Showing papers in "Macromolecules in 2021"

Neighboring Group Participation and Internal Catalysis Effects on Exchangeable Covalent Bonds: Application to the Thriving Field of Vitrimer Chemistry

Abstract: 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 resis...

Stable Co-Continuous PLA/PBAT Blends Compatibilized by Interfacial Stereocomplex Crystallites: Toward Full Biodegradable Polymer Blends with Simultaneously Enhanced Mechanical Properties and Crystallization Rates

Abstract: 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 strength can be acquired, resulting from the i-SC-induced co-continuous morphology; and fourth, i-SC can function as a “rigid” supporting layer between phases even above 200 °C, resulting in significantly enhanced morphology stability in melt. The versatile, facile, and practical strategy offers an industrially relevant technique to fabricate super-robust and fully biobased polymer materials.

Plastic Pollution: A Material Problem?

Abstract: 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 polymers in everyday life and the increased imbalance between their use and efficient end-of-life options, which leads to critical issues such as plastic pollution. We touch on these issues, propose possible solutions, and offer a perspective on the recycling (mechanical, (bio)chemical, and organic), environmental biodegradation of polymers, and promising material design concepts. Our discussion highlights that plastic pollution is not solely a material problem but an issue that the whole of society must bear the responsibility to solve together.

Optimizing Morphology to Trade Off Charge Transport and Mechanical Properties of Stretchable Conjugated Polymer Films

Abstract: 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 large-area printing is discussed in terms of film uniformity and crystallization control. Finally, we summarize the challenges and opportunities in microstructure control of stretchable conjugated polymer films.

Upcycling of Polyethylene Terephthalate to Continuously Reprocessable Vitrimers through Reactive Extrusion

Abstract: 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 sacrificing the original processability.

Chemical Functionalization of Carbon Nanotubes with Polymers: A Brief Overview

Abstract: Carbon nanotubes (CNTs), the one-dimensional allotropes of carbon, have attracted noteworthy research interest since their discovery in 1991 owing to their large aspect ratio, low mass density, and unique chemical, physical, and electronic properties that provide exciting possibilities for nanoscale applications. Nonetheless, two major issues should be considered when working with this sort of nanomaterial: their strong agglomerating tendency, since they are typically present as bundles or ropes of nanotubes, and the metallic impurities and carbonaceous fragments that go along with the CNTs. The successful utilization of CNTs in a wide variety of applications—in particular, in the field of polymer composites—depends on their uniform dispersion and the development of a strong chemical interaction with the polymeric matrix. To achieve these aims, chemical functionalization of their sidewalls and tips is required. In this article, a brief overview of the different approaches for CNT modification using polymers is provided, focusing on the covalent functionalization via “grafting to” or “grafting from” strategies. The characteristics and advantages of each approach are thoroughly discussed, including a few typical and recent examples. Moreover, applications of polymer-grafted CNTs as biosensors, membranes, energy storage substances, and EMI shielding are briefly described. Finally, future viewpoints in this vibrant research area are proposed.

Importance of Broad Temperature Windows and Multiple Rheological Approaches for Probing Viscoelasticity and Entropic Elasticity in Vitrimers

Abstract: Model polydimethylsiloxane telechelic vitrimers with dynamic boronic ester bonds were synthesized to investigate the viscoelastic properties of dynamic networks with extremely low Tg via multiple rheological approaches. Frequency sweeps and stress relaxation tests, conducted at more than 120 °C above Tg, 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 dynamics. Our work points to the importance of assessing a broad temperature window and using multiple approaches in probing vitrimers and dynamic networks.

Hydrogen-Bond Association-Mediated Dynamics and Viscoelastic Properties of Tough Supramolecular Hydrogels

Abstract: 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 el...

Arresting Elevated-Temperature Creep and Achieving Full Cross-Link Density Recovery in Reprocessable Polymer Networks and Network Composites via Nitroxide-Mediated Dynamic Chemistry

Abstract: 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 industrial-grade natural rubber/carbon black composites, exhibit full cross-link density recovery and essentially no creep at 80 °C, where alkoxyamine cross-links are nearly static, after multiple molding cycles at 140/160 °C, where alkoxyamine cross-links are dynamic. This capability to “turn on” and “arrest” dynamic chemistry over a relatively narrow temperature window is attributed to the high activation energy (∼120 kJ/mol) and thus strong temperature dependence of the alkoxyamine dissociation reaction. With this key element of high activation energy for the dissociation reaction in systems undergoing dynamic reversion or the dynamic exchange reaction in vitrimers, it is possible to design covalent network materials with acute temperature response allowing for reprocessability with outstanding elevated-temperature creep resistance.

A Lewis Pair as Organocatalyst for One-Pot Synthesis of Block Copolymers from a Mixture of Epoxide, Anhydride, and CO2

Abstract: Ring-opening copolymerization (ROCOP) of epoxide/anhydride/CO2 for the synthesis of CO2-based copolymers is a promising process, because CO2 is an abundant, nontoxic, and low-cost monomer and has b...

Molecular Dynamics Simulations of Ion-Containing Polymers Using Generic Coarse-Grained Models

Abstract: 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 p...

Nonlinear Shear Rheology of Entangled Polymer Rings

Abstract: Steady-state shear viscosity η(γ) of unconcatenated ring polymer melts as a function of the shear rate γ is studied by a combination of experiments, simulations, and theory. Experiments using pol...

Unentangled Vitrimer Melts: Interplay between Chain Relaxation and Cross-link Exchange Controls Linear Rheology

Abstract: Vitrimers are polymer networks that engage in dynamic associative exchange reactions. Their covalent cross-links preserve network connectivity but permit topology fluctuations, making them both ins...

Secondary Structures of Polypeptide-Based Diblock Copolymers Influence the Microphase Separation of Templates for the Fabrication of Microporous Carbons

Abstract: In this study, we synthesized two forms of the polypeptide-based rod–coil diblock copolymer poly(ethylene oxide-b-γ-benzyl-l-glutamate) (PEO-b-PBLG), comprising hydrophilic PEO as the coil segment ...

Lithium Salt-Induced In Situ Living Radical Polymerizations Enable Polymer Electrolytes for Lithium-Ion Batteries

Abstract: Herein, polymer electrolytes (PEs) were designed and fabricated through lithium salt-induced in situ living radical copolymerization of poly(ethylene glycol) methacrylate (PEGMA) and various (meth)acrylates monomers (methyl methacrylate (MMA), n-butyl acrylate (BA), n-butyl methacrylate (BMA), or styrene) with 18-crown-6-ether (18CE6) as both the solvent of copolymerization and the plasticizer of PEs. The lithium salt plays a dual role of activator for alkyl halides (R–X, X = Br or I) initiators, and lithium-ion source. The polymer electrolyte in situ formed in the Li/LiFePO₄ cell with a cellulose membrane showed excellent compatibility with electrode materials. The Li/P(PEGMA-co-MMA)-based PE/LiFePO₄ cell possessed an initial discharge capacity of 166.5 mAh g–¹ at 0.2C and maintained a capacity of 155.3 mAh g–¹ at 0.2C after 290 cycles. The lithium salt-induced in situ polymerization offers a new strategy toward polymer electrolytes for high-performance lithium-ion batteries.

Biosourced Acetal and Diels–Alder Adduct Concurrent Polyurethane Covalent Adaptable Network

Abstract: 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-l...

Ion Correlations and Their Impact on Transport in Polymer-Based Electrolytes

Abstract: Author(s): Fong, KD; Self, J; McCloskey, BD; Persson, KA | Abstract: 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 insights from such analysis can inform design rules for improved systems.

Effect of Solvent Quality on the Phase Behavior of Polyelectrolyte Complexes

Abstract: The role of polyelectrolyte–solvent interactions, among other non-Coulomb interactions, in dictating the thermodynamics and kinetics of polyelectrolyte complexation is prominent yet sparingly studi...

Mechanical and Structural Consequences of Associative Dynamic Cross-Linking in Acrylic Diblock Copolymers

Abstract: The composition of low-Tg n-butylacrylate-block-(acetoxyaceto)ethyl acrylate block polymers is investigated as a strategy to tune the properties of dynamically cross-linked vinylogous urethane vitrimers. As the proportion of the cross-linkable block is increased, the thermorheological properties, structure, and stress relaxation evolve in ways that cannot be explained by increasing cross-link density alone. Evidence is presented that network connectivity defects such as loops and dangling ends are increased by microphase separation. The thermomechanical and viscoelastic properties of block copolymer-derived vitrimers arise from the subtle interplay of microphase separation and network defects.

Enhanced Photocatalytic Performance of Donor–Acceptor-Type Polymers Based on a Thiophene-Contained Polycyclic Aromatic Unit

Abstract: 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.

Polyelectrolyte Complex Coacervation across a Broad Range of Charge Densities.

Abstract: 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 (wP,c) of wP,c ∼ f0.37±0.01 was extracted; this trend was in good agreement with accompanying simulation predictions. Below f = 0.50, wP,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 (co)polyelectrolytes.

Alkali Metal Carboxylates: Simple and Versatile Initiators for Ring-Opening Alternating Copolymerization of Cyclic Anhydrides/Epoxides

Abstract: In this contribution, we thoroughly investigated the ring-opening alternating copolymerization (ROAC) of cyclic anhydride and epoxide by using commercially available alkali metal carboxylates (AMCs...

Protein-Mimetic Self-Assembly with Synthetic Macromolecules

Abstract: Protein-mimetic amphiphiles have significant promise as a platform to access the complex functions of natural biological materials and incorporate the tunability and environmental resilience of syn...