Chemistry Reference
In-Depth Information
H
2
N
NH
2
H
2
N
NH
2
1-39
This polymer is practically as resistant as pyrolitic graphite to high tempera-
tures and high-energy radiation.
Ladder polymers are double-strand linear polymers. Their permanence proper-
ties are superior even to those of conventional network polymers. The latter are
randomly cross-linked, and their molecular weight can be reduced by random
scission events. When a chemical bond is broken in a ladder polymer, however,
the second strand maintains the overall integrity of the molecule and the frag-
ments of the broken bond are held in such close proximity that the likelihood of
their recombination is enhanced.
Space limitations do not permit the description of other varieties of rigid chain
macromolecules, such as semiladder and spiro structures, which are of lesser cur-
rent commercial importance.
1.7
Thermoplastics and Thermosets
A thermoplastic is a polymer that softens and can be made to flow when it is
heated. It hardens on cooling and retains the shape imposed at elevated tempera-
ture. This heating and cooling cycle can usually by repeated many times if the
polymer is properly compounded with stabilizers. Some of the polymers listed
earlier that are thermoplastics are polystyrene (1-1), polyethylene (1-3), poly
(vinyl chloride) (1-4), poly(ethylene terephthalate) (1-5), and so on.
A thermosetting plastic is a polymer that can be caused to undergo a chemical
change to produce a network polymer, called a thermoset polymer. Thermosetting
polymers can often be shaped with the application of heat and pressure, but the
number of such cycles is severely limited. Epoxies, for which cross-linking reac-
tions are illustrated in
Eqs. (1-9) and (1-10)
, are thermosetting polymers. The
structurally similar phenoxies (1-22) are usually not cross-linked and are consid-
ered to be thermoplastics.
A thermoset plastic is a solid polymer that cannot be dissolved or heated to
sufficiently high temperatures to permit continuous deformation, because chemi-
cal decomposition intervenes at
lower temperatures. Vulcanized rubber is an
example.
The classification into thermoplastic and thermosetting polymers is widely
used although the advances of modern technology tend to blur the distinction
between the two. Polyethylene and poly(vinyl chloride) wire coverings and pipe
can be converted to thermoset structures by cross-linking their molecules under
