245
pared to a 10% increase for randomly oriented nanocomposites.
In addition, aligned MWCNTs/polystyrene nanocomposites were
also shown to exhibit an increased yield strength and ultimate
yield strength compared to a pristine polystyrene polymer. Due to
the fact that CNTs exhibit high electrical conductivities, the application of a magnetic field or electric field has also been used to
induce CNT alignment. It was observed that the elastic modulus
measured in the direction parallel to the magnetic field was greater
than for the direction perpendicular to the field. This fact suggests
that aligned MWCNTs contribute significantly to an increase in the
elastic modulus in the direction parallel to the aligned CNTs.
Finally, the interfacial bonding between the CNTs and the polymer
matrix needs to be considered. It is believed that the efficiency of
load transfer in nanocomposites is controlled by the interfacial
characteristics of the reinforcing phase and the matrix. In the case of
polymer-matrix nanocomposites reinforced with CNTs, it is believed
that the main mechanisms of load transfer include mechanical interlocking, chemical bonding, and nonbonded interactions between
the CNTs and the matrix. In particular, it is believed that the key
factor in forming a strong bond between the polymer matrix and a
CNT lies in the polymer morphology, especially its ability to form
large diameter helices around individual CNTs. The strength of the
interface is then due to molecular-level entanglements of CNTs and
matrix as well as long-range order of the polymer.
The inclusion of CNTs in polymer-matrix nanocomposites can
also be used to improve the thermal conductivity of these materials. The CNTs create a percolation network that allows the nanocomposite to conduct heat with conductivities up to 3.5 times the
conductivity of the pristine polymer. However, the thermal conductivity of a nanocomposite is still far from the theoretical value
for an isolated CNT, which has been predicted to be on the order
of 10
3 W/m.K. The main reason for this discrepancy is the large
thermal resistance that exists between the polymer matrix and the
CNT surface. Because CNTs possess high surface-to-volume ratios,
when CNTs are dispersed in the polymer matrix, the large interfacial area creates a significant resistance. This effect has been attributed to differences in phonon frequency between the CNTs and
the polymer matrix. Some solutions have been suggested to decrease
the thermal resistance of the polymer/CNT interface. For example,
it has been proposed to covalently bond the CNTs to the polymer
matrix to improve the phonon/phonon coupling. It has also been
suggested to use MWCNTs instead of SWCNTs because the former
have smaller aspect ratios and therefore less interfacial area. The use
Special Cases
pared to a 10% increase for randomly oriented nanocomposites.
In addition, aligned MWCNTs/polystyrene nanocomposites were
also shown to exhibit an increased yield strength and ultimate
yield strength compared to a pristine polystyrene polymer. Due to
the fact that CNTs exhibit high electrical conductivities, the application of a magnetic field or electric field has also been used to
induce CNT alignment. It was observed that the elastic modulus
measured in the direction parallel to the magnetic field was greater
than for the direction perpendicular to the field. This fact suggests
that aligned MWCNTs contribute significantly to an increase in the
elastic modulus in the direction parallel to the aligned CNTs.
Finally, the interfacial bonding between the CNTs and the polymer
matrix needs to be considered. It is believed that the efficiency of
load transfer in nanocomposites is controlled by the interfacial
characteristics of the reinforcing phase and the matrix. In the case of
polymer-matrix nanocomposites reinforced with CNTs, it is believed
that the main mechanisms of load transfer include mechanical interlocking, chemical bonding, and nonbonded interactions between
the CNTs and the matrix. In particular, it is believed that the key
factor in forming a strong bond between the polymer matrix and a
CNT lies in the polymer morphology, especially its ability to form
large diameter helices around individual CNTs. The strength of the
interface is then due to molecular-level entanglements of CNTs and
matrix as well as long-range order of the polymer.
The inclusion of CNTs in polymer-matrix nanocomposites can
also be used to improve the thermal conductivity of these materials. The CNTs create a percolation network that allows the nanocomposite to conduct heat with conductivities up to 3.5 times the
conductivity of the pristine polymer. However, the thermal conductivity of a nanocomposite is still far from the theoretical value
for an isolated CNT, which has been predicted to be on the order
of 10
3 W/m.K. The main reason for this discrepancy is the large
thermal resistance that exists between the polymer matrix and the
CNT surface. Because CNTs possess high surface-to-volume ratios,
when CNTs are dispersed in the polymer matrix, the large interfacial area creates a significant resistance. This effect has been attributed to differences in phonon frequency between the CNTs and
the polymer matrix. Some solutions have been suggested to decrease
the thermal resistance of the polymer/CNT interface. For example,
it has been proposed to covalently bond the CNTs to the polymer
matrix to improve the phonon/phonon coupling. It has also been
suggested to use MWCNTs instead of SWCNTs because the former
have smaller aspect ratios and therefore less interfacial area. The use
Special Cases