The 50 most-cited articles in Macromolecules, 2023

In the past decade, stimuli-responsive hydrogels are increasingly studied as biomaterials for tissue engineering and regenerative medicine purposes. Smart hydrogels can not only replicate the physicochemical properties of the extracellular matrix but also mimic dynamic processes that are crucial for the regulation of cell behavior. Dynamic changes can be influenced by the hydrogel itself (isotropic vs anisotropic) or guided by applying localized triggers. The resulting swelling-shrinking, shape-morphing, as well as patterns have been shown to influence cell function in a spatiotemporally controlled manner. Furthermore, the use of stimuli-responsive hydrogels as bioinks in 4D bioprinting is very promising as they allow the biofabrication of complex microstructures. This perspective discusses recent cutting-edge advances as well as current challenges in the field of smart biomaterials for tissue engineering. Additionally, emerging trends and potential future directions are addressed.

High Resolution Image Download MS PowerPoint Slide The role of the molecular weight in the crystallization and melt memory of poly(ε-caprolactone) (PCL) was investigated. To this end, 10 PCL samples of synthetic and commercial origin and different chain ends, covering a number-average molecular weight ( M n ) range between 0.48 and 70.5 kg/mol, were analyzed. Polarized light optical microscopy, differential scanning calorimetry, and small-angle X-ray scattering (SAXS) were employed for in-depth characterization. The thermal transitions, morphology, crystallization kinetics, structural parameters, and memory effects were evaluated as a function of M n . The melting temperature and the equilibrium melting temperature saturate at a particular M n . Instead, the crystallization temperature and the degree of crystallinity display an optimum M n at which the parameters reach a maximum, describing a bell-shaped behavior as a function of M n . Similarly, the primary nucleation rate, spherulitic growth rate, and overall crystallization rate exhibit a bell-shaped behavior as a function of M…

Polymer colloids are complex materials that have the potential to be used in a vast array of applications. One of the main reasons for their continued growth in commercial use is the water-based emulsion polymerization process through which they are generally synthesized. This technique is not only highly efficient from an industrial point of view but also extremely versatile and permits the large-scale production of colloidal particles with controllable properties. In this perspective, we seek to highlight the central challenges in the synthesis and use of polymer colloids, with respect to both existing and emerging applications. We first address the challenges in the current production and application of polymer colloids, with a particular focus on the transition toward sustainable feedstocks and reduced environmental impact in their primary commercial applications. Later, we highlight the features that allow novel polymer colloids to be designed and applied in emerging application areas. Finally, we present recent approaches that have used the unique colloidal nature in…

High Resolution Image Download MS PowerPoint Slide A crucial modern dilemma relates to the ecological crisis created by excess plastic waste production. An emerging technology for reducing plastic waste is the production of “ chemically recyclable ” polymers. These polymers can be efficiently synthesized through ring-opening polymerization (ROP/ROMP) and later recycled to pristine monomer by ring-closing depolymerization, in an efficient circular-type system. This Perspective aims to explore the chemistry involved in the preparation of these monomer/polymer systems, while also providing an overview of the challenges involved, including future directions.

Carbon dioxide (CO 2 ) is considered a renewable one-carbon building block for chemical synthesis owing to its abundance, low toxicity, nonflammability, and availability. The incorporation of CO 2 into the polymer backbone affords various ester- or carbonyl-containing moieties, which create desired material properties and impart unique degradability after use under microbial conditions. In this Perspective, we focus on critical advances in CO 2 -derived polymers and highlight catalytic strategies, novel conceptual routes, and potential application fields. The outlook and perspective are provided to describe the future challenges and new strategies for establishing CO 2 -based polymers with desired performances and economic competitiveness comparable to that of petroleum-based materials.

Hydrogen bonding plays a very important role in the construction and stabilization of natural and synthetic polymeric materials. Urea is generally considered and used as a hydrogen bond breaker. No successful examples have been provided to show that it can function as a strong hydrogen bond producer to fabricate polymeric materials with excellent mechanical properties. Here, we show that hydrogen-bonded poly(vinyl alcohol) (PVA)–urea hydrogels with extraordinary mechanical properties can be prepared by drying PVA–urea solutions at an elevated temperature and then swelling in water. The drying process leads to the close contact of PVA chains and urea molecules and hence enables the formation of multiple hydrogen bonds between a urea molecule and neighboring PVA chains, i.e., urea functions as a bridging molecule. The tensile strength and elastic modulus of the PVA–urea hydrogels reach 23.8 and 11.28 MPa, respectively, which are much higher than those of neat PVA hydrogels with similar water contents. Structural characterizations prove that the added urea is mostly retained in the…

Polymers with dynamic covalent bonds are an exciting class of materials with properties and potential applications that rapidly gain significant attention from diverse scientific communities. While the number of publications on this emerging topic increases exponentially, navigating through this literature reveals a series of physical inconsistencies in previously established concepts and some misconceptions in data analysis and interpretation. The current paper presents an analysis of practices often applied to the characterization of systems with dynamic bonds and emphasizes particular misconceptions that may lead to critical divergences and misinterpretations of the results. We particularly focus on practices that should be avoided in the interpretation of stress–relaxation and viscoelastic data and in estimates of the bond dissociation energy. Instead, we suggest using traditional approaches that have been applied to the analysis of polymer viscoelastic properties for decades. Lastly, we emphasize the current challenges in a full understanding of the microscopic behavior of…

High Resolution Image Download MS PowerPoint Slide Cationic polymers are widely used materials in diverse biotechnologies. Subtle variations in these polymers’ properties can change them from exceptional delivery agents to toxic inflammatory hazards. Conventional screening strategies optimize for function in a specific application rather than observing how underlying polymer–cell interactions emerge from polymers’ properties. An alternative approach is to map basic underlying responses, such as immunogenicity or toxicity, as a function of basic physicochemical parameters to inform the design of materials for a breadth of applications. To demonstrate the potential of this approach, we synthesized 107 polymers varied in charge, hydrophobicity, and molecular weight. We then screened this library for cytotoxic behavior and immunogenic responses to map how these physicochemical properties inform polymer–cell interactions. We identify three compositional regions of interest and use confocal microscopy to uncover the mechanisms behind the observed responses. Finally, immunogenic activity…

Large-scale applications of flame-retardant epoxy resins have caused sustainability concerns on the use of renewable resources and end-of-life wastes. New strategies are required to address chemical recycling of fire-safe epoxy resins. Here, we demonstrated recyclable flame-retardant epoxy resins (FREPs) using itaconic acid-derived hyperbranched epoxy resin (IA-EHBP) and (1,3,5-hexahydro- s -triazine-1,3,5-triyl) benzyl mercaptan (HT-BM). The unique structure enabled a closed-loop chemical recyclable network that can be degraded into monomers with up to 86% yield of recovered monomers. The hyperbranched topological structure of IA-EHBP also contributed to a significant improvement in the strength and toughness of the FREPs. By using positron annihilation lifetime spectroscopy, the effects of free-volume hole size and relative fractional free-volume on the mechanical performance and dynamic mechanical properties of FREPs were studied, and the function relations were therefore established based on the Williams–Landel–Ferry equation. The incorporation of IA-EHBP effectively promoted…

With environmental concerns increasing, the development of durable, degradable, and reprocessable elastomers is crucial for the advancement of environmentally friendly wearable electronics. Herein, self-healing, degradable, and reprocessable polyimine elastomers with a cross-linked network constructed by dynamic imine bonds were synthesized under room temperature for a flexible strain sensor. Owing to the formation of a microphase separation structure by polyetheramine as soft segments and adipic acid dihydrazide as hard segments, the obtained polyimine elastomer exhibited superior mechanical properties with elongation at break of 1158% and tensile strength of 2.51 MPa. Moreover, the cross-linked network of dynamic imine bonds endowed the elastomer with outstanding self-healing capability and reprocessability. The healing efficiency reached 96.0% after healing at room temperature for 24 h, and the reprocessed elastomer maintained 80.6% of the original tensile strength after three reprocessing cycles. In addition, the polyimine elastomer was able to completely degrade into monomers…

Plastics offer several advantages, but their production and disposal processes have severe environmental implications. To overcome these issues, there is a need to switch from the linear to a circular economy by recycling plastic waste and by utilizing renewable resources to create bioplastics. However, this is challenging in the case of nonbiodegradable polyolefins (POs), which form the largest fraction of produced polymers and the least recycled one. Mechanical recycling, chemical recycling, and PO bioplastics are the three pillars of PO circular economy. Although mechanical recycling is an environmentally and economically viable option, it often results in the degradation and downgrading of POs. Nonetheless, innovations in mechanical recycling, such as the use of (nano)fillers or compatibilization with olefin block copolymers, attempt to mitigate these issues. Furthermore, the development of covalent adaptable networks improves the mechanical properties of recycled PO thermoplastics and provides recyclable PO elastomers. If mechanical recycling fails to meet the desired…

High Resolution Image Download MS PowerPoint Slide Predicting and understanding the phase equilibria or phase separation in polymer–solvent solutions represent unresolved fundamental problems in polymer science. The phase behavior and thermodynamics of polymer miscibility depend on the inter- and intramolecular interactions of a polymer with a certain molecular weight distribution mixed with a solvent. Here, we develop a machine-learning framework to achieve highly generalized and robust prediction of Flory–Huggins χ parameters for polymer–solvent solutions. The model was trained using experimentally observed temperature-dependent χ parameters for 1190 samples, comprising 46 unique polymers and 140 solvent species. However, the difficulty was that the data set was quantitatively limited and qualitatively biased owing to technical issues in determining the Flory–Huggins χ parameters. To overcome these limitations, we produced an in-house data set of χ parameters obtained from quantum chemical calculations for thousands of polymer–solvent pairs and a large list of soluble and…

In general, the network structure formed in epoxy resins is heterogeneous, and its extent is dependent on the curing temperature. Based on Fourier-transform infrared spectroscopy in conjunction with coarse-grained molecular dynamics simulations, we herein discuss the origin of mesoscopic heterogeneity in an epoxy resin composed of a typical epoxy base and diamine hardener, which react with each other in two steps. The rate balance of the first and second step reactions, which led to chain extension and branching, respectively, depended on the curing temperature; at a higher temperature, the second-step reaction became faster. Since the reaction proceeded more slowly at lower temperatures, a homogeneous domain was formed as unreacted substances were incorporated into the network. At higher temperatures, on the other hand, the reaction proceeded rapidly before unreacted substances were incorporated into the network, leaving isolated small fragments and nanoscale voids or free spaces in the domain. We also found that an actively reacting domain with a length scale of several tens of…

Polymer brushes have been theoretically described, their behavior under diverse conditions has been modeled, and their preparation via surface-initiated polymerizations (SIPs) has been progressively refined. However, despite three decades of research endeavors by chemists and materials scientists, an array of challenges and questions characterizing the synthesis, characterization, and understating of polymer brushes remains and motivates this Perspective. How do the main structural parameters of polymer brushes link to their unique properties? Can we synthesize multifunctional brushes in a more practical and scalable manner? Can we successfully overcome the intrinsic challenges that enable their comprehensive characterization? While being closer than ever to the translation of polymer brushes into technologically relevant materials, we have tried to answer these questions by simultaneously outlining the status of research in this subfield of polymer science and the evolving perspectives in their synthesis and characterization.

Epoxy resins, which possess a three-dimensional network obtained by a curing reaction between epoxy and amine compounds, are in a glassy state at room temperature. They are therefore generally brittle and exhibit rather poor resistance to crack propagation, which is one of the greatest drawbacks for their practical application. To solve this problem, a better understanding of their fracture behavior, which is related to long-term durability, is strongly desired. We herein report on how the extent of network heterogeneity affects the fracture behavior of epoxy resins. Three types of epoxy resins were obtained under different curing conditions, which generated different extents of heterogeneity. The fracture behavior of the three epoxy resins was examined by in situ scanning electron microscopy in conjunction with microbeam X-ray scattering experiments. When the epoxy resin in which a notch was pre-introduced was stretched under a small strain, the internal network structure was deformed, and the extent was more significant with increasing heterogeneity. As further stretching was…

Vitrimers are associative covalent adaptable networks that undergo reversible bond-exchange reactions while maintaining a fixed cross-linking density with changing temperature. To date, experimental studies that rely on macroscopic rheology have not been able to reveal topological changes and microscopic dynamics in these materials. Here, coarse-grained molecular dynamics simulations combined with a Monte Carlo method are implemented to investigate the topological structural changes, microscopic dynamics, and linear rheology of unentangled side-chain-linked vitrimers in conjunction with the sticky Rouse model (SRM). We find that there is a minor variation in the topological structure with temperature. The dynamic heterogeneities of the bond-exchange behavior and the system dynamics increase remarkably when approaching the topological freezing transition temperature T v . Quantitative agreement between the simulation results and the SRM predictions is observed for the stress relaxation, elastic and loss moduli, and the relative mean-squared displacement, especially at the…

We extend previous systematic investigations of a family of model coarse-grained polymer network-forming materials by focusing on the influence of varying molecular rigidity on basic conformational properties (i.e., radius of gyration R g and persistence length l p ), segmental dynamics (structural relaxation time τ α ), and mechanical properties (shear G and bulk B moduli) of this broad class of materials. We find that increasing molecular rigidity increases the radius of gyration R g and persistence length l p of the network chains, as observed before in polymer solutions. The increase in molecular rigidity leads to a significant slowing down in the segmental dynamics and a strong increase in the characteristic temperatures (i.e., onset temperature T A, glass transition temperature T g, and Vogel temperature T 0 ) and fragility of glass formation. We also find that the structural relaxation time τ α, along with G and B, does not exhibit the near-universal scaling with reduced temperature T / T g as found previously for fully flexible cross-linked networks having variable…

≤ 1.23) for a wide targeted DP range (50-800). In situ chain extension and block copolymerization confirmed the preserved chain end functionality. In addition, polymerization was triggered/halted by turning on/off a green light, showing temporal control. The optimized conditions also enabled controlled polymerization of various hydrophilic acrylate monomers, such as 2-hydroxyethyl acrylate, 2-(methylsulfinyl)ethyl acrylate), and zwitterionic carboxy betaine acrylate. Notably, the method allowed the synthesis of well-defined acrylate-based protein-polymer hybrids using a straightforward reaction setup without rigorous deoxygenation.

Polyglycolic acid (PGA) is considered a promising competitor in green packaging due to its superior strength and outstanding barrier properties, but its applications are limited by its inherent brittleness. Traditional toughening methods are usually accompanied by sacrifices in both strength and barrier properties. In this work, robust and high-barrier PGA/poly(butylene adipate- co -terephthalate) (PBAT) films were prepared by combining reactive compatibilization, biaxial drawing, and constrained annealing processes. The abundant uniformly oriented nanocrystals with the c -axis parallel to the film surface formed in biaxial drawing and constrained annealing increase the tensile strength as reinforcing elements and improve the barrier properties as a barrier wall. Biaxial drawing also induces a reinforced chain entanglement network, which facilitates the improvement of ductility. Importantly, reactive compatibilization plays a key role by increasing interfacial interactions to prevent cavitation and to maintain a robust entanglement network during annealing, consequently further…

A vitrimer has covalent cross-links that preserve network connectivity but permit topology fluctuations through dynamic exchange reactions. In this work, we investigate the linear rheology of polybutadiene (PB) vitrimers bearing cross-links that exchange via dioxaborolane metathesis. PB vitrimers are cross-linked in solution using photoinitiated thiol-ene click chemistry. As the targeted cross-link density is increased, both the insoluble fraction and glass transition temperature increase. Linear viscoelasticity is studied using a combination of small amplitude oscillatory shear (SAOS), creep, and stress relaxation measurements. In SAOS, the elastic modulus is approximately constant while the viscous modulus increases as angular frequency decreases. In creep, the compliance displays power law behavior that persists for at least 8 hrs. In stress relaxation, the modulus transitions from a rubbery plateau into a power law regime. Both SAOS and creep data are visually superposed into master curves using horizontal shift factors that exhibit Arrhenius behavior. However, the activation…

Thermochromic polymers, as a highly applicable intelligent category of innovative materials, are capable of changing their color with temperature variations. Different characteristics of the matrix and specially its polarity can be influential factors in the responsivity of thermochromic polymers. In this study, polystyrene, poly(methyl methacrylate), and poly(dimethylaminoethyl methacrylate) with different polarities were synthesized via reverse atom transfer radical polymerization. Thermochromic polymers were prepared by physical incorporation of the N -(rhodamine-B)lactam-ethylenediamine leuco dye and HCl color developer into the three polymers. The effect of polarity of the polymers on their thermochromism and fluorescence properties was investigated. The thermochromic polymers changed their color from colorless to a colored state with increasing temperature due to the intermolecular proton transfer in the system. UV–vis and fluorescence spectroscopies were used to evaluate the thermochromism and fluorescence properties of the polymer systems, respectively, at different…

The widespread use of polymer materials and their improper disposal have resulted in significant environmental pollution and wastage of resources. Therefore, the design of chemically recyclable, environmentally compatible, and applicable polymers represents a new approach for a green and circular polymer economy, and it has become a major focus worldwide. Herein, we efficiently synthesized a seven-membered ether lactone, 3,4-dihydro-2 H -benzo[ b ][1,4]dioxepin-2-one (BDXO), through a one-step reaction and successfully realized its controlled ring-opening polymerization (ROP). The ROP of BDXO in bulk and solvent conditions catalyzed by Sn(Oct) 2 was investigated systematically, and the thermodynamic parameters obtained were Δ S P 0 = −30.90 J·mol –1 ·K –1 and Δ H P 0 = −14.52 kJ·mol –1 . The PBDXO polymer exhibits excellent thermal properties, applicable mechanical properties, excellent biocompatibility, and complete hydrolytic degradability. Notably, not only was the direct chemical recycling of PBDXO to BDXO monomer realized with high efficiency through bulk thermal…

Here, we report the first study of the reprocessability and properties of non-isocyanate polythiourethane (NIPTU) networks in which the cross-links are of two types, thionourethane and disulfide, the latter obtained by auto-oxidation of pendant thiol groups. Our two NIPTU networks are biowaste based because starting materials can be derived from cashew nutshells or rice husks. We use dynamic mechanical analysis, tensile testing, and water sorption studies to compare our NIPTU networks with structurally analogous polyhydroxyurethane (PHU) networks, which are non-isocyanate polyurethanes (PUs). With significant advantages in reactivity, mechanical properties, and cross-link density, NIPTU networks can be favorable alternatives to PHU networks. Our biobased NIPTU networks also exhibit better water resistance, with a factor of ∼3 reductions in water sorption relative to their PHU analogues. Due to their dynamic covalent cross-links, our NIPTUs show excellent reprocessability with complete recovery of cross-link density after multiple reprocessing steps as well as potential as…

High Resolution Image Download MS PowerPoint Slide A necessary transformation for a sustainable economy is the transition from fossil-derived plastics to polymers derived from biomass and waste resources. While renewable feedstocks can enhance material performance through unique chemical moieties, probing the vast material design space by experiment alone is not practically feasible. Here, we develop a machine-learning-based tool, PolyID, to reduce the design space of renewable feedstocks to enable efficient discovery of performance-advantaged, biobased polymers. PolyID is a multioutput, graph neural network specifically designed to increase accuracy and to enable quantitative structure–property relationship (QSPR) analysis for polymers. It includes a novel domain-of-validity method that was developed and applied to demonstrate how gaps in training data can be filled to improve accuracy. The model was benchmarked with both a 20% held-out subset of the original training data and 22 experimentally synthesized polymers. A mean absolute error for the glass transition temperatures of…

Chemically recyclable thermosets are an ideal substitute for traditional thermosets in the development of a circular economy and sustainable environment. However, the development of efficient and easy-to-achieve chemical recycling strategies remains challenging. Herein, a series of supramolecular thermosets that can be chemically recycled under mild acid conditions at room temperature are fabricated by cross-linking the polyimine polymers with dynamic boroxines (PI x -Boroxine). By tailoring the molar content of boroxines, the PI 1.2 -boroxine can exhibit a tensile strength of ∼30.6 MPa, a tensile yield strength of 33.0 MPa, an elongation at break of ∼111.6%, and a Young’s modulus of ∼679.6 MPa. Because of the dynamic nature of boroxines and imine bonds, the PI x -boroxine supramolecular thermoset exhibits fast stress relaxation behavior, which enables them to have good reprocessing ability. These unique features can also guarantee the PI x -boroxine supramolecular thermosets to be a high-performance reusable hot-melt adhesive. The maximum lap shear strength of the PI x…

Imparting polylactide (PLA) films with great heat resistance, transparency, and balanced mechanical performance simultaneous is a big challenge in academia and industry due to mutual exclusiveness of properties. In this work, a newfound strategy that combines elongational stress field and thermal field is proposed to reorganize the crystalline and amorphous structure of PLA films. It is discovered that the oriented PLA films with high draw ratios form a unique structure of the oriented amorphous phase restricted by the oriented nanosized crystal during thermal treatment at an appropriate temperature. Superb stiffness–toughness balance of the film is achieved as the elongation at break increases from 16 to 91.9% after the thermal treatment at 150 °C, while the yield strength increases from 50.0 to 76.0 MPa. It is worth noting that the PLA film also has excellent transparency, i.e., a transmittance as high as 86.6% (550 nm), owing to crystallites formed on the nanometer scale. Excellent heat resistance is also obtained, with the onset decline temperature increasing from 63.7 (for the…

The present work studies the mechanisms controlling the energy barrier for ion hopping in conducting polymers. Polymer electrolytes usually show Arrhenius-like temperature dependence of the conductivity relaxation time (characteristic time of local ion rearrangements) at temperatures below their glass transition T g . However, our analysis reveals that the Arrhenius fit of this regime leads to unphysically small prefactors, τ 0 ≪ 10 –13 s. Imposing a value of 10 –13 s for this parameter renders the fairly unexpected result that the energy barrier for charge transport in these polymers has strong temperature dependence even below T g . Our study also reveals significant temperature variations of the dielectric permittivity and the instantaneous shear modulus in the glassy state of these polymers. Using the Anderson and Stuart model, we demonstrate that these variations provide strong justifications for the temperature variation of energy barrier for ion hopping. Most importantly, the proposed approach reveals that the energy barrier controlling ion hopping in polymer electrolytes is…

To ensure the stable performance and extended service life of high-integrity and high-power electronic devices, it is imperative for dielectric materials to consistently exhibit a low dielectric constant ( D k ) and dissipation factor ( D f ) across a wide temperature range. Here, we have undertaken the synthesis of two distinct polyimides (PIs), namely, 6FDA-GA and 6FDA-GB, featuring varying biphenyl side groups. These selected side groups have been intentionally incorporated to impede the interchain stacking of PI macromolecules and enhance the twist rigidity of the PI mainchain. It was substantiated that the smaller side-chain groups in 6FDA-GB are notably effective in constraining the rotation of the imide ring within the main chain owing to their closer proximity. This constraint has unequivocally resulted in a substantial increase in the twist rigidity of the main chain. Thus, the resulting film was endowed wtih an expanded free volume and concurrently curtailed dipole motion. These combined effects have led to a reduction in both D k and D f . The 6FDA-GB film exhibits a D k…

High Resolution Image Download MS PowerPoint Slide Alicyclic quaternary ammonium cations having all the β-protons in a strain-free ring structure are in general highly base-resistant and are thus very attractive to employ for anion exchange membrane (AEM) applications. However, tethering cations such as N, N -dimethylpiperidinium (DMP) to polymer backbones without introducing any weak links is quite challenging. In the present study, we have attached pairs of piperidine rings in their 4-position to fluorene and 2,7-diphenylfluorene via methylene bridges using straightforward S N 2 reactions. These fluorenes were subsequently utilized as monomers in polyhydroxyalkylations to prepare poly(fluorene alkylene)s with different contents of the piperidine groups. AEMs were cast after quaternizing the piperidine groups to introduce DMP and spirocyclic 6-azonia-spiro-[5,5]undecane-6-ium (ASU) cations, respectively. The AEMs reached very high hydroxide ion conductivities, 100–156 mS cm –1 at 80 °C, in the ion exchange capacity range 1.8–2.4 mequiv g –1 . X-ray scattering showed ionomer peaks…

Branched polymers have aroused great interest due to their less chain entanglement, high concentration of chain terminals, good solubility, and low viscosity in melts and solutions. In this Perspective, we highlight various synthesis strategies for branched polymers with different structures and discuss their applications in the field of solid polymer electrolytes for lithium batteries, coatings and membranes, surfactants, hydrogels, and so on. Here, we expect to arouse more synthesis approaches such as enzyme-catalyzed living radical polymerization and photoinitiated living radical polymerization and also to copolymerize monomers with different properties or to modify branched polymers in order to obtain functionalized branched polymers that can be produced under mild conditions.

High Resolution Image Download MS PowerPoint Slide The effect of composition and morphology on mechanochemical activation in nanostructured block copolymers was investigated in a series of poly(methyl methacrylate)- block -poly( n -butyl acrylate)- block -poly(methyl methacrylate) (PMMA- b -PnBA- b -PMMA) triblock copolymers containing a force-responsive spiropyran unit in the center of the rubbery PnBA midblock. Triblock copolymers with identical PnBA midblocks and varying lengths of PMMA end-blocks were synthesized from a spiropyran-containing macroinitiatior via atom transfer radical polymerization, yielding polymers with volume fractions of PMMA ranging from 0.21 to 0.50. Characterization by transmission electron microscopy revealed that the polymers self-assembled into spherical and cylindrical nanostructures. Simultaneous tensile tests and optical measurements revealed that mechanochemical activation is strongly correlated to the chemical composition and morphologies of the triblock copolymers. As the glassy (PMMA) block content is increased, the overall activation increases,…

The correlations between the molecular structures of four Tröger’s base (TB)-based polyimides (PIs) and two non -TB containing analogues and physical properties including thermal conductivity (λ) and dielectric properties both at low and high frequencies were investigated in detail. The TB-based PI films exhibited low dielectric constants ( D k = 2.25–2.80) at 10 GHz. They possessed much lower λ values (0.035–0.145 W/mK) compared to the commercial PI Kapton (0.240 W/mK). The influences of incorporating TB units into chain backbones on aggregation structures and physical properties of PIs were identified. Incorporating TB units into chain backbones effectively reduced the degree of chain orientation and increased fractional free volume, leading to both low D k and low λ values for the resulting PI films. Also, introducing TB units enhanced molecular weights, toughness, and glass-transition temperature ( T g ) of the resulting PIs. Therefore, the TB-based PIs can be promising heat-insulating and low- k dielectric materials.

We are facing a rapidly growing plastic pollution crisis, which is adversely affecting communities and ecosystems across the globe. Polymer scientists have an urgent obligation to develop and implement strategies for reutilizing plastic waste streams instead of continually relying on virgin feedstocks in plastic manufacturing. Postconsumer and postindustrial plastic items should be treated as valuable resources instead of discarding them. In order to do this, technological outlets must be developed to produce renewed materials using plastic waste as an input. Polyesters represent one of the largest plastic waste streams because they are commonly used in single-use beverage and food containers as well as textiles. Polyesters are also ideal candidates to exploit through chemical diversification, owing to the wide variety of functional derivatives that can be synthetically accessed via ester reactivity. This work highlights some recent developments related to diversifying polyester waste, with a focus on poly(ethylene terephthalate) (PET) as waste stream. The strategies encompass the…

Ring polymers have fascinated theorists, simulators, and experimentalists as they are the simplest polymer without ends, giving rise to important topology related properties. We present the state of the art of recent synthetic efforts and investigations into the structure and dynamics of dense nonconcatenated ring polymer systems. Analyzing the existing knowledge, we identify challenges for future research: In the realm of synthesis the creation of well-defined high molecular weight rings based on different monomers are highly desirable. In the field of ring conformations, the existence of double folded structures that are at the basis of many theoretical approaches was severely challenged and further scrutiny is needed. Similarly, the issue of the size dependent transition to mass fractal structures, or the effect of local stiffness on the conformation and dynamics are open questions. The role of ring–ring threading in the terminal dynamics of rings needs to be further elucidated. In particular, the novel proposed topological glass transition for very high molecular weight ring…

High Resolution Image Download MS PowerPoint Slide High refractive index polymers are essential in next-generation flexible optical and optoelectronic devices. This paper describes a simple synthetic method to prepare polymeric optical coatings possessing high refractive indexes. Poly(4-vinylpyridine) (P4VP) thin films prepared using initiated chemical vapor deposition are exposed to highly polarizable halogen molecules to form stable charge-transfer complexes: P4VP-IX (X = I, Br, and Cl). Fourier transform infrared spectroscopy was used to confirm the formation of charge-transfer complexes. Characterized by spectroscopic ellipsometry, the maximum refractive index of 2.08 at 587.6 nm is obtained for P4VP-I 2 . For P4VP-IBr and P4VP-ICl, the maximum refractive indexes are 1.849 and 1.774, respectively. By controlling the concentration of charge-transfer complexes, either through the halogen incorporation step or polymer composition through copolymerization with ethylene glycol dimethacrylate, the refractive indexes of the polymer thin films can be precisely controlled. The…

Research into 3D printing using reversible addition–fragmentation chain transfer (RAFT) polymerization has garnered interest since it was first reported in 2019. This technique was initially developed to expand the scope of light-based 3D printing technologies by producing materials that can be modified postprinting, termed “living” 3D printing. The livingness can be achieved by incorporating reactivatable RAFT functionalities within the polymer networks, enabling 3D materials to be modified after printing. As the field of RAFT-mediated 3D printing has progressed, further studies have revealed its applications in advanced materials. These include spatially resolved surface functionalization and patterning, self-healing, welding, and nano- and microscale structuring of 3D polymers. Additionally, RAFT-mediated 3D printing enables the production of scaffolds with controlled interconnected channel-pore architecture, suitable for customized drug delivery. This Perspective provides a review of the chemical mechanisms employed in RAFT-mediated 3D printing and highlights the advanced…

Polymers are under increasing demand as thermal management materials for electronic devices such as integrated circuits and electrical machines. However, the intrinsic thermal conductivity of polymers is typically low, around 0.2 W/(m K). Although crystallinity is qualitatively known to have a positive correlation with thermal conductivity, the quantitative relationship is unclear because, in most cases, changes in crystallinity are accompanied by differences in the chemical structure of the polymer. In this work, vitrimers with a fixed chemical structure and slow crystallization kinetics are investigated to reveal the relationships between crystallinity and various physical properties relevant to heat transport. As slow crystallization occurs over the span of one week, the physical properties of the vitrimers also evolve. Changes in thermal conductivity are dramatic from 0.10 to 1.0 W/(m K). Quantitative relationships among crystallinity, thermal conductivity, speed of sound, and chain conformation are elucidated by a combination of in situ measurements.

High Resolution Image Download MS PowerPoint Slide Chemical recycling to monomers (CRMs) of A–B–A block copolymers is governed by the chemical structure and thereby the thermodynamic behavior of different block constituents. Here, we show how a thermodynamic toolkit based on a cyclic monomer structure and solvent properties can be utilized in the design of recyclable A–B–A block copolymers with varying material properties. By combining four cyclic monomers lactide, ε-decalactone, 2,2-diethyltrimethylene carbonate, and trimethylene carbonate, three different block copolymers were created, suitable for different CRM scenarios. The chemical structure of the soft midblock (ε-decalactone or trimethylene carbonate) appeared to have a critical impact both on the ring-closing depolymerization behavior and mechanical properties, where changing from a polyester to a polycarbonate soft block increased Young’s modulus from 14 to 200 MPa. Hence, this work demonstrates the complexity as well as the opportunities in the design of macromolecular structures for a circular economy.

Hydrogen bonding in thermosetting resins can have a significant influence on the polymerization processes and the properties of corresponding thermosets, but its role in the polymerization of benzoxazine resins remains unclear. Here, we synthesized two novel phosphaphenanthrene-functionalized benzoxazine monomers from 2-6-oxido-6 H -dibenz-[ c, e ][1,2]oxaphosphorin-1,4-dihydroxy phenylene, aniline/furfurylamine, and paraformaldehyde and investigated the structures by 1 H, 13 C, and 31 P NMR, Fourier transform infrared (FT-IR), elemental analysis, and high-resolution mass spectrometry. Ring-opening polymerizations of both monomers were then studied by differential scanning calorimetry (DSC) and in situ FT-IR spectroscopy, and the resulting thermosets exhibited high thermal stability and low flammability. Density functional theory (DFT) calculations suggested that intramolecular hydrogen bonds are preferably formed between the phenolic −OH and the P═O in the phosphaphenanthrene functionality for both monomers, which is in line with the experimental results. We proposed a…

High Resolution Image Download MS PowerPoint Slide We herein report the synthesis of commercially attractive low molar mass polycarbonate polyols obtained through the ring-opening copolymerization of CO 2 and epoxides, using a series of borinane-based bifunctional organocatalysts in the presence of chain transfer agents (CTAs). These catalysts enable CO 2 /epoxide copolymerizations with high linear vs cyclic selectivity and outstanding productivity for both poly(cyclohexane carbonate) polyols (18.2 kg/g catalyst) and poly(ether propylene carbonate) polyols (1.1 kg/g catalyst). These copolymerizations exhibit all features of living processes; the molar mass of the resulting polycarbonates could be precisely controlled by varying the [monomer]/CTA ratio. The high performance of these catalysts implying a low loading shows a great potential for applications in large-scale preparation of CO 2 -based polyols.

The motion of nanoparticles in a polymer matrix is dictated by the intricate coupling of the nanoparticles and surrounding polymers. Various length- and time-scale dependent features of nanoparticle–polymer coupling in a polymer matrix have been delineated in the past decade by combining scaling theory and molecular simulations. Representative scenarios of nanoparticle dynamics in polymers, which embody the roles of polymer matrix topology, the polymers grafted to nanoparticle surface, the anisotropic shape of nanoparticles, and an external driving force, are reviewed. The systems examined demonstrate both the richness of the physics in the nanoparticle–polymer coupling and the capability of the scaling-level description in providing unique insights into the size- and time-dependence of nanoparticle mobility. Following a review of recent work, more scenarios of nanoparticles in polymer matrices, which reflect new pieces of physics in the nanoparticle–polymer coupling, are discussed. Together with the advances in the chemical synthesis of nanoparticles and polymers as well as in the…

We present a modified Lake–Thomas theory that accounts for the molecular details of network connectivity upon crack propagation in polymer networks. This theory includes not only the energy stored in the breaking network strands (bridging strands) but also the energy stored in the tree-like structure of the strands connecting the bridging strands to the network continuum, which remains intact as the crack propagates. The energy stored in each of the generations of this tree depends nonmonotonically on the generation index due to the nonlinear elasticity of the stretched network strands. Further, the energy required to break a single bridging strand is not necessarily dominated by the energy stored in the bridging strand itself but in the higher generations of the tree. We describe the effect of mechanophores with stored length on the energy stored in the tree-like structure. In comparison with the “strong” mechanophores that can only be activated in the bridging strand, “weak” mechanophores that can be activated both in the bridging strand and in other generations could provide more…

Formation of stereocomplex crystals (SC) is an effective way to improve the heat resistance and mechanical performance of poly(lactic acid) products. However, at all but the slowest cooling rates, SC crystallization of a high-molecular-weight poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemate stops at a high temperature or does not even start, leaving the remaining melt to crystallize into homochiral crystals (HC) or an SC-HC mixture on continuous cooling. To understand this intriguing phenomenon, we revisit the SC crystallization of both high- and low-molecular-weight PLLA/PDLA racemates. Based on differential scanning calorimetry (DSC), supplemented by optical microscopy and X-ray scattering, we concluded that what stops the growth of SC is the accumulation of the nearly pure enantiomer, either PDLA or PLLA, that is rejected from the SC ahead of its growth front. The excess enantiomer is a result of random compositional fluctuation present in the melt even if the average composition is 1:1. The situation is more favorable if the initial polymer is not fully molten or is…

Achieving ultrafast resilience comparable to that of irreversible cross-linking elastomers remains a formidable challenge for self-healing supramolecular elastomers. Herein, we construct a π-type tweezer structure formed by imidazolidinylurea and aromatic imine to stabilize dynamical hard domains (SDHDs) in polyurethane (PU) elastomers. SDHDs endow the dynamic PU network with high binding energy and minimal stress relaxation at room temperature, similar to irreversible covalent cross-links. As such, the elastomer demonstrates rapid resilience comparable to vulcanized natural rubber as well as high mechanical strength and toughness. Meanwhile, SDHDs can be readily activated upon heating, enabling exceptional healing ability (∼100% healing efficiency) under mild conditions (50 °C) and complete recovery of mechanical properties after recycling. More interestingly, the PU elastomer exhibits significant elastocaloric effects with an adiabatic temperature change of −13.5 °C, surpassing that of state-of-the-art vulcanized natural rubber (−9.4 °C). Therefore, this work presents a new…

Polymeric membranes for gas separations have experienced rapid growth over the past few decades, and extensive research has been performed to develop high-performance polymer membrane materials with outstanding stability and durability. Cross-linking of polymers has been demonstrated as an effective approach to improve the resistance to physical aging as well as plasticization induced by condensable gases like CO 2 . In this Perspective, we examine recently reported, innovative strategies that have produced cross-linked polymeric membranes with attractive separation performance as well as resistance to plasticization and physical aging. The discussion will cover the state-of-the-art design strategies that are focused on cross-linking chemistry and innovative cross-linked network structures, with an emphasis on the effect of cross-linking on membrane properties. New research directions are proposed for advancing the development of cross-linked polymer membrane materials to realize the full potential of this class of membrane materials for gas separation applications and beyond.

Polyurethane (PU) thermosets are usually landfilled at the end of their service lifetimes and cannot be recycled through conventional means. Although dibutyltin dilaurate (DBTDL) is an effective catalyst for reprocessing PU thermosets, organotins are immunotoxic and teratogenic, which is concerning for catalysts embedded within covalent adaptable networks (CANs). Here, we identify Zr(acac) 4 and Zr(tmhd) 4 as catalysts to reprocess PU CANs more sustainably. Thermoset PU foams containing Zr-based catalysts exhibit characteristic stress relaxation times between 6 and 200 s, similar or faster than DBTDL. These catalysts are capable of reprocessing a PU foam for four or five cycles. Dynamic mechanical thermal analysis indicates that zirconium-based catalysts preserve glass transition temperature and crosslink density between cycles and are promising catalysts for bulk reprocessing of thermoset PUs. Furthermore, a solvent-free reprocessing method was developed by co-milling the PU foam and the Zr catalyst. Ultimately, these reprocessing methods using Zr-based catalysts will impart…

The cross-linked low-dielectric-constant material has garnered much attention in the rapid development of advanced electronics packaging. However, few studies have systematically explored the effects of cross-linking density and cross-linking degree on dielectric properties of benzocyclobutene (BCB)-based polymeric materials. Herein, a series of BCB-based polycarbosiloxanes with varying cross-linking densities were synthesized through Piers–Rubinsztajn polycondensation and post-Heck modification. The relationship between cross-linking behavior and the dielectric characteristics of cured materials was systematically explored using both molecular dynamics (MD) simulation and experimental verification. The results revealed that higher cross-linking density and cross-linking degree resulted in more microbranched structures, which effectively reduced the dielectric constant and dielectric loss of the materials. Moreover, high cross-linking density enhanced the thermal and dimensional stability and enabled a facile tuning of the mechanical property of the materials. Overall, our work…

It is attractive to develop chemically recyclable polymers with desirable performances from commercial monomers to avoid the expensive and time-consuming scalable process for completely new monomers. In this contribution, we present the rapid and controlled ring-opening polymerization (ROP) of bio-sourced commercial alkyl-δ-lactones to produce closed-loop recyclable polyesters in the presence of an organophosphazene base ( t Bu-P 2 )/urea binary catalyst. The obtained polyesters are capable of recycling back to pristine monomers by simply heating them in bulk in the presence of stannous octanoate (Sn(Oct) 2 ) as the catalyst. The effects of alkyl substituent length on the polymerization kinetics, thermodynamics, as well as the properties of resultant poly(alkyl-δ-lactone)s were investigated. Well-defined triblock copolymers consisting of poly(alkyl-δ-lactone)s as soft middle block and PLLA as hard end block were successfully prepared by one-pot, sequential ROP of alkyl-δ-lactones and l -lactide ( l -LA). These triblock copolymers can be used as pressure-sensitive adhesives without…

Quasi-macromolecules (QMs) have achieved both highly efficient and stable devices along with a low batch-to-batch variation, which favors the commercialization of organic solar cells (OSCs). However, understanding of the relationship between molecular structure design for QMs and their photovoltaic properties is still limited. In this work, we reported a three-arm star-shaped QM acceptor TBT with a benzo[1,2- b:3,4- b ′:5,6- b ″] trithiophene (BTT) unit as the electron-donating central core and three A–DA′D–A type acceptors as the electron-deficient conjugated arms. Meanwhile, DBT with conventionally linear molecular skeletons was also synthesized for comparison. TBT exhibits an optical bandgap (1.42 eV) and energy levels similar to those of DBT, but with better light-absorption capability, an alleviated tendency to aggregate, and different packing behavior. When blended with PM6, TBT-based OSC achieved a higher power conversion efficiency of 18.04% than its DBT (16.53%) counterpart, which is one of the highest value afforded by star-shaped acceptors in binary OSCs so far. Moreover,…

How do the degrees of surface modification on nanoparticles affect their enhancement to the mechanical performance of composites? This issue is generally overlooked with a consensus that surface modification can promote the compatibility between nanoparticles and polymeric matrix, which therefore advances mechanical performances of obtained composites. In this study, we designed a composite system of castor oil (CO)-based polyurethane (PU) as the matrix enhanced by cellulose nanocrystals (CNCs) with changing surface modifications of low, moderate, and high grafting of the CO chains as nanofillers. The compatibility between modified CNCs and PU was quantified by the Hansen solubility parameters, with the gradual decrease of Ra values from 14.1 to 6.1. The interfacial compatibility between nanofillers and matrix directly determined the dispersion and microstructures of composite systems and further influenced mechanical performances of materials, exhibiting the improvement on Young’s modulus, tensile strength, work of fracture, and storage modulus for the modified CNC-enhanced…