Carbon Nanotubes in Acrylic Bone Cement - Biologically Responsive Biomaterials for Tissue Engineering - page 190

the MWCNT resulting in the formation of agglomerations within the cement
matrix. These agglomerations acted as stress concentrations within the cement
microstructure, therefore providing a mechanism for premature failure of the
cement when subjected to load. In contrast, low loadings (£0.25 wt.%) of MWCNT
were more readily disentangled by the application of ultrasonic energy and
homogenously dispersed in the resulting nanocomposite. The presence of well-
dispersed MWCNT in PMMA cement was effective in bridging the initial crack
and preventing crack propagation, thus further enhancing the mechanical integrity
of the cement mantle. Dunne et al. [ 22 ] recorded that the increased loading of
MWCNT extended the time at which polymer gelation occurred, as well as
extended the time before the onset of polymerisation. They attribute this to a
chemical interaction between the functional chemical groups on the MWCNT, in
addition to the higher loadings of MWCNT preventing the polymer chains from
forming [ 22 ] .
Gojny et al. [ 31 ] reported that the addition of chemical functional groups to
MWCNT can provide a negative charge to the MWCNT and thus reduce agglom-
eration. This in turn improves interaction between the nanotubes and the host
polymer. The results of these studies by Ormsby et al. [ 67, 68 ] concurred with
the findings of Gojny et al. [ 31 ]. The PMMA bone cement with unfunctionalised
MWCNT exhibited the least significant improvements for all mechanical proper-
ties measured. This reduced improvement was attributed to poor MWCNT dis-
persion within the cement matrix, resulting in the occurrence of MWCNT
agglomerations. The unfunctionalised MWCNT provided a degree of mechani-
cal reinforcement at lower loadings (£0.25 wt.%), largely due to the reduced
tendency for MWCNT agglomerations to occur. In the research conducted by
Ormsby et al. [ 67, 68 ], MWCNTs chemically functionalised using either car-
boxyl or amine groupings were chosen, as it has been reported that the mechani-
cal properties of generic PMMA can be improved using MWCNT with these
functional groups [ 71 ]. MWCNT functionalised with carboxyl groups provided
the most significant improvements in all mechanical properties of the PMMA
cement. It is proposed that these significant improvements are a result of homo-
geneous MWCNT dispersion within the PMMA matrix, aided by negatively
charged carboxyl groups. This homogeneous dispersion, in tandem with interfa-
cial interactions between the functionalised MWCNT and PMMA matrix, could
provide improved mechanical properties within the resultant nanocomposite.
MWCNT-PMMA composite cements incorporating amine functional groups
also exhibited improved mechanical properties. These improvements were less
signi fi cant ( p -value £0.01) however, compared to the PMMA cement containing
carboxyl functionalised MWCNT. It was postulated that this was due to the lower
level of functional groups present on the amine functionalised MWCNT when
compared with the carboxyl functionalised MWCNT (that is 0.5 % versus 4.0 %,
functional groups, respectively). This lower concentration of amine functional
groups may result in a more heterogeneous MWCNT dispersion within the
cement matrix, therefore resulting in a less successful transfer of stress through
the cement mantle.