Sacrificial Mechanical Bond is as Effective as a Sacrificial Covalent Bond in Increasing Cross-Linked Polymer Toughness

Molecular Release by the Rotaxane and Pseudorotaxane Approach

The controlled release of target molecules is a relevant application in several areas, such as medicine, fragrance chemistry and catalysis. Systems which pursue this implementation require a fine-tune of the start and rate of the release, among other properties. In this scenario, rotaxane- and pseudorotaxane-based systems are postulated as ideal scaffolds to accomplish a precise cargo release, due to the special features provided by the intertwined arrangement. This short review covers advances towards the controlled release of different molecules using rotaxane- and pseudorotaxane-based systems, both in solution and in the solid state.

Molecular Origin of the Stretchability and Fatigue-Resistance of Rotaxane-Based Mechanically Interlocked Polymer Networks

Rotaxane-based polymer networks leveraging host-guest recognition have recently emerged as a versatile platform for developing smart materials. Despite numerous studies on these polymers, their unique mechanical properties are mostly associated with the sliding motion of the macrocycle along the axle, leaving the impact of the presence or absence of interlocked structures on the mechanical performance of materials yet to be directly demonstrated. In this work, we present a densely (pseudo)rotaxane-based supramolecular polymeric network (SPN) and a mechanically interlocked network (MIN) as model systems to explore how the mechanical interlocking unit dominates the material properties. Specifically, we have achieved a significant transition from SPN to MIN by finely tuning the stopper size, just substituting a methyl with a dimethyl group attached to the phenyl ring. Although their stereochemical structures are similar, a subtle increase in the stopper size can lead to striking improvements in stretchability and anti-fatigue performance. The stopper size-relevant dethreading behavior, as evidenced by a combined approach of solid-state NMR spectroscopy and rheology, is the underlying molecular mechanism for the difference in the macroscopic mechanical properties. We anticipate that the fundamental understanding gained from this work will advance the development of rotaxane-based materials with emergent functions and applications.

Tunable Mechanically Interlocked Semi-Crystalline Networks

High-performance polymers based on dynamic chemistry have been widely explored for multi-field advanced applications. However, noncovalent sacrificial bond-mediated energy dissipation mechanism causes a trade-off between mechanical toughness and resilience. Herein, we achieved the synchronous boost of seemingly conflicting material properties including mechanical robustness, toughness and elasticity via the incorporation of mechanical chemistry into traditional semi-crystalline networks. Detailed rheological tests and all-atom molecular dynamics simulation reveal that the excellent mechanical robustness and toughness are attributed to the dissociation of crystalline domains threading through the sieve-shape macrocycles. Reversible nano-crystalline domains and ring-sliding-effect accelerated segment motion efficiently reduce energy dissipation to achieve instantaneous resilience. Moreover, the model polymers demonstrate that the multiple dynamic components endow the resulting polymer with excellent reprocessability under mild conditions. This mechanically interlocked semi-crystalline polymer exhibits potential applications as a thermal/photo actuator. This work reveals the synergic effects of mechanically interlocked sites and tunable crystalline domains, thus providing a reliable guide for the comprehensive improvement of material performance.

Polyurethane Elastomers Strengthened by Pseudo[1]rotaxanes Based on Pillararenes

As a unique property of the interlocked structures, rotaxane allows for intramolecular motions between its wheel and axle components. Introduction of rotaxanes into polymers can endow them with distinctive macroscopic features and outstanding mechanical properties. Here, we prepare a copillar[5]arene with a hydroxyl and an amino-group on each end, which can spontaneously form a pseudo[1]rotaxane through intramolecular hydrogen bonds. This pseudo[1]rotaxane possesses a releasable extra alkyl chain, which is then incorporated into a linear polyurethane by reacting with a diisocyanate to prepare polyurethane elastomers with spring-like structures. The results of stress-strain test and dynamic mechanical analysis all indicate that sliding motions of the axle part on the pseudo[1]rotaxane in the polymer skeleton can greatly dissipate energy, which endows the elastomers with higher toughness and better fatigue resistance. Moreover, the addition of moderate amount of cuprous bromide to form cuprous-thioether coordination in the polymers can further improve the mechanical properties.

Weak Covalent Bonds and Mechanochemistry for Synergistic Self-Strengthening of Elastomers

The macroscopic properties of elastomers are intimately linked to their molecular reactivity and mechanisms. Here, we propose a new strategy for designing strengthening materials based on the synergy of weak covalent bonds and mechanochemistry. After mechanical treatment, the failure strength and toughness of the elastomer increased from 2.37 ± 0.05 MPa and 11.34 ± 0.30 MJ/m3 to 6.02 ± 0.04 MPa and 18.40 ± 0.30 MJ/m3, respectively, while maintaining excellent tensile properties. Notably, experimental tests, theoretical calculations, and small-molecule reaction model results show that the sulfur-carbon bond is more prone to homolysis, and the reactive sites are between sulfur radicals and the end-positioned carbon of the vinyl. The C-S weak bond of spirothiopyran (STP) first undergoes homolysis to dissipate energy suffering from external stress, and the radical-mediated click reaction leads to the interchain cross-linking, thus enhancing the mechanical strength. In the end, the prepared elastomer is further used to construct a photonic elastomer, which exhibits not only mechanical force-enhanced strength but also mechanochromism. The present work provides an opportunity for innovative design of self-strengthening materials, and the prepared novel self-strengthening elastomer has broad applications in visualized strain monitoring, electronic skin, soft robots, and other fields.

Autonomic Self-Healing of Polymers: Mechanisms, Applications, and Challenges

Mechanical loads degrade polymers by enabling mechanochemical fragmentation of macromolecular backbones. In most polymers, this fragmentation is irreversible, and its accumulation leads to the appearance and propagation of cracks and, ultimately, fracture of the material. Self-healing describes a diverse and loosely defined collection of approaches that aim at reversing this damage. Most reported synthetic self-healing polymers are non-autonomic, i.e., they require the user to input free energy (in the form of heat, irradiation, or reagents) into the damaged material to initiate its repair. Here, we critically discuss emerging chemical approaches to autonomic self-healing that rely on regenerating the density of load-bearing, dissociatively-inert backbone bonds either after the load on a partially damaged material dissipated or continuously and in competition with the mechanochemically driven loss of backbones in the loaded material. We group the reported chemistries into three broad types whose analysis yields a set of criteria against which the potential of a prospective approach to yield practically relevant self-healing polymers can be assessed quantitatively. Our analysis suggests that the direct chain-to-chain addition in mechanically loaded unsaturated polyolefins is the most promising chemical strategy reported to date to achieve autonomic synchronous self-healing of practical significance.

Hydrogels with high sacrifice efficiency of sacrificial bonds and with high strength and toughness due to dense entanglements of polymer chains

Introducing sacrificial bonds is a common method for increasing the toughness of hydrogels. Many sacrificial bonds have been extensively investigated, but the sacrifice efficiency has never been studied. In this study, polyacrylamide hydrogels with highly entangled polymer chains containing carboxyl-zirconium (-COO--Zr4+) sacrificial bonds are prepared to study the effect of polymer chain entanglement on the sacrificial bond efficiency. Unlike chemical crosslinking points, the dense physical entanglements do not affect the toughness (∼43 MJ/m3) of hydrogels but significantly improve the tensile strength (by two times) and Young's modulus (by six times). Physical entanglements enable the chains to slide and adjust the network structure under stress, which enables more polymer chains and sacrificial bonds to participate in the deformation process. Therefore, dense entanglements will greatly improve the sacrifice efficiency. However, a high density of chemical crosslinking points will limit the improvement in the sacrifice efficiency, which is attributed to the sliding limitations because of physical entanglement. The highly entangled polyacrylamide hydrogels toughened by -COO--Zr4+ have an excellent load-bearing capacity. This study provides a novel strategy for designing hydrogels with ultra-high strength and toughness, which paves the way for the development of many hydrogels used in engineering materials.

Strong, tough, and freeze-tolerant all-natural cellulose-based ionic conductor enabled by multiscale cellulose networks

Soft ionic conductors are widely used in flexible electronics. However, the simultaneous enhancement of their mechanical properties and ionic conductivity remains challenging. This paper reports the successful development of a strong and tough cellulose-based ionic conductor with exceptional mechanical properties and high ionic conductivity by in situ dissolution and reorganization of the fiber matrix of filter paper to create a multiscale structure. The resulting ionic conductor exhibits a fracture strength of 14.13 MPa and a fracture energy of up to 2.84 MJ/m3, exceeding most reported ionic conductors. It also exhibits an impressive ionic conductivity of up to 76.3 mS/cm. Results of experiments on its use in a flexible quasi-solid-state zinc-hybrid supercapacitor show its remarkable features, such as a high capacity of 218 mAh/g, an energy density of 217 Wh/kg, and a power density of 17,520 W/kg. Furthermore, it exhibits excellent temperature resistance, working effectively even at -60 °C. In addition, by incorporating kirigami structures, we fabricated a strain sensor with the cellulose-based ionic conductor with a high gauge factor, as well as a piezoresistive sensor for handwriting recognition and a capacitance pressure sensor for force mapping with wide range and sensitivity. This study opens up new possibilities for fabricating flexible electronics with superior performance using sustainable and renewable resources.

Mechanical Regulation of Polymer Gels

The mechanical properties of polymer gels devote to emerging devices and machines in fields such as biomedical engineering, flexible bioelectronics, biomimetic actuators, and energy harvesters. Coupling network architectures and interactions has been explored to regulate supportive mechanical characteristics of polymer gels; however, systematic reviews correlating mechanics to interaction forces at the molecular and structural levels remain absent in the field. This review highlights the molecular engineering and structural engineering of polymer gel mechanics and a comprehensive mechanistic understanding of mechanical regulation. Molecular engineering alters molecular architecture and manipulates functional groups/moieties at the molecular level, introducing various interactions and permanent or reversible dynamic bonds as the dissipative energy. Molecular engineering usually uses monomers, cross-linkers, chains, and other additives. Structural engineering utilizes casting methods, solvent phase regulation, mechanochemistry, macromolecule chemical reactions, and biomanufacturing technology to construct and tailor the topological network structures, or heterogeneous modulus compositions. We envision that the perfect combination of molecular and structural engineering may provide a fresh view to extend exciting new perspectives of this burgeoning field. This review also summarizes recent representative applications of polymer gels with excellent mechanical properties. Conclusions and perspectives are also provided from five aspects of concise summary, mechanical mechanism, biofabrication methods, upgraded applications, and synergistic methodology.

Productive chemistry induced by mechanochemically generated macroradicals

Large or repeated mechanical loads degrade polymeric materials by accelerating chain fragmentation. This mechanochemical backbone fracture usually occurs by homolysis of otherwise inert C-C, C-O and C-S bonds, generating highly reactive macroradicals. Because backbone fracture is detrimental on its own and the resulting macroradicals can initiate damaging reaction cascades, a major thrust in contemporary polymer mechanochemistry is to suppress it, usually by mechanochemical release of "hidden length" that dissipates local molecular strain. Here we summarize an emerging complementary strategy of channelling mechanochemically generated macroradicals in reaction cascades to form new load-bearing chemical bonds, which enables local self-healing or self-strengthening, and/or to generate mechanofluorescence, which could yield detailed quantitative molecular understanding of how material-failure-inducing macroscopic mechanical loads distribute across the network. We aim to identify generalizable lessons derivable from the reported implementations of this strategy and outline the key challenges in adapting it to diverse polymeric materials and loading scenarios.

Hydrogel morphogenesis induced by force-controlled growth

Morphogenesis is one of the most marvelous natural phenomena. The morphological characteristics of biological organs develop through growth, which is often triggered by mechanical force. In this study, we propose a bioinspired strategy for hydrogel morphogenesis through force-controlled chemical reaction and growth under isothermal conditions. We adopted a double network (DN) hydrogel with sacrificial bonds. Applying mechanical force to the gel caused deformation and sacrificial bond rupture. By supplying monomers to the gel, the radicals generated by the bond rupture triggered the formation of a new network inside the deformed gel. This new network conferred plasticity to the elastic gel, allowing it to maintain its deformed shape, along with increased volume and strength. We demonstrated that sheet-shaped DN hydrogels rapidly adopted various three-dimensional shapes at ambient temperature when subjected to forces such as drawing and blowing. This mechanism enables morphogenesis of elastic hydrogels and will promote the application of these materials in biomedical fields and soft machines.

TRPM4-Inspired Polymeric Nanochannels with Preferential Cation Transport for High-Efficiency Salinity-Gradient Energy Conversion

Biological ion channels exhibit switchable cation transport with ultrahigh selectivity for efficient energy conversion, such as Ca2+-activated TRPM4 channels tuned by cation-π interactions, but achieving an analogous highly selective function is challenging in artificial nanochannels. Here, we design a TRPM4-inspired cation-selective nanochannel (CN) assembled by two poly(ether sulfone)s, respectively, with sulfonate acid and indole moieties, which act as cation-selective activators to manage Na+/Cl- selectivity via ionic and cation-π interactions. The cation selectivity of CNs can be activated by Na+, and thereby the Na+ transference number significantly improves from 0.720 to 0.982 (Na+/Cl- selectivity ratio from 2.6 to 54.6) under a 50-fold salinity gradient, surpassing the K+ transference number (0.886) and Li+ transference number (0.900). The TRPM4-inspired nanochannel membrane enabled a maximum output power density of 5.7 W m-2 for salinity-gradient power harvesting. Moreover, a record energy conversion efficiency of up to 46.5% is provided, superior to most nanochannel membranes (below 30%). This work proposes a novel strategy to biomimetic nanochannels for highly selective cation transport and high-efficiency salinity-gradient energy conversion.

Mechanochemical Approaches to Fundamental Studies in Soft-Matter Physics

Stretching a segment of a polymer beyond its contour length makes its (primarily backbone) bonds more dissociatively labile, which enables polymer mechanochemistry. Integrating some backbone bonds into suitably designed molecular moieties yields mechanistically and kinetically diverse chemistry, which is becoming increasingly exploitable. Examples include, most prominently, attempts to improve mechanical properties of bulk polymers, as well as prospective applications in drug delivery and synthesis. This review aims to highlight an emerging effort to apply the concepts and experimental tools of mechanochemistry to fundamental physical questions in soft matter. A succinct summary of the state-of-the-knowledge of the field, with emphasis on foundational concepts and generalizable observations, is followed by analysis of 3 recent examples of mechanochemistry yielding molecular-level details of elastomer failure, macromolecular chain dynamics in elongational flows and kinetic allostery. We conclude with reasons to assume that the highlighted approaches are generalizable to a broader range of physical problems than considered to date.

Synthesis of 'Impossible' Rotaxanes

'Impossible' rotaxanes, which are constituted by interlocked components without obvious binding motifs, have attracted the interest of the mechanically interlocked molecules (MIMs) community. Within the synthetic efforts reported in the last decades towards the preparation of MIMs, some innovative protocols for accessing 'impossible' rotaxanes have been developed. This short review highlights different selected synthetic examples of 'impossible' rotaxanes, as well as suggests some future directions of this research area.