Cross-linked Elastomers Research Articles

Mechanics of multi-stimuli temperature-responsive hydrogels

The temperature-sensitivity of a certain class of elastomeric gels (cross-linked elastomers keen to uptake a fluid into their network, shown for example by poly-N-isopropylacrylamide (pNIPAm) hydrogels), is an interesting property allowing a temperature-controlled swelling. Being the elastomer-fluid affinity sensible to the temperature variation, this implies the possibility to control the amount of fluid uptaken by the material by properly changing its temperature. This capability is particularly interesting in biomedical applications or in the development of sensors whose responsiveness is often required to depend on the environmental temperature. Further, the incorporation of photo-thermal particles into the gel enables the use of light for controlling the material response. In this way, smart untethered multi-stimuli sensors or actuators can be obtained.In the present study, we consider the mechanical behavior of temperature-sensitive hydrogels and, relying on a theoretical multiphysics-based model accounting for light diffusion, heat generation and transfer, fluid absorption, and mechanics, the response of morphing elements is studied. Light-induced morphing due to photo-thermal effect is also considered and mathematically modeled. Validation and parametric simulations of the emerging deformations confirm the soundness of the approach and demonstrate the wide range of morphing functionalities obtainable by harnessing the temperature-dependent sensitivity of pNIPAm.

Dual and triple shape memory properties of poly(ε-caprolactone)-based cross-linked polymer elastomers

As a smart material, Shape Memory Polymer (SMP) can perceive changes in the external environment in which it is located, and make corresponding instantaneous active responses to the changes. In this study, polyurethane polymer was synthesized with polycaprolactone (PCL) as the soft segment, 2,2-dihydroxymethyl propionic acid (DMPA) and diisocyanate as the hard segment. DMA is used to evaluate the shape memory performance of the polymer, and results indicate that the synthesized polymer has a wide transition temperature and exhibits excellent thermally induced double and triple shape memory. The polymer with a 1:1 M ratio of PCL and DMPA has a shape fixation rate of 80% and a recovery rate of 99%. After 3 cycles, the film can still quickly return to its original shape. The findings of the study show that the composite material has great application potential under high and low temperature environments drives.

Noncovalently Cross-Linked Polymeric Materials Reinforced by Well-Designed In Situ-Formed Nanofillers.

Noncovalently cross-linked polymeric materials generally exhibit lower mechanical robustness than traditional polymeric materials. Therefore, it is important to improve the mechanical properties of noncovalently cross-linked polymeric materials using an efficient and generalized approach. In this Perspective, we systematically summarized the recent development of noncovalently cross-linked polymeric materials reinforced by in situ-formed nanofillers. The synergy of high-density noncovalent interactions and in situ-formed rigid nanofillers provided an effective means for the fabrication of noncovalently cross-linked plastics with high mechanical strength. The design of in situ-formed tough nanofillers, which could deform and dissociate, endowed the noncovalently cross-linked hydrogels and elastomers with high toughness, excellent stretchability, elasticity, damage resistance, and damage tolerance. Benefiting from the well-designed in situ-formed nanofillers, these noncovalently cross-linked polymeric materials with enhanced mechanical strength still exhibited satisfactory healing, recycling, and reprocessing properties. Outlooks were provided to envision the remaining challenges to the further development and practical application of noncovalently cross-linked polymeric materials reinforced with in situ-formed nanofillers.

Boronic ester bonds crosslinked vitrimer elastomers with mechanical robustness, shape memory, self-healing and recyclability properties

Permanently cross-linked networks confer chemical and mechanical stability to cross-linked elastomers, making their recycling a serious environmental issue. Recyclable covalently crosslinked elastomers have recently been developed by building covalently adaptive networks in elastomers. Here, we demonstrate a simple and general strategy to construct covalent adaptable networks with mechanical robustness, self-healing, shape memory, and recyclability. The dynamic boronic ester linkage between silica and epoxidized natural rubber endows the material with high stretchability and high tensile strength. The boronic ester bond exchange enables the cross-linked network to achieve self-healing, remodeling, and shape memory through network structural rearrangement. The materials can be recovered by hot pressing or dissolution, and the structure of the sample recovered by dissolution remains basically unchanged. Furthermore, a self-healing flexible electronic device can be fabricated by spraying conductive graphite on the surface of these elastomers. Considering that this interfacial boronic ester bond cross-linking can be easily achieved between different commercial epoxy polymers and hydroxyl-rich nanoparticles, we believe that this work has great potential in the development of rubber-based vitrimer elastomers and flexible wearable devices.

Bioinspired Ultra Tear-Resistant Elastomer with a Slidable Double-Network Structure.

Tear resistance is of vital importance in the fabrication and application of synthetic soft materials. However, the paradox of simultaneously improving the tearing energy and elasticity remains a huge challenge for conventional approaches. Here, inspired by the skin, we successfully constructed an extraordinary tear-resistant, superelastic elastomer by the introduction of nanosized polycyclodextrin into the elastomer network to form a slidable interpenetrate double network structure. The tearing energy of the SDEP elastomer is up to 274 KJ/m2, which is comparable to metals and alloys and increased more than 100 times compared with the chemically cross-linked elastomer. The fracture strain exceeded 3300%, which is hardly achieved by other materials with high tearing energy. This comprehensive improvement of antitearing and super elasticity property was achieved by (i) a slide ring effect to dissipate energy and blunt a crack tip; (ii) straightening and reorientation of the slidable double network to deflect the advancing of a crack tip; (iii) a double network sharing the load. These results provide a novel strategy to fabricate elastic, tear-resistant soft material, which may contribute to the practical application as tear-resistant flexible electronics and irregular-shaped stretchable devices.

Thermoreversible Cross-Linked Rubber Prepared via Melt Blending and Its Nanocomposites.

A covalent adaptable network based on the thermoreversible cross-linking of an ethylene–propylene rubber through Diels–Alder (DA) reaction was prepared for the first time through melt blending as an environmental-friendly alternative to traditional synthesis in organic solvents. Functionalization of the rubber with furan groups was performed in a melt blender and subsequently mixed with different amounts of bismaleimide in a microextruder. Cross-linking was confirmed by FT-IR spectroscopy and insolubility at room temperature, while its thermoreversible character was confirmed by a solubility test at 110 °C and by remolding via hot-pressing. Mechanical and thermomechanical properties of the obtained rubbers showed potential to compete with conventionally cross-linked elastomers, with stiffness in the range 1–1.7 MPa and strain at break in the range 200–500%, while allowing recycling via a simple melt processing step. Nanocomposites based on the thermoreversible rubber were prepared with reduced graphene oxide (rGO), showing significantly increasing stiffness up to ca. 8 MPa, ∼2-fold increased strength, and thermal conductivity up to ∼0.5 W/(m K). Results in this paper may open for industrially viable and sustainable applications of thermoreversible elastomers.

Tailoring Electromechanical Properties of Natural Rubber Vitrimers by Cross-Linkers

The growing demand for smart polymeric transducers such as dielectric elastomer actuators and energy harvesters has urged the use of sustainable and recyclable elastomeric materials. Vitrimer chemistry has shed light on future reprocessable and recyclable thermosets and elastomers. In this work, epoxidized natural rubber (ENR) vitrimers were prepared using diacid or triacid cross-linkers and formed covalently cross-linking networks via thermally triggered reversible β-hydroxy ester bonds. The cross-linked ENR elastomers exhibited Arrhenius-type viscoelastic behavior with a complete stress relaxation between 140 and 160 °C, that is, vitrimer characteristics, which were highly dependent on the cross-linking temperature. The mechanical and dielectric properties of the ENR vitrimers can be tuned by varying the molecular structure and concentration of the cross-linkers. Among the diacid and triacid cross-linkers, Pripol 1017 fatty polyacid (P1017) and 3,3′-dithiopropionic acid (DTPA) had similar effects on the cross-linking density and mechanical properties of the ENR vitrimers. The highest tensile strength of 8.70 ± 1.9 or 15.6 ± 2.6 MPa was obtained at 6 mol % of P1017 or DTPA, respectively. While for diamide-based diacid cross-linker (DME), 8 mol % was needed to reach the highest tensile strength of 13.1 ± 2.7 MPa for the elastomer. The three ENR vitrimers showed increased relative permittivity ε′ = 5∼7 at 1 kHz while maintaining low dielectric losses compared to traditional dicumyl peroxide-cured ENR, with ε′ = 3.57 at 1 kHz. With the optimized acidic cross-linker concentrations of P1017 at 6 mol %, DTPA at 6 mol %, and DME at 8 mol %, the ENR vitrimers exhibited improved actuation capabilities at lower electrical fields. Utilizing dynamic cross-linkers to tune the electromechanical properties of dielectric elastomers and the reversibly cross-linked polymer networks will open new opportunities for smart and sustainable dielectric elastomer devices.

Room temperature Self-healable and extremely stretchable elastomer with improved mechanical Properties: Exploring a simplistic Metal-Ligand interaction

The dynamic supramolecular crosslinking reactions like imine bond formation, disulfide metathesis reactions, Diels Alder reactions, metal–ligand interactions, H-bonding, and ionic interactions are broadly used for the self-healing application of elastomers. Herein, we have developed a new self-healable and extremely stretchable elastomer based on robustness and dynamic metal–ligand coordination bonds between the API ligand and the Zn2+ metal ion moieties incorporated onto the XNBR rubber backbone. The FT-IR analysis confirmed the formation of amide linkage and metal–ligand coordination bonds into the XNBR rubber backbone. The healing performance and mechanical properties of various rubber compounds depend on the dosages of metal ions and the type of crosslinking systems. The metal–ligand crosslink compounds exhibit excellent healing performance over the sulfur crosslinking system. Differential scanning calorimetry, rubber process analyzer, crosslinking density, and morphology studies are indicated to characterize the metal–ligand interactions in the XNBR rubber matrix. The metal–ligand cross-linked XNBR rubber with 3 parts of Zn2+ metal ion demonstrates an excellent healing performance of 91.2%, with the tensile strength of 5.7 ± 0.8 MPa at room temperature for 24 h is much higher than the covalently cross-linked elastomers. The metal–ligand coordination bonds are entirely dynamic and thermoreversible during the rebuilding process and obtain an excellent self-healing property than the sulfur curing system.

Dual physical cross-linked self-healing elastomer for the triple shape memory

Dual physical cross-linked self-healing elastomer for the triple shape memory

A physics-informed multi-agents model to predict thermo-oxidative/hydrolytic aging of elastomers

This paper introduces a novel physics-informed multi-agents constitutive model to propose prediction in quasi-static constitutive behavior of cross-linked elastomer and the loss of mechanical performance during environmental aging. The presented model is used to simulate the effect of single-mechanism chemical aging (i.e. thermal-inducedor hydrolytic aging) on the behavior of the material in this hybrid framework. Those environmental single-mechanism damages change the polymer matrix over time due to massive chain scission, chain formations, and changing the arrangement of molecules in the polymer matrix. We propose a data-driven super-constrained machine-learned engine to represent damage in the polymer matrix and capture the changes in material behavior, including its inelastic features such as Mullins effect and permanent set in the course of aging. We have simplified the 3D stress–strain tensor mapping problem into a small number of super-constrained 1D mapping problems by means of a sequential order reduction. An assembly of multiple replicated conditional neural-network learning-agents (L-agents) is trained to systematically simplify the high-dimensional mapping problem into multiple 1D problems, each represented by a different type of agent. Our hybrid framework is designed to capture the effect of deformation history, aging time, and aging temperature. The model is validated with respect to a comprehensive set of experiments specifically designed to benchmark model capabilities and also against available data in the literature. Thermodynamic consistency and frame independency have been verified. Besides acceptable predictive abilities, a significant reduction of computational cost to predict behavior at multiple states of deformation is the most significant feature of this model.

Experimental characterisation and modelling of the strain rate dependent mechanical response of a filled thermo-reversible supramolecular polyurethane

• A sugar-filled healable polyurethane composite was fabricated, characterized and modelled. Composites consisting of a polymer binder filled with various particles are widely used in industrial applications, and key to their engineering design is the ability to produce mechanical models of these materials. One limitation of many composites is the use of cross-linked elastomers as matrix materials, which reduces re-usability and recyclability. In this research, a recently developed reusable and low temperature processable supramolecular polyurethane was mixed with sugar particles to produce a model composite material with different particle sizes or volume fractions, which were subsequently characterised and modelled. Large strain mechanical properties of the material were analysed in compression at strain rates up to approximately 1800 s −1 , supported by measurements of the small strain viscoelastic response at different temperatures and frequencies. A model was developed that combined the known viscoelastic response of the supramolecular polyurethane with filler reinforcement and strain activated damage equations calibrated against quasi-static experiments. The model provided a good prediction of the high strain rate behaviour, for which the effect of adiabatic heating in the sample was also considered. The model parameters were related back to the filler particle size and volume fraction. Finally, a preliminary study of geometry and strength recovery was performed.

Heterogeneous network design strategy toward mechanically robust and recyclable elastomers

It is well-accepted that thermoplastic vulcanizates (TPVs) integrate the fascinating recyclability of thermoplastics and the mechanical robustness comparable to nano-filled elastomers, deriving from the particular heterogeneous network structure. However, the elasticity of TPVs is inferior to covalently cross-linked elastomers, since the irreversible deformation or even collapse of plastic phases of TPVs will certainly give rise to considerable hysteresis loss and therefore deteriorate the elasticity. Herein, we proposed a facile network design strategy toward covalently cross-linked elastomer with mechanical robustness and recyclability by engineering heterogeneous blend networks into diene-rubber system. Specifically, two conventional diene-rubbers, ethylene propylene diene terpolymer (EPDM) and styrene-butadiene rubber (SBR), were adopted as thermodynamic incompatible pair and then directly admixed. Due to the significant difference in the cross-linking reactivity between the pair, the resulting system assembled into a microphase-separated structure with strong interfacial adhesion upon in situ co-cross-linking. Upon deformation, the overall chain orientation of the networks is promoted by the efficient interfacial stress transfer and a striking reinforcement of the networks is achieved accordingly. More importantly, by virtue of the slightly cross-linking attribute of EPDM component, the resulting system featured with desirable recyclability and high elasticity in comparison with conventional TPVs. We envisage that this work will provide important inspiration for the non-filling reinforcement of elastomers and the development of a new generation of thermoplastic vulcanizates with considerably low hysteresis through manipulating the covalent network heterogeneity.

Dynamic Covalent Bond Cross-Linked Luminescent Silicone Elastomer with Self-Healing and Recyclable Properties.

Two aldehyde-modified tetraphenylene derivatives with different functionality are synthesized and exhibit different fluorescence properties. By incorporating tetraphenylene derivatives into polydimethylsiloxane (PDMS) networks, two elastomers are prepared through dynamic covalent cross-linking. The elastomers show excellent fluorescence properties, mechanical properties, thermal stability as well as self-healing and recycle properties. At the same time, the mechanical properties of the elastomers are influenced by the functionality of the tetraphenylene derivatives and the molecular weight of the PDMS. The self-healing process takes place quickly and the recycling process can be carried out by solution processing and hot pressing. It shows the similar tensile properties between the pristine and healed samples.

A room temperature self-healing and thermally reprocessable cross-linked elastomer with unprecedented mechanical properties for ablation-resistant applications

High-performance ablative composites demonstrate promising applications in the field of aerospace for thermal protection. In this work, a mechanically strong, fast room temperature self-healing and thermally recyclable crosslinked elastomer was prepared by constructing high-density hydrogen bonds and dynamic disulfide bonds. The tensile strength of the prepared poly(urea-urethane) elastomers (PIDA) reached as high as 31.0 MPa after self-healing for 48 h at room temperature, exhibiting a self-healing efficiency of 93.6%. The PIDA-based composite with 10 wt% hollow phenolic microspheres possessed a tensile strength of 8.67 MPa and exceptional ablative performance with a line ablation rate of 0.0643 mm/s. The extraordinary mechanical properties of PIDA-based materials were related to the presence of reversible disulfide bonds, meticulously engineered hydrogen bonds and chemical cross-linking sites consisting of 2,4-diamino-6-hydroxypyrimidine (DAHP) and urethane moieties that linked by flexible alicyclic hexatomic spacers. Incorporating DAHP allowed the rapid formation of hydrogen bonds at the crosslinking sites, which increased the interaction force of the repair surface during the initial stages of self-healing and provided support for further exchange reactions of disulfide bonds. This work opens an avenue toward developing ultra-robust room temperature self-healing materials for potential thermal protection in aerospace industry.

Poly(β-hydroxyl amine)s: Valuable Building Blocks for Supramolecular Elastomers with Tunable Mechanical Performance and Superior Healing Capacity.

Supramolecular elastomers integrated with high mechanical toughness and excellent self-healing ability offer attractive applications in various fields such as biomedical materials and wearable electronics. However, the multistep preparation process for creating functional polymer precursors and the expensive stock materials required are two factors that limit the widespread use of supramolecular elastomers. Herein, for the first time, poly(β-hydroxyl amine)s generated by amine-epoxy polymerization were used in the development of supramolecular polymer materials. Based on the novel silicon-containing poly(β-hydroxyl amine)s synthesized by the polymerization between 1,3-bis(3-glycidyloxypropyl)tetramethyldisiloxane and 3-amino-1,2-propanediol, dually cross-linked supramolecular elastomers with both hydrogen bonding and metal coordination were achieved, displaying adjustable mechanical properties with the tensile strength varying from 0.70 MPa to 2.52 MPa, respectively. Thanks to the dynamic nature of the supramolecular interactions, these elastomers exhibited favorable hot-pressing reprocessability and excellent self-healing performance, with the healing efficiency reaching up to 98% at 60 °C for 48 h. Potential applications for photoluminescent materials and flexible electronic devices were demonstrated. We believe that its simplicity of synthesis, adjustable mechanical properties, and robust self-healing capacities bode well for future applications of this new supramolecular elastomer.

Photoswitchable Covalent Adaptive Networks Based on Thiol-Ene Elastomers.

Covalent adaptive networks combine the advantages of cross-linked elastomers and dynamic bonding in a single system. In this work, we demonstrate a simple one-pot method to prepare thiol-ene elastomers that exhibit reversible photoinduced switching from an elastomeric gel to fluid state. This behavior can be generalized to thiol-ene cross-linked elastomers composed of different backbone chemistries (e.g., polydimethylsiloxane, polyethylene glycol, and polyurethane) and vinyl groups (e.g., allyl, vinyl ether, and acrylate). Photoswitching from the gel to fluid state occurs in seconds upon exposure to UV light and can be repeated over at least 180 cycles. These thiol-ene elastomers also exhibit the ability to heal, remold, and serve as reversible adhesives.

Construction of durable superhydrophobic and anti-icing coatings via incorporating boroxine cross-linked silicone elastomers with good self-healability.

The fragility of the micro-nano structure makes superhydrophobic coatings highly susceptible to stress, resulting in a decrease in their superhydrophobic and anti-icing performance. In this work, we proposed a new insight to improve durability by incorporating a thin layer of self-healable elastomer with a dynamic network on the micro-nano structure. We constructed superhydrophobic coatings (EP/SiO2/BFVSE) with a three-layered structure of the epoxy resin/silica nanoparticle/silicon elastomer. The silicon elastomer (BFVES) with a B-O dynamic cross-linked network and fluorinated moieties was synthesized by graft polymerization on vinyl silicon oil. The preparation route is facile and convenient for mass production. BFVES has rapid self-healing properties for scratches at room-temperature, underwater and at -18 °C. EP/SiO2/BFVSE preserved apparently higher CAs after being immersed in pH = 1, pH = 13, and NaCl solutions for 96 h as compared with the EP/SiO2 coating. In a water striking environment, the CA of EP/SiO2/BFVSE was slightly decreased to 153°. SEM images further reveal that the recovery of superhydrophobicity and icephobicity is attributed to the self-healing behavior of the boroxine-containing silicon elastomer. The EP/SiO2/BFVSE coating also possesses additional self-healing ability under chemical oxidation. The high durability of the self-healable superhydrophobic coating enables great application potential in aircraft, marine vessels, and outdoor facilities in harsh environments.

Controlling foamability of polypropylene/γ-irradiated ethylene acrylic elastomer blends by extent of crosslinking and domain microstructure of elastomer

Foams of polypropylene/elastomer blends can often lead to softer foams which may not be desirable every time. Incorporating rigidity to the foams can often be made possible by preferentially crosslinking the elastomer phase prior to blending. Although foamability of polypropylene/elastomer blends has been understood in the scientific community, the influence of the extent of crosslinking in the elastomer phase is not yet understood well. The purpose of this investigation is to identify the influence of the extent of elastomer crosslinking and the blend morphological attributes (achieved by varying screw speed during melt mixing) on foamability of polypropylene/partially crosslinked elastomer blends. Crosslinking of ethylene-acrylic elastomer is carried out using gamma radiation with several doses (0, 12.5, 25, 50 kGy) before melt blending and, subsequently, 10 wt.% of the irradiated elastomers (prior optimized) are mixed with polypropylene in a micro-compounder at three different screw speeds. The microstructure development in blends is characterized by scanning electron microscopy. Frequency sweep rheological analysis is done for selected blends to identify the ease of foamability among the series of blends. Foaming of blends is done with supercritical carbon dioxide in batch mode at three different temperatures. The density reduction along with the microcellular morphology development of blends with foaming is analyzed with the screw speed, the extent of crosslinking, and foaming temperature; furthermore, the individual input parameters (the elastomer domain size, controlled by the screw speed and the extent of crosslinking, controlled by gamma radiation dose) are optimized based on the foam morphology. A uniform and good foamability were achieved at 155 and 160°C for blends with elastomers, irradiated at 12.5 and 25 kGy radiation doses. The lowest density foam (0.37 g/cc) was obtained for polypropylene with 12.5 kGy irradiated crosslinked elastomer mixed at 200 rpm at 160°C foaming temperature. The final elastomer domain dispositions within the foam morphologies are characterized and the plausible foaming mechanism is proposed.

Biomimetic integration of tough polymer elastomer with conductive hydrogel for highly stretchable, flexible electronic

Flexible electronics is booming and attracts considerable interest in monitoring physiological activities and health conditions. The development of human-friend flexible electronics with both excellent human movement monitoring and flexible electrode functions at a wide working range remains highly challenging. Herein, we report a biomimetic double-layered multifunctional flexible electronic device composed of a stretchable, tough elastomer covalently coupled with a conductive, double-network hydrogel for monitoring physiological motions. The introduction of optimal physical crosslinking including hydrogen bond and metal coordination in the covalently cross-linked polyurethane elastomer effectively provides favorable flexibility and stretchability. The elastomer-hydrogel integration (EHI) shows ultra-fast responsiveness (10 ms) and resilience (246 ms) as a skin sensor and could effectively detect diverse muscle and joint movements in a highly sensitive and signal waveform-interpreting manner. Further, such a polymer integration with skin shape-adaptive hydrogel and low contact impedance enables real-time high-quality detection of electrocardiogram (ECG) by serving as flexible electrode compared to commercial Ag/AgCl electrodes, and was successfully applied to measure electrooculogram (EOG) and electromyogram (EMG). EHI-based electrodes were feasible and effective for electroencephalogram (EEG) signal detection in brain-computer interfaces (BCI) system. Altogether, this biomimetic EHI electronic device provides an advanced platform for the design of next-generation flexible healthcare diagnostic monitors and soft robots.

Dynamically Cross-Linked Polyolefin Elastomers with Highly Improved Mechanical and Thermal Performance

Dynamically Cross-Linked Polyolefin Elastomers with Highly Improved Mechanical and Thermal Performance