Chemistry Reference
In-Depth Information
In some cases, grafting is used to form the copolymers. Examples of
chains combined with PDMS in this way include poly(ethylene oxide),
120-123
fluorinated chains,
124
alkyl acrylates
125
and alkyl methacrylates,
126
poly(vinyl
alcohol),
127
poly(ether sulfones),
128
and polyurethanes.
129
Grafting has also
been used to introduce
t
-butylamine and
t
-butylammonium biocidal func-
tionalities,
130
and to improve the adhesion of silicone rubber to polyure-
thane.
131
In other cases, the siloxane groups are simply placed on the ends of
chains, such as onto some polybenzoxazines.
132
In related work, NMR studies have been carried out on PDMS chains
onto which octyl groups had been grafted.
133
Other examples of grafted
systems include poly(ethylene oxide),
123
polyurethanes,
131
poly(butyl
methacrylate),
126
and gelatin.
134
8.3 INTERPENETRATING NETWORKS
In this type of material, two networks are formed, either simultaneously
or sequentially, in such a way as to interpenetrate one another. The net-
works thus “communicate” with one another through interchain physical
forces and entanglements, rather than through covalent bonds. A particu-
larly simple example is the simultaneous formation of two PDMS net-
works, one by a condensation end-linking reaction and the other by an
addition end-linking reaction, with the two types of chains mixed at the
molecular level.
135,
136
A more complex example, with more novel properties, is the prepara-
tion of interpenetrating networks (IPNs) between PDMS and an organic
thermoplastic polymer such as a nylon, polyurethane, or polyester. The
preparation of this material, called Rimplast® by its developers,
137
in-
volves a chemical reaction between a vinyl-functional polysiloxane
blended into the thermoplastic, and a hydride-functional PDMS blended
into more of the same thermoplastic. Small amounts of platinum catalyst
are also present in both types of components. The polysiloxane content is
typically around 10%. Pellets of both components are placed in an ex-
truder or other high-temperature processor where they melt into a uni-
form mass at approximately 300°C. The reaction between the two
complementary types of PDMS results in a network evenly distributed
throughout the thermoplastic as a “semi-IPN” in the sense that there is
only one IPN (PDMS). The resulting composite has the desirable proper-
ties of both the PDMS (good lubricity, abrasion resistance, and dielectric
properties), and the thermoplastic (good mechanical strength and mold-
ing characteristics).
