Surface Modification Of Nanoparticles

Composite technology is aimed at producing a combination of properties that cannot be achieved with either of the constituents acting alone. Composition, component properties, microstructure, and interactions are the main factors determining the characteristics of composite materials. As compared with the other factors, interactions in composites play the most important role. Two basic types of interactions must be considered: particle-particle and particle-matrix interactions [55]. The effect of the former usually decreases the composite's performance, whereas that of the latter leads to the development of an interphase with properties different from those of the components.

It is known that the interaction between neighboring particles is governed by the competitive contributions made by attraction and repulsion due to van der Waals and electrostatic forces [56]. With decreasing particle size, a reduction in repulsion is much more significant than the decrease in attraction. As a result, attraction between the particles plays the leading role under a certain critical particle diameter. It accounts for the strong tendency of nanoparticles to aggregate.

The structure of a nanoparticle assembly may be classified according to the terminology used in the powder industry. That is, the ultimate primary particles (the smallest particulate pieces) exist in both strongly and weakly bound structures, which are commonly referred to as aggregates and agglomerates, respectively [57]. Agglomerates consist of an assembly of aggregates and have a larger, often more open structure. They behave like a single particle and can be disrupted by considerable force.

In a recent report by Pukanszky and Fekete [58], the forces acting on particulate fillers incorporated in a polymer (fluid or melt) are discussed in detail based on the works of Kendall [59] and Adams and Edmondson [60]. Although many types of forces (e.g., viscous, capillary, electrostatic, etc.) have certain effects, the adhesive force, Fa, which acts as an attractive force, and the hydrodynamic force, taking the role of a separating force, Fh, are the most important ones:

where R denotes the radius of the particles, ^ab is the reversible work of adhesion between the solid and the fluid, ^ is the viscosity of the fluid, and y is the applied shear rate. It was revealed that agglomeration is determined by the relative magnitudes of attractive and separating forces. The predominant factors that influence the homogeneity of composites are the size of the particles, their surface tensions, and the shear forces acting during their homoge-nization [61]. It should be noted that the interparticle interaction might easily prevail over the shear force and favors the development of agglomerated structures in the case of smaller particles such as nanoparticles. Hence surface modification of the particles becomes critical to minimizing particle-particle interaction and enhancing particle-matrix interaction in composites.

In general, particulate surface modification can be carried out by utilizing physical and chemical interactions between particles and modifiers. The resulting bonds are often a mixture of secondary and chemical bonds. A great understanding of (a) treatment techniques, (b) interfacial characterization, and (c) the effect of interphase on composite properties has been acquired for composites incorporating inorganic fillers [62]. In the following, attention is focused on specific surface modification methods that have been intentionally used to introduce organic coatings onto nanosized filler surfaces.

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