8 Nanocomposites Based on Thermosetting Polyurethane Matrix. . .
145
the nature in which those carboxylic groups are arranged and the strength of
their interaction with the MWCNT surface is vitally important. Hemin being noncovalently bound to the surface of the carbon nanotube solely interacts with the
nanotube surface through van der Waals forces. Whilst immobilised hemin improves
the “functionalisation” equivalent of MWCNT-ox and also their dispersibility and
greater bonding interaction with the PU without sacrificing the structural integrity
of the carbon nanotube, it is apparent that the surface force interaction between the
MWCNT and hemin molecule is weaker than the covalent system of MWCNT-ox.
The underlying mechanical integrity of MWCNTs is therefore not reflected in the
tensile performance of the non-covalent bonded systems.
8.4 Conclusions
Nanocomposites based on PU matrix and MWCNTs, containing carboxyl, lactone
or phenol groups on the surface, were prepared and investigated. MWCNTs were
incorporated into the PU matrix in the process of PU synthesis either through
covalent or van der Waals forces. The structural peculiarities, the thermodynamic
miscibility, dynamic and static mechanical properties and segmental dynamics of
composites have been investigated. Overall, it was found that the functional groups
for PU attachment that were covalently bonded to the MWCNT lattice possessed
superior mechanical performance to those functional groups that were immobilised
through van der Waals forces to the MWCNT surface. The necessity to remove acidoxidative lattice fragments (fulvic acids) from MWCNT-ox samples to ensure only
covalent bonding with PU matrix is shown.
The vapour sorption by nanocomposites and by nanofiller was studied, and
thermodynamic affinity of polymer components to the MWCNT was estimated.
It was shown that free energy of interaction between the carbon nanotubes with
functionalized surfaces and with polyurethane matrix is negative for all types
of nanofillers. This indicates the thermodynamic stability of filled polyurethane
samples and suggests high adhesion of polyurethane to carbon nanotubes with
functionalized surfaces.
The dynamics in PU-MWCNT nanocomposites with low filler contents was
performed using a combined DMA-CRS approach. The strong dependence between
matrix dynamics and variations in the nanotube surface chemistry was demonstrated. It was found that the interface between the nanotube and a van der Waals
immobilized intermediate leads to unchanged dynamics and mechanically weaker
composites. Only direct covalent bonding of matrix to carbon nanotube lattice,
which is free from fulvic acids, results in the dramatic changes in the glass transition
dynamics of PU matrix even at low filler content. As a result, the displacement
of main PU relaxation maxima in the complicated DMA and creep rate spectra,
manifesting a pronounced dynamic heterogeneity within the glass transition, from
around 0 to 80–140 ◦ C was shown.
145
the nature in which those carboxylic groups are arranged and the strength of
their interaction with the MWCNT surface is vitally important. Hemin being noncovalently bound to the surface of the carbon nanotube solely interacts with the
nanotube surface through van der Waals forces. Whilst immobilised hemin improves
the “functionalisation” equivalent of MWCNT-ox and also their dispersibility and
greater bonding interaction with the PU without sacrificing the structural integrity
of the carbon nanotube, it is apparent that the surface force interaction between the
MWCNT and hemin molecule is weaker than the covalent system of MWCNT-ox.
The underlying mechanical integrity of MWCNTs is therefore not reflected in the
tensile performance of the non-covalent bonded systems.
8.4 Conclusions
Nanocomposites based on PU matrix and MWCNTs, containing carboxyl, lactone
or phenol groups on the surface, were prepared and investigated. MWCNTs were
incorporated into the PU matrix in the process of PU synthesis either through
covalent or van der Waals forces. The structural peculiarities, the thermodynamic
miscibility, dynamic and static mechanical properties and segmental dynamics of
composites have been investigated. Overall, it was found that the functional groups
for PU attachment that were covalently bonded to the MWCNT lattice possessed
superior mechanical performance to those functional groups that were immobilised
through van der Waals forces to the MWCNT surface. The necessity to remove acidoxidative lattice fragments (fulvic acids) from MWCNT-ox samples to ensure only
covalent bonding with PU matrix is shown.
The vapour sorption by nanocomposites and by nanofiller was studied, and
thermodynamic affinity of polymer components to the MWCNT was estimated.
It was shown that free energy of interaction between the carbon nanotubes with
functionalized surfaces and with polyurethane matrix is negative for all types
of nanofillers. This indicates the thermodynamic stability of filled polyurethane
samples and suggests high adhesion of polyurethane to carbon nanotubes with
functionalized surfaces.
The dynamics in PU-MWCNT nanocomposites with low filler contents was
performed using a combined DMA-CRS approach. The strong dependence between
matrix dynamics and variations in the nanotube surface chemistry was demonstrated. It was found that the interface between the nanotube and a van der Waals
immobilized intermediate leads to unchanged dynamics and mechanically weaker
composites. Only direct covalent bonding of matrix to carbon nanotube lattice,
which is free from fulvic acids, results in the dramatic changes in the glass transition
dynamics of PU matrix even at low filler content. As a result, the displacement
of main PU relaxation maxima in the complicated DMA and creep rate spectra,
manifesting a pronounced dynamic heterogeneity within the glass transition, from
around 0 to 80–140 ◦ C was shown.