144
L. V. Karabanova et al.
a noticeable drop compared with 0.01 and 0.25 wt%, which could be connected with
changes in structure of polyurethane matrix induced by the addition of MWCNT-ox
(Fig. 8.3). The presence of 0.1% MWCNT-ox in the nanocomposites results in a
dramatic change in the structure of polyurethane where the complete disappearance
of the hard and soft domains could be observed (Fig. 8.3).
Although the highest additive of MWCNTs studied in our investigation was
0.25 wt. %, our results align well with studies carried out by Wei Chen et al.
[45], where it was found that increasing the MWCNT fraction in a PU matrix
also increased the tensile strength and modulus [45]. However, the authors [45]
used the thermoplastic polyurethane elastomer as a matrix for the generation
of nanocomposites and introduced the minimal amount of MWCNTs 5.6 wt.%.
Significant improvement in Young’s modulus and tensile strength in this work were
achieved for MWCNT content of 9.3 wt.% only [45].
The comparison of covalent and ionic bonding between MWCNTs and PU has
been previously explored by H-C Kuan et al. [46], but their MWCNT-ox did not
include the procedure of removal of in situ generated and surface-immobilised fulvic
acids. In this study, it was found that MWCNT-ox provide the higher Young moduli
than MWCNT-red and also the non-covalent systems, but these results could be
obtained because of the higher content of nanotubes (4%) [46].
McClory et al. [22] used thermosetting PU for synthesizing MWCNT nanocomposites with nanotube content up to 1 wt. % and also recorded significant enhancements in stiffness, strength and toughness. Wang et al. [11] created PU-MWCNT
nanocomposites with good mechanical properties due to covalent cross-linking of
MWCNTs to the PU matrix using poly(acryloyl chloride).
Given that hemin interacts with the MWCNT in the same manner as fulvic acids,
generated by the lattice degradation through acid digestion under oxidation, it can
be inferred that the presence of such fulvic acids on improperly treated and purified
MWCNT-ox can lead to a retardation of the mechanical performance of the final
nanocomposites.
In the case of MWCNT-red, the number of available alcohol sites increases
over the number of carboxylic groups for MWCNT-ox, and yet the mechanical
performance, by comparison, diminishes. The number of surface acidic groups on
oxidised and reduced MWCNTs should be approximately the same given that the
lithium aluminium hydride treatment does not exhibit any signs of surface group
removal or lattice degradation and preferentially forms alcohol groups that should
promote connectivity to PU during polymerisation. The increased performance
of MWCNT-ox implies that either carboxylic groups on MWCNT-ox are more
reactive than the alcohol (and phenol) groups on MWCNT-red or the formation of a
urethane group through the now pendant aliphatic alcohol group (on MCWNT-red)
is mechanically weaker than the linkage through the carboxylic group (on MWCNTox) in the reaction of the isocyanate monomer with surface acidic groups.
Given that the number of carboxylic groups on MWCNT-hemin is greater than
MWCNT-hemin-red and MWCNT-ox and between the two hemin species there is
little to separate their mechanical performance, it strongly suggests that mechanical
strength does not solely come from a greater quantity of carboxylic groups, but
L. V. Karabanova et al.
a noticeable drop compared with 0.01 and 0.25 wt%, which could be connected with
changes in structure of polyurethane matrix induced by the addition of MWCNT-ox
(Fig. 8.3). The presence of 0.1% MWCNT-ox in the nanocomposites results in a
dramatic change in the structure of polyurethane where the complete disappearance
of the hard and soft domains could be observed (Fig. 8.3).
Although the highest additive of MWCNTs studied in our investigation was
0.25 wt. %, our results align well with studies carried out by Wei Chen et al.
[45], where it was found that increasing the MWCNT fraction in a PU matrix
also increased the tensile strength and modulus [45]. However, the authors [45]
used the thermoplastic polyurethane elastomer as a matrix for the generation
of nanocomposites and introduced the minimal amount of MWCNTs 5.6 wt.%.
Significant improvement in Young’s modulus and tensile strength in this work were
achieved for MWCNT content of 9.3 wt.% only [45].
The comparison of covalent and ionic bonding between MWCNTs and PU has
been previously explored by H-C Kuan et al. [46], but their MWCNT-ox did not
include the procedure of removal of in situ generated and surface-immobilised fulvic
acids. In this study, it was found that MWCNT-ox provide the higher Young moduli
than MWCNT-red and also the non-covalent systems, but these results could be
obtained because of the higher content of nanotubes (4%) [46].
McClory et al. [22] used thermosetting PU for synthesizing MWCNT nanocomposites with nanotube content up to 1 wt. % and also recorded significant enhancements in stiffness, strength and toughness. Wang et al. [11] created PU-MWCNT
nanocomposites with good mechanical properties due to covalent cross-linking of
MWCNTs to the PU matrix using poly(acryloyl chloride).
Given that hemin interacts with the MWCNT in the same manner as fulvic acids,
generated by the lattice degradation through acid digestion under oxidation, it can
be inferred that the presence of such fulvic acids on improperly treated and purified
MWCNT-ox can lead to a retardation of the mechanical performance of the final
nanocomposites.
In the case of MWCNT-red, the number of available alcohol sites increases
over the number of carboxylic groups for MWCNT-ox, and yet the mechanical
performance, by comparison, diminishes. The number of surface acidic groups on
oxidised and reduced MWCNTs should be approximately the same given that the
lithium aluminium hydride treatment does not exhibit any signs of surface group
removal or lattice degradation and preferentially forms alcohol groups that should
promote connectivity to PU during polymerisation. The increased performance
of MWCNT-ox implies that either carboxylic groups on MWCNT-ox are more
reactive than the alcohol (and phenol) groups on MWCNT-red or the formation of a
urethane group through the now pendant aliphatic alcohol group (on MCWNT-red)
is mechanically weaker than the linkage through the carboxylic group (on MWCNTox) in the reaction of the isocyanate monomer with surface acidic groups.
Given that the number of carboxylic groups on MWCNT-hemin is greater than
MWCNT-hemin-red and MWCNT-ox and between the two hemin species there is
little to separate their mechanical performance, it strongly suggests that mechanical
strength does not solely come from a greater quantity of carboxylic groups, but