Page 274 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Copolymerization 237

crystalline thermoplastics while the soft segments or phases are amorphous. In continuous chains
containing blocks of hard and soft segments, the molecular arrangement normally contains crystal-
line regions where there is sufficient length in the hard segment to form the crystalline regions or

phases where the soft segments form amorphous regions.
Such hard/soft scenarios can also be achieved through employing grafts where the pendant group
typically acts as the hard segment with the backbone acting as the soft segment. Below, 7.37 is
a representation of a typical graft copolymer chain. For an effective network to be formed, each
“A” chain needs to have at least two “B” grafts to allow for formation of a continuous interlinked
network. While there has been a lot of research done with such graft materials, they have not yet
become very important commercially.

-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-
| | | (7.37)
(B) n (B) n (B) n

Thermoplastic elastomers can also be achieved through physical mixing of hard and soft seg-

ments. These are fine dispersions of a hard thermoplastic polymer and an elastomer. The two mate-
rials generally form interdispersed cocontinuous phases. Often the physical combining is achieved
through intense mechanical mixing but in some cases, such as with polypropylene and EPMs the
effect of blending is achieved through polymerizing the finely dispersed elastomer phase (EPM)

simultaneously with the hard polypropylene.
At times, the phases are cross-linked during the mechanical mixing. This process is referred to

as “dynamic vulcanization” and produces a finely dispersed discontinuous cross-linked elastomer
phase. The products are referred to as thermoplastic vulcanizates or dynamic vulcanizates. The
products of this process have an insoluble elastomer phase giving the material greater oil and sol-
vent resistance. The cross-linking also reduces or eliminates the flow of this phase at high tempera-

tures and/or under high stress. This allows the material better resistance to compression set.
Styrene–butadiene–styrene block copolymers differ structurally from the random copolymer of
styrene and butadiene (SBR). Because styrene and butadiene blocks are incompatible, they form sep-
arate phases joined at the junctions where the various blocks are connected. This gives an elastomeric
material where the butadiene blocks form the soft segments and the styrene blocks form hard blocks.
The block copolymer made from connecting blocks of polystyrene with blocks of polybutadiene
illustrates another use of soft and rigid or hard domains in thermoplastic elastomers. The polysty-
rene blocks give rigidity to the polymer while the polybutadiene blocks act as the soft or fl exible
portion. The polystyrene portions also form semicrystalline domains that add to the strength of the
copolymer and these domains also act as “physical crosslinks” allowing the soft portions to respond
in an “elastomeric” manner, while the semicrystalline domains give the material the “elastomeric”
memory. The polybutadiene blocks act as the continuous phase while the polystyrene blocks act as
the discontinuous phase. Heating the material above the T of the polystyrene domains allows whole
m
chain mobility, allowing processability of the virgin material and subsequent reprocessability of
used material. Upon cooling, the rigid domains reform. Block polystyrene–polybutadiene copoly-
mers are used in the soles of many of the athletic shoes.
Worldwide sales of thermoplastic elastomers are on the order of one and a half million tons with
a value of about $5 billion.

7.8 BLENDS
There is an ongoing search for new materials and materials that exhibit needed properties. Blends
are one of the major avenues of achieving these new materials without actually synthesizing new
polymers. Polymer blends are physical mixtures of two or more polymers though sometimes the var-
ious phases are chemically bonded together. These blended mixtures may offer distinct properties,

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