Chemistry Reference
In-Depth Information
Fiber
axis
Folded chain
crystallites
Disordered
domains
Extended
noncrystalline molecules
FIGURE 4.7
Schematic representation of structure of a microfibril in an oriented fiber.
make up the structure of oriented, semicrystalline, synthetic fibers.
Figure 4.7
is a
simplified model of such a structure. The gross fiber is made up of interwoven
microfibrils that may branch, bend, and fuse together.
The mechanical properties of polymer crystallites are anisotropic. Strengths
and stiffnesses along the molecular axis are those of the covalent bonds in the
polymer backbone, but intermolecular cohesive forces in the transverse directions
are much weaker. For example, in the chain direction the modulus of polyethyl-
ene is theoretically
10
9
Pa), while the moduli of the crys-
tallites in the two transverse directions are
200 GPa (i.e., 200
3
B
2 GPa. Oriented extended chain
structures are produced by very high orientations. In conventional spinning of
semicrystalline fibers or monofilaments (the distinction is primarily in terms of
the diameters of these products) the polymer melt is extruded and cooled, so that
stretching of the solid polymer results in permanent orientation. The degree to
which a high-molecular-weight polymer can be stretched in such a process is lim-
ited by the “natural draw ratio” of the polymer, which occurs because entangle-
ments in the material prevent
B
its extension beyond a certain extent without
