8 Nanocomposites Based on Thermosetting Polyurethane Matrix. . .
117
Functionalized graphene sheets (FGSs) were recently used as nanosized conductive
fillers in thermoplastic PU [21]. Therein, the nanocomposite with 1 part of FGSs
per 50 parts of PU had an electrical conductivity of 10 7 times increase over that of
pristine PU, and DMA showed that the FGSs efficiently reinforced the PU matrix.
Information on molecular dynamics, as estimated from DMA, is rather limited
for thermoplastic PU-MWCNT composites. According to Kwon et al. [15], the
storage modulus and mechanical loss factor peak temperature (the glass transition
temperature T g ) of the nanocomposites increased with increasing functionalized
MWCNT content, whereas Raja et al. [14] found only small changes in the glass
transition or even slight decrease in T g with increasing carbon nanotubes content
for thermoplastic PU-MWCNT composites as compared with that for the neat
matrix. For the thermosetting PU-non-functionalized MWCNT nanocomposites
with CNT content of 0.1–1 wt%, McClory et al. [22] showed that CNTs were
highly dispersed in the PU matrix, and the substantial enhancements in stiffness,
strength and toughness were found for the nanocomposites compared with unfilled
PU. Meantime, DMA showed only slight changes in glass transition dynamics in
these nanocomposites.
The role of chemical processes at the interfaces and the character of the interfacial interactions are of a special significance in ascertaining the exact mechanism of
nanocomposite performance including thermosetting PU-MWCNT nanocomposites
that undoubtedly requires an in-depth study. Whilst this has been explored in a
cursory sense [23–27], the direct comparison of CNT influence on dynamics of
polymers, when nanotubes were functionalized through van der Waals interactions
with an intermediate bridge to the polymer or those were directly covalently linked
to the polymer, has not been made up to now. The significance of such studies is
conditioned with a following problem. The acid oxidation procedure, as a traditional
way for chemically functionalising CNTs, generates lattice oxidative fragments
that behave similar to fulvic acids. Therein, these species are hydrophobic under
acid conditions and therefore their creation prevents to immediate interaction of
the matrix with the surface of CNTs. These lattice fragments, or fulvic acids, are
connected with CNTs via van der Waals forces but simultaneously are extensively
functionalised with acidic groups, which also participate in chemical reactions at
interfaces and can account for up to 50% of the total number of acidic groups
in oxidized CNT system [24, 25]. Their removal under base washing or reflux
will leave purely covalent sites for target synthesis (direct covalent CNT-polymer
bonding); however, the numerous works on CNT-polymer composites that involved
acid-oxidized CNTs have neglected to include such a purification strategy and
therefore raise questions about the mixture of covalent and van der Waals bonding
at the interfaces and its relation to the final properties of the composite.
The goal of this study is the investigation of the structure and final mechanical
performance of the thermosetting PU-based nanocomposites containing MWCNTs
possessing acidic and other functional groups generated on the MWCNT surface
and attached either through van der Waals forces or covalently bound to the CNT
lattice at low filler content varying from 0.01 to 0.25 wt. %. The investigations of
the thermodynamic parameters of polymer-filler interactions within formation of the
117
Functionalized graphene sheets (FGSs) were recently used as nanosized conductive
fillers in thermoplastic PU [21]. Therein, the nanocomposite with 1 part of FGSs
per 50 parts of PU had an electrical conductivity of 10 7 times increase over that of
pristine PU, and DMA showed that the FGSs efficiently reinforced the PU matrix.
Information on molecular dynamics, as estimated from DMA, is rather limited
for thermoplastic PU-MWCNT composites. According to Kwon et al. [15], the
storage modulus and mechanical loss factor peak temperature (the glass transition
temperature T g ) of the nanocomposites increased with increasing functionalized
MWCNT content, whereas Raja et al. [14] found only small changes in the glass
transition or even slight decrease in T g with increasing carbon nanotubes content
for thermoplastic PU-MWCNT composites as compared with that for the neat
matrix. For the thermosetting PU-non-functionalized MWCNT nanocomposites
with CNT content of 0.1–1 wt%, McClory et al. [22] showed that CNTs were
highly dispersed in the PU matrix, and the substantial enhancements in stiffness,
strength and toughness were found for the nanocomposites compared with unfilled
PU. Meantime, DMA showed only slight changes in glass transition dynamics in
these nanocomposites.
The role of chemical processes at the interfaces and the character of the interfacial interactions are of a special significance in ascertaining the exact mechanism of
nanocomposite performance including thermosetting PU-MWCNT nanocomposites
that undoubtedly requires an in-depth study. Whilst this has been explored in a
cursory sense [23–27], the direct comparison of CNT influence on dynamics of
polymers, when nanotubes were functionalized through van der Waals interactions
with an intermediate bridge to the polymer or those were directly covalently linked
to the polymer, has not been made up to now. The significance of such studies is
conditioned with a following problem. The acid oxidation procedure, as a traditional
way for chemically functionalising CNTs, generates lattice oxidative fragments
that behave similar to fulvic acids. Therein, these species are hydrophobic under
acid conditions and therefore their creation prevents to immediate interaction of
the matrix with the surface of CNTs. These lattice fragments, or fulvic acids, are
connected with CNTs via van der Waals forces but simultaneously are extensively
functionalised with acidic groups, which also participate in chemical reactions at
interfaces and can account for up to 50% of the total number of acidic groups
in oxidized CNT system [24, 25]. Their removal under base washing or reflux
will leave purely covalent sites for target synthesis (direct covalent CNT-polymer
bonding); however, the numerous works on CNT-polymer composites that involved
acid-oxidized CNTs have neglected to include such a purification strategy and
therefore raise questions about the mixture of covalent and van der Waals bonding
at the interfaces and its relation to the final properties of the composite.
The goal of this study is the investigation of the structure and final mechanical
performance of the thermosetting PU-based nanocomposites containing MWCNTs
possessing acidic and other functional groups generated on the MWCNT surface
and attached either through van der Waals forces or covalently bound to the CNT
lattice at low filler content varying from 0.01 to 0.25 wt. %. The investigations of
the thermodynamic parameters of polymer-filler interactions within formation of the