Chemistry Reference
In-Depth Information
Fig. 6.4
Illustration of
polystyrene and
polybutadiene domains
to triblock ones, like styrene-butadiene-styrene:
n
o
m
A typical triblock copolymer may consist of about 150 styrene units at each end of the macromol-
ecule and some 1,000 butadiene units in the center. The special physical properties of these block
copolymers are due to inherent incompatibility of polystyrene with polybutadiene or polyisoprene
blocks. Within the bulk material, there are separations and aggregations of the domains.
The polystyrene domains are dispersed in continuous matrixes of the polydienes that are the major
components. At ambient temperature, below the
T
g
of the polystyrene, these domains are rigid and
immobilize the ends of the polydiene segments. In effect they serve both as filler particles and as
cross-links. Above
T
g
of polystyrene, however, the domains are easily disrupted and the material can
be processed as a thermoplastic polymer. The separation into domains is illustrated in Fig.
6.4
.
These thermoplastic elastomers are prepared by anionic solution polymerization with organome-
tallic catalysts. A typical example of such preparation is polymerization of a 75/25 mixture of
butadiene/styrene in the presence of
-butyllithium in a hydrocarbon-ether solvent blend.
At these reaction conditions butadiene blocks form first and when all the butadiene is consumed,
styrene blocks form. In other preparations, monomers are added sequentially, taking advantage of the
“living” nature of these anionic polymerizations.
These block copolymers have very narrow molecular weight distributions. Also, the sizes of the
blocks are restricted to narrow ranges to maintain optimum elastomeric properties.
sec
6.11.2 GR-N Rubber
Butadiene-acrylonitrile rubbers are another group of useful synthetic elastomers. These copolymers
were originally developed in Germany where they were found superior in oil resistance to the
butadiene-styrene rubbers. Commercially, these materials are produced by free-radical emulsion
