As a new member of carbon family, graphene has attracted considerable attention in the field of graphene/polymer nanocomposites (GPNs), due to its excellent physical properties. GPNs have become one of the most promising applications of graphene. However, some important factors like dispersion and interfacial interactions between graphene and polymer matrices still restrict the achievement of better performance for GPNs, especially in the nonpolar polyolefin. Polypropylene (PP), one of the largely consumed plastic, is widely used in daily life and industry. However, its inherent inflammability has restricted its application in some fields. Therefore, it is imperative to improve its flame retardancy. The application of nanotechnology in flame retardant polymer has been regarded as one of the most important advancements in this research field. The potential application of graphene as a flame retardant nanomaterial has been explored. However, its flame retardant performance is not marked as compared with conventional layered materials, such as montmorillonite. The relatively low flame retardant efficiency for graphene may be ascribed to its inferior dispersion, poor resistance to oxidation and simple flame retardant mechanism.

In this dissertation, non-covalent and covalent functionalization approaches are adopted to modify surface feature of graphene, and then to improve the flame retardancy of PP. The graphitizable modifiers including melamine (MA) and polyaniline (PANI) are in-situ decomposed to carbon-based protective layer coating on graphene nanosheets, which strengthen its barrier performance. Covalent functionalization of graphene with polymer can decrease the cohesive energy between graphene nanosheets and render the uniform dispersion. Enhanced radical trapping ability of graphene by the absorbed phosphomolybdic acid (PMoA) is expected to endow its PP nanocomposites with improved thermal oxidative stability and flame retardancy. Phosphorus and nitrogen-containing flame retardant and transition metal nanomaterials with catalytic carbonization function are decorated on graphene to endow it with different flame retardant mechanisms. Graphene is combined with intumescent flame retardant (IFR) and the flammability and combustion behavior of the composites at different fire scenarios are investigated. The research work in this dissertation is composed of the following parts.

1. MA and PMoA are employed to non-covalently functionalize graphene for preparing PP nanocomposites with enhanced flame retardant properties. It can be observed that the functionalized graphene oxide (FGO) is homogeneously dispersed in PP matrix with intercalation and exfoliation microstructure. The FGO/PP nanocomposite exhibits higher thermal stability and flame retardant property than those of the GO counterpart. The g-C3N4 nanosheets formed by condensation of MA provide protective layer on graphene and enhance its barrier effect. PMoA is immobilized on the graphene nanosheets via electrostatic interactions. The enhancements in thermal-oxidative stability and flame retardant properties of PP nanocomposites containing graphene nanohybrid are predominantly ascribed to the barrier effect of graphene and enhanced radical capturing property of the nanohybrid.

2. Using p-phenylenediamine (PPD) and cyanuric chloride as modifiers, the grafting of maleic anhydride grafted polypropylene onto graphene nanosheets is achieved by the reactions between amino groups in the grafted PPD and maleic anhydride groups. Because of the presence of the polymeric compatibilizer, homogeneous dispersion of FGO and strong interfacial interactions are achieved. A significant enhancement in the thermal stability of the nanocomposites is obtained at low FGO loadings. However, the elongation at break of nanocomposites is decreased, and tensile strength shows no change with increasing loading of FGO. The detailed mechanism for the decreased elongation at break and the unchanged tensile strength of nanocomposites is proposed according to the study on the mobility of polymer chain and lamellae. In addition, the flame retardant property of the nanocomposites is improved to a certain extent.

3. To enhance barrier effect and dispersion of graphene simultaneously, graphitizable PANI nanofibers are used to modify graphene surface using "grafting from" approach. The PANI-RGO can reduce obviously the flammability of PP, including the reductions in peak heat release rate (PHRR) and total heat release. The smoke release of the nanocomposites during the combustion in cone tests is also decreased, indicating enhanced fire safety by the incorporation of PANI-RGO. Carbon nanofibers formed by PANI are in-situ coated on the graphene nanosheets, thus, the barrier effect of the graphene is reinforced. Therefore, the heat release rate of the nanocomposites is greatly decreased and the smoke emission is also retarded.

4. A phosphorus and nitrogen-containing flame retardant, polyphosphazene, is covalently grafted onto the surfaces of GO via the "grafting from" strategy. Then, Ni(OH)2 are loaded on the nanosheets through the strong interactions between Ni2+ and the amino groups in the phosphazene flame retardant. This work is to take advantages of high flame retardant efficiency of this flame retardant, transition metal with catalytic carbonization effect and collaborative dispersion effects of two different nanofillers. The modification strategy of FGO2 combines several flame retardant mechanisms: physical barrier of graphene, phosphorus-nitrogen synergy and catalytic carbonization effect, resulting in significant improvement in flame retardancy.

5. Graphene is expected to act as a synergistic agent with IFR to further reduce the flammability of PP. The flammability tests and fire behavior are investigated under different fire scenarios. In the small flame tests, the incorporation of graphene results in the gradually decreased limiting oxygen index values and deteriorated UL-94 rating. However, in the combustion tests under forced-flaming scenario, the loading of graphene (0.5 and 1 wt%) can further reduce the PHRR and improve fire safety. The higher graphene content (2 wt%) in the IFR system results in the worse flame retardancy than that of neat IFR/PP. Due to the excellent barrier effect of graphene, the release of pyrolysis gas products including flammable and noninflammable gases is greatly inhibited. The graphene is observed to enhance the compactness and graphitization degree of the final char. Owing to the barrier function of graphene, more phosphorus and nitrogen are available for char formation. Graphene can considerably increase the melt viscosity of IFR/PP system. The low loading of graphene is observed to enhance the intumescence of the char, and the swelling of char is inhibited, when higher content of graphene is added. The mechanisms for these abnormal flame retardant performances are concluded. When optimum loading of graphene is added, compactness, mechanical properties of the char and melt viscosity of the matrix are appropriately enhanced, resulting in the enhancements in the swelling of char and flame retardancy. The superabundant graphene is harmful to reduce heat release rate, due to excessively strengthened char.