The 50 most-cited articles in Macromolecules, 2019

Abstract Molecular modeling and simulations are invaluable tools for the polymer science and engineering community. These computational approaches enable predictions and provide explanations of experimentally observed macromolecular structure, dynamics, thermodynamics, and microscopic and macroscopic material properties. With recent advances in computing power, polymer simulations can synergistically inform, guide, and complement in vitro macromolecular materials design and discovery efforts. To ensure that this growing power of simulations is harnessed correctly, and meaningful results are achieved, care must be taken to ensure the validity and reproducibility of these simulations. With these considerations in mind, in this Perspective we discuss our philosophy for carefully developing or selecting appropriate models, performing, and analyzing polymer simulations. We highlight best practices, key challenges, and important advances in model development/selection, computational method choices, advanced sampling methods, and data analysis, with the goal of educating potential polymer…

Abstract Emulsion templating presently extends far beyond the original hydrophobic porous polymers that were synthesized within surfactant-stabilized water-in-oil high internal phase emulsions (HIPEs) by using free radical polymerization. This Perspective presents the extraordinary versatility of emulsion templating that has emerged with the growing numbers of HIPE systems, HIPE stabilization strategies, monomers, polymerization chemistries, multicomponent materials, and surface functionalities. Emulsion templating now goes far beyond “porous polymers” by encompassing the encapsulation of aqueous solutions, ionic melts, and organic liquids as well as by encompassing porous carbons and porous inorganics. Herein, we present comprehensive pictures of the state-of-the-art, of the prospective large-scale and niche applications, of the advantages and challenges for industrial scale-up, and of the crucial directions that should be pursued in future work. We demonstrate that it is emulsion templating’s considerable and versatile parameter space that offers opportunities for pioneering work,…

Covalent cross-linking of rubbers is essential for obtaining high resilience and environmental resistance but prevents healing and recycling. Integrating dynamic covalent bonds into cross-linked rubbers can resolve the trade-off between permanent cross-linking and plasticity. The state-of-the-art elastomer-based dynamic covalent networks require either intricate molecular makeup or present poor mechanical properties. In this work, we demonstrate a simple way to prepare mechanically robust yet healable and recyclable elastomeric vitrimers by engineering dynamic dual cross-links of boronic esters and coordination bonds into a commercial rubber. Specifically, epoxidized natural rubber is covalently cross-linked with a boronic ester cross-linker carrying dithiol through chemical reaction between epoxy and thiol groups. The covalently cross-linked networks are able to alter the topologies through boronic ester transesterifications, thereby conferring them with healing ability and reprocessability. In particular, the mechanical properties can be remarkably enhanced by introducing…

Cross-linked networks feature exceptional chemical and mechanical resilience but consequently lack recyclability. Vitrimers have emerged as a class of materials that feature the robustness of thermosets and the recyclability of thermoplastics without compromising network integrity. Most examples of vitrimers have involved new polymers with exchangeable bonds within their backbones. In pursuit of a more universal, commercially viable route, we propose a method utilizing commercially available and inexpensive reagents to prepare vitrimers from vinyl monomer-derived prepolymers that contain cross-linkable β-ketoester functional groups. Controlled radical copolymerization of methyl methracrylate and (2-acetoacetoxy)ethyl methacrylate afforded linear prepolymers that were converted into vitrimers in a single step by treatment with a trifunctional amine. These materials displayed the characteristic features and reprocessability of vitrimers over as many as six (re)processing cycles. Critically, the networks prepared through this process largely retain the chemical and thermal properties…

High-performance lignin-containing polyurethane elastomers (LPUes) were successfully synthesized via partially substituting petroleum-derived polyols with depolymerized enzymatic hydrolysis lignin (DEL). The influences of DEL on the structure, thermostability, mechanical performance, and thermal reprocessability of LPUes were systematically studied. The tensile strength and toughness of PUes were significantly enhanced after the introduction of DEL, with the maximum tensile strength reaching up to 60.7 MPa and the toughness up to 263.6 MJ/m 3 . The enhancement for the strength and toughness was attributed to the dual cross-linking network structure and the interfacial hydrogen bonds between lignin and the PU matrix, which were demonstrated to facilitate the orientation of chain segments and lead to strain-induced crystallization and self-reinforcement. LPUes also exhibited much better elasticity than the control sample without lignin and could maintain excellent mechanical performance after being hot reprocessed.

The number of biomedical applications of hydrogels is increasing rapidly on account of their unique physical, structural, and mechanical properties. The utility of hydrogels as drug delivery systems or tissue engineering scaffolds critically depends on the control of diffusion of solutes through the hydrogel matrix. Predicting or even modeling this diffusion is challenging due to the complex structure of hydrogels. Currently, the diffusivity of solutes in hydrogels is typically modeled by one of three main theories proceeding from distinct diffusion mechanisms: (i) hydrodynamic, (ii) free volume, and (iii) obstruction theory. Yet, a comprehensive predictive model is lacking. Thus, time and capital-intensive trial-and-error procedures are used to test the viability of hydrogel applications. In this work, we have developed a model for the diffusivity of solutes in hydrogels combining the three main theoretical frameworks, which we call the multiscale diffusion model (MSDM). We verified the MSDM by analyzing the diffusivity of dextran of different sizes in a series of poly(ethylene…

Vitrimers are polymer networks able to change their topology through degenerate exchange reactions. As a result, they behave like elastic solids when the topology of the network is frozen and like viscoelastic liquids at high temperatures. Controlling the dynamics of molecular exchange is essential to regulate the solid-to-liquid transition temperature in elastomeric vitrimers. In this study, such materials were prepared by the radical grafting of a bis-thiol dioxaborolane onto a low molar mass unentangled polybutadiene. All the resulting elastomeric vitrimers fully relax stress and are reprocessable and recyclable. Because of the low molar mass of the thermoplastic precursor, the low number of cross-links per chain, and the dynamics of the dioxaborolane exchange reaction, these vitrimers dissolve in tetrahydrofuran after prolonged immersion time at room temperature. Creep-recovery experiments at various temperatures show that both the viscosity and the viscosity activation energy of vitrimers can be controlled by manipulating the cross-linking density. This feature offers a new…

Polythiourethane (PTU) can be synthesized by a type of click chemistry involving the reaction of thiols with isocyanates. To our knowledge, thiourethane dynamic chemistry has not been significantly explored from a fundamental standpoint and has only begun to be studied regarding its use in producing recyclable, reprocessable substitutes for traditional cross-linked polyurethane networks. Using model compounds, we demonstrated the dual nature of the mechanism associated with catalyzed thiourethane dynamic chemistry: at elevated temperature, thiourethane groups undergo exchange reactions with free thiol groups and thermal reversion to thiols and isocyanates. We used this chemistry to synthesize cross-linked PTU elastomers which, upon optimization, achieve full recovery of cross-link density and tensile properties after multiple, relatively rapid remolding cycles. We characterized stress relaxation as a function of temperature and stoichiometric imbalance to provide insight into the mechanism of the network structure change. A small level (10 mol %) of excess thiol groups reduces…

Abstract The current approach to regulating mechanical properties of elastomeric materials is predominately based on the exploratory mixing of different polymers, solvents, and fillers─which is both inflexible in application and imprecise in property control. Here we overview a new materials design approach that harnesses well-defined molecular codes of independently controlled architectural parameters to program grand variation of mechanical “phenotypes”. This design-by-architecture approach generates a set of universal correlations between the molecular architecture and the physical behavior of elastomers. In turn, this will lead to novel solvent-free materials that closely mimic the mechanical behavior of biological tissues, ranging from soft fat tissue to firm skin, and fundamentally change high-impact technologies such as soft robotics, wearable electronics, and biomedical devices.

Theoretical approaches addressing the mechanism of polymer crystallization remain the great challenge in polymer science. Numerous different, or even conflicting, models/theories have been proposed during the past several decades. However, none of them can fully satisfy the whole community. In this Perspective, we first trace the roots of these models/theories back to the classical and nonclassical nucleation theories. The correlation between these theories and milestone theoretical works in polymer crystallization is elucidated together with their intrinsic drawbacks. Then the newly proposed two-step nucleation scenarios, with either bond-orientational order or density fluctuation as precursors, are introduced, which, in our view, may stimulate the development of polymer crystallization theory. Afterward, the peculiarities of polymer crystallization due to chain flexibility and connectivity are discussed. A personal outlook on the ultimate polymer crystallization theory is given at last, which is suggested to address the following three questions: (i) How do flexible chains…

Intelligent skinlike materials have recently attracted tremendous research interests for employing in electronic skin, soft robotics, and wearable devices. Because the traditional soft matters are restricted in unsatisfactory mechanical performances or short-term usage, these materials are adverse to practical applications. Here, intriguing conductive hydrogel materials with multifunctionality (MFHs) are fabricated by using poly(acrylic acid) (PAA), dopamine-functionalized hyaluronic acid (DHA), and Fe 3+ as ionic cross-linker. The mussel-inspired networks with delicate combination of physical and chemical cross-linking possess synergistic features of inherent viscoelasticity, high stretchability (800%), and durable self-adhesiveness to various substrates. Owing to the abundant hydrogen bonds and multiple metal coordination interactions between Fe 3+, catechol, and carboxylic groups, the matrix reveals repeatable thermoplasticity and autonomous self-healing property both mechanically and electrically (98% recovery in 2 s). When served as strain sensors, the MFHs can distinctly…

This work shows that the environmental-friendly biorenewable and biodegradable poly(lactic acid) (PLLA) can be made to have superior mechanical and thermal characteristics and thus show promise to replace conventional petroleum-based polymers such as polyethylene terephthalate. Using time-resolved polarized optical microscopy (POM), we have investigated how and why conventional crystallization tends to cause deterioration of ductility in semicrystalline PLLA. Specifically, the POM study based on partially crystallized PLLA samples promotes the idea that the spherulitic crystals are mechanically weaker than glassy noncrystalline domains whose cohesive strength stems from the chain networking because of intermolecular uncrossability. By removing the large spherulitic crystal formation and inducing nanocrystal formation through melt-stretching of PLLA in its amorphous state, we identified a completely transparent crystalline state of PLLA that is extremely tough and resistant against heat.

Elastomeric anion exchange membranes (AEMs) were prepared by acid-catalyzed Friedel–Crafts alkylation of the polystyrene block of polystyrene- b -poly(ethylene- co -butylene)- b -polystyrene (SEBS) using bromoalkylated tertiary alcohols and triflic acid as a catalyst, followed by amination with trimethylamine. This simple one-step bromoalkylation allowed convenient control of both the degree of functionalization and cation tether length by changing the molar ratio and the structure of the bromoalkylated tertiary alcohol. The resulting quaternary ammonium-functionalized ionic triblock SEBS copolymers showed a microphase-separated morphology on the 35 nm length scale. A series of AEMs with different ion exchange capacities and ion tether lengths were systematically investigated by comparing swelling and anion conductivity. Because the SEBS AEMs showed high swelling and low dimensional stability in water due to the rubbery nature of SEBS, the hard segment PS units were cross-linked by 1,6-hexanediamine for practical use. The cross-linking of SEBS AEMs reduced water uptake significantly…

Supertoughened and heat-resistant poly( l -lactide) (PLLA)/elastomer blends were prepared by controlling the distribution of stereocomplex (sc) crystallites and the morphological change from sea–island structure to cocontinuous-like structure. To control the distribution of sc and the phase morphology, poly( d -lactide) (PDLA) was first blended with ethylene–vinyl acetate–glycidyl methacrylate elastomer (EVMG) to prepare EVMG/PDLA masterbatch comprising both free PDLA and EVMG- g -PDLA copolymers. The free and grafted PDLA would collaborate with PLLA to form sc in the matrix and at the interface, respectively, during subsequent melt-blending of the masterbatch with PLLA. Consequently, the in situ formed sc crystallites not only enhanced the interfacial adhesion but also increased the melt viscosity and crystallization rate of the PLLA matrix. The sc crystallites amount can be tuned via PDLA content to achieve designable properties and cocontinuous-like morphology, leading to highly improved toughness of PLLA; e.g., the notched impact strength of PLLA was increased by 84 times.…

Abstract This Perspective provides a current survey on the synthesis, self-assembly, and vast variety of applications possible from block copolymer (BCP) systems containing at least one segment of either poly(2-vinylpyridine), P2VP, or poly(4-vinylpyridine), P4VP. Strategies and insight toward P2VP or P4VP as the segment of choice and their influence on material properties, effective interaction parameters, and hosting a variety chemistries from small molecules to nanoparticles are provided. It is with hope that this work serves as a modern field guide to the versatility and utility of these chemistries for those new to or continuing research with these BCP systems.

An original design strategy for the preparation of polymers with a low dielectric constant is presented. The key to this design strategy is taking the most advantage of the secondary relaxation behavior of the polymer chains to obtain more free volume in the bulk, which can effectively reduce the dielectric constant of the polymer. By using this design strategy, we have successfully synthesized a novel polyimide TmBPHF with a pendant group that consists of a biphenyl unit attached to the meta-position of a phenyl ring that is part of a triaryl unit. The intrinsic k and dielectric loss values of the TmBPHF are 2.09 and 0.0012 at 10 kHz, respectively. More importantly, such outstanding low- k performance remains stable up to 300 °C. The excellent low- k performance of TmBPHF is mainly due to the secondary relaxation, especially the β relaxation, which occurs from the rotation of the pendant group. The TmBPHF film shows an ultralow moisture rate (∼0.17%), which is able to maintain the low- k property stability in different humid environments. Meanwhile, the TmBPHF film also shows…

In this research, we demonstrate effects of cross-link density on the bond-exchange property of trans-esterification-based vitrimers. In most of the past literature, the change of cross-link density is inevitably accompanied by the change of concentration of reactive groups, which disturbs extraction of pure effects of cross-link density. For pursuing fair comparison, the concentrations of free OH groups (i.e., the reactive group for trans-esterification) and catalysts are here kept the same among samples with different cross-link densities. This is achieved by using an amorphous low- T g polyester possesing multiple COOH side groups (PE-COOH), di-epoxy compound (1,4-butanediol diglycidyl ether), and mono-epoxy compound (butyl glycidyl ether). The di-epoxy acts as a cross-linker agent that reacts with COOH groups in PE-COOH, while the mono-epoxy reacts but does not work as the cross-linker, and thus the cross-link density can be tuned by the fraction of the di-epoxy compound ( f di-epoxy ) in the total epoxy compounds. Importantly, both epoxy compounds generate a free OH group in…

Abstract Conditions of rapid processing often drive polymers to adopt nonequilibrium molecular conformations, which, in turn, can give rise to structural, dynamical, and mechanical properties that are significantly different from those in thermodynamic equilibrium. However, despite the possibility to control the desired nonequilibrium properties of polymers, a rigorous microscopic understanding of the processing–property relations is currently lacking. In an attempt to stimulate progress along this topical direction, we focus here on three prototypical and apparently different cases: spin-coated polymer films, rapidly drawn polymer fibers, and sheared polymer melts. Inspired by the presence of common observations in the chosen cases, we search for order parameters as, for example, topological correlations and heterogeneities, which may allow characterizing the processing-induced behavior of polymers. We highlight that such approaches, necessitating concerted efforts from theory, simulations, and experiments, can provide a profound understanding leading to predictable and tunable…

Polylactide (PLA) films with an excellent balance of toughness and stiffness were realized by extensional stress efficiently. For the relatively low extensional stress, gauche–gauche conformers that originated from the oriented amorphous chains lead to super-toughening behavior. Among higher extensional stress, strain-induced orientation and crystallization act as the driving force of reinforcement. This mechanism is evidenced by the pronounced enhancement in the elongation at break from 16.9 up to 294.9% accompanying the variation yield strength from 45.3 up to 135.5 MPa. The highest elongation at break results from the early stretching stages, whereas the highest yield strength is obtained from a high draw ratio. More impressively, PLA films show temperature-invariant super-ductility and reinforcement at low temperatures (0 and −20 °C). This work provides a preferable and scalable method to fabricate competitive PLA materials, expanding the practical application of sustainable polymers served at a wide temperature range or in harsh environments.

It is highly desired yet challenging to develop hydrogels with a combination of excellent mechanical properties and good stability for potential applications. Here we report κ-carrageenan/polyacrylamide double-network (DN) hydrogels with remarkable mechanical performances and high stability in water, which are achieved using zirconium ions to further cross-link the first physical network of κ-carrageenan by forming coordination complexes. Thus, obtained DN hydrogels in equilibrated state with water content of 83–91 wt % were highly transparent and showed tensile breaking stress of 1.5–3.2 MPa, breaking strain of 300–2200%, Young’s modulus of 0.2–2.2 MPa, and tearing fracture energy of 0.4–18.5 kJ/m 2 . We found that dual-cross-linking of the κ-carrageenan network, i.e., the coiled-coil junctions and the metal-coordination bonds, was indispensable for the combined mechanical properties and stability of these gels. Essential reasons were rationally discussed based on the results of control experiments. The strategy we described should be applicable to other biopolymer-based hydrogels…

The reprocessing or recycling of cross-linked polymers by incorporating dynamic covalent cross-links has the potential to increase their usable lifetimes and reduce their environmental impact. Polyurethanes (PUs) are among the largest class of cross-linked polymers, making up 31% of the thermosetting materials market in 2012; however, their direct recycling into similar value materials is not well developed. We demonstrate that several Lewis acid catalysts mediate urethane exchange, likely via a dissociative mechanism, selectively and under mild conditions. Incorporating these catalysts into cross-linked polyether and polyester PUs with structures similar to commercial PU thermosets gives cross-linked materials that can completely relax stress in 100 s at temperatures as low as 140 °C. The dynamic polymers were reprocessed via compression molding to provide materials with similar cross-linking densities to as-synthesized materials. Because these systems are based on commercially available PU monomers and inexpensive Lewis acid catalysts, we anticipate that these findings will enable…

It remains a challenge to fabricate healable and recyclable polymeric materials with simultaneously enhanced tensile strength, stretchability, and toughness. Herein, we report a simple approach to fabricate high-performance polymer hydrogels that not only integrate high tensile strength, stretchability, and toughness but also possess self-healing and recycling capabilities. The polymer hydrogels are fabricated by mixing a positively charged polyelectrolyte mixture of poly(diallyldimethylammonium chloride) (PDDA)/branched poly(ethylenimine) (PEI) with a negatively charged polyelectrolyte mixture of poly(sodium 4-styrenesulfonate) (PSS)/poly(acrylic acid) (PAA) in an aqueous solution followed by molding, drying, and rehydration. The (PDDA/PEI)–(PSS/PAA) hydrogels with in situ-formed PDDA–PSS nanoparticles have a tensile strength, strain at break, and toughness of 1.26 ± 0.06 MPa, 2434.2 ± 150.3%, and 19.53 ± 0.48 MJ/m 3, respectively. The toughness of the (PDDA/PEI)–(PSS/PAA) hydrogels is ∼5.2 and ∼108 times higher than that of the PEI–PAA and PDDA–PSS hydrogels, respectively.…

Polymers that depolymerize end-to-end upon cleavage of their backbones or end-caps, often termed “self-immolative” polymers (SIPs), have garnered significant interest in recent years. They can be distinguished from other degradable and stimuli-responsive polymers by their ability to provide amplified responses to stimuli, as a single bond cleavage event is translated into the release of many small molecules through a cascade of reactions. Here, the synthesis and properties of the major classes of SIPs including poly(benzyl carbamate)s, poly(benzyl carbonate)s, poly(benzyl ether)s polyphthalaldehydes, polyglyoxylates, polyglyoxylamides, and poly(olefin sulfone)s are presented. In addition, their advantages and limitations as well as their recent applications in areas including sensors, drug delivery, micro- and nanopatterning, transient devices and composites, coatings, antibacterial, and recyclable plastics are described. Finally, the challenges associated with the development of new SIP backbones and their translation into commercial products are discussed.

Unorthodox luminogenic polymers without aromatic luminogens have attracted great interest in recent years; however, the low fluorescence efficiency is still a big drawback. In this paper, we synthesized a fluorescent hyperbranched polysiloxane with both carbonyl and vinyl groups ( P1 ). Surprisingly, it exhibited nontraditional intrinsic luminescence with the highest quantum yield up to 43.9% among the reported silica-containing hyperbranched fluorescent polymers to date. Reference oligomers P2 and P3, theoretical calculations, and transmission electron microscopy were employed to explore the fluorescence mechanism. The high fluorescence quantum yield is ascribed to the synergism of vinyl and carbonyl groups as well as the Si–O group-promoted through-space conjugation. Thus, the supramolecular hyperbranched polysiloxane was assembled by conjugation to increase the oscillator strength and decrease the band gap. Moreover, the solvent effect and pH dependency properties of P1 and its application as an Fe 3+ probe were also studied.

Stereocomplexation of enantiomeric poly( l -lactide)/poly( d -lactide) (PLLA/PDLA) chains opens up a great opportunity toward sustainable PLA engineering plastic with exceptional heat resistance and durability. However, the processing and applications of stereocomplex-type PLA (SC-PLA) are significantly blocked by its inferior melt stability (i.e., the weak melt memory effect in triggering complete SC crystallization, which makes it hard to obtain exclusive formation of SC crystallites in melt-processed products) and inherent brittleness. In this contribution, we demonstrate an unprecedented strategy to address these obstacles by one-pot reactive melt blending of the equimolar PLLA/PDLA blend with reactive poly(ethylene–methyl acrylate–glycidyl methacrylate) (E-MA-GMA) in the presence of catalyst, where both the stereocomplexation and the grafting of some PLLA/PDLA chains onto E-MA-GMA backbones take place simultaneously and competitively. Intriguingly, the E-MA- graft -PLA copolymer in situ formed can substantially improve the melt stability of SC-PLA matrix as compatibilizer, and…

Dynamic vulcanization has been demonstrated to be a versatile and efficient way of improving impact toughness of PLA. However, the existing vulcanization routes usually suffer from complicated presynthetic procedures and use of nonrenewable modifiers as well as markedly enhanced melt-viscosity. Herein, using both biomass-derived hydrogenated dimer acid (HDA) and an excess molar amount of l -lysine ethyl ester diisocyanate (LDI) as toughening monomers, we have developed a facile yet highly effective diisocyanate method for the design of fully bio-based PLA blends with excellent impact toughness and melt-flowability. The in situ formation and self-cross-linking of flexible biopolyamide (HDAPA) toughener, as well as its reactive compatibilization with PLA, were accomplished in a single melt-blending step. When incorporating the amount of HDAPA from 15 to 20 wt % or higher, the resulting blends evolved from the network-like morphology to the bi-continuous one with the cross-linked HDAPA network. At HDAPA content higher than 10 wt %, a sharp and persistent brittle–ductile transition…

Some amphiphilic block copolymers exhibit sol–gel transition in water upon heating, which affords a promising injectable thermogel in clinic and an interesting soft matter system. While ABA-, BAB-, and AB-type block copolymers have all been reported, little is known about the comparative study of supermolecular structures of these polymer types in the physical hydrogels, which hinders the understanding of the universal mechanism of structural changes during thermogelling. Herein, a thermogellable aqueous system of ABA triblock copolymer poly( d, l -lactide- co -glycolide)- b -poly(ethylene glycol)- b -poly( d, l -lactide- co -glycolide) was investigated by both experiments and computer simulations, with the corresponding AB diblock copolymer and BAB triblock copolymer as controls. The copolymers were synthesized via ring-opening polymerization, and their thermogelling behaviors in water were analyzed with transmission electron microscopy, three-dimensional dynamic light scattering, and so forth. Fluorescence resonance energy transfer, temperature-dependent 13 C NMR, and rheological…

This work demonstrates a series of epoxy vitrimers/polymers whose properties can be tuned by incorporating a catechol moiety as a part of the starting bisphenol motif. Glycidylation of the catechol containing bisphenol yielded a single-epoxide derivative ( E1 ) and a triepoxide ( E3 ) with tunable ratios through a two-step synthetic approach. E1 creates a dangling group (benzodioxane) within the network. By comparison, E3 with higher functionality ( n = 3) affords networks with improved cross-links. By controlling the E1 / E3 ratio of epoxy prepolymers, the properties of the resulting vitrimers/polymers can vary significantly. Polymers with the E1 / E3 ratio of ca. 6.1 (86:14) exhibited high strain, excellent self-healing properties, and fast stress relaxation that made them readily reconfigurable and could be used as hot-melt adhesives that permitted debonding on demand. Networks with higher content of E3 (e.g., E1 / E3 ratio of ca. 0.6 (36:64)), by comparison, exhibited significantly improved thermomechanical properties like tensile strength and modulus.

We present a conceptual framework for adding molecular details of chain extension and force-coupled bond dissociation to the Lake–Thomas model of tear energy in rubbery crack propagation. Incorporating data reported from single-molecule force spectroscopy experiments provides an estimate for the stored energy per bond at fracture of ∼60 kJ mol –1 for typical hydrocarbon polymers, well below the typical carbon–carbon bond dissociation energy in these systems. Opportunities to test and exploit the role of molecular extension and covalent bond scission in experimental systems are proposed.

Continuous-flow chemistry is emerging as an enabling technology for the synthesis of precise polymers. Recent advances in this rapidly growing field have hastened a need for a fundamental understanding of how fluid dynamics in tubular reactors influence polymerizations. Herein, we report a comprehensive study of how laminar flow influences polymer structure and composition. Tracer experiments coupled with in-line UV–Vis spectroscopy demonstrate how viscosity, tubing diameter, and reaction time affect the residence time distribution (RTD) of fluid in reactor geometries relevant for continuous-flow polymerizations. We found that the breadth of the RTD has strong, statistical correlations with reaction conversion, polymer molar mass, and dispersity for polymerizations conducted in continuous flow. These correlations were demonstrated to be general to a variety of different reaction conditions, monomers, and polymerization mechanisms. Additionally, these findings inspired the design of a droplet flow reactor that minimizes the RTD in continuous-flow polymerizations and enables the…

Bromoalkyl-functionalized poly(olefin)s were synthesized by copolymerization of 4-(4-methylphenyl)-1-butene with 11-bromo-1-undecene using Ziegler–Natta polymerization. The resulting bromoalkyl-functionalized poly(olefin)s were converted to quaternary ammonium-containing anion-conductive copolymers by reacting the pendant bromoalkyl group with trimethylamine or a custom-synthesized tertiary amine containing pendant quaternary ammonium moieties. Poly(olefin)-based AEMs with three cations per side chain showed considerably higher hydroxide conductivities, up to 201 mS/cm at 80 °C in liquid water, compared to that of samples with only one cation per bromoalkyl site (68 mS/cm, 80 °C, liquid water), likely due to phase separation in the triple-cation structure. More importantly, triple-cation side-chain poly(olefin) AEMs exhibited higher hydroxide conductivity under relative humidity conditions (50%–100%) than typical AEMs based on benzyltrimethylammonium cations. The triple-cation the triple-cation side-chain poly(olefin)-based AEM exhibited an ionic conductivity as high as 115 mS/cm…

We prepared acrylamide monomers with permethylated cyclodextrins (PM-CDAAmMe) or peracetylated cyclodextrins (PAc-CDAAmMe). PM-CDAAmMe and PAc-CDAAmMe are soluble in various hydrophobic liquid acrylate monomers, and they can form inclusion complexes with guest monomers such as adamantane or fluoroalkyl groups tethered to a vinyl residue. The bulk polymerization of the liquid acrylate monomers with the PM-CDAAmMe or PAc-CDAAmMe monomers and the guest monomers gave highly flexible and tough elastomers. Tensile tests on the obtained supramolecular elastomers showed fracture strains of over 800% and fracture energies that were 12 times larger than those of covalently cross-linked conventional elastomers, indicating that the host–guest cross-linking made the supramolecular elastomers quite tough. During the deformation process, the applied stress is dispersed into the supramolecular elastomers by dissociation and recombination of the reversible host–guest complex. Moreover, these host–guest complexes also allow the adhesion of fractured pieces of the supramolecular elastomers without…

Alkaline stability of anion exchange membranes (AEMs) is an essential requirement for the practical application of alkaline membrane fuel cells. In this study, we investigate the alkaline stability of Diels–Alder polyphenylenes (DAPPs) under various stability testing conditions. Structural analysis and membrane properties of the DAPPs indicated that different chemical structural changes of quaternized DAPPs occur depending on test conditions. Benzyltrimethylammonium-functionalized DAPPs degraded rapidly via nucleophilic benzyl substitution under a relatively mild condition at 80 °C with 4 M NaOH, whereas DAPPs with hexyltrimethylammonium (HTMA DAPP) exhibited much better alkaline stability under the same condition. While only cross-linking of unreacted alkyl bromides of HTMA DAPP occurred at 80 °C, we observed the degradation of the cations via β elimination after long-term testing. At higher temperature or reduced relative humidity (RH) conditions, for example, 160 °C with 8 M NaOH or RH 10% at 100 °C, the rapid degradation of the cation in HTMA DAPP occurred via nucleophilic…

The linear viscoelastic responses for a series of polyelectrolyte complexes, PECs, made from pairs of poly[3-(methacryloylamino)propyltrimethylammonium chloride], a polycation, and poly(sodium methacrylate), a polyanion, having various molecular weights were measured. Time–temperature superposition (TTS) for broad and narrow molecular weight distributions revealed entangled behavior at low salt concentration for the longer polyelectrolytes studied. All characteristic lifetimes were slowed by “sticky” dynamics of positive, Pol + and negative, Pol –, pairing. Time–temperature–salt doping superposition (TTSS) was achieved by considering the dual effects of increasing salt concentration on PECs: the partner lifetimes of Pol + and Pol – were inversely proportional to [NaCl], as was the population of Pol + Pol – pairs. Relaxation times for polymer partnering, entanglement, and reptation were measured directly on some systems. Whereas the intrinsic (in the absence of salt ions) lifetime for Pol + Pol – pairs was determined to be on the order of 1 × 10 –4 s, salt doping provided a faster,…

Easy preparation, high stability, prominent activity, and excellent recyclability are four key elements for high-performance heterogeneous photocatalysts. Developing covalent organic framework (COF)-based heterogeneous photocatalysts possessing all of these traits is highly challenging. In this study, we successfully synthesized an imine-linked BBO-COF by the “two-in-one” strategy featuring the above four merits. Highly crystalline and porous BBO-COF can be easily prepared in at least 11 different simplex solvents independent of their polarity and boiling points and even under an air atmosphere. Moreover, BBO-COF exhibits extraordinary chemical stability and photostability in strong acid (12 M HCl), corrosive base (12 M NaOH), and visible light for 7 days. Furthermore, BBO-COF exhibited prominent photocatalytic activity toward oxidative hydroxylation reaction of arylboronic acids with excellent substrates tolerance and reusability. This “two-in-one” design strategy open a new avenue for facile constructing novel functional COFs with tailor-made properties.

Herein, we designed a series of well-defined bottlebrush-like macromolecules with short and densely grafted poly(ethylene oxide) (PEO) side chains through several controlled radical polymerization methods to prepare solid polymer electrolytes (SPEs). The PEO side chains can work as sensational conductors for lithium salt. Moreover, these macromolecules own lithium cations linked on the backbone, which can facilitate the transport of Li ions and thus improve the lithium-ion transference number. The prepared SPE exhibits a considerable ion conductivity of 4.49 × 10 –5 S cm –1 at 30 °C owing to its specific structure. Furthermore, the high capacity and excellent cycle performance demonstrate the foreseeable application of the obtained electrolyte in the lithium-ion battery.

The role of water in the tribochemical mechanisms of ultralow wear polytetrafluoroethylene (PTFE) composites was investigated by studying 10 and 20 wt % polyether ether ketone (PEEK)-filled and 5 wt % αAl 2 O 3 -filled PTFE composites. These composites were run against stainless-steel substrates in humidity, water, and dry nitrogen environments. The results showed that the wear behavior of both composites was significantly affected by the sliding environment. Both composites achieved remarkably low wear rates in humidity because of tribochemically generated carboxylate end groups that anchored the polymer transfer films to the steel substrate. In nitrogen, PTFE–PEEK outperformed PTFE−αAl 2 O 3 because of polar carbonyl groups on PEEK, which increased the surface energy of PEEK, aiding it in adhering to the substrate and resulting in a transfer film. Both composites in water exhibited high wear. The water oversaturated the functional groups at the end of the polymer chains and prevented the formation of a transfer film.

Carbon dioxide has emerged as a new and innovative “trigger” for stimuli-responsive materials. It is abundant, inexpensive, nontoxic, and environmentally benign. Carbon dioxide-switchable polymers are a class of stimuli-responsive polymers for which CO 2 is a trigger used to reversibly switch the polymer properties, typically from hydrophobic (in the absence of CO 2 ) to hydrophilic (in the presence of CO 2 ). Recent years have witnessed a surge in interest in polymers incorporating CO 2 -switchable moieties, where the change in properties upon switching has enabled the development of a new generation of stimuli-responsive materials, including switchable particles, surfaces, catalysts, and a plethora of nonspherical polymer particle morphologies such as wormlike micelles, vesicles, tubules, and gels. This Perspective introduces the principles of CO 2 -switching, highlights recent major developments, and presents our personal insights into challenges with commercialization of CO 2 -switchable polymers and future opportunities for new research directions and materials applications.

Synthetic polypeptides, the analogues of natural proteins, are important biomaterials that have found broad biomedical applications. Ring-opening polymerization of N -carboxyanhydrides offers a reliable way to prepare high-molecular-weight polypeptides in large scale with diverse side-chain functionalities. The past two decades have seen significant advances in the polypeptide field, with the development of various controlled polymerization methodologies, the deeper understanding on secondary structures, and the discovery of new assembly behaviors and applications. In this Perspective, we highlight several key advances in the chemical synthesis and materials application of synthetic polypeptides and discuss promising future directions in this area.

Oppositely charged polyelectrolytes in solution spontaneously associate into hydrated complexes or coacervates, PECs. The morphology, stability, and properties of PECs depend strongly on their ion content, which moderates the “sticky” reversible interactions between Pol + and Pol – oppositely charged repeat units. Here, it is shown that the distribution of ions between a PEC and the aqueous solution in which it is immersed is accurately predicted by the Donnan equilibrium. For ideal, stoichiometric mixing of polyelectrolytes, corresponding to an enthalpy of complexation Δ H PEC → 0, the salt, MA, concentration inside the PEC, [MA] PEC, is equal to the solution salt concentration, [MA] s . Isothermal calorimetry measurements along a Hofmeister series show that if mixing is exothermic, [MA] PEC < [MA] s, while for endothermic association of Pol + and Pol –, [MA] PEC > [MA] s . A set of simple self-consistent expressions illustrate PEC salt response without consideration of net Coulombic or electrostatic forces between charged species. Δ H PEC exactly predicts deviations from ideal…

High-temperature-resistant dielectric films, the heart of energy storage components in film capacitors, are key elements to ensure that the capacitors operate properly in harsh environments. Herein, a kind of flexible thermal-resistant poly(arylene ether nitrile) (TR-PEN) dielectric film with controllable high-temperature resistance is fabricated through post self-crosslinking of PEN at different temperatures for different times. The dielectric films can present extremely high thermal stability ( T g > 370 °C), a long service life at 300 °C (4.5 × 10 4 min, 95 wt % of the residual weight), and a low temperature coefficient of dielectric constant from 50 to 300 °C (<5 × 10 –4 °C –1 ). The retention of the energy density of TR-PEN350 at 300 °C is higher than 80% compared with that at room temperature. These results indicate that the TR-PEN films can be used over a long term as capacitor films at temperatures up to 300 °C. Besides, the simple and controllable fabrication technique for the TR-PEN can be easily industrialized.

Carbosilane side chain-equipped isoindigo–bithiophene semiconducting polymers (PII2T) have been designed and synthesized for stretchable electronics applications. Systematically tailoring the length and branch position of carbosilane side chains (C6 to C10) offers an effective route to optimize charge-transport behavior and improve the mechanical properties of semiconducting polymer thin films. The basic polymer properties, surface morphology, electrical characteristics, and strain-dependent performance of polymers with various lengths of carbosilane side chains were explored. The series of polymers exhibited a field-effect mobility over 2 cm 2 V –1 s –1, and an odd–even effect was observed relating to the length of side chains. On the other hand, when the longer side chain was incorporated, a lower thin-film modulus was reached because the extended side chain can dilute the volume of the rigid polymer backbone and open up the space between polymer chains (i.e., larger lamellar spacing). Surprisingly, PII2T-C10 thin films possess desirable electrical and mechanical properties,…

In this study, the effect of different process conditions on the material properties of a single UV-cured layer of methacrylate resin, typically used in the stereolithography (SLA) process, is assessed. This simplified approach of the SLA process gives the opportunity to study the link between process conditions and mechanical properties without complicated interactions between different layers. Fourier-transform infrared analysis is performed to study the effect of light intensity, curing time, and initiator concentration on the monomer conversion. A model is developed based on the reaction kinetics of photopolymerization that describes and predicts the experimental data. The effect of curing time and light intensity on the glass-transition temperature is studied. A unique relation exists between conversion and glass-transition temperature, independent of the light intensity and curing time. Tensile tests on UV-cured resin show an increase in yield stress with increasing curing time and a linear relation between glass-transition temperature and yield stress. However, a lower light…

The dynamic mechanical and rheological behavior of polyelectrolyte coacervates and complex precipitates is of interest for many applications ranging from health to personal care. Hydration is an important factor, but its effect on the dynamic properties of polyelectrolyte complexes (PECs) is poorly understood. Here, we describe the dynamic behavior of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) complex precipitates at varying relative humidity values and temperatures using both dynamic mechanical analysis (DMA) and all-atom molecular dynamics simulations. To mirror the experimental system via simulation, the water content within the PEC is measured and used as the parameter of interest rather than relative humidity. In the experimental DMA, the modulus decreases with both increasing water content and temperature. The data are superimposed into a super master hydrothermal curve using the time–temperature superposition principle and the time–water superposition principle for the first time. The temperature-dependent shift factor ( a T ) follows an Arrhenius…

A theory-based understanding of perfluorosulfonic acid (PFSA) class of ionomer aggregation in different solvents is crucial for transitioning from empirically guided fabrication of high-performance electrodes of fuel cells and other electrochemical devices. Considering Nafion as a model PFSA ionomer, herein, we report the details of its molecular assembly in various solvents (dielectric constant, ε r = 2.38–109) using fully atomistic molecular dynamics simulation. In contrast to the long-held view of elongated aggregate structure of Nafion, our study shows that a variety of structures exist depending on solvent polarity. The work revealed the existence and clustering of ion pairs in Nafion dispersions, a phenomenon reported for the first time for PFSA ionomer dispersions. We introduce a new aggregation phase diagram for Nafion in different solvents: (a) type-I aggregate: aggregates with ionomer chains physically cross-linked via ionic clusters in low dielectric solvents [toluene, isopropyl alcohol (IPA), ethanol, glycerol]; (b) type-II aggregate: lamella like two-dimensional…

Development of mechanically strong and adhesive hydrogels with self-recovery and self-healing properties is important for many applications but has proven to be very challenging. Here, we reported a double-network design strategy to synthesize a fully physically cross-linked double-network (DN) hydrogel, consisting of the first gelatin network and the second poly( N -hydroxyethyl acrylamide) network where both networks were mainly cross-linked by hydrogen bonds. The resultant gelatin/pHEAA hydrogels exhibited high mechanical property (tensile stress of 1.93 MPa, tensile strain of 8.22, tearing energy of 4584 J/m 2 ), fast self-recovery at room temperature (toughness/stiffness recovery of 70.2%/68.0% after 10 min resting), and good self-healing property (self-healed tensile stress/strain of 0.62 MPa/3.2 at 60 °C for 6 h). More importantly, gelatin/pHEAA hydrogels also exhibited strong surface adhesion on different hydrophilic solid surfaces, as indicated by high adhesion energy (i.e., interfacial toughness) of 645 J/m 2 on glass, 867 J/m 2 on Al, 702 J/m 2 on Ti, and 579 J/m 2 on…

Multiple computational and experimental techniques are used to understand the nanoscale morphology and water/proton transport properties in a series of sulfonated Diels–Alder poly(phenylene) (SDAPP) membranes over a wide range of temperature, hydration, and sulfonation conditions. New synthetic methods allow us to sulfonate the SDAPP membranes to much higher ion exchange capacity levels than has been previously possible. Nanoscale phase separation between the hydrophobic polymer backbone and the hydrophilic water/sulfonic acid groups was observed for all membranes studied. We find good agreement between structure factors calculated from atomistic molecular dynamics (MD) simulations and those measured by X-ray scattering. With increasing hydration, the scattering ionomer peak in SDAPP is found to decrease in intensity. This intensity decrease is shown to be due to a reduction of scattering contrast between the water and polymer and is not indicative of any loss of nanoscale phase separation. Both MD simulations and density functional theory (DFT) calculations show that as hydration…

Per- and poly fluorinated alkyl substances (PFASs), notably perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid, contaminate many ground and surface water resources and are environmentally persistent. Furthermore, there are many other PFASs in use that are persistent and contaminate fresh water resources. A polymer consisting of β-cyclodextrin (β-CD) cross-linked with decafluorobiphenyl ( DFB-CDP ) has shown promise for sequestering PFOA at environmentally relevant concentrations, though its efficacy to remove other PFASs from water has not yet been explored. Additionally, although the DFB-CDP was designed to sequester PFASs on the basis of favorable fluorous interactions, the rationale for its relatively high affinity for PFOA compared to other previously synthesized β-CD polymers remains unknown. In this study, we explored cross-linker chemistry as a potential determinant of PFAS affinity for β-CD polymers. We synthesized three DFB-CDP derivatives with varying degrees of phenolation in the cross-linker (to evaluate effects of polymer surface charge) along with two β-CD…

Diketopyrrolopyrrole (DPP)-based donor–acceptor conjugated polymers, with increasing amount of weak H-bonding units, namely 2,6-pyridinedicarboxamide (PDCA), inserted as end groups in alkyl side chains were prepared and investigated. In contrast to previously reported DPP polymers containing PDCA units as conjugation breakers along the polymer backbone, PDCA in the alkyl side chains readily produced almost quantitative formation of intermolecular H-bonding even at low PDCA unit content (<10 mol %) as shown by Fourier transform infrared spectroscopy (FTIR). The efficient intermolecular H-bonding was further supported by the appearance of a pronounced vibronic shoulder in the UV–vis spectra and a reduction of interlamellar spacing (from 24.02 to 22.87 Å) compared to the neat DPP polymer. Increasing mol % of PDCA units in side chains of DPP conjugated polymers also has a clear effect on the thermal and mechanical properties of the films as investigated by dynamic mechanical analysis (DMA). Polymers with a high loading of PDCA showed a linear increase in both tan delta intensity and…

The growing trend in antimicrobial resistance is a potential threat to our society. Due to this, the development of new antimicrobial compounds is urgently required. High-throughput compositional analysis, combined with recent advances in polymerization protocols, allows for rapid production of potentially antimicrobial compounds with minimal expertise. This can provide the impetus for correlating activity with composition and functionality. In this study, we have used high-throughput photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization to test the combinations of seven different monomers using 120 different formulations against three distinct bacterial species: Gram-negative Pseudomonas aeruginosa, Gram-positive Staphylococcus aureus, and Mycobacterium smegmatis . Through variations in composition, we have demonstrated the potential of high-throughput PET-RAFT for highly reproducible products, as well as simultaneous testing of multiple variables. Results indicate that primary amines work best against Gram-negative P. aeruginosa,…