Page 88 - Handbook of Properties of Textile and Technical Fibres

Properties of wool 69

amorphous matrix model proposed by Feughelman (1959). Feughelman later refined
this model to include two regions of thermal and mechanical stability in the crystalline
phase, the so-called series-zone model, which accounted for the change in slope of the
curve in the postyield region (Feughelman and Haly, 1959, 1960a). The less stable X-
zones were deemed to open freely and be responsible for extension in the yield region,
whereas the more highly cross-linked Y-zones were more difficult to extend, giving
rise to the increased slope in the postyield region. On the basis of studies on porcupine
quills the matrix was deemed to show thixotropic behavior in water, consistent with a
gelesol transition (Feughelman and Druhala, 1975). Later Wortmann and Zahn (1994)
offered a structural explanation for the X and Y zones, viz., two distinctly different and
well-defined portions of the monomer of the intermediate filament, with disulfide
bonds influencing one of the portions and resulting in increased resistance to
extension.
In the same year, Feughelman proposed a new model, based on the matrix consist-
ing of water together with globular high sulfur protein (Feughelman, 1994). The post-
yield region was explained in terms of the intermediate filaments coming together on
extension of the fiber and jamming the globular protein once the more mobile water
had moved out of the space between the intermediate filaments. This seems the least
likely of the proposed models.
Concurrent with these developments, Chapman (1969a) proposed a model, later
refined and developed by Chapman, Hearle, and others (Hearle and Chapman,
1968a,b; Chapman and Hearle, 1970, 1971; Hearle et al., 1971; Hearle and Susutoglu,
1985), in which the intermediate filaments show a slight extension up to 2% strain but
then a constant stress as the a-helix unfolds to form the extended beta structure. Based
on supercontraction experiments (Chapman, 1970) the matrix is modeled as a highly
cross-linked rubber and accounts for the increased stiffness in the postyield region. All
the models propose that the Hookean region results primarily from stretching of the
a-helix in the intermediate filaments and the yield region from the unfolding of the
a-helix. They differ in the explanation of the response of the fiber in the postyield re-
gion and the properties of the matrix. It is worth noting that the matrix properties used
in Feughelman’s series zone model are based on stressestrain curves for African
porcupine quill, measured in a direction at right angles to the direction of growth.
The assumption is that, like wool fibers, the intermediate filaments are parallel to
the growth direction and therefore would not be expected to contribute to the lateral
tensile properties. More recent studies (Maxwell, 2002) confirmed the axial and trans-
verse tensile properties of porcupine quill; however, transmission electron microscope
(TEM) images of the cross-section of a quill (Fig. 3.11) showed that the intermediate
filaments are oriented in all directions not just parallel to the direction of growth.
Consequently, lateral tensile tests will not be a measure of the matrix properties alone.
Furthermore, TEM and SPM studies revealed that the structure of the quill is complex,
showing features quite different from those observed in wool fiber cross-sections.
Hence, the validity of using porcupine quill to explain the deformation of wool fibers
during mechanical stretching needs to be questioned.
More detail on this topic can be obtained from the book by Feughelman (1997), the
review article by Chapman (1969b), and the chapter (Hearle, 2002) and recent reviews