Environmental Engineering Reference
In-Depth Information
technological attention because clay is environmentally benign, abundantly available, and
there is wide knowledge available on clay-intercalation chemistry.
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20.2.1 Polymer-Clay Nanocomposite Formation
Any physical mixture of a polymer and clay does not necessarily form a nanocompos-
ite with improved properties. Immiscibility leads to poor physical attraction between the
polymer and the clay particles, and that consequently leads to relatively poor mechanical
properties. Generally, clays are hydrophilic, and this means that it is necessary to select
a hydrophilic polymer to enable miscibility when preparing nanocomposites. However,
hydrophilic polymers are not compatible with all composite-fabrication procedures.
Fortunately, the clay can be modiied accordingly to enhance compatibility. In many cases,
a surfactant is used and the resulting modiied clay is called organoclay. Many compounds
have been used to modify clay. For example, dimethyl sulfoxide, methanol, and octadecyl-
amine can be used. The “popular” Cloisite
®
organoclays are prepared by modifying mont-
morillonite clay with salts of dimethyl dehydrogenated tallow quaternary ammonium.
The concept of modifying the clay takes advantage of the exchangeable ions in the gallery
of the clay. The general structure of a 2:1 dioctahedral smectite clay is shown in Figure 20.1.
The structure shows that the clay possesses lithium (Li), sodium (Na), rubidium (Rb),
and cesium (Cs) ions sandwiched between two layers. These ions can be replaced with
surfactants under appropriate conditions to form an organoclay. The replacement of the
exchange cations in the cavities of the layered clay structure by alkylammonium surfac-
tants can render compatible the surface chemistry of the clay and a hydrophobic polymer
matrix. When natural montmorillonite is used, the dispersion of the clay in the polyvinyl
chloride (PVC) matrix is typically poor, and when organically modiied montmorillonite
is used an improved dispersion is observed.
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Therefore, organic modiication of the clay
is very important and it has an effect on the nanocomposite properties. It should be men-
tioned that the type of surfactant, the chain length, and the packing density might play
an important role
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in determining the suitability of the surfactant to modify the clay.
Replacing the exchangeable ions in the cavity of the clay does not only serve to alter the
hydrophilic or polarity properties of the clay but also widens the intergallery spacing to
Al, Fe, Mg, Li
OH
O
Li, Na, Rb, Cs
Te trahedral
Octahedral
Te trahedral
Exchangeable cations
FIGURE 20.1
(See color insert.)
Layered silicate structure. (Adapted from G. Beyer,
Plast. Addit. Compd.
, 4, 22, 2002.)
