Biomedical Engineering Reference
In-Depth Information
etical and experimental studies have appeared which deal with these complex
issues in an attempt to make it possible to identify some universal rules. Re-
sults obtained in the context of silicone/polymer blends which are reviewed
here are compared as far as possible to the general principles of polymer
blends. The reason we focus on silicone blends is because of their potential
in a wide range of applications thanks to their very specific properties. The
main characteristics of polydimethylsiloxanes include for example their low
glass transition temperatures, heat stability, waterproof properties, resistance
to oxidation, stability at high and low temperatures, great molecular flexi-
bility, high impact resistance, good electrical insulation, resilience, oxygen
permeability, biocompatibility, low surface energy and relative insensitivity to
UV light etc.
As a result of these important properties, different blends have been
studied: thermoplastics or thermosets with silicone, hydrogel/silicones. These
materials can be more or less efficiently produced thanks to the choice of
routes: simple and compatibilized blends, as well as IPNs.
2
Silicone/Polymer Bicomponent Blends
2.1
Unmodified Silicone/Polymer Blends
Polysiloxane is well known to be immiscible with most polymers. The re-
sulting materials usually present a two-phase morphology. A good dispersion
can be obtained when a special mixing procedure is applied to the blend but
it still appears to be unstable. Hence literature describing simple blends of
a silicone with another polymer using the usual processes such as extruder or
batch mixing is relatively poor. Historically, some patents claimed blends of
polysiloxane with polycarbonate [1] which presented good elongation, flexi-
bility and resistance to basic hydrolysis. No exudation of the polysiloxane
was observed. The major drawback observed was the difficulty in incorpo-
rating the silicone into the thermoplastic. Bostick et al. [2] claimed to avoid
this problem by using octaphenylcyclotetrasiloxane instead of a polysilox-
ane, but the blend properties were not specified. Another example dealt
with the incorporation of polydimethylsiloxane into polyamide [3]. Although
a better impact strength may have been measured on the freshly extruded
material with 6 wt %, the homogeneity is not discussed in the patent and
no indication concerning the evolution of the blend is given. Except for
these limited examples, exudation and loss of mechanical properties are usu-
ally observed. So, in general, the amount of silicone incorporated is limited
and should not exceed 2 wt % [4-6]. Only a few percent of polysiloxane is
needed to achieve wear-resistant properties and therefore it was used with
