The 50 most-cited articles in Macromolecules, 2021

The field of polymer science recently celebrated its 100th anniversary. The excellent material properties of a wide range of plastics (generally used as a synonym for polymers) have opened many new opportunities in lightweight packaging, agricultural fields for improved crop production, cosmetics, detergents, and more advanced applications, among many others. During the journey of polymer science, various topics have attracted particular attention because they have promised technological advancements or new challenges relevant to the needs of the time. Each has led to intense research, development, and scientific discussion. Since the early 1970s, awareness has increasingly been drawn to the pollution arising from plastic disposal and the related material solutions. The topic has become urgent in the past few years, becoming the focus of widespread interest and debate beyond polymer science boundaries. An important question is, can we hold the material properties of polymers responsible for the plastic pollution issue? In this Perspective, we consider the role and benefits of…

Low intrinsic thermal conductivity coefficients (λ) of polyimide (PI) films limit their application in heat-prone flexible electronics. By optimizing the liquid crystal range to fit the curing temperature, novel kinds of intrinsically highly thermally conductive liquid crystalline PI (LC-PI) films are fabricated. When the molar ratio of 4,4′-diaminodiphenyl ether (ODA) to 1, 4-bis(4-aminophenoxy)benzene (TPE-Q) is 1:3 (sample no. IV), liquid crystal range of pre LC-PI IV films is 272∼388 °C, including curing temperature of 350 °C. The LC-PI IV films cured in liquid crystal range retain liquid crystal texture to room temperature, and the in-plane λ (λ ∥ ) and through-plane λ (λ ⊥ ) at room temperature reach 2.11 W/(m·K) and 0.32 W/(m·K), respectively, significantly higher than λ ∥ (0.77 W/(m·K)) and λ ⊥ (0.15 W/(m·K)) of LC-PI I films which are cured out of the liquid crystal range. Meanwhile, LC-PI IV films possess excellent mechanical and thermal properties, showing application prospects in high-heating flexible electronics.

Constructing stable co-continuous morphology of commercial immiscible polymer blends remains an ongoing challenge in terms of complex presynthetic routes, multiple parameter dependency, and intrinsic instability of phase morphology. Herein, we demonstrate a full biodegradable polymer blend, poly(lactic acid) (PLA) and poly(butylene adipate- co -terephthalate), where hitherto inaccessible co-continuous with asymmetric compositions (70/30) can be obtained with the assistance of interfacial stereocomplex crystallites (i-SCs) through reactive blending. By taking full advantages of this unprecedented compatibilizer, nanostructured co-continuous blends with synergistically enhanced comprehensive performance are achieved. First, due to the “rigid” i-SC, co-continuous morphology is induced through a simple melt blending procedure; second, considerable augmentation of the crystallization rate of the PLA matrix is accomplished on account of the in situ formed nucleation agent ( i.e., i-SC); third, a super toughened material with simultaneously enhanced tensile strength, ductility, and impact…

Vitrimers constitute a fascinating class of polymer materials that make the link between the historically opposed 3D networks (thermosets) and linear polymers (thermoplastics). Their chemical resistance, reshaping ability, and unique rheological behavior upon heating make them promising for future applications in industry. However, many vitrimers require the use of high catalyst loadings, which raises concerns for their durability and limits their potential applications. To cope with this issue, internal catalysis and neighboring group participation (NGP) can be used to enhance the reshaping ability of such materials. A few studies report the effect of activating groups on the exchange reactions in vitrimers. Nevertheless, knowledge on this topic remains scarce, although research on vitrimers would greatly benefit from NGP already known in organic chemistry. The present Perspective presents the different types of exchangeable bonds implemented in vitrimers and discusses chemical groups known to have or potentially capable of an enhancing effect on exchange reactions. This analysis…

Synthetic hydrogels with hydrophobic interactions, which show excellent mechanical performance and good anti-swelling ability in saltwater, have great potential in various industries, such as soft robots, 3D printing, and wearable sensors. Normally, hydrophobic molecules inside a hydrophobic hydrogel tend to aggregate to form a large hydrophobic domain, leading to a phase separation phenomenon because water is a poor solvent of the hydrophobic domain. This aggregation, however, inhibits the adhesion of the hydrophobic hydrogel to various dry materials and thus limits its application in device and sensor industries. In this study, we report the synthesis of hybrid hydrogels with ionically and hydrophobically cross-linked networks. This novel hybrid hydrogel can strongly adhere to various substrates, such as glass, polypropylene, silicone, wood, and polytetrafluoroethylene, with a maximum adhesion strength measured to be 100 kPa. Meanwhile, this hybrid hydrogel can be stretched beyond 8–10 times of its initial length. We attribute this observed strong adhesion and high toughness…

Polymer adhesion is ubiquitous in both the natural world and human technology. It is also a complex multiscale phenomenon, such that the solution of adhesion problems requires a convergence of chemistry, physics, and engineering. In this Perspective, we provide an overview of some of the fundamental concepts that have emerged in the field of polymer adhesion, discuss recent work, and identify challenges in three specific areas: (a) theories and simulations, with an emphasis on problems involving chain scission; (b) experimental methods for measuring forces and characterizing interfaces at the molecular scale; and (c) strategies inspired by living organisms to generate underwater adhesion.

Forming robust associative interactions has been an effective strategy for the design of tough hydrogels. However, the role of associative interactions in the dynamics of hydrogels still remains elusive. Here, we report a series of poly(acrylamide- co -methacrylic acid) hydrogels with moderate water contents and excellent mechanical properties that are facilely synthesized by free-radical copolymerization. The mechanical properties of these hydrogels vary with the feeding molar fraction of acrylamide ( f am ). The gels with f am of 0.2–0.35 exhibit high toughness and good stability in water, which is related to the dense hydrogen bonds and relatively high segment rigidity of the matrix. Dynamic modulus spectra extended by time-temperature superposition and relaxation measurements indicate that the gels undergo glassy-to-rubbery transition with decreased frequency, and the robust hydrogen bonds, whose density is 1–3 times that of entanglements, retard chain disentanglement and contribute to the plateau modulus of the gels at low frequencies. The activation energy for the dissociation…

We describe the first examples of benzoxazine (BZ)-linked tetraphenylethylene (TPE)- and pyrene (Py)-containing conjugated microporous organic polymers (CMPs)—TPE-TPE-BZ and Py-TPE-BZ, respectively—having high surface areas and pore volumes, prepared through multistep syntheses (Schiff base, reduction, Mannich, and Sonogashira–Hagihara coupling), and their solid-state transformations through ring-opening polymerizations (ROPs) to give BZ linkages have never been reported previously. The chemical structures and, particularly, the presence of BZ units in the TPE-TPE-BZ and Py-TPE-BZ CMPs were confirmed through Fourier transform infrared and solid-state nuclear magnetic resonance spectroscopic analyses. The TEP-TPE-BZ CMP had a high BET specific surface area and a high total pore volume (325 m 2 g –1 and 0.69 cm 3 g –1, respectively). Because BZ units can undergo ROP through simple thermal treatment without the need for a curing agent or catalyst, the CMPs readily underwent one-step chemical transformations in the solid state to form new CMPs featuring phenolic OH and Mannich bridges…

The development of next-generation polymer-based electrolytes for energy storage applications would greatly benefit from a deeper understanding of transport phenomena in these systems. In this Perspective, we argue that the Onsager transport equations provide an intuitive but underutilized framework for analyzing transport in polymer-based electrolytes. Unlike the ubiquitous Stefan-Maxwell equations, the Onsager framework generates transport coefficients with clear physical interpretation at the atomistic level and can be computed easily from molecular simulations using Green-Kubo relations. Herein we present an overview of the Onsager transport theory as it applies to polymer-based electrolytes and discuss its relation to experimentally measurable transport properties and the Stefan-Maxwell equations. Using case studies from recent computational work, we demonstrate how this framework can clarify nonintuitive phenomena such as negative cation transference number, anticorrelated cation-anion motion, and the dramatic failure of the Nernst-Einstein approximation. We discuss how…

Vitrimers are expected to combine features of thermosets and thermoplastics but their continuous reprocessing is still a challenge; poly(ethylene terephthalate) (PET) has been widely used in our daily life, while its cross-linking upcycle contradicts with processability. Herein, we combined polyol with a tertiary amine structure and diepoxy to transform PET to continuously reprocessable vitrimers through an industrial twin-screw extruder. The cross-linking of PET was determined by swelling and rheology experiments, and the vitrimer feature was characterized by stress relaxation and oscillatory frequency sweep experiments. Creep resistance and mechanical properties of PET vitrimers were improved greatly relative to neat PET. Meanwhile, PET vitrimers exhibited excellent reprocessability via compression, extrusion, and injection molding suitable for industrial production. According to this work, any thermoplastics containing ester bonds should be able to be upgraded to vitrimers for additional advantages such as creep resistance, dimensional stability, insolubility, etc., without…

Model polydimethylsiloxane telechelic vitrimers with dynamic boronic ester bonds were synthesized to investigate the viscoelastic properties of dynamic networks with extremely low T g via multiple rheological approaches. Frequency sweeps and stress relaxation tests, conducted at more than 120 °C above T g, show the anticipated Arrhenius behavior of relaxation time with inverse temperature and give the same activation energy for a fixed molecular weight, obtained using a variety of analysis methods. Time–temperature superposition demonstrates that the flow regime is thermorheologically simple, while the modulus of the plateau regime increases with increasing temperature, consistent with a conserved network topology and associative bond exchange. As relaxation times decrease, the rubbery plateau modulus increases, indicating a decoupling of terminal dynamics from mechanics. Below 40 °C, a second Arrhenius regime with lower activation energy emerges, which is attributed to a transition from relaxation dominated by reaction exchange kinetics to relaxation dictated by local polymer…

Using this theoretical approach, we explore the influence of molecular structure and temperature on vitrimer linear viscoelasticity. We observe that vitrimers with uniform and random cross-link distributions exhibit larger viscosities and relaxation times than gradient and blocky types. Polydimethylsiloxane vitrimer (which has a flexible backbone) shows an Arrhenius temperature dependence for viscosity, while polystyrene vitrimers (which has rigid backbones) are only Arrhenius at high temperatures. During stress relaxation, the short time dynamics represent monomer friction, while the long time dynamics encompass a combination of network strand relaxation and cross-link exchange. Because of the different temperature dependences of the processes, time-temperature superposition fails. We also show that the effective rheological activation energy can be estimated a priori using only the cross-link exchange activation energy and the backbone Williams-Landel-Ferry parameters. (Submitted to Macromolecules)

Conjugated polymers are a promising candidate for large-area stretchable electronics because of their tunable electrical and mechanical properties, light weight, and low-cost solution processing. Significant research progress has been achieved in stretchable polymer electronics by synthesizing novel conjugated polymer materials and designing new device geometries. However, the inherent competition between high charge mobility and good mechanical compliance has long existed for conjugated polymer films. In this Perspective, we will provide an understanding of how to balance the electrical properties and mechanical properties from the point of view of multiple length scale microstructures of conjugated polymers. After a brief introduction of microstructure features, charge transport, and mechanical properties in thin films, we focus on how to design the percolation morphology with the aggregates, tie chains, and amorphous phase via controlling solution preaggregation and film-formation dynamics. Furthermore, the rational transfer of film morphology from small-area coating to…

High Resolution Image Download MS PowerPoint Slide Polyelectrolyte complex coacervates of homologous (co)polyelectrolytes with a near-ideally random distribution of a charged and neutral ethylene oxide comonomer were synthesized. The unique platform provided by these building blocks enabled an investigation of the phase behavior across charge fractions 0.10 ≤ f ≤ 1.0. Experimental phase diagrams for f = 0.30–1.0 were obtained from thermogravimetric analysis of complex and supernatant phases and contrasted with molecular dynamics simulations and theoretical scaling laws. At intermediate to high f, a dependence of polymer weight fraction in the salt-free coacervate phase ( w P,c ) of w P,c ∼ f 0.37±0.01 was extracted; this trend was in good agreement with accompanying simulation predictions. Below f = 0.50, w P,c was found to decrease more dramatically, qualitatively in line with theory and simulations predicting an exponent of 2/3 at f ≤ 0.25. Preferential salt partitioning to either coacervate or supernatant was found to be dictated by the chemistry of the constituent…

Conventional polyurethane (PU) thermosets are built by nonrenewable petrifaction resources and are burdensome to be recycled, reprocessed, and reshaped in virtue of their permanent covalent cross-links. Here, for the first time, we synthesized a novel acetal precursor from abundant and renewable furfural and produced PU covalent adaptable networks (CANs) combining excellent reprocessability, controlled degradability, as well as outstanding high-temperature creep resistance. The properties could be further adjusted from soft to hard via the Diels–Alder reaction with bismaleimide. In addition, a small-molecule model study and theoretical calculations with density functional theory (DFT) certified that acetal linkage substituted by a furan ring (or a benzene ring) went through a dissociative exchange reaction via a carbocation mechanism. We envisage that the uncovering of the catalyst-free metathesis of acetal linkage substituted by a furan ring enriches the dynamic chemistry of acetal and greatly promotes the development of acetal-based CANs. Meanwhile, the acetal CANs provide a prime…

Covalent adaptable networks (CANs) represent a transition material combining favorable features of thermosets and thermoplastics. However, it is still a huge challenge to simultaneously achieve fast reprocessability and high performance for CANs. Here, we designed catechol-based acetal CANs to achieve continuous reprocessing without sacrificing thermal and mechanical properties. A small-molecule model study demonstrated the significantly accelerated acetal exchange by neighboring group participation (NGP) of phenolic hydroxyl. Using this internally catalyzed acetal chemistry, a series of CANs with a broad range of properties were simply prepared from bio-based epigallocatechin gallate (EGCG) and tri(ethylene glycol) divinyl ether (TEGVE) via one-step “click” cross-linking without using catalysts or releasing small-molecule byproducts. The dynamic nature of the CANs was confirmed via stress relaxation and multiple recycling methods including extrusion. While the dense cross-link density and high rigidity of the network provided high solvent resistance and mechanical properties. This…

The discovery of the Frank–Kasper σ phase in diblock copolymer and tetrablock terpolymer melts catalyzed a renewed interest over the past decade in understanding particle-forming phases in block polymer systems. This Perspective provides a concise overview of the Frank–Kasper phases seen to date in block polymers (A15, σ, and the C14 and C15 Laves phases) and mechanisms known to produce them: conformational asymmetry in neat diblock copolymer melts, interfacial segregation effects in diblock copolymer blends, particle swelling in diblock copolymer/homopolymer blends, and matrix segregation effects in neat tetrablock terpolymer melts. While a qualitative understanding of the emergence of Frank–Kasper phases in block polymer systems has been achieved, a number of outstanding questions remain, in particular those arising from the low degree of polymerization used in experiments, nonequilibrium effects during thermal processing, and the large design space available in blends and multiblock systems. This Perspective discusses potential avenues for future research related to these areas…

Recently, the research activity in the field of the stereospecific polymerization of conjugated dienes has focused on the use of catalysts based on organometallic complexes having a well-defined structure and containing ligands of various types (e.g., phosphines, bis-imines, and pyridylimines), since it has been observed that the nature of the ligand, as expected on the basis of the diene polymerization mechanism proposed several years ago by Porri, is able to exert a strong influence on the polymerization regio- and stereoselectivity, on the polymer molecular weights and their distribution, and in some cases to impart a living feature to the catalysts themselves. In this Perspective we highlight the most recent results obtained in this field and discuss possible future developments in this area.

Biomass-derived materials possess vast potential for material science and industry in the next decades. Dwindling fossil resources and an increasing environmental awareness increase the demand for sustainable feedstock-based alternatives. In addition to natural rubber ( cis -1,4-polyisoprene), the class of terpenes offers a large variety of renewable monomers, like the 1,3-diene monomers β-myrcene and β-farnesene. Living anionic polymerization of bio-based 1,3-diene monomers enables the synthesis of well-defined, high molecular weight block- and statistical copolymers with unique control over molecular weights, polymer architecture, and polydiene microstructure. The resulting materials can be used for a variety of applications. For instance, polyfarnesene has been introduced as an additive in tire mixtures and replaces fossil resource-based rubbery building blocks in styrenic thermoplastic elastomers. In addition, the unsaturated nature of polymyrcene and polyfarnesene renders them accessible for functionalization by a variety of postmodification reactions, which results, for…

Vitrimer materials that obscure the line between ″thermoset″ and ″thermoplastic″ have already attracted a great deal of interest since they exhibit dynamic properties of stress relaxation or reprocessability without losing their permanently cross-linked networks. While much research has been aimed at developing new exchangeable dynamic bonds to broaden the scope, there is a dearth of effort in exploring the key factors that influence the dynamic properties of vitrimer materials. These explorations are not only useful in the exact design and synthesis of vitrimer materials but also quite important in their theory establishment and computer simulation. However, only a few ways including catalyst control have been confirmed to be the key factors in controlling the dynamic properties of classical vitrimer materials like the polyester-based epoxy vitrimer. Here, we proposed that the density of exchangeable ester bonds (υ) in networks also has a crucial role in adjusting the dynamic properties of epoxy vitrimers. Four polyester-based epoxy vitrimers were synthesized from the tricarboxylic…

Ring-opening copolymerization (ROCOP) of epoxide/anhydride/CO 2 for the synthesis of CO 2 -based copolymers is a promising process, because CO 2 is an abundant, nontoxic, and low-cost monomer and has been considered as one of the most attractive renewable C1-resources. Although considerable progress has been made in the past decade, synthesis of block copolymers from mixed monomers through a one-pot polymerization is still a great challenge. Selective terpolymerizations of epoxide/anhydride/CO 2 have been exclusively catalyzed by metal-based catalysts. There has been no success in using organocatalysts that can bridge ROCOP of epoxide/CO 2 and ROCOP of epoxide/anhydride to produce CO 2 -based block copolymers, although a few organocatalysts have been independently used for either ROCOP. In this contribution, the PPNCl/TEB Lewis pair was selected as the organocatalyst to conjoin these two processes for one-pot ROCOP of epoxide/anhydride/CO 2 . NMR investigations of cyclohexene oxide (CHO)/phthalic anhydride (PA)/CO 2 copolymerization revealed that polyester production was much faster…

The incorporation of weak and reversible dynamic bonds is a key strategy for enhancing the mechanical properties of polymers. However, the quantitative relationship between the molecular characteristics of dynamic bonds and the resultant macroscopic mechanical properties has not been elucidated fully. Here, we systematically investigated the effect of hydrogen bonds (H-bonds) on the mechanical properties of polymer networks. Three types of H-bonding groups were homogeneously introduced into identical polymer networks with a nearly ideal structure. The number density and thermodynamic characteristics, that is, binding energy of H-bonds, controlled properties such as stiffness and stretchability of polymer networks. On the other hand, the activation energy of chain motion and the rate of recovery after mechanical loading depended on the kinetic aspects of the H-bonds, that is, the energy barriers to dissociation and re-association. These findings provide a principle for selecting appropriate dynamic bonds to design materials with desired properties.

The solution state of a conjugated polymer plays a critical role in the film morphology and charge carrier mobility of the solution process organic field-effect transistors (OFETs). However, it remains challenging to establish the relationship between solution-state and solid-state morphologies. Herein, we report an effective strategy to control the film formation process and film morphology by regulating the solution state, specifically, through the interaction between the branched side chain of poly[4-(4,4-bis(2-octyldodecyl)-4 H -cyclopenta[1,2- b:5,4- b ′]dithiophen-2-yl)- alt -[1,2,5]-thiadiazolo[3,4- c ]pyridine] (PCDTPT-ODD) and the nonpolar solvent. A precise adjustment of the solution state using the Hansen solubility parameter distance ( R a ) has been achieved. Compared to the polar solvent, ordered aggregates were formed in the solution formed by nonpolar solvents. Interestingly, a highly ordered chain arrangement was observed in solutions with suitable R a ’s (8.2 and 9.4 MPa 1/2 for hexane and isooctane, respectively). Importantly, solution-state aggregation was…

The advances in understanding the dynamics of both the physically and chemically reversible networks are summarized from a rheological perspective. The focus is placed on linear viscoelasticity, which is related to thermodynamics and reversible kinetics at quasi-equilibrium states. We first explain basic assumptions and predictions of the reversible-network theories, including the reversible gelation, sticky-Rouse, and sticky-reptation theories. We then summarize the dynamic features of the state-of-the-art materials that have not been fully incorporated into the current theoretical framework: (1) cooperative motion of the nearby stickers significantly increases the dissociation energy of physically reversible networks, (2) the degree of gelation becomes highly T -dependent when the Gibbs free energy approaches the thermal energy for both the physically and chemically reversible networks, and (3) the exchange rate of dynamic cross-links of chemically reversible networks is reaction-controlled and depends on the chemical species and concentrations of the reactants. Finally, some…

Stretchable conductive elastomers play an irreplaceable role in flexible electronic devices. However, stretchable conductive elastomers are usually soft and susceptible to damage. In this study, inspired from skin, highly stretchable and elastic conductive elastomers integrated with damage resistance, damage tolerance, and healability are fabricated by loading ionic liquids (ILs) within the polyurethane (PU) elastomers of the multiblock polymers of poly(dimethylsiloxane) (PDMS)/polycaprolactone (PCL) coordinated with Zn 2+ ions. The mechanically robust conductive elastomer, with a tensile strength of ∼15.2 MPa and a stretchability of ∼2668%, has a satisfactory ionic conductivity of 2.9 × 10 –4 S cm –1 . The conductive elastomer exhibits exceptional strain-adaptive stiffening, with an ∼100-fold increase in modulus when being fully stretched. The strain-adaptive stiffening endows the elastomer with excellent damage resistance. Meanwhile, the conductive elastomer has a record-high fracture energy of ∼33.8 kJ m –2 . The notched conductive elastomer can prevent the propagation of the…

A poly( n -butyl methacrylate) macroinitiator with terminal chlorine chain-end functionality (CEF) was depolymerized by ATRP mediated by a copper(II) chloride/tris(2-pyridylmethyl)amine (CuCl 2 /TPMA) catalyst at 170 °C. Depolymerization reactions with solid loadings between 8 and 21 wt % recovered >40% monomer within 10 min and up to 67% monomer at 8 wt % solid loading. This method was selective to n -butyl methacrylate monomer by concurrent depolymerization and distillation on a rotary evaporator. Control experiments confirmed that the reactions stopped due to the loss of CEF before equilibrium was established at the equilibrium monomer concentration. Incubation of the macroinitiators showed evidence of alkyl halide decomposition via lactonization of the chain end, leading to lower initiation efficiencies and an increase in the thermal stability of the polymer.

Epoxy resins with a network structure, which are obtained by curing reactions of epoxy and amine compounds, are an important class of thermosetting resins. We here report the segmental dynamics of epoxy resins in which the three-dimensional network was well-defined and systematically varied. What we found by experiments in conjunction with atomistic molecular dynamics simulation was that when the cross-linking density in the epoxy resins increased, the glass transition temperature increased with an associated reduction in the spatial scale of the cooperative rearranging region and the increase in the dynamic heterogeneity. The knowledge here obtained should be useful for a better understanding of the physical properties of thermosetting polymers.

Thermosets and thermoset composites constitute an extraordinary challenge for recycling and participation in a circular economy because their permanent covalent cross-links prevent spent thermosets from being melt processed into new products. With annual world-wide production in the tens of billions of kilograms, the inability to recycle thermosets into high-value products represents major economic and sustainability losses. While recent research into polymer networks with dynamic covalent cross-links has indicated promise for reprocessability at common melt-state processing temperatures, a crucial shortcoming has been identified: such reprocessable networks and network composites commonly exhibit creep at use conditions due to their dynamic nature, which may prevent their use in applications that require long-term dimensional stability. Here, we use a strategy based on nitroxide-mediated polymerization to synthesize reprocessable networks and network composites containing alkoxyamine dynamic bonds. The resulting networks, including those synthesized from lab-grade polybutadiene and…

Polymer informatics tools have been recently gaining ground to efficiently and effectively develop, design, and discover new polymers that meet specific application needs. So far, however, these data-driven efforts have largely focused on homopolymers. Here, we address the property prediction challenge for copolymers, extending the polymer informatics framework beyond homopolymers. Advanced polymer fingerprinting and deep-learning schemes that incorporate multitask learning and meta learning are proposed. A large data set containing over 18 000 data points of glass transition, melting, and degradation temperature of homopolymers and copolymers of up to two monomers is used to demonstrate the copolymer prediction efficacy. The developed models are accurate, fast, flexible, and scalable to more copolymer properties when suitable data become available.

High Resolution Image Download MS PowerPoint Slide Predictive computational modeling of polymer materials is necessary for the efficient design of composite materials and the corresponding processing methods. Molecular dynamics (MD) modeling is especially important for establishing accurate processing–structure–property relationships for neat resins. For MD modeling of amorphous polymer materials, an accurate force field is fundamental to reliable prediction of properties. Reactive force fields, in which chemical bonds can be formed or broken, offer further capability in predicting the mechanical behavior of amorphous polymers subjected to relatively large deformations. To this end, the reactive interface force field (IFF-R) has been recently developed to provide an efficient means to predict the behavior of materials under these conditions. Although IFF-R has been proven to be consistent for some crystalline organic and inorganic systems, it has not yet been proven to be consistent for amorphous polymer systems. The objective of this study is to use IFF-R to predict the…

High Resolution Image Download MS PowerPoint Slide A universal method for determining the modulus of the interlamellar amorphous phase of semicrystalline polymers was proposed. The basics of the method were presented on the example of high-density polyethylene (HDPE). The local deformation of the amorphous component was induced by the introduction of a swelling agent (hexane). The swelling-induced local strain and local stress of the interlamellar amorphous phase were estimated based on the changes of the long period and the yield stress, respectively. The determined modulus of the interlamellar amorphous phase of HDPE (≈40 MPa) was an order of magnitude higher than the modulus of the bulk rubbery amorphous phase of HDPE (≈3 MPa). The modulus of the interlamellar amorphous phase of other semicrystalline polymers, such as polypropylene (PP), low-density polyethylene (LDPE), and an ethylene–octane copolymer (EOC), was also determined and amounted to 50.3, 11.8, and 4.2 MPa, respectively. For polyethylene materials (HDPE, LDPE, and the EOC), a linear increase in the modulus of the…

Deuterium, a stable hydrogen isotope, has been playing important roles in many scientific areas, including polymer science. The developments of deuteration science and polymer science have been interwoven for the past 60 years. The unique characteristics of the scattering lengths for hydrogen isotopes (protium and deuterium) result in the dramatic differences of scattering length densities for protiated and deuterated compounds omnipresent in polymers, rendering neutron scattering a powerful tool for the investigation of polymeric structures and dynamics that are inaccessible to other tools. While the role of deuteration in neutron scattering of polymeric materials can hardly be overstated, it has also enabled the addressing of many questions using other approaches, such as NMR and vibrational spectra. In this Perspective, we provide a brief account of deuterium in the synthesis, properties, and application of various polymers. We also offer an outlook of the potentials in this ever-evolving and exciting area.

Thermally recyclable semiaromatic polyesters with good mechanical properties are prepared by ring-opening polymerization (ROP) of benzene-fused caprolactones. Three constitutional isomers of benzo-thia-caprolactones ( BOTO-I, BOTO-II, and BOTO-III ) were synthesized. Their organocatalytic ROPs were systematically investigated and correlated with their structures. The diphenyl phosphate (DPP)-catalyzed ROP of BOTO-II is controllable and can reach high monomer conversion. BOTO-I has a high equilibrium monomer concentration ([ M ] e = 0.15 M) at 30 °C, whose ROP can only be achieved with 1,5,7-triazabicyclo[4.4.0] dec-5-ene (TBD) as the catalyst, but with poor control. BOTO-III can be polymerized with TBD or a binary catalytic system of 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) and 1-(3,5-bis(trifluoromethyl)phenyl)-3-cyclohexylthiourea (TU) with poor control. Polymers P(BOTO-I) and P(BOTO-II) are thermally stable up to 270 °C, but P(BOTO-III) starts to decompose at 180 °C. These three polymers are semicrystalline materials with slow crystallization rates. P(BOTO-I) displays the…

Steady-state shear viscosity η(γ̇) of unconcatenated ring polymer melts as a function of the shear rate γ̇ is studied by a combination of experiments, simulations, and theory. Experiments using polystyrenes with Z ≈ 5 and Z ≈ 11 entanglements indicate weaker shear thinning for rings compared to linear polymers exhibiting power law scaling of shear viscosity η ∼ γ̇ –0.56 ± 0.02, independent of chain length, for Weissenberg numbers up to about 10 2 . Nonequilibrium molecular dynamics simulations using the bead-spring model reveal a similar behavior with η ∼ γ̇ –0.57 ± 0.08 for 4 ≤ Z ≤ 57. Viscosity decreases with chain length for high γ̇. In our experiments, we see the onset of this regime, and in simulations, which we extended to Wi ∼ 10 4, the nonuniversality is fully developed. In addition to a naive scaling theory yielding for the universal regime η ∼ γ̇ –0.57, we developed a novel shear slit model explaining many details of observed conformations and dynamics as well as the chain length-dependent behavior of viscosity at large γ̇. The signature feature of the model is the…

Vitrimers have captured broad interest in academia and industry because they offer a compelling combination of mechanical performance, thermal stability, and recyclability. They are permanent polymer networks that contain dynamic covalent bonds that undergo thermally activated exchange reactions without decreasing the connectivity of the network. The introduction of such functionality to commercial thermoplastics via reactive processing is envisioned to be an economical way to access high-performance, recyclable materials. Polyolefins are of particular interest because they are the most widely used class of polymers today. Although reactive processing is widely practiced in the industry, it remains a nascent approach for accessing polyolefin vitrimers. Herein, we report a dimaleimide bis(dioxaborolane) that allows for commercial polyethylene (PE) to be transformed into a vitrimer in a single step without any small-molecule byproducts. The miscibility of the grafting agent with the PE melt is a critical consideration for optimizing the grafting protocol, as efficient mixing is…

Depolymerization of vinyl polymers into monomers is energy-intensive due to the high thermal and chemical stability of the backbone. Depolymerizations of methacrylic polymers are typically conducted above the ceiling temperature and thermal degradation temperature to degrade polymers by bond scission. This work investigates the catalyzed depolymerization of a Cl-capped poly(poly(dimethylsiloxane) methacrylate) (P(PDMS 11 MA-Cl)) polymer mediated by an atom transfer radical polymerization catalyst: copper(II) chloride/tris(2-pyridylmethyl)amine (CuCl 2 /TPMA) at 170 °C. The depolymerization yield, rate, and selectivity were improved by increasing the ratio of [TPMA]/[CuCl 2 ]. Electron transfer from the ligand contributed to the Cu(I) activator (re)generation at high temperature ( T > 130 °C), as proven by ultraviolet–visible spectroscopy. The bottlebrush could be partially depolymerized and repolymerized over a few cycles.

High Resolution Image Download MS PowerPoint Slide Poly(ester-carbonate) copolymers were synthesized through triethylborane (TEB)-activated, one-pot copolymerization of epoxides with anhydrides (AH) and CO 2 . Depending upon the feeding ratio of AH to the epoxides, poly(ester-carbonate) copolymers with random and tapered ester structures could be derived due to the much higher reactivity of AH toward growing oxanions compared to the reactivity of CO 2 . Anhydrides like succinic anhydride (SA) and phthalic anhydride (PA) when tried in terpolymerization with epoxides, like propylene oxide (PO) and cyclohexene oxide (CHO), and CO 2 indeed exhibited such high reactivities in comparison to CO 2 that tapered block structures were eventually obtained for high feeding ratios of AH to PO and random copolymers for the AH-to-epoxide feeding ratio lower than 10%. Terpolymerization with different feeding ratios of anhydrides to epoxides was thus systematically investigated. In an alternate path, diblock poly(ester- b -carbonate)s were prepared by sequential copolymerization of epoxide/anhydride…

Although polymer dielectrics show wide application prospects in electronic and electrical fields, a low relative dielectric constant and a low energy density restrict their rapid developments. To overcome these shortcomings, a strategy is proposed to facilitate orientational polarization through the incorporation of the π–π stacked biphenyl side groups in dipolar polymers. In this study, sulfonyl-containing poly(norbornene)s with and without biphenyl groups were synthesized by ring-opening metathesis polymerization, denoted as PBTMD-SO 2 and PTMD-SO 2, respectively. Their dielectric behaviors and energy-storage properties were investigated. The dielectric constant of PBTMD-SO 2 with the biphenyl side groups was as high as 11.1 at 25 °C and 1 kHz, which was nearly 35% higher than that of PTMD-SO 2 due to the stronger orientational polarization. In addition, the dielectric loss in the range of 1 kHz to 1 MHz increased from 0.061 to 0.172 for PTMD-SO 2 and from 0.075 to 0.110 for PBTMD-SO 2 . Electric displacement–electric field loops studies indicated that the discharge energy density…

We provide a perspective on polymer glass formation, with an emphasis on models in which the fluid entropy and collective particle motion dominate the theoretical description and data analysis. The entropy theory of glass formation has its origins in experimental observations relating to correlations between the fluid entropy and liquid dynamics going back nearly a century ago, and it has entered a new phase in recent years. We first discuss the dynamics of liquids in the high-temperature Arrhenius regime, where transition state theory is formally applicable. We then summarize the evolution of the entropy theory from a qualitative framework for organizing and interpreting temperature-dependent viscosity data by Kauzmann to the formulation of a hypothetical “ideal thermodynamic glass transition” by Gibbs and DiMarzio, followed by seminal measurements linking entropy and relaxation by Bestul and Chang and the Adam–Gibbs (AG) model of glass formation rationalizing the observations of Bestul and Chang. These developments laid the groundwork for the generalized entropy theory (GET),…

Polymerization-induced phase separation enables fine control over thermoset network morphologies, yielding heterogeneous structures with domain sizes tunable over 1-100 nm. However, the controlled chain-growth polymerization techniques exclusively employed to regulate morphology at these length scales are unsuitable for a majority of thermoset materials typically formed through step-growth mechanisms. By varying the composition of a binary curing agent mixture in a classic rubber-toughened epoxy thermoset, where the two curing agents are selected based on disparate compatibility with the rubber, we demonstrate facile tunability over morphology through a single compositional parameter. Indeed, this method yields morphologies spanning nano-scale to macro-scale, controlled by the relative reactivities and thermodynamic compatibility of the network components. We further demonstrate a profound connection between chain dynamics and microstructure in these materials, with the tunable morphology enabling exquisite variations in glass transition. In addition, previously unattainable control…

Molecular dynamics simulations with generic bead–spring models have been instrumental in revealing the molecular-scale behavior that underlies structure–property relationships of various types of polymeric systems. The generic, coarse-grained modeling approach does not include atomistic detail and is not focused on quantitatively matching the properties of a particular chemical system, though typically several parameters are included that can be tuned to consider different types of chemistries. Besides allowing access to longer length and time scales due to computational efficiency, this type of approach is advantageous in that the physical insight gained is often relevant across the entire class of related materials. The connectivity and number of beads in a generic bead–spring models can be adjusted in an obvious manner to describe the most basic homopolymer features (e.g., to consider different chain lengths and linear versus branched architectures). In uncharged copolymers or solvent containing systems, the chemical interactions between components are considered by adjusting the…

It is of significance to develop stretchable conductors for flexible electronics. Although intrinsically conducting polymers like poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) can exhibit high conductivity, they have limited mechanical stretchability because of the rigid conjugated backbone and strong interchain interaction. Here, we report the significant enhancement in the mechanical stretchability of PEDOT:PSS films by adding a compound of two or more hydroxyl groups like glycerol, malic acid, 1,2,6-hexanetriol, or triethylene glycol. The elongation at break can be enhanced from <10% of pristine PEDOT:PSS films to >50%. The enhancement in the mechanical stretchability is less significant when other compounds with only one hydroxyl or no hydroxyl group are used. The effect of the compounds with multiple hydroxyl groups on the stretchability of PEDOT:PSS is related to the Hansen solubility parameter (HSP) δ h . A compound with a higher δ h value can give rise to a more significant plasticization of PEDOT:PSS. The mechanism is attributed to the destruction of…

Block copolymers are used to construct covalent adaptable networks that employ associative exchange chemistry (vitrimers). The resulting vitrimers display markedly different nanostructural, thermal and rheological properties relative to those of their statistical copolymer-derived counterparts. This study demonstrates that prepolymer sequence is a versatile strategy to modify the properties of vitrimers.

The controllable introduction of dynamical chemical cross-linking into polymers permits the development of recyclable polymer products with high performance. However, challenges lie in chemically inert polyolefins, which are the largest polymer product in the world to date. Here, we demonstrate the preparation of polyolefin elastomers (POEs) with tunable dynamic chemical cross-linking, compatible with their industrial production. Boronic ester bond cross-linked POEs (BPOEs) present high mechanical performances with tensile strength (σ) up to 23.8 MPa, Young’s modulus ( E ) up to 29.0 MPa, and toughness ( U T ) up to 69.6 MJ·m –3, far superior to those reported elastomers. BPOEs also demonstrate good thermal stability maintaining a steady storage modulus plateau of 2.9 MPa above 100 °C, instead of 0.2 MPa for pristine POEs. After reprocessing three times, BPOEs show a good reprocessability with a σ recovery of 85.6%, an E of 108.5%, a U T of 95.7%, and an elongation at break (ε) of 97.1%, even higher than that of commercial POEs such as POE-8150 of Dow Company. Our method thus…

Epoxy/dicarboxylic acid vitrimer was prepared by the solvent-free reaction of diglycidyl ether of bisphenol A (DGEBA) and 1,4-cyclohexane dicarboxylic acid (CHDA) with the addition of monobutyltin oxide (Sn) as a catalyst. By tailoring the catalyst content (≥5 mol %), an effective conversion of functional groups during cure demonstrated the network polymerization mechanisms and a sequence of the side reactions. Indeed, the manufactured vitrimers exhibit creep and full stress relaxation thanks to catalytic transesterifications. By changing the epoxy/diacid ratio, the thermo-mechanical properties and mechanical behavior of the epoxy/acid vitrimers can be tuned while keeping self-healing ability. At high epoxy excess, both glass-transition temperature ( T g ) and solid–liquid viscoelastic transition temperature ( T v ) shift to a higher temperature. At vitrimer formulations 1:0.6 and 1:0.5 (epoxy/acyl), a remarkable improvement of fracture toughness ( K Ic ) is observed, indicating the transition from stiff to relatively ductile materials at 1:0.6. This is attributed to the altered…

Precisely and conveniently controlling the handedness and magnitude of circularly polarized luminescence (CPL) is a key issue in developing CPL-active materials. In this work, a series of chiral helical polyacetylenes bearing pendant pyrene groups were synthesized. CPL inversion occurred when the polymer solutions fabricated into thin solid films. More interestingly, amplification of CPL was also observed during this process; that is, the polymer solution showed a luminescence dissymmetry factor ( g lum ) of 1 × 10 –3, while over 100-fold magnitude amplification of g lum values is obtained in the polymer films. Significantly, the g lum values of the films can be regulated from 10 –1 to 10 –3 by simply changing the film-forming solvents. The cause of the above phenomena is due to the presence of two different chirality transfer channels: intermolecular interactions in the solutions and the “matching rule” in the films. This study provides a new strategy to construct CPL materials with tunable handedness and a large dissymmetry factor.

Nonvolatile and durable ionogels are emerging and promising stretchable ionic conductors for wearable electronics. However, the construction of reconfigurable and recyclable ionogels with high mechanical robustness, high stretchability, and autonomous healability, while heavily demanded, is very challenging. Here, we present a gradient-responsive cross-linking strategy for preparing a highly stretchable and reconfigurable thermoplastic engineering ionogel (TPEI). The design of both microcrystalline and dense hydrogen bonds in TPEI contributed to the formation of a unique gradient-responsive network. When the TPEI was melt-processed under heating, the microcrystalline network structure was destroyed to form entangled polymer chains, while the high-density hydrogen-bonded network structure was only partially destroyed. The remaining hydrogen-bonded network allowed the TPEI to have a high viscosity for melt processing. When the TPEI was cooled upon melting injection, extrusion, and spinning, the hydrogen-bonded network was rapidly reconstructed in tens of seconds, allowing it to be…

Semiconducting polymers with high mobility and mechanical robust properties are strongly dependent on their molecular weight. However, the relationship between molecular weights and solution chain entanglements, film microstructures, charge carrier mobility, and mechanical properties for donor–acceptor conjugated polymers remains less understood. Herein, P(NDI2OD-T2) with a weight-average molecular weight ( M w ) from 34.0 to 280 kDa was investigated as a model system. The polymer chain exhibited three regions in chloroform solutions: fewer entanglements (34.0–77.7 kDa), enhanced entanglements (170 kDa), and severe entanglements (280 kDa). This chain solution behavior resulted in three distinct film microstructures: (1) 34.0–77.7 kDa, liquid-crystalline-like morphologies with highly ordered chain arrangements and large crystallite lengths ( l c ) yet relatively low tie-chain densities that increased with M w; (2) 170 kDa, small fibril morphology with less ordered chain arrangements and a decreased l c of only 5.6 nm yet a high tie-chain density; and (3) 280 kDa, a seemingly…

Dense polymeric films based on network-forming cellulose nanofibrils (CNFs) have excellent mechanical properties but are limited by moisture sensitivity. Here, interfibrillar effects from CNF surface properties are investigated. TEMPO-oxidized CNFs and two native CNFs are prepared with a similar length and width, to exclude geometrical effects. The CNFs have different surface properties in terms of sorbed hemicellulose content, hemicellulose molar mass, and surface charge. Moisture sorption, structural changes, and mechanical properties at different relative humidities are characterized. The presence of sorbed hemicelluloses in the interfibrillar interphase has favorable effects on the mechanical tensile properties. Surface-charged carboxyls increased moisture sorption and film thickness swelling and reduced the mechanical properties. A comparison with biaxially oriented polyethylene terephthalate films provides a perspective into the structure and properties of CNF films. The present study shows the importance of the interfibrillar interface and interphase region for mechanical…

Donor–acceptor (D–A)-type photocatalysts containing the polycyclic aromatic donor ( BTT-CPP ) and nonpolycyclic aromatic donor ( TTB-CPP ) are designed and synthesized. Their physicochemical properties are thoroughly investigated. The photocatalytic hydrogen generation experiments show that both polymers exhibit distinct photocatalytic performance. BTT-CPP displays an attractive hydrogen evolution rate (HER) of 37,866 μmol g –1 h –1 without further addition of the metal co-catalyst, one of the highest values reported for organic photocatalysts. The optimal HER of TTB-CPP is 8466 μmol g –1 h –1 . These results highlight the important role of the donor unit on the performance of D–A-type photocatalysts and demonstrate that thiophene-contained polycyclic aromatic unit BTT is an excellent donor unit for high HER photocatalysts.