Chemistry Reference
In-Depth Information
applications. The same chemical species can be used as a plastic, without fiber-
like axial orientation. Thus most fiber-forming polymers can also be used as plas-
tics, with adjustment of molecular size if necessary to optimize properties for par-
ticular fabrication conditions and end uses. Not all plastics can form practical
fibers, however, because the intermolecular forces or crystallization tendency may
be too weak to achieve useful stable fibers. Ordinary polystyrene is an example
of such a plastic material, while polyamides, polyesters, and polypropylene are
prime examples of polymers that can be used in both areas.
Elastomers are necessarily characterized by weak intermolecular forces.
Elastic recovery from high strains requires that polymer molecules be able to
assume coiled shapes rapidly when the forces holding them extended are released.
This rules out chemical species in which intermolecular forces are strong at the
usage temperature or which crystallize readily. The same polymeric types are
thus not so readily interchangeable between rubber applications and uses as fibers
or plastics.
The intermolecular forces in polyolefins like polyethylene (1-3) are quite low,
but the polymer structure is so symmetrical and regular that the polymer segments
in the melt state are not completely random. The vestiges of solid-state crystallites
that persist in the molten state serve as nuclei for the very rapid crystallization
that occurs as polyethylene cools from the molten state. As a result, solid polyeth-
ylene is not capable of high elastic deformation and recovery because the crystal-
lites prevent easy uncoiling or coiling of the macromolecules. By contrast,
random copolymers of ethylene and propylene in mole ratios between about 1/4
and 4/1 have no long sequences with regular geometry. They are therefore non-
crystallizing and elastomeric.
1.9
Miscellaneous Terms
Chain. A linear or branched macromolecule is often called a chain because the
repeating units are joined together like links in a chain. Many polymers are poly-
merized by chain reactions, which are characterized by a series of successive
reactions initiated by a single primary event. Here the term chain is used to desig-
nate a kinetic sequence of reaction events that results in the production of a
molecular chain composed of linked repeating units.
Resin. In polymer technology the term resin usually means a powdered or
granular synthetic polymer suitable for use, possibly with the addition of other
nonpolymeric ingredients. Although the meaning is very ill defined, it is listed
here because it is widely used.
Condensation and addition polymers. The explanation of these two widely
used terms is postponed to
Section 7.1
where polymerization processes are con-
sidered for the first time in this text.
