Page 270 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology En012c-604 July 26, 2001 16:2

Polymers, Thermally Stable 779

and an unsubstituted bispyrene—using the relevant cat-
alyst. The product from these reactions is a crys-
talline intractable material that is extremely difficult to
process. The introduction of phenyl substituents (Q)
into the polymer (II) has resulted in soluble, noncrys-
talline materials with relatively high molecular weight
4
(3–6 × 10 ).

Poly-p-xylylene coatings (Parylenes) involving both
unsubstituted and chloro-substituted products have been
successfully marketed in electrical/electronic coatings ap-
plications used in air and inert atmospheres at around 150
◦
and 220 C, respectively. A comparison of properties, par-
Highly branched polyphenylenes have been produced ticularlyelectrical,oftypicalParylene,silicone,andepoxy
involving, however, different reaction schemes than those coatings formulations is shown in Table I. The deleterious
used for linear systems. Polymer (III) is obtained via effect, however, of the alkylene ( CH 2 CH 2 ) linking
cure (274 C) of a soluble, processible intermediate while unit, particularly on thermooxidative stability, is clearly
◦
oligomer (IV) cures (cross-links) at temperatures above apparent from the isothermal TGA curves (Fig. 2).
melt flow (>150 C). Both materials have been used in
◦
high-temperature resin applications, the former specially
as the matrix in carbon fiber-reinforced composites. B. Polyamides
The extensive development of aromatic polyamides (ara-
mids) as high-temperature polymers, particularly as
thermally stable fibers, followed naturally from the
commercial successes of the wholly aliphatic systems
(nylon; nylon 6,6 etc.). Aramids (VI and VII) are
produced from amino acids (A–B condensations) and
from diacid/diamine combinations (AA–BB condensa-
tions), respectively, by low-temperature interfacial or
high-temperature solution polymerization techniques.

The use of highly polar solvents such as dimethylfor-
mamide (DMF), hexa-methylphosphoramide (HMP), and
Poly-p-xylene (V) has been most successfully pre- N-methyl-pyrrolidone (NMP), among several others, has
pared by the controlled (steam/950 C) pyrolysis of been critical to the success of many high-temperature
◦
p-xylene/chlorinated p-xyleneviaacyclicdimerthatrear- polymerization reactions used to produce both aromatic
ranges (in vacuo) to yield the crystalline high-molecular- and heteroaromatic polymers.
weight (∼500,000) polymer as a hard, impervious A large number of aramids have been obtained from
coating. widely varied A–B and AA–BB precursors. Generally,