Elastomeric Networks Research Articles

Complex Network Representation of the Structure-Mechanical Property Relationships in Elastomers with Heterogeneous Connectivity

SummaryThe complicated structure-property relationships of materials have recently been described using a methodology of data science that is recognized as the fourth paradigm in materials science. In network polymers or elastomers, the manner of connection of the polymer chains among the crosslinking points has a significant effect on the material properties. In this study, we quantitatively evaluate the structural heterogeneity of elastomers at the mesoscopic scale based on complex network, one of the methods used in data science, to describe the elastic properties. It was determined that a unified parameter with topological and spatial information universally describes some parameters related to the stresses. This approach enables us to uncover the role of individual crosslinking points for the stresses, even in complicated structures. Based on the data science, we anticipate that the structure-property relationships of heterogeneous materials can be interpretatively represented using this type of “white box” approach.

Reversible Soft Mechanochemical Control of Biaryl Conformations through Crosslinking in a 3D Macromolecular Network

AbstractTuning the dihedral angle (DA) of axially chiral compounds can impact biological activity, catalyst efficiency, molecular motor performance, or chiroptical properties. Herein, we report gradual, controlled, and reversible changes in molecular conformation of a covalently linked binaphthyl moiety within a 3D polymeric network by application of a macroscopic stretching force. We managed direct observation of DA changes by measuring the circular dichroism signal of an optically pure BINOL‐crosslinked elastomer network. Stretching the elastomer resulted in a widening of the DA between naphthyl rings when the BINOL was doubly grafted to the elastomer network; no effect was observed when a single naphthyl ring of the BINOL was grafted to the elastomer network. We have determined that ca. 170 % extension of the elastomers led to the transfer of a mechanical force to the BINOL moiety of 2.5 kcal mol−1 Å−1 (ca. 175 pN) in magnitude and results in the opening of the DA of BINOL up to 130°.

Reversible Soft Mechanochemical Control of Biaryl Conformations through Crosslinking in a 3D Macromolecular Network.

Tuning the dihedral angle (DA) of axially chiral compounds can impact biological activity, catalyst efficiency, molecular motor performance, or chiroptical properties. Herein, we report gradual, controlled, and reversible changes in molecular conformation of a covalently linked binaphthyl moiety within a 3D polymeric network by application of a macroscopic stretching force. We managed direct observation of DA changes by measuring the circular dichroism signal of an optically pure BINOL-crosslinked elastomer network. Stretching the elastomer resulted in a widening of the DA between naphthyl rings when the BINOL was doubly grafted to the elastomer network; no effect was observed when a single naphthyl ring of the BINOL was grafted to the elastomer network. We have determined that ca. 170 % extension of the elastomers led to the transfer of a mechanical force to the BINOL moiety of 2.5 kcal mol-1 Å-1 (ca. 175 pN) in magnitude and results in the opening of the DA of BINOL up to 130°.

Structure-property relationship of highly crosslinked rubber-iron oxide composite based on chloroprene rubber (CR) as well as on nitrile rubber (NBR); a comparative study using different models

Highly crosslinked elastomer-iron oxide composite for grinding as well as for polishing application. With the recent introduction of organic acid-based coolants in polishing applications, the designed composites should have good resistance to oils. This investigation reports the preparation and properties of high crosslinked elastomer-iron oxide composites based on Chloroprene Rubber (CR) as well as on Nitrile Rubber (NBR) as main elastomer matrix and their comparative study. In NBR system, a small amount of natural rubber (NR) was used to improve the abrasion resistance. The crosslink density (CLD) was determined from the plateau modulus in DMA using Nielsen’s model. CLD was also determined based on the equilibrium-swelling ratio by using Flory-Rehner model. The CLD at lower cure time estimated by both methods was substantially different. Nevertheless, it converges to a common value at highly crosslinked state. Highly crosslinked CR shows 500% higher modulus at high temperature when compared to the NBR system. Highly crosslinked elastomeric composites based on Chloroprene Rubber (CR) as well as Nitrile Rubber (NBR) with high iron oxide content were prepared. This investigation gives insights into the fabrication of composites and evaluates the network structure of highly crosslinked composites. Different models were used to characterize the elastomeric network structures in the composites.

Architected elastomer networks for optimal electromechanical response

Dielectric elastomers (DEs) that couple deformation and electrostatics have the potential for use in soft sensors and actuators with applications ranging from robotic, biomedical, energy, aerospace and automotive technologies. However, currently available DEs are limited by weak electromechanical coupling and require large electric fields for significant actuation. In this work, a statistical mechanics-based model of DE chains is applied to elucidate the role of a polymer network architecture in the performance of the bulk material. Given a polymer network composed of chains that are cross-linked, the paper examines the role of cross-link density, orientational density of chains, and other network parameters in determining the material properties of interest including elastic modulus, electrical susceptibility, and the electromechanical coupling. From this analysis, a practical strategy is presented to increase the deformation and usable work derived from (anisotropic) dielectric elastomer actuators by as much as 75−100%.

Transesterification in Epoxy–Thiol Exchangeable Liquid Crystalline Elastomers

The incorporation of vitrimer bond-exchange chemistry into liquid crystalline elastomer networks produces “exchangeable liquid crystal elastomers” (xLCE). These materials offer a facile method of m...

Singular Nature of the Elastocapillary Ridge

The functionality of soft interfaces is crucial to many applications in biology and surface science. Recent studies have used liquid drops to probe the surface mechanics of elastomeric networks. Experiments suggest an intricate surface elasticity, also known as the Shuttleworth effect, where surface tension is not constant but depends on substrate deformation. However, interpretations have remained controversial due to singular elastic deformations, induced exactly at the point where the droplet pulls the network. Here we reveal the nature of the elastocapillary singularity on a hyperelastic substrate with various constitutive relations for the interfacial energy. First, we finely resolve the vicinity of the singularity using goal-adaptive finite element simulations. This confirms the universal validity, also at large elastic deformations, of the previously disputed Neumann's law for the contact angles. Subsequently, we derive exact solutions of nonlinear elasticity that describe the singularity analytically. These solutions are in perfect agreement with numerics, and show that the stretch at the contact line, as previously measured experimentally, consistently points to a strong Shuttleworth effect. Finally, using Noether's theorem we provide a quantitative link between wetting hysteresis and Eshelby-like forces, and thereby offer a complete framework for soft wetting in the presence of the Shuttleworth effect.

Influence of pre-polymerisation atmosphere on the properties of pre- and poly(glycerol sebacate)

Poly(glycerol sebacate) (PGS) is a versatile biodegradable biomaterial on account of its adjustable mechanical properties as an elastomeric polyester. Nevertheless, it has shown dissimilar results when synthesised by different research groups under equivalent synthesis conditions. This lack of reproducibility proves how crucial it is to understand the effect of the parameters involved on its manufacturing and characterize the polymer networks obtained. Several studies have been conducted in recent years to understand the role of temperature, time, and the molar ratio of its monomers, while the influence of the atmosphere applied during its pre-polymerisation remained unknown. The results obtained here allow for a better understanding about the effect of inert (Ar and N2) and oxidative (oxygen, dry air, and humid air) atmospheres on the extent of the reaction. The molecular pattern of intermediate pre-polymers and the gelation time and morphology of their corresponding cured PGS networks were studied as well. Overall, inert atmospheres promote a rather linear growth of macromers, with scarce branches, resulting in loose elastomers with long chains mainly crosslinked. Conversely, oxygen in the latter atmospheres promotes branching through secondary hydroxyl groups, leading to less-crosslinked ‘defective’ networks. In this way, the pre-polymerisation atmosphere could be used advantageously to adjust the reactivity of secondary hydroxyls, in order to modulate branching in the elastomeric PGS networks obtained to suit the properties required in a particular application.

A constitutive model for multi network elastomers pre-stretched by swelling

Multi network elastomers (MNEs), composed of a single sacrificial network and other matrix networks, exhibit appealing mechanical properties. In this paper, we develop a constitutive model for MNEs prepared by multiple swellings. Firstly, the swelling process is analyzed. The free energy of a swollen elastomer consists of the strain energy of deformed polymer chains and the energy of mixing. We analyze the mechanical equilibrium and chemical potential equilibrium during the swelling of MNEs. The degree of swelling of MNEs is affected by the preparation conditions, like the component of the solution, as well as the microscopic physical quantities, such as the chain length and density of polymer chains of the networks. Secondly, the free energy of the completed MNE subjected to external loading is composed of the strain energy of each network. The chains of the sacrificial network (first network) can be destroyed gradually when the applied load increases. We adopt the network alteration theory to describe the above progressive damage of the sacrificial network. In addition, the matrix networks are fully elastic. We verify this model using the experimental data including stress–stretch curves of elastomers with double and triple networks, as well as the step cycle curves of triple network elastomers (TNEs). This model predicts well for the swelling-induced pre-stretches of each network under specified preparation conditions. It relates the stress to the stretch of MNEs under external loading, and indicates the stress contribution of each network, and the damage evolution of the sacrificial network. Our model is instructive for designing MNEs with desired mechanical properties.

Double Network Elastomers: Self‐Assembly Strategy for Double Network Elastomer Nanocomposites with Ultralow Energy Consumption and Ultrahigh Wear Resistance (Adv. Funct. Mater. 34/2020)

Double Network Elastomers: Self‐Assembly Strategy for Double Network Elastomer Nanocomposites with Ultralow Energy Consumption and Ultrahigh Wear Resistance (Adv. Funct. Mater. 34/2020)

The Impact of Vitrimers on the Industry of the Future: Chemistry, Properties and Sustainable Forward-Looking Applications.

Thermosets are known to be very reliable polymeric materials for high-performance and light-weight applications, due to their retained dimensional stability, chemical inertia and rigidity over a broad range of temperatures. However, once fully cured, they cannot be easily reshaped or reprocessed, thus leaving still unsolved the issues of recycling and the lack of technological flexibility. Vitrimers, introduced by Leibler et al. in 2011, are a valiant step in the direction of bridging the chasm between thermoplastics and thermosets. Owing to their dynamic covalent networks, they can retain mechanical stability and solvent resistance, but can also flow on demand upon heating. More generally, the family of Covalent Adaptable Networks (CANs) is gleaming with astounding potential, thanks to the huge variety of chemistries that may enable bond exchange. Arising from this signature feature, intriguing properties such as self-healing, recyclability and weldability may expand the horizons for thermosets in terms of improved life-span, sustainability and overall enhanced functionality and versatility. In this review, we present a comprehensive overview of the most promising studies featuring CANs and vitrimers specifically, with particular regard for their industrial applications. Investigations into composites and sustainable vitrimers from epoxy-based and elastomeric networks are covered in detail.

Transparent Stretchable Dual-Network Ionogel with Temperature Tolerance for High-Performance Flexible Strain Sensors.

A stretchable transparent double network ionogel composed of physically cross-linked poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP)) and chemically cross-linked poly(methyl methacrylate-co-butylmethacrylate) (P(MMA-co-BMA)) elastomer networks within [EMIM][TFSI] ionic liquid was fabricated through a facile one-pot thermal polymerization. The dual-network (DN) ionogel presents good mechanical performance (failure tensile stress 2.31 MPa, strain 307%) with a high loading of ionic liquid (70 wt %) for achieving required ionic conductivity (>0.1 S/m at room temperature). The transparent chemical cross-linked P(MMA-co-BMA) elastomer network endows high transparency (>93%) and high stretchability to the DN ionogel. The DN ionogel maintains good toughness, elasticity, and transparency in a wide temperature range (-40 to 80 °C) for the application in a harsh environment. In addition, the sensitivity of the DN ionogel to the change of environment temperature and deformation was detected and described. The practical potential of the DN ionogel in flexible electronic devices is further revealed by fabricating DN ionogel strain sensors to detect the movement of different human limbs including the bending of the finger, wrist, and elbow as well as the slight throat jitter during the swallowing and vocalization, showing fast response, high sensitivity, and good repeatability.

Self‐Assembly Strategy for Double Network Elastomer Nanocomposites with Ultralow Energy Consumption and Ultrahigh Wear Resistance

AbstractOne of the environmental crises facing the world is pollution due to rubber auto tire destruction. The use of tires in vehicles consumes 6% of the world's energy and causes 5% of carbon dioxide emissions; it accounts for up to 10% of the microplastic pollution found in oceans. Here, a new rubber nanocomposite self‐assembled from hard and soft elastomer matrixes is designed: polybutadiene with its two hydroxy chain ends reacts with 4,4'‐diphenylmethane diisocyanate to form segmented polyurethane. This system first undergoes self‐assembly, forming well‐defined nanoscale hard domains distributed in the soft matrix. Then, cross‐linking between the soft segments is accomplished by a controlled radiation method, resulting in the double network elastomer (DN‐E). Remarkably, the DN‐E exhibits the lowest reported loss factor value at 60 °C. The index of energy dissipation in the rolling tire demonstrates a prominent reduction of 72%, accomplished with an 88% decrease in energy loss, and 85% less wear loss, as compared with best earlier reported commercial tires. These new double‐network materials open a new prospective for the design and fabrication of ultralow energy‐consumption and strong abrasion‐resistance elastomers, which establishes a milestone for the development of the next generation of green low‐pollution tires causing much less energy dissipation.

A Tailorable Family of Elastomeric‐to‐Rigid, 3D Printable, Interbonding Polymer Networks

AbstractSoft materials with widely tailorable mechanical properties throughout the material's volume can shape the future of soft robotics and wearable electronics, impacting both consumer and defense sectors. Herein, a platform of 3D printable soft polymer networks with unprecedented tunability of stiffness of nearly three orders of magnitude (MPa to GPa) and an inherent capability to interbond is reported. The materials are based on dynamic covalent polymer networks with variable density of crosslinkers attached to prepolymer backbones via a temperature‐reversible Diels–Alder (DA) reaction. Inherent flexibility of the prepolymer chains and controllable crosslinking density enable 3D printed networks with glass transition temperatures ranging from just a few degrees to several tens of degrees Celsius. Materials with an elastomeric network demonstrate a fast and spontaneous self‐healing behavior at room temperature both in air and under water—a behavior difficult to achieve with other crosslinked materials. Reversible dissociation of DA networks at temperatures exceeding ≈120 °C allows for reprintability, while control of the stereochemistry of DA attachments enables reprogrammable shape memory behavior. The introduced platform addresses current major challenges including control of polymer interbonding, enhanced mechanical performance of printed parts, and reprocessability of 3D‐printed crosslinked materials in the absence of solvent.

Catalytic Control of Plastic Flow in Siloxane-Based Liquid Crystalline Elastomer Networks.

Liquid crystalline elastomer networks cross-linked by dynamic covalent bonds (xLCE) have the ability to be (re)processed during the plastic flow. However, the current bond-exchange strategies that are used to induce plastic flow in xLCE lack the efficient method to control the elastic-plastic transition. Here we describe a straightforward method to manipulate the transition to plastic flow via the choice of catalyst in xLCE cross-linked by siloxane. The nature and the amount of catalyst have a profound effect on the elastic-plastic transition temperature, and the stress relaxation behavior of the network. The temperature of fast plastic flow and the associated bond-exchange activation energy varied from 120 °C and 83 kJ/mol in the "fastest" exchange promoted by triazobicyclodecene (TBD) to 240 °C and 164 kJ/mol in the "slowest" exchange with triphenylphosphine (PPH), with a range of catalysts in between. We have identified the optimum conditions for programming an aligned monodomain xLCE, high programming temperature (230 °C) and low nematic to isotropic transition (60 °C), to achieve thermally and mechanically stable actuators.

Biocompatible liquid-crystal elastomers mimic the intervertebral disc

The hierarchical and anisotropic mechanical behavior requirement of load-bearing soft tissues limits the utility of conventional elastomeric materials as a replacement for soft-tissue materials. Liquid-crystal elastomers (LCEs) have the potential to excel in this regard owing to its unique combination of mesogenic order in an elastomeric network. In this study, the mechanical behavior of the LCEs relevant to load-bearing biomedical applications was explored. LCEs with different network orientations (i.e., mesogen alignments) were investigated by fabricating the LCEs with polydomain and monodomain configurations. The polydomain and monodomain LCEs with the same degree of network crosslinking demonstrated diverse mechanical behavior, ranging from highly stiff and elastic nature to high damping capacity, depending on the loading direction with respect to the network alignment. The LCEs were also capable of matching the anisotropic mechanical behavior of an intervertebral disc. Additional studies were conducted on the in vivo biological response of LCEs upon subcutaneous implantation, as well as on the effect of the exposure to an in vitro simulated physiological environment on the mechanical behavior. The LCEs’ mechanical response was negligibly affected when exposed to biomedically relevant conditions. Furthermore, the solid and porous LCEs did not show any adverse effect on the surrounding tissues when implanted subcutaneously in rats. The biological response allows for tissue ingrowth and helps illustrate their utility in implantable biological devices. Finally, the utility of LCEs to mimic the mechanical function of biological tissue such as intervertebral disc was demonstrated by fabricating a proof of concept total disc replacement device.

Highly transparent, self-healing conductive elastomers enabled by synergistic hydrogen bonding interactions

• Highly transparent, self-healing conductive elastomers were prepared. • Synergistic H-bonding interactions were employed for the self-healing mechanism. • Poly(PDES)-PA elastomers were used to monitor the human activities. It is attractive, though it is very challenging, to synthesize conductive elastomers with high transparency and self-healing ability, simultaneously with facile and environmentally friendly preparation process. Here we demonstrate the fabrication of highly transparent, self-healing conductive elastomers based on synergistic hydrogen bonding interactions between poly(polymerizable deep eutectic solvent) (poly(PDES)) and phytic acid (PA) molecules. The hybrid network elastomers showed a fast self-healing property (within 2 s) without any other external stimuli and excellent self-healing efficiency (91.5%) within 48 h. The poly(PDES)-PA elastomers were highly transparent (an average transmittance of 93%), intrinsically conductive (an ionic conductivity of 7.8 × 10 −4 S cm −1 ) and stretchable (strains up to 1300%). Furthermore, the abilities of healable & conductive poly(PDES)-PA elastomers to monitor the human activities retained no matter before or after being damaged. We believe this kind of dynamic bond manipulation is a powerful strategy to design and synthesize various transparent, self-healing, conductive elastomers for versatile applications.

3D printing of a biocompatible double network elastomer with digital control of mechanical properties.

The majority of 3D-printed biodegradable biomaterials are brittle, limiting their potential application to compliant tissues. Poly (glycerol sebacate) acrylate (PGSA) is a synthetic biodegradable and biocompatible elastomer, compatible with light-based 3D printing. In this work we employed digital-light-processing (DLP)-based 3D printing to create a complex PGSA network structure. Nature-inspired double network (DN) structures with two geometrically interconnected segments with different mechanical properties were printed from the same material in a single shot. Such capability has not been demonstrated by any other fabrication technique. The biocompatibility of PGSA after 3D printing was confirmed via cell-viability analysis. We used a finite element analysis (FEA) model to predict the failure of the DN structure under uniaxial tension. FEA confirmed the soft segments act as sacrificial elements while the hard segments retain structural integrity. The simulation demonstrated that the DN design absorbs 100% more energy before rupture than the network structure made by single exposure condition (SN), doubling the toughness of the overall structure. Using the FEA-informed design, a new DN structure was printed and the FEA predicted tensile test results agreed with tensile testing of the printed structure. This work demonstrated how geometrically-optimized material design can be easily and rapidly achieved by using DLP-based 3D printing, where well-defined patterns of different stiffnesses can be simultaneously formed using the same elastic biomaterial, and overall mechanical properties can be specifically optimized for different biomedical applications.

Ionoelastomer junctions between polymer networks of fixed anions and cations.

Soft ionic conductors have enabled stretchable and transparent devices, but liquids in such devices tend to leak and evaporate. In this study, we demonstrate diodes and transistors using liquid-free ionoelastomers, in which either anions or cations are fixed to an elastomer network and the other ionic species are mobile. The junction of the two ionoelastomers of opposite polarity yields an ionic double layer, which is capable of rectifying and switching ionic currents without electrochemical reactions. The entropically driven depletion of mobile ions creates a junction of tough adhesion, and the stretchability of the junction enables electromechanical transduction.

Ionic elastomeric material electronics

Materials Science Wearable devices often need to be soft or flexible, and ideally, these properties would extend beyond packaging material to also include the electronics. Some soft ionic conductors have been made in the form of flexible, stretchable, and transparent devices, but leaks from these materials is a concern. Kim et al. demonstrate ionic elastomeric diodes and transistors that harness ionic double layers to rectify and switch ionic currents (see the Perspective by Gao and Lee). This is achieved without trapped liquids by fixing the anions or cations to an elastomer network while the other species of ions remain mobile. Science , this issue p. [773][1]; see also p. [735][2] [1]: /lookup/doi/10.1126/science.aay8467 [2]: /lookup/doi/10.1126/science.aba6270