Chemistry Reference
In-Depth Information
weights of polystyrene and cis-polybutadiene for domain formation in AB block
copolymers of these species are about 5000 and 40,000, respectively
[32]
.
The properties of block copolymers that are most affected by molecular archi-
tecture are elastomeric behavior, melt processability, and toughness in the solid
state. The effects of such copolymers in polymer blends can obviously also be
strongly influenced by the same factors.
When one component of the block polymer is elastomeric, a thermoplastic
rubber can be obtained. This occurs only when the block macromolecules include
at least two hard (T
g
.
usage temperature) blocks. A diblock structure pins only
one end of the rubbery segment, and true network structures can therefore not be
produced in such AB species. The volume fraction of the hard block must be
sufficiently high (
20%) to provide an adequate level of thermally labile cross-
linking for good recovery properties. If the volume fraction of hard material is
too high, however, the rigid domains may change from spherical, particular
regions to an extended form in which elastic recovery is restricted.
A block copolymer is expected to be superior to a graft copolymer in stabiliz-
ing dispersions of one polymer in another because there will be fewer conforma-
tional restraints to the penetration of each segment type into the homopolymer
with which it is compatible. Similarly, diblock copolymers might be more effec-
tive than triblock copolymers, for the same reason, although tri- and multiblock
copolymers may confer other advantages on the blend because of the different
mechanical properties of these copolymers.
Block copolymers serve as blending agents with simple homopolymers as well as
stabilizing agents for mixtures of homopolymers. Blends of a homopolymer with an
AB-type block copolymer will be weak if the elastomeric segment of the block poly-
mer forms the sole continuous phase or one of the continuous phases. This problem
can be circumvented by cross-linking the rubber after the blend is made or by using
an ABA block copolymer in which the central segment (B) is rubbery and the termi-
nal, glassy (A) segments serve to pin both ends of the center portions.
When block copolymers are used in rubber mixes there is no particular advan-
tage to a triblock or multiblock species because the final mixture will be vulca-
nized in any event.
Linear ABA and (AB)
n
block copolymers can form physical networks that per-
sist at temperatures above the glassy regions of the hard segments. Very high melt
elasticities and viscosities are therefore sometimes encountered. The accompanying
processing problems can often be alleviated by blending with small proportions of
appropriate homopolymers. For example, when styrene
$
styrene tri-
block rubbers are used as thermoplastic elastomers it is common practice to extend
the rubbery phase with paraffinic or naphthenic oils to decrease the cost and viscos-
ity of the compound. (Aromatic oils are to be avoided as they will lower the T
g
of
the polystyrene zones.) The accompanying decrease in modulus is offset by the addi-
tion of polystyrene homopolymer which also reduces elasticity during processing.
While copolymers are generally used in blends to modify the properties of
homopolymers or mixtures of homopolymers, the reverse situation also occurs.
butadiene
