Materials for engineering, 3rd Edition - (Malestrom)

Structure of engineering materials 31
secondary bonds melt so that the polymer can flow like a viscous liquid,
allowing it to be formed. The material regains its strength, reversibly, when
it is cooled, and such polymers are known as thermoplastics.
Thermosets are made by mixing two components (a resin and a hardener)
which react and harden. During polymerization, chemical bonds are formed
which cross-link the polymer chains. These strong covalent bonds between
adjacent chains produce polymers of higher rigidity than the thermoplastics,
which cannot be resoftened by heating once the network of cross-linking
primary bonds has been established. A familiar example of this type of
polymer is epoxy, which is used as an adhesive and as the matrix of fibreglass
composites.
Elastomers may be classified as linear thermoset polymers with occasional
cross-links which, after removal of the load, enable the material to return to
its original shape. The common elastomers are based on the structure:
⎛ H H ⎞
|
|
⎜ —C —C ==C —C—
⎟
⎜ | | | | ⎟
⎝ H H R H ⎠
n
with the site ‘R’ occupied by CH 3 (in natural rubber), H (in synthetic rubber),
or Cl (in neoprene, which is used for seals because of its oil-resistance).
Polymer crystals
It is possible for some long-chain molecular solids to crystallize if the chains
happen to be packed closely together. Molecules with ordered, regular structures
with no bulky side groups and a minimum of chain branching will usually
crystallize. Thus the chains of an isotactic polymer (with its side-groups, if
any, symmetrically placed on the backbone) may, if slowly cooled, be pulled
by the secondary bonds into parallel bundles, often forming chain-folded
crystal structures (Fig. 1.25).
Even if this crystallinity is good enough to enable such a polymer to
diffract X-rays, even the most crystalline of polymers is only 98% crystal.
The crystalline parts are separated by amorphous regions. In melt-crystallized
polymers, the plate-like, chain-folded regions may organize themselves into
spherulites. These are highly crystalline units that grow with spherical symmetry
to diameters of the order of 0.01 mm until they impinge on one another.
Spherulites scatter light easily, so that transparent polymers become translucent
when crystalline and, under polarized light, the spherulitic structure may be
revealed (Fig. 1.26).
The degree of crystallinity may be determined from measurements of
specific volume or density. Control of the degree of crystallinity must be
exercised in the production of artefacts from these materials, because this