Biomedical Engineering Reference
In-Depth Information
values were systematically found to be above the experimental values. This
couldbeexplainedbythefactthatthe
values of the PS/PDMS pair were far
from unity considering that Eqs. 3 and 4 give better results for equiviscous
polymer blends. But, the Utracki model [15] (Eq. 5) also gave overestimated
values although it is even more dedicated to blends having viscosity ratios
far from unity. Chuai et al. suggest that a model taking elastic behaviour into
account could give better results.
λ
=
(
[
η
]
Φ
m
Φ
m
-
Φ
2,PI
)
λ
.
(5)
(
Φ
m
-
Φ
1,PI
)
In Eq. 5, [
η
] is the intrinsic viscosity of the dispersed phase and
Φ
m
is the
Φ
m
=1-
Φ
cr
,
Φ
cr
is the
maximum packing volume fraction (in most cases,
critical volume fraction or percolation threshold).
The morphology is also dependant on thermodynamical factors such as
the interfacial tension (
): in the case of a polymer dispersed in another one,
the size of the droplets was found to be proportional to the interfacial tension.
Furthermore, the interfacial tension gives direct information on the equi-
librium state of the blend since it is linked to the free energy (
γ
G
)ofthe
blend via the Flory-Huggins interaction parameter. Anastasiadis et al. [16]
developed the use of an automated pendant drop apparatus coupled with
a shape analysis algorithm to obtain the
∆
values for polymer blends, in par-
ticular for a polybutadiene(PB)-polydimethylsiloxane system. The interfacial
tension was found to decrease slightly linearly with temperature. It was also
found to increase with the PDMS molecular weight (keeping the PB molecu-
lar weight constant) in relative good agreement with a Mn
-1
/
2
expression
instead of the Mn
-2
/
3
expressed by other researchers with simpler systems
(e.g.
n
-alkanes vs. low molecular weight PDMS) [17, 18]. The deviation was
attributed to the polydispersity of the PDMS. Standard
D
4
(octamethylcy-
clotetrasiloxane) polymerization usually leads to silicone polymers having
a polydispersity index around 2. The other explanation was the proximity of
the critical point where a Mn
-1
/
2
relationship is expected.
γ
2.2
Functionalized Silicone/Polymer Blends
Functionalized polysiloxanes are attractive because, with only small mod-
ification of the polysiloxane properties (e.g. density, yield strength, etc.),
they allow reduction in the interfacial tension thanks to better interac-
tions with the other homopolymer. Two studies involving PB/PDMS and
polyisoprene (PIP)/PDMS are of particular interest [19, 20]. The PDMS end
groups were either amine (- NH
2
)oracid(- COOH). It was first observed
that the PB/PDMS-NH
2
system exhibits a 30% reduction in interfacial ten-
sion compared to the equivalent PB/PDMS system. A preliminary reduction
