Dynamic covalent polymer networks represent a rapidly emerging class of polymeric materials, capable of self-repairing when mechanically damaged. These materials also possess the ability to being dissolved and reformed, conferring upon objects made of such materials a longer service life, with positive economic and environmental impacts. While most such materials developed to date have a poorly-defined structure, as they are randomly cross-linked, better-defined dynamic covalent polymer networks comprising model building blocks attract increasing interest, both because of enhanced mechanical properties and offering themselves for more precise studies. This investigation presents the development of model dynamic covalent polymer networks, cross-linked via acylhydrazone bonds, and based on end-linked star oligomers, that is, having a size intermediate between polymeric stars and monomers. After their appropriate end-functionalization and purification, the oligomeric star building blocks were used to form polymeric networks in an organic solvent (organogels), which were subsequently characterized in terms of their swelling, mechanical, and dynamic properties. 相似文献
Increasing the flexibility of polymer chains is a common method of increasing the deformability of solid polymeric materials. Here, the effects of “conjugation‐break spacers” (CBSs)—aliphatic units that interrupt the sp2‐hybridized backbone of semiconducting polymers—on the mechanical and photovoltaic properties of a diketopyrrolopyrrole‐based polymer are described. Unexpectedly, the tensile moduli and cracking behavior of a series of polymers with repeat units bearing 0%, 30%, 50%, 70%, and 100% of the CBS are not directly related to the percent incorporation of the flexible unit. Rather, the mechanical properties are a strong function of the order present in the film as determined by grazing‐incidence x‐ray diffraction. The effect of the CBSs on the photovoltaic performance of these materials, on the other hand, is more intuitive: it decreases with increasing fraction of the flexible units. These studies highlight the importance of solid‐state packing structure—as opposed to only the flexibility of the individual molecules—in determining the mechanical properties of a conjugated polymer film for stretchable, ultraflexible, and mechanically robust electronics.
The development of more sustainable materials with a prolonged useful lifetime is a key requirement for a transition towards a more circular economy. However, polymer materials that are long-lasting and highly durable also tend to have a limited application potential for re-use. This is because such materials derive their durable properties from a high degree of chemical connectivity, resulting in rigid meshes or networks of polymer chains with a high intrinsic resistance to deformation. Once such polymers are fully synthesised, thermal (re)processing becomes hard (or impossible) to achieve without damaging the degree of chemical connectivity, and most recycling options quickly lead to a drop or even loss of material properties. In this context, both academic and industrial researchers have taken a keen interest in materials design that combines high degrees of chemical connectivity with an improved thermal (re)processability, mediated through a dynamic exchange reaction of covalent bonds. In particular vitrimer materials offer a promising concept because they completely maintain their degree of chemical connectivity at all times, yet can show a clear thermally driven plasticity and liquid behavior, enabled through rapid bond rearrangement reactions within the network. In the past decade, many suitable dynamic covalent chemistries were developed to create vitrimer materials, and are now applicable to a wide range of polymer matrices. The material properties of vitrimers, however, do not solely rely on the chemical structure of the polymer matrix, but also on the chemical reactivity of the dynamic bonds. Thus, chemical reactivity considerations become an integral part of material design, which has to take into account for example catalytic and cross-reactivity effects. This mini-review will aim to provide an overview of recent efforts aimed at understanding and controlling dynamic cross-linking reactions within vitrimers, and how directing this chemical reactivity can be used as a handle to steer material properties. Hence, it is shown how a focus on a fundamental chemical understanding can pave the way towards new sustainable materials and applications.In this minireview, we survey recent advances in the development of vitrimer materials. Focus on how to chemically control their material properties is used to highlight challenges for boosting the potential of this emerging class of polymer materials.相似文献
In the past decades a shift in paradigm took place in industrial polymer research for structural materials. Only a few new polymers based on new monomeric building blocks were developed. The main focus is now on tailoring improved “old polymers” with well-defined structure and properties based on a set of low cost “old” monomers using controlled polymerization mechanisms. 相似文献
Most of the industrial plastics used up to now consist of flexible macromolecules in which the repeat units are joined together in a nonlinear fashion by rotating bonds. Stiff-chain polymers, which in the ideal case have a rodlike shape, have received much less attention. The reason for this is that such polymers usually exhibit an exceedingly low solubility. Very often, they are absolutely insoluble and do not melt. However, in recent years these polymers have aroused technical interest because high-modulus fibers and moldings can be fabricated from them. These favorable mechanical properties are directly related to the nearly parallel arrangement of the rodlike macromolecules in the solid state which is achieved through spinning or extrusion from a liquid crystalline (nematic) state. It is therefore evident that a thorough understanding of the phase behavior and the structure is required for a universal technical utilization of these materials. The great variety of methods used in polymer science today has led to a deeper understanding of stiff-chain polymers. Experimental results together with theoretical modeling of the phase behavior have direct implications for the practical use of these macromolecular materials. 相似文献
Supramolecular polymers[1] are introduced as a new approach to come to materials in which the repeating units are not connected by covalent bonds but by specific secondary interactions. Self-complementary quadruple hydrogen bonded structures with high association constants are presented as easy to synthesize fragments in supramolecular polymers. Some of the many possibilities of equilibrium polymers are discussed, while it is shown that these supramolecular polymers can obtain materials properties normally only obtained with macromolecules. 相似文献
Photoresponsive materials that change in response to light have been studied for a range of applications. These materials are often metastable during irradiation, returning to their pre-irradiated state after removal of the light source. Herein, we report a polymer gel comprising poly(ethylene glycol) star polymers linked by Cu24L24 metal–organic cages/polyhedra (MOCs) with coumarin ligands. In the presence of UV light, a photosensitizer, and a hydrogen donor, this “polyMOC” material can be reversibly switched between CuII, CuI, and Cu0. The instability of the MOC junctions in the CuI and Cu0 states leads to network disassembly, forming CuI/Cu0 solutions, respectively, that are stable until re-oxidation to CuII and supramolecular gelation. This reversible disassembly of the polyMOC network can occur in the presence of a fixed covalent second network generated in situ by copper-catalyzed azide-alkyne cycloaddition (CuAAC), providing interpenetrating supramolecular and covalent networks. 相似文献
Photoresponsive materials that change in response to light have been studied for a range of applications. These materials are often metastable during irradiation, returning to their pre‐irradiated state after removal of the light source. Herein, we report a polymer gel comprising poly(ethylene glycol) star polymers linked by Cu24L24 metal–organic cages/polyhedra (MOCs) with coumarin ligands. In the presence of UV light, a photosensitizer, and a hydrogen donor, this “polyMOC” material can be reversibly switched between CuII, CuI, and Cu0. The instability of the MOC junctions in the CuI and Cu0 states leads to network disassembly, forming CuI/Cu0 solutions, respectively, that are stable until re‐oxidation to CuII and supramolecular gelation. This reversible disassembly of the polyMOC network can occur in the presence of a fixed covalent second network generated in situ by copper‐catalyzed azide‐alkyne cycloaddition (CuAAC), providing interpenetrating supramolecular and covalent networks. 相似文献
Dynamic covalent bonds (DCBs) have received significant attention over the past decade. These are covalent bonds that are capable of exchanging or switching between several molecules. Particular focus has recently been on utilizing these DCBs in polymeric materials. Introduction of DCBs into a polymer material provides it with powerful properties including self‐healing, shape‐memory properties, increased toughness, and ability to relax stresses as well as to change from one macromolecular architecture to another. This Minireview summarizes commonly used powerful DCBs formed by simple, often “click” reactions, and highlights the powerful materials that can result. Challenges and potential future developments are also discussed. 相似文献
Multiblock copolymers represent a fascinating class of materials that sits at the very heart of industrial applications and fundamental polymer science. They are most often made of a linear succession of incompatible “soft” and “hard” segments that microphase separate at room temperature while they can be easily re-homogenized upon heating. This thermoreversible character provides them with decisive advantages with respect to other rubber-based materials such as vulcanized elastomers, making them indispensable for the development of a more sustainable polymer industry. Beyond practical opportunities, tailoring the multiblock copolymers morphology has a pivotal role to play in the fundamental understanding of the structure–properties relationship of polymer-based systems. It notably serves to comprehend complex materials such as semicrystalline homopolymers and nanocomposites. Aside from the thorough work developed on well-defined diblock copolymers for half a century, this article review aims to guide the reader into the more intricate world of multiblock copolymers by providing him/her quantitative tools to connect chemical nature, microstructure and mechanical properties. 相似文献
Elastomers of controlled molecular structure were prepared from hydroxyl-terminated atactic poly(propylene oxide) (PPO) chains having number-average molecular weights Mn in the range 800–4360 g mole?1. The chains were end-linked into noncrystallizable trifunctional networks using a specially prepared aromatic triisocyanate. The networks thus obtained were studied with regard to their stress–strain isotherms in the unswollen state, in elongation at 25°C, and with regard to their equilibrium swelling in benzene at 61°C. Values of the modulus in the limit at high deformation were in good agreement with corresponding results previously obtained on networks of poly(dimethylsiloxane) (PDMS). This is of considerable importance since use of the widely used “plateau modulus” as a measure of interchain entangling would suggest that the networks of PPO would have a much higher density of such entanglements than would the corresponding networks of PDMS. The close similarity between the moduli of the two types of networks therefore argues against the idea that such entanglements make large contributions to the equilibrium elastomeric properties of a polymer network. These values of the high deformation modulus are also in good agreement with recent molecular theories as applied to the nonaffine deformation of a “phantom” network. The values of the low deformation modulus were considerably smaller than the values predicted for an affine deformation, however, suggesting that the junction points were not firmly embedded in the network structure. This is presumably due to the relatively low degree of chain-junction entangling in the case of relatively short network chains. The swelling equilibrium results were in very good agreement with the new theory of network swelling developed by Flory. 相似文献
The glass phase of most polymers is typically considered amorphous. However, many commercially important polymer glasses do contain a subtle level of structural order intermediate between the crystalline and truly amorphous phases. These include a number of commercially important polymers. This “Intermediate Order” is distributed homogeneously throughout the glass phase and is therefore quite different than the morphology of semi-crystalline materials. Typical computer simulation methods can fail to accurately reproduce the structure of polymers with intermediate order, because they are often based on the assumption that the polymer structure is truly amorphous. Given the subtle nature of this order, computer simulation is important in understanding this structural order and the unique material properties commonly associated with it. We outline the critical issues required for the successful simulation of polymer glasses with intermediate order, and discuss the results of some continuing research designed to improve the accuracy of such simulations. 相似文献
Structural modularity of polymer frameworks is a key advantage of covalent organic polymers, however, only C, N, O, Si, and S have found their way into their building blocks so far. Here, the toolbox available to polymer and materials chemists is expanded by one additional nonmetal, phosphorus. Starting with a building block that contains a λ5-phosphinine (C5P) moiety, a number of polymerization protocols are evaluated, finally obtaining a π-conjugated, covalent phosphinine-based framework (CPF-1) through Suzuki–Miyaura coupling. CPF-1 is a weakly porous polymer glass (72.4 m2 g−1 BET at 77 K) with green fluorescence (λmax=546 nm) and extremely high thermal stability. The polymer catalyzes hydrogen evolution from water under UV and visible light irradiation without the need for additional co-catalyst at a rate of 33.3 μmol h−1 g−1. These results demonstrate for the first time the incorporation of the phosphinine motif into a complex polymer framework. Phosphinine-based frameworks show promising electronic and optical properties, which might spark future interest in their applications in light-emitting devices and heterogeneous catalysis. 相似文献