HIGH-PERFORMANCE POLYESTER FIBERS—PEN AND LCPS

The polyester derived from ethylene glycol and naphthalene-2,6-dicarboxylic acid was first
discovered by ICI in the late 1940s [43]. It has a much higher Tg than PET and gives strong,
high-modulus fibers, but the inaccessibility of the diacid was an insurmountable problem until
recently. Now, firms like Amoco (now Solvay) are able to supply the dimethyl ester of 2,6-
NDA, and the polymer (PEN) is increasingly used in high-performance polyester films and
for high-softening-point blow-molded bottles and containers. Recently, Honeywell have
started producing a high-performance PEN fiber under the name PENTEX. This absorbs
UV light owing to the naphthalene ring. In Japan, stretch-blow-molded PEN bottles are used
to package vitamins and baby food, which would otherwise be adversely affected by UV light.

FIBERS FROM MAIN-CHAIN THERMOTROPIC POLYESTERS—LCPS
It was recognized early in the development of polymer science that the tensile modulus of
polymers should correlate with both the chemical and physical structures, and that maximum
property levels would be achieved when all the molecular chain backbone bonds were lined up
in the direction of measurement [44,45]. The all-aromatic main chain thermotropic polyesters
are semirodlike molecules that naturally organize into nematic liquid crystal domains and
many variants are commercially available in resin and fiber form (see literature and websites
of Ticona and DuPont). The nematic state can be viewed as similar to ‘‘logs floating in a
river,’’ leading to ease of flow parallel to the molecular axis (low elongational viscosity) and
an extended chain structure in the solid state. Hence, the nematic state in polymers brings
both processing ease and high axial tensile properties. Figure 1.10 illustrates the processing of
LCPs and the morphology produced, in contrast to conventional polymers such as PET or
nylon.
All of the LCPs are composed of stiff, highly aromatic molecules and are characterized by a
very high local molecular orientation in the solid state (orientation function >0.95). If processed
into fibers, the local orientation is transformed to global. These globally oriented LCP
fibers are further characterized by very high specific tensile properties and intrinsically low
density when compared with metals, ceramics, and carbon. The highly anisotropic nature of
these oriented LCP fibers, causing inherent weakness in shear and compression, limiting their
use almost exclusively to applications in tension, is not shown in Figure 1.10. It should also be
noted that, in the absence of global orientation, the tensile properties of the thermotropic
polymers are similar to filled plastics, and compressive behavior is less of a critical issue.

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