Page 362 - Handbook of Properties of Textile and Technical Fibres

Regenerated cellulosic fibers 335

found to be in the range of 130e140 GPa and 80e90 GPa, respectively (Eichhorn
et al., 2005; Gindl et al., 2006; Northolt et al., 2001; Kong et al., 2008). The modulus
of shear between polymer chains is found to be in the range of 1.5 GPa and 2.5 GPa in
Cellulose I and II, respectively (Northolt, 1985; Northolt et al., 2001). But, the
crystalline form alone does not dictate fiber tensile properties.
The other features of the supramolecular structure that have a direct bearing on
polymeric fiber strength include the following (Hearle, 2008; Lenz et al., 1993):

• Degree of crystallinity or order: a higher degree of crystallinity lends greater stability to the
supramolecular structure and thus contributes to improving fiber strength.
• Aspect ratio (length/width ratio) of crystalline domains: a high ratio promotes higher initial
moduli in fibers and increases the tenacity but reduces the extensibility.
• Degree of orientation: a higher degree of orientation parallel to the fiber axis of the crystal-
line domains, as well as of polymer chains in the noncrystalline domains, improves the
tenacity, reduces extensibility, and improves the initial modulus.
• Size of crystalline domains: larger crystallite sizes impede stress transfer through the supra-
molecular structure and thus reduce the fiber strength.
• Molecular extent: the higher this parameter, which is a measure of how far individual poly-
mer chains extend along the length of the fiber, the greater the propensity for interactions
between neighboring chains and thus lower the propensity for chain slippage and higher
the strength.
The tenacity also varies in direct proportion to the degree of polymerization in the
fibers and is attributed to the higher propensity of chain entanglement (Chen et al.,
2015). The effects of orientation and structural aspects (chain length, degree of crys-
tallinity, etc.) on the tensile stressestrain properties of fibers are illustrated in Fig. 10.6.
The arrangement of crystalline and noncrystalline domains in the supramolecular
structure of fibers also influences fiber stressestrain behavior and different models
for their distribution are proposed. These range from, on one end, a “series” model
where the crystalline and noncrystalline domains alternate with each other along the

0 S

Stress

Strain
Figure 10.6 Schematic illustration of the effects of orientation (O) and structural aspects (S) on
the tensile stressestrain properties of fibers.
Reproduced from Hearle JWS: Fibers, 2. Structure. In Wiley-VCH, editor: Ullmann’s fibers.
Volume 1. Fiber classes, production and characterization, Weinheim, Germany, 2008,
Wiley-VCH Verlag GmbH & Co. KGaA with kind permission of the publisher.