Page 405 - Handbook of Properties of Textile and Technical Fibres

378 Handbook of Properties of Textile and Technical Fibres

between parallel chains. Therefore, the amide and the methylene groups are no longer
in the same plane. This structure is described as pseudohexagonal lattice (Stepaniak
et al., 1979; Miyasaka and Ishikawa, 1968; Hiramatsu and Hirakawa, 1982). This
chain conformation is similar to the pleated sheet conformation of polypeptide chains.
PA 4 and PA 6 are unusual because they are able to crystallize in both a and g
phases. By application of axial tension or high pressure the g form in PA 6 can be
simply converted into a form (Miyasaka and Ishikawa, 1968; Hiramatsu and

Hirakawa, 1982). In PA 6, beside stable a (melting point 222 C) and g (melting point

214 C) forms, there is also a metastable b form (Auriemma et al., 1997). The b form
exhibits a hexagonal symmetry, with hydrogen bonds between parallel and antiparallel
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chains. The density of this form (1140 kg/m ) is lower than the a (1230 kg/m ) or the g
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(1180 kg/m ) form and closer to the density of the amorphous form (1080 kg/m ).
Heating above 130 C changes the b form into the a form.

In PA 66 there exist a I and a II forms. The stable a I form obtained by solution
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crystallization is triclinic, with a higher density (1240 kg/m ) and a higher melting

point (265 C) (Starkweather et al., 1963). The a II metastable form is obtained at lower
temperatures or by quenching. Therefore, the crystalline lattice is less perfect and a
fraction of the hydrogen-bonds shifts from the methylene zigzag plane, leading to a
pseudohexagonal lattice. The crystalline density of the a II form is lower (1160 kg/
3

m ) and the melting point is lower (262 C) as well.
Under normal working conditions, an intermediate structure a I II appears. During
aging at ambient temperatures or in a humid environment, the a I II slowly changes
towards the thermodynamically stable a I form (Vergelati. et al., 1993).
The elementary unit cell lengths of PA 6 and PA 66 are both approximately
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1.72 nm. The cell volume of PA 6 a forms is 0.122 nm (density 1232 kg/m ) and
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for PA 66 a forms, it is 0.303 nm (density 1220 kg/m ). The cell volume of PA 6
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g forms is 0.645 nm (density 1160 kg/m ).
In the g form, the balance of energies is primarily between the van der Waals
interactions in the crystal lamellae and the twisting of the amide groups out of the
plane. Hence, the hydrogen bonds always stay in their minimum energy conformation
and enlarged methylene packing leads to a shortening of the unit cell chain axis
because of this twist (Najem, 2009).
In PA 66 a broad, so called Brill transition, of the crystal structure between 177
and 217 C appears. This is, in fact, a gradual transformation from triclinic to hexag-

onal symmetry, accompanied by a 12% increase in unit cell volume (Starkweather
and Jones, 1981). The increase in crystal symmetry, at the Brill transition, causes a
marked increase in segmental mobility of the methylene groups and is accompanied
by changes in the thermal and mechanical properties (Starkweather and Jones,
1981). PA 6 does not show a distinct Brill transition to a high-temperature pseudohex-
agonal crystal phase, mainly because of its 40 C lower melting temperature than PA

66 when crystallized from the melt (220K), but a diffuse transition is observed close to
its melting point (Starkweather and Jones, 1981). Wang, Tsou, and Lin (Wang et al.,
2012) found a Brill transition in electrospun PA fibers created from formic acid.
As-spun fibers exhibit a unique crystalline phase with a melting temperature of

235 C, i.e., higher than the equilibrium melting temperature of PA 6. The content