Page 306 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Composites and Fillers 269

The ultimate strength and properties of many materials is dependent upon the intimate contact
between the various members. Thus, for ceramics, nanosized particles allow a more homogeneous
structure, resulting in stronger ceramic materials.
As noted before, nanocomposites have also been with us since almost the beginning of time.
Our bones are examples of nanocomposites. The reinforcement material is plate-like crystals of
hydroxyapatite, Ca (PO ) (OH) , with a continuous phase of collagen fibers. The shell of a mol-

10 4 6 2
lusk is microlaminated containing as the reinforcement aragonite (a crystalline form of calcium
carbonate) and the matrix is a rubbery material. Allowing nature to be a source of ideas is a con-

tinuing theme in synthetic polymer science, including modification of natural polymers. Much of
the renewed interest in nanocomposite materials is the direct result of the availability of new nano-
building blocks.
Within a composite material, much of the ultimate strength comes from the intimate contact the

fiber has with the matrix material. Nanofibers allow more contact between the fiber (on a weight

basis) and the matrix, resulting in a stronger composite because of an increased fi ber surface–matrix
interface.
A number of inorganic/organic nanocomposites have been made. These include nanoinorganics
including nanofibers from silicon nitride, titanium (IV) oxide, zirconia (ZrO ), alumina (Al O ),

2 2 3
titanium carbide (TiC), and tungsten carbide (WC). It also includes the use of special clays (layered
silicates) mixed with nylons to form nanocomposites. The clay layers are separated giving plate-
lets about 1 nm thick. These nylon–clay microcomposites are used to make the air intake cover
of some of the Toyota automobiles. These individual clay platelets have also been used to form
nearly single-layer polymer chain sheets similar to lignin. The interaction with the silicate surface
encourages the polymer chains to take different arrangements. To be effective, the hydrophilic
silicate surface is generally modifi ed to accommodate the more hydrophobic common monomers
and polymers.

While carbon fiber (thickness on the order of 1,000 nm) composites offer very strong materials,
carbon nanotubes make even stronger composites. These carbon nanotubes have aspect ratios of
more than 1,000 (ratio of length to diameter). Further, because some carbon nanotubes are electri-
cally conductive, composites containing them can be made to be conductive. A number of carbon
nanotube matrixes have been made, including using a number of engineering resins such as polyes-
ters, nylons, polycarbonates, and poly(phenylene ether).

Individual polymer chains can be more flexible than groups of chains (bulk) even when the poly-
mer is generally considered to be rigid. This is presumably because single chains have less torsional
strain imparted by near neighbors and various chain entanglements and associations are not present.
Compared with carbon fi bers, carbon nanotubes imbedded within a polymer matrix can withstand
much greater deformations before they break. Further, nanomaterials are generally more effi cient
in transferring applied load from the nanomaterial to the matrix. These factors contribute to the
greater strength of carbon nanotube composites.
As noted before, adhesion between the reinforcing agent and matrix is important. Some matrix
materials do not adhere well with certain fibers. This is partially overcome through the introduc-

tion of defects or functional groups onto the nanomaterials that act as hooks to anchor them to the
matrix material.
Research with tires continues to be active with the end goals of reducing tire weight and increas-
ing tire lifetime. For example, truck tires are now capable of running 750,000 miles with the carcass
married to four tire threads. The truck tire liners weight about 9 pounds. A decrease in weight of
50% would translate into a significant increase in mileage of 3–5 mpg. Solutions should be inexpen-

sive employing readily available and abundant materials.
Nanocomposites are being used in tires, in particular tire inner liners. Here, less permeable inner
layers are achieved by the introduction of “clad” layers that allow the use of a thinner inner liner,
resulting in an overall lighter tire. Tire inner layers are typically derived from butyl rubber, often
halogenated butyl rubber.

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