and inorganic materials, such as high strength, heat stability, and chemical
resistance. The incorporation of organic/inorganic hybrids can yield materials
possessing excellent stiffness, strength, and gas barrier properties with far less
inorganic content than is used in conventionally filled polymer composites: the
higher the degree of delamination in polymer/clay nanocomposites, the greater
the enhancement of these properties. Nanocomposites such as these can also
be employed as scratch- and abrasive-resistant hard coatings, nonlinear optical
materials, and reinforcements for elastomers and plastics (LeBaron et al. 1999;
Vaia 2000).
Nanofillers can be categorized on the basis of their dimensions, such as
one-dimensional fillers (nanotubes and nanowires), two-dimensional fillers
(clays and graphene), and three-dimensional fillers (spherical and cubic nanoparticles). Among them, clays have sandwich structures with one octahedral Al
sheet and two tetrahedral Si sheets, the so-called phyllosilicates. There are many
types of phyllosilicates, including kaolinite, montmorillonite (MMT), hectorite,
saponite, and synthetic mica. These clays consist of stacked silicate sheets with
lengths of about 46 nm for hectorite, 170 nm for saponite, 218 nm for MMT,
1230 nm for synthetic mica, and so on. They have the same sheet thickness of
1 nm. Therefore, the only difference among them is the length of the silicate sheets
(Yano et al. 1997; Garcia et al. 2000).
To overcome problems of macro- and micro-phase separation between organic
polymers and inorganic clays, organic/inorganic polymer hybrids have mostly been
synthesized using three methods: solution intercalation, melt intercalation, and
in-situ intercalation polymerization. Additionally, other approaches, such as the
sol–gel process and monomer/polymer grafting to clay layers, have resulted in
organic/inorganic polymer hybrids (Ishida et al. 2000; Shen et al. 2002).
Solution intercalation is based on a solvent system in which the polymer is
soluble and the clay layers swell. The layered clay is first swollen in a solvent,
such as N,N
0 -dimethylacetamide (DMAc). When the polymer and clay solutions are
mixed, the polymer chains intercalate and displace the solvent from between the
layers of clay. Upon solvent removal, the intercalated structure remains, resulting in
hybrids with nanoscale morphology. In the process of melt intercalation, the layered
silicate is mixed with a molten polymer matrix. If the silicate surfaces are sufficiently
compatible with the chosen polymer, the polymer can enter the interlayer space and
form an intercalated or an exfoliated nanocomposite. Finally, in-situ intercalation
polymerization is based on the use of one or more monomers that may be in-situ
linearly polymerized or crosslinked and was the first method used to synthesize
polymer-layered silicate nanocomposites based on nylon 6. In situ intercalation
relies on swelling of the organoclay due to the monomer, followed by in-situ
polymerization initiated thermally or by the addition of a suitable compound.
Chain growth in the clay galleries triggers clay exfoliation and nanocomposite
formation. Thus, an advantage of the in-situ method is the preparation of polymer
hybrids without physical or chemical interactions between the organic polymer and
the inorganic material (Min and Chang 2012).
580
T. Y. Ha et al.
resistance. The incorporation of organic/inorganic hybrids can yield materials
possessing excellent stiffness, strength, and gas barrier properties with far less
inorganic content than is used in conventionally filled polymer composites: the
higher the degree of delamination in polymer/clay nanocomposites, the greater
the enhancement of these properties. Nanocomposites such as these can also
be employed as scratch- and abrasive-resistant hard coatings, nonlinear optical
materials, and reinforcements for elastomers and plastics (LeBaron et al. 1999;
Vaia 2000).
Nanofillers can be categorized on the basis of their dimensions, such as
one-dimensional fillers (nanotubes and nanowires), two-dimensional fillers
(clays and graphene), and three-dimensional fillers (spherical and cubic nanoparticles). Among them, clays have sandwich structures with one octahedral Al
sheet and two tetrahedral Si sheets, the so-called phyllosilicates. There are many
types of phyllosilicates, including kaolinite, montmorillonite (MMT), hectorite,
saponite, and synthetic mica. These clays consist of stacked silicate sheets with
lengths of about 46 nm for hectorite, 170 nm for saponite, 218 nm for MMT,
1230 nm for synthetic mica, and so on. They have the same sheet thickness of
1 nm. Therefore, the only difference among them is the length of the silicate sheets
(Yano et al. 1997; Garcia et al. 2000).
To overcome problems of macro- and micro-phase separation between organic
polymers and inorganic clays, organic/inorganic polymer hybrids have mostly been
synthesized using three methods: solution intercalation, melt intercalation, and
in-situ intercalation polymerization. Additionally, other approaches, such as the
sol–gel process and monomer/polymer grafting to clay layers, have resulted in
organic/inorganic polymer hybrids (Ishida et al. 2000; Shen et al. 2002).
Solution intercalation is based on a solvent system in which the polymer is
soluble and the clay layers swell. The layered clay is first swollen in a solvent,
such as N,N
0 -dimethylacetamide (DMAc). When the polymer and clay solutions are
mixed, the polymer chains intercalate and displace the solvent from between the
layers of clay. Upon solvent removal, the intercalated structure remains, resulting in
hybrids with nanoscale morphology. In the process of melt intercalation, the layered
silicate is mixed with a molten polymer matrix. If the silicate surfaces are sufficiently
compatible with the chosen polymer, the polymer can enter the interlayer space and
form an intercalated or an exfoliated nanocomposite. Finally, in-situ intercalation
polymerization is based on the use of one or more monomers that may be in-situ
linearly polymerized or crosslinked and was the first method used to synthesize
polymer-layered silicate nanocomposites based on nylon 6. In situ intercalation
relies on swelling of the organoclay due to the monomer, followed by in-situ
polymerization initiated thermally or by the addition of a suitable compound.
Chain growth in the clay galleries triggers clay exfoliation and nanocomposite
formation. Thus, an advantage of the in-situ method is the preparation of polymer
hybrids without physical or chemical interactions between the organic polymer and
the inorganic material (Min and Chang 2012).
580
T. Y. Ha et al.