The structure of hexagonal boron nitride (h-BN) is similar to that of graphite before functionalization and exfoliation. For applications in polymer nanocomposites, chemical exfoliation is a more economically attractive route to few-layer h-BN nanosheets. A thermal oxidation process of h-BN powder could achieve large scale exfoliation and hydroxylated functionalization, as described in prior literature. In this report, hydroxylated h-BN (BNO) was prepared by heating h-BN under air, and then covalently incorporated into epoxy resin modified with (3-isocyanatopropyl)triethoxysilane to prepare epoxy resin (EP) nanocomposites by sol–gel process. The structure and morphology of BNO were well characterized. BNO was dispersed in the EP matrix with the form of mainly exfoliated and intercalated structures, and formed strong interfacial interaction with the matrix. Thermogravimetric analysis results revealed that BNO significantly improved thermal stability and thermal oxidative resistance of EP nanocomposites at high temperature. The char yield and the temperature at 50 wt% mass loss were increased and the maximum mass loss rate was remarkably reduced. Moreover, the addition of 3 wt% BNO led to extremely high T_g of EP nanocomposite, 42.7 °C higher than that of pure EP, due to improved crosslinking density and confinement effect of BNO sheets on the mobility of polymer networks. Cone calorimeter test results indicated that fire safety properties of EP nanocomposites were also enhanced by the addition of BNO, such as 53.1% reduction in peak heat release rate and 32.6% decrease in total heat release, and decreased release of smoke and toxic gases. The mechanism for enhanced fire retardancy is that thermally stable condensed barrier consisting of h-BN sheets and silicon dioxide for heat and mass transfer protects the matrix from further combustion.

Functionalized reduced graphene oxide (FRGO) wrapped with a phosphorus and nitrogen-containing flame retardant (FR) was successfully prepared via a simple one-pot method and well characterized. Subsequently, FRGO was covalently incorporated into epoxy resin (EP) to prepare flame retardant nanocomposites. The FRGO was well dispersed in the matrix and formed strong interfacial adhesion. Thermogravimetric analysis results revealed that the presence of RGO, FR or FRGO in an EP matrix led to a slight thermal destabilization effect under air and nitrogen, which increased the char yield at 700 °C and reduced the maximum mass loss rate. Furthermore, the glass transition temperature of the FRGO/EP nanocomposite with an FRGO loading of 4 wt% (FRGO/EP4) was remarkably increased by 29.6 °C, probably due to the improved crosslinking density and confinement effect of graphene sheets on the mobility of polymer networks. The evaluation of combustion behavior demonstrated that a 43.0% reduction in the peak heat release rate (PHRR) for the FRGO/EP nanocomposite containing 2 wt% FRGO and a 30.2% reduction in the total heat release (THR) for FRGO/EP4 over pure EP were achieved by the addition of FRGO. These notable reductions in fire hazards were mainly due to the synergistic effect of FRGO and the flame retardant: the wrapped flame retardant accelerated the degradation of the EP matrix, promoting the formation of additional char residues; the flame retardant improved the thermal oxidative resistance of the graphene; a high-thermal-stability char layer, consisting of graphene sheets, retarded the permeation of heat and the escape of volatile degradation products.

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 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.

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.

Polyurethane (PU) composites reinforced with graphene nanosheets (GNSs) were prepared by in situpolymerization. Graphene nanosheets, which were derived from chemically reduced graphite oxide (GO) nanosheets, were characterized by solid-state ¹³C MAS NMR, XRD and FTIR. A morphological study showed that, due to the formation of chemical bonds, the GNS was dispersed well in the PU matrix. With the incorporation of 2.0 wt% of GNSs, the tensile strength and storage modulus of the PU increased by 239% and 202%, respectively. The nanocomposites displayed high electrical conductivity, and good thermal stability of PU was also achieved. The facile and rapid technique presented here will provide an effective and promising method of preparing graphene-based polymer composites.