering, and single cell analysis areas due to their multiple favorable properties.

Simultaneous reinforcement and toughness improvement of an epoxy-phenolic network with a hyperbranched polysiloxane modifier.

An epoxy–phenolic network is modified with hyperbranched polysiloxane (HBPSi). The addition of HBPSi-2, which has medium molecular weight, can significantly decrease the viscosity of the uncured epoxy–phenolic system and increase the crosslinking density and homogeneity of the cured crosslinking network. With 10% HBPSi-2, the mechanical properties of the samples are improved comprehensively: tensile modulus and maximum strength increase by 11.4% and 36.2%, respectively, while elongation at break and impact strength increase by 153.8% and 186.7%, respectively. The comprehensive improvements in the mechanical properties are attributed to combined effects of crosslinking density, network rigidity, cohesive density and the matrix-modifier compatibility. What is more, HBPSi-2 also significantly increases the char yield of the material and decreases the thermal weight loss rate, indicating an improved thermal stability. All these results may provide a new strategy for toughness and strength improvement of the epoxy–phenolic network.

In situ crosslinking of poly (vinyl alcohol)/graphene oxide-glutamic acid nano-composite hydrogel as microbial carrier: Intercalation structure and its wastewater treatment performance

Poly (vinyl alcohol) (PVA)/graphene oxide (GO) nano-composite hydrogels were prepared through in situ chemical cross-linking by incorporation with glutamic acid (Glu) as a compatibilizer. Through covalent and hydrogen bonding interaction, much more PVA molecules were grafted onto GO surface, while GO was exfoliated and uniformly distributed in the PVA matrix with increasing average layer thickness. Addition of GO-Glu hybrid could improve the storage modulus and the effective crosslinking density (ve) of the composite hydrogels, and the network structure with GO as crosslinking point formed. Moreover, the specific surface area and the biocompatibility of PVA hydrogel were improved significantly by compositing with GO-Glu, resulting in a remarkable increase of loading capacity of microorganism. And therefore the COD removal rate could be improved when the hydrogel was applied in wastewater treatment.

Thermal Transport in Soft PAAm Hydrogels.

As the interface between human and machine becomes blurred, hydrogel incorporated electronics and devices have emerged to be a new class of flexible/stretchable electronic and ionic devices due to their extraordinary properties, such as softness, mechanically robustness, and biocompatibility. However, heat dissipation in these devices could be a critical issue and remains unexplored. Here, we report the experimental measurements and equilibrium molecular dynamics simulations of thermal conduction in polyacrylamide (PAAm) hydrogels. The thermal conductivity of PAAm hydrogels can be modulated by both the effective crosslinking density and water content in hydrogels. The effective crosslinking density dependent thermal conductivity in hydrogels varies from 0.33 to 0.51 Wm−1K−1, giving a 54% enhancement. We attribute the crosslinking effect to the competition between the increased conduction pathways and the enhanced phonon scattering effect. Moreover, water content can act as filler in polymers which leads to nearly 40% enhancement in thermal conductivity in PAAm hydrogels with water content vary from 23 to 88 wt %. Furthermore, we find the thermal conductivity of PAAm hydrogel is insensitive to temperature in the range of 25–40 °C. Our study offers fundamental understanding of thermal transport in soft materials and provides design guidance for hydrogel-based devices.

Discussion of paper by J. Zhao, K. Mayumi, C. Creton and T. Narita, entitled ‘Rheological properties of tough hydrogels based on an associating polymer with permanent and transient crosslinks: Effects of crosslinking density’

Discussion of paper by J. Zhao, K. Mayumi, C. Creton and T. Narita, entitled ‘Rheological properties of tough hydrogels based on an associating polymer with permanent and transient crosslinks: Effects of crosslinking density’

Rheological properties of tough hydrogels based on an associating polymer with permanent and transient crosslinks: Effects of crosslinking density

We studied the rheological and mechanical properties of a series of poly(vinyl alcohol) dual crosslink hydrogels, permanently crosslinked by glutaraldehyde and transiently crosslinked by borate ions, at various crosslinking densities. The contribution of the chemical and physical crosslinks to the dynamic moduli was first determined with linear oscillatory tests, and the additivity of the two contributions was validated. The single relaxation time of the system was interpreted as the physical bond breaking time which can be slowed down by reassociation of the open physical bonds before full relaxation. By comparing the Rouse relaxation time of the strand between two adjacent crosslinks with the healing time of the physical bonds, an increase in the relaxation time with decreasing density of physical crosslinks was explained. In all cases, the extensibility and stress at break of the gels were found to increase with the addition of physical crosslinks, but the extensibility increased the most for the smallest amount of chemical and physical crosslink densities while the stress at break increased the most for the most physically crosslinked system with a low chemical crosslink density. This strongly suggests that the toughening mechanism due to the presence of transient bonds can only be active on a network with a low density of chemical crosslinks. In large strain experiments, the tensile stress could be separated into a strain-dependent and a time-dependent term for all the crosslinking densities studied, and master curves of the reduced stress as a function of time were obtained. This showed that the chemical and physical crosslinks independently and additively contribute to the viscoelasticity of the dual crosslink gel.

Temperature-Induced Morphological Transitions of Poly(dimethylacrylamide)–Poly(diacetone acrylamide) Block Copolymer Lamellae Synthesized via Aqueous Polymerization-Induced Self-Assembly

Aqueous dispersion polymerization of diacetone acrylamide (DAAM) by chain extension from a hydrophilic poly(N,N-dimethylacrylamide) (PDMA30) macromolecular chain transfer agent (macro-CTA) to produce PDMA30–PDAAMx block copolymer nano-objects was investigated in detail by systematically varying solids content and degree of polymerization of the core-forming PDAAM, leading to the formation of pure lamellae, mixed lamellae/vesicles, and pure vesicles as revealed by dynamic light scattering (DLS), transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). PDMA30–PDAAMx lamellae were found to span an unprecedented wide space in the morphology phase diagram. Moreover, in situ cross-linking of lamellae via statistical copolymerization of DAAM with an asymmetric cross-linker allyl acrylamide and the effect of cross-linking density on the colloidal and morphological stabilities were studied, representing the first report on in situ cross-linking of lamellae duri...

Super-Resolution Fluorescence Imaging of Spatial Organization of Proteins and Lipids in Natural Rubber

Natural rubber (NR) with proteins and lipids has superior mechanical properties to its synthetic counterpart, polyisoprene rubber. However, it is a challenge to unravel the morphology of proteins and lipids. Here we used two-color stochastic optical reconstruction microscopy (STORM) to directly visualize the spatial organization of proteins and lipids in NR. We found that the proteins and lipids form an interdispersed stabilizing layer on the surface of NR latex particles. After drying, the proteins and lipids form aggregates of up to 300 nm in diameter. The aggregates physically interact with the terminal groups of polyisoprene chains, leading to the formation of a network, which contributes to the high elasticity and mechanical property of NR. If we remove proteins in NR, the large phospholipid aggregates disintegrate into small ones. However, it does not decompose the network but rather reduces the effective cross-linking density, thus the deproteinized NR is still elastic-like with decreased mechanical property. Removing both proteins and lipids wholly decomposes the network, thus, results in a liquid-like behavior of the rubber. The STORM measurements in this paper enable more insight into the structure-property relationship of NR, which also shows a great potential of STORM in studying the fine structure of polymeric materials and nanocomposites.

Reversible Shape-Memory Effect in Cross-Linked Linear Poly(ε-caprolactone) under Stress and Stress-Free Conditions

The effect of cross-link density on the reversible shape-memory effect (SME) under constant load was systematically studied in cross-linked linear poly(e-caprolactone) (PCL). A remarkable reversible SME under stress-free conditions was observed in PCL with the highest achieved cross-link density. Thermal properties as well as morphology, size, and orientation of the nanocrystalline structure formed in covalent networks of PCL under load were compared with those in PCL crystallized under stress-free conditions. As shown, the oriented growth of crystals is the origin of both the reversible SME under and without load. Furthermore, a significant rise of crystallinity and crystal thickness was detected in PCL crystallized under constant load. The fitting curves of the temperature-dependent strain as well as the quantities of crystallinity, type of crystalline structure, size, and orientation of the crystals got by modeling the reversible SME in PCL under stress well correspond to their values obtained experime...

Mechanical properties of biocompatible clay/P(MEO2MA-co-OEGMA) nanocomposite hydrogels

The effects of crosslinking density, polymer concentration and monomer ratio on the mechanical properties (tensile and compressive properties) of biocompatible clay/P(MEO2MA-co-OEGMA) nanocomposite (NC) hydrogels were investigated. These novel NC hydrogels, composed of inorganic/organic networks, were prepared via in-situ free radical polymerization. The results showed that with increasing inorganic crosslinking agent, i.e. clay concentration, an increase in the tensile strength, elongation at break and compressive strength was observed. Similarly, with increasing polymer concentration, the tensile strength and compressive strength of the NC hydrogels increased while the elongation at break decreased. Increasing the molar concentration of OEGMA in the comonomer led to an increase in the tensile strength of the NC hydrogels but a reduction in the compressive strength. Moreover, clay/P(MEO2MA-co-OEGMA) NC hydrogels presented good biocompatibility bolstering their application as tissue engineering scaffolds.