Chemistry Reference
In-Depth Information
If molecules are restrained by entanglement with other chains or by actual
chemical bonds (cross-links) between chains, deformation is still possible because
of cooperative motions of local segments. This presupposes that the number of
chain atoms between such restraints is very much larger than the average size of
segments involved in local motions. Ordinary vulcanized natural rubber contains
0.5
5 parts (by weight) of combined sulfur vulcanizing agent per 100 parts of
rubber. Approximately one of every few hundred monomer residues is cross-
linked in a typical rubber with good properties (the molecular weight of the chain
regions between cross-links is 20,000
25,000 in such a hydrocarbon rubber). If
the cross-link density is increased, for example, by combining 30
50 parts of sul-
fur per 100 parts rubber, segmental motion is severely restricted. The product is a
hard, rigid nonelastomeric product known as “ebonite” or “hard rubber.”
High elasticity is attributed to a shortening of the distance between the ends of
chain molecules undergoing sufficient thermal agitation to produce rotations about
single bonds along the main chains of the molecules. The rapid response to applica-
tion and removal of stress which is characteristic of rubbery substances requires
that these rotations take place with high frequency at the usage temperatures.
Rotations about single bonds are never completely free, and energy barriers
that are encountered as substituents on adjacent chain atoms are turned away
from staggered conformations (Fig. 1.6). These energy barriers are smallest for
molecules without bulky or highly polar side groups. Unbranched and relatively
symmetrical chains are apt to crystallize on orientation or cooling, however, and
this is undesirable for high elasticity because the crystallites hold their constituent
chains fixed in the lattice. Some degree of chain irregularity caused by copoly-
merization can be used to reduce the tendency to crystallize. If there are double
bonds in the polymer chain as in 1,4-polydienes like natural rubber, the
cis
con-
figuration produces a lower packing density; there is more free space available
for segmental jumps and the more irregular arrangement reduces the ease of crys-
tallization. Thus
cis-
polyisoprene (natural rubber) is a useful elastomer while
trans
-polyisoprene is not.
The molecular requirements of elastomers can be summarized as follows:
1.
The material must be a high polymer.
2.
Its molecules must remain flexible at all usage temperatures.
3.
It must be amorphous in its unstressed state. (Polyethylene is not an
elastomer, but copolymerization of ethylene with sufficient propylene reduces
chain regularity sufficiently to eliminate crystallinity and produce a useful
elastomer.)
4.
For a polymer to be useful as an elastomer, it must be possible to introduce
cross-links in such a way as to bond a macroscopic sample into a continuous
network. Generally, this requires the presence of double bonds or chemically
functional groups along the chain.
Polymers that are not cross-linked to form infinite networks can behave elasti-
cally under transient stressing conditions. They cannot sustain prolonged loads,
