Page 410 - Handbook of Properties of Textile and Technical Fibres

The chemistry, manufacture, and tensile behavior of polyamide fibers 383

packing of fibrils. Most microfibrils have their ends at the outer boundary of the
fibril. This high local concentration of structural defects is important for crack
propagation. The coalescence of microcracks along the boundary of the fibril tends
to produce long, axially oriented microvoids. These microvoids are often the initiation
of fiber tensile failure. The number of taut tie molecules is relatively small, compared
with intrafibrillar tie molecules. Their portion is substantially increased during plastic
deformation of the fibrous structure.
During the drawing of polyamides, firstly the necks propagate where the spheru-
litic structure is changed into a fibrillar arrangement. Further deformation is uniform,
chain orientation is improved, but the long period remains unchanged. The microfi-
brils and fibrils remain basically unaffected and the plastic deformation can proceed
only by pulling the adjacent elements in the draw direction (Puffr and Kubanek,
1991a).
The interfibrillar tie molecules extend and the inhomogeneities at the ends of
microfibrils are leveled. This results in increased mechanical resistance of the fibrillar
structure. The extended-chain interfibrillar domains composed of taut tie molecules are
the strongest elements of the fiber structure, and have an important effect on the fiber
strength.
A model of the structure of a polyamide, containing phases of taut tie molecules, is
shown in Fig. 12.17.
For the characterization of fiber morphology it is sufficient to know the volume
fraction and orientation of crystalline and amorphous regions (Prevorsek and Butler,
1972; Prevorsek et al., 1973) and the proportion of taut tie molecules (see Fig. 12.18).
This type of three-phase model from Takaynagy can be used for the prediction of
some physical properties of fibers from a knowledge of the properties for individual
phases. The relative volume portion of taut tie molecules is v TT , the relative volume
portion of the crystalline phase is v c ¼ l c (1 v TT ), and the relative volume portion
of the amorphous phase is v a ¼ (1 l c )(1 v TT ). A three-phase model from
Fig. 12.18 can be used for the simple prediction of some mechanophysical properties
of fibers P (initial modulus E, tenacity s, thermal resistance etc.). The property P is
equal to:

1

l c 1 l c
P ¼ v TT P TT þð1 v TT Þ þ (12.1)
P c P a
where P a is the property of the amorphous phase, P c is the property of the crystalline
phase, and P TT is the property of taut tie molecules. In the case of the initial modulus
and tenacity it can be assumed that taut tie molecules have the same property values as
the crystalline phase, i.e., if P TT ¼ P c .Ifl c ¼ v c /(1 v TT ) and P TT ¼ P c is substituted
into Eq. (12.1), the result will be:

2 P c P a
P ¼ v TT P c þð1 v TT Þ (12.2)
P c ð1 v TT v c Þþ P a v c