The 50 most-cited articles in Macromolecules, 2022

Epoxy vitrimers with dynamic covalent networks enable reprocessing and recycling of epoxy thermosets. However, achieving high mechanical performance remains a challenge. In this work, ferulic acid-based hyperbranched epoxy resin (FEHBP) was synthesized to produce closed-loop recyclable and catalyst-free epoxy vitrimers without compromising its thermal and mechanical properties. The incorporation of FEHBP with a hyperbranched topological structure improved the tensile strength, modulus, and toughness of epoxy vitrimers through an in situ reinforcing and toughening mechanism. The hydroxyls of FEHBP catalyzed the dynamic transesterification and accelerated the reprocessing of epoxy vitrimers. Thus, the obtained epoxy vitrimers demonstrated excellent weldability, malleability, and programmability. Epoxy vitrimers with 10 phr FEHBP exhibited high tensile strength (126.4 MPa), usable T g (94 °C), fast stress relaxation (a relaxation time of 45 s at 140 °C) and a retention of tensile strength (above 88.3%) upon recycling. The degradation products were reused to produce new epoxy vitrimers…

The low thermal conductivity coefficient (λ) of polyimide (PI) films are limiting their application in high-power electronic equipment, and the disordered alignment of fillers discourage efficient improvement of λ for PI-based composite films. Herein, polyethylene glycol trimethylnonyl ether is used to perform liquid crystalline modification on graphene fluoride (LC-GeF) to achieve ordered alignment. Intrinsically thermally conductive liquid crystalline PI (LC-PI) matrix is utilized to fabricate thermally conductive LC-GeF/LC-PI composite films. In-plane λ (λ ∥ ) and through-plane λ (λ ⊥ ) of 15 wt % LC-GeF/LC-PI composite films reach 4.21 and 0.63 W/(m·K), 446.8% and 320.0% higher than λ ∥ (0.77 W/(m·K)) and λ ⊥ (0.15 W/(m·K)) of normal PI films, 99.5% and 96.9% higher than λ ∥ (2.11 W/(m·K)) and λ ⊥ (0.32 W/(m·K)) of LC-PI films, also higher than 15 wt % GeF/LC-PI composite films. Additionally, LC-GeF/LC-PI composite films possess more excellent insulating, mechanical, and thermal properties than GeF/LC-PI composite films.

Polyethylenes enjoy wide applications due to their many superior properties. The incorporation of some polar functional groups into the otherwise nonpolar polyethylene backbone can significantly improve many important properties and further broaden their applications. During the past few decades, many catalyst systems have been demonstrated with the capabilities of copolymerizing ethylene with polar monomers. As a result, numerous polar functionalized polyethylenes have been prepared bearing various polar groups. However, the studies into material properties of functional polyethylenes from transition-metal-catalyzed ethylene–polar monomer copolymerization have not received much attention until recently. After some brief discussions of selected examples of potent catalysts and suitable polar monomers, this Perspective summarizes some recent advances in the improvements in surface and compatibilities properties (hydrophilicity, adhesion, dyeability, and compatibility with other types of polymers) induced through polar group incorporation as well as some custom-made properties such as…

While transesterification has been largely explored by chemists, only a few studies comparatively dealt with its “sluggish cousin”, transcarbamoylation. Originally suggested 70 years ago to explain the stress decay observed at high temperature in polyurethane chemical networks, transcarbamoylation─also called transurethanization or urethane exchange─is still underexploited in both organic and polymer chemistry. This is mainly related to the use of toxic reactants such as isocyanates and tin-based catalysts involved during the preparation of molecular carbamates or polyurethanes (PUs) as well as harsh conditions required to observe such exchanges in molecular transformations. Many studies have focused their interest in alternatives to the use of isocyanates, underestimating the potential of transcarbamoylation to produce, recycle, or reprocess PUs (wastes). This Perspective presents the latest developments and plausible future directions to prepare sustainable PU-based materials by using transcarbamoylation with a focus on covalent dynamic networks.

Elastomers play an irreplaceable role in industry and daily life; however, they are usually soft and susceptible to damage. In this study, skin-like poly(urethane-urea) elastomers with high mechanical strength, stretchability, elasticity, and excellent damage resistance, damage tolerance, and healability are fabricated by cross-linking polycaprolactone (PCL) chains with hydrogen-bond arrays. The elastomer, which is denoted as PU-ASC, has a tensile strength of ∼72.6 MPa, recovery strain of ∼500%, and fracture energy of ∼161 kJ m –2 . Moreover, the PU-ASC elastomer exhibits unique strain-adaptive stiffening, which endows the elastomer with the capacity to resist damage. The skin-like PU-ASC-IL conductors can be conveniently fabricated by loading ionic liquids (ILs) into the PU-ASC elastomers. The healable, stretchable, elastic, damage-resistant, and damage-tolerant PU-ASC-IL conductors show record-high mechanical performance, with tensile strength, toughness, and fracture energy values of ∼22.8 MPa, ∼164.2 MJ m –3, and ∼73.6 kJ m –2, respectively. The damage resistance and damage…

Polymer electrolyte membranes are charged polymers in the membrane shape, which can separate the anode reaction from the cathode reaction, allowing the fabrication of compact and highly efficient electrochemical devices. In recent years, great success has been witnessed in the development of advanced polymer electrolyte membranes, driven by the fast development of fuel cells that promise a clean and highly efficient sustainable energy supply. However, drawbacks limiting the widespread adoption of polymer electrolyte membranes still exist. These include the high cost of fluorinated polymer electrolyte membranes, the poor ion transport capability of non-fluorinated aromatic polymer-based polymer electrolyte membranes (especially under low-humidity conditions), the insufficient durability, and the function-led design of advanced polymer electrolyte membranes for applications beyond fuel cells. Herein we briefly discuss several important challenges that should be solved before polymer electrolyte membranes could be extensively adopted in real-world applications. These challenges include…

The structural manipulation of the electroactive β phase of poly(vinylidene fluoride) (PVDF) is particularly important in sensor and actuator applications. Herein, an efficient way to enhance dielectric and ferroelectric properties of PVDF films by annealing preoriented PVDF films through thermal treatment combined with the pressure field is proposed. During annealing processing, an appropriate pressure is attributed to the efficient dipole rotation and results in complete phase transformation of the nonpolar α phase (TGTG′) into the polar β phase (TTTT). Moreover, the appropriate pressure and temperature fields synergistically promote a more perfect alignment of the main chain in β-crystallites along the stretching direction. Thus, the pure β phase with ultrahigh orientation (Herman’s orientation factor >0.97) is successfully obtained. Moreover, the enhanced mobility of molecular chains with the increase of temperature contributes to the perfection of β-crystallites with high crystallinity. The relaxation of oriented chains in the amorphous region at high temperature during the…

Vitrimers are polymer networks in which associative exchange reactions can take place under specific conditions, thus conferring reprocessability to insoluble materials. Recently, catalyst-free vitrimers have emerged as a new generation of vitrimers able to overcome potential leaching, aging, and sintering issues of catalysts and to ensure the preservation of vitrimer properties after numerous reshaping processes. Here, a catalyst-free epoxy vitrimer featuring α-CF 3 -substituted ester functions is reported. First, a new tetra-acid precursor was synthesized via a catalyst-free thia-Michael addition. A catalyst-free ring-opening polymerization was then performed on two different diepoxy monomers (DGEBA and BDGE) to obtain polymer networks composed of ester linkages. Curing was evaluated by rheology, differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR) monitoring of the polymerization kinetics. A gel content of over 70% was measured after 24 h in tetrahydrofuran (THF). Finally, the accelerating effect of the α-CF 3 group on transesterification…

Recycling of polymers has become increasingly relevant with the focus on a circular economy. Controlled radical polymerizations create an avenue for polymerization and depolymerization through activation of the dormant polymer chain-end functionality. Here, the depolymerizations of poly(methyl methacrylate) (PMMA) and poly( n -butyl methacrylate) (PBMA) macroinitiators with terminal chlorine chain-end functionality mediated by iron chloride salts and iron powder at 170 °C are reported. The depolymerizations were conducted with polystyrene as internal standard for kinetics measurements by GPC. Higher conversion and depolymerization rates were achieved in tetra(ethylene glycol) dimethyl ether than in 1,2,4-trichlorobenzene. Depolymerization with zerovalent Fe 0 as a supplemental activator and reducing agent was the most effective, reaching >70% conversion at 10 wt % solid polymethacrylate loadings with fast reaction rates. The monomer isolated during the depolymerization was recovered through distillation.

Although bio-based benzoxazines (BZs) have been explored widely as sustainable thermosetting resins, few high-performance BZs have been prepared completely from natural renewable resources. In this study we synthesized a fully bio-based multifunctional bisbenzoxazine (AP-fa-BZ) in high yield and purity from apigenin (AP), furfurylamine (fa), and benzaldehyde by using both solvent and solventless approaches. Fourier transform infrared (FTIR) spectroscopy, high-resolution mass spectrometry, and one- and two-dimensional nuclear magnetic resonance spectroscopy confirmed the chemical structure of AP-fa-BZ. We then used dynamic mechanical analysis, differential scanning calorimetry (DSC), thermogravimetric analysis, and in situ FTIR spectroscopy to examine the thermal characteristics of AP-fa-BZ before and after its ring-opening polymerization (ROP). DSC revealed that the temperature required for the formation of poly(AP-fa-BZ) through ROP (236.3 °C) was significantly lower than that of a typical 4-phenyl-3,4-dihydro-2 H -1,3-benzoxazine (Pa-type) monomer due to the positive catalytic…

In this paper, we describe dual crown ether (CE)- and benzoxazine (BZ)-linked porous organic polymers (CE-BZ-POPs) incorporating pyrene (Py) and tetraphenylethylene (TPE) units, synthesized through a multistep process involving Schiff-base formation, reduction, Mannich condensation, and Sonogashira–Hagihara coupling, with their structures confirmed using Fourier transform infrared and solid-state nuclear magnetic resonance spectroscopy. The presence of both CE and BZ units in POPs has never been explored previously. Here, the BZ units underwent solid-state chemical transformations through thermal ring-opening polymerization without a thermal curing agent or any catalyst. The resulting Mannich bridges and phenolic units facilitated CO 2 capture, mediated through strong acid/base and/or intermolecular hydrogen bonding interactions; furthermore, the CE units bound strongly with metal ions through specific metal–ligand interactions, suggesting that these CE-BZ-POPs might be useful for wastewater treatment.

Plastics recycling has become an important issue based on the principle of sustainability and circular economy. Therefore, the development of a valuable chemical recycling method, to achieve the recycling process of end-of-life polylactide (PLA) waste without loss of material properties, meets unprecedented opportunities. In this regard, we propose a “DE–RE polymerization” strategy to achieve the recycling process from end-of-life PLA plastic waste to new virgin-quality PLA materials in a “polymer to polymer” mode. This approach improves the efficiency of PLA recycling and promotes the development of plastics reutilization.

In this work, tough and fast light-controlled healable lignin-containing polyurethane elastomers (LPUes) with a dynamic dual-crosslinking network were successfully synthesized. The network consisted of dynamic noncovalent bonds (hydrogen bonds and Zn 2+ -based coordination bonds) and dynamic carbamate bonds, which endowed LPUes with excellent mechanical properties (31.4 MPa, 1528%), reprocessability, and healing properties. After 4 times of reprocessing, the retention ratios of tensile strength and elongation at break for LPUes maintained above 85 and 95%, respectively. The incorporation of Zn 2+ -based coordination bonds significantly enhanced the photothermal conversion capability of LPUes, with the maximum surface temperature reaching above 160 °C in 2 min under the near-infrared (NIR) illumination of 1.2 W/cm 2 . The prepared LPUes exhibited rapid and outstanding light-controlled healing performance with 98% healing efficiency of mechanical properties under 10 min of NIR irradiation. This work demonstrated the novel application of biomass lignin in recyclable and light-driven…

Nature shows us that complex molecular architectures lead to unique material properties, and these observations have driven polymer scientists to synthesize complex architectures in an effort to discover how topology influences properties in synthetic polymers. In this Perspective, we discuss a variety of complex architectures synthesized using ring-opening metathesis polymerization (ROMP), including multiblock linear polymers, bottlebrush homopolymers and (multi)block copolymers, dendronized polymers, star polymers, and polymer–biomolecule conjugates. Traditional and recently developed synthetic methods, including polymerization-induced self-assembly, copolymerization to create gradient structures, and engineering approaches to making complex topologies using ROMP, are also reviewed. In this context, we highlight emerging applications stemming from these materials, including drug delivery vehicles, nanoscale constructs, and components in light refraction or energy storage, among others. Finally, we conclude with an in-depth discussion on practical considerations in ROMP that enable…

Glycopolymers are synthetic polymers with pendant sugars, which hold promise for a range of biomedical applications ranging from tissue engineering to sensing. The known specific interaction of glycopolymers with lectins has inspired researchers to use these polymers to deliver drugs to cells that overexpress lectin receptors. As a result, many glycopolymers─based on mannose, galactose, fructose, or other saccharides–have been used for the targeted delivery of drugs, ranging from traditional anticancer drugs to nucleic acid-derived therapeutics. For drug delivery purposes, glycopolymers are typically processed into nanoparticles that form a matrix to entrap the drug safely, such as micelles, polyplexes, polyion complex micelles, or other nanosized carriers. In vitro and in vivo studies have shown that drugs can indeed be delivered selectively to specific cells by leveraging the selective recognition of surface bound lectins. The key to the interaction between glycopolymers and lectins is the presence of strong intermolecular forces such as hydrogen bonding. The formation of strong…

1,4;3,6-Dianhydrohexitols (isohexides) consisting of isosorbide, isomannide, and isoidide are produced from cellulose and can be incorporated into superengineered plastics while retaining characteristics that stem from their bulky and rigid alicyclic structures. In this study, seven types of polyimides (PIs) incorporating a chiral isosorbide (ISS) skeleton in the main chain (ISS-PIs) were prepared using ISS and a tetracarboxylic dianhydride derived from trimellitic anhydride. ISS-PIs exhibited excellent optical transparency, low refractive indices, low dielectric constants, and small birefringence compared to wholly aromatic PIs. This is because the in-plane orientation of the main chain and dense molecular packing between the PI chains were suppressed by the bulky alicyclic ISS structure and the bent and flexible PI main chains with reduced intermolecular interactions. ISS-PIs with excellent optical properties also demonstrated lower dielectric constants ( D k ) with moderate dissipation factors ( D f ) at 10 and 20 GHz. Although the structure dependence of D k is similar to that…

Castor oil (CO)-derived polyurethanes (PUs), denoted CPUs, are drastically limited because they cannot be reprocessed, reshaped, or recycled and because they show low strength, poor toughness, and insufficient functionality due to their permanently cross-linked structure with a high cross-link density and a soft backbone. Herein, we developed novel reversible piperazine–urea bonds (PaUBs) from inexpensive piperazines and isocyanates and incorporated the PaUBs into CPUs to fabricate sustainable and reprocessable poly(urethane urea) covalent adaptable networks (CANs). We synthesized the CANs from CO, N -(2-hydroxyethyl) piperazine (HEP), and 4-methylenedicyclohexyl diisocyanate (HMDI) via a catalyst- and solvent-free approach. The CANs showed variable tensile behavior over a wide range from a soft elastomer to tough and hard plastics. They also exhibited excellent malleability, stable reprocessability, and multiple recyclability, in which their original properties were recovered after several thermal recycling or remolding cycles. The CANs possessed excellent shape memory performance…

Covalent adaptable networks (CANs) relying on dynamic cross-links have been developed to make the cross-linked polymeric materials and composites degradable. However, due to the reversibility of dynamic bonds, the CANs and composites suffer accidental degradation and failure upon a certain stimulus (moisture, acid/base, reductant/oxidant, etc.) in the application environment. Herein, inspired by parallel circuits, interlocked covalent adaptable networks (ICANs) were prepared by one-pot reactions from epoxy monomers and two curing agents that contained different dynamic bonds of aromatic disulfide and aromatic imine bonds, resulting in dual dynamic parallel cross-links in homogeneous epoxy networks. The ICANs exhibited outstanding mechanical properties and improved stability, relying on the topological interlocking structure. The ICANs could be unlocked and became degradable only when two stimuli were both applied to completely break the cross-links of disulfide bonds and imine bonds. When applying ICANs as a matrix to form carbon fiber-reinforced polymer (CFRP) composites, the…

In the quest for sustainable development, degradable polymers are attractive alternatives to traditional fossil-based plastics. As monocomponent degradable polymers often suffer from unsatisfactory performances, the utilization of corresponding copolymers with tailored block compositions is viable for improved properties. To this end, the emerging field of self-switchable polymerization offers a facile access to the on-demand synthesis of degradable copolymers with controlled sequences, in which the polymerizations were regulated by the monomers themselves. This Perspective outlines state-of-the-art self-switchable polymerization systems based on ring-opening (co)polymerization with various types of monomers. The scope of catalysts and monomer types are summarized. Comprehensive comparison between different types of monomers is offered along with an outlook on possible monomers in the future.

Polystyrene (PS) is a major commodity polymer, widely used in various applications ranging from packaging to insulation thanks to its low cost, high stiffness and transparency as well as its relatively high softening temper-ature. Similarly to all polymers prepared by radical polymerization, PS is constituted of a C-C backbone and thus is not degradable. To confer degradability to such materials, the copolymerization of vinyl monomers with cyclic monomer that could undergo radical ring-opening is an efficient methodology to introduce pur-posely cleavable bonds into the polymer backbone. Dibenzo[c,e]-oxepane-5-thione (DOT) is a cyclic thionolac-tone monomer known for its efficient copolymerization with acrylate derivatives but so far could not be incor-porated into PS backbones. From a theoretical study combining DFT and kinetic models using the PREDICI software, we showed that modifying experimental conditions could overcome these limitations and that high molar mass degradable polystyrene (Mw close to 150,000 g.mol-1) could be prepared via statistical insertion of thioester groups…

The molecular chemical structure has a decisive influence on physicochemical properties of polymer materials, which becomes more complex for hydrogels with dense associative interactions. Recently, we have developed a series of tough supramolecular hydrogels with robust hydrogen-bond associations, and the properties of gels severely depend on the composition of copolymers. Here, we examine the influence of the molecular structure on the mechanical and viscoelastic behaviors of these supramolecular hydrogels synthesized from copolymerization of acrylamide/methacrylamide and acrylic acid/methacrylic acid with a varied α -methyl group on the basic unit. The obtained four kinds of the as-prepared hydrogels exhibit distinct mechanical performances and dynamics, evolving from elastic to glassy state with the increase in the number of α -methyl groups. Rheological dynamic spectra of these hydrogels follow the time–temperature superposition principle, which are used to elucidate the correlation between the chemical structure and the dynamics of gel materials. The incorporation of a…

The optoelectronic properties of conjugated polymers are dictated by their chain conformations, which depend on the interplay of delocalization of electrons along the π-conjugated backbone and the intrachain interactions of pendant side chains. Here, we leverage small-angle neutron scattering to measure the chain shapes of several classes of commonly used, high mobility donor–acceptor conjugated polymers in dilute solution. We find that these model conjugated polymers are semiflexible with persistence lengths ranging from several to hundreds of nanometers, dependent on the molecular structure of the polymer, indicating the importance of repeat unit geometry, particularly side-chain size and branching, on the overall chain conformations. The measured persistence lengths show good agreement with those calculated according to dihedral distributions predicted from density functional theory. Larger persistence lengths are shown to correlate with increased charge-carrier mobility, signifying the importance of rational molecular design to obtain high persistence length organic…

It is of greater significance to develop amine-cured epoxy vitrimers than conventional epoxy-carboxylic acid vitrimers because amines are the most widely curing agents than carboxylic acids. Considering that the reaction of amines with glycidyl esters can skillfully create dynamic hydroxyl esters, so we synthesized aliphatic and aromatic glycidyl esters and then cured them by amine-curing agents containing disulfide bonds. Here, besides the generated hydroxyl esters, the dynamic disulfide bonds are also integrated into the cross-linked network to form dual dynamic vitrimers. The synthesized glycidyl esters show very high-efficiency curing reactivity due to the lower activation energy. The properties of dual dynamic vitrimers can be easily tuned via the proportion of aromatic and aliphatic glycidyl esters to realize the transition from the rubber state to glass state. Moreover, the combinations of enhancement and toughening are achieved via the addition of aromatic glycidyl esters into dual dynamic vitrimers without sacrificing the curing activity and reprocessing property. Compared…

Activating innate immunity in a controlled manner is necessary for the development of next-generation therapeutics. Adjuvants, or molecules that modulate the immune response, are critical components of vaccines and immunotherapies. While small molecules and biologics dominate the adjuvant market, emerging evidence supports the use of immunostimulatory polymers in therapeutics. Such polymers can stabilize and deliver cargo while stimulating the immune system by functioning as pattern recognition receptor (PRR) agonists. At the same time, in designing polymers that engage the immune system, it is important to consider any unintended initiation of an immune response that results in adverse immune-related events. Here, we highlight biologically derived and synthetic polymer scaffolds, as well as polymer-adjuvant systems and stimuli-responsive polymers loaded with adjuvants, that can invoke an immune response. We present synthetic considerations for the design of such immunostimulatory polymers, outline methods to target their delivery, and discuss their application in therapeutics.…

Conventional gas separation membranes made of glassy polymers often exhibit increased segmental motion and reduced selectivity when exposed to high-pressure condensable gases. Decarboxylation cross-linking can effectively improve the plasticization resistance and gas permeability of polymer membranes, but this usually comes at the expense of the gas selectivity. In this work, enhanced π–π interactions and decarboxylation cross-linking are synergistically designed among polymer chains by introducing benzimidazole units and carboxyl side groups into the 6FDA-PABZ:DABA polyimide (PI-Im-COOH). After the thermal treatment, a cross-linked structure was formed through the decarboxylation reaction as confirmed by FTIR spectroscopy and thermal analysis. The π–π interactions between benzimidazole moieties were simultaneously enhanced as proven by WAXD, UV–vis, and fluorescence spectroscopy. The gas separation performance and plasticization resistance of the decarboxylated PI-Im-COOH membranes were enhanced significantly. In particular, the decarboxylated PI-Im-COOH membranes exhibit higher…

Three fluorinated ladder diamines with norbornyl benzocyclobutene (N2BC) segments were prepared by catalytic norbornene-arene annulation (CANAL) reactions of norbornadiene with fluorinated bromoanilines. NMR results revealed that two regioisomers were obtained for each diamine, with syn -to- anti ratios of ∼2:1. High-temperature solution polycondensation of these diamines with commercial dianhydrides produced a series of colorless polyimides (CPIs). These polymers exhibited extremely high glass-transition temperatures ( T g ) of 390–479 °C, reasonable coefficients of thermal expansion of 36–61 ppm K –1, and excellent optical transparency ( T 400 > 78% and b * < 2.2). These outstanding properties were attributed to the incorporation of fluorinated substituents as well as the rigid but contorted N2BC segments. For certain dianhydrides, the CANAL-F/CF 3 -derived CPIs exhibited the best optical properties due to their highest fluorine contents, while the CANAL-2F-derived CPIs exhibited the best thermal and mechanical properties, despite the lowest optical transmittance as a result of…

Vitrimers are functional cross-linked materials, exhibiting reprocessability, recyclability, and healability, and thus these are expected for application as sustainable materials. The functionalities of vitrimers are attributable to their associative bond exchange mechanism that is activated at a certain high temperature. The construction of a tuning method for the bond exchange properties must be useful for coming practical application of the vitrimer concept. Here, we prepare transesterification-based vitrimers via the thiol–epoxy click reaction to elucidate the essential effects of the branch numbers ( f ) at the cross-link point on their bond exchange properties, where f can be readily tuned via the functionality of the starting materials. The temperature-ramp creep and stress–relaxation tests then demonstrate that the vitrimer properties, such as the softening and stress–relaxation behaviors, vary depending on f . The experimental results derive some empirical relationships between f and the relaxation time and between f and activation energy of the bond exchange. In addition,…

Designing materials with tunable modulus and viscosity is key for applications such as three-dimensional (3D) printing, sound damping, and wearable devices. Vitrimers provide an ideal platform for viscoelastic design because their dynamic, conserved bond exchange allows for control of both cross-link density and exchange kinetics. Here, multiple boronic acid cross-linkers with different functionalities and kinetics were reacted with silicone diols to form poly(dimethylsiloxane) (PDMS) vitrimers. Networks cross-linked with boric acid or two phenyl-substituted boric acids exhibited relaxation times and viscosities within one order of magnitude of each other. Conversely, a cross-linker with nitrogen neighboring groups led to a four order of magnitude acceleration in network relaxation time while still exhibiting a similar modulus to the slower systems. All of these samples demonstrate an increase in moduli with temperature due to entropic elasticity. To understand the effect of more than one dynamic bond on the viscoelastic response, multiple cross-linkers were then combined into a…

To develop structure–property relationships for cross-linked thermosetting polymers, it is crucial to better understand key factors that control their segmental dynamics and macroscopic properties. Here, we employ a coarse-grained (CG) polymer model to systematically explore the combined effect of varying the cohesive energy (ε) and cross-link density ( c ) on the segmental relaxation time and mechanical properties for a model cross-linked glass-forming thermoset material. We find that increasing c increases both the glass transition temperature T g and fragility of glass formation, while the fragility decreases with an increase in ε. These competing effects of ε and c on fragility are practically important since fragility determines the overall temperature width of the glass formation over which the non-Arrhenius temperature dependence is observed. Our simulation results show that the basic mechanical properties ( i.e., bulk and shear moduli) of cross-linked thermosets are mainly influenced by ε. More interestingly, the macroscopic mechanical properties are found to be strongly…

Recently, the Diels–Alder reaction between furan and maleimide functional groups has been exploited in dynamic reversible covalent networks, introducing a healing ability and reprocessing in various applications. Both applications and (re)processing techniques impose specific requirements, which can be met by the large tunability of the material properties of Diels–Alder-based networks. The extensive set of structure–property relations presented in this paper allows for designing Diels–Alder networks meeting a broad range of material properties. Three major network design parameters are defined that influence these properties: the Diels–Alder concentration, the degree of functionality of the reactive maleimide and furan groups on the monomers, and the stoichiometric ratio between these reactive groups. Their individual influences on the kinetics and the thermomechanical, rheological, and healing properties were investigated via tensile testing, differential scanning calorimetry, dynamic mechanical analysis, and dynamic rheometry. By tuning the design parameters independently,…

We provide a Perspective on recent findings in extensional rheology for polymer melts and solutions with different macromolecular architectures. For linear polymer systems, we focus on the argument of flow-induced friction reduction and provide our comments from an experimental point of view. For branched polymers, we provide our understanding on how to control extensional viscosity for polymers with different zero-shear-rate viscosity based on recent experimental results of star and comb polymer melts. For ring polymers, we summarize the recent progress in extensional rheology and provide our view on what would be interesting for further investigation. On the basis of the above results and discussions, we show that extensional rheology is complicated for even very simple molecular architectures. More data from both experiments and simulations are important for improvement of a fundamental understanding and development of theories, which is essential for molecular design of polymer materials.

In this work, fumed silica was compounded with silicone rubber and processed under two different extraction conditions to quantitatively analyze bound rubber for studying filler–rubber interaction. When nanocomposites were treated with toluene at 90 °C under ultrasonic (US) irradiation, physisorbed polymer chains were more substantially removed when compared to the traditional room temperature (RT) extraction method. The bound rubber fraction of silicone rubber filled with 40 phr of silica was 32.85% by RT treatment, while the US-treated one was reduced to 6.20%. A thin layer of tightly bound rubber adsorbed on the silica surface could be obtained by US treatment as observed by scanning electron microscopy and transmission electron microscopy. Low-field 1 H NMR results confirmed that the obtained tightly bound rubber chains were strongly constrained. The attenuated total reflectance-infrared spectra showed that the formation of tightly bound rubber caused redshift of the Si–O–Si characteristic peaks of silica, and the obtained tightly bound rubber has lower total surface energy than…

Sustainable and energy-efficient molecular separation requires membranes with high permeability and selectivity. Among various membranes, the mixed matrix membranes (MMMs) prepared from two-dimensional MXene materials with well-defined nanochannels are expected to be used for the precise separation of molecules. However, the stacking of a large number of 2D MXene nanosheets creates gas mass transfer barriers, resulting in a significant reduction in gas permeability. To solve this issue, herein, a high-performance self-supporting Pebax/CMC@MXene MMM was prepared by a combination of 2D MXene nanosheets with branched carboxymethyl cellulose (CMC) for CO 2 gas separation from natural gas and flue gas. Gas separation experimental results show that the as-prepared self-supporting Pebax/CMC@MXene MMMs can significantly improve gas permeance and selectivity. When the loading of MXene nanosheets in the CMC@MXene filler was fixed at 1.5 mg/mL, the CO 2 permeance of Pebax/CMC@MXene MMMs was found to be 521 GPU, and the CO 2 /N 2 selectivity was calculated to be 40.1. Meanwhile, the CO 2…

A mathematical model that can provide good predictions of the solute diffusion coefficient in hydrogels would be highly beneficial in designing hydrogels for biomedical and industrial applications, and a number of such models have been derived. Mesh size plays a prominent role in determining the solute diffusion coefficient within a hydrogel. However, in assessing the predictive ability of models derived for this purpose, we have employed various values of the mesh size, i.e., the correlation length or the mesh radius. Herein, a systematic examination of the use of the correlation length or the mesh radius as the mesh size was performed in assessing the predictive quality of four recent models: a semiempirical Cukier hydrodynamic model, an obstruction model, an obstruction-exclusion model, and a combined free volume/obstruction model. The use of the correlation length as the mesh size along with the obstruction model yielded the most consistent agreement between experimental data and model predictions. In contrast, use of the mesh radius did not yield good agreement with the…

Three-dimensional (3D) printing based on photoinduced reversible addition–fragmentation chain transfer (RAFT) polymerization is emerging as a versatile and powerful method to prepare “living” 3D objects, which can be postmodified with various functionalities. However, an additional photoinitiator or photocatalyst is necessary in these systems, which is toxic and will cause negative effects on the properties of the prepared materials. Here, we report oxygen-tolerant and rapid living 3D printing based on photoiniferter RAFT polymerization, which does not need additional photoinitiators or photocatalysts. A xanthate, O -ethyl-S-2-ethyl propionate, was chosen as both the photoinitiator and RAFT agent in this process. Various monomers and RAFT agents were screened in this system. Materials with different properties were prepared utilizing the postfunctionalization of the printed living objects. Furthermore, a polymer welding method was proposed by painting fresh monomers between two living objects for post-photocuring. This photoiniferter RAFT polymerization-based living 3D printing…

Recyclability and reprocessability of permanently cross-linked polymeric materials have received considerable scientific and technological attention in view of the environmental pollution and sustainable development. By introducing dynamic covalent bonds, vitrimers are emerging as a promising attempt to address this pressing challenge. However, there is still a lack of thermodynamic and kinetic understanding of the bond exchange reactions (BERs) of vitrimers at the molecular level. Herein, by employing coarse-grained molecular dynamics simulations, we successfully construct a model vitrimer system composed of a polymer network formed from linear chains, which can rearrange the network topology via BERs. In this study, we examine the effect of the bond swap energy barrier (Δ E sw ) on a variety of mechanical properties. We find that Δ E sw critically controls the dynamics of the linear chains and the reactive beads located on the linear chain. Our results indicate that the best mechanical performance characteristics are achieved at an intermediate value of Δ E sw . Meanwhile, stress…

Dynamic networks couple the robust nature of thermosets with the shapeability and recyclability of thermoplastics. Though a wealth of exchange chemistries can be leveraged to fabricate dynamic networks, reversibly cross-linked systems that are comprised primarily of chains with nonexchangeable backbones increase the network complexity and render flow behavior difficult to predict. Here, we report the flow behavior of polymethacrylate-based vitrimers prepared by cross-linking β -ketoester-containing polymers with multifunctional amines and demonstrate that the molecular weight and dispersity of the primary chains can be tuned to actively tailor the dynamics of the resulting networks. Vitrimers of similar composition and cross-link density made from polymers below and well above the entanglement molecular weight exhibited distinct viscoelastic flow properties with energies of activation ranging from 93 to 264 kJ/mol. Resistance to creep increased dramatically with increasing primary chain length. Finally, blending of prepolymers to skew M n above/below M e was demonstrated, showing a…

It is highly desirable to develop chemically recyclable polymers to address the challenge in establishing a sustainable circular polymer economy. Despite the mass production and widespread applications, there are limited reported examples for the polyurethanes capable of chemical recycling to monomers with high efficiency and purity. In this contribution, we report the “living”/controlled ring-opening polymerization (ROP) of bio-renewable δ-caprolactone (δCL) at room temperature in bulk using an organobase in combination with urea as a catalyst. The telechelic PδCL diol precursor with well-defined terminal groups can be prepared using catalyst loading as low as 0.05 mol %. A one-pot strategy by cascade ROP of δCL and step-growth polymerization of PδCL diol precursors with diisocyanate under solvent-free conditions produced thermoplastic polyurethane elastomers with excellent elastic recovery, tensile strength, ultimate elongation, and low residue strain. Remarkably, the polyurethanes can be chemically recycled to recover δCL with high purity and excellent yield (∼99%) by simple…

Stretchable ionogels have been considered as ideal materials for constructing flexible electronics. However, current ionogels suffer from the well-known trade-off between mechanical strength and conductivity. Here, we develop a simple strategy based on polymerization-induced phase separation to simultaneously enhance the mechanical performance and conductivity of the ionogels by randomly copolymerizing a hydrophobic and a hydrophilic monomer in a hydrophobic ionic liquid (IL). The polymerization process induced the formation of a bicontinuous network containing a polymer-rich phase and a solvent-rich phase. The polymer-rich domains with hydrogen bonds can bear loading, greatly improving the mechanical strength; meanwhile, the solvent-rich domains form conductive nanochannels to enhance the conductivity. The resulting copolymer ionogel is highly stretchable (500% strain), and the optimal fracture stress and conductivity are 0.29 MPa and 3.4 mS/cm, achieving 7.8- and 2.3-fold enhancements compared with that of the prepared homogeneous (pure PMEA) ionogel at the same IL content,…

The development of new technologies for the removal of a family of manufactured chemicals, the per- and polyfluoroalkyl substances (PFAS), from the environment is urgently needed to safeguard public and environmental health. Here we report a fundamental study of the binding mechanisms driven by fluorine–fluorine and electrostatic interactions between perfluorooctanoic acid (PFOA), an important PFAS molecule, and three types of block copolymer sorbents containing individually perfluoropolyether (PFPE) or quaternized ammonium groups, or both functional segments in combination. The results show that both the fluorine–fluorine interactions between the PFPE segment of the block copolymer and the fluorinated tail of the PFOA as well as electrostatic attraction between the quaternized ammonium group and the anionic PFOA headgroup are crucial to achieve effective PFOA sorption from aqueous solutions. The fluorine–fluorine interactions contribute to recognition of PFOA molecules via fluorophilicity, with fast exchange between bound and free PFOA being observed, while the electrostatic…

Sulfur-containing silane coupling agents (SSCAs) are widely incorporated into silica-filled rubber composites to improve silica dispersion and enhance interfacial adhesion, maximizing the potential of silica. However, SSCA synthesis involves multistep procedures and requires organic solvents. In addition, modifying rubber composites with SSCAs has persistent issues, such as low coupling efficiency and uncontrollable structure. Herein, we report the facile synthesis of alkoxysilyl-functionalized polysulfides (SPVs) with a tunable molecular structure (sulfur rank and alkoxysilyl fraction) via inverse vulcanization of vinyltriethoxysilane (VTES) with a copolymer of sulfur and styrene. SPVs serve as interfacial modifiers in silica-filled rubber composites by separately reacting with the silica and rubber chains. In particular, the multiple alkoxysilyl sites in the SPV chain increase the probability of silanization, yielding high coupling efficiency in the rubber composite. Although the molar content of alkoxysilyl groups in SPV1 (the mass feed ratio of VTES and copolymer is 1) is half…

We implement a hybrid molecular dynamics/Monte Carlo simulation to study the microscopic dynamics and the macroscopic rheology of vitrimers with a fast bond exchange rate. We show that the linear viscoelastic properties and mean squared displacement of the vitrimers collapse onto master curves by applying the same shift factors that follow the Williams–Landel–Ferry equation at low temperatures and Arrhenius-like behavior at high temperatures. The linkage between the microscopic dynamics and the linear rheology of vitrimers is established using the generalized Stokes–Einstein relationship, which efficiently extends the timescale of simulations and predicts the viscoelasticity. The values of the shift factors are related to the characteristic decay time of the intermediate scattering function, which is accessible in scattering experiments. The same results hold in the case of an all-atom model of an ionic liquid. Our methodology provides a microscopic basis for the time-superposition principle and predicts the macroscopic rheology of thermo-rheologically simple vitrimers.

Elastomer toughening of plastics is extensively acknowledged as a robust route to develop high-performance materials. However, the drastic sacrifice of tensile strength resulting from overloading of the elastomer remains unresolved. So far, the elastomers used as toughening agents are mainly non-crosslinked since linear chains are usually considered to exhibit superiority in adsorbing the impact energy. Herein, we found that the micro-crosslinking of a commercially available linear elastomer, ethylene-acrylic ester-glycidyl methacrylate terpolymer (EGMA), is significantly beneficial for the toughening of polylactide (PLLA). The reactive micro-crosslinked EGMA (RMCE) with various crosslinking densities was feasibly fabricated through melt processing and used as a more efficient toughener for the brittle PLLA than the linear elastomer. The notched impact strength of the PLLA/RMCE was 35.9 kJ/m 2, which was more than 5 times that of PLLA/EGMA (7.0 kJ/m 2 ) at the same 15 wt % elastomer loadings. At the same time, the tensile strength and Young’s modulus kept high values of 51 MPa and…

When mixed, solutions of positive and negative polyelectrolytes may spontaneously phase-separate into blended, hydrated complexes or coacervates (PECs). Charge-pairing interactions between oppositely-charged polyelectrolytes within PECs are weakened with the addition of salt MA. With a sufficiently high concentration of MA, the PEC may dissociate back into the individual polyelectrolytes, reversing the liquid–liquid phase separation induced by charge pairing and other interactions. This critical salt concentration (CSC), or “salt resistance,” has been extensively used to compare the stability and strength of association in PECs. However, the CSC is not always observed, and it shows a strong dependence on the type of ions comprising MA. In addition, the CSC is more likely to be observed with PECs assembled from polycarboxylates, a type of weak polyelectrolyte. Here, it is shown that a lack of experimental CSC is correlated with the preferred role of ions M + and A – in the PEC, counterion versus co-ion, or the specificity of a particular ion for a particular polyelectrolyte repeat…

High Resolution Image Download MS PowerPoint Slide The work explains the mechanism of the improvement of oxygen barrier performance of cold-crystalized polylactide (PLA) modified with low molecular weight compounds (triethyl citrate, TEC; glycerol, Gly; and polyethylene glycol, PEG). It was found that the incorporation of small amounts of modifier (0.5–1.5 wt %) into the PLA matrix greatly reduces the oxygen transport properties. On the other hand, a further increase in the modifier content in PLA blends led to a deterioration of these properties. Based on the thermal analysis, it was shown that the modifier present in the PLA matrix did not affect the degree of crystallinity. Positron annihilation lifetime spectroscopy (PALS) and density measurements were used to correlate the observed changes in the parameters of oxygen permeability with the molecular packing in the interlamellar amorphous region. Overall, it was found that the nanostructure of the amorphous phase, largely dependent on the content and compatibility of the modifier with the polymer matrix, is of key importance in…

To overcome the current scarcity of fresh water sustainably, new technologies will be required that produce potable water from a range of sources, including seawater and moisture from the atmosphere. Moreover, we must recover and reuse water from wastewater streams to reduce our global water footprint. To date, there remain significant concerns about the environmental/ecological impact, high energy consumption, and extensive maintenance costs of current technologies that might prevent their transition to more sustainable routes of potable water generation. One class of material that can enable low-energy water production is thermoresponsive polymers. Due to their unique phase behavior, production flexibility, and biocompatibility, these materials may allow for sustainable routes to fresh water in current and new technologies. In this Perspective, we specifically summarize the design and application of poly(N-isopropylacrylamide)- (PNIPAm-) based thermoresponsive microgels and hydrogels. In particular, we show how these materials have been used for water purification, including…

We describe the synthesis of ultrahigh molecular weight water-soluble polymers via photoiniferter polymerization under inverse miniemulsion conditions. The catalyst-free heterogeneous process is mediated using low-intensity UV irradiation and offers rapid polymerization rates, excellent molecular weight control, polymer end-group fidelity, temporal control, advanced architectures, and most notably, viscosity control. We refined the polymerization conditions by considering the nature of the surfactant, costabilizer, and iniferter agent to achieve acrylamido homopolymers and block copolymers of molecular weights exceeding 1,000,000 Da at ambient temperature. This approach to well-defined ultrahigh molecular weight polymers overcomes the complications of high viscosity to facilitate eventual scale-up.

A series of 50 oxime esters bearing various substituents were designed and synthesized as photoinitiators (48 of them were never synthesized before and only B1 and B10 were reported). Good light absorption properties in the visible range were observed for these oxime esters. Some structures exhibited better photoinitiation abilities than diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) upon exposure to a LED@405 nm. Interestingly, PIs with methyl substituent on the oxime ester group have better photoinitiation performance than the others. Substituent effects were investigated through molecular orbitals calculations, the detection of CO 2, and the investigation of the generated free radicals. The results demonstrate that the substituents on the oxime ester group exert major effects on the photoinitiation ability via the decarboxylation reaction. In addition, a chemical mechanism was proposed. 3D printed objects were successfully obtained by using the most reactive oxime ester as photoinitiator, and their thermal initiation behaviors of several oxime esters were also studied…

Natural rubber (NR) has a strategic position in the rubber industry due to its excellent properties, but it faces the problem of insufficient output and difficulty in recycling. Here, a high-performance elastomer was prepared by the simple epoxidation modification of bio-based Eucommia ulmoides gum (EUG). For the first time, it was found that epoxidized EUG (EEUG) with an epoxy degree of 12.3%–17.4% no longer crystallized at room temperature but had the ability to strain-induced crystallization (SIC). As a result, its vulcanizates showed excellent tensile strength (17 MPa), toughness (37.8 MJ m –3 ), and elasticity (strain recovery values >97%) without filler reinforcement, which is comparable with vulcanized NR. The SIC of EEUG was studied by synchrotron radiation wide-angle X-ray diffraction (SR-WAXD), and its performance was also compared with NR under the reinforcement of carbon black. In addition, through cross-linking by disulfide bonds, EEUG could be used to prepare high-performance (tensile strength of 10.5 MPa, toughness of 30.7 MJ m –3 ), recyclable green elastomers. This…

Current synthetic epoxy thermosets exhibit a trade-off between stiffness and toughness. Despite the urgent demands for highly stiff and tough epoxy thermosetting resins, existing technologies have struggled to attain resins that satisfy this balance. To circumvent this inherent trade-off, a new strategy to obtain a double load-sharing epoxy thermoset network via hierarchical curing process without adding any other filler is proposed. In the epoxy resin bearing acetyl ester side groups cured by an aromatic amine, the “in situ” step-growth phase-separated morphologies and hydrogen-bonding interactions between hydroxyl groups and the ester groups, which endow the resin with high strength and toughness, can be easily controlled by simply tuning the feed ratio of the amine curing agent. Notably, previously unachievable control of mechanical properties is attained, the processing performance is improved, and the product resists hydrothermal aging. The successful application of this strategy provides promising perspectives for construction versatile and high-performance thermosetting.