Facile mechanochemical cycloreversion of polymer cross-linkers enhances tear resistance

Unusually long polymers crosslinked by domains of physical bonds

Polymers crosslinked by covalent bonds suffer from a conflict: dense covalent crosslinks increase modulus but decrease fatigue threshold. Polymers crosslinked by physical bonds commonly have large hysteresis. Here we simultaneously achieve high modulus, high fatigue threshold, and low hysteresis in a network of unusually long polymer chains crosslinked by domains of physical bonds. When the network without precrack is pulled by a moderate stress, chains in the domains slip negligibly, so that the domains function like hard particles, leading to high modulus and low hysteresis. When the network with a precrack is stretched, the chains in the domains at the crack tip slip but do not pull out. This enables high tension to transmit over long segments of chains, leading to a high fatigue threshold.

Designing High-Mechanical-Property Organic Polymeric Crystals: Insights from Stress Dispersion and Energy Dissipation Strategies

Despite recent significant advancements in the applications of organic polymeric crystals (OPCs), a comprehensive understanding of the design principles for high-mechanical-property crystals remains somewhat elusive. Here, we investigate the mechanical properties of OPCs from the perspectives of stress dispersion and energy dissipation by examining crystals of a macrocycle and three analogous polymers with different solvent fillings, utilizing a novel research platform constructed via dative B-N bonds. Through a thorough mechanical study and investigation into the molecular mechanisms of these model topologies, it was demonstrated that structural expansion and solvent filling are effective pathways for enhancing the mechanical performance of the OPCs by employing stress dispersion and energy dissipation strategies. Overall, our research showcases precise control over the molecular topology of the OPC materials and elucidates specific pathways for stress dispersion and energy dissipation in modulating their mechanical performance, offering a broader design perspective for efficiently enhancing the mechanical properties of other crystalline polymers, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs).

Fabrication of Microgel-Reinforced Hydrogels via Vat Photopolymerization

We report a facile method for the vat photopolymerization (i.e., digital light processing, DLP) of microgel-reinforced hydrogels that leverages both light and dark polymerization for curing. As an example, norbornene modified hyaluronic acid (NorHA) microgels at varying volume fractions swollen in acrylamide monomer are implemented as resins. When processed with DLP, acrylamide polymerization and cross-linking results in the formation of a secondary, continuous network that percolates through the microgels. At even low volume fractions (e.g., 30% v/v), the addition of microgels results in up to 4-fold increases in the stress at failure and work of fracture and a reduction in hydrogel swelling. The microgel-reinforced hydrogels are 3D printed into intricate shapes (e.g., metamaterial lattices) while maintaining uniform microgel distributions, and microgels with varied cross-link densities, cross-linkers, and fabrication methods are also investigated. This work expands the potential of microgel-reinforced hydrogels across applications where geometric freedom is essential.

Photo-Tunable Elastomers Enabling Reversible, Broad-Range Modulation of Mechanical Properties Via Dynamic Covalent Crosslinkers

Modulating the mechanical properties of soft materials with light is essential for achieving customizable functionalities. However, existing photo-responsive materials suffer from limited mechanical performance and a restricted tunable range. Here, a photo-tunable elastomer is developed by incorporating a urethane acrylate network with selenosulfide-based dynamic covalent crosslinkers, achieving high tensile strength exceeding 1.2 MPa in their stiff state and variable Young's modulus within a 0.8 MPa range. These crosslinkers undergo selenosulfide photo-metathesis, gradually breaking under ultraviolet light and reforming under visible light, enabling fine control over the modulus, strength, and stretchability of the elastomer. In terms of controllability, the design supports multiple tunable states, which allow for the use of intermediate mechanical properties. Moreover, by modeling the crosslinking density changes with reaction kinetics, modulus variation is predicted as a function of light exposure time. The light-induced modulation facilitates localized mechanical property adjustments, generating transformable multi-material structures and enhancing fracture resistance. Integrating these crosslinkers into different polymer networks provides a strategy for creating various photo-tunable elastomers and gels.

Highly Entangled Hydrogels by Photoiniferter-Mediated Polymerization

We report the synthesis of ultra-high molecular weight (UHMW) poly(N,N-dimethylacrylamide) (PDMAm) hydrogels with extremely low crosslinking densities by trithiocarbonate photoiniferter-mediated reversible deactivation radical polymerization (RDRP). Fixing the photoiniferter to crosslinker ratio and gradually increasing the targeted degree of polymerization (DPtarget) allowed for simultaneous control over the crosslinking density and the average molecular weight (Mn) of the primary chains, both below and above the critical molecular weight of entanglement (Mc). Interestingly, a plateau in storage moduli (G') was observed for UHMW PDMAm hydrogels with a sufficiently high DPtarget (>5,000), indicating a transition to the entanglement-dominated regime, with no contribution from crosslinks to the overall modulus, thus indicating the formation of highly entangled hydrogels. These hydrogels exhibit enhanced properties such as high toughness and resistance to swelling despite their vanishingly small crosslinking densities. Furthermore, even when equipped with cleavable crosslinkers, the UHMW PDMAm hydrogels resist degradation due to dense entanglements which act as transient crosslinks preventing the gels from swelling, while sparse covalent crosslinks help to maintain their structural integrity and avoid chain disentanglement. This approach allows simple synthesis of elastic and tough hydrogels with a well-defined structure and tuneable contributions from both crosslinks and entanglements.

Elastomer Mechanics of Cross-Linked Linear-Ring Polymer Blends

Cross-linking a blend of linear and ring polymers creates a new topology-based dual-network elastomer in which the two components differ significantly in their topology. We use molecular simulations and topological analysis to examine key mechanical properties as functions of ring polymer volume fraction ϕR. For ϕR < ϕR*, where the rings begin to overlap, the network shear modulus G and the maximum stretch ratio λp are weakly dependent on ϕR. For ϕR > ϕR*, entanglements trapped in the network are diluted as the rings overlap, leading to a significant decrease in G and an increase in λp with increasing ϕR. The peak tensile stress, σp, exhibits a maximum around ϕR*, indicating an enhancement of network strength due to the stronger cohesion from the entanglements between linear and ring polymers.

Robust Mechanically Interlocked Network Ionogels

Ionogels have attracted considerable attention as versatile materials due to their unique ionic conductivity and thermal stability. However, relatively weak mechanical performance of many existing ionogels has hindered their broader application. Herein, we develop robust, tough, and impact-resistant mechanically interlocked network ionogels (IGMINs) by incorporating ion liquids with mechanical bonds that can dissipate energy while maintain structural stability. Profiting from the dynamic yet stable nature of the mechanically interlocked networks, IGMINs exhibit high tensile strength (9.6 MPa), fracture energy (39 kJ/m2), and toughness (25.9 MJ/m3), along with a high elongation rate (473 %) and excellent impact resistance and shape memory, resulting in overall performance that surpasses most reported ionogels. Furthermore, in the application of strain sensors for monitoring the gait of crawling robots, the toughness and robustness of IGMINs ensure their ability to consistently output stable electrical signals during the stretching and contraction processes, thereby highlighting their practical application potential. Our work provides a new research strategy for toughening ionogels and promotes the development of mechanically interlocked materials.

Rapid self-strengthening in double-network hydrogels triggered by bond scission

The scission of chemical bonds in materials can lead to catastrophic failure, with weak bonds typically undermining the materials' strength. Here we demonstrate how weak bonds can be leveraged to achieve self-strengthening in polymer network materials. These weak sacrificial bonds trigger mechanochemical reactions, forming new networks rapidly enough to reinforce the material during deformation and significantly improve crack resistance. This rapid strengthening exhibits strong rate dependence, dictated by the interplay between bond breaking and the kinetics of force-induced network formation. As the network formation is generally applicable to diverse monomers and crosslinkers with different kinetics, a wide range of mechanical properties can be obtained. These findings may inspire the design of tough polymer materials with on-demand, rate-dependent mechanical behaviours through mechanochemistry, broadening their applications across various fields.

Network design for soft materials: addressing elasticity and fracture resistance challenges

Soft materials, such as elastomers and gels, feature crosslinked polymer chains that provide stretchable and elastic mechanical properties. These properties are derived from entropic elasticity, which limits energy dissipation and makes the material susceptible to fracture. To address this issue, network designs that dissipate energy through the plastic zone have been introduced to enhance toughness; however, this approach compromises elasticity, preventing the material from fully recovering its original shape after deformation. In this review, we describe the trade-off between fracture resistance and elasticity, exploring network designs that overcome this limitation to achieve both high toughness and low hysteresis. The development of soft materials that are both elastic and fracture-resistant holds significant promise for applications in stretchable electronics, soft robotics, and biomedical devices. By analyzing successful network designs, we identify strategies to further improve these materials and discuss potential enhancements based on existing limitations.

Tough and self-healing waterborne polyurethane elastomers via hydrogen bonds and oxime-carbamate design for wearable flexible strain sensors

Self-healing waterborne polyurethane elastomers, as functional materials, have been widely applied in flexible wearable devices. However, inevitable crack damage during use significantly limits their service life. Achieving an optimal balance between mechanical strength and self-healing ability remains a major challenge in the development of high-performance self-healing polyurethane materials. In this study, a rigid-soft phase-separated structure with multilayered rigid and flexible supramolecular segments was designed using isophorone isocyanate (IPDI), dimethylglyoxime (DMG), and 6-methyl-1,3,5-triazine-2,4-diamine (AGM) as hard segments, and polytetramethylene ether glycol (PTMEG) as soft segments. AGM establishes a multi-hydrogen bonding network with urethane groups, and DMG introduces dynamically reversible oxime-carbamate bonds, enabling the PU elastomers to achieve a self-healing efficiency of up to 95.5%. Benefiting from the gradient rigidity of the polyurethane, the strong hydrogen bond formed by the urea-carbamate bond and the triazine ring, the elastomer exhibited excellent mechanical properties, with a tensile strength of 33.04 MPa, an elongation at break of 954.79%, and a toughness of 90.66 MJ m-3. Moreover, when the composite conductor was used as an electronic skin, it possessed good self-healing properties with electrical response properties.

Enabling Selective Mechanochemical Scission of Network Crosslinks by Exchanging Single Carbon Atoms for Silicon

The tearing of a polymer network arises from mechanochemically coupled bond-breaking events in the backbone of a polymer chain. An emerging research area is the identification of molecular strategies for network toughening, such as the strategic placement of mechanochemically reactive groups (e.g., scissile mechanophores) in the crosslinks of a network instead of in the load-bearing primary strands. These mechanically labile crosslinkers have typically relied on release of ring strain or weak covalent bonds for selective covalent bond scission. Here, we report a novel chemical design for accelerated mechanochemical bond scission based on replacing a single carbon atom in a crosslinker with a silicon atom. This single-atom replacement affords up to a two-fold increase in the tearing energy. We suggest a mechanism, validated by computational modeling, for accelerated mechanochemical Si-C bond scission based on minimizing the energy required to distort the starting material toward the transition-state geometry. We demonstrated the seamless incorporation of these scissile carbosilanes to toughen 3D-printed networks, which demonstrates their suitability for additive manufacturing processes.

Two dimensional MoS2 accelerates mechanically controlled polymerization and remodeling of hydrogel

Self-remodeling material can change their physical properties based on mechanical environment. Recently, mechanically controlled polymerization using mechanoredox catalyst enabled composite materials to undergo a permanent structural change, thereby enhancing their mechanical strength. However, a significant delay in material's response was observed due to the sluggish activation of the bulk catalyst for polymerization. Herein, we report a fast, mechanically controlled radical polymerization of water soluble monomers using 2D MoS2 as the mechanoredox catalyst, studied under various mechanical stimuli, including ultrasound, ball milling and low frequency vibrations. Our strategy enables complete polymerization within several minutes of work. This accelerated process can be utilized to create composite hydrogels with the ability to alter their mechanical and electrical properties in response to mechanical stimuli. This strategy has potential for applications in smart materials such as hydrogel sensors, artificial muscles, and implantable biomaterials.

Selective Depolymerization for Sculpting Polymethacrylate Molecular Weight Distributions

Chain-end reactivation of polymethacrylates generated by reversible-deactivation radical polymerization (RDRP) has emerged as a powerful tool for triggering depolymerization at significantly milder temperatures than those traditionally employed. In this study, we demonstrate how the facile depolymerization of poly(butyl methacrylate) (PBMA) can be leveraged to selectively skew the molecular weight distribution (MWD) and predictably alter the viscoelastic properties of blended PBMA mixtures. By mixing polymers with thermally active chain ends with polymers of different molecular weights and inactive chain ends, the MWD of the blends can be skewed to be high or low by selective depolymerization. This approach leads to the counterintuitive principle of the "destructive strengthening" of a material. Finally, we demonstrate, as a proof of concept, the encryption of information within polymer mixtures by linking Morse code with the MWDs before and after selective depolymerization, allowing for the encoding of data within blends of synthetic macromolecules.

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.

Force-Activated Spin-Crossover in Fe2+ and Co2+ Transition Metal Mechanophores

Transition metal mechanophores exhibiting force-activated spin-crossover are attractive design targets, yet large-scale discovery of them has not been pursued due in large part to the time-consuming nature of trial-and-error experiments. Instead, we leverage density functional theory (DFT) and external force explicitly included (EFEI) modeling to study a set of 395 feasible Fe2+ and Co2+ mechanophore candidates with tridentate ligands that we curate from the Cambridge Structural Database. Among nitrogen-coordinating low-spin complexes, we observe the prevalence of spin crossover at moderate force, and we identify 155 Fe2+ and Co2+ spin-crossover mechanophores and derive their threshold force for low-spin to high-spin transition (FSCO). The calculations reveal strong correlations of FSCO with spin-splitting energies and coordination bond lengths, facilitating rapid prediction of FSCO using force-free DFT calculations. Then, among all Fe2+ and Co2+ spin-crossover mechanophores, we further identity 11 mechanophores that combine labile spin-crossover and good mechanical robustness that are thus predicted to be the most versatile for force-probing applications. We discover two classes of mer-symmetric complexes comprising specific heteroaromatic rings within extended π-conjugation that give rise to Fe2+ mechanophores with these characteristics. We expect the set of spin-crossover mechanophores, the design principles, and the computational approach to be useful in guiding the high-throughput discovery of transition metal mechanophores with diverse functionalities and broad applications, including mechanically activated catalysis.

Constructing Strong and Tough Polymer Elastomers via Photoreversible Coumarin Dimer Mechanophores

Advanced elastomers with outstanding strength, toughness, and reusability hold significant potential for diverse applications. Using photochemistry and mechanochemistry to develop such materials has become a very effective strategy. Here, we report that photoreversible coumarin-based mechanophores that can make force-/light-triggered cycloreversion are chemically incorporated into polyurethane elastomers to simultaneously enhance their strength and toughness. Coumarin dimer mechanophore cross-linkers are formed in the polyurethane elastomer after exposure to 365 nm irradiation, leading to networks with dramatically enhanced mechanical properties. The tensile strength of the HNA-PU2000 elastomer with coumarin dimer mechanophore cross-linkers could increase from 13.9 to 26.9 MPa, elongation at break from 726 to 1053%, and toughness from 25.6 to 119.7 MJ·m-3, in contrast to its counterpart HNA-PU2000 elastomer without coumarin dimer mechanophores. Additionally, the polyurethane elastomer exhibits a decent reusable capability via force-/light-triggered cycloreversion of coumarin dimer mechanophores under mechanical stress or 254 nm irradiation. This mechano-/photochemical strategy to improve strength, toughness, and reusable capability of elastomers provides a facile approach for the development of high-performance elastomer materials.

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.

Design and characterization of high-performance energetic hydrogels with enhanced mechanical and explosive properties

Polymeric hydrogels, known for their excellent mechanical properties and pre-cross-linking flowability, provide a promising solution for recycling waste propellants, ensuring safety and maintaining explosive performance. This study developed a double cross-linked network energetic hydrogel that effectively combines mechanical strength with explosive capabilities. Using a Ford 4 Cup, temperature data logger, universal testing machine, and detonation performance tests, we examined the impacts of kinematic viscosity, cross-linking time, compressive strength, and explosive properties. The optimal kinematic viscosity for stabilizing hollow glass microspheres (GM) was found to be 129.7 mm2/s. Cross-linking time was negatively correlated with initiator, catalyst levels, and reaction temperature, but positively correlated with retarder content. Compressive strength increased with acrylamide (AM) content and showed an initial rise before decreasing with N,N'-methylenebisacrylamide (MBAA) content and reaction temperature. The maximum compressive strength was achieved with 5% MBAA (of AM mass fraction) at 40 °C. Detonation velocity and steel plate damage decreased with increasing AM content and initially increased then decreased with GM content. A balance of mechanical and explosive properties was achieved with 6% AM and 4% GM, resulting in a detonation velocity of 4536 m/s. This hydrogel shows significant potential for waste munitions management.

A Versatile Microporous Design toward Toughened yet Softened Self-Healing Materials

Realizing the full potential of self-healing materials in stretchable electronics necessitates not only low modulus to enable high adaptivity, but also high toughness to resist crack propagation. However, existing toughening strategies for soft self-healing materials have only modestly improves mechanical dissipation near the crack tip (ГD), and invariably compromise the material's inherent softness and autonomous healing capabilities. Here, a synthetic microporous architecture is demonstrated that unprecedently toughens and softens self-healing materials without impacting their intrinsic self-healing kinetics. This microporous structure spreads energy dissipation across the entire material through a bran-new dissipative mode of adaptable crack movement (ГA), which substantially increases the fracture toughness by 31.6 times, from 3.19 to 100.86 kJ m-2, and the fractocohesive length by 20.7 times, from 0.59 mm to 12.24 mm. This combination of unprecedented fracture toughness (100.86 kJ m-2) and centimeter-scale fractocohesive length (1.23 cm) surpasses all previous records for synthetic soft self-healing materials and even exceeds those of light alloys. Coupled with significantly enhanced softness (0.43 MPa) and nearly perfect autonomous self-healing efficiency (≈100%), this robust material is ideal for constructing durable kirigami electronics for wearable devices.

A universal strategy for decoupling stiffness and extensibility of polymer networks

Since the invention of polymer networks such as cross-linked natural rubber in the 19th century, it has been a dogma that stiffer networks are less stretchable. We report a universal strategy for decoupling the stiffness and extensibility of single-network elastomers. Instead of using linear polymers as network strands, we use foldable bottlebrush polymers, which feature a collapsed backbone grafted with many linear side chains. Upon elongation, the collapsed backbone unfolds to release stored length, enabling remarkable extensibility. By contrast, the network elastic modulus is inversely proportional to network strand mass and is determined by the side chains. We validate this concept by creating single-network elastomers with nearly constant Young's modulus (30 kilopascals) while increasing tensile breaking strain by 40-fold, from 20 to 800%. We show that this strategy applies to networks of different polymer species and topologies. Our discovery opens an avenue for developing polymeric materials with extraordinary mechanical properties.

The Restoring Force Triangle: A Mnemonic Device for Polymer Mechanochemistry

In polymer mechanochemistry, mechanophores are specific molecular units within the macromolecular backbone that are particularly sensitive to tension. To facilitate understanding of this selective responsiveness, we introduce the restoring force triangle (RFT). The RFT is a mnemonic device intended to provide intuitive insight into how external tensile forces (i.e., stretching) can selectively activate scissile bonds, thereby initiating mechanically driven chemical reactions. The RFT utilizes two easily computable parameters: the effective bond stiffness constant, which measures a bond's resistance to elongation, and the bond dissociation energy, which is the energy required to break a bond. These parameters help categorize reactivity into thermal and mechanical domains, providing a useful framework for developing new mechanophores that are responsive to force but thermally stable. The RFT helps chemists intuitively understand how tensile force contributes to the activation of a putative mechanophore, facilitating the development of mechanochemical reactions and mechano-responsive materials.

Toughening and Imparting Deconstructability to 3D-Printed Glassy Thermosets with "Transferinker" Additives

Thermoset toughness and deconstructability are often opposing features; simultaneously improving both without sacrificing other mechanical properties (e.g., stiffness and tensile strength) is difficult, but, if achieved, could enhance the usage lifetime and end-of-life options for these materials. Here, a strategy that addresses this challenge in the context of photopolymer resins commonly used for 3D printing of glassy, acrylic thermosets is introduced. It is shown that incorporating bis-acrylate "transferinkers," which are cross-linkers capable of undergoing degenerative chain transfer and new strand growth, as additives (5-25 mol%) into homemade or commercially available photopolymer resins leads to photopolymer thermosets with substantially improved tensile toughness and triggered chemical deconstructability with minimal impacts on Young's moduli, tensile strengths, and glass transition temperatures. These properties result from a transferinker-driven topological transition in network structure from the densely cross-linked long, heterogeneous primary strands of traditional photopolymer networks to more uniform, star-like networks with few dangling ends; the latter structure more effectively bear stress yet is also more easily depercolated via solvolysis. Thus, transferinkers represent a simple and effective strategy for improving the mechanical properties of photopolymer thermosets and providing a mechanism for their triggered deconstructability.

Mechanochemically Promoted Functionalization of Postconsumer Poly(Methyl Methacrylate) and Poly(α-Methylstyrene) for Bulk Depolymerization

We describe a methodology of post-polymerization functionalization to enable subsequent bulk depolymerization to monomer by utilizing mechanochemical macro-radical generation. By harnessing ultrasonic chain-scission in the presence of N-hydroxyphthalimide methacrylate (PhthMA), we successfully chain-end functionalize polymers to promote subsequent depolymerization in bulk, achieving up to 82 % depolymerization of poly(methyl methacrylate) (PMMA) and poly(α-methylstyrene) (PAMS) within 30 min. This method of depolymerization yields a high-purity monomer that can be repolymerized. Moreover, as compared to the most common methods of depolymerization, this work is most efficient with ultra-high molecular weight (UHMW) polymers, establishing a method with the potential to address highly persistent, non-degradable all-carbon backbone plastic materials. Lastly, we demonstrate the expansion of this depolymerization method to commercial cell cast PMMA, achieving high degrees of depolymerization from post-consumer waste. This work is the first demonstration of applying PhthMA-promoted depolymerization strategies in homopolymer PMMA and PAMS prepared by conventional polymerization methods.

Mechanical Properties of Epoxy Networks with Metal Coordination Bonds: Insights from Temperature and Molar Mass Variation

We investigate the thermal and mechanical properties of poly(ethylene glycol), PEG, networks with either solely covalent epoxy bonds (single networks, SNs) or coexisting epoxy and iron-catecholate bonds (dual networks, DNs). The latter has recently been shown to be a promising material that combines mechanical strength with significant deformability. Here, we address the previously unexplored effects of the temperature and PEG precursor molar mass on the mechanical properties of the networks. We focus on PEG molar masses of 500 g/mol, where crystallization is suppressed, and 1000 g/mol, where some weak crystals are formed. SNs soften with an increasing PEG molar mass. Heating reversibly softens the DN, but it has a minimal effect on SNs. Nonlinear shear deformation of the DN breaks iron-catecholate bonds, and subsequent recovery upon shear cessation occurs to a long-time steady-state modulus whose value is almost triple the original one, likely due to the formation of tris-complexes versus initial sterically or kinetically trapped bis-complexation. The response under elongation indicates that the DN with sacrificial bonds is stiffer and more extensible than the other networks. These results may provide guidelines for designing dual networks with tunable mechanics at the molecular level.

Effect of dynamic bond concentration on the mechanical properties of vitrimers

The presence of dynamic covalent bonds allows vitrimers to undergo topology alterations and display self-healing properties. Herein, we study the influence of varying the concentration of dynamic bonds on the macroscopic properties of hybrid vitrimer networks by subjecting them to triaxial stretching tests using molecular simulations. Results show that the presence of dynamic bonds allows for continuous stress relaxation in the hybrid networks leading to delayed craze development and higher stretching as compared to permanently crosslinked networks. The work highlights the ability of glassy vitrimer networks to relax tensile stress during deformation successfully.

Carbohydrate-Based Reprocessable and Healable Covalent Adaptable Biofoams

Polymeric foams derived from bio-based resources and capable of self-healing and recycling ability are of great demand to fulfill various applications and address environmental concerns related to accumulation of plastic wastes. In this article, a set of polyester-based covalent adaptable biofoams (CABs) synthesized from carbohydrates and other bio-derived precursors under catalyst free conditions to offer a sustainable alternative to conventional toxic isocyanate-based polyurethane foams is reported. The dynamic β-keto carboxylate linkages present in these biofoams impart self-healing ability and recyclability to these samples. These CABs display adequate tensile properties especially compressive strength (≤123 MPa) and hysteresis behavior. The CABs swiftly stress relax at 150 °C and are reprocessable under similar temperature conditions. These biofoams have displayed potential for use as attachment on solar photovoltaics to augment the output efficiency. These CABs with limited swellability in polar protic solvents and adequate mechanical resilience are suitable for other commodity applications.

Mechanochemistry: Fundamental Principles and Applications

Mechanochemistry is an emerging research field at the interface of physics, mechanics, materials science, and chemistry. Complementary to traditional activation methods in chemistry, such as heat, electricity, and light, mechanochemistry focuses on the activation of chemical reactions by directly or indirectly applying mechanical forces. It has evolved as a powerful tool for controlling chemical reactions in solid state systems, sensing and responding to stresses in polymer materials, regulating interfacial adhesions, and stimulating biological processes. By combining theoretical approaches, simulations and experimental techniques, researchers have gained intricate insights into the mechanisms underlying mechanochemistry. In this review, the physical chemistry principles underpinning mechanochemistry are elucidated and a comprehensive overview of recent significant achievements in the discovery of mechanically responsive chemical processes is provided, with a particular emphasis on their applications in materials science. Additionally, The perspectives and insights into potential future directions for this exciting research field are offered.

Photochemical [2 + 2] Cycloaddition Enables the Synthesis of Highly Thermally Stable and Acid/Base-Resistant Polyesters from a Nonpolymerizable α,β-Conjugated Valerolactone

We report a simple strategy to transform a nonpolymerizable six-membered α,β-conjugated lactone, 5,6-dihydro-2H-pyran-2-one (DPO), into polymerizable bicyclic lactones via photochemical [2 + 2] cycloaddition. Two bicyclic lactones, M1 and M2, were obtained by the photochemical [2 + 2] cycloaddition of tetramethylethylene and DPO. Ring-opening polymerization (ROP) of M1 and M2 catalyzed by diphenyl phosphate (DPP), La[N(SiMe3)2]3, and 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris (dimethylamino) phosphoranylide-namino]-2λ5, 4λ5-catenadi(phosphazene) (tBu-P4) were conducted. M1 is highly polymerizable, either DPP or La[N(SiMe3)2]3 could catalyze its living ROP under mild conditions, affording the well-defined PM1 with a predictable molar mass and low dispersity. M2 could only be polymerized with tBu-P4 as the catalyst, also generating the same polymer PM1. PM1 has high thermal stability, with a Td,5% being up to 376 °C. Ring-opening copolymerization (ROcP) of M1 and δ-valerolactone (δ-VL) catalyzed by La[N(SiMe3)2]3 afforded a series of random copolymers with enhanced thermal stabilities. Both PM1 and the copolymer containing 10 mol % M1 exhibited excellent resistance to acidic and basic hydrolysis. Our results demonstrate that direct photochemical [2 + 2] cycloaddition of α,β-conjugated valerolactone is not only a strategy to tune its polymerizability, but also allows for the synthesis of highly thermally stable aliphatic polyesters, inaccessible by other methods.

Programming Mechanical Properties through Encoded Network Topologies

Polymer networks remain an essential class of soft materials. Despite their use in everyday materials, connecting the molecular structure of the network to its macroscopic properties remains an active area of research. Much current research is enabled by advances in modern polymer chemistry providing an unprecedented level of control over macromolecular structure. At the same time, renewed interest in self-healing, dynamic, and/or adaptable materials continues to drive substantial interest in polymer network design. As part of a special issue focused on research performed in the Polymer Science and Engineering Department at the University of Massachusetts, Amherst, this review highlights connections between macromolecular structure of networks and observed mechanical properties as investigated by the Tew research group.

Molecular control via dynamic bonding enables material responsiveness in additively manufactured metallo-polyelectrolytes

Metallo-polyelectrolytes are versatile materials for applications like filtration, biomedical devices, and sensors, due to their metal-organic synergy. Their dynamic and reversible electrostatic interactions offer high ionic conductivity, self-healing, and tunable mechanical properties. However, the knowledge gap between molecular-level dynamic bonds and continuum-level material properties persists, largely due to limited fabrication methods and a lack of theoretical design frameworks. To address this critical gap, we present a framework, combining theoretical and experimental insights, highlighting the interplay of molecular parameters in governing material properties. Using stereolithography-based additive manufacturing, we produce durable metallo-polyelectrolytes gels with tunable mechanical properties based on metal ion valency and polymer charge sparsity. Our approach unveils mechanistic insights into how these interactions propagate to macroscale properties, where higher valency ions yield stiffer, tougher materials, and lower charge sparsity alters material phase behavior. This work enhances understanding of metallo-polyelectrolytes behavior, providing a foundation for designing advanced functional materials.

Molecular Ball Joints: Mechanochemical Perturbation of Bullvalene Hardy-Cope Rearrangements in Polymer Networks

The solution-state fluxional behavior of bullvalene has fascinated physical organic and supramolecular chemists alike. Little effort, however, has been put into investigating bullvalene applications in bulk, partially due to difficulties in characterizing such dynamic systems. To address this knowledge gap, we herein probe whether bullvalene Hardy-Cope rearrangements can be mechanically perturbed in bulk polymer networks. We use dynamic mechanical analysis to demonstrate that the activation barrier to the glass transition process is significantly elevated for bullvalene-containing materials relative to "static" control networks. Furthermore, bullvalene rearrangements can be mechanically perturbed at low temperatures in the glassy region; such behavior facilitates energy dissipation (i.e., increased hysteresis energy) and polymer chain alignment to stiffen the material (i.e., increased Young's modulus) under load. Computational simulations corroborate our work that showcases bullvalene as a reversible "low-force" covalent mechanophore in the modulation of viscoelastic behavior.

Ferried Albumin-Inspired Bioadhesive With Dynamic Interfacial Bonds for Emergency Rescue

Emergency prehospital wound closure and hemorrhage control are the first priorities for life-saving. Majority of bioadhesives form bonds with tissues through irreversible cross-linking, and the remobilization of misalignment may cause severe secondary damage to tissues. Therefore, developing an adhesive that can quickly and tolerably adhere to traumatized dynamic tissue or organ surfaces in emergency situations is a major challenge. Inspired by the structure of human serum albumin (HSA), a branched polymer with multitentacled sulfhydryl is synthesized, then, an instant and fault-tolerant tough wet-tissue adhesion (IFA) hydrogel is prepared. Adhesive application time is just 5 s (interfacial toughness of ≈580 J m-2), and favorable tissue-adhesion is maintained after ten cycles. IFA hydrogel shows unchangeable adhesive performance after 1 month of storage based on the internal oxidation-reduction mechanism. It not only can efficiently seal various organs but also achieves effective hemostasis in models of the rat femoral artery and rabbit-ear artery. This work also proposes an effective strategy for controllable adhesion, enabling the production of asymmetric adhesives with on-demand detachment. Importantly, IFA hydrogel has sound antioxidation, antibacterial property, hemocompatibility, and cytocompatibility. Hence, the HSA-inspired bioadhesive emerges as a promising first-aid supply for human-machine interface-based health management and non-invasive wound closure.

Metal Ion-Induced Acid Hydrolysis Strategy for the One-Step Synthesis of Tough and Highly Transparent Hydrolyzed Polyacrylamide Hydrogels

Polyacrylamide (PAM) hydrogel is hard to enhance through coordination bonds because amide groups rarely coordinate with metal ions strongly in an aqueous solution. It is known that the aqueous solution of ZrOCl2.8H2O can be strongly acidic depending on its concentration. Consequently, through a facile one-step metal ion-induced acid hydrolysis strategy (MIAHS), tough and highly transparent hydrolyzed PAM physical hydrogels are prepared by using ZrOCl2.8H2O in this work. The formation of the partially hydrolyzed PAM physical hydrogels elucidates that the side reaction of imidization during common acid hydrolysis of PAM can be perfectly overcome because the structure of the Zr(IV) ion and its interaction with amide groups promote selective acidic hydrolysis from amide to carboxyl groups. Compared to most coordination cross-linked hydrogels, which need at least two-step fabrication, the hydrolyzed PAM hydrogel via MIAHS can be obtained by one-step synthesis due to the weak interaction between amide groups and Zr(IV). The obtained PAM hydrogel cross-linked by hydrogen bonds and coordination bond between Zr(IV) and carboxyl is a multibond network (MBN) and can achieve hierarchical energy dissipation, which exhibits excellent mechanical properties (tensile strength of 3.15 MPa, elongation at break of 890%, and toughness of 17.0 MJ m-3), high transparence (transmittance of 95%), and outstanding conductivity (5.6 S m-1) at water content of 80 wt %. The high gauge factor (from 2.24 to 12.8 as the strain increases from 0 to 400%) endows the hydrolyzed PAM hydrogels with promising application as strain sensors. Furthermore, in addition to ZrOCl2.8H2O, the fact that various hydrolyzable compounds of Ti(IV), Zr(IV) Hf(IV), and Sn(IV) can also fabricate tough hydrolyzed PAM hydrogels verifies the universality of MIAHS. Therefore, the simple, efficient, and universal MIAHS will shed new light on preparing functional PAM-based hydrogels.

Effect of the Activation Force of Mechanophore on Its Activation Selectivity and Efficiency in Polymer Networks

In recent decades, more than 100 different mechanophores with a broad range of activation forces have been developed. For various applications of mechanophores in polymer materials, it is crucial to selectively activate the mechanophores with high efficiency, avoiding nonspecific bond scission of the material. In this study, we embedded cyclobutane-based mechanophore cross-linkers (I and II) with varied activation forces (fa) in the first network of the double network hydrogels and quantitively investigated the activation selectivity and efficiency of these mechanophores. Our findings revealed that cross-linker I, with a lower activation force relative to the bonds in the polymer main chain (fa-I/fa-chain = 0.8 nN/3.4 nN), achieved efficient activation with 100% selectivity. Conversely, an increase of the activation force of mechanophore II (fa-II/fa-chain = 2.5 nN/3.4 nN) led to a significant decrease of its activation efficiency, accompanied by a substantial number of nonspecific bond scission events. Furthermore, with the coexistence of two cross-linkers, significantly different activation forces resulted in the almost complete suppression of the higher-force one (i.e., I and III, fa-I/fa-III = 0.8 nN/3.4 nN), while similar activation forces led to simultaneous activations with moderate efficiencies (i.e., I and IV, fa-I/fa-IV = 0.8 nN/1.6 nN). These findings provide insights into the prevention of nonspecific bond rupture during mechanophore activation and enhance our understanding of the damage mechanism within polymer networks when using mechanophores as detectors. Besides, it establishes a principle for combining different mechanophores to design multiple mechanoresponsive functional materials.

Filled Elastomers: Mechanistic and Physics-Driven Modeling and Applications as Smart Materials

Elastomers are made of chain-like molecules to form networks that can sustain large deformation. Rubbers are thermosetting elastomers that are obtained from irreversible curing reactions. Curing reactions create permanent bonds between the molecular chains. On the other hand, thermoplastic elastomers do not need curing reactions. Incorporation of appropriated filler particles, as has been practiced for decades, can significantly enhance mechanical properties of elastomers. However, there are fundamental questions about polymer matrix composites (PMCs) that still elude complete understanding. This is because the macroscopic properties of PMCs depend not only on the overall volume fraction (ϕ) of the filler particles, but also on their spatial distribution (i.e., primary, secondary, and tertiary structure). This work aims at reviewing how the mechanical properties of PMCs are related to the microstructure of filler particles and to the interaction between filler particles and polymer matrices. Overall, soft rubbery matrices dictate the elasticity/hyperelasticity of the PMCs while the reinforcement involves polymer-particle interactions that can significantly influence the mechanical properties of the polymer matrix interface. For ϕ values higher than a threshold, percolation of the filler particles can lead to significant reinforcement. While viscoelastic behavior may be attributed to the soft rubbery component, inelastic behaviors like the Mullins and Payne effects are highly correlated to the microstructures of the polymer matrix and the filler particles, as well as that of the polymer-particle interface. Additionally, the incorporation of specific filler particles within intelligently designed polymer systems has been shown to yield a variety of functional and responsive materials, commonly termed smart materials. We review three types of smart PMCs, i.e., magnetoelastic (M-), shape-memory (SM-), and self-healing (SH-) PMCs, and discuss the constitutive models for these smart materials.

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.

Self-Amplified HF Release and Polymer Deconstruction Cascades Triggered by Mechanical Force

Hydrogen fluoride (HF) is a versatile reagent for material transformation, with applications in self-immolative polymers, remodeled siloxanes, and degradable polymers. The responsive in situ generation of HF in materials therefore holds promise for new classes of adaptive material systems. Here, we report the mechanochemically coupled generation of HF from alkoxy-gem-difluorocyclopropane (gDFC) mechanophores derived from the addition of difluorocarbene to enol ethers. Production of HF involves an initial mechanochemically assisted rearrangement of gDFC mechanophore to α-fluoro allyl ether whose regiochemistry involves preferential migration of fluoride to the alkoxy-substituted carbon, and ab initio steered molecular dynamics simulations reproduce the observed selectivity and offer insights into the mechanism. When the alkoxy gDFC mechanophore is derived from poly(dihydrofuran), the α-fluoro allyl ether undergoes subsequent hydrolysis to generate 1 equiv of HF and cleave the polymer chain. The hydrolysis is accelerated via acid catalysis, leading to self-amplifying HF generation and concomitant polymer degradation. The mechanically generated HF can be used in combination with fluoride indicators to generate an optical response and to degrade polybutadiene with embedded HF-cleavable silyl ethers (11 mol %). The alkoxy-gDFC mechanophore thus provides a mechanically coupled mechanism of releasing HF for polymer remodeling pathways that complements previous thermally driven mechanisms.

High Strength and Toughness Polymeric Triboelectric Materials Enabled by Dense Crystal-Domain Cross-Linking

Lightweight, easily processed, and durable polymeric materials play a crucial role in wearable sensor devices. However, achieving simultaneously high strength and toughness remains a challenge. This study addresses this by utilizing an ion-specific effect to control crystalline domains, enabling the fabrication of a polymeric triboelectric material with tunable mechanical properties. The dense crystal-domain cross-linking enhances energy dissipation, resulting in a material boasting both high tensile strength (58.0 MPa) and toughness (198.8 MJ m-3), alongside a remarkable 416.7% fracture elongation and 545.0 MPa modulus. Leveraging these properties, the material is successfully integrated into wearable self-powered devices, enabling real-time feedback on human joint movement. This work presents a valuable strategy for overcoming the strength-toughness trade-off in polymeric materials, paving the way for their enhanced applicability and broader use in diverse sensing applications.

Mechanically Triggered Polymer Deconstruction through Mechanoacid Generation and Catalytic Enol Ether Hydrolysis

Polymers that amplify a transient external stimulus into changes in their morphology, physical state, or properties continue to be desirable targets for a range of applications. Here, we report a polymer comprising an acid-sensitive, hydrolytically unstable enol ether backbone onto which is embedded gem-dichlorocyclopropane (gDCC) mechanophores through a single postsynthetic modification. The gDCC mechanophore releases HCl in response to large forces of tension along the polymer backbone, and the acid subsequently catalyzes polymer deconstruction at the enol ether sites. Pulsed sonication of a 61 kDa PDHF with 77% gDCC on the backbone in THF with 100 mM H2O for 10 min triggers the subsequent degradation of the polymer to a final molecular weight of less than 3 kDa after 24 h of standing, whereas controls lacking either the gDCC or the enol ether reach final molecular weights of 38 and 27 kDa, respectively. The process of sonication, along with the presence of water and the existence of gDCC on the backbone, significantly accelerates the rate of polymer chain deconstruction. Both acid generation and the resulting triggered polymer deconstruction are translated to bulk, cross-linked polymer networks. Networks formed via thiol-ene cross-linking and subjected to unconstrained quasi-static uniaxial compression dissolve on time scales that are at least 3 times faster than controls where the mechanophore is not covalently coupled to the network. We anticipate that this concept can be extended to other acid-sensitive polymer networks for the stress-responsive deconstruction of gels and solvent-free elastomers.

Clots reveal anomalous elastic behavior of fiber networks

The adaptive mechanical properties of soft and fibrous biological materials are relevant to their functionality. The emergence of the macroscopic response of these materials to external stress and intrinsic cell traction from local deformations of their structural components is not well understood. Here, we investigate the nonlinear elastic behavior of blood clots by combining microscopy, rheology, and an elastic network model that incorporates the stretching, bending, and buckling of constituent fibrin fibers. By inhibiting fibrin cross-linking in blood clots, we observe an anomalous softening regime in the macroscopic shear response as well as a reduction in platelet-induced clot contractility. Our model explains these observations from two independent macroscopic measurements in a unified manner, through a single mechanical parameter, the bending stiffness of individual fibers. Supported by experimental evidence, our mechanics-based model provides a framework for predicting and comprehending the nonlinear elastic behavior of blood clots and other active biopolymer networks in general.

Effect of Predamage on the Fracture Energy of Double-Network Hydrogels

Double-network (DN) hydrogels are tough soft materials, and the high fracture resistance can be attributed to the formation of a large damage zone (internal fracture of the brittle first network) around the crack tip. In this work, we studied the effect of predamage in the brittle network on the fracture energy Γc of DN hydrogels. The prestretch of the first network was induced by prestretching the DN gels to prestretch ratio λpre. Depending on the λpre in relative to the yielding stretch ratio λy, above which the brittle first network starts to break into discontinuous fragments inside DN gels, two regimes were observed: Γc decreases monotonically with λpre in the regime of λpre < λy, mainly due to the decreasing contribution from the bulk internal damage, while Γc increases with λpre in the regime of λpre > λy. The latter can be understood by the release of the hidden length of the stretchable network strands by the rupture of the brittle network, whereby the broken fragments of the brittle network could serve as sliding cross-links to further delocalize the stress-concentration near the crack tip and prevent chain scissions.

Polymer mechanochemistry: from single molecule to bulk material

The field of polymer mechanochemistry has experienced a renaissance over the past decades, primarily propelled by the rapid development of force-sensitive molecular units (i.e., mechanophores) and principles governing the reactivity of polymer networks for mechanochemical transduction or material strengthening. In addition to fundamental guidelines for converting mechanical energy input into chemical output, there has also been increasing focus on engineering applications of polymer mechanochemistry for specific functions, mechanically adaptive material systems, and smart devices. These endeavors are made possible by multidisciplinary approaches involving the development of multifunctional mechanophores for mechanoresponsive polymer systems, mechanochemical catalysis and synthesis, three-dimensional (3D) printed mechanochromic materials, reasonable design of polymer network topology, and computational modeling. The aim of this minireview is to provide a summary of recent advancements in covalent polymer mechanochemistry. We specifically focus on productive mechanophores, mechanical remodeling of polymeric materials, and the development of theoretical concepts.

Ultraflexible, cost-effective and scalable polymer-based phase change composites via chemical cross-linking for wearable thermal management

Phase change materials (PCMs) offer great potential for realizing zero-energy thermal management due to superior thermal storage and stable phase-change temperatures. However, liquid leakage and solid rigidity of PCMs are long-standing challenges for PCM-based wearable thermal regulation. Here, we report a facile and cost-effective chemical cross-linking strategy to develop ultraflexible polymer-based phase change composites with a dual 3D crosslinked network of olefin block copolymers (OBC) and styrene-ethylene-butylene-styrene (SEBS) in paraffin wax (PW). The C-C bond-enhanced OBC-SEBS networks synergistically improve the mechanical, thermal, and leakage-proof properties of PW@OBC-SEBS. Notably, the proposed peroxide-initiated chemical cross-linking method overcomes the limitations of conventional physical blending methods and thus can be applicable across diverse polymer matrices. We further demonstrate a portable and flexible PW@OBC-SEBS module that maintains a comfortable temperature range of 39-42 °C for personal thermotherapy. Our work provides a promising route to fabricate scalable polymer-based phase change composite for wearable thermal management.

Dual Ratiometric Fluorescence Monitoring of Mechanical Polymer Chain Stretching and Subsequent Strain-Induced Crystallization

Tracking the behavior of mechanochromic molecules provides valuable insights into force transmission and associated microstructural changes in soft materials under load. Herein, we report a dual ratiometric fluorescence (FL) analysis for monitoring both mechanical polymer chain stretching and strain-induced crystallization (SIC) of polymers. SIC has recently attracted renewed attention as an effective mechanism for improving the mechanical properties of polymers. A polyurethane (PU) film incorporating a trace of a dual-emissive flapping force probe (N-FLAP, 0.008 wt %) exhibited a blue-to-green FL spectral change in a low-stress region (<20 MPa), resulting from conformational planarization of the probe in mechanically stretched polymer chains. More importantly, at higher probe concentrations (∼0.65 wt %), the PU film showed a second spectral change from green to yellow during the SIC growth (20-65 MPa) due to self-absorption of scattered FL in a short wavelength region. The reversibility of these spectral changes was demonstrated by load-unload cycles. With these results in hand, the degrees of the polymer chain stretching and the SIC were quantitatively mapped and monitored by dual ratiometric imaging based on different FL ratios (I525/I470 and I525/I600). Simultaneous analysis of these two mappings revealed a spatiotemporal gap in the distribution of the polymer chain stretching and the SIC. The combinational use of the dual-emissive force probe and the ratiometric FL imaging is a universal approach for the development of soft matter physics.

Thermoresponsive Helical Dendronized Poly(phenylacetylene)s: Remarkable Stabilization of Their Helicity via Photo-Dimerization of the Dendritic Pendants

Dynamic helical polymers can change their helicity according to external stimuli due to the low helix-inversion barriers, while helicity stabilization for polymers is important for applications in chiral recognition or chiral separations. Here, we present a convenient methodology to stabilize dynamic helical conformations of polymers through intramolecular cross-linking. Thermoresponsive dendronized poly(phenylacetylene)s (PPAs) carrying 3-fold dendritic oligoethylene glycol pendants containing cinnamate moieties were synthesized. These polymers exhibit typical features of dynamic helical structures in different solvents, that is, racemic contracted conformations in less polar organic solvents and predominantly one-handed stretched helical conformations in highly polar solvents. This dynamic helicity can be enhanced through selective solvation by increasing the polarity of the organic solvents or simply via their thermally mediated dehydration in water. However, through photocycloaddition of the cinnamate moieties between the neighboring pendants via UV irradiation, these dendronized PPAs adopt stable helical conformations either below or above their phase transition temperatures in water, and their helical conformations can even be retained in less polar organic solvents. Spectroscopic and atomic force microscopy measurements demonstrate that photocycloaddition between the cinnamate moieties occurs on the individual molecular level, and this is found to be helpful in restraining the photodegradation of the PPA backbones. Molecular dynamics simulations reveal that the spatial orientation of the pendants along the rigid polyene backbone is crucial for the photodimerization of cinnamates within one helix pitch.

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

Sacrificial chemical bonds have been used effectively to increase the toughness of elastomers because such bonds dissociate at forces significantly below the fracture limit of the primary load-bearing bonds, thereby dissipating local stress. This approach owes much of its success to the ability to adjust the threshold force at which the sacrificial bonds fail at the desired rate, for example, by selecting either covalent or noncovalent sacrificial bonds. Here, we report experimental and computational evidence that a mechanical bond, responsible for the structural integrity of a rotaxane or a catenane, increases the elastomer's fracture strain, stress, and energy as much as a covalent bond of comparable mechanochemical dissociation kinetics. We synthesized and studied 6 polyacrylates cross-linked by either difluorenylsuccinonitrile (DFSN), which is an established sacrificial mechanochromic moiety; a [2]rotaxane, whose stopper allows its wheel to dethread on the same subsecond time scale as DFSN dissociates when either is under tensile force of 1.5-2 nN; a structurally homologous [2]rotaxane with a much bulkier stopper that is stable at force >5.5 nN; similarly stoppered [3]rotaxanes containing DFSN in their axles; and a control polymer with aliphatic nonsacrificial cross-links. Our data suggest that mechanochemical dethreading of a rotaxane without failure of any covalent bonds may be an important, hitherto unrecognized, contributor to the toughness of some rotaxane-cross-linked polymers and that sacrificial mechanical bonds provide a mechanism to control material fracture behavior independently of the mechanochemical response of the covalent networks, due to their distinct relationships between structure and mechanochemical reactivity.

Naphthopyran molecular switches and their emergent mechanochemical reactivity

Naphthopyran molecular switches undergo a ring-opening reaction upon external stimulation to generate intensely colored merocyanine dyes. Their unique modularity and synthetic accessibility afford exceptional control over their properties and stimuli-responsive behavior. Commercial applications of naphthopyrans as photoswitches in photochromic ophthalmic lenses have spurred an extensive body of work exploring naphthopyran-merocyanine structure-property relationships. The recently discovered mechanochromic behavior of naphthopyrans has led to their emergent application in the field of polymer mechanochemistry, enabling advances in the design of force-responsive materials as well as fundamental insights into mechanochemical reactivity. The structure-property relationships established in the photochemical literature serve as a convenient blueprint for the design of naphthopyran molecular force probes with precisely tuned properties. On the other hand, the mechanochemical reactivity of naphthopyran diverges in many cases from the conventional photochemical pathways, resulting in unexpected properties and opportunities for deeper understanding and innovation in polymer mechanochemistry. Here, we highlight the features of the naphthopyran scaffold that render it a powerful platform for the design of mechanochromic materials and review recent advances in naphthopyran mechanochemistry.