Chemistry Reference
In-Depth Information
M
1
M
1
M
1
ABA block copolymer
(M
1
M
1
M
1
M
2
M
2
M
2
z
x
y
11-3
M
3
M
3
M
3
(M
1
M
1
M
1
M
2
M
2
M
2
ABC block copolymer
k
i
j
11-4
multiblock
copolymer
M
1
M
1
M
1
M
2
M
2
M
2
etc.
(M
1
M
1
M
1
M
2
M
2
M
2
d
a
b
c
11-5
stereoblock copolymer
M
1
M
1
M
1
M
1
M
1
M
1
isotactic
syndiotactic
11-6
The properties of block copolymers differ from those of a mixture of the
corresponding homopolymers or a statistical copolymer with the same net compo-
sition. An important example is the ABA-type styrene/butadiene/styrene thermo-
plastic elastomers. Normal elastomers are cross-linked by covalent bonds to
preserve the shape of stressed articles. A rubber tire, for example, would flow
eventually into a pancake shape under the weight of an automobile if the elasto-
mer were not vulcanized. Cross-linked rubbers are thermosets. When such materi-
als are heated, chemical degradation and charring occurs at temperatures at which
the rubbers are still not soft enough to permit reshaping. Solid thermoplastic styr-
ene
butadiene triblock elastomers consist of glassy, rigid, polystyrene domains
linked together by rubbery polybutadiene segments (11-7). The polystyrene
regions serve as effective cross-links and stabilize the structure against moderate
stresses. They can be softened sufficiently at temperatures over 100
C, however,
that these products can be molded and remolded many times. When they are
cooled the rigid polystyrene zones reform, and the polybutadiene links confer a
rubbery character on the solid product.
polybutadiene
regions
polystyrene
domains
11-7
