Polymer Mixtures - Polymer Science and Engineering - page 263

This is illustrated by the foregoing example and also by mixtures of poly(pheny-
lene oxide) (1-14) polymers and styrene styrene triblock thermoplas-

tic elastomers. Minor proportions of the block copolymers can be usefully added
to the phenylene oxide polymer to improve the impact strength and processability
of the latter. This is analogous to the use of polystyrene or HIPS in such applica-
tions. It is interesting also that the incorporation of poly(phenylene oxide) ele-
vates the usage temperature of the thermoplastic elastomer by raising the
softening point of the hard zones [33] .
A number of studies have been conducted to determine the conditions for pro-
duction of transparent films when an AB block copolymer is mixed with a hom-
polymer that is chemically similar to one of the blocks and the blend is cast from a
common solvent. All agree that the homopolymer is solubilized into the corre-
sponding domain of the block copolymer when the molecular weight of the homo-
polymer does not exceed that of the same segment in the block polymer. If the
molecular weight of the homopolymer is greater than the molecular weights of the
appropriate segments in the block polymer, the system will separate into two
phases. When high-molecular-weight polystyrene is added to a styrene

butadiene

butadiene
block copolymer with styrene blocks that are shorter than those of the homopoly-
mer, separate loss modulus transitions can be detected for the polystyrene homo-
polymer zones and the polystyrene domains in the block copolymer [34] .
The behavior observed depends also on the morphology of the block polymer.
Thus when the block polymer texture consists of inclusions of poly-B in a contin-
uous matrix of poly-A, addition of homopolymer A will result only in its inclu-
sion in the matrix regardless of the molecular weight of the homopolymer.
However, the addition of increasing amounts of poly-B can lead to a whole series
of morphologies that eventually include separate zones of poly-B.
When a block copolymer is blended with a homopolymer that differs in
composition from either block,

the usual result is a three-phase structure.

Miscibility of the various components is not necessarily desirable. Thus

styrene styrene block copolymers are recommended for blending

with high density polyethylene to produce mixtures that combine the relative high
melting behavior of the polyolefin with the good low temperature properties of
the elastomeric midsections of the block polymers.
It

butadiene

butadiene
diblock copolymers is augmented by melt blending the components in the pres-
ence of peroxides. The grafting and cross-linking that occur are an instance
of

is claimed that the toughening of polystyrene by styrene

dynamic cross-linking processes described earlier. Rubbery triblock

styrene butadiene styrene copolymers toughen polystyrene without the need

for cross-linking, for reasons mentioned above.
The compounding of styrene styrene triblock polymers with graft

polymer high impact polystyrene is also interesting. Blends of polystyrene and
the thermoplastic rubber show worthwhile impact strength increases only when
the elastomer is present at a volume fraction

butadiene

25%. But when the thermoplas-
tic rubber is added to high-impact polystyrene, which already has about 25 vol%

. B