The 50 most-cited articles in Macromolecules, 2024

The large production and indiscriminate disposal of plastics have resulted in serious resource and global environmental crises, which has raised a demand to develop a more sustainable and circular plastics economy. An ideal strategy to address the end-of-life issue of plastics is to develop next-generation polymers with closed-loop life cycles, which can be selectively depolymerized back to monomers at the end of their service life. Aliphatic polyesters prepared by ring-opening polymerization (ROP) of moderately strained lactones have shown great potential in closed-loop recyclable polymers. This Perspective highlights the recent achievements for closed-loop recyclable polyesters that are derived from four-, five-, six-, and seven-membered lactones by focusing on the discussion of thermodynamic and kinetic considerations, monomer design principles and polymer preparations, material properties, and chemical recyclability. Finally, the current challenges and possible directions for closed-loop recyclable polymers are also discussed.

High-performance elastomers that possess a combination of high mechanical toughness and fast healability have garnered extensive interest because of their diverse application potential. Inspired by the unique multiple hydrogen bond (H-bond) structure of spider silk and the rapid dynamic exchange of hindered urea bonds (HUBs), a self-healing polyurea elastomer with ultrahigh toughness was designed by incorporating dynamic sextuple H-bonds and HUBs into the polymer chain. Such a design affords high stretchability (1586%), excellent toughness (45.53 MJ m –3 ), good self-healing efficiency (91.6%), fracture energy (39.68 kJ m –2 ), and recyclability. The high mechanical performance and good healability are attributed to the presence of reversibly cross-linked noncovalent sextuple H-bonds and dynamically covalent HUBs, which have been validated by stress relaxation tests. Meanwhile, by substituting the chain extender adipic dihydrazide with hexamethylenediamine, which possesses a comparable structure but fewer amide bonds, the effect of sextuple H-bonds on elastomers was confirmed. More…

This Perspective aims to present the current status and future opportunities for polymer science in battery technologies. Polymers play a crucial role in improving the performance of the ubiquitous lithium ion battery. But they will be even more important for the development of sustainable and versatile post-lithium battery technologies, in particular solid-state batteries. In this article, we identify the trends in the design and development of polymers for battery applications including binders for electrodes, porous separators, solid electrolytes, or redox-active electrode materials. These trends will be illustrated using a selection of recent polymer developments including new ionic polymers, biobased polymers, self-healing polymers, mixed-ionic electronic conducting polymers, inorganic-polymer composites, or redox polymers to give some examples. Finally, the future needs, opportunities, and directions of the field will be highlighted.

Breakthrough to the extreme properties of polymer networks relies on new insights into their molecular dynamics. Time-salt concentration superposition has been discovered in polyelectrolyte coacervate systems, proving instrumental in tuning the mechanical performance of polyelectrolyte hydrogels. However, time-salt type superposition has never been mentioned in the existing literature. Herein, we reported that the mechanical properties of poly(methacrylamide) (PMAm) hydrogel can be systematically regulated in a vast range by treating with different salts, for example, Young’s modulus can be tuned from 10 –2 to 10 3 MPa. The unusual behavior of salt-stiffening arises from the salt-enhanced phase separation of the polymer network and a subsequent glassy transition of the polymer-rich phase. Rheological results demonstrate that the dynamic behavior of the hydrogels can be superposed onto “time-salt type” master curves with the salt types aligning along the Hofmeister series. The time-salt type shift factor exhibits correlation with the mobility of water molecules as revealed by…

Traditional π-conjugated luminescent macromolecules typically suffer from aggregation-caused quenching (ACQ) and high cytotoxicity, and they require complex synthetic processes. In contrast, nonconventional luminescent macromolecules (NCLMs) with nonconjugated structures possess excellent biocompatibility, ease of preparation, unique luminescence behavior, and emerging applications in optoelectronics, biology, and medicine. NCLMs are currently believed to produce inherent luminescence due to through-space conjugation of overlapping electron orbitals in solid/aggregate states. However, as experimental facts continue to exceed expectations or even overturn some previous assumptions, there is still controversy about the detailed luminous mechanism of NCLMs, and extensive studies are needed to further explore the mechanism. This Perspective highlights recent progress in NCLMs and classifies and summarizes these advances from the viewpoint of molecular design, mechanism exploration, applications, and challenges and prospects. The aim is to provide guidance and inspiration for the huge…

Nonsolvent-induced phase separation (NIPS) is a facile and cost-effective technique for the efficient fabrication of multifunctional polyvinylidene fluoride (PVDF) membranes. However, the polymorphic transition of PVDF during this process is not well understood, although different interpretations have been proposed. Here, the polymorphism of solution-cast PVDF films and the corresponding NIPS membranes has been investigated. Results show that the use of polar aprotic solvents is normally unable to induce the α → β phase transformation in the cast films, whereas immersing the cast wet films in a nonsolvent water bath (forming the NIPS membranes), the β phase emerges, revealing that both solvent and nonsolvent are crucial for polymorphic transition during NIPS. Further experiments show that (1) a low-temperature bath, a long immersion duration, and a suitable solution concentration can promote the β phase formation; (2) solvents having large dipole moment (μ), hydrogen-bond accepting power (β HBA ), Hansen solubility parameter disparity (δ t,P–S ), and viscosity (η S ) tend to produce…

The development of polyimide (PI) films with both a low dielectric constant and low dissipation factor at high frequency is an urgent requirement in a new generation of mobile communication technology. Although some breakthroughs have been made in the study of low-dielectric constant polyimide, the main influencing factors and regulation of the high-frequency dissipation factors are still unclear, which severely limits its development and application, and they deserve further study. In this work, we prepared a series of low-dielectric constant polyimide films and measured their high-frequency dielectric properties at different humidity levels by using a homemade system. The free volume and water adsorption of these polyimide films were precisely evaluated by positron annihilation lifetime spectroscopy and the dynamic vapor sorption method, respectively. Our study shows that the main factor affecting the high-frequency dissipation factor of low-dielectric constant PI films is the water adsorption, and the influence of water adsorption capacity and water binding intensity on the…

With the rapid expansion of product forms of flexible displays and electronics, the replacement of brittle glass-based substrates with transparent polymer materials is strongly demanded. Colorless polyimides (CPIs) are reckoned as the most promising transparent polymer substrate materials due to their inherent thermal stability, electrical insulation, high transparency, excellent foldability, and proven roll-to-roll processing capability. However, CPIs prepared based on current common strategies have obvious weakness of either transparency or/and thermal/mechanical stability, all of which cannot fulfill the harsh requirements of high-temperature device fabrication. Through great effort, we have developed a series of CPIs based on newly designed highly fluorinated aromatic diamines. The strong electron-withdrawing nature and high bond energy of Ar–F and Ar–CF 3 enable a high level of optical transparency while retaining high thermal decomposition stability. This is realized importantly due to the restricted dihedral rotation effect in specific diamines, where multiple –F or –CF 3…

The development of sustainable composites necessitates biobased epoxy resins that are highly recyclable and degradable; however, the integration of mechanical, reprocessing, and rapid degradation properties into a single epoxy resin remains a significant challenge. The present study proposes a straightforward approach to overcoming the problem by combining two labile covalent bonds into an epoxy resin. The combination of acetal and disulfide bonds demonstrates a synergistic effect on the degradation performance of epoxy resin, leading to an ultrafast degradation of the epoxy polymer. The carbon fiber-reinforced composite with the epoxy resin matrix shows a tensile strength exceeding 630 MPa, but the epoxy resin degrades completely within just 8 min, while the recovered carbon fibers display nondestructive characteristics similar to those of the original material. Moreover, the epoxy resin that we designed shows good self-healing and reprocessing ability. Scratches on the resin surface can be completely self-healed by heating, while the powdered resin can be reshaped under a hot…

Crosslink heterogeneity and topological defects have been shown to affect the moduli of polymer networks in the low strain regime. Probing their role in the high strain regime, however, has been difficult because of premature network fracture. Here, we address this problem by using a double network approach to investigate the high-strain behavior of both randomly and regularly crosslinked networks with the same backbone chemistry. Randomly crosslinked poly(n-butyl acrylate) networks with target molecular weights between crosslinks of 5-30 kg/mol were synthesized via free-radical polymerization, while regularly-crosslinked poly(n-butyl acrylate) networks with molecular weights between crosslinks of 7-38 kg/mol were synthesized via crosslinking of tetrafunctional star polymers. Both types of networks were then swollen in a monomer/crosslinker mixture, polymerized to form double networks, and characterized via uniaxial tensile testing. Interestingly, the onset of strain stiffening occurred later in regular networks than in random networks with the same modulus, but was well-predicted…

Introducing dynamic covalent bonds in polymers is a promising solution to improve their recyclability and self-healing abilities minimally compromising the mechanical properties. Imine bonds are exciting candidates as dynamic functionalities because of their rapid exchange and easy chemical synthesis. Although the exchange mechanisms of imines in small molecules are well-established, some misconceptions still exist regarding their behavior in polymers. Hence, we investigate the dynamic and viscoelastic properties of imine-bearing model systems and highlight that imine bond exchange only happens when mediated by unreacted amine groups. Therefore, imine metathesis is unlikely at temperatures below polymeric decomposition, and transimination is the major mechanism of bond exchange. Surprisingly, the energy barrier for this mechanism depends on the concentration of functional groups forming imine bonds. We hypothesize that the hydrogen bonds between amine and aldehyde groups might affect this process, but further work is needed to solve this puzzle.

Highly designable architectures of polymers endow the same polymer with different properties. In principle, diverse architectures should enrich the self-assembly behavior of block copolymers, providing a possibility for the formation of many novel ordered phases. However, it is infeasible to simply search the huge library of architectures for the formation of desired new phases, and it requires some valid guiding principles for efficiently designing block copolymers. In the past decade, some sophisticated mechanisms have been proposed in addition to the common mechanism that the self-assembly of block copolymers is dictated by the competition between the interaction energy and the stretching energy. To realize these sophisticated mechanisms in the architectures constitutes the guiding principles for the design of block copolymers. A series of theoretical works have demonstrated that one or more sophisticated mechanisms can be derived for desired novel phases, and thus the architectures of block copolymers can be properly designed to achieve these phases. To date, only the…

High-strength elastomers capable of room-temperature healing hold promising applications in flexible electronics. However, the unwanted spontaneous merging for the incorrect contact (such as folding or stacking) during the use process will prevent the widespread promotion of these elastomers. By simulating the strengthening and healing processes of natural skin, a series of robust and kinetically stable poly(urea-urethane) elastomers (PUU) were synthesized by integrating three reversible interactions, including dynamic disulfide bonds, hierarchical hydrogen bonds, and strain-induced crystallization, into polymer chains. Among these elastomers, PUU-DTDA9 exhibits high strength (40 MPa), exceptional transient room-temperature self-healing ability (97.71% healing efficiency of tensile strength), and good kinetic stability (no merging occurs at the contact area after it is kept at room temperature for 24 h). The exchangeability of disulfide bonds was locked by the hydrogen bonds with moderate density in the polymer chains, ensuring a high kinetic stability of the elastomer under service…

Developing a promising and scalable strategy for simultaneously improving ductility, stiffness, heat resistance, as well as transparency of polylactide (PLA) films is of great significance but remains a big challenge. In this work, the manipulation of the amorphous chain network and oriented nanocrystals simultaneously reconciles mechanical properties, heat resistance, and transparency of isotropic PLA films via the “biaxial orientation-confined crystallization” process. The biaxial extensional stress with a high strain rate induces the oriented homocrystallites (HCs), which can act as physical cross-links, and strengthens the amorphous chain entanglement network. Subsequent confined crystallization is utilized to promote the perfection of oriented HCs and simultaneously induce the formation of stereocomplex crystals (SCs), further strengthening the chain entanglement and restricting chain relaxation. Thus, superior stiffness–ductility balance is achieved as the elongation at break and yield strength increase from 12.5% and 56.2 MPa to 106.1% and 84.2 MPa, respectively. Moreover,…

Transition metal-catalyzed olefin polymerization dominates in the polyolefin field, and we still have limited control over a catalyst’s behavior. In addition to traditional steric and electronic modifications, developing new design strategies for transition metal catalysts is highly intriguing and may open avenues for the highly efficient synthesis of new high-performance polyolefins. In this Perspective, recent advances in weak noncovalent interactions including hydrogen bonding, π–π interactions, metal–aryl π-interactions, and electrostatic metal–X interactions (X = heteroatoms) for modulating late transition metal-catalyzed polymerization of olefins are reviewed with an emphasis on their influences on (co)polymerization of olefins. It provides a practical consultation for polymer synthesis chemists that are interested in developing weak noncovalent interaction-assisted polymerization of olefins.

Thanks to many promising properties, including biocompatibility and the ability to experience large deformations, poly(ethylene glycol) diacrylate (PEGDA) hydrogels are excellent candidate materials for a wide range of applications. Interestingly, the polymerization of PEGDA leads to a network microstructure that is fundamentally different from that of the "classic" polymeric gels. Specifically, PEGDA hydrogels comprise PEG chains that are interconnected by multifunctional densely grafted rod-like polyacrylates (PAs), which serve as cross-linkers. In this work, we derive a microstructurally motivated model that captures the essential features which enable deformation in PEGDA hydrogels: (1) entropic elasticity of PEG chains, (2) deformation of PA rods, and (3) PA-PA interactions. Expressions for the energy-density functions and the stress associated with each of the three contributions are derived. The model demonstrates the microstructural evolution of the network during loading and reveals the role of key microscopic quantities. To validate the model, we fabricate and compress…

Silica-filled rubber composite is the main component of the “green tire” featured with a high fuel economy. Nowadays, incorporating sulfur-containing silane coupling agents (SSCAs) to improve the compatibility between silica and rubber is essential to optimizing the comprehensive properties of the resultant composites. However, SSCA synthesis requires multistep procedures and organic solvents and many intractable issues, such as low modification efficiency and volatile organic compound (VOC) emissions, cannot be resolved with regard to the use of SSCAs. Herein, we presented the facile synthesis of an epoxy-engineered polysulfide (SPVE) via inverse vulcanization of styrene, 1,2-epoxy-4-vinylcyclohexane, and sulfur and utilized it as a new-brand modifier for silica-filled rubber composite. The coupling mechanism of SPVE is revealed. Specifically, the polysulfides of SPVE can graft onto rubber chains; meanwhile, the epoxy groups of SPVE readily react with silanol groups on the silica surface, creating molecular linkages between rubber and silica. Consequently, incorporating SPVE…

Polymer processing is an essential step in achieving the desired shape and properties of polymer parts that are being broadly used in areas from packaging to aerospace. However, the design of the processing for new products is a matter of trial and error in practice. This is partly due to a limited understanding of polymer crystallization under realistic processing conditions, where strong shear flow and elevated pressure are coupled in an inhomogeneous manner. Unlike the well-established research topics that focus on polymer crystallization under the sole effect of pressure or shear flow, the subject of this Perspective is crystallization under processing conditions which include both, i.e., flow-induced crystallization under pressure (FICP). Here we focus particularly on recent advances in this field. Starting with a brief introduction of the special experimental devices for studying FICP, we discuss the remarkable interplay between flow and pressure and its effect on crystallization kinetics, crystal structure, and morphology. Accelerated crystallization, multiphase diagrams, and…

As a widely used functional polyolefin, the industrial ethylene (E)-methyl acrylate (MA) copolymer (EMA), prepared by free radical polymerization under harsh conditions, is a lightly branched copolymer with in-chain and terminal MA incorporation. Metal-catalyzed E/MA copolymerization as an alternative polymerization technique is highly attractive and challenging. Herein, we first report efficient direct E/MA copolymerization using “sandwich” dibenzobarrelene-derived α-diimine nickel catalysts. The produced lightly branched EMA copolymers with in-chain, pendant, and terminal MA units have a chain structure very similar to industrial EMA, which is not prepared using the previously reported nickel and palladium catalysts. Experimental results support that noncovalent Ni–phenyl interactions in α-diimine nickel catalysts promote E/MA copolymerization. Density functional theory studies clearly illuminate the crucial role of Ni–phenyl interactions in E/MA copolymerization and well explain the formation of in-chain, pendant, and terminal MA units in the EMA copolymer chain.

High Resolution Image Download MS PowerPoint Slide This study introduces a straightforward and efficient route for synthesizing self-healable polybenzoxazine networks by utilizing dynamic imine bond exchanges under mild conditions. The process involves combining polyethylenimines with aldehyde-functional bisbenzoxazine, which was produced from vanillin, using a Sc(OTf) 3 catalyst and subjecting them to moderate heating at 150 °C. Remarkably, the resulting polybenzoxazine films exhibit good self-healing capabilities at low temperatures and pressures, without requiring any additional additives to facilitate the healing process. The degree of recovery was assessed through tensile tests, while rheologic measurements were utilized to analyze stress relaxation and activation energy of dynamic bonding, providing insights into the self-healing process. Additionally, comprehensive spectral characterizations and investigations of thermal behaviors were conducted to gain a deeper understanding of the material’s properties and performance. Moreover, the polybenzoxazine networks demonstrated…

Dynamic covalent thermosets have emerged as a next-generation sustainable plastic that combines the advantages of traditional thermosets and thermoplastics. However, dynamic covalent thermosets generally suffer from the trade-off between strength/stiffness and ductility/toughness. Herein, a strategy to simultaneously strengthen and toughen dynamic covalent thermosets, incorporation of hydrogen-bonded (H-bonded) cross-links into the dynamic covalent networks, is reported. We designed a polyimine network cross-linked by both the dynamic imine bonds and H-bonds, resulting in a remalleable and recyclable polyimine thermoset with high yield strength (ca. 55.2 MPa) and Young’s modulus (ca. 1.7 GPa) as well as high ductility (ca. 72%) and toughness (ca. 32.7 MJ m –3 ). When the H-bonded cross-links are removed from the polyimine network or replaced by the imine bonds, the mechanical strength or toughness of the polyimines is significantly declined, respectively. Therefore, this work provides an effective design principle for strong and tough dynamic covalent thermosets with modularity and…

Ethylene-based polyolefin elastomers (EPOEs) represent a novel class of polyolefin material prepared using ethylene as the only feedstock and bear comparable mechanical and elastic properties versus traditional polyolefin elastomers (POEs). EPOEs are prepared from late-transition-metal (nickel or palladium)-catalyzed chain walking polymerization of ethylene. However, the literature-reported potent catalysts are homogeneous systems that operate at relatively low temperatures (<90 °C), while the industrial production of POE materials requires high temperatures (>120 °C). In this contribution, we install some OM (M = H, Na, and K) tags in an α-diimine ligand framework, enabling facile heterogenization of the homogeneous nickel complexes onto a MgCl 2 -based solid support. The resulting heterogeneous nickel complexes are highly active in ethylene polymerization and demonstrate much better thermal stability (up to 150 °C) than their homogeneous counterparts. More importantly, the polyethylene products demonstrate super-stretchability (up to 2160%) while maintaining high tensile strength…

Insufficient charge separation and sluggish exciton transport seriously restrict the practical application of photocatalytic uranium extraction from seawater. In this study, a D-π-A conjugated microporous polymer is synthesized using perylene, phosphonate-containing fluorene, and benzothiadiazole as D, π-linker, and A units, respectively, as novel uranium extraction photocatalysts. Both experimental and theoretical studies have demonstrated that the D-π-A structure simultaneously expands π-electron delocalization and promotes intramolecular charge transfer, thus accelerating the photocatalytic reaction. More importantly, phosphate ester and benzothiadiazole together act as uranyl-affinity “hooks” in the skeleton, adding the asymmetry and expanding the built-in electric field, thereby enhancing the driving force of photogenerated charge separation and elevating the charge separation efficiency (84.8%). The results show that the photocatalytic uranium extraction capacity of CMP-D-π′-A reaches 11.68 ± 0.21 mg g –1, exceeding most reported photocatalysts. These findings provide a…

We formulate a statistical mechanical theory for how dynamic bond exchange influences the activated hopping-driven relaxation of Kuhn segments in dynamically cross-linked networks or vitrimers over a wide range of temperatures and cross-link densities. The key new methodological aspect is to address in a self-consistent manner the dynamic consequences of bond exchange on the Kuhn segmental alpha relaxation, and vice versa. The predicted temperature dependence of the segmental alpha time of vitrimers at high temperatures remains the same as that of permanent networks, but at lower temperatures, a significant acceleration of relaxation occurs due to bond exchanges. From a mechanistic perspective, the vitrimer local cage barrier is very weakly affected by bond exchange, while the collective elastic barrier contribution decreases significantly in the deeply supercooled regime. The vitrimer glass transition temperature is predicted to grow linearly with the square root of the cross-link density, as previously found for permanent networks. Material-specific chemical effects such as…

Herein we report a carboxylated nitrile butadiene rubber (XNBR) elastomer design that merges high stretchability, toughness, and self-healing through Cu 2+ -COOH-TEA and Dy 3+ -COOH-TEA coordination (TEA denotes triethylamine). The TEA molecules as extra ligands stiffen the coordination complexes. The Cu 2+ -COOH-TEA coordination leads to the formation of fast relaxation cross-links, mainly contributing to energy dissipation. In contrast, the Dy 3+ -COOH-TEA coordination results in the generation of slow relaxation cross-links, mainly contributing to the mechanical strength. The theoretical simulations demonstrate the multiple breaking/reformation modes of the coordination cross-links during stretching, where Dy 3+ -mediated cross-links decrease but become more robust, while Cu 2+ -mediated cross-links break rapidly but can be almost fully recovered in the form of more dynamic cross-links. Finally, the XNBR/TEA/Cu/Dy elastomer achieves high stretchability (∼5000%), high strength (6.6 MPa), and temperature-adaptable self-healing. The coexistence of Cu 2+ and Dy 3+ coordination…

Poor toughness limits the use of biobased and biodegradable polylactic acid (PLA). The conventional method of blending with flexible polymers toughens PLA with a significant loss of strength and incompatibility. In this work, carbon dioxide-based biodegradable polypropylene carbonate (PPC) was used as the flexibilizer, and PPC-TiO 2 -epoxy Janus nanosheets (JNs) synthesized by grafting PPC-diols and epoxy groups onto each side of titanium dioxide nanosheets respectively were used as compatibilizers. PLA/PPC/JNs blends were then prepared by melt processing. The in situ reaction between the epoxy groups on one side of the JNs and the carboxylic acid groups on the PLA occurs, forming a solid covalent bond. The grafted PPC chains on the other side of the JNs were tightly entangled with those of the PPC flexibilizer. Moreover, due to the Pickering effect of JNs, they can stably anchor at the interface, which reduces the interfacial tension and significantly enhances compatibility. Compared with PLA/PPC, the addition of as low as even 0.3 wt % JNs reduced the PPC domain size from 3.25 to…

High Resolution Image Download MS PowerPoint Slide The commonly used nonsolvent-induced phase separation process for creating polymer membranes lacks microscopically solidified mechanisms. This study employs dissipative particle dynamics simulations to investigate the solidification dynamics and the impact of nonsolvent–solvent affinity on membrane morphology and microstructure. Strong nonsolvent–solvent affinity triggers active nonsolvent–solvent exchange and membrane solidification via nonsolvent-induced precipitation, resulting in a traversing pore structure. In contrast, weak affinity restricts exchange, leading to solidification primarily through solvent loss-induced oversaturation and resulting in a closed void structure. The membrane’s microstructure is closely linked to the solidified polymer conformations, with smaller polymer sizes observed in membranes with low crystallite content compared to those with high crystallite content. Polymer sizes are smaller in coil-like conformations compared to those in interfolding conformations. Increasing nonsolvent–solvent affinity…

Our recent studies departed from the conventional description of polymer fracture behavior while maintaining consistency with the principles of material mechanics including linear elastic fracture mechanics (LEFM). In traditional fracture mechanics of brittle materials, crack resistance is quantified in terms of toughness G c, which represents the critical energy release per unit fracture surface area. This perspective suggests that high G c involved high energy dissipation. A new perspective has surfaced, proposing a fundamental yet underexplored, seemingly universal fracture mechanism and explaining the origin of high G c within an alternative framework. According to this view, for unfilled plastics and elastomers, fracture initiates when local tensile stress surpasses the polymer’s fracture strength σ F(inh), and high toughness is a consequence of high fracture strength. Remarkably, for polymers in both plastic and elastomeric states, their inherent strength σ F(inh) appears to be of a comparable magnitude to the nominal tensile strength σ b . Spatial–temporal resolved polarized…

High Resolution Image Download MS PowerPoint Slide Vitrimers are covalently cross-linked polymers that can be reshaped and recycled, triggered by the temperature. Well above the canonical glass transition temperature T g, they gain malleable, thermoplastic-like properties; below, they behave as thermosets. The responsible molecular mechanism is enabled by dynamic covalent bond exchange. However, the underlying physics of the transition from thermoplastic to thermoset properties is rather unclear and the subject of current scientific debate. In this work, we address the questions of what extent the temperature-dependent malleability of vitrimers is reflected in the thermal expansion behavior and, conversely, to what extent the thermal expansion behavior is suited to characterize the respective creep behavior of vitrimers. It will be shown that at least in the case of the used model vitrimer, only the canonical glass transition is able to stop the vitrimer to undergo irreversible shape changes and that the often-discussed topology freezing temperature of vitrimers T v is purely…

Polymer ionic conductors have shown great promise as iontronic sensors for flexible wearable devices and intelligent machines. A series of exquisitely designed hydrogels, ionogels, and ionic elastomers have good mechanical properties, such as super stretchability and high elasticity. However, most gels tend to exhibit softening or linear mechanoresponsive behavior when subjected to stress, which is completely different from the strain-stiffening behavior of the biological tissues. Therefore, designing polymer ionic conductors with good mechanical properties and strain-stiffening ability remains a challenge, which is critical in improving the reliability and durability of iontronic sensing. Here, we propose a strong/weak ionic interaction strategy to develop poly(ionic liquid) elastomers (PILEs) through the copolymerization of imidazolium ionic liquid monomers and acrylate monomers. The design allows weak ionic interactions to impart softness to the polymer network, while strong ionic interactions stiffen the network during stretching. The resulting transparent PILE possesses…

It is well-known that the phase behavior and physicochemical and adhesive properties of complex coacervates are readily tuneable with the salt concentration of the medium. For toxicity reasons, however, the maximum applicable salt concentration in biomedical applications is typically low. Consequently, other strategies must be implemented in order to optimize the properties of the resulting complex coacervates. In this work, the effect of the charge density of a strong polyanion on the properties of complex coacervates was studied. To control this charge density, statistical anionic/charge-neutral hydrophilic copolymers were synthesized by means of an elegant protection/deprotection strategy and subsequently complexed with a strong polycation. The resulting complexes were observed to have an increasing water content as well as faster relaxation dynamics, with either increasing salt concentration or decreasing charge density. Time-salt and time-salt-charge density superpositions could be performed and showed that the relaxation mechanism of the complex coacervates remained unchanged.…

High Resolution Image Download MS PowerPoint Slide Improving the heat resistance of epoxy resins remains an important and challenging issue across a wide variety of applications. One strategy to address this challenge is the design of cured resins based on multifunctional epoxy. Compared to conventional difunctional epoxy resins, multifunctional epoxy resins are expected to easily form a highly cross-linking structure, which is anticipated to contribute to enhanced heat resistance. However, there is a lack of information about the detailed mechanisms of the formation of such cross-linking structures and their effects on the physical properties. We herein tracked the kinetics of curing reactions of difunctional and trifunctional epoxies with an amine hardener. Despite the identical reactivity of epoxy groups in both base monomers, the trifunctional epoxy system cured faster. In addition, in the difunctional epoxy system, gelation was followed by vitrification, while in the trifunctional epoxy system, gelation and vitrification occurred simultaneously. The accelerated curing reaction…

Shish crystals are a crucial crystal structure in ultrahigh molecular weight polyethylene (UHMWPE), composed of fully extended, nonentangled molecular chains, enabling the exceptional mechanical properties of low-dimensional UHMWPE products. During the molding process, precise control of the processing temperature allowed for the fabrication of high-entanglement UHMWPE films containing shish crystals. Rheological characterization revealed that the introduction of nonentangled shish crystals contributed to a partial reduction in the degree of entanglement in the UHMWPE films. Using in situ small-angle X-ray scattering (SAXS)/ultrasmall-angle X-ray scattering (USAXS)/wide-angle X-ray diffraction (WAXD), combined with ex situ SEM and DSC measurements, the structural evolution in high-entanglement UHMWPE films with reserved shish crystals was investigated during hot stretching. The results indicated that the introduction of shish crystals contributed to the reduction of inclined lamellar stacking caused by high entanglement, thereby facilitating the transformation of inclined lamellae…

Polymers with exceptional heat resistance are critically valuable in numerous domains, particularly as essential components of flexible organic light-emitting diodes. Among these, polyimides (PIs) demonstrate significant potential as substrate candidates for these next-generation flexible displays due to their robustness. However, traditional Edisonian approaches struggle to navigate the vast chemical space of PIs and also pose challenges of small data, which constrains the learnable chemical space for machine learning (ML). In this study, we propose a chemical-knowledge-based strategy to facilitate the design of PIs with high glass transition temperature ( T g ) utilizing an atom-wise graph neural network and small data. Inspired by chemical intuition, our strategy leverages the available data on the same property (i.e., T g ) from other polymers, which is beneficial for expanding the chemical space used for ML. The trained ML model achieves an impressive performance in predicting T g of polymers. We have also investigated the impact of the chemical space encompassed by the data…

At present, there is still a lack of in-depth research into the relationship between structural changes during the healing process and self-healing efficiency of self-healing polymers. In the current work, molecular dynamics (MD) simulations and machine learning methods were applied to investigate the microscopic mechanisms of self-healing in polymers. Based on MD simulations, it was found that chain segments and the entire chains diffuse into the crack region during healing, together with the reformation of reversible interactions. Segment diffusion is closely related to the reconstruction of reversible interactions, and chain diffusion can lead to the interpenetration of chains. Based on machine learning methods, it was demonstrated that chain diffusion (the interpenetration of chains) plays the most crucial role in influencing the self-healing efficiency of polymers. Furthermore, based on the Random Forest method, a quantitative relationship between the structural changes in the healing process and the self-healing efficiency can be established.

Automation and machine learning techniques are poised to dramatically accelerate the development of new materials while simultaneously increasing our understanding of the physics and chemistry that underlie the formation of such materials. In particular, the convergence of accessible machine learning tools, the availability of high-quality data, and the advent of accessible experimental automation platforms have led to a number of closed-loop autonomous experimentation platforms or “self-driving labs”. Such platforms integrate robotic experimenters with AI-guided experiment planning to autonomously perform large numbers of experiments without human input. After briefly reviewing the state of the field and the broad classes of autonomous efforts, this perspective outlines several high-value focus areas for future ML-guided characterization efforts. Among many advantages, we expect that autonomous approaches will allow the systematic study of rare and nonequilibrium phenomena, provide dramatically greater measurement efficiency through targeting of cutting-edge, resource-intensive…

Natural rubber (NR) material is indispensable in various fields of our daily life, while its preparation usually involves toxic cross-linking agents and energy-consuming vulcanization processes. The permanent cross-linking network of NR products also makes its degradation and recycling difficult. Herein, a fully biosourced, vulcanization-free NR material is developed by simply introducing carrageenan (KC) in the rubber matrix via a facile aqueous mixing method. The nonrubber components (NRCs) on the rubber latex particles are of crucial importance for this art as they can interact with KC by forming hydrogen bonding between the functional groups of proteins in NRCs and the abundant hydroxyl and sulfonate groups on KC molecules. As a result, KC is selectively dispersed between the latex particles and forms a segregated filler network. The special KC network and the good interfacial interaction between KC and rubber promote strain-induced crystallization and endow rubber composites with great mechanical properties. Moreover, the obtained NR/KC composites exhibit an interesting…

A diamine monomer 5-amino-2-(4-hydroxy-3-aminobenzene)-benzimidazole (OH-BIA) containing both benzimidazole and hydroxyl groups was synthesized through a facile two-step reaction and polymerized with four commercially available dianhydrides to yield four polyimides. The solubility, chain packing, mechanical performance, thermal stability, and gas transport properties of the synthesized polyimides were investigated in detail. The synthesized polyimides exhibited good solubility, excellent mechanical properties (tensile strengths ≥103.3 MPa), high thermal stability and glass-transition temperatures (≥396 °C), and high gas selectivity due to the simultaneous introduction of benzimidazole and hydroxyl groups. In particular, because of their enhanced size-sieving ability, these polyimide membranes exhibited higher ideal selectivity for some gas pairs such as CO 2 /CH 4 (up to 116) compared to commercial Matrimid 5218 (selectivity for CO 2 /CH 4 is 36) and other reported polyimide dense membranes (selectivity for CO 2 /CH 4 is up to 63). This work provides useful insights into the design…

Recognizing that interfacial adhesion plays a key role in determining the mechanical properties of nanocomposites, we attempted to synthesize thermoplastic polyurethane (TPU)/MXene nanocomposites with covalently bonded TPU chains on the surfaces of MXene nanosheets through an in situ polymerization method. Different analyses showed that some TPU chains start to grow from the MXene surfaces, leading to significant improvements in the mechanical properties of the TPU/MXene nanocomposites compared to neat TPU and a solution-mixed TPU/MXene nanocomposite. In particular, the in situ synthesized TPU/MXene nanocomposite exhibited a 55% higher elastic modulus and approximately 110% higher tensile strength than neat TPU, as well as a 30% higher elastic modulus and approximately 180% higher tensile strength than the solution-mixed TPU/MXene nanocomposite with the same MXene content (0.5 wt %). This simultaneous improvement in the stiffness and toughness of the materials demonstrates that the in situ synthesis of TPU/MXene nanocomposites, without any surface treatments of the MXene, is a…

Thermosetting composites such as carbon fiber-reinforced epoxy resins are vital for various industries but face sustainability challenges due to their permanent cross-linking nature. Recent advances in dynamic covalent chemistry offer promise for reprocessing, welding, and repairing under mild conditions, yet closed-loop recycling remains elusive. Achieving high flame retardancy introduces further complexities, as traditional treatments hinder recyclability and raise environmental concerns. From this perspective, innovative thermosets incorporating dynamic chemical bonds show potential for reconciling flame retardancy with recyclability. Through clever molecular design, a variety of functional groups have been introduced into dynamic covalent thermosets in different ways. These diverse functional groups not only impart flame retardancy to the target resins and their composites, but also participate directly or indirectly in dynamic covalent exchange reactions. However, challenges persist in balancing dynamic exchange reactions with cured material properties. Despite these…

Recyclable elastomers based on adaptable covalent networks fabricated via different types of dynamic bonds have been designed and prepared to resolve the urgent problems generated by waste elastomers. However, due to inevitable side reactions during thermal recycling (e.g., oxidation, permanent cross-linking), mechanical recovery efficiency after thermal recycling is normally <90% for most recyclable elastomers, making it difficult to achieve comparable performance. Herein, we report a novel semi-interpenetrating network design that achieves mechanical reinforcement after thermal recycling via network topology isomerization and formation of new cross-linking points. The designed semi-interpenetrating network includes an aromatic disulfide bond-containing polyurethane network and long linear polyurethane chains with side-chain vinyl groups. Triggered by heat during reprocessing, the disulfide bonds inside the cross-linked network break and the generated phenyl sulfur radicals undergo thiol–ene reactions with the side-chain vinyl groups in the linear polyurethane chains, resulting in…

Reactive processing provides a simple approach for grafting dynamic covalent cross-linkers onto linear or branched polymers, resulting in covalent adaptable networks (CANs). We synthesized poly( n -hexyl methacrylate) (PHMA) CANs from neat, entangled PHMA using radical-based reactive processing to graft the dynamic covalent cross-linker called BiTEMPS methacrylate (BTMA) between PHMA side chains. By tuning the BTMA loading, we achieved a range of cross-link densities and characterized how stress relaxation, elevated-temperature creep, and reprocessability are affected by cross-link density. The grafting of dynamic covalent cross-links to the PHMA side chains allowed for a novel comparison of our CANs to the reversible, entangled polymers described by sticky reptation theory, in which long-time relaxation occurs by the unraveling of backbone entanglements, a process enabled by the dissociation and subsequent exchange of side-chain “stickers.” We observed two stress relaxation regimes in our CANs that required their fitting to a linear combination of two stretched exponential decay…

The design of cyclic monomers is crucial for the development of polymers with ideal thermal and mechanical properties by ring-opening polymerization. Herein, we provide a systematic investigation into the thio-modification effects on the polymerization behavior of cyclic ether–ester monomers and the final properties of the corresponding poly(ether- alt -ester)s. The position of thio-modification significantly affected the polymerization thermodynamics and thus could regulate the ceiling temperature ( T c ). O-to-S substitutions in the monomer′s ether/ester sites would increase the monomer′s α-H acidity, and the catalytic system strictly determined the chain initiation process as well as the chain-end groups. Density functional theory calculations and experimental studies revealed that O-to-S substitutions at the ester site would significantly accelerate the polymerization under the same conditions, thanks to the high reactivity of the thioester group and strong nucleophilicity of the chain end. The resulting poly(ether- alt -ester)s exhibited crystallinity, precisely tunable…

High Resolution Image Download MS PowerPoint Slide Understanding and predicting the mechanical and conformational properties of conjugated polymer (CP) thin films are a central focus in flexible electronic device research. Employing molecular dynamics simulations with an architecture-transferable chemistry-specific coarse-grained (CG) model of poly(3-alkylthiophene)s (P3ATs), developed by using an energy renormalization approach, we investigate the mechanical and conformational behavior of P3AT thin films during deformation. The density profiles and measures of local mobility identify a softer interfacial layer for all films, the thickness of which does not depend on M w or side-chain length. Remarkably, Young’s modulus measured via nanoindentation is more sensitive to M w than for tensile tests, which we attribute to distinct deformation mechanisms. High- M w thin films show increased toughness, whereas longer side-chain lengths of P3AT resulted in lower Young’s modulus. Fractures in low- M w thin films occur through chain pullout due to insufficient chain entanglement and crazing…

We report the successful synthesis of ultrahigh-molecular-weight (UHMW, 10 6 –10 7 g/mol) ABA triblock copolymers through photoiniferter polymerization using solution and inverse miniemulsion conditions. The photoiniferter process was first employed in homogeneous aqueous media to prepare telechelic UHMW poly( N, N -dimethylacrylamide). The effect of the R-group of the iniferter (ZC(═S)SR) on the polymerization control and rate was investigated. The chain-end fidelity of the resulting polymers was verified through sequential chain extensions in solution. Various iniferters were investigated for their hydrolytic stability and partitioning between aqueous and organic phases. Relying on these learnings, the photoiniferter synthesis of UHMW ABA triblock copolymers was achieved under inverse miniemulsion conditions, and the approach was expanded in monomer scope to incorporate 3-acrylamidophenylboronic acid units to impart stimuli-responsive behavior.

Natural hydrogels, such as cartilage, have a multiscale hierarchical structure composed of multiple modulus-contrasting building blocks, bringing about their extraordinary mechanical properties. Conventional tough engineering hydrogels, such as Double Network (DN) hydrogel, lack cartilage’s high-order aggregated structure across multiple length scales, although they have a molecular composition similar to cartilage. This work focuses on high-order architecture in DN hydrogels through superimposing microphase separation and nanocrystalline domains in interpenetrating molecular networks of stiff chitosan and soft poly(vinyl alcohol) by using freezing-thawing-assisted alkali out. The constructed High-order Double Network (HDN) architecture has a molecular composition identical to initial interpenetrating molecular networks but exhibits hierarchical multiscale discrepancies, including bicontinuous phase at microscale, nanocrystalline domains at nanoscale, and polymer chain packing at subnanoscale. We find that these structural differences are strongly correlated with the macroscopic…

Polyurethane (PU) has an excellent mechanical property due to its unique microphase separated structure, but the molecular mechanism is still unclear. In this work, a multiscale model and the corresponding potential functions of PU are first developed via an inverse Boltzmann iterative method. Following it, the fracture toughness of PU is explored via a triaxial deformation, which exhibits a rise with the increase in the content of hard segments (HS). By analyzing the stress decomposition, morphology, and conformation of two phases, the HS phase can act as physical cross-links due to the strong phase strength, which mainly improves the tensile stress at the low strain. The soft segment (SS) phase contributes to the stress enhancement at high strain due to the large elongation. This microstructural evolution can be visualized by characterizing the von Mises strain and the local bead stress, which denote the microscopic strain and stress, respectively. Finally, the volume fraction, number, and surface area of voids are quantified to record their nucleation positions and growth…

Olefin block copolymers (OBCs), which exhibit improved mechanical and thermal properties over conventional random copolymers, are high-performance polyolefin materials with promising applications in cutting-edge fields. Herein, we reported a series of isotactic polypropylene ( i PP)-based OBCs with both high melting temperatures ( T m s) and tunable mechanical properties, prepared via coordinative chain transfer polymerization, using a dimethyl(pyridylamido)hafnium/[Ph 3 C][B(C 6 F 5 ) 4 ]/Al i Bu 3 catalytic system. The prepared OBCs were composed of a semicrystalline i PP segment with T m > 146 °C and a propylene/α-olefin copolymer segment with comonomer content up to 17 mol %. The block microstructure of the OBCs was confirmed by gel permeation chromatography along with cross-fractionation chromatography. By properly tuning the block length and block composition, these OBCs exhibited tunable mechanical properties ranging from tough plastics (breaking strength reaching 26 MPa, elongation at break reaching 1170%) to ductile elastomers (breaking strength reaching 14 MPa, elastic…

In this report, we demonstrate that polymethacrylates can undergo appreciable depolymerization, with mild conditions and without reactive end groups, using ball-mill grinding (BMG). For instance, the important commercial polymer, poly(methyl methacrylate) (PMMA) underwent up to 41% depolymerization to monomer within 8 min of grinding. We explored the factors that influence the depolymerization conversion and found temperature (heating to only 43 °C), addition of a small amount of liquid (μL of tert -butyl alcohol), and ball number/size to be the most important parameters. We also applied this method to other polymethacrylates and found the pendent substituent to influence the extent of depolymerization. Addition of a liquid was found to reverse the effects of the substituent that were observed with bulk grinding. For instance, among the polymethacrylates studied, PMMA had the lowest depolymerization conversion in bulk and one of the highest depolymerization conversions in a slurry. Notably, the molecular weight of the residual polymer is remarkably low, ca. 1 kDa, demonstrating the…

Solid-state nuclear magnetic resonance (ssNMR) has been playing an indispensable role in revealing the interplay of structure and molecular dynamics in polymers at different states. In this Perspective, we first provide an overview about the fundamental spin interactions in ssNMR and then highlight some recent progress on sensitivity-enhanced ssNMR spectroscopy and in situ NMR. Moreover, we highlight ssNMR applications in the field of polymer crystallization, molecular dynamics, chemical reactions, supramolecular polymers, energy materials, and so on. Finally, our personal perspective is given on the future development at the crossroad of ssNMR and polymer science.