Biomedical Engineering Reference
In-Depth Information
tions. The consequence of this imposed irremovable structure is the forced
compatibilization of the polymers. This limits the phase separation via a seg-
regation phenomenon that normally occurs when two incompatible polymers
are blended. The phase separation limitation enables one to obtain materials
with well-defined properties, for example mechanical properties which are re-
quired for many applications. Another example is the transparency which is
directly dependant on the phase separation so that process has to be well con-
trolled to obtain a phase domain size smaller than the light wavelength (about
a few hundreds of nanometers) resulting in translucent materials. Although
transparency is a useful indication of the quality of the interpenetration of the
networks, Tg measurements (DMA, DSC, etc.) and more recently
13
CNMR
cross-polarization give quantitative information. IPNs can be produced using
different processes. On the one hand, simultaneous non-interfering polymer-
ization and crosslinking leads to a
simultaneous IPN
. On the other hand,
polymerization and crosslinking of the first polymer by forming the first net-
work in the presence of the second monomer or prepolymer (by swelling or in
solution) with subsequent polymerization and crosslinking in a second step
leads to a
sequential IPN
. This can also be done in an emulsion media where
the IPN obtained is referred to as a
latex IPN
. Overindulgently, the concept of
interpenetration has been extended to blends where only one of the two com-
ponents form a network, the other one being a non-branched polymer. These
blends are called
semi-IPNs
(occasionally named
pseudo-IPNs
).
4.1
Silicone Crosslinking Methods
Silicone crosslinking can be obtained via different reactions which have been
classified by Brook into six categories [74]: (1) “alcoholysis”, (2) room tem-
Table 1
Polysiloxane crosslinking systems
Cure system
Chemical entities
Catalysts
R
3
SiH + HOSiR
3
1
“alcoholysis”
Pt complex, tin carboxylates
2 :
il l + i
3
or SiX
4
Tin, titanate, acidic
room-temperature
X = Cl, carboxylates,
or basic catalysts
vulcanization
oximes, alkoxides
3
HTV:
Polysiloxane or vinyl
Radical generator
high temperature
containing polysiloxane
(chiefly peroxide)
vulcanization
4
Radiation and UV
Polysiloxane or vinyl
containing polysiloxane
Pt based complexes
a
5
Hydrosilylation [117]
R-SiH + vinyl group
a
Other metal complexes (rhodium, palladium . . .) are seldom used in our IPN synthesis
