Page 70 - Nanotechnology an introduction

example ultrahard nanoparticles might be added to a relatively soft polymer matrix to create a hard, plastic material.
6.5.1. Polymer–Nano-Object Blends

In fact, most of the recognized successes in nanomaterials so far have been not in the creation of totally new materials through mechanosynthesis
(see Section 8.3), which is still an unrealized goal, but in the more prosaic world of blending, which is the simplest form of combination. For
example, one adds hard particles to a soft polymer matrix to create a hard, abrasion-resistant coating. As with atomically-based
mechanosynthesis, the results are, to a first approximation, additive. Thus we might again write a sum like

This is not actually very new. Paint, a blend of pigment particles in a matrix (the binder), has been manufactured for millennia. What is new is the
detailed attention paid to the nanoparticulate additive. Its properties can now be carefully tailored for the desired application. If one of components
is a recognized nanosubstance—a nanoparticle or nanofiber, for example—it is acceptable to describe the blend as a nanostructured material.
Other applications of this kind include ultralow permeability materials for food packaging, for which it is often undesirable to restrict the ingress of
oxygen. The main principle here is to blend particles of a very high aspect ratio (fragments of nanoplates rather than nanoparticles) into the polymer
such that the principal plane of the objects is perpendicular to the route of ingress. The tortuosity of gas diffusion is vastly increased in such
materials. Very often it is sufficient to apply an ultrathin coating onto a conventional flexible polymer substrate to achieve the desired diminution of
gas permeability. The design of friction damping composites requires careful consideration of the viscoelastic properties of the material. “Self-
cleaning” and anti-grafitti coatings rely on the active, typically photo-induced, chemistry taking place at the surface of the nanoparticles included in
the matrix.

The Paradigm of Paint
The biggest range of applications for such nanocomposites is in thin film coatings—in other words paint. This is a much older composite than most
other nanomaterials. Paint consists of a pigment (quite possibly made of nanoparticles) dispersed in a matrix of varnish. Paint can be said to
combine the opacity of the pigment with the film-forming capability of the varnish.

Traditional pigments may comprise granules in the micrometer size range; grinding them a little bit more finely (if possible—see Section 6.1.1)
turns them into nano-objects. Compared with transparent varnish, paint combines the attribute of protection from the environment with the attribute
of color. The principle can obviously be (and has been) extended practically ad libitum: by adding very hard particles to confer abrasion resistance;
metallic particles to confer electrical conductivity; tabular particles to confer low gas permeability, and so on.
The purpose of adding materials to a polymer matrix is, then, to enhance properties such as stiffness, heat resistance, fire resistance, electrical
conductivity, gas permeability, and so forth; the object of any composite is to achieve an advantageous combination of properties. If the matrix is a
metal, then we have a metal–matrix composite (MMC, Section 6.5.2). A landmark was Toyota's demonstration in 1991 that the incorporation of a
few weight percent of a nanosized clay into a polyamide matrix greatly improved the thermal, mechanical (e.g., doubled the tensile modulus) and
gas permeability (barrier) properties of the polymer compared with the pure polymer or the conventional composite made with micrometer-sized
additive. This was the first commercial nanocomposite.
Another mineral–polymer composite is the material from which many natural seashells are constructed: platelets of aragonite dispersed in a protein
matrix (see also Section 6.5.5). In this case, however, the “matrix” only constitutes a few percent of the volume of the composite.

There is no general theory suggesting that the advantage scales inversely with additive size; whether a nanocomposite is commercially viable
depends on all the parameters involved. There is such a huge variety of materials that it is perhaps futile to attempt a generalization. However, the
very small size of individual nanoparticles would make it feasible to incorporate a greater variety of materials within the matrix for a given additive
weight percent. Very often, ensuring wetting of the particle by the matrix presents a significant technological hurdle. Most successful composites
require the additive to be completely wetted by the matrix. Wetting behavior can be predicted using the Young–Dupré approach (see Section 3.2);
if, however, the particle becomes very small, the surface tension will exhibit a curvature-dependent deviation from the bulk value appropriate for a
planar interface.
The three main fabrication routes for polymeric nanocomposites are:

1. Blending preformed nanoparticles with the matrix, which is typically in the molten state.
2. Dispersing preformed nanoparticles in monomer and polymerizing the matrix around the nanoparticles (the Toyota composite mentioned
above followed this route: modified clay was swollen in the presence of caprolactam followed by polymerization).
3. Synthesizing the nanoparticles in situ within the matrix.
In each case the goal is to disperse the composites uniformly in the matrix. Hence the chemistry of the particle–composite interface is very
important.
Nanocomposites can in principle be substituted for pure polymer in most applications. Nevertheless, there is persistent reluctance to use them in
structurally vital components since extensive data on their long-time performance is still lacking. A general difficulty is that the improved strength,
stiffness, etc. of the composites inevitably results in their being subjected to increased designed stress—otherwise there would be no advantage in
using them—and hence diminished tolerance to damage.
Polymers with Added Electro-Active Materials
This is a typical functionality-enhancing technology. Antimony tin oxide (ATO) has become a popular additive for applications such as protection
against electromagnetic interference (EMI) and electrostatic discharge (ESD). Antistatic coatings incorporate additives with both hydrophobic and
hydrophilic radicals, ideally concentrated in the interfacial zone between the polymer and the air: the hydrophobic radicals are oriented towards the