Biomedical Engineering Reference
In-Depth Information
Another way to achieve crosslinking of the silicone is via condensation. Fu
and Mark [143] obtained a silicone network by mixing hydroxy-terminated
PDMS and tetraethoxysilane. The reaction was catalyzed by a stannous com-
pound. The network obtained was then swollen with styrene whose polymer-
ization was initiated by benzoyl peroxide. Ultimate strength and elongation
at break were both enhanced. This crosslinking system has also been used
in other patented systems involving for example poly(arylene sulphide) [144,
145]. Fujiki et al. [146] prepared silicone/PMMA semi-IPNs where the silicone
is crosslinked by condensation but with an acidic catalyst to avoid the use of
organotin compounds. For example, a semi-IPN was obtained by mixing to-
gether hydroxyl-terminated-PDMS, tetraethoxysilane, an acidic catalyst such
as acetic acid, MMA monomer and AIBN before heating and curing.
Arkles [147] combined the hydrosilylation and condensation mechanism
to obtain a secondary crosslinked semi-IPN. The polyamide thermoplastic
was blended with a vinyl- and alkoxy-containing silicone. The hydrosilyla-
tion first occurred to yield a semi-IPN. In a second step, this semi-IPN was
exposed to moisture and underwent the second crosslinking stage. Gilmer
et al. [148] prepared a silicone network/PMMA system by condensation of
chlorosilane and/or silanol. The mechanical properties were studied and the
IPNs were found to be tougher when the degree of crosslinking was higher.
The silicone was based on an aliphatic and aromatic silane, so that the authors
obtained an IPN with an aromatic silane which had a transmittance of 79%
whereas that of PMMA is 92% (aliphatic silicone-based IPN has 0%transmit-
tance).
Finally, the “alcoholysis” (
RSiOH
+
HSiR
) catalyzed by metal carboxylate,
amine or quaternary ammonium was patented by Gornowicz [149]. Various
thermoplastics such as PA, PP, SBS and PE were described in the different
examples.
4.2.5
Mixing Method
To enhance the efficiency and the stability of the semi-IPNs, their preparation
can include a second system of compatibilization.
4.2.5.1
Preformed Compatibilizer
Some authors combined the IPN concept with the use of compatibi-
lizers similar to that mentioned in Section 2. These may be a coup-
ling agent such as epoxy-functionalized polysiloxane, polysiloxane copoly-
mers or an organofunctional grafted polyolefin such as poly(ethylene-co-
methacrylate) or MA-g-EPDM (maleic anhydride grafted EPDM) [124, 125,
133]. Knaub et al. [150] studied a poly(urethane-ureas)/PDMS semi-IPN in
