Page 279 - 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

788 Polymers, Thermally Stable

resistance. PEEK in particular offers toughness under high possible to achieve an acceptable solubility and proces-
levels of impact loading. A recent comparative study of sibility without intervention of a prepolymer. Synthetic
PEEK(APC-2) and PPS studied the effect of drop-weight activity was greatest during the 1960s and early 1970s;
impact and compression after impact. Although PPS ex- since then the effort has concentrated on applicational
hibited a high resistance to perforation, PEEK showed an developments and, as a result, it has been necessary to
ability to confine the damage and hence had a markedly compromise between thermal/thermooxidative stability
improved damage tolerance. and processibility.
Most current applications for PEEK composites are in With the notable exception of the polyimides and aryl
aerospace. In both fixed- and rotary-wing aircraft, rela- cyanate ester resins, as well as to a lesser extent the ordered
tively small components such as access doors and leading (rigid-rod) polybenzazoles (see below), during the past
edge structures are in service. A number of larger, more decade there has been a marked decrease in published
complex demonstrator structures involving main fuselage references to the heteroaromatic polymer systems, some
and tailplane are under development. of which are mentioned below. The continued presence
of such systems in this account is, however, intended to
F. Liquid Crystal Polymers (LCPs) reflect the intense activity directed into the development
of these thermally stable polymers during the 1960s and
Initial (pre-1985) commercial developments of LCPs in-
1970s.
cluded Aramid fiber (Kevlar), based on poly(1,4-phenyl-
eneterephthalamide), which is a lyotropic (solvent-
A. Ring-Chain
processed) material, and the thermotropic Ekkcell
I-200 (Xydar) based on p-hydroxybenzoic acid. Since 1. Poly(1,3,4-Oxadiazoles)
1985 thermotropic LCPs have been developed by duPont
Several routes exist to aromatic poly(1,3,4-oxadiazoles)
and Hoechst-Celanese based on aromatic and condensed
(XVII), however, cyclization of preformed polyhydra-
aromatic (naphthalene-based) copolyesters which com-
zides has proved most productive of useful (fibers, films)
bine a high level of processability with durability and
materials.
stiffness. Both amorphous and crystalline LCPs—the
Aromatic polyhydrazides are soluble in polar sol-
amorphous-crystalline terminology refers to secondary
vents and can be solution-cast or spun into films and
thermal transitions detected by TGA—have been pro-
fibers, which on cyclodehydration yield insoluble in-
cessed by injection/blow-molding, extrusion (into film
tractable poly(1,3,4-oxadiazoles) in the same material
and sheet), and thermoforming techniques. The largest
form. The wholly aromatic system (XVII;R = m-C 6 H 4 ,
outlets for the LCPs are currently in the field of electronic 1
R = p-C 6 H 4 ) in the form of fiber or film exhibits a
and electrical components in which unreinforced and
◦
60% strength retention in air after 24 hr at 400 C,
reinforced (glass- and mineral-filled) have been used.
50% after 700 hr at 300 C. Introduction of flexible
◦
Surface-mount applications of these components require
groups midchain leads to an increase in tractability but
high dimensional and thermal stability provided by the
a decline in thermal/thermooxidative stability. Aliphatic-
LCPs during vapor phase or infrared soldering.
linked poly(1,3,4-oxadiazoles), for example, are readily
soluble in polar solvents with melt temperatures around
III. HETEROCYCLIC AROMATIC 150–200 C (aromatics > 400 C) and major (TGA) weight
◦
◦
(HETEROAROMATIC) SYSTEMS loss in both inert and oxidizing atmospheres between 300
◦
and 350 C (aromatics 400–480 C).
◦
Heteroaromatic systems have been influential in the
development of thermally stable organic polymers. Some
40 to 50 polymers have been produced in which pheny-
lene rings alternate or are condensed with predominantly
nitrogen containing 5- or 6-membered heterocyclic rings.
Significantly, it has been possible to develop excep-
tionally high levels of stability in often insoluble and
intractable polymers that have themselves been obtained
from high-molecular-weight soluble and processable
precursor polymers via “postpolymerization cyclization”
reactions. Even so in some instances, for example,
with certain polyquinoxalines and polybenzimidazoles,
structural “tailoring” of the final macromolecule made it