Page 275 - Carrahers_Polymer_Chemistry,_Eighth_Edition

238 Carraher’s Polymer Chemistry

one set of properties related to one member of the blend, and another set of properties related to the
second member of the blend. The blended mixtures may also offer some averaging of properties.
The property mix of polymeric blends is dependent on a number of factors, one of the major being
the miscibility of the polymers in one another. This miscibility is in turn dependent on the nature of
the polymers composing the blend and the amount of each component in the blend. Here polymer
blends will be divided into miscible and immiscible polymer blends.
Extent of mixing is related to time since mixing requires sufficient time to allow the polymer

chains to mix. Thus, for miscible blends particular structures can be “frozen-in” by rapid cooling
when the desired mixing is achieved. Here, micelles of particular structures can cause the mixture
to perform in one manner governed by the particular grouping that may not occur if more total
mixing occurs.
Miscibility/immiscibility can be described in simple thermodynamic terms as follows, at con-
stant temperature. Mixing occurs if the free energy of mixing is negative.
∆G = ∆H – T∆S (7.38)
mixing mixing mixing

Mixing is exactly analogous with polymer solubility. The driving force for mixing and solubil-
ity is the entropy or random-related term. The entropy-related term must overcome the opposing
enthalpy energy term. In a more complete treatment, temperature and volume fraction must be
considered.

7.8.1 IMMISCIBLE BLENDS
Immiscible combinations are all about us. Oil and water is an immiscible combination; as is the
lava in the so-called lava-lamps; and chicken broth in chicken soup. Immiscible blends are actually
a mis-naming at the molecular level since they are not truly mixed together. But at the macrolevel
they appear mixed, so the name immiscible blends.
Immiscible blends are said to be phase separated, that is, there are different phases mixed
together. Both phases are solid in behavior.
Because PS is brittle with little impact resistance under normal operating conditions, early work
was done to impart impact resistance. The best known material from this work is called high-impact
polystyrene or HIPS. HIPS is produced by dispersing small particles of butadiene rubber in with the
styrene monomer. Bulk or mass polymerization of the styrene is begun producing what is referred
to as prepolymerization material. During the prepolymerization stage styrene begins to polymerize
with itself forming droplets of polystyrene with phase separation. When nearly equal phase volumes
of polybutadiene rubber particles and polystyrene are obtained, phase inversion occurs and the
droplets of polystyrene act as the continuous phase within which the butadiene rubber particles are
dispersed. The completion of the polymerization generally occurs employing either bulk or aqueous
suspension conditions.
Most HIPS has about 4%–12% polybutadiene in it so that HIPS is mainly a polystyrene intense
material. The polymerization process is unusual in that both a matrix composition of polystyrene
and polybutadienes is formed as well as a graft between the growing polystyrene onto the polybu-
tadiene is formed. Grafting provides the needed compatibility between the matrix phase and the
rubber phase. Grafting is also important in determining the structure and size of rubber particles
that are formed. The grafting reaction occurs primarily by hydrogen abstraction from the polybu-
tadiene backbone either by growing polystyrene chains or alkoxy radicals if peroxide initiators are
employed.
High-impact polystyrene is an immiscible blend that is used in many applications and used to
be employed as the material for many of the automotive bumpers. The polystyrene portion is strong

and inflexible while the polybutadiene particles are flexible, allowing an impact to be distributed
over a larger area. The polybutadiene rubbery portion allows the bumper to bend and indent and

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