Materials for engineering, 3rd Edition - (Malestrom)

30
Materials for engineering
(a)
C
C
(b)
C
C
(c)
C
C
1.23 Schematic representation of the arrangement of side groups in
linear polymers with carbon chain C—C: (a) isotactic, (b) syndiotactic
and (c) atactic.
Many apparently linear polymers are in fact branched, Fig. 1.24, as a
result of subsidiary reactions occurring during polymerization. Polyethylene
is available with a wide range of structures and hence properties. The low
density (LDPE) types are extensively branched, while high density polyethylene
(HDPE) is essentially linear. Medium density (MDPE) types fall between
these extremes and grades are used according to their application, ranging
from packaging for the flexible lower density types to semi-structural for the
stiffer high-density polyethylenes.
Although the directions of the C—C bonds in the chain are rigidly fixed,
rotation about these bonds is relatively easy. The chain is thus flexible and
the bulk plastic consists of a tangled network of highly kinked chains which
are locally linked together by the much weaker secondary bonds of the van
der Waals or hydrogen bond type. When the temperature is raised, these
1.24 Diagram of a branched linear polymer.