Page 520 - Handbook of Properties of Textile and Technical Fibres

Tensile failure of polyester fibers 493

Hindered rotation

Interlayer slippage
Figure 13.33 Modes of naphthalene ring motion.

PEN, films show necking behavior at 145 C, which is between T g (117 C) and the
cold crystallization temperature T cc (195 C) (Cakmak and Lee, 1995; Higashioji

and Bhusdan, 2002). This is usually observed in PET only when it is stretched close
to or below T g . At temperatures between T g and T cc PET stretches rather uniformly
(Cakmak and Lee, 1995).
PEN molecules contain naphthalene rings, which are stiffer than those of PET. The
PET chains are more flexible in comparison with PEN. The reason is the collinear
attachment of ethylene glycol diester residues to the phenyl rings of PET, compared
with the parallel, but noncollinear attachment to the naphthyl rings in PEN (Tonelli,
2002). The important aspect of PEN is the influence of the increased chain stiffness
on the mechanical and thermal properties of the polymer. The naphthalene portions
exhibit higher creep compliance as well.
PEN polymer, like PET, can be formed into an amorphous form by quenching from
the melt or it can be crystallized either by slow cooling from the melt or by stretching
between the glass transition temperature and the cold crystallization temperature T cc .
Some authors reported that this neck formation is a result of a highly co-operative
orientation of the naphthalene planes parallel to the surface of the film. This behavior
resembles an isotropic to nematic structural transition, which occurs in highly localized
regions of the sample.
In contrast to PET, which crystallizes into only one crystal modification, PEN has

two different crystal modifications (Buchner et al., 1989). Crystallizing at 180 C
3

yields the triclinic a modification with density 1407 kg/m . Crystallizing at 240 C
yields the b form. This form is triclinic as well but with different cell parameters
3
and higher density 1439 kg/m . The molecules in this modification are therefore
densely packed but are not fully extended along the chain direction.
Crystallization may occur during fiber formation. By high-speed spinning the fibers
containing b-crystals of a very high perfection were obtained. The diffraction data of
these fibers revealed a monoclinic PEN b structure (Van den Heuvel and Klop, 2000).
X-ray analysis revealed the presence of a mesophase in addition to the crystal
forms. In this mesophase structure, the molecular chains are in registry with each other