Chemistry Reference
In-Depth Information
4.5
Rubber Elasticity
4.5.1
Qualitative Description of Elastomer Behavior
Unvulcanized rubber consists of a large number of flexible long molecules with a
structure that permits free rotation about single bonds in the primary chain. On
deformation the molecules are straightened, with a decrease in entropy. This
results in a retractive force on the ends of the polymer molecules. The molecular
structure of the flexible rubber molecules makes it relatively easy for them to
take up statistically random conformations under thermal motion. This property is
a result of the weak intermolecular attractive forces in elastomers and distin-
guishes them chemically from other polymers which are more suitable for use as
plastics or fibers.
It is important to understand that flow and deformation in high polymers
result from local motion of small segments of the polymer chain and not from
concerted, instantaneous movements of the whole molecule. High elasticity
results from the ability of extended polymer chains to regain a coiled shape rap-
idly. Flexibility of segments of the molecule is essential for this property, and this
flexibility results from relative ease of rotation about the axis of the polymer
chain.
Figure 4.12
illustrates the mechanism of a segmental jump by rotation
about two carbon
carbon bonds in a schematic chain molecule
[4]
. The hole in
the solid structure is displaced to the right, in this scheme, as the three-carbon
segment jumps to the left. Clearly, such holes (which are present in wastefully
packed, i.e., noncrystalline polymers) can move through the structure.
FIGURE 4.12
Schematic representation of a segmental jump by rotation about two carbon
carbon
bonds in a macromolecular chain
[4]
.
