Biomedical Engineering Reference
In-Depth Information
increase in the amount of added ceramic will enhance conversion in the first
region, where high rates of polymerization occur. A subsequent decrease in
polymerization in the second region is observed (Fig. 9.6(a), curves 2-8). It
is interesting to note that the extent of conversion, which is reached in the
first region of the kinetic curves, is linearly dependent upon the amount of
added ceramic (Fig. 9.6(b)). From the data shown in this figure, the quantity
of ceramic needed to achieve complete polymer conversion in the region
where the high rate of polymerization occurs was estimated. As expected
(Fig. 9.6(a), curve 8), addition of 90% by weight of Y
1
Ba
2
Cu
3
O
7
x
assures
complete polymerization in the active polymerization centers on the ceramic
surface.
The mechanism and topochemistry of the process can alternatively be
explained if the active polymerization centers formed and fixed on the
surface of the SC ceramic grains are of the radical type. Initiation of
polymerization (in the absence of AIBN) begins from the surface of the
ceramic when the filler content in the reaction mixture is too high (90% by
weight). In such cases, the polymerization process is characterized by a high
rate and, presumably,
is localized at the ceramic-monomer interstitial
boundary.
It is known that fixation of the macromolecule ends on the surface of the
filler sharply decreases their mobility and, correspondingly, changes the
kinetic parameters of polymerization. In particular, it substantially
decreases the constant rate of the bimolecular chain rupture. This is the
main reason for the steadiness of the process. In the reaction mixture, a
second region appears in the kinetic curves of polymerization with an
increase in monomer concentration. This suggests a decrease in the rate of
the process. This is possibly the result of accumulated polymer on the
surface of the ceramic, hindering the accessibility of the monomer molecules
to the active polymerization centers. Occlusion of the active centers by the
macromolecules occurs and the growth rate is controlled by diffusion.
Furthermore, because of transference of the chain to the monomer
(analogous to blocked polymerization) the kinetics of the process become
salient at a certain MMA concentration (20-25% by weight). This facilitates
the possible transition of radicals from the surface of the filler into the bulk.
Presumably, in this case, the rate of blocked polymerization is lower than in
the initial stages of implanted polymerization. This is because the diffusion
retardation imposed by the filler surface is less conspicuous. Although the
present qualitative model of the mechanism and topochemistry of the
surface MMA satisfactorily describes the observed peculiarities, this topic
remains open for further study and discussion.
