Biomedical Engineering Reference
In-Depth Information
Initiation
O
O
CH
3
O
+
H
3
C
N
O
O
CH
3
O
BPO
DmpT
Benzoyl radical
CH
3
Chain growth
C
+
n
H
2
C
CH
3
CH
3
CH
3
CH
3
H
2
COOCH
3
+
H
2
C
R
C
H
3
C
C
RCH
2
C
C
C
n
COOCH
3
COOCH
3
COOCH
3
COOCH
3
Chain recombination
CH
3
CH
3
CH
3
H
2
2R
CH
2
C
n
RCH
2
C
C
C
n
R
COOCH
3
H
3
COOC
COOCH
3
8.4
Synthesis of PMMA-based bone cements.
of the cement, thus it is necessary for irradiated polymers to have a much
higher molecular weight before the sterilization.
29
Another important factor is the shrinkage of the bone cement.
30
this is
the reason why it is not possible to use only MMA for bone cements, as
the polymerization shrinkage would be extremely high. For example, pure
MMA shrinks by 21%, which means that 1 litre of MMA results in 790 ml
PMMA. on the other hand, the use of a pre-polymerized powder component
leads to a theoretical shrinkage of 6-7% which in reality is lower owing to
the cement porosity.
8.3.2 Thermal properties and transitions
As has been previously discussed, plastics change their physical state with
rising temperature from glass-like/brittle to rubber-elastic. the temperature
range in which this change occurs is characterized by the glass transition
temperature (
T
g
).
in the dry state, PMMA bone cements have a relatively high glass transition
temperature (about 90-100°c) compared to the body temperature. However,
water uptake by the polymer softens the polymer and therefore
T
g
decreases.
the softening effect is due to micro-Brownian movement and leads to a
