Xiaming Feng

Self-assembly of Ni–Fe layered double hydroxide/graphene hybrids for reducing fire hazard in epoxy composites

Ni–Fe layered double hydroxide/graphene hybrids were synthesized by a one-pot in situ solvothermal route. X-ray diffraction and X-ray photoelectron spectroscopy analyses showed that the formation of Ni–Fe layered double hydroxide (Ni–Fe LDH) and the reduction of graphene oxide occurred simultaneously during the one-pot solvothermal process. TGA results showed that the incorporation of Ni–Fe LDH significantly improved the thermal stability of the graphene. Subsequently, Ni–Fe LDH/graphene hybrids were introduced into epoxy resins for reducing their fire hazard. With the incorporation of 2.0 wt% of Ni–Fe LDH/graphene, the onset thermal degradation temperature of the epoxy composite was significantly increased by 25 °C compared to that of pure epoxy. Also, the addition of Ni–Fe LDH/graphene hybrids imparted excellent flame retardant properties to the epoxy matrix, evidenced by the dramatically reduced peak heat release rate and total heat release values obtained from a micro combustion calorimeter and cone calorimeter. This dramatically reduced fire hazard was mainly attributed to the synergistic effects of Ni–Fe LDH/graphene hybrids: the adsorption and barrier effect of graphene slowed down the thermal degradation of the polymer matrix, inhibited the heat and flammable gas release and promoted the formation of graphitized carbons, while Ni–Fe LDH improved the thermal oxidative resistance of the char layer.

Functionalization of graphene with grafted polyphosphamide for flame retardant epoxy composites: synthesis, flammability and mechanism

A polyphosphamide (PPA) was synthesized and covalently grafted onto the surface of graphene nanosheets (GNSs) to obtain a novel flame retardant, PPA-g-GNS, and subsequently PPA-g-GNS was incorporated into epoxy resins (EPs) to enhance the fire resistance. The chemical structures and morphology of the precursors and target product were confirmed using 1H-NMR spectroscopy, Fourier transform infrared spectroscopy and atomic force microscopy. The tensile results showed that the mechanical strength and modulus of the PPA-g-GNS/EP composite were higher than those of pure EP and PPA/EP, owing to the outstanding reinforced effect of graphene. The evaluation of the thermal properties demonstrated that the addition of PPA or PPA-g-GNS to epoxy had a thermal destabilization effect below 400 °C, but led to a higher char yield at higher temperatures. Furthermore, the PPA-g-GNS/EP composite exhibited superior fire resistant performance, such as higher LOI values and reduced PHRR and FIGRA values, compared to pure EP and PPA/EP, which was probably attributed to the higher char yield during combustion. A possible flame retardant mechanism was speculated according to the direct pyrolysis-mass spectrometry results: the phosphate species degraded from PPA catalyzed the decomposition of the PPA-g-GNS/EP composites to generate various pyrolysis products; the pyrolysis products were absorbed and propagated on the graphene which served as a template of micro-char, and thus a continuous and compact char layer was formed; such a char layer provided effective shields to protect the underlying polymers against flame.

In situ synthesis of a MoS2/CoOOH hybrid by a facile wet chemical method and the catalytic oxidation of CO in epoxy resin during decomposition

In this work, a new MoS2/CoOOH hybrid material was successfully synthesized by a facile wet chemical method, and its structure was confirmed by X-ray diffraction and Raman spectroscopy. A morphological study showed that, due to the different sizes of the two components, the resulting MoS2/CoOOH hybrid displayed a disordered structure in which large MoS2 sheets had many independent and separate CoOOH nanoplatelets on the surface. The catalytic oxidation effect of MoS2/CoOOH hybrids on the thermal decomposition of epoxy resin was studied by thermogravimetric analysis-infrared spectrometry. It was found that the amount of organic volatiles of epoxy resin significantly decreased and non-flammable CO2 was generated after incorporating MoS2/CoOOH hybrids, which implied the reduced toxicity of the volatiles and obvious smoke suppression. Meanwhile, the incorporation of MoS2/CoOOH hybrids also resulted in a remarkable increase in the char residue of the epoxy composite, indicating the efficient catalytic carbonization of MoS2/CoOOH hybrids. Based on the X-ray diffraction and Fourier transform infrared results of the char residue, the possible mechanism of the reduced fire hazards and high char formation of the epoxy composites was proposed as the combination of the adsorption and synergistic catalytic effect of the MoS2/CoOOH catalyst, which would provide promising applications in the development of fire safety polymer materials.

Flame-retardant-wrapped polyphosphazene nanotubes: A novel strategy for enhancing the flame retardancy and smoke toxicity suppression of epoxy resins.

The structure of polyphosphazene nanotubes (PZS) is similar to that of carbon nanotubes (CNTs) before modification. For applications of CNTs in polymer composites, surface wrapping is an economically attractive route to achieve functionalized nanotubes. Based on this idea, functionalized polyphosphazene nanotubes (FR@PZS) wrapped with a cross-linked DOPO-based flame retardant (FR) were synthesized via one-step strategy and well characterized. Then, the obtained FR@PZS was introduced into epoxy resin (EP) to investigate flame retardancy and smoke toxicity suppression performance. Thermogravimetric analysis indicated that FR@PZS significantly enhanced the thermal stability of EP composites. Cone calorimeter results revealed that incorporation of FR@PZS obviously improved flame retardant performance of EP, for example, 46.0% decrease in peak heat release rate and 27.1% reduction in total heat release were observed in the case of epoxy composite with 3wt% FR@PZS. The evolution of toxic CO and other volatile products from the EP decomposition was significantly suppressed after the introduction of FR@PZS, Therefore, the smoke toxicity associates with burning EP was reduced. The presence of both PZS and a DOPO-based flame retardant was probably responsible for this substantial diminishment of fire hazard.

Studies on Synthesis of Electrochemically Exfoliated Functionalized Graphene and Polylactic Acid/Ferric Phytate Functionalized Graphene Nanocomposites as New Fire Hazard Suppression Materials

Practical application of functionalized graphene in polymeric nanocomposites is hampered by the lack of cost-effective and eco-friendly methods for its production. Here, we reported a facile and green electrochemical approach for preparing ferric phytate functionalized graphene (f-GNS) by simultaneously utilizing biobased phytic acid as electrolyte and modifier for the first time. Due to the presence of phytic acid, electrochemical exfoliation leads to low oxidized graphene sheets (a C/O ratio of 14.8) that are tens of micrometers large. Successful functionalization of graphene was confirmed by the appearance of phosphorus and iron peaks in the X-ray photoelectron spectrum. Further, high-performance polylactic acid/f-GNS nanocomposites are readily fabricated by a convenient masterbatch strategy. Notably, inclusion of well-dispersed f-GNS resulted in dramatic suppression on fire hazards of polylactic acid in terms of reduced peak heat-release rate (decreased by 40%), low CO yield, and formation of a high graphitized protective char layer. Moreover, obviously improvements in crystallization rate and thermal conductivities of polylactic acid nanocomposites were observed, highlighting its promising potential in practical application. This novel strategy toward the simultaneous exfoliation and functionalization for graphene demonstrates a simple yet very effective approach for fabricating graphene-based flame retardants.

High-Performance Poly(ethylene oxide)/Molybdenum Disulfide Nanocomposite Films: Reinforcement of Properties Based on the Gradient Interface Effect

Herein, the molybdenum disulfide (MoS2) was simultaneously exfoliated and noncovalently functionalized by ultrasonication in a Pluronic aqueous solution and then was used to prepare the poly(ethylene oxide) (PEO) based nanocomposite films. The homogeneous dispersion of MoS2 and strong nanosheets/matrix interfacial adhesion were confirmed by representative electron microscopes. The considerable barrier action of the effective MoS2 nanosheets obviously restricted the ordering of crystal lamellae and the motion of polymer chains and then resulted in the formation of the devastated spherocrystal structure and morphological alterations in the nanocomposites, which were confirmed by polarized optical microscopy and the high value of the glass transition temperature. Importantly, MoS2 nanosheets hold great promise in reinforcing the thermal stability and mechanical property of polymer by increasing the effective volume of MoS2 nanosheets. A substantial reinforcement effect of PEO/MoS2 composite films was achieved: even at a relatively low loading level (0.9 wt %), 88.1% increase in Youngs modulus, 72.7% increase in stress-at-failure, and 62.1 °C increment of the temperature corresponding to half weight loss were obtained. These significant reinforcements can be attributed to the gradient interface region, which could effectively transfer the stress from the weak polymer chains to the robust nanosheets, thus endowing the PEO/MoS2 composite films with excellent properties.

MoS2/Polymer Nanocomposites: Preparation, Properties, and Applications

ABSTRACT The discovery of graphene has aroused enormous scientific interests in fabrication and application of new two-dimensional (2D) materials in the past decade. As a typical layered graphene-like material, molybdenum disulfide (MoS2) shows high band gap that overcomes the zero-band gap shortcoming of graphene. The unique properties enable MoS2 to exhibit great potential applications in the fields of electronic and optoelectronic devices. Despite the enormous scientific interests aroused by MoS2, little attention has been focused on the progress in fabrication, properties, and applications of MoS2/polymer nanocomposites up to now. In this review, we first present production routes to exfoliated MoS2 with an emphasis on physical and chemical exfoliations, as well as chemical synthesis. Then solvent- and melt-based strategies to disperse exfoliated MoS2 nano-platelets in various polymer matrices are discussed. We also review electrical, thermal, mechanical, tribological, and flame retardant properties of the MoS2/polymer nanocomposites. An overview of potential applications for these nanocomposites associated with current challenges is provided for future perspectives of this promising new class of nanocomposites.

A facile and cost-effective approach to the reduction of exfoliated graphite oxide using sodium hypophosphite under acidic conditions

Herein we demonstrate that graphene production could be implemented through the chemical reduction of an exfoliated graphite oxide (GO) suspension using a sodium hypophosphite–hydrochloric acid mixture. The structure and composition of the resultant reduced GO (RGO) were confirmed by XRD, XPS and Raman spectra. The RGO exhibited high C/O atomic ratio (13.5), excellent thermal stability and high electrical conductivity (up to 380 S m−1). We speculate that the strong reducibility of hypophosphorous acid derived from a sodium hypophosphite–hydrochloric acid solution is the main mechanism for oxygen reducing and graphite restructuring of GO sheets. Considering that all the raw materials used are of very low toxicity and widely available, this facile and green technique presented here will provide a promising method for the production of graphene on an industrial scale.

Mussel-inspired functionalization of electrochemically exfoliated graphene: based on self-polymerization of dopamine and its suppression effect on the fire hazards and smoke toxicity of thermoplastic polyurethane

The suppression effect of graphene in the fire hazards and smoke toxicity of polymer composites has been seriously limited by both mass production and weak interfacial interaction. Though the electrochemical preparation provides an available approach for mass production, exfoliated graphene could not strongly bond with polar polymer chains. Herein, mussel-inspired functionalization of electrochemically exfoliated graphene was successfully processed and added into polar thermoplastic polyurethane matrix (TPU). As confirmed by SEM patterns of fracture surface, functionalized graphene possessing abundant hydroxyl could constitute a forceful chains interaction with TPU. By the incorporation of 2.0 wt % f-GNS, peak heat release rate (pHRR), total heat release (THR), specific extinction area (SEA), as well as smoke produce rate (SPR) of TPU composites were approximately decreased by 59.4%, 27.1%, 31.9%, and 26.7%, respectively. A probable mechanism of fire retardant was hypothesized: well-dispersed f-GNS constituted tortuous path and hindered the exchange process of degradation product with barrier function. Large quantities of degradation product gathered round f-GNS and reacted with flame retardant to produce the cross-linked and high-degree graphited residual char. The simple functionalization for electrochemically exfoliated graphene impels the application of graphene in the fields of flame retardant composites.

The effect of doped heteroatoms (nitrogen, boron, phosphorus) on inhibition thermal oxidation of reduced graphene oxide

The doping of nitrogen into reduced graphene oxide (NRGO) is achieved by reduction of graphene oxide with hydrazine hydrate and ammonium hydroxide. Boron (BRGO) or phosphorus (PRGO) doped reduced graphene oxide (RGO) is obtained by annealing of RGO (prepared by reduction with sodium borohydride) with boric acid or phosphoric acid, respectively. The successful preparation of the doped RGO is confirmed by Fourier transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. A remarkable enhancement in thermal oxidative stability of RGO is achieved by doping of these heteroatoms. The enthalpy values of BRGO and PRGO during the thermal oxidation decrease remarkably compared with that of RGO, indicating the reduced heat release by the doped heteroatoms. The mechanism for improvement in thermal oxidative resistance by doping of heteroatoms is demonstrated clearly. Doping of boron atom not only lowers the electrons density on the reactive carbon atoms and the Fermi level of carbon, but also contribute to graphitization of RGO, leading to inhibition of thermal oxidation of RGO. Phosphorus containing complexes, in the forms of metaphosphates, C–O–PO3 and C–PO3 groups, can poison the active sites on RGO and function as physical barrier for the access of oxygen. Retarding oxidation of RGO against air may be strengthened by forming more thermostable structure in NRGO, for example: pyrrolic-N (NC2), pyridinic-N (NC2) and graphitic-N (NC3). The doping of heteroatoms will provide an important strategy for broadening RGO application at the elevated temperature.