Toward Robust, Tough, Self-Healable Supramolecular Elastomers for Potential Application in Flexible Substrates

Smart Polydimethylsiloxane Materials: Versatility for Electrical and Electronic Devices Applications

Bio-inspired autonomous smart polydimethylsiloxane (PDMS) and its composite materials hold immense promise for a wide range of applications in electrical and electronic devices. These materials mimic natural protective mechanisms with self-healing, self-reporting, and self-cleaning properties, enabling innovative and efficient device design. Smart PDMS materials autonomously activate repair mechanisms in response to mechanical or electrical damage, achieving rapid structural and functional recovery and preventing failure due to the accumulation of minor damage. These materials can intuitively report their status through striking color changes, fluorescence, or luminescence when exposed to external stimuli, providing efficient and practical visual feedback for device health monitoring and fault warning. They also have the capacity to effectively eliminate contaminants and ice deposits from their surfaces, thereby ensuring stable device operation. This review aims to introduce the current research progress in self-healing, self-cleaning, and self-reporting PDMS materials. The review systematically discusses the principles, methodological innovations, mechanistic analysis, and applications of these materials, highlighting their significant potential for applications in the field of electrical and electronic devices. Moreover, the review provides an in-depth analysis of the key challenges facing current research and offers insights into future research directions and strategies.

Robust Self-Healing PDMS-Based Elastomers Featuring Tunable Mechanical Properties Enabled by Dual Non-Covalent Interactions

It is desirable to develop highly efficient self-healing polydimethylsiloxane (PDMS) elastomers with excellent mechanical properties, which can be solved by introducing dual non-covalent interactions. However, most of the self-healing PDMS elastomers reported so far require harsh conditions, while the improvement of self-healing ability often compromises the mechanical properties. Moreover, hydrogen bonds of traditional urea derivatives tend to crystallize excessively, adversely affecting the stretchability and toughness of elastomers. In this work, strong Zn2+ coordination is introduced into a thiourea hydrogen bond network, which is unlikely to crystallize, resulting in a series of robust and efficient self-healing elastomers. By changing the content of Zn2+ ions, the mechanical properties of materials can be strategically tuned from superior stretchability (≈6000%) to high strength (≈4.2 MPa). In addition, the elastomers also possess favorable self-healing ability. The surface scratches can be completely healed at room temperature for 24 h, and the self-healing efficiency of mechanical properties under mild conditions (60 °C, 6 h) generally reached more than 90%. In addition, the applications of PDMS-BDTI-Zn on hydrophobic coatings are tentatively explored in view of the remarkable hydrophobicity of PDMS.

Development of highly robust polyurethane elastomers possessing self-healing capabilities for flexible sensors

Traditional flexible electronic sensing materials have fallen short in meeting the diverse application needs and environments of modern times. Hence, we require a multi-functional elastomer material to improve the overall performance and expand the functionality of flexible electronic sensors. In this study, we fabricated a multi-block polyurethane (PU) elastomer based on semi-crystalline polycaprolactone (PCL) chain segments and highly flexible polydimethylsiloxane (PDMS) chain segments, which showcases outstanding mechanical properties, self-healing capabilities, and recyclability. By adjusting the ratio parameters of the chain segments, we were able to modulate the thermodynamic behavior, hydrophobicity, mechanical behavior, and self-healing properties of the designed PU elastomers. The optimized ratios exhibited good tensile strength (16.26 MPa), high elongation at break (3300.84%), good toughness (278.82 MJ m-3, fracture energy ≈ 234.96 KJ m-2), high self-repairing (≈100%, at room temperature for 12 h), efficient recyclability, and puncture resistance. Self-healing is accomplished through the interactions between dynamic disulfide bonds, dynamic boron-oxygen bonds, and hydrogen bonds. The conductive ink (PEDOT:PSS) was encapsulated within this elastomer to construct a flexible electronic sensor, attaining excellent sensing performance (stable output for 1000 cycles). This multi-functional polyurethane elastomer acts as an ideal matrix material for flexible electronic sensors, offering novel concepts and perspectives for the next generation of green electronic flexible materials, electronic flexible robots, and other stimulus-responsive materials.

Self-Healing, Degradable, and Biobased Polyurethane Elastomer for High-Performance Piezoresistive Pressure Sensors with a Hump-like Microstructure

Flexible sensors are widely applied in the fields of electronic skins and wearable devices, yet it is still a big challenge to effectively prolong the lifespan of the damaged sensors and reduce environmental pollution caused by discarded sensors after updating and upgrading. Herein, we proposed a self-healing, degradable, and biobased polyurethane elastomer for high-performance flexible pressure sensors. The elastomer synthesized using fatty diamine as a chain extender possessed a high tensile strength of 13.25 MPa and an elongation at break of 830%, and the self-healing efficiency reached up to 109.2%. Additionally, the elastomer could be fully degraded within 7 days in a 1 mol L-1 NaOH solution with the assistance of ethanol. The elastomer-based pressure sensor with a hump-like microstructure was fabricated with reduced graphene oxide as the conductive material via a simple template method. The sensor showed a high sensitivity of 9.448 kPa-1, a large sensing range of 0-300 kPa, a short response/recovery time of 40/80 ms, and a good sensing stability of 14,000 cycles. Moreover, the sensor was utilized to monitor different human motions, including muscle contraction, joint bending, swallowing, voice recognition, and pulse beat. Importantly, even after being severely damaged, the sensor was able to recover its function in detecting human motions. The findings of this research provide a strategy for the sustainable development of environmentally friendly and functional elastomers and flexible sensors.

Innovative Synthesis of Photo-Responsive, Self-Healing Silicone Elastomers with Enhanced Mechanical Properties and Thermal Stability

Photo-responsive materials have garnered significant interest for their ability to react to non-contact stimuli, though achieving self-healing under gentle conditions remains an elusive goal. In this research, an innovative and straightforward approach for synthesizing silicone elastomers is proposed that not only self-heal at room temperature but also possess unique photochromic properties and adjustable mechanical strength, along with being both transparent and reprocessable. Initially, aldehyde-bifunctional dithiophene-ethylene molecules with dialdehyde groups (DTEM) and isocyanurate (IPDI) is introduced into the aminopropyl-terminated polydimethylsiloxane (H2N-PDMS-NH2) matrix. Subsequently, palladium is incorporated to enhance coordination within the matrix. These silicone elastomers transition to a blue state under 254 nm UV light and revert to transparency under 580 nm light. Remarkably, they demonstrate excellent thermal stability at temperatures up to 100 °C and show superior fatigue resistance. The optical switching capabilities of the silicone elastomers significantly affect both their mechanical characteristics and self-healing abilities. Notably, the PDMS-DTEM-IPDI-@Pd silicone elastomer, featuring closed-loop photo-switching molecules, exhibits a fracture toughness that is 1.3 times greater and a room temperature self-healing efficiency 1.4 times higher than its open-loop counterparts. This novel photo-responsive silicone elastomer offers promising potential for applications in data writing and erasure, UV protective coatings, and micro-trace development.

High toughness, healable, self-cleaning polydimethylsiloxane elastomers with "rigid-while-flexible" mutual network structure

Polydimethylsiloxane (PDMS) elastomers with high mechanical and healing properties are developed as smart materials for electrical power systems and electronic devices to address electrical or mechanical damage. However, the challenge is to reconcile the conflicting molecular mechanisms of mechanical and healing properties in the development of PDMS elastomers. This study adopts the "rigid-while-flexible" mutual network structure by copolymerizing the rigid polyimide (PI) with flexible segments with dynamic reversible crosslinking designed on the PDMS backbones. This elastomer (designated PSiPI) exhibits high toughness, tensile strength and elongation at break, as well as excellent healing efficiency and recyclability. Moreover, the PSiPI elastomer also exhibits good insulation and corona damage healing properties. Taking advantage of the recyclability and healing properties of PSiPI elastomers, healable superhydrophobic coatings with contact angles greater than 150° have been prepared by compositing PSiPI elastomers with SiO2. Likewise, combining the elastomer with conductive materials can create a healing flexible conductor. This "rigid-while-flexible" design approach provides important inspiration for the development of high-performance, sustainable and environmentally friendly PDMS elastomers for electrical and electronic applications.

Mechanical activation-enhanced metal-organic coordination strategy to fabricate high-performance starch/polyvinyl alcohol films by extrusion blowing

The production of high-performance starch-based packaging films by extrusion blowing is challenging, ascribed to poor processability of the blend precursors. In this study, a new strategy of mechanical activation (MA)-enhanced metal-organic coordination was proposed to improve the processability of starch (St)/polyvinyl alcohol (PVA) blend precursor, with calcium acetate (CA) as a chelating agent and glycerol as a plasticizer. MA pretreatment activated the hydroxyl groups of starch and PVA for constructing strong metal-organic coordination between CA and St/PVA during reactive extrusion, which effectively enhanced the melt processing properties of the blend precursor, contributing to the fabrication of high-performance St/PVA films by the extrusion-blowing method. The as-prepared St/PVA films exhibited excellent mechanical properties (tensile strength of 34.5 MPa; elongation at break of 271.8 %), water vapor barrier performance (water vapor permeability of 0.704 × 10-12 g·cm-1·s-1·Pa-1), and oxygen barrier performance (oxygen transmission rate of 0.7 cm3/(m2·day·bar)), along with high transmittance and good uniformity. These outstanding characteristics and performances can be attributed to the improved interfacial interaction and compatibility between the two matrix phases. This study uncovers the mechanism of MA-enhanced metal-organic coordination for improving the properties of starch-based films, which provides a convenient and eco-friendly technology for the preparation of high-performance biodegradable films.

Insight into the fracture energy dissipation mechanism in elastomer composites via sacrificial bonds and fillers

Most tough elastomer composites are reinforced by introducing sacrificial structures and fillers. Understanding the contribution of fillers and sacrificial bonds in elastomer composites to the energy dissipation is critical for the design of high-toughness materials. However, the energy dissipation mechanism in elastomer composites remains elusive. In this study, using a tearing test and time-temperature superposition, we investigate the effect of fillers and sacrificial bonds on the energy dissipation of elastomer composites consisting of poly(lipoic acid)/silver-coated Al fillers. We found that the fillers and sacrificial bonds mutually enhance both the intrinsic fracture energy and the bulk energy dissipation, and moreover the sacrificial bonds play a more important role in enhancing fracture toughness than the fillers. It is unreasonable to rely solely on the loss factor for bulk energy dissipation. The addition of sacrificial bonds results in a chain segment experiencing greater binding force compared to the addition of fillers. This suggests that the chain segment consumes more energy during its movement. By calculating the length of the Kuhn chain segment and the Kuhn number, it is evident that the addition of sacrificial bonds results in a greater binding force for the chain segment than the addition of fillers, and this enhanced binding force increases the energy consumption during the motion of the chain segment.

Dual Reversible Network Nanoarchitectonics for Ultrafast Light-Controlled Healable and Tough Polydimethylsiloxane-Based Composite Elastomers

It is highly desirable to develop polydimethylsiloxane (PDMS) elastomers with high self-healing efficiency and excellent mechanical properties. However, most self-healable materials reported to date still take several hours to self-heal and improving the self-healing property often comes at the expense of mechanical properties. Herein, a simple design strategy of dual reversible network nanoarchitectonics is reported for constructing ultrafast light-controlled healable (40 s) and tough (≈7.2 MJ m-3) PDMS-based composite elastomers. The rupture reconstruction of dynamic bonds and the reinforcement effect of carbon nanotubes (10 wt %) endowed our composite elastomer with excellent fracture toughness that originated from a good yield strength (≈1.1 MPa) and stretchability (≈882%). Moreover, carbon nanotubes can quickly and directly heat the damaged area of the composite to achieve its ultrafast repair with the assistance of dynamic polymer/filler interfacial interaction, greatly shortening the self-healing time (12 h). The self-healing performance is superior to that of reported self-healable PDMS-based materials. This novel strategy and the as-prepared supramolecular elastomer can inspire further various practical applications, such as remote anti-icing/deicing materials.

Flexible, sensitive and rapid humidity-responsive sensor based on rubber/aldehyde-modified sodium carboxymethyl starch for human respiratory detection

Natural polymers with abundant hydrophilic groups are potential candidates for humidity sensor designing. Unfortunately, most of natural polymers lack essential stretchability and high conductivity, which hinder their development in the field of flexible humidity sensor. Cooperation with rubbers and conductive nanometer materials is an effective method to make the best use of natural polymers in flexible humidity sensor. In this paper, a flexible and sensitive sensor with rapid response to humidity change is fabricated based on aldehyde-modified sodium carboxymethyl starch (ACMS), carboxylated styrene-butadiene rubber (XSBR) and Ag nanoflakes through film-forming method. The pre-prepared ACMS owns a better dispersibility in the aqueous phase and serves as reducing agent for formation of Ag nanoflakes. After the film-forming process, the composite film shows a strength of 5.66 MPa and a high stretchability with strain of 367 %. Besides, our sensor shows a rapider response to humidity change than the commercial electronic hygrometer that it takes only 1 s to respond to the humidity change from 25 % RH to 27 % RH. Therefore, the XSBR/ACMS/Ag sensor possesses an impressive sensitive response to slight sweat on human skin and breath, which could find applications in monitoring people's health and distinguish their physical condition.

Scalable Manufacturing of Environmentally Stable All-Solid-State Plant Protein-Based Supercapacitors with Optimal Balance of Capacitive Performance and Mechanically Robust

The development of advanced biomaterial with mechanically robust and high energy density is critical for flexible electronics, such as batteries and supercapacitors. Plant proteins are ideal candidates for making flexible electronics due to their renewable and eco-friendly natures. However, due to the weak intermolecular interactions and abundant hydrophilic groups of protein chains, the mechanical properties of protein-based materials, especially in bulk materials, are largely constrained, which hinders their performance in practical applications. Here, a green and scalable method is shown for the fabrication of advanced film biomaterials with high mechanical strength (36.3 MPa), toughness (21.25 MJ m^-3 ), and extraordinary fatigue-resistance (213 000 times) by incorporating tailor-made core-double-shell structured nanoparticles. Subsequently, the film biomaterials combine to construct an ordered, dense bulk material by stacking-up and hot-pressing techniques. Surprisingly, the solid-state supercapacitor based on compacted bulk material shows an ultrahigh energy density of 25.8 Wh kg^-1 , which is much higher than those previously reported advanced materials. Notably, the bulk material also demonstrates long-term cycling stability, which can be maintained under ambient condition or immersed in H₂ SO₄ electrolyte for more than 120 days. Thus, this research improves the competitiveness of protein-based materials for real-world applications such as flexible electronics and solid-state supercapacitors.

Multivalent Design of Low-Entropy-Penalty Ion-Dipole Interactions for Dynamic Yet Thermostable Supramolecular Networks

Dynamic supramolecular networks are constantly accompanied by thermal instability. The fundamental reason is most reversible noncovalent bonds quickly decay at elevated temperatures and dissociate below 100 °C. Here, in this paper, we realize a reversible ion-dipole interaction with high-temperature stability exceeding 150 °C. The resultant supramolecular network can simultaneously possess mechanical strength of 1.32 MPa (14.8 times that of pristine material), dynamic self-healing capability, and a stable working temperature of up to 200 °C. From the prolonged characteristic relaxation time of 600 s even at 100 °C, our material represents one of the most thermally stable dynamic supramolecular polymers. These remarkable performances are achieved by using a new multivalent yet low-entropy-penalty molecular design. In this way, the noncovalent bond can reach a high enthalpy while minimizing the entropy-dominated thermal dissociations.

Surface Hydrophobization Provides Hygroscopic Supramolecular Plastics Based on Polysaccharides with Damage-Specific Healability and Room-Temperature Recyclability

Supramolecular materials with room-temperature healability and recyclability are highly desired because they can extend materials lifetimes and reduce resources consumption. Most approaches toward healing and recycling rely on the dynamically reversible supramolecular interactions, such as hydrogen, ionic and coordinate bonds, which are hygroscopic and vulnerable to water. The general water-induced plasticization facilitates the healing and reprocessing process but cause a troubling problem of random self-adhesion. To address this issue, here it is reported that by modifying the hygroscopic surfaces with hydrophobic alkyl chains of dodecyltrimethoxysilane (DTMS), supramolecular plastic films based on commercial raw materials of sodium alginate (SA) and cetyltrimethylammonium bromide (CTAB) display extraordinary damage-specific healability. Owing to the hydrophobic surfaces, random self-adhesion is eliminated even under humid environment. When damage occurs, the fresh surfaces with ionic groups and hydroxyl groups expose exclusively at the damaged site. Thus, damage-specific healing can be readily facilitated by water-induced plasticization. Moreover, the films display excellent room-temperature recyclability. After multiple times of reprocessing and re-modifying with DTMS, the rejuvenated films exhibit fatigueless mechanical properties. It is anticipated that this approach to damage-specific healing and room-temperature recycling based on surface hydrophobization can be applied to design various of supramolecular plastic polysaccharides materials for building sustainable societies.

Malleable and self-healing rubbers covalently crosslinked by reversible boronic ester bonds

Covalent cross-linking of rubbers is essential to obtain high elasticity, which plays a significant role in practical applications. Integrating dynamic covalent bonds into rubbers is a reliable way to endow them with self-healing capacity to repair damage and extend their service life. Herein, we propose a facile and effective method to introduce a crosslinking network composed of dynamic boronic ester bonds into epoxidized natural rubber (ENR). The reaction between dopamine-modified ENR and the hydrolysate of boric acid in weak alkaline solution formed a cross-linking network with reversible boronic ester bonds. Owing to the rearrangement of the crosslinked network initiated by the boronic ester exchange reaction, covalently crosslinked rubbers exhibit relatively better malleability and excellent self-healing ability under moderate conditions (60 °C/24 h).

Strengthened self-healable natural rubber composites based on carboxylated cellulose nanofibers participated in ionic supramolecular network

Cellulose, as a green reinforcing agent for rubber, has excellent improvement on the tensile strength but usually accompany with a deterioration of extensibility and self-healing property. Herein, we report an efficient method to prepare robust and self-healable natural rubber/zinc dimethacrylate/carboxylated cellulose nanofibers (NR/ZDMA/CNC) composites which are constructed by a CNC participated ionic supramolecular network. Ionic supramolecular network in NR is generated by the polymerization of ZDMA during a controlled peroxide-initiated vulcanization of NR. Interestingly, NR with massive ion clusters has strong affinity with CNC, which facilitates the uniform dispersion of CNC and the compatibility between CNC and NR. Meanwhile, CNC participates into the supramolecular network via non-covalent interaction with NR chains equipped with ionic crosslinks. This greatly reduces the adverse effect of CNC on the dynamic characteristics of supramolecular network. As a result, the tensile strength of NR/ZDMA composite with 20 phr CNC could reach 4.13 MPa, while its self-healing efficiency still maintains at >80 %. Thus, NR composites with non-covalent interaction between CNC and supramolecular network display improved strength, maintained extensibility, and excellent self-healing capability. This study thus demonstrates a feasible approach to reduce the negative effect of reinforcing fillers on a self-healing rubber based on supramolecular networks.

Highly conductive, Transparent, Adhesive and Self-healable Ionogel Based on a Deep Eutectic Solvent with Widely Adjustable Mechanical Strength

Ionogels have attracted intensive attentions as promising flexible conductive materials. However, simultaneous integration of excellent mechanical properties, high conductivity, outstanding self-healing ability and strong adhesiveness is still challenging. Here, an ingenious composition design is proposed to address this long-standing challenge of ionogels. High-performance PEI/PAA/CMC ionogels, consisted of a loosely cross-linked poly(acrylic acid) (PAA) network, dynamically cross-linked network based on polycationic polyethyleneimine (PEI) and polyanionic PAA, and carboxymethyl cellulose (CMC) reinforcing filler, are formed in a deep eutectic solvent (DES) composed of choline chloride and urea. Benefiting from the loose PAA network and dynamic noncovalent interactions, ionogels with both highly enhanced mechanical robustness and excellent conductivity are obtained at high loading of DES, overcoming the strength-ductility/conductivity trade-off dilemma. By adjusting PEI/PAA mass ratio, the tensile strength and strain of PEI/PAA/CMC ionogels are effectively controlled in a wide range of 0.15-7.9 MPa and 232-1161%, respectively, while maintaining the desirable conductivity of ∼10^-4 S cm^-1 . Besides, healed tensile strength over 2.1 MPa and adhesion strength up to 0.2 MPa are achieved for the PEI_0.06 /PAA_0.25 /CMC_0.01 ionogel. The delicate design strategy provides a feasible approach to prepare ionogels with outstanding comprehensive performance, which have potential for applications in flexible electronics. This article is protected by copyright. All rights reserved.

Interfacial Coordination Interaction Enables Soft Elastomer Composites High Thermal Conductivity and High Toughness

Soft elastomers have attracted wide applications, such as soft electronic devices and soft robotics, due to their ability to undergo large deformation with a small external force. Most elastomers suffer from poor toughness and thermal conductivity, which limits their use. The addition of inorganic fillers can enhance the thermal conductivity and toughness, but it deteriorates the softness (low Young's modulus and high stretchability). Integrating thermal conductivity, toughness, and softness into one elastomer is still a challenge. Here, we report a strategy of interfacial coordination interaction to achieve soft elastomer composites with high thermal conductivity and high toughness. We demonstrate the strategy by using poly(lipoic acid) elastomer and silver-coated aluminum filler as model, where silver-sulfur coordination cross-links are formed at the interface. The resultant elastomer composite shows high streachability (450%), high thermal conductivity (2.35 W m-1 K-1), low modulus (321 kPa), and high toughness (3496 J m-2), which cannot be achieve in existing elastomers. The time domain thermoreflectance technique demonstrates that the silver-sulfur coordination interaction lowers the interfacial thermal resistance, resulting in enhanced thermal conductivity of the elastomer composites. The excellent softness stems from lower bonding energy of the silver-sulfur coordination cross-links compared with covalent chemical cross-links. The high toughness also benefits from the interfacial silver-sulfur coordination interaction that can dissipate more energy upon deformation. We further demonstrate the potential application of the thermally conductive, tough, and soft elastomer composites for thermal management of chip and soft electronic devices.

Design and synthesis of mechanochromic poly(ether-ester-urethane) elastomer with high toughness and resilience mediated by crystalline domains

Mechanochromic elastomers play an important role in stain sensing, materials damage alarming and stress detecting, etc. Low activation strain and stress, high toughness and resilience, and self-recovery ability are essential...

Antimicrobial PDMS Surfaces Prepared through Fast and Oxygen-Tolerant SI-SARA-ATRP, Using Na2SO3 as a Reducing Agent

Poly(dimethylsiloxane) (PDMS) is an attractive, versatile, and convenient material for use in biomedical devices that are in direct contact with the user. A crucial component in such a device is its surface in terms of antimicrobial properties preventing infection. Moreover, due to its inherent hydrophobicity, PDMS is rather prone to microbial colonization. Thus, developing an antimicrobial PDMS surface in a simple, large-scale, and applicable manner is an essential step in fully exploiting PDMS in the biomedical device industry. Current chemical modification methods for PDMS surfaces are limited; therefore, we present herein a new method for introducing an atom transfer radical polymerization (ATRP) initiator onto the PDMS surface via the base-catalyzed grafting of [(chloromethyl)phenylethyl]trimethoxysilane to the PDMS. The initiator surface was grafted with poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) brushes via a surface-initiated supplemental activator and reducing agent ATRP (SI-SARA-ATRP). The use of sodium sulfite as a novel reducing agent in SI-SARA-ATRP allowed for polymerization during complete exposure to air. Moreover, a fast and linear growth was observed for the polymer over time, leading to a 400 nm thick polymer layer in a 120 min reaction time. Furthermore, the grafted PDMAEMA was quaternized, using various alkylhalides, in order to study the effect on surface antimicrobial properties. It was shown that antimicrobial activity not only depended highly on the charge density but also on the amphiphilicity of the surface. The fast reaction rate, high oxygen tolerance, increased antimicrobial activity, and the overall robustness and simplicity of the presented method collectively move PDMS closer to its full-scale exploitation in biomedical devices.

Tough double-network elastomers with slip-rings

In order to surmount the inherent trade-off between toughness and stiffness for most elastomers, we developed a strategy which let two polymer networks form an interpenetrated structure through introducing slip-rings by a very simple one-step synthesis method.