124
L. V. Karabanova et al.
introduce functional groups for the attachment of PU, where the functional groups
were either surface immobilised on or covalently bound directly to the surface
of carbon nanotubes. MWCNT-ox and MWCNT-red both possess covalent sites
directly on the lattice and were purified free of fulvic acids to ensure no competition
with surface-immobilised functional groups resulted. Therein, connecting atomic
units (fragments) of PU network are directly attached to the MWCNT lattice.
MWCNT-hemin and MWCNT-hemin-red also have covalent sites for attachment,
but only through the hemin molecules that decorate the surface of the nanotubes
through non-covalent bonding. Ultimately this approach will facilitate a direct
comparison across the type of MWCNTs to assess which type of interactions
may reflect superior mechanical performance and also elucidate whether acidoxidative lattice fragments (fulvic acids) can exert a detrimental effect to the
final mechanical performance of the composite. Moreover, it is recognised that
in chemically modifying the carbon lattice, the surface tension of the MWCNTs
will also change and will alter the dispersibility of MWCNTs into a polymerising
solution/matrix and in turn affect the size and distribution of the hard and soft
domains of PU.
The oxidation of MWCNTs in nitric acid digests part of the outer walls,
introducing acidic groups directly to the lattice, as well as generating fulvic acids,
which are removed through the reflux in alkali solution. The types of groups
identified on the MWCNT surface include carboxylic, lactone and phenolic groups
as the chief moieties. Acid anhydride, carbonyl, alcohol and ether groups may
also be present, though only the first would be accounted in the titrations using
the standard three bases of NaHCO 3 , Na 2 CO 3 and NaOH [30]. The carboxylic,
phenolic and alcohol groups are of particular interest as they can participate in
the polymerisation reaction with the (di)isocyanate monomers to form an amide
and two urethane bonds, respectively [31]. Therefore, this study may also make
inference to the reactivity comparison of carboxylic and phenolic groups by using
MWCNTs with the same degree of oxidation, where carboxylic acids are predominant in MWCNT-ox and phenolic and alcohol groups are the principal ones in
MWCNT-red.
Titrations of the surface acidic groups show (Table 8.1) the increase in the number of carboxylic, lactone and phenolic groups when acid-oxidising pMWCNTs.
pMWCNTs exhibit a low number of functional groups, which are recognised as
being generated through their CVD synthesis, representing terminal groups near
lattice vacancies [32]. On reduction of MWCNT-ox to MWCNT-red, the number of
carboxylic groups decreases by around 70%. The increase in the number of lactone
groups may result from the reduction of acid anhydride groups. There was no
increase in the number of phenolic groups; however, it was determined in a previous
study by XPS analysis that aliphatic alcohol groups, undetectable via the titrations
used, are present [33].
Hemin is a polycyclic aromatic hydrocarbon with an Fe 3+ core and two
carboxylic groups at the outer edges of the cyclic structure. Its enforced surface
immobilisation (through non-covalent interaction) to the surface of unfunctionalised
carbon nanotubes in the presence of an acid renders the molecule hydrophobic and
L. V. Karabanova et al.
introduce functional groups for the attachment of PU, where the functional groups
were either surface immobilised on or covalently bound directly to the surface
of carbon nanotubes. MWCNT-ox and MWCNT-red both possess covalent sites
directly on the lattice and were purified free of fulvic acids to ensure no competition
with surface-immobilised functional groups resulted. Therein, connecting atomic
units (fragments) of PU network are directly attached to the MWCNT lattice.
MWCNT-hemin and MWCNT-hemin-red also have covalent sites for attachment,
but only through the hemin molecules that decorate the surface of the nanotubes
through non-covalent bonding. Ultimately this approach will facilitate a direct
comparison across the type of MWCNTs to assess which type of interactions
may reflect superior mechanical performance and also elucidate whether acidoxidative lattice fragments (fulvic acids) can exert a detrimental effect to the
final mechanical performance of the composite. Moreover, it is recognised that
in chemically modifying the carbon lattice, the surface tension of the MWCNTs
will also change and will alter the dispersibility of MWCNTs into a polymerising
solution/matrix and in turn affect the size and distribution of the hard and soft
domains of PU.
The oxidation of MWCNTs in nitric acid digests part of the outer walls,
introducing acidic groups directly to the lattice, as well as generating fulvic acids,
which are removed through the reflux in alkali solution. The types of groups
identified on the MWCNT surface include carboxylic, lactone and phenolic groups
as the chief moieties. Acid anhydride, carbonyl, alcohol and ether groups may
also be present, though only the first would be accounted in the titrations using
the standard three bases of NaHCO 3 , Na 2 CO 3 and NaOH [30]. The carboxylic,
phenolic and alcohol groups are of particular interest as they can participate in
the polymerisation reaction with the (di)isocyanate monomers to form an amide
and two urethane bonds, respectively [31]. Therefore, this study may also make
inference to the reactivity comparison of carboxylic and phenolic groups by using
MWCNTs with the same degree of oxidation, where carboxylic acids are predominant in MWCNT-ox and phenolic and alcohol groups are the principal ones in
MWCNT-red.
Titrations of the surface acidic groups show (Table 8.1) the increase in the number of carboxylic, lactone and phenolic groups when acid-oxidising pMWCNTs.
pMWCNTs exhibit a low number of functional groups, which are recognised as
being generated through their CVD synthesis, representing terminal groups near
lattice vacancies [32]. On reduction of MWCNT-ox to MWCNT-red, the number of
carboxylic groups decreases by around 70%. The increase in the number of lactone
groups may result from the reduction of acid anhydride groups. There was no
increase in the number of phenolic groups; however, it was determined in a previous
study by XPS analysis that aliphatic alcohol groups, undetectable via the titrations
used, are present [33].
Hemin is a polycyclic aromatic hydrocarbon with an Fe 3+ core and two
carboxylic groups at the outer edges of the cyclic structure. Its enforced surface
immobilisation (through non-covalent interaction) to the surface of unfunctionalised
carbon nanotubes in the presence of an acid renders the molecule hydrophobic and