290
W. Qiu and X.-Y. Liu
that a 1% ratio of TiO 2 nanoparticles greatly enhanced both the breaking strength
and elongation of the composite fibers [69]. In addition, these functionalized TiO 2
particles can also endow the composite fibers with anti-ultraviolet properties; this is
extremely important for the long-term storage of silk fibers [69]. Similarly, Wang
et al. [70]. obtained mechanically enhanced silk fibers by feeding B. mori larval
silkworms SWNTs and graphene. In addition, the incorporation of graphene and
SWNTs can increase the conductivity of these fibers [70].
To quantify the influence of additives, a structural survey was also carried out,
particularly for the second level. Because the α-helix and random coil are relatively
softer than the β-sheet, a higher α-helix/random coil structure content should result
in a material with an improved breaking tensile strength and modules with greater
toughness. However, the β-sheet content in the composite fibers was found to be
relatively smaller than that of the natural control fibers, which suggests that the
presence of SWNTs and graphene in the silk matrix may hinder the conformational
transition from random coil/α-helix to β-sheet [70]. To some degree, this result is
reasonable because the relatively large dimensions of SWNTs and graphene might
induce a steric hindrance effect and consequently prevent the crystallization of SF
molecules. However, this finding conflicts with the hierarchical network structure
model proposed in this chapter, which claims that a lower β-sheet content should be
accompanied by smaller breaking stress. Nevertheless, the enhancement of breaking
strength is reportedly considerably attributable to the presence of relatively stronger
mechanical additives (i.e., the SWNTs and graphene) [70]. Specifically, it is hypothesized that these additives in the silk matrix may act as knots (similar slipknot structures are widely found in other biological structures such as proteins and DNA
strands) [71]. They may also act as a key frictional element, reshaping the entire
fiber and dissipating the additional fracture energy, which consequently results in
enhanced mechanical properties in the fibers. The inclusion of additional additives
in the composite fibers may not necessarily produce stronger fibers; on the contrary,
it is determined that if excessive additives are incorporated into the silk matrix, such
additives tend to aggregate and cause defects, which eventually results in silk fibers
with inferior mechanical performance [69, 70]. In summary, this natural feeding
method is not only straightforward but also effective. More importantly, this method
can be scaled up easily and can thus shed light on the massive production of reinforced
silk fibers.
Forced Reeling of Natural Silk Fiber: As discussed, with an increase in the force
reeling speed of both silkworm silk (Fig. 6.26a and d) and spider MA fibers
(Fig. 6.26b), the mechanical performance, and especially the breaking stress values,
can be improved accordingly. Structural characterization reveals that it is the mesoreconstruction of the hierarchical structures, especially the level 3 and level 4
structures, that results in mechanical enhancement. Specifically, the crystallite size
decrease monotonously with the acceleration of the reeling speed. Meanwhile,
the orientation function and the density of crystallites increased (Fig. 6.26c) [18].
Recently, Xu et al. [32] have revealed that the key structural parameters, i.e., f (orientation function), n β (the number of crystallites at the cross section of a nanofibril),
W. Qiu and X.-Y. Liu
that a 1% ratio of TiO 2 nanoparticles greatly enhanced both the breaking strength
and elongation of the composite fibers [69]. In addition, these functionalized TiO 2
particles can also endow the composite fibers with anti-ultraviolet properties; this is
extremely important for the long-term storage of silk fibers [69]. Similarly, Wang
et al. [70]. obtained mechanically enhanced silk fibers by feeding B. mori larval
silkworms SWNTs and graphene. In addition, the incorporation of graphene and
SWNTs can increase the conductivity of these fibers [70].
To quantify the influence of additives, a structural survey was also carried out,
particularly for the second level. Because the α-helix and random coil are relatively
softer than the β-sheet, a higher α-helix/random coil structure content should result
in a material with an improved breaking tensile strength and modules with greater
toughness. However, the β-sheet content in the composite fibers was found to be
relatively smaller than that of the natural control fibers, which suggests that the
presence of SWNTs and graphene in the silk matrix may hinder the conformational
transition from random coil/α-helix to β-sheet [70]. To some degree, this result is
reasonable because the relatively large dimensions of SWNTs and graphene might
induce a steric hindrance effect and consequently prevent the crystallization of SF
molecules. However, this finding conflicts with the hierarchical network structure
model proposed in this chapter, which claims that a lower β-sheet content should be
accompanied by smaller breaking stress. Nevertheless, the enhancement of breaking
strength is reportedly considerably attributable to the presence of relatively stronger
mechanical additives (i.e., the SWNTs and graphene) [70]. Specifically, it is hypothesized that these additives in the silk matrix may act as knots (similar slipknot structures are widely found in other biological structures such as proteins and DNA
strands) [71]. They may also act as a key frictional element, reshaping the entire
fiber and dissipating the additional fracture energy, which consequently results in
enhanced mechanical properties in the fibers. The inclusion of additional additives
in the composite fibers may not necessarily produce stronger fibers; on the contrary,
it is determined that if excessive additives are incorporated into the silk matrix, such
additives tend to aggregate and cause defects, which eventually results in silk fibers
with inferior mechanical performance [69, 70]. In summary, this natural feeding
method is not only straightforward but also effective. More importantly, this method
can be scaled up easily and can thus shed light on the massive production of reinforced
silk fibers.
Forced Reeling of Natural Silk Fiber: As discussed, with an increase in the force
reeling speed of both silkworm silk (Fig. 6.26a and d) and spider MA fibers
(Fig. 6.26b), the mechanical performance, and especially the breaking stress values,
can be improved accordingly. Structural characterization reveals that it is the mesoreconstruction of the hierarchical structures, especially the level 3 and level 4
structures, that results in mechanical enhancement. Specifically, the crystallite size
decrease monotonously with the acceleration of the reeling speed. Meanwhile,
the orientation function and the density of crystallites increased (Fig. 6.26c) [18].
Recently, Xu et al. [32] have revealed that the key structural parameters, i.e., f (orientation function), n β (the number of crystallites at the cross section of a nanofibril),