Syntheses and flame retarding properties of DOPO polymers,melamine polymers,and DOPO‐melamine copolymers

The 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) polymers, melamine polymers, and DOPO‐melamine copolymers have been successfully synthesized, and their flame retarding properties have also been investigated by blending with polypropylene (PP)/styrene‐ethylene‐butylene‐styrene (SEBS) alloys. Experimental results establish that all of them are good polymeric flame retardants. No blooming or color stains occur when they are incorporated into PP/SEBS alloys. Among lab‐made polymeric flame retardants, DOPO‐ melamine copolymers exhibit the best thermal stability and nonflammability. PP/SEBS alloys containing DOPO‐melamine copolymers display comparable thermal resistance and flame retarding behavior (T_d = 290°C; char yield: 15.6%, LOI: 23, and flammability: UL‐94 V0) as the alloys containing common commercial flame retardants (i.e., DOPO, melamine, and ammonium polyphosphate). Copyright © 2013 John Wiley & Sons, Ltd.     

Poly(phenylene oxide) modified cyanate resin for self‐healing

Self‐healing cyanate ester resins (CE) were developed by adding low molecular weight poly(phenylene oxide) (PPO) resin, yielding a high performance CE/PPO system via a low‐temperature process. The addition of PPO improved the flexural strength and fracture toughness of the CE matrix without sacrificing thermal properties. CE/PPO formulations with 5, 10, and 15 wt.% PPO showed 43%, 65%, and 105% increase in fracture toughness due to a combination of crack deflection, crack pinning, and matrix cavitation around second‐phase particles. When PPO was introduced into the CE, dielectric properties were either unchanged or declined. During thermal treatment to heal damaged CE, liquid PPO flowed into cracks, and during subsequent cooling, solidified to bond the crack surfaces. The self‐healing efficiency for CE with 15 wt.% PPO after heating to 220°C for 1 h exhibited a recovery of 73% in toughness and 81% in microtensile strength. Copyright © 2014 John Wiley & Sons, Ltd.     

Improving the flame retardancy of polyamide 6 by incorporating hexachlorocyclotriphosphazene modified MWNT

A nitrogen‐, phosphorus‐ and chlorine‐containing flame retardant, hexachlorocyclotriphosphazene (HCTP), has been covalently grafted onto the surface of multi‐wall carbon nanotubes (MWNT) to obtain MWNT‐HCTP. Polyamide 6 (PA6)/MWNT composites were then prepared via melt compounding. The flammability of PA6/MWNT composite was characterized by cone calorimetry, limiting oxygen index (LOI) and UL‐94 tests. The results showed that peak heat release rate of samples containing 3 wt% MWNT‐HCTP was only 460 kW/m², which decreased by 35.2% compared with that of a neat PA6 sample. The LOI value was increased from 22.7% to 26.5%, and UL‐94 test performance was also significantly improved by the presence of MWNT‐HCTP. Scanning electron microscope (SEM) and optical microscope analysis showed that modified MWNT had a better dispersion and compatibility in PA6 than unmodified MWNT. The composition of residue chars and volatile products was investigated by SEM/energy dispersive X‐ray spectroscopy, Fourier transform infrared spectroscopy (FTIR) and thermogravimetric‐FTIR, respectively. It was proposed that grafted HCTP was mainly functioned in the condensed phase, where P, N can synergistically promote char formation and Cl element can catch free radicals to terminate the chain reaction during combustion of the PA6 composite. Copyright © 2014 John Wiley & Sons, Ltd.     

Morphology and mechanical behavior of isotactic polypropylene with different stereo‐defect distribution in injection molding

Large amount of work has been published on the isotacticity–properties relationship of isotactic polypropylene (iPP). However, the stereo‐defect distribution dependence of morphology and mechanical properties of iPP injection molding samples is still not clear. In this study, two different isotactic polypropylene (iPP) resins (PP‐A and PP‐B) with similar average isotacticity but different stereo‐defect distribution were selected to investigate the morphology evolution and mechanical properties (tensile and notching) of their injection molding samples using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), 2D wide angle X‐ray diffraction (2D‐WAXD), and scanning electron microscope (SEM). The results of DMA showed that the molecular movement ability of PP‐A (with less uniform distribution of stereo‐defect) was stronger than that of PP‐B, meanwhile the analysis of DSC and SEM suggested that after injection molding, smaller spherullites, and crystals with higher perfection had formed in the specimens of PP‐A. The resulting of tensile properties of PP‐A were found to be better than that of PP‐B. The results of morphology evolution by SEM observation and 2D‐WAXD showed that PP‐A is more likely to occur interspherulite deformation and can disperse the tensile stress more efficiently, and therefore, its crystal structure can withstand a greater force when tensile stress is applied. On the other hand, PP‐B has larger spherulites and boundaries, and low perfection of lamellaes, and the intraspherulte deformation tend to take place. It is easier for the crystal of PP‐B to be broken up and reoriented along the tensile direction. Copyright © 2014 John Wiley & Sons, Ltd.     

miR-98 suppresses melanoma metastasis through a negative feedback loop with its target gene IL-6

Dysregulated microRNA (miRNA) expression has a critical role in tumor development and metastasis. However, the mechanism by which miRNAs control melanoma metastasis is unknown. Here, we report reduced miR-98 expression in melanoma tissues with increasing tumor stage as well as metastasis; its expression is also negatively associated with melanoma patient survival. Furthermore, we demonstrate that miR-98 inhibits melanoma cell migration in vitro as well as metastatic tumor size in vivo. We also found that IL-6 is a target gene of miR-98, and IL-6 represses miR-98 levels via the Stat3-NF-κB-lin28B pathway. In an in vivo melanoma model, we demonstrate that miR-98 reduces melanoma metastasis and increases survival in part by reducing IL-6 levels; it also decreases Stat3 and p65 phosphorylation as well as lin28B mRNA levels. These results suggest that miR-98 inhibits melanoma metastasis in part through a novel miR-98-IL-6-negative feedback loop.     

Non-isothermal crystallization kinetics and morphology of wollastonite-filled β-isotactic polypropylene composites

To obtain wollastonite-filled β-iPP composites, the wollastonite with β-nucleating surface (β-wollastonite) was prepared through chemical reaction between wollastonite with α-nucleating surface (α-wollastonite) and pimelic acid. The formation of calcium pimelate on the surface of wollastonite was proved using Fourier transform infrared spectrometry and scanning electron microscopy. The crystallization behavior, melting characteristics, non-isothermal crystallization kinetics, and crystalline morphologies of α- and β-wollastonite-filled iPP composites were studied by differential scanning calorimetry and polarizing optical microscopy. It is found that the crystallization peak temperatures of β-wollastonite-filled iPP composites were higher than that of α-wollastonite-filled iPP composites, which indicated that wollastonite with β-nucleating surface has stronger heterogeneous nucleation than that of wollastonite with α-nucleating surface. Although the crystallization temperatures of iPP and iPP composites decreased with increasing cooling rates, α-wollastonite-filled iPP composites mainly crystallized in α-spherulite and β-wollastonite-filled iPP composites formed β-spherulite. In addition, the spherulite size of β-wollastonite-filled iPP composites was smaller than that of α-wollastonite-filled iPP composites. Jeziorny and Mo methods were applicable to study the non-isothermal crystallization kinetics of wollastonite-filled iPP composites. The activation energy (?E) and the nucleation efficiency (EN) of non-isothermal crystallization were calculated by Kissinger method and the equation proposed by Fillon, respectively. The β-wollastonite-filled iPP composites exhibited higher crystallization rate, activation energy, and EN than that of α-wollastonite-filled iPP composites.     

Growth and downconversion luminescence of Ho3+/Yb3+ codoped α‐NaYF4 single crystals by the Bridgman method using a KF flux

Downconversion (DC) luminescence with emission at about 1000 nm under excitation of 448‐nm light in Ho³⁺/Yb³⁺ codoped α‐NaYF₄ single crystal is realized. The crystal was grown by the Bridgman method using KF as an assisting flux in a NaF‐YF₃ system. The energy‐transfer process and quantum cutting (QC) mechanisms are presented through the analysis of the spectra. The energy‐transfer processes of first‐ and second‐order cooperative DC are responsible for the increase of the emission intensity at 1000 nm, and it is the first‐order cooperative DC that is dominant for the DC process. When the Ho³⁺ concentration is fixed at about 0.8 mol%, the optimal concentration for ∼1000 nm emission is 3.02 mol% Yb³⁺ in the current research. The energy‐transfer efficiency and the total quantum efficiency are analyzed through the luminescence decay curves. The maximum quantum cutting efficiency approaches to 184.4% in α‐NaYF₄ single crystals of 0.799 mol% Ho³⁺ and 15.15 mol% Yb³⁺. However, the emission intensity at 1000 nm decreases while the energy‐transfer efficiency from Ho³⁺ to Yb³⁺ increases, which may result from the fluorescence quenching between Ho³⁺ and Yb³⁺ ions, Yb³⁺ and Yb³⁺ ions.     

Effect of pyrolysis characteristics on ignition mechanism and NO emission of pulverized coal during oxy-fuel combustion

The effect of pyrolysis behavior on the ignition mechanism was investigated by thermogravimetric technique. The pyrolysis tests show that Datong bituminous coal (DT) pyrolyzes earlier and releases volatiles faster than does Guohua bituminous coal (GH). During oxy-fuel combustion, more volatiles accumulated around DT particles can be ignited easily with increasing oxygen concentration which results in the heterogeneous ignition transforming to homogeneous ignition, while for GH, less volatile is released during devolatilization and the coal particles are more likely ignited heterogeneously. After the transformation of ignition mechanism, the ignition temperature of DT decreases significantly, but the combustibility index S is not appreciably affected. The effect of pyrolysis characteristics on NO emissions was studied by a fixed-bed reactor. It is found that compared to GH, DT released NO more quickly and intensively which leads to more fuel-N converting to NO. With the rise in oxygen concentration, the NO yields of both coals reach the peak values at 40 % oxygen concentration and then decline mainly due to the enhanced homogeneous NO reductions at higher oxygen concentration. With the rise in furnace temperature, the NO yields of coal samples increase first and then decrease with a maximum at 900 °C which is possibly a result of the competing reactions of volatile-N oxidation and reduction in the process of NO formation.     

Biosynthesis of Streptolidine Involved Two Unexpected Intermediates Produced by a Dihydroxylase and a Cyclase through Unusual Mechanisms

Streptothricin‐F (STT‐F), one of the early‐discovered antibiotics, consists of three components, a β‐lysine homopolymer, an aminosugar D ‐gulosamine, and an unusual bicyclic streptolidine. The biosynthesis of streptolidine is a long‐lasting but unresolved puzzle. Herein, a combination of genetic/biochemical/structural approaches was used to unravel this problem. The STT gene cluster was first sequenced from a Streptomyces variant BCRC 12163, wherein two gene products OrfP and OrfR were characterized in vitro to be a dihydroxylase and a cyclase, respectively. Thirteen high‐resolution crystal structures for both enzymes in different reaction intermediate states were snapshotted to help elucidate their catalytic mechanisms. OrfP catalyzes an Fe^II‐dependent double hydroxylation reaction converting L ‐Arg into (3R,4R)‐(OH)₂‐L ‐Arg via (3S)‐OH‐L ‐Arg, while OrfR catalyzes an unusual PLP‐dependent elimination/addition reaction cyclizing (3R,4R)‐(OH)₂‐L ‐Arg to the six‐membered (4R)‐OH‐capreomycidine. The biosynthetic mystery finally comes to light as the latter product was incorporation into STT‐F by a feeding experiment.     

A Chiral Gold Nanocluster Au20 Protected by Tetradentate Phosphine Ligands

The chirality of a gold nanocluster can be generated from either an intrinsically chiral inorganic core or an achiral inorganic core in a chiral environment. The first structural determination of a gold nanocluster containing an intrinsic chiral inorganic core is reported. The chiral gold nanocluster [Au₂₀(PP₃)₄]Cl₄ (PP₃=tris(2‐(diphenylphosphino)ethyl)phosphine) has been prepared by the reduction of a gold(I)–tetraphosphine precursor in dichloromethane solution. Single‐crystal structural determination reveals that the cluster molecular structure has C₃ symmetry. It consists of a Au₂₀ core consolidated by four peripheral tetraphosphines. The Au₂₀ core can be viewed as the combination of an icosahedral Au₁₃ and a helical Y‐shaped Au₇ motif. The identity of this Au₂₀ cluster is confirmed by ESI‐MS. The chelation of multidentate phosphines enhances the stability of this Au₂₀ cluster.