Basic Technology of Plastics - Introduction to Polymeric Composites with Rice Hulls - page 15

There are various ways that monomers can arrange during polymerisation but
they can be broken down into two categories: crosslinked and uncrossed-linked.
Furthermore, uncrossed-linked polymers can be subdivided into linear and branched
polymers. The most common examples of uncrossed-linked polymers that present
the various degrees of branching are polyethylene (PE). Another important family of
un-crosslinked polymers are copolymers.
Copolymers are polymeric materials with two monomer types in the same chain. A
polymer that is formed by three monomer groups is called a terpolymer . Depending
on how the different monomers are arranged in the polymer chain, the patterns are
distinguished as random, alternating, block or graft copolymers.
Although thermoplastics (soften on heating and can be re-used) can crosslink under
specific conditions such as gel formation, if PE is exposed to high temperatures for
prolonged periods of time, thermosets (harden on heating and cannot be re-used)
and some elastomers are polymeric materials that can crosslink. This crosslinking
causes the material to become resistant to heat after it has solidified. Usually, this
crosslinking is a result of double bonds that break, allowing the molecules to link with
their neighbours. One of the oldest thermosetting polymers is phenol-formaldehyde
(also known as phenolic resin). The reaction in which phenol molecules react with
formaldehyde molecules to create a three-dimensional (3D) crosslinked network that
is stiff and strong (thereby leaving water as the byproduct of this chemical reaction)
is called condensation polymerisation .
2.2.6 Polymer Structures
The properties of plastics are affected by the arrangement of atoms and molecules.
The atomic arrangement of the polymers can be classified as crystalline , amorphous
or molecular . Crystallinity is the three-dimensional arrangement of atoms, ions and
molecules in a regular pattern. Diamonds and table salt are two common materials
with crystalline structures that can be seen readily. Because of the considerable length
of polymer molecules, there could be less crystallisation. Because the molecular chains
are only partly ordered, most crystalline plastics are not transparent in the solid
state. Partially crystalline (semi-crystalline) polymers such as the linear polyolefins,
polyacetals and polyamides (PA) are, as a rule, translucent-to-opaque.
The crystalline content of some polymers may be >95%, whereas the remainder is
amorphous. Crystallinity of low-density polyethylene (LDPE) is ≈65%, whereas high-
density polyethylene (HDPE) may be >95%. The crystallisation of bulk polymers
is characterised by the formation of large crystalline aggregates called spherulites .