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1 Introduction

Figure 1.16: Structures of carbon compounds, all based on aromatic rings.

these structures are lightweight but strong considering their weight, thus carbon nan-

otubes (as well as larger carbon fibers) have been used as reinforcement or strengthen-

ing agents for polymers. Fibers only reinforce materials, though, when they are evenly

distributed, rather than phase separated, thus “coupling agents” need to also be added

for the strengthening effect. Coupling agents are surfactants, amphiphiles that bind on

one side to the fiber, on the other side to the polymer matrix. They reduce the surface

energy between the two phases and allow the fibers to mix more easily with the matrix.

Mechanical properties of materials. The strength of the material is an important

property. First, let us remind ourselves about mechanical forces. Force is simply mass

times acceleration, F = ma. The more force I put on an object, the more it will accelerate.

The heavier the object, the more force I need to use to accelerate it.

In the case of machines, even on the nanoscale, usually the force has to be trans-

ferred somewhere to have an effect. Basically, we are talking about a lever that trans-

mits the force (Figure 1.17). One of the requirements to transfer force, though, is that the

lever material is stiff. If the material were flexible, the effect of the force would be to

bend the lever; no force would be transferred to the other side of the lever. If the lever

were elastic like a spring, it would be even worse: the lever would resist and then force

your hand back to where it started (Figure 1.18).

Elasticity is an important property for a material, assuming it is not the material

for a lever, of course. Elasticity is one of the reasons why some materials don’t break

so easily, they just deform and then return to their original form when a force acts on

them. Brittle materials often break more easily.