SciCombinator

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Concept: Thermosetting polymer

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Nitrogen-based thermoset polymers have many industrial applications (for example, in composites), but are difficult to recycle or rework. We report a simple one-pot, low-temperature polycondensation between paraformaldehyde and 4,4'-oxydianiline (ODA) that forms hemiaminal dynamic covalent networks (HDCNs), which can further cyclize at high temperatures, producing poly(hexahydrotriazine)s (PHTs). Both materials are strong thermosetting polymers, and the PHTs exhibited very high Young’s moduli (up to ~14.0 gigapascals and up to 20 gigapascals when reinforced with surface-treated carbon nanotubes), excellent solvent resistance, and resistance to environmental stress cracking. However, both HDCNs and PHTs could be digested at low pH (<2) to recover the bisaniline monomers. By simply using different diamine monomers, the HDCN- and PHT-forming reactions afford extremely versatile materials platforms. For example, when poly(ethylene glycol) (PEG) diamine monomers were used to form HDCNs, elastic organogels formed that exhibited self-healing properties.

Concepts: Polymer chemistry, Monomer, Plastic, Carbon fiber, Thermoplastic, Polyethylene terephthalate, Thermosetting plastics, Thermosetting polymer

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Here we report the synthesis of thermosetting resins from low molar mass Kraft lignin fractions of high functionality, re-fined by solvent extraction. Such fractions were fully characterized by 31P-NMR, 2D-HSQC NMR, SEC and DSC in order to obtain a detailed description of the structures. Reactive oxirane moieties were introduced on the lignin backbone under mild reaction conditions and quantified by simple 1H-NMR analysis. The modified fractions were chemically cross-linked with a flexible polyether diamine (Mn≈2000), in order to obtain epoxy thermosets. Epoxies from different lignin fractions, studied by DSC, DMA, tensile tests and SEM, demonstrated substantial differences in terms of thermo-mechanical prop-erties. For the first time, strong relationships between lignin structures and epoxy properties could be demonstrated. The suggested approach provides unprecedented possibilities to tune network structure and properties of thermosets based on real lignin fractions, rather than model compounds.

Concepts: Chemical reaction, Polymer chemistry, Lignin, Plastic, Thermoplastic, Kraft process, Thermosetting plastics, Thermosetting polymer

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Design and direct fabrication of high-performance thermosets and composites via 3D printing are highly desirable in engineering applications. Most 3D printed thermosetting polymers to date suffer from poor mechanical properties and low printing speed. Here, a novel ink for high-speed 3D printing of high-performance epoxy thermosets via a two-stage curing approach is presented. The ink containing photocurable resin and thermally curable epoxy resin is used for the digital light processing (DLP) 3D printing. After printing, the part is thermally cured at elevated temperature to yield an interpenetrating polymer network epoxy composite, whose mechanical properties are comparable to engineering epoxy. The printing speed is accelerated by the continuous liquid interface production assisted DLP 3D printing method, achieving a printing speed as high as 216 mm h-1 . It is also demonstrated that 3D printing structural electronics can be achieved by combining the 3D printed epoxy composites with infilled silver ink in the hollow channels. The new 3D printing method via two-stage curing combines the attributes of outstanding printing speed, high resolution, low volume shrinkage, and excellent mechanical properties, and provides a new avenue to fabricate 3D thermosetting composites with excellent mechanical properties and high efficiency toward high-performance and functional applications.

Concepts: Polymer chemistry, Plastic, Composite material, Inkjet printer, Epoxy, Thermoplastic, Thermosetting plastics, Thermosetting polymer

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Many research groups have developed unique micro/nano-structured dry adhesives by mimicking the foot of the gecko with the use of molding methods. Through these previous works, polydimethylsiloxane (PDMS) has been developed and become the most commonly used material for making artificial dry adhesives. The material properties of PDMS are well suited for making dry adhesives, such as conformal contacts with almost zero preload, low elastic moduli for stickiness, and easy cleaning with low surface energy. From a performance point of view, dry adhesives made with PDMS can be highly advantageous but are limited by its low productivity, as production takes an average of approximately two hours. Given the low productivity of PDMS, some research groups have developed dry adhesives using UV-curable materials, which are capable of continuous roll-to-roll production processes. However, UV-curable materials were too rigid to produce good adhesion. Thus, we established a PDMS continuous-production system to achieve good productivity and adhesion performance. We designed a thermal roll-imprinting lithography (TRL) system for the continuous production of PDMS microstructures by shortening the curing time by controlling the curing temperature (the production speed is up to 150 mm min-1). Dry adhesives composed of PDMS were fabricated continuously via the TRL system.

Concepts: Plastic, Materials science, Material, Continuity, Adhesion, Adhesive, Productivity, Thermosetting polymer

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Anisotropic polymers with aligned macromolecule chains exhibit directional strengthening of mechanical and physical properties. However, manipulating the orientation of polymer chains in a fully cured thermoset is almost impossible due to its permanently crosslinked nature. In this paper, we demonstrate that rearrangeable networks with bond exchange reactions (BERs) can be utilized to tailor the anisotropic mechanical properties of thermosetting polymers. When a constant force is maintained at BER activated temperatures, the malleable thermoset creeps in the direction of stress, and macromolecule chains align themselves in the same direction. The aligned polymer chains result in an anisotropic network with a stiffer mechanical behavior in the direction of creep, while with a more compliant behavior in the transverse direction. The degree of network anisotropy is proportional to the amount of creep strain. A multi-length scale constitutive model is developed to study the creep-induced anisotropy of thermosetting polymers. The model connects the micro-scale BER kinetics, orientation of polymer chains, and directional mechanical properties of network polymers. Without any fitting parameters, it is able to predict the evolution of creep strain at different temperatures and anisotropic stress-strain behaviors of CANs after creep. Predictions on the chain orientation are verified by molecular dynamics (MD) simulation. Based on parametric studies, it is shown that the influences of creep time and temperature on the network anisotropy can be generalized into a single parameter, and the evolution of directional modulus follows an Arrhenius type time-temperature superposition principle (TTSP). The presented work provides a facile approach to transform isotropic thermosets into anisotropic ones using simple heating, and their directional properties can be readily tailored by the processing conditions.

Concepts: DNA, Polymer, Polymer chemistry, Temperature, Plastic, Thermoplastic, Isotropy, Thermosetting polymer

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The unique capability of topological rearrangement for dynamic covalent polymer networks has enabled various unusual properties (self-healing, solid-state plasticity, and reprocessability) that are not found in conventional thermosets. Achieving these properties in one network in a synergetic fashion can open up new opportunities for shape memory polymer. To accomplish such a goal, the freedom to tune topological rearrangement kinetics is critical. This is, however, challenging to achieve. In this work, two sets of dynamic bonds (urethane and hindered urea) are incorporated into a hybrid network for synthesizing shape memory poly(urea-urethane). By changing the bond ratio, networks with highly tunable topological rearrangement kinetics are obtained. Combining self-healing, solid-state plasticity, and reprocessability in such one shape memory network leads to unusual versatility in its shape shifting performance.

Concepts: Electron, Polymer chemistry, Plastic, Bond, Bonds, Lancashire, The Bond, Thermosetting polymer

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The synthesis of polymers from renewable resources is a burning issue that is actively investigated. Polyepoxide networks constitute a major class of thermosetting polymers and are extensively used as coatings, electronic materials, adhesives. Owing to their outstanding mechanical and electrical properties, chemical resistance, adhesion, and minimal shrinkage after curing, they are used in structural applications as well. Most of these thermosets are industrially manufactured from bisphenol A (BPA), a substance that was initially synthesized as a chemical estrogen. The awareness on BPA toxicity combined with the limited availability and volatile cost of fossil resources and the non-recyclability of thermosets implies necessary changes in the field of epoxy networks. Thus, substitution of BPA has witnessed an increasing number of studies both from the academic and industrial sides. This review proposes to give an overview of the reported aromatic multifunctional epoxide building blocks synthesized from biomass or from molecules that could be obtained from transformed biomass. After a reminder of the main glycidylation routes and mechanisms and the recent knowledge on BPA toxicity and legal issues, this review will provide a brief description of the main natural sources of aromatic molecules. The different epoxy prepolymers will then be organized from simple, mono-aromatic di-epoxy, to mono-aromatic poly-epoxy, to di-aromatic di-epoxy compounds, and finally to derivatives possessing numerous aromatic rings and epoxy groups.

Concepts: DNA, Polymer chemistry, Chemical substance, Bisphenol A, Plastic, Epoxy, Thermosetting plastics, Thermosetting polymer

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Poly(urea-urethane) thermosets containing the 1-tert-butylethylurea (TBEU) structure feature a reversible dissociation/association process of their covalent linkages under mild conditions. Unlike conventional thermosets, TBEU-based poly(urea-urethane) thermosets maintain their malleability after curing. Under high temperature (100 °C) and applied pressure (300 kPa), ground TBEU thermoset powder can be remolded to bulk after 20 min.

Concepts: Aluminium, Steel, Ductility, Thermosetting polymer

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A novel method is presented to fabricate high-resolution, high-aspect ratio metal wires embedded in a plastic substrate for flexible electronics applications. In a sequential process, high-resolution channels connected to low-resolution reservoirs are first created in a thermosetting polymer by imprint lithography. A reactive Ag ink is then inkjet-printed into the reservoirs and wicked into the channels by capillary forces. These features serve as a seed layer for copper deposition inside the channels via electroless plating. Highly conductive wires (>50% bulk metal) with minimum line width and spacing of 2 and 4 μm, respectively, and an aspect ratio of 0.6 are obtained. The embedded wires exhibit good mechanical flexibility, with minimal degradation in electrical performance after thousands of bending cycles.

Concepts: Metal, Copper, Plastic, Aspect ratio, Plating, Printed circuit board, Flexible, Thermosetting polymer

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A novel bio-based epoxy monomer with conjugated double bonds, glycidyl ester of eleostearic acid (GEEA) was synthesized from tung oil fatty acids and characterized by 1H-NMR and 13C-NMR. Differential scanning calorimeter analysis (DSC) and FT-IR were utilized to investigate the curing process of GEEA with dienophiles and anhydrides. DSC indicated that GEEA could crosslink with both dienophiles and anhydrides through Diels-Alder reaction and epoxy/anhydride ring-opening reaction. Furthermore, Diels-Alder crosslink was much more active than the ring-opening of epoxy and anhydride in the curing process. FT-IR also revealed that GEEA successively reacted with dienophiles and anhydrides in both crosslinking methods. Dynamic mechanical analysis (DMA) and mechanical tensile testing were used to study the thermal and mechanical properties of GEEA cured by maleic anhydride, nadic methyl anhydride and 1,1'-(methylenedi-4,1-phenylene)bismaleimide. Due to the independence between the curing agents, dienophile and anhydride, a series of thermosetting polymers with various properties could be obtained by adjusting the composition of these two curing agents.

Concepts: Polymer, Polymer chemistry, Fat, Carboxylic acid, Differential scanning calorimetry, Thermoplastic, Dynamic mechanical analysis, Thermosetting polymer