Chenlu Bao

Preparation of graphene by pressurized oxidation and multiplex reduction and its polymer nanocomposites by masterbatch-based melt blending

Graphene is prepared from graphite by pressurized oxidation and multiplex reduction. The pressurized oxidation is advantageous in easy operation and size-control, and the multiplex reduction, based on ammonia and hydrazine, produces single-atom-thick graphene (0.4–0.6 nm thick) which can be directly observed by atomic force microscopy. A masterbatch strategy, which is feasible in “soluble” thermoplastic polymers, is developed to disperse graphene into poly(lactic acid) by melt blending. The graphene is well dispersed and the obtained nanocomposites present markedly improved crystallinity, rate of crystallization, mechanical properties, electrical conductivity and fire resistance. The properties are dependent on the dispersion and loading content of graphene, showing percolation threshold at 0.08 wt%. Graphene reinforces the nanocomposites but cuts down the interactions among the polymer matrix, which leads to reduced mechanical properties. Competition of the reinforcing and the reducing causes inflexions around the percolation threshold. The roles of the heat barrier and mass barrier effects of graphene in the thermal degradation and combustion properties of the nanocomposites are discussed and clarified.

In situ preparation of functionalized graphene oxide/epoxy nanocomposites with effective reinforcements

In order to obtain homogeneous dispersion and strong filler-matrix interface in epoxy resin, graphene oxide was functionalized via surface modification by hexachlorocyclotriphosphazene and glycidol and then incorporated into epoxy resin to obtain nanocomposites via in situ thermal polymerization. The morphology of nanocomposites was characterized by scanning electron microscopy and transmission electron microscopy, implying good dispersion of graphene nano-sheets. The incorporation of functionalized graphene oxide effectively enhanced various property performances of epoxy nanocomposites. The storage modulus of the epoxy nanocomposites was significantly increased by 113% (2% addition) and the hardness was improved by 38% (4% addition). Electrical conductivity was improved by 6.5 orders of magnitude. Enhanced thermal stability was also achieved. This work demonstrates a cost-effective approach to construct a flexible interphase structure, strong interfacial interaction and good dispersion of functionalized graphene in epoxy nanocomposites through a local epoxy-rich environment around graphene oxide sheets, which reinforces the polymer properties and indicates further application in research and industrial areas.

Poly(vinyl alcohol) nanocomposites based on graphene and graphite oxide: a comparative investigation of property and mechanism

Polymer/graphene nanocomposites have generated intensive interest due to their unique properties. Dispersion and interface interactions between graphene and the polymer matrix are two key factors to obtain property enhancements. According to the open literature, in poly(vinyl alcohol) (PVA) nanocomposites, graphene usually obtains more significant property enhancements than graphite oxide (GO), although GO can much more easily form a good dispersion and strong interaction in the PVA matrix because of its oxygenated functionalities, and the reason has not been well documented yet. In this work, graphene and GO were successfully incorporated into PVA; the properties and the mechanism for the property enhancements were investigated. GO formed better dispersion and exfoliation while graphene caused more property enhancements including mechanical properties, electrical conductivity and thermal stability. The mechanical strength of the graphene/GO nano-layers is attributed to be the fundamental cause for the enhancements in crystallinity and mechanical properties; the hydrogen bond among the PVA molecules is the key factor to influence the glass transition temperatures; the hydrogen bond between the graphene/GO nano-layers and PVA matrix is the decisive factor for the exfoliation and dispersion of graphene/GO; the conducting network is the explanation for the increased electrical conductivity; the physical barrier effect of graphene nano-sheets is the main cause for improved thermal stability. This work investigates the mechanisms for property enhancements, clarifies the roles of the hydrogen bond and the mechanical strength of the graphene/GO nano-layers, and explains why graphene usually achieves more property enhancements than GO.

Preparation of poly(vinyl alcohol) nanocomposites with molybdenum disulfide (MoS2): structural characteristics and markedly enhanced properties

As a graphene-like layered nano-material, molybdenum disulfide (MoS2) has gained much attention from the materials fields. In our research, MoS2/poly(vinyl alcohol) (PVA) nanocomposites are prepared by solvent blending method. The morphology, thermal properties, fire resistance properties and mechanical properties of the PVA/MoS2 nanocomposites are studied. MoS2 is homogeneously dispersed and partially exfoliated in the PVA matrix as indicated by X-ray diffraction (XRD) pattern, Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM) characterization. The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results indicate improved the thermal decomposition temperature and the glass transition temperature (Tg). The thermal degradation temperature is increased by 20–40 °C. Meanwhile, the peak of heat release rate (pHRR) and total heat release (THR) are decreased by 33% and 20%, respectively. Storage modulus at 40 °C is increased by 28%, and the tensile strength is increased by 24% upon addition of 1 wt% and 5 wt% MoS2. The improvements in the thermal properties, fire resistance properties and mechanical properties of PVA nanocomposites are attributed to the good dispersion of MoS2, physical barrier effects of MoS2 and strong interactions between PVA and MoS2.

Functionalized graphene oxide for fire safety applications of polymers: a combination of condensed phase flame retardant strategies

Graphene is promising for the fire safety applications of polymers, but the ease of burn out limits further developments. A novel strategy based on functionalized graphene oxide (FGO) is developed to overcome this challenge. Graphene oxide is functionalized with char-catalyzing agents and reactive compounds and incorporated into polystyrene. When FGO–polystyrene composites are degraded or burned, FGO catalyzes the char formation from polystyrene (Char A). Char A protects FGO from burning out and then FGO acts as a graphitic char (Char B). Because of the combination of Char A, Char B, the physical barrier effects of FGO, and the strong interfacial interactions of FGO and polymers, the fire safety properties of the FGO–polystyrene composites are improved, including decreased peak CO release rate (66% decrease), decreased peak CO2 release rate (54% decrease), decreased peak heat release rate (53% decrease), decreased thermal degradation rate (30% decrease), decreased total heat release (38% decrease), and increased char formation (7 times), etc. This strategy combines several condensed phase flame retardant strategies such as the nanocomposite technique, intumescent flame retardant systems and phosphorus–nitrogen synergism systems, and hence results in more significant improvements as compared with prior work.

Graphite oxide, graphene, and metal-loaded graphene for fire safety applications of polystyrene

Graphite oxide, graphene, ZrO2-loaded graphene and β-Ni(OH)2-loaded graphene (joint appellation: Gs) were prepared and incorporated into polystyrene so as to improve the fire safety properties of polystyrene. By the masterbatch-melt blending technique, Gs nanolayers were well dispersed and exfoliated in polystyrene as thin layers (thickness 0.7–2 nm). The fire safety properties were visibly improved, including an increased thermal degradation temperature (18 °C, PS/Ni–Gr-2), decreased peak heat release rate (40%, PS/Zr–Gr-2) and reduced CO concentration (54%, PS/Ni–Gr-2). The mechanism for the improved thermal stability and fire safety properties was investigated based on this study and previous works. The physical barrier effect of graphene, the interaction between graphene and polystyrene, and the synergistic effect of the metal compounds are the causes for the improvements.

Influence of g-C3N4 nanosheets on thermal stability and mechanical properties of biopolymer electrolyte nanocomposite films: a novel investigation.

A series of sodium alginate (SA) nanocomposite films with different loading levels of graphitic-like carbon nitride (g-C3N4) were fabricated via the casting technique. The structure and morphology of nanocomposite films were investigated by X-ray powder diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. Thermogravimetric analysis results suggested that thermal stability of all the nanocomposite films was enhanced significantly, including initial thermal degradation temperature increased by 29.1 °C and half thermal degradation temperature improved by 118.2 °C. Mechanical properties characterized by tensile testing and dynamic mechanical analysis measurements were also reinforced remarkably. With addition of 6.0 wt % g-C3N4, the tensile strength of SA nanocomposite films was dramatically enhanced by 103%, while the Youngs modulus remarkably increased from 60 to 3540 MPa. Moreover, the storage modulus significantly improved by 34.5% was observed at loadings as low as 2.0 wt %. These enhancements were further investigated by means of differential scanning calorimetry and real time Fourier transform infrared spectra. A new perspective of balance was proposed to explain the improvement of those properties for the first time. At lower than 1.0 wt % loading, most of the g-C3N4 nanosheets were discrete in the SA matrix, resulting in improved thermal stability and mechanical properties; above 1.0 wt % and below 6.0 wt % content, the aggregation was present in SA host coupled with insufficient hydrogen bondings limiting the barrier for heat and leading to the earlier degradation and poor dispersion; at 6.0 wt % addition, the favorable balance was established with enhanced thermal and mechanical performances. However, the balance point of 2.0 wt % from dynamic mechanical analysis was due to combination of temperature and agglomeration. The work may contribute to a potential research approach for other nanocomposites.

Silicon nanoparticle decorated graphene composites: preparation and their reinforcement on the fire safety and mechanical properties of polyurea

Reduced graphene oxide (rGO) was decorated with organic/inorganic nanoparticles through an in situ sol–gel process with various thicknesses. The presence of organic/inorganic nanoparticles made the rGO lipophilic, as evidenced by the good dispersion of the nanoparticles–rGO in dimethyl formamide solvent (DMF). The thickness of the nanoparticles–rGO could be varied by adjusting the amount of the silicane additive, as evidenced by the AFM results. The nanoparticles–rGO was then incorporated into polyurea in different ratios via in situ polymerization and the property enhancement of the nanocomposites was investigated. The TEM morphological study showed that, due to the good interfacial interaction between the nanoparticles–rGO and polyurea, nanoparticles–rGO was dispersed well in the polyurea matrix. Compared with the rGO, the nanoparticles could significantly improve the thermal stability and thermal conductivity of polyurea, implying that the good dispersion of rGO and the functional groups on the surface of rGO had a significant effect on the thermal stability and thermal conductivity of polyurea. The peak heat release rate (pHRR) of nanoparticles–rGO/polyurea nanocomposites was significantly reduced, which indicated that the combustible gas releasing rate of polyurea was reduced. Moreover, the storage modulus and tensile strength of the nanocomposites with 0.2 wt% have been enhanced by about 60% and 110% in comparison with those of neat polyurea, respectively. This simple and effective approach, decorating the rGO with organic/inorganic nanoparticles, is believed to offer possibilities for broadening the graphene applications in the polymer materials and make it possible to decorate the graphene with other functional groups and vary the aspect ratio of decorated graphene according to its application.