Page 272 - 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 781

introduction of additional flexible links mid-chain [e.g., The latter process has produced a wide range of
O , S , SO 2 , and (CH 2 ) ] as well as substitution polyesters from which it was possible to assess property–
in the aromatic rings increased solubility but reduced poly- structure relationships. The replacement of wholly para-
mermelttemperature.Moresuccessfully,thedevelopment linked unsubstituted rings in a homopolymer system by
and application of alternating ordered copolyamides con- meta-links, incorporation of midchain flexible groups
taining a high proportion of para-substituted phenylene, [e.g., C(CF 3 ) 2 , C(CH 3 ) 2 , CH 2 , O ] or interpo-
and heteroaromatic rings yielded soluble, high melt sys- sition of ordered alternating copolymer structures resulted
tems in which a high level of crystallinity could be in- in an increased solubility/processibility paralleled, how-
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duced, most significantly after the spinning process. The ever, by a reduced thermal stability [as much as 150 C
thermal/thermooxidative stability and retention of fiber (TGA) in air or nitrogen].
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tenacity at temperature (300 C) for extended periods in- The commercial development of useful high-tempera-
creases in the alternating co-aramids as the content of ture aromatic polyesters has been based on A–B type melt
para-substitution increases. condensations of p-hydroxybenzoic acid derivatives and
In spite of the considerable activity referred to above is limited to a homopolymer [Ekonol; VIII (p-linked)]
only two aramid systems have been recognized commer- and two related copolymer compositions, Ekkcel C-1000
cially. The first poly-m-phenyleneisophthalamide (VII, and Ekkcel I-2000. A comparison of the thermal stability

Ar = Ar = m-C 6 H 4 ), marketed as Nomex for its prin- (isothermal weight loss in air) of the three polymer sys-
cipal use in flame-resistant fabrics, the second origi- tems is demonstrated in Fig. 3. Ekonol, because of its very
nated as poly (p-benzamide) (VI, p-C 6 H 4 ), marketed high crystallinity/melting, is processed under exceptional
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as Fiber B/PRD-49, but a later ultrahigh modulus ma- conditions of compressive sintering (370 C/70 MPa) or
terial development (Kevlar 49) was based on poly(1,4- plasma spray while Ekkcell C-100 and I-200 are com-

phenyleneterephthalamide) (VII,Ar = Ar = p-C 6 H 4 ). pression and injection molded, respectively. Comparative
These para-linked aramids form lyotropic liquid crys- property data for Ekonol versus selected high-temperature
talline solutions which can be spun to high-strength/high- alternatives are detailed in Table II.
stiffness fibers. A comparison of the thermal stability
(TGA) underlines the superiority in both inert and oxi-
D. Poly(Phenylene Sulfide)
dizing atmospheres of the para/para-linked aramid (rapid
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degradation begins above 500 C) as opposed to the meta/ Although other synthetic approaches have been reported,
meta isomer (rapid degradation begins above 400 C). the most successful and also commercial route to linear
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poly (phenylene sulfide) (X) is illustrated in the following
reaction sequence:
C. Polyesters
Like the aramids, an impetus for the assessment of
aromatic polyesters in a thermally stable role was the
commercial success generated in the aliphatic series, in
this case poly(ethyleneterephthalate), a fiber- and film-
As prepared, the polymer is moderately crystalline
forming material. Research effort was directed to a wide
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(∼65%) exhibiting both a T g (85 C) and T m (285 C) and
area of aromatic polymers, (VIII) and (IX), produced by
A–BorAA–BB condensation processes. elevated temperature cure involving cross-linking and
chain extension processes results in an insoluble but duc-
tile network polymer. The stabilizing effect of oxidative
cross-linking processes is well illustrated in a compari-
son of isothermal weight loss in air and nitrogen (Fig. 4).
Poly(phenylene sulfide) (X) in its commercial form (Ry-
ton) can be used in molding and laminating resin and
surface coating applications. As a “base” material for
bearings it exhibits considerable high-temperature ben-
efits over conventional epoxy resins (Fig. 5); carbon fiber
laminates incorporating poly(phenylene sulfide) as a ma-
trix resin maintain a high level of tensile and flexural hot
and hot/wet property retention (Table III).
Further research activity has now produced a
series of related aromatic polysulfides. However,