Page 276 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Copolymerization 239

protects the PS from fracturing while the PS phase resists further deformation. This combination
gives a strong fl exible material.
In general, the morphology on a molecular level varies with the fraction of each component in the
mixture. In general terms, we can talk about a continuous and discontinuous phase. For a combina-
tion of polymers A and B, such as PS and polybutadiene, at low amounts of A, polymer A will typi-
cally act as a discontinuous phase surrounded by B. Thus, at low amounts of PS, the PS congregates
as small particles in a “sea” or continuous phase of B, the polybutadiene. As the fraction of polymer
A increases, the spheres eventually become so large as to join together forming a continuous phase,
and so two continuous phases are present. As the fraction of A continues to increase, polymer B
becomes the discontinuous phase, being surrounded by the continues phase polymer A.
The discontinuous phase generally takes the rough shape of a sphere to minimize surface area
exposure to the other phase. The size of the spheres influences the overall properties and varies

with concentration. In general, because of the affinity of like polymer chains, spheres tend to grow.
Larger sphere sizes are promoted because they give less relative contact area with the other phase.
As noted above, immiscible blends can exhibit different properties. If the domains are of suffi -
cient size, they may exhibit their own T and T values. Many commercially used immiscible blends
g m
have two separate T and/or T values.
g m
Blends can also offer variable physical properties as already noted for HIPS. Consider a blend of
polymer PS and butadiene where butadiene is the major component. PS is the stronger material with the
blend weaker than PS itself. In some cases, the blend can be stronger than the individual polymers. Heat
and pressure can result in the change of the discontinuous phase becoming flattened out when pressed

against a mold. The spheres can also be caused to elongate forming rod-like structures with the result-
ing structure similar to composites where the rod-like structures strengthen the overall structure.
The strength can also be increased by using about the same amounts of the two polymers so
that they form two continuous phases. Here, both phases can assist the blend to be strong. Another
approach is to use compatibilizers. Compatibilizers are materials that help bind together the phases
allowing stress–strain to be shared between the two phases. Many compatibilizers are block copo-
lymers where one block is derived from polymers of one phase and the second block composed of
units derived from polymers of the second phase. The two blocks get “locked” into the structures of
the like phases and thus serve to connect the two phases.
Graft copolymers are also used as compatibilizers to tie together different phases. HIPS contain
polystyrene grafted onto polybutadiene backbones. This allows stress/strain to be transferred from
the PS to the polybutadiene phase transferring energy that might break the brittle PS to the more

flexible polybutadiene phase. That is why HIPS is stronger than PS itself.
Compatibilizers also act to modify the tendency to form large spheres. The formation of large
spheres is a result of the two polymer components trying to segregate. The compatibilizer causes
the two phases to come together minimizing the tendency to form large spheres. For instance, for a
mixture of 20:80 PS:polybutadiene the sphere size is about 5–10 μm, whereas addition of about 9%
PS-polyethylene (polyethylene is enough like polybutadiene to be incorporated into the polybutadi-
ene phases) block copolymer results in PS spheres of about 1 micron. This increases the interface
between the two phases resulting in better mechanical properties because stress–strain can be more
effectively transferred from one phase to the other.

7.8.2 MISCIBLE BLENDS

Miscible blends are not as easy to achieve as immiscible blends. As noted above, entropy is the
major driving force in causing materials to mix. Because polymer chains are already in a state of
relatively high order, increases in randomness are not easily achieved so that immiscible blends are
often more easily formed. To make matters worse, for amorphous polymers the amount of disorder
in the unmixed polymer is often higher than for blends that tend to arrange the polymer chains in a
more ordered fashion.

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