Silica nanoparticle reinforced polymer composites

On the other hand, nanometer-metal-particule/polymer composite materials have been prepared. Wizel et al. [124] prepared the cobalt-polymer and iron-polymer composites by means of ultrasound radiation. The amorphous cobalt and iron particles measuring 10-30 nm and 50-90 nm in size, respectively, were mixed with distilled methylacrylate or methylmethacrylate (MA or MMA) monomers in dry N, N’-dimethylformamide (DMF) using sonication cell without exposure to air [125]. The sonication product was a colloidal solution, and the polymer was precipitated from the colloidal solution by adding, under N2

atmosphere, an excess of cold methanol at the end of the sonication. In view of the magnetic properties of these nanoparticles reinforced polymer composites, it was found that the Fe-PMMA (Fe-Poly(methyl methacrylate)) composite shows superparamagnetic behavior due to single-domain particles. Moreover, the Fe-PMMA did not show saturation of the magnetization and also lacked hysteresis in its magnetization loops. The Co-PMA also showed the same behavior. But the magnetization values measured for the cobalt particles were always lower than those of the iron.

of modulus with increasing deformation ratio, can be generally explained in terms of the breakdown process occurring in the agglomerates. Moreover, it has also been proposed that the silica network structure in molten state has the memory of the silica structure in solid state. As a consequence, a complete volumetric expansion of the resulting composite does not allow to undergo unless a large strain amplitude being applied during dynamic mechanical testing.

Lee et al [128]. incorporated the hydrophobic silica filler of 12 nm in diameter into the blends of liquid crystalline polymers (LCPs) and polypropylene (PP) by mean of twin-screw extruder. It is found that the nanofiller could act as a fibrillation enhancer for in-situ LCP/PP composites. As a consequence, the inclusion of nano-sized silica by mean of twin-screw extruder could induce high aspect ratio LCP fibrils through shear flow. Moreover, the resulting composites exhibited better tensile strength and modulus accompanied with only a small reduction in failure strain.

Musto et al [129]. reported that the morphology of the silica/polyimide hybrid could be controlled by using a coupling agent such as γ-glycidyloxypropyltrimethoxysilane (GOTMS).

Conventionally, micro-sized particulate composites could be produced in the absence of GOTMS. However, nano-structured and co-continuous nanocomposites would be obtained by introducing the coupling agent in the precursor solution for the silica phase. It is found that the presence of the inorganic phase reduces the extent of plastic flow of the polyimide phase and, as a result, fracture takes place at progressively lower strain with increasing silica content. In addition, the silica/polyimide composites with the use of compatibilizing agent (GOTMS) can increase their tensile stress at the silica contents up to 15 – 20% by weight. As expected, a co-continuous phase morphology with high adhesion between the phases could bring about significant improvements of the tensile properties. Through SEM observations,

morphologically, the introducing GOTMS can form fine interconnected or co-continuous phases morphology. It is known that in polymeric composites the external stress is transferred from the continuous polymeric matrix to the discontinuous reinforcing phase. As a result, the ultimate properties of the composite materials are dependent on the extent of bonding between the two phases.

Organic/inorganic hybrid polymer composites can also be prepared by polymerization compounding [130]. Ultra fine fumed silica particles, with the specific surface area (BET), particle size and density of ~390 m²/g, 7 nm, and 2.3 g/cm³, respectively, were firstly attached with tert-butyl hydroperoxide to impart the surface activation of the modified silica particles. Then, the monomers and the initiator were grafted onto the activated silica sites. By means of radical polymerization, the polymer chain segments could in situ grow up at the activated sites of the porous silica. Accordingly, this polymerization compounding method could obtain polymers with narrow molecular weight distribution as well as block copolymers. Moreover, because of the growth of polymer chain segments onto the porous and activated silica particles, little agglomeration could be occurred. Consequently, through polymerization compounding technique, polymers or copolymers can be successfully grafted on the silica surface. With a more controlled morphology, such as tethered chains, polymer

‘brushes’ or patterned film could be obtained.

On the other hand, modified nano-sized silica particles could react with a silane-coupling agent, such as 3-(trimethoxysilyl) propyl methacrylate, and therefore graft this coupling agent on the silica particles [131]. It was proved that the amount of the grafted coupling agent on the silica particles was about 2 molecules per nm². Mixing these grafted silica particles with poly((meth)acrylate) and then coating this mixture on the substrate of polycarbonate would increase the hardness of poly((meth)acrylate) film. TEM (transmission

electron microscope) images taken from the silica-(meth)acrylate hybrid coating show the coating film having a separated and clear-cut domain of silica particle. In addition, Tan et al.

[132] also used the above silane-coupling agent to react with polyethercarbonate via a free radical reaction to obtain thealkoxysilane-containing copolymer precursors, which could be used in subsequent sol-gel process to result in the polyethercarbonate-silica nanocomposites.

In addition to layer clay, nanoscale colloidal silica has been considered as inorganic fillers for the preparation of polymer nanocomposites by means of the sol-gel process [133-136] or in situ polymerization technique [130]. Even since the sol-gel technique was applied to result in polymer nanocomposites, silane coupling agents were commonly employed to improve the inorganic-organic interfacial compatibility [129]. However, during the curing stage such as epoxy compound, the evaporation of solvent and the gelation reaction of the hydrolyzed alkoxysilane occurred simultaneously. As a consequence, the solution process of sol-gel technique is not practical in the processing of epoxy molding compounds. The simultaneously released volatiles from the gelation of alkoxysilane sol would certainly bring about undesirable effects to the epoxy resin, as well as difficulties during epoxy curing reaction. Another drawback in using the sol-gel process in polymer-silica nanocomposites is its harm to the initial thermal stability of the resulting nanocomposites. This effect is mainly due to the residual of the silanoxy group in the resulting polymer-silica nanocomposites, and these silanoxy groups might perform dehydration reaction at high temperatures in the processing or service period of the nanocomposites.