Page 294 - 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 803

Polyquinoxalines (LVIII,Z = H) (PQs) are formed dition/cure processes have involved use of acetylenic-end
from a bisglyoxal dihydrate (LIX,Z = H). Wholly aro- groups. Intramolecular cross-link cure (see polyimides)
matic systems with limited solubility and fusibility, how- of vicinal pendant groups increases T g (∼120 C) solely
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ever, have been successfully evaluated as metal-to-metal by chain-stiffening effects induced by an increase in the
(steel and titanium adherends) adhesives and as high- content of fused rings.
temperature matrix resins in advanced carbon and boron Acetylene-terminated phenyl quinoxalines (LX, ATQs)
fiber reinforced composites (Table IX). The high process- chain extend/cross-link by intermolecular cycloaddition.
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ing temperatures (>400 C) for these resins (T g values Carbon fiber-reinforced ATQ laminates retain approxi-
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> 300 C) have been reduced by the incorporation of flex- mately 90% of their room temperature flexural strength
ible midchain units, for example, either linkages (T g val- and modulus at 260 C and, unlike carbon/epoxy lami-
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ues < 200 C) associated with, however, a concomitant nates, these properties are maintained under hot wet con-
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reduction in thermo-oxidative stability (ITGA/400 C). ditions (Fig. 12).
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Polyphenylquinoxalines (LVIII,Z = C 6 H 5 ) (PPQs) with
improved thermooxidative stability (Fig. 12) and proces-
C. Ladder/Highly Fused Ring Polymers
sibility compared to the PQs have been obtained on re-
placement of bisglyoxals by more stable dibenzils (LIX Ladder systems have extended the concept of het-
Z = C 6 H 5 ) in the reaction sequence shown above. Ad- eroaromatic polymers into those “double-strand” struc-
vanced PPQ-based unidirectional carbon fiber reinforced tures incorporating essentially linear rather than cross-
composites and metal-to-metal (stainless-steel and tita- linked chains of fused rings. Despite wide predictions that
nium adherends) adhesive formulations exhibit superior such systems would exhibit improved thermal/thermo-
retention of strength properties over the PQs at temper- oxidative stability allied to superior high-temperature
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atures > 300 C. However despite T g values > 300 C for properties, in practice they show little or no improve-
the wholly aromatic PPQs, long-term use above 250 C ment over the best of the “conventional heteroaromatic”
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requires a postprocessing increase in the initial level of polymers. Numerous ladder polymers based on indi-
T g in order to minimize the reduced property levels asso- vidual heterocyclic units have been synthesized; typi-
ciated with thermoplasticity/creep deterioration. Several cally a wholly ladder polyquinoxaline (LXI) exhibited a
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volatileless cross-linked systems have been investigated TGA weight loss of 10% at 440 and 585 C in air and
including cyano- or cyanato-chain-pendant groups (T g in- nitrogen, respectively, with a 38% loss in nitrogen at
crease on cure ∼100 C), but the most effective cycload- 800 C.
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