Environmental Engineering Reference
In-Depth Information
allow unhindered intercalation of the polymer chains in the clay. By deinition, intercala-
tion is the insertion of polymer chains between two galleries of clay. This enlarges the
intergallery spacing of the clay.
The expansion of the gallery spacing by the organic modiier allows proper formation
of a nanocomposite during fabrication. Three strategies are mainly employed in the prep-
aration of nanocomposites: in situ polymerization, 11 the melt-blending method, and the
solution-blending method. 9,12
20.2.1.1 In Situ Polymerization
In the in situ polymerization procedure, monomers are mixed with the clay before poly-
merization. The main goal is to disperse the clay layers in the polymer matrix to obtain a
nanocomposite with homogeneous exfoliated structures. Uniform dispersion of the clay in
this method is attributed to the low monomer viscosity. According to Lingaraju et al., 13 this
procedure was used in the preparation of the “irst” nanocomposite whereby researchers
at the Toyota Central Research Laboratories synthesized the nylon 6-clay nanocomposite.
For most technologically essential polymers, in situ polymerization is limited because
a suitable monomer-silicate solvent system is not always available, and it is not always
compatible with current polymer-processing techniques. 14,15
20.2.1.2 Solution Blending
The solution-blending technique involves the selection of a solvent that is capable of swell-
ing the clay and dissolving the polymer matrix. A homogeneous three-component mix-
ture of appropriate composition of the polymer, clay, and solvent is prepared through
heating and mechanical and/or ultrasonic stirring. Depending on the interactions of the
solvent and nanoparticles, the nanoparticle aggregates can be disintegrated in the solvent
owing to the weak van der Waals forces that stack the layers together. 9 Polymer chains can
then be adsorbed onto the nanoparticles. The inal step of the procedure involves removal
of the solvent through evaporation or by precipitation in a nonsolvent. There is entropy
gain associated with the removal of the solvent molecules, and it is thought to compensate
for the entropic loss of the intercalated polymer chains. 16 Therefore, it can be speculated
that entropy-driven intercalation might be expected to occur even in the absence of an
enthalpy gain due to favorable interactions between the macromolecules and the surface
of the clay layers. 17
This technique is used especially with water-soluble polymers, poly(acrylic acid) (PAA), 18
poly(ethylene oxide), 19 poly(ethylene vinyl alcohol), etc. The polarity of such polymers is
believed to contribute an enthalpy gain helping intercalation. 20 The solution-blending pro-
cedure requires that the selected polymer be compatible with the selected solvent. An
example of an organic solvent and hydrophobic polymers like polypropylene, 21 have also
been given consideration. Notably, this method produces a high degree of intercalation
only for certain polymer/clay/solvent systems, implying that for a given polymer one has
to ind the right clay, organic modiier, and solvents.
20.2.1.3 Melt Blending
The melt-blending procedure, also known as melt intercalation, came to prominence in
the late 1990s. 22 This method involves the physical mixing of the polymer matrix and the
clay in the molten state of the polymer. This process is attractive to researchers since it
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