Polymeric nanocomposites are a class of materials in which nanoscale particulates such as layered clays or spherical inorganic minerals are dispersed within polymeric matrices. Compared to pure polymers, polymeric nanocomposites are claimed to exhibit markedly improved properties, such as modulus, strength, stiffness, flame retardancy, dimensional stability, electrical conductivity, barrier performance, solvent and heat resistance, wettability and dyeability depending on type and content of nanoparticles used .
Processing of such polymeric nanocomposites are more difficult compared to the corresponding pure polymers since such inorganic nanoparticles have strong tendencies to agglomerate. To overcome such difficulties, use is made of sol-gel processing, in situ polymerization and melt compounding. The last method is still the most cost effective, simple, feasible and environmentally benign process for the mass production of polymeric nanocomposite .
Poly(ethylene terephthalate) (PET) is a semicrystalline polymer and its composites are widely used in packaging, construction, automobile, household, electrical and textile industries. Considerable efforts have been devoted to improve various physical, mechanical and barrier properties of PET through mixing it with nanoclays to produce layered clay-incorporated . Other approaches for improving such properties are incorporation of spherical inorganic nanoparticles such as nano-SiO 2 into the PET matrix .
Fumed silica is characterized by its extremely small particle size and large surface area. The surface of fumed silica has three chemical groups of isolated hydroxy, hydrogen-bonded hydroxy, and siloxane groups. Thus, the surface generally is hydrophilic, although the siloxane groups are hydrophobic. The hydrophilic surface of fumed silica, however, can be rendered hydrophobic by reacting its surface hydroxyl groups with hydrophobic reagents such as polydimethylsiloxane, dimethyldichlorosilane and hexamethyldisilane .
Many research works have focused on incorporation of nanosilica into various polymers however only a few deal with preparation of PET-silica nanocomposites . Researchers claim that nano-silica particle impact higher stiffness, tensile strength, modulus, impact strength, toughening, crystallinity, viscosity, creep resistance, and interfacial adhesion in polyethylene, polypropylene and thermoplastic elastomer nanocomposites, depending on surface properties of such nano-silica particles . The addition of different nano-silica particles improves coefficient of friction, wear resistance and toughness of nylon 6 and nylon 6,6 nanocomposites . Polyvinyl alcohol-SiO 2 interactions highly influenced the composite and particle distribution within the polymer matrix depending on pH of solution .
In the case of PET nanocomposites, experimental evidence indicates that nano-silica does not behave as a nucleating agent