Biomedical Engineering Reference
In-Depth Information
The observed change in the kinetic character of MMA polymerization is
understandable if it is considered that some specific interaction takes place
for the ceramic, both with the monomer and with the initiator. It is known
[33] that perovskites of the Y
1
Ba
2
Cu
3
O
7
x
type possess catalytic properties
for numerous chemical reactions. In this case, it could be suggested that
some separate localities of the ceramic grains can interact with MMA and
AIBN. In this instance, if the rate of interaction between the ceramic and the
initiator is far higher than with the monomer, then some part of the initiator
interacts with the active centers of the Y
1
Ba
2
Cu
3
O
7
x
grain surface, filling
them fully and thus excluding the possibility of interaction with MMA.
Thus, some part of the initiator is 'blocked' by the ceramic's surface and
takes no part in the initiation reaction.
In this particular case, increasing the amount of ceramic added enhances
the share of the 'blocked' initiator and, consequently, reduces the rate of
polymerization (Fig. 9.6(a), curves 2-5). As to the kinetics of polymerization
obtained in the absence of AIBN, it seems that the monomer interacts with
the active localities of the ceramic grains. This leads to the formation of
primary active centers of polymerization, which are fixed on the solid
Y
1
Ba
2
Cu
3
O
7
x
surface.
It is hard to apply the conventional mechanisms of chain growth and
rupture for this example because the macromolecule end groups, which
would otherwise by capable of participating in reactions, are non-mobile in
this particular case. This excludes the possibility of chain quenching. From
this knowledge, the sharp increase in the rate of polymerization at the initial,
fast stages of the reaction becomes understandable. As to the mechanism of
the growth of the chains, the monomer reacts with an active center while
approaching it. It is apparent that elongation of the macromolecule captures
these centers and consequently diminishes the accessibility of the centers to
monomers. Finally, the kinetics of the chain growth reactions on the
ceramic surface turn into diffusion control, leading to a decrease in the rate
of polymerization.
The second region of the kinetic curves (Fig. 9.6(a), curves 2-7) shows a
sharp decrease in the polymerization rate, and a stepwise increase is
observed. This is linked with polymerization that occurs not on the surface,
but in the bulk monomer - the layer of macromolecules that, upon reaching
a definite dimension, encompasses the active chain and partially desorbs
from the surface of the ceramic. The active end of the chain is thus
uncovered and when these uncovered ends accumulate further, the rate of
homogeneous polymerization increases up to the point of complete
utilization of the monomer. It should therefore be noted that the rate of
bimolecular chain termination in the bulk is low because the furled and
uncovered macro chains have little mobility.
For this mechanism of chain initiation and growth, it is obvious that an
