Biomedical Engineering Reference
In-Depth Information
3.2.3
In-Situ Branched Copolymer Formation
The two principal in-situ syntheses of branched copolymers are by step
growth or radical chemistry. It should be noted that crosslinking of the same
phase can also occur in addition to branching. This crosslinking is the basic
principle of IPN formation. Hence, in this section, we will only refer to re-
ports where crosslinking of the same phase appears to be a side reaction and
not the expected one.
Shih et al. [60] studied the modification of a novolac-type epoxy resin
with PDMS to overcome brittleness and poor impact resistance. This kind of
resin is typically cured via their epoxy functions. The authors also introduced
isocyanate monofunctionalized PDMS. Hence, the common treatment with
MDA (4,4
-methylene dianiline) not only cured the resin on the one hand,
but also made it possible to form the branched copolymer. Mechanical and
thermal analyses showed that an optimum in isocyanate-terminated PDMS
content was required to reach good thermal and physical properties and low
moisture absorption.
Radical chemistry has been used for producing either PDMS/polyolefin
blends or PDMS/unsaturated polymer blends. Grafting polyolefins via a rad-
ical mechanism is widespread due to the absence of usable functional groups
and is particularly useful for silylated compounds [61-64]. Jana et al. [65]
worked with LDPE and vinyl-containing PDMS which they cured with DCP
(Fig. 7). Using DSC, they determined a first-order curing reaction. The pro-
posed mechanism is depicted in Fig. 8. They also showed that the activation
energy depends on both the amount of peroxide and the blend composition.
Mechanical properties also varied with the DCP content and PDMS content.
The optimum DCP content for the best tensile strength was found to be
1.5 wt %. Elongation at break decreased with increasing peroxide and PDMS.
Kole et al. [66, 67] studied a silicone-EPDM rubber blend. They enhanced
the compatibility of the materials by the introduction of interactive [66] or re-
active [67] functions grafted onto the two polymers. In both studies, PDMS
Fig. 7
Branched LDPE/ PDMS system, Jana et al. [65]
