chaPter 7 nanomaterials: Properties
248
fire resistance through char formation are also interesting properties exhibited by these nanocomposites. In addition, they can also
display superior resistance to permeability of gases. In terms of
mechanical properties, the increase in the elastic modulus of nylonbased nanocomposites seems to be related to the average length of
the layers and therefore their aspect ratio. It has been shown that
the exfoliated layers are the main factor responsible for the stiffness
improvement. However, in a pure elastomeric matrix, exfoliation
does not seem to be a prerequisite to enhance the material stiffness.
In addition, a large increase in the tensile modulus for an exfoliated
structure is also observed for thermoset polymer matrices.
On the other hand, the stress at failure may vary strongly depending
on the nature of the interactions between the matrix and the nanoscale filler in thermoplastic-based (intercalated or exfoliated) nanocomposites. For example, exfoliated nylon-6-based nanocomposites
or intercalated PMMA-based nanocomposites exhibit an increase
in the stress at break, which is usually explained by the presence
of polar and ionic interactions between the polymer and the silicate layers. On the contrary, polypropylene-based nanocomposites
show negligible or slight improvement in the stress at failure due
to the lack of interfacial adhesion between the nonpolar polypropylene and the polar layered silicates, and polystyrene-intercalated
nanocomposites exhibit a decrease in the stress at failure. In terms
of ductility, the addition of layered silicates to polymer-based nanocomposites leads to different results. In intercalated PMMA and
polystyrene the elongation at break is reduced. However, such a loss
in ultimate elongation does not occur in elastomeric epoxy. Rather,
the addition of the nanoclay induces an increase in elongation.
The incorporation of layered silicate into a polymer matrix can
also affect the thermal properties of nanocomposites. In terms of
thermal conductivity, it seems that one of the effects of the layered
silicates is to change the cell density and cell size. In fact, nanoclay/polyurethane nanocomposites show an increase in cell density
and a decrease in cell size with the addition of nanoclay. This effect
is attributed to the nanoclay acting as cell nucleation sites leading
to higher cell densities and small cells, which in turn reduce the
thermal conductivity. The second effect is due to the fact that the
chemically bonded silicate layers inhibit heat and mass transport.
However, the greatest benefit of using nanoclays as fillers for nanocomposites is in improving the thermal stability of the polymer
matrix. This is seen in various polymer systems. The factors that
determine the extent of thermal stabilization in nanocomposites
arise from the restriction of thermal motion of polymer molecules
248
fire resistance through char formation are also interesting properties exhibited by these nanocomposites. In addition, they can also
display superior resistance to permeability of gases. In terms of
mechanical properties, the increase in the elastic modulus of nylonbased nanocomposites seems to be related to the average length of
the layers and therefore their aspect ratio. It has been shown that
the exfoliated layers are the main factor responsible for the stiffness
improvement. However, in a pure elastomeric matrix, exfoliation
does not seem to be a prerequisite to enhance the material stiffness.
In addition, a large increase in the tensile modulus for an exfoliated
structure is also observed for thermoset polymer matrices.
On the other hand, the stress at failure may vary strongly depending
on the nature of the interactions between the matrix and the nanoscale filler in thermoplastic-based (intercalated or exfoliated) nanocomposites. For example, exfoliated nylon-6-based nanocomposites
or intercalated PMMA-based nanocomposites exhibit an increase
in the stress at break, which is usually explained by the presence
of polar and ionic interactions between the polymer and the silicate layers. On the contrary, polypropylene-based nanocomposites
show negligible or slight improvement in the stress at failure due
to the lack of interfacial adhesion between the nonpolar polypropylene and the polar layered silicates, and polystyrene-intercalated
nanocomposites exhibit a decrease in the stress at failure. In terms
of ductility, the addition of layered silicates to polymer-based nanocomposites leads to different results. In intercalated PMMA and
polystyrene the elongation at break is reduced. However, such a loss
in ultimate elongation does not occur in elastomeric epoxy. Rather,
the addition of the nanoclay induces an increase in elongation.
The incorporation of layered silicate into a polymer matrix can
also affect the thermal properties of nanocomposites. In terms of
thermal conductivity, it seems that one of the effects of the layered
silicates is to change the cell density and cell size. In fact, nanoclay/polyurethane nanocomposites show an increase in cell density
and a decrease in cell size with the addition of nanoclay. This effect
is attributed to the nanoclay acting as cell nucleation sites leading
to higher cell densities and small cells, which in turn reduce the
thermal conductivity. The second effect is due to the fact that the
chemically bonded silicate layers inhibit heat and mass transport.
However, the greatest benefit of using nanoclays as fillers for nanocomposites is in improving the thermal stability of the polymer
matrix. This is seen in various polymer systems. The factors that
determine the extent of thermal stabilization in nanocomposites
arise from the restriction of thermal motion of polymer molecules