Functional Elastomers Research Articles

From filler to structure: Designing 3D-printable silicone elastomers with broadband electromagnetic interference shielding

Material extrusion has revolutionized the fabrication of silicone elastomers with intricate and customized structures. However, the trade-off between the ink printability and functional filler compounding impedes the advancement of 3D-printed silicone elastomers for applications such as electromagnetic interference (EMI) shielding and thermal management. In this study, we present a novel approach to fabricating functional silicone elastomers, focusing on the design of fillers, inks and structures. Ink printability was achieved by modified nanosheets, which conferred the thixotropy and self-support capacity to inks by constructing dynamic interfacial interactions within the silicone matrix. Additionally, modified nanosheets exhibited a “lubricating” effect under high shear rates owing to their layered structure, thereby facilitating a smooth extrusion process. Utilizing EMI shielding simulations of periodic porous structures as a guide, we successfully printed broadband EMI shielding silicone elastomers. Furthermore, the versatility of our approach was demonstrated through the creation of customized 3D-printed shielding boxes and wearable thermal management films, showcasing the diverse potential applications of the 3D-printed silicone elastomers. We anticipate that our innovative design approach will bridge the gap between functional elastomers and 3D printing technology, opening up new avenues for their applications in various fields.

Self-healing and reprocessable biobased non-isocyanate polyurethane elastomer with dual dynamic covalent adaptive network for flexible strain sensor

Owing to the nontoxicity of monomers, non-isocyanate polyurethane elastomers have attracted considerable attention, yet the synthesis and functionalization are still challenging. Herein, self-healing and reprocessable biobased non-isocyanate polyurethane (NIPU) elastomers with dual dynamic covalent adaptive network (DDCAN) by disulfide bonds and dynamic imine bonds were synthesized as substrate for the construction of flexible strain sensor with MXene as conductive substance. The tensile strength and elongation at break of the NIPU elastomer were 3.22 MPa and 234 %, respectively. Thanks to the formation of DDCAN, the NIPU elastomer showed high healing efficiency of 97.5 % after healing at room temperature for 24 h. Interestingly, the NIPU elastomer possessed superior reprocessing capability and the tensile strength after three reprocessing cycles reached 136.1 % of the original one, which was attributed to the increase of crosslinking points caused by topological rearrangement. In addition, the NIPU elastomer-based flexible strain sensor exhibited fast response (response time = 60 ms) and excellent repeatability (1000 bending-releasing cycles) and was successfully applied for human motion detection and speech recognition. The findings in this work conceivably stand out as a new methodology for the preparation of biobased and functional elastomers, which will greatly promote the sustainable development and application of flexible electronics.

Synthesis of Bottlebrush Polymers with Spontaneous Self-Assembly for Dielectric Generators.

Cross-linked bottlebrush polymers received significant attention as dielectrics in transducers due to their unique softness and strain stiffening caused by their structure. Despite some progress, there is still a great challenge in increasing their dielectric permittivity beyond 3.5 and cross-linking them to defect-free ultrathin films efficiently under ambient conditions. Here, we report the synthesis of bottlebrush copolymers based on ring-opening metathesis polymerization (ROMP) starting from a 5-norbornene-2-carbonitrile and a norbornene modified with a poly(dimethylsiloxane) (PDMS) chain as a macromonomer. The resulting copolymer was subjected to a postpolymerization modification, whereby the double bonds were used both for functionalization with thiopropionitrile and subsequent cross-linking via a thiol-ene reaction. The solutions of both bottlebrush copolymers formed free-standing elastic films by simple casting. DMA and broadband impedance spectroscopy revealed two glass transition temperatures uncommon for a random copolymer. The self-segregation of the nonpolar PDMS chains and the polynorbornane backbone is responsible for this and is supported by the interfacial polarization observed in broadband impedance spectroscopy and the scattering peaks observed in small-angle X-ray scattering (SAXS). Additionally, the modified bottlebrush copolymer was cross-linked to an elastomer that exhibits increased dielectric permittivity and good mechanical properties with significant strain stiffening, an attractive property of dielectric elastomer generators. It has a relative permittivity of 5.24, strain at break of 290%, elastic modulus at 10% strain of 380 kPa, a breakdown field of 62 V μm-1, and a small actuation of 5% at high electric fields of 48.5 V μm-1. All of these characteristics are attractive for dielectric elastomer generator applications. The current work is a milestone in designing functional elastomers based on bottlebrush polymers for transducer applications.

Hybrid Cross-Linking to Construct Functional Elastomers.

ConspectusElastomers have been extensively used in diverse industrial sectors such as footwear, seals, tires, and cable jacketing and have attracted more and more attention in emerging fields such as regenerative medicine, soft robotics, and stretchable electronics. Global consumption of natural and synthetic elastomers amounted to nearly 27 million metric tons in 2020. In addition, to further enhance the common properties of elastomers, it is highly desired to endow elastomers with functionalities such as reprocessability, biomimetic mechanical properties, self-healing ability, bioactivity, and electrical conductivity, which will significantly broaden their applications. The covalent or noncovalent cross-linked structure is the essential factor for the elasticity of elastomers. Traditional elastomers usually comprise a single type of cross-linked molecular network, for which it is difficult to modulate the properties and introduce functionalities. Inspired by the simultaneous existence of multiple cross-linked structures in proteins, researchers have employed a hybrid cross-linking strategy to construct elastomers. Various noncovalent interactions (e.g., hydrogen bonds, metal-ligand coordination, ionic interactions, and chain folding) and dynamic covalent bonds (e.g., disulfide bonds, oxime-urethane bonds, and urea bonds) have been integrated in elastomers. Accordingly, the properties and functionalities of elastomers can be tuned by regulating the types, ratios, and distributions of cross-links. The hybrid cross-linking strategy provides a versatile and effective way to construct diverse functional elastomers for broad applications in various important fields.In this Account, we present our recent progress on functional elastomers constructed by a hybrid cross-linking strategy, including their design, preparation, properties, and diverse applications. First, we provide a brief introduction of the basic concept of functional elastomers and outline general strategies and mechanics for functional elastomers constructed by hybrid cross-linking. Then, we classify hybrid cross-linked elastomers by their design strategies, including multiple cross-linking, topological design, chemical coupling, and multiple networks. The relationships between the functionalities and hybrid cross-linked structures are summarized. At the same time, we also introduce diverse applications of these hybrid cross-linked elastomers in biomedicine, flexible electronics, soft robotics, 3D printing, and so on. Finally, we discuss our perspective on open challenges and future development trends of this rapidly evolving field. This Account highlighting the diverse hybrid cross-linked elastomers not only provides insights into strategies for elastomer functionalization but also provides new ideas for material design and inspires a variety of new applications.

Low-temperature oil resistance ester polysiloxane with excellent reprogrammable, reprocessable performance

With ever-increasing environmental concerns, functional elastomers (oil-resistance, conductive, etc.) that processing reprogramming, and reprocess ability is desirable yet challenging, particularly those with low-temperature elasticity. Herein, we report an ester polysiloxane elastomer (EPS) synthesized via ring-opening polymerization (ROP) and efficient thiol-ene post-polymerization. The EPS exhibited tunable Tg from −61 °C to −90 °C. Furthermore, −60 °C relaxation-recovery cycles were programmed on DMA, demonstrating EPS-30-F excellent low-temperature elasticity. The swelling behavior of EPS-F in contrast to several commercially available oil-resistant rubber varieties confirmed its superiority in both solvent and oil resistance. Moreover, the first combination of siloxane equilibration chemistry with the currently commonly utilized hot-vulcanization molding process endows EPS with a reprogramming and reprocessing ability. The results from this study may provide an approach for the accessible preparation of low-temperature oil resistance elastomers with reprogrammablility and reprocessability to respond to sustainable development concerns.

Multi-scale-filler reinforcement strategy enabled stretchable silicone elastomer with synergistically enhanced thermal conductivity and mechanical strength

As soft electronic devices and robotics advance towards high power density and miniaturization, integrating superior thermal and mechanical properties has become a big challenge for functional elastomers. Herein, a multi-scale-filler reinforcement strategy was reported for constructing stretchable, mechanically strengthened and thermally conductive silicone elastomers. The multi-scaled Al2O3/vinyl and methyl co-modified fumed SiO2 (VM-SiO2) co-filled liquid silicone rubber composite exhibited a high thermal conductivity of 1.25 W m−1 K−1 at ∼55 vol% filler loading, 555.7% higher than that of the matrix. The finite element simulation demonstrated that the “bridging” effect of small-scaled fillers between gaps of large-scaled spheres in the matrix lowered the interfacial thermal resistance. Meanwhile, stemming from the strong interfacial interactions between the VM-SiO2 and matrix, up to 12.4 times higher elongation at break (7.17%) and 317.7% increase of tensile strength (3.00 MPa) were reached. We further demonstrated the potential application of the multi-scaled Al2O3/VM-SiO2 co-filled LSR composites for thermal management materials.

Covalent adaptable network in an anthracenyl functionalised non-olefinic elastomer; a new class of self-healing elastomer coupled with fluorescence switching

Fabrication of elastomers with covalent adaptive networks (CANs) leads to easy recyclability and self-healing characteristics leading to extended service life. Herein, a commercially available non-olefinic elastomer, poly(ethylene-co-vinyl acetate-co-glycidyl methacrylate) (EVA-GMA), was functionalised to endow CANs based on thermoreversible Diels-Alder (DA) reaction. For this purpose, the epoxy functionalised elastomer was initially modified with fluorescence active anthracene moieties. Next, the anthracenyl-functionalised elastomer was individually crosslinked with 1,1′-(methylenedi-4,1-phenylene) bismaleimide (BM) and bifunctional 1,2,4-triazoline-3,5-dione (bis-TAD) derivatives to derive the rubbery CANs via DA chemistry. Notably, bis-TAD-derived CANs were obtained much faster (<10 min, r.t.) than maleimide-derived CANs elastomer (96 h at 70 °C). Differential scanning calorimetry (DSC) analysis revealed that functional elastomers with both DA-type CANs are thermoreversible above 130 °C. Noticeably, in the case of the TAD-derived CANs, the (re)processing and healing timings were relatively faster than the maleimide-derived CANs. DA-derived CANs elastomers exhibited a significant healing and recycling efficiency of up to 90 %. The dynamic nature of DA-type CANs elastomers was studied via DMA analysis. Moreover, the fluorescence switching (ON-OFF) behaviour of the anthracenyl functionalised and DA elastomers was studied under UV (λ = 365 nm) light and showed switching behaviour at 140 °C. These functionalised EVA-GMA CANs showed significant self-healing, good recyclability, and well-defined switching properties, indicating their potential applications in speciality rubber products, chemosensors, functional paints, and coatings.

Designing Supertough and Ultrastretchable Liquid Metal-Embedded Natural Rubber Composites for Soft-Matter Engineering.

Functional elastomers with incredible toughness and stretchability are indispensable for applications in soft robotics and wearable electronics. Furthermore, coupled with excellent electrical and thermal properties, these materials are at the forefront of recent efforts toward widespread use in cutting-edge electronics and devices. Herein, we introduce a highly deformable eutectic-GaIn liquid metal alloy-embedded natural rubber (NR) architecture employing, for the first time, industrially viable solid-state mixing and vulcanization. Standard methods of rubber processing and vulcanization allow us to fragment and disperse liquid metals into submicron-sized droplets in cross-linked NR without compromising the elastic properties of the base matrix. In addition to substantial boosts in mechanical (strain at failure of up to ∼650%) and elastic (negligible hysteresis loss) performances, the tearing energy of the composite was enhanced up to 6 times, and a fourfold reduction in the crack growth rate was achieved over a control vulcanizate. Moreover, we demonstrate improved thermal conductivity and dielectric properties for the resulting composites. Therefore, this work provides a facile and scalable pathway to develop liquid metal-embedded soft elastomeric composites that could be instrumental toward potential applications in soft-matter engineering.

Reversible addition–fragmentation chain transfer polymerization of myrcene derivatives: an efficient access to fully bio-sourced functional elastomers with recyclable, shape memory and self-healing properties

The rubber industry has been faced with critical challenges including unsustainable fossil-based monomer sources, lack of functionality and growing environmental concerns of waste vulcanizates.

Versatile value-added application of hyperbranched lignin derivatives: Water-resistance adhesive, UV protection coating, self-healing and skin-adhesive sensing

Bio-based copolymers with versatile properties and good processability are highly desired for practical applications. Herein we describe novel hyperbranched adhesive, UV protective coatings and functional elastomers with 6.1–9.5 wt% lignin synthesized by a one-pot RAFT grafting polymerization. The unique macromolecular structure and sufficient hydrogen bonds of these Lignin-graft-poly(n-butyl acrylate-co-1-vinylimidazole) copolymers (Lignin-BVs) resulted in fascinating and controllable features. Specifically, incorporating 13.9 wt% imidazole (VI) effectively improved the adhesive performances with maximal adhesive force and strength up to 34.0 N and 367.8 N·s, respectively, which were better than the commercial 3 M double-sided adhesive. Owing to water-resistance and UV absorption capacity, our Lignin-BVs could be served as excellent adhesive coatings for future outdoor decorative materials, safety clothing or beach umbrella. Moreover, upon increasing VI content to 19.4 wt%, Lignin-BV19.4 elastomer showed automatically room temperature self-healing behavior and could be fabricated into skin-adhesive strain sensor. Our studies provided a green and effective method in the preparation of the high value-added lignin copolymers, which could be appealing to both frontier research and industrial development.

Bio-based self-healing Eucommia ulmoides ester elastomer with damping and oil resistance

Eucommia ulmoides gum (EUG) is an essential bio-based material with a similar chemical structure to natural rubber, but it is rigid plastic at room temperature due to crystallization, which limits its wide application. Herein, we report a new method for synthesis of functional Eucommia ulmoides ester (EUET) elastomer by chemical modification of EUG. First, EUG was epoxidized by performic acid formed from hydrogen peroxide and formic acid in situ in toluene solution and then subjected to ring-opening reaction with acetic acid. It was found that EUET contained both α-hydroxyl ester and β-hydroxyl ester in the molecular chain. Unlike EUG, EUET was no longer crystallized due to the reduction of regularity, and it has been successfully converted into elastomer, which was confirmed by the elastic strain recovery experiment. Furthermore, because of the presence of polar groups and the hydrogen bond between them, EUET could self-heal under the condition of 50 °C, and their vulcanizates possessed excellent mechanical properties, resilience, oil resistance and damping properties. This study may promote the application of EUG in the field of bio-based functional elastomers.

Dynamic magneto-mechanical properties of magneto-sensitive elastomers determined using a new experimental test involving forced longitudinal vibration

A new experimental method involving forced longitudinal vibration is presented for experimentally determining the dynamic magnetomechanical properties of a magnetosensitive elastomer in a magnetic field. A cylindrical sample is attached to a vibration platform and placed in a longitudinal magnetic field generated by a solenoid electromagnet, and a resonant technique is used to obtain the dynamic magnetomechanical properties of the tested sample. The results indicate that the resonant frequency (i) increases with the intensity of the applied magnetic field and the content of the magnetic-particle filler in the matrix but (ii) decreases with the sample length. The dynamic properties of the storage and loss moduli depend significantly on the excitation frequency and the magnetic-particle content. The testing process shows that it is simple and easy to evaluate the dynamic properties using forced longitudinal vibration, with the additional advantage of it being a nondestructive technique. This method could be extended to characterize the coupled magnetomechanical behavior of magnetosensitive functional elastomers such as Terfenol-D/epoxy composites.

Enhanced dual-responsive shape memory nanocomposites with rapid and efficient self-healing capability

High-strength nanocomposites were prepared by in situ polymerization of 2-methoxyethyl acrylate and N,N′-dimethylacrylamide with the existence of TiO2 nanoparticles. Based on the reversible hydrogen bond network, the resultant materials showed enhanced mechanical strength, excellent shape memory effect, and superior self-healing capability. To accomplish the goal of rapid and efficient self-healing for the materials, the elevated temperature was chosen as the healing temperature, and the fracture sample could be completely repaired within 10 min with the fracture strength of 13.56 MPa and high healing efficiency of ~ 100% (MD50-T5). Combining the high strain and water-absorbing nature of materials, the recovery of the complex shape of helix structure and self-lacing process in water under physiological temperature could be programmed and finished. Thus, the dual functional elastomers could be used in coating, suturing, and possess the wide application prospects in bioengineering and smart devices.

Bioinspired engineering of sacrificial bonds into rubber networks towards high-performance and functional elastomers

Abstract Elastomers with unique high elasticity are recognized as strategic materials. The development of elastomers with the combination of high-performance and functionality is of great importance. In recent years, inspired by the structural characteristics of natural materials, the concept of sacrificial bonding has been successfully applied in the design of elastomeric materials, aiming at significantly improving mechanical and functional properties. In this short review, followed by brief description of the principle of sacrificial bonding, we summarize the main progress of the design of the elastomers with sacrificial bonds, and the main performance characteristics. The sacrificial units, including the covalent and non-covalent sacrificial bonds, may be engineered in rubber chains or the interfaces of rubber nanocomposites. In addition, the mechanisms of sacrificial bonding in conferring the functionalities have been described. Several important functionalities, including shape memory effects, damping and self-healing capability combined with robustness, are introduced. Based on the state of the art of the elastomers with sacrificial bonds, the prospective and challenges of are accordingly presented and discussed.

A kind of bonding functional energetic thermoplastic elastomers based on glycidyl azide polymer

A kind of bonding functional energetic thermoplastic elastomers (ETPEs) were synthesized as the binder of solid propellants using the mixture of chain extenders including diethyl bis(hydroxymethyl) malonate (DBM). The results showed that the mechanical properties of ETPEs decreased with increasing percent of DBM in the chain extenders. On the contrary, the work of adhesion between solid ingredients hexogen (RDX) and ETPEs increased. In order to test the comprehensive impacts of two elements, the RDX/ETPE propellants were prepared. The results showed when the percent of DBM was 25%, the overall properties of the propellants were optimum.

Novel tri-block copolymers of poly(acrylic acid)-b-poly(2,2,3,3,4,4,4-hexafluorobutyl acrylate)-b-poly(acrylic acid) prepared via two-step RAFT emulsion polymerization

A facile method for preparing ABA tri-block functional elastomers via two-step RAFT emulsion polymerization is introduced in this work.

Hard and Soft: Toughening Concrete by Impregnation with Functional Elastomers

Toughening is a concept frequently used in brittle materials in order to prevent premature failure. For concrete, toughening is almost exclusively achieved by pre-stressing, i.e. by creating residual stresses that force cracks to close. However, this can only be accomplished with certain element geometries and is particularly unsuitable for thin specimens.An alternative method of toughening is to deflect the crack or to absorb the crack tip by means of a suitable material, in particular viscoelastic polymers. In the present study, the concrete samples are transformed into hybrid inorganic/organic composites, in which the organic phase exerts the crack-stopping properties. To do so, the pore system is filled with a mixture of methyl methacrylate and 2-hydroxyethyl methacrylate, which is then polymerised to form a functional copolymer. Witha 1:1 ratio of the two monomers, a 4.5-fold increase in flexural strength and 2.3-fold increase in bending modulus was observed compared to the reference concrete. It is assumed that the additional increase compared to concrete impregnated with polymers not containing 2-hydroxyethyl methacrylate is the direct consequence of the interfacial interaction provided by the presence of pendant hydroxyethyl groups in the polymer.

Effect of carbon nanotube reinforcement on the properties of the recycled poly(ethylene terephthalate)/poly(ethylene naphthalate) (r-PET/PEN) blends containing functional elastomers

In this study, the mechanical, thermomechanical, thermal, electrical properties and the morphology of the composites, based on blends of recycled poly(ethylene terephthalate) (r-PET) and poly(ethylene naphthalate) (PEN) that were mixed with functional elastomers and multi walled carbon nanotube (CNT) were investigated. Two types of functional elastomers; terpolymer of ethylene–ethyl acrylate–maleic anhydride (E-EA-MAH) and terpolymer of ethylene–methyl acrylate–glycidyl methacrylate (E-MA-GMA), were used to ensure the miscibility between PET and PEN during the preparation of the blends and composites. All composite and blend samples were extruded by using a laboratory scale twin screw microcompounder. Test samples were prepared via laboratory scale injection molding machine. According to the results of the thermomechanical tests, usage of both elastomers enhanced the miscibility between r-PET and PEN. Morphological analyses showed that the blends and composites which contain E-EA-MAH exhibited better elastomer phase dispersion with smaller domain sizes when compared with the samples with E-MA-GMA. Samples prepared with E-EA-MAH had better mechanical properties than the ones containing E-MA-GMA due to the better elastomer phase dispersion. Moreover, addition of CNT also improved the mechanical properties of the samples for both elastomer types. In contrast to mechanical test results, samples prepared with E-MA-GMA had higher electrical conductivity values when compared with those of the ones containing E-EA-MAH due to the differences in the selective distribution of CNT particles between the polymer phases in the samples.

Synthesis of a New, Low-Tg Siloxane Thermoplastic Elastomer with a Functionalizable Backbone and Its Use as a Rapid, Room Temperature Photoactuator

A new, low-Tg siloxane thermoplastic elastomer with a functionalizable backbone was synthesized via sequential anionic polymerization and coupling, and its utility as a platform to produce functional elastomers was demonstrated by the attachment of a photoresponsive liquid crystal to produce a rapid, room temperature photoactuator. Polystyrene was used as a hard glassy end block, and poly(vinylmethylsiloxane) served as the soft middle segment in a polystyrene-b-poly(vinylmethylsiloxane)-b-polystyrene ABA triblock copolymer. The vinyl side chain was used to attach a side-on oriented mesogen to the siloxane backbone, and the resulting liquid crystal triblock copolymer was characterized with reversible photocontraction tests, where it was shown to be both elastomeric and rapidly photoresponsive at room temperature. Rather than simply undergoing a bending mechanism, an oriented thin cast film of the elastomer was observed to contract reversibly at a tensile strain of 3.3% against 25.7 kPa of applied stress in...

Functional Elastomers via Sequential Selective Diene Copolymerization/Hydrophosphorylation Catalysis

AbstractAn effective preparation of new tailor‐made macromolecular materials via the combination of two fully atom‐efficient catalytic transformations is reported. First, new isoprene‐co‐1,3,7‐octatriene [P(IP‐co‐OT)] copolymers with controlled composition (2–21 mol% triene incorporated), molecular weight (Mn=23–102,000 g mol−1) and microstructure (1,4‐cis rich) have been prepared using the homogeneous Ziegler–Natta catalyst system composed of diallylneodynium chloride, magnesium chloride, tetrahydrofuran and methylaluminoxane [Nd(allyl)2Cl(MgCl2)2(THF)4/MAO]. Next, the pendant vinyl moieties in those P(IP‐co‐OT) copolymers have been selectively transformed into phosphonate groups by hydrophosphorylation in the presence of homogeneous rhodium phosphine catalysts. The latter hydrophosphorylation reaction has been optimized in terms of catalytic activity and selectivity, to define the conditions for an effective and safe procedure that does not affect the macromolecular architecture. All polymer materials have been microstructurally analyzed by 1H, 13C, and 31P NMR spectroscopy, to diagnose the catalyst selectivities in the copolymerization and hydrophosphorylation processes.