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The flame retardancy for polymers is very demanding as a lot of polymers are flammable and decompose into small and volatile hydrocarbons upon pyrolysis. In the past, most effective flame retardants for polymers are halogen-based. With the increasing regulatory requirements on safety and environments, halogen-based flame retardants are gradually being replaced with non-halogenated flame retardants. Inorganic metal hydroxides, mainly aluminum hydroxide (ATH) and magnesium hydroxide (MDH), are the largest class of flame retardants by volume. Other non-halogenated flame retardants, such as phosphorus-, nitrogen- and silicon-based, show promises for this property. The up-to-date technique gaps are: (1) for inorganic flame retardants, high loading of 40 - 60 wt% is required, leading to worse mechanical properties and heavy weight, (2) for current non-halogenated ones, most of them are small molecules. Such small flame retardant molecules have relatively poor thermal stability and mechanical strength, and will slowly leach out from the composites surface, leading to performance deterioration and possible environmental pollution. The development of new flame retardants with higher efficacy, lower loading, higher strength and high stability are therefore in need. Targeting this, we have developed new flame retardants with polymeric nature, which theoretically possess the above advantages over the current ones, and demonstrated their effectiveness in various polymers like polypropylene, polyamide. The flame retardant long chains will entangle with polymer matrix chains, stabilizing the whole composites without leaching issues.

MATERIALS FOR HUMANITY (MH 21)

Polymeric Flame Retardants: a Promising Category for Enhanced Performance

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MH 21 is arranged around 6 themes that focus attention on pressing needs that should be addressed to enhance the well-being of communities and provide a forum for presenting breakthrough discoveries.

technical paper

Compatible polymer blends with synergetic performance play an integral part in modern society from household, automobiles to aerospace. Conventional miscible polymer systems often relies on strong chemical interactions, while the inherent entropy effects are often neglected. Using a combination of experiments and simulation, we show that entropy driving forces could control the phase structure and mechanical properties of polymer blends from commercial plastics, creating ultrahigh extensibility (~146%), while maintaining its transparency and high strength (~70MPa).

Entropy-driven Ultratough Biodegradable Polymer Blends

With the increasing demand for the high strength and high fracture toughness composites in aerospace, automobile and other structural applications, great efforts have been devoted to the development of carbon fiber reinforced thermoplastic composites. However, the majority of commercial carbon fibers are sized for reactive thermosetting epoxy matrix, and therefore cannot be used directly for thermoplastic resin impregnation due to the high processing temperatures and poor interfacial interactions with polymer matrix. In this work, novel surfactant-free aqueous dispersions were derived from commercial polyimide resins through emulsion/solvent evaporation technique. The obtained nano-dispersions with uniform particle size around 100 nm and a high Zeta potential of -48.1 mV were deposited on carbon fibers via electrochemical deposition and dip coating method, respectively. The sized carbon fibers were characterized by scanning electron microscopy (SEM) and thermogravimetric (TGA) analysis, and the sizing performance was assessed by single fiber pull-out testing using polyetherimide (PEI) as the polymer matrix. The results show that the prepared polyimide sizing presents a positive effect on the interfacial adhesion of carbon fiber/PEI composites. Moreover, the carbon fibers sized by electrochemical deposition method exhibit higher interfacial shear strength (IFSS) compared to the dip-coated ones due to the increased van der Waals forces between the PEI matrix and sizing layer as well as the chemical bonding between the sizing layer and carbon fiber surface. The developed aqueous polyimide nano-dispersions exhibit great potential for application in the manufacturing of continuous carbon fiber reinforced thermoplastic prepregs.



Novel polyimide-based aqueous nano-dispersions as sizing agents for carbon fiber-reinforced thermoplastic composites

PVA is a polymer containing a large number of hydroxyl groups, which can be used by microorganisms as a source of carbon and energy. Water-soluble PVA has three major advantages: good abrasion resistance, high oil resistance, and high viscosity. Narrow intermolecular gaps and strong intermolecular forces caused the PVA hydroxyl groups for etherification and acetalization reactions to form a flexible film. In this research, The polyvinyl alcohol (PVA) nanofibers with good water resistance and high orientation were prepared by electrospinning technology, and PVA fiber self reinforced PVA resin matrix composite membrane with high strength and high transmittance was successfully prepared by solution impregnation method and self reinforcing concept. The PVA composite film prepared in this paper has good light transmittance, and the light transmittance is more than 85% at the wavelength of 400-800 nm. Compared with pure PVA films, the strength and toughness of random PVA nanofiber reinforced composites (R-PVAF) (30 wt%) and oriented PVA nanofiber reinforced composites (U-PVAF-1) (30 wt%) were both improved. Compared with R-PVA fiber, the strength and toughness of PVA composites can be further improved with the introduction of U-PVA fiber. The tensile strength of pure PVA film is about 30.1 MPa, while the tensile strength of U-PVAF-3 (50 wt%) can reach 70.0 MPa after the self-strengthening of PVA nanofiber, and the tensile strength of U-PVAF-3 (50 wt%) is increased by about 132.6%, indicating that the self-strengthening method can greatly improve the mechanical properties of PVA film.

Tensile properties of PVA nanofiber self-reinforced transparent PVA composite film

The unique characteristics of perovskite nanocrystals (NCs) comprise excellent luminous emissions, flexibility, and remarkable optoelectronic properties. Unfortunately, the fatal drawbacks of perovskite NCs include poor stability and chemical and thermal degradation. Several studies have demonstrated that the stability of CsPbBr3 colloidal NCs could be improved through the room temperature ligand-assisted re-precipitation (LARP) method. Additionally, analysis towards CsPbBr3 NCs surface will aid in developing long-term stability of colloidal NCs based devices. To date, no comprehensive study on the surface chemistry of CsPbBr3 colloidal NCs has been attempted nor initiated.

The Surface Structure of Size-tuneable Colloidal CsPbBr3 Nanocrystals

1) A new technique for synthesis of Au nanoparticle embedded inPoly(methyl-methacrylate-PMMA) and Poly(styrene - PS) freestanding (FS) films using biomimetic templates of black plum andlemon grass leaves is introduced. 2) The black plum leaf is hydrophilic while lemongrass ishydrophobic in nature. 3) The demonstration of these microstructurally replicated surfacesfor Surface enhanced Raman scattering (SERS) and wettabilityapplications is presented. 4) The wettability of the FS films is mainly hydrophilic with angledependent on surface roughness and precursor concentration.5) Detection of 4- aminothiophenol (4-ATP), L-Aspartic Acid andChebulinic acid by the FS films using is demonstrated. Enhancementfactors of the order of 104 are achieved.

Biomimetic-templated growth of gold nanoparticle-Poly(methyl-methacrylate) and Poly(styrene) films for different applications

The future of green electronics possessing great strength and toughness proves to be a promising area of research in this technologically advanced society. This work developed the first fully bendable and malleable toughened PLA green composite by incorporating a multi-functional PHB-rubber copolymer filler that acts as an effective nucleating agent to accelerate the PLA crystallization and performs as a dynamic plasticizer to generate massive polymer chain movement. The resultant biocomposite exhibits a 24-fold and 15-fold increment in both elongation and toughness respectively while retaining its elastic modulus at > 3 GPa. Mechanism study shows the toughening effect is due to an amalgamation of massive shear yielding, crazing, and nano-cavitation in the highly-dense PLA matrix. Uniquely distinguished from the typical flexible polymer that stretches and recovers, this biocomposite is the first report of PLA that can be “bend, twist, turn and fold” at room temperature and exhibit excellent mechanical robustness even after a 180 ° bend, attributes to the highly interconnected polymer network of innumerable nano-cavitation complemented with an extensively unified fibrillar bridges. This unique trait certainly opens up a new horizon to future sustainable green electronics development.

Bend, Twist and Turn: First Bendable and Malleable Toughened PLA Green Composites

Motivation: 1) Greater moisture stability of 2D and quasi-2D layered perovskites than 3D perovskites 2) Reduced phase impurity with control of composition of quasi-2D perovskites for improved optical properties and their application to optoelectronics devices 3) Double hole injection layer for reduced leakage current in optoelectronics device

Optimization of Composition with Reduced Phase Impurity in Quasi-2D Perovskite for Optoelectronics

Magnetic skyrmions are nanoscale spin structures which can be stabilized at room temperature in multilayer films [1, 2]. In particular, the amorphous multilayer film presents a pinning-free material platform, enabling efficient spin-orbit-torque manipulation of skyrmion dynamics. The device-relevant configuration of dots is of immense interest for realizing skyrmionic tunnel junctions for next-generation memory and computing applications [3]. Here, we first establish an amorphous multilayer composite stack comprising [Pt/CoB(x)/Ir]3/[Pt/CoB(y)/MgO], where x: 1.7-2.2 nm and y: 1-1.4 nm, with demonstrable skyrmions of sizes ~100 nm stabilized at zero field (ZF). By varying both the bottom (x) and top (y) CoB thicknesses, the magnetic anisotropy, Keff, can be tuned by an order from -0.03 to 0.126 MJ/m3. Next, we present ZF stabilization of skyrmions confined in such multilayer nanodots over a range of Keff and geometric sizes (w: 300 – 1000nm). Kerr magnetometry characterization of these nanodot arrays shows a monotonic increase in squareness with decreasing w. Expectedly, at ZF, we observe a gradual evolution in magnetic textures from the labyrinthine stripe to skyrmion phase, and eventually to the uniform magnetisation phase with w reduction (Figure 1). Notably, the stabilization of ZF skyrmions is observed over a wide w range from 400 to 700 nm across samples and skyrmion multiplets are stabilized in dots with low Keff. Finally, we demonstrate that at intermediate OP applied fields, skyrmion multiplets can be stabilized across varying w. Micromagnetic simulations are performed to model the geometric confinement effect of the nanodots on the skyrmion nucleation probability and size. These results provide a platform for harnessing the properties of nanoscale skyrmions in confined geometries for device applications.

Study of Confinement Effects on Skyrmions in Amorphous Multilayer Nanodots

Emulating advanced synaptic functions of the human brain with electronic devices contributes an important step toward constructing high-efficiency neuromorphic systems. Ferroelectric materials are promising candidates as synaptic weight elements in neural network hardware due to their controllable polarization states. However, the increased depolarization field at the nanoscale and the complex fabrication process of the traditional ferroelectric materials hamper the development of high-density, low power and highly sensitive synaptic devices. Here we report the implementation of 2D ferroelectrics α-In2Se3 as an active channel material to emulate typical synaptic functions. The α-In2Se3-based synaptic device features multi-mode operations, enabled by the coupled ferroelectric polarization under various voltage pulses applied at both drain- and gate-terminals. Moreover, the energy consumption can be reduced to ~ 1 pJ by using high-κ dielectric (Al2O3). The successful control of ferroelectric polarizations in α-In2Se3 and its application in artificial synapses are expected to inspire the implementation of 2D ferroelectric materials for future neuromorphic systems. （Acknowledgment: This work was supported by the Science and Engineering Research Council of A*STAR (Agency for Science, Technology and Research) Singapore, under Grant No. A20G9b0135.

Intrinsic Polarization Coupling in 2D α-In2Se3 toward Artificial Synapse with Multi-mode Operations

Two-dimensional (2D) van der Waals heterostructure (vdWH)-based floating gate devices show great potential for next-generation nonvolatile and multilevel data storage memory. However, high program voltage induced substantial energy consumption, which is one of the primary concerns, hinders their applications in low-en- ergy-consumption artificial synapses for neuromorphic computing. In this study, we demonstrate a three-terminal floating gate device based on the vdWH of tin disul- fide (SnS2), hexagonal boron nitride (h-BN), and few-layer graphene. The large electron affinity of SnS2 facilitates a significant reduction in the program voltage of the device by lowering the hole-injection barrier across h-BN. Our floating gate device, as a nonvolatile multilevel electronic memory, exhibits large on/off current ratio (~10 ), good retention (over 10 s), and robust endurance (over 1000 cycles). Moreover, it can function as an artificial synapse to emulate basic synaptic func- tions. Further, low energy consumption down to ~7 picojoule (pJ) can be achieved owing to the small program voltage. High linearity (<1) and conductance ratio (~80) in long-term potentiation and depression (LTP/LTD) further contribute to the high pattern recognition accuracy (~90%) in artificial neural network simulation. The proposed device with attentive band engineering can promote the future development of energy-efficient memory and neuromorphic devices.

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