247
the polymer matrix. However, smectic clays are highly hydrophilic,
and thus individual layers do not easily disperse in relatively hydrophobic species, such as an organic polymer.
To overcome this problem, an ion exchange reaction is usually
carried out whereby organic cations open the clay layers and make
them hydrophobic enough, resulting in an organically modified
clay. At this point, the organically modified clay can be intercalated
with the polymer through various techniques, such as solventbased, melt-blending, and polymerization-based methods. In the
case of the solvent-based method, low concentrations of organoclays are dispersed in a solvent in which a polymer is soluble (see
Figure 7.51). Due to the weak forces between the silicate layers,
the solvent separates the layers, thereby allowing the polymer to
adsorb onto the surfaces of individual silicate platelets. The solvent
is then evaporated, causing the individual platelets to reagglomerate. Highly polar polymers, such as nylon and polyimides, can be
more readily intercalated than nonpolar polymers.
Another technique that is frequently used to form polymer-matrix
nanocomposites involves melt blending of organoclays with polymers using conventional extrusion equipment. The studies show
that intercalation of polymer chains between the layered clays can
occur spontaneously by heating a mixture of polymer and clay
powder above the glass transition temperature or melting temperature. Once sufficient polymer mobility is achieved, the chains
diffuse into the clay layers, thereby producing a swelled polymer/
clay layered structure.
Finally, an in situ polymerization can also be employed. Here,
monomers are directly intercalated into the organically modified
clay, followed by polymerization. In this way, nylon, polyimide,
polystyrene, and rubber-based nanocomposites have been developed. Once the clays have been intercalated with a polymer, the
nanocomposites can be processed by conventional melt-processing techniques. The resulting nanocomposites can exhibit various
microstructures, namely intercalated nanocomposites, when the
polymer diffuses into the clay layers, thereby expanding the distance between the clay layers, and exfoliated nanocomposites when
individual delaminated silicate layers are dispersed in a polymer
matrix (see Figure 7.52).
Nanoscale layered silicates have proven to induce great improvements in the properties of conventional polymers. An unexpected
large increase in the elastic and flexural modulus was achieved with
small amounts of silicates (as low as 1 wt%). Thermal stability and
Figure 7.51
Intercalation solvent-based process.
+
Intermolecular
interactions
Silicate layer
Polymer
Alphadic
chains
Figure 7.52
Intercalated and exfoliated polymer
nanocomposites.
Conventional
composite
Intercalated
nanocomposite
Exfoliated
nanocomposite
+
Polymer
Layered clay
Special Cases
the polymer matrix. However, smectic clays are highly hydrophilic,
and thus individual layers do not easily disperse in relatively hydrophobic species, such as an organic polymer.
To overcome this problem, an ion exchange reaction is usually
carried out whereby organic cations open the clay layers and make
them hydrophobic enough, resulting in an organically modified
clay. At this point, the organically modified clay can be intercalated
with the polymer through various techniques, such as solventbased, melt-blending, and polymerization-based methods. In the
case of the solvent-based method, low concentrations of organoclays are dispersed in a solvent in which a polymer is soluble (see
Figure 7.51). Due to the weak forces between the silicate layers,
the solvent separates the layers, thereby allowing the polymer to
adsorb onto the surfaces of individual silicate platelets. The solvent
is then evaporated, causing the individual platelets to reagglomerate. Highly polar polymers, such as nylon and polyimides, can be
more readily intercalated than nonpolar polymers.
Another technique that is frequently used to form polymer-matrix
nanocomposites involves melt blending of organoclays with polymers using conventional extrusion equipment. The studies show
that intercalation of polymer chains between the layered clays can
occur spontaneously by heating a mixture of polymer and clay
powder above the glass transition temperature or melting temperature. Once sufficient polymer mobility is achieved, the chains
diffuse into the clay layers, thereby producing a swelled polymer/
clay layered structure.
Finally, an in situ polymerization can also be employed. Here,
monomers are directly intercalated into the organically modified
clay, followed by polymerization. In this way, nylon, polyimide,
polystyrene, and rubber-based nanocomposites have been developed. Once the clays have been intercalated with a polymer, the
nanocomposites can be processed by conventional melt-processing techniques. The resulting nanocomposites can exhibit various
microstructures, namely intercalated nanocomposites, when the
polymer diffuses into the clay layers, thereby expanding the distance between the clay layers, and exfoliated nanocomposites when
individual delaminated silicate layers are dispersed in a polymer
matrix (see Figure 7.52).
Nanoscale layered silicates have proven to induce great improvements in the properties of conventional polymers. An unexpected
large increase in the elastic and flexural modulus was achieved with
small amounts of silicates (as low as 1 wt%). Thermal stability and
Figure 7.51
Intercalation solvent-based process.
+
Intermolecular
interactions
Silicate layer
Polymer
Alphadic
chains
Figure 7.52
Intercalated and exfoliated polymer
nanocomposites.
Conventional
composite
Intercalated
nanocomposite
Exfoliated
nanocomposite
+
Polymer
Layered clay
Special Cases