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W. Qiu and X.-Y. Liu
predicted stress–strain profiles of silkworm silk fibers are in agreement with the
results of experimental measurements. According to their reports, during the silk
fiber stretching process, the extension linearly increases with the responsive force in
the linear region, until the force reaches a threshold, after which the β-crystallites start
to split. The splitting of β-sheets (within β-crystallites) results from the breakage of
H-bondings, and assuming that the occurrence of H-bondings (along the β-strands) is
uniform everywhere, a larger amount of force is required to extract longer molecules
from β-crystallites because more H-bondings must be broken [44]. In other words, if
the fibrous axis (the c direction) of a β-crystallite is larger, such crystallites become
more robust. However, the beneficial effect attributable to crystallite size in the c
direction (i.e., L c ) on the mechanical stability is limited. In contrast, Xu et al. [32]
have shown that if the crystallite size L c is larger than 6 nm, the influence of size on
the splitting force becomes negligible. In natural silkworm silk fibers, the measured
L c values were more than 6 nm (Table 6.1). Thus, the splitting force of β-crystallites
in silk materials should not be correlated to the crystallite size along the c direction
[32]. However, Keten et al. [45] performed a series of large-scale molecular dynamics
(MD) simulations to investigate the effects of crystallite size (in the b direction) and
reported that small β-crystallites with a size of only a few nanometers can give rise
to greater strength, stiffness, and toughness compared to large β-crystallites [45].
According to the results of their simulation study, in the crystal network, if the
length of β-crystallites is 2–4 nm along the interchain direction (L b ), the mechanical
properties are optimal [45]. This experimental finding is in accordance with the
reported results; the strength and toughness of silk fibers are enhanced when the βcrystallite size is reduced from 10 nm to 6.5 nm and further reduced to 3 nm. This can
be attributed to the difference in geometry and stress distribution pathways of small
and large β-crystallites. For large crystals, the hydrogen bonds are directly stretched
because of tension (i.e., the β-crystallites are pulled in a direction parallel to that of the
hydrogen bonds). In contrast, for small crystals that can be deformed when exposed
to shear forces, hydrogen bonds are pulled orthogonal to the bonding direction. This
shear deformation pathway can optimally utilize hydrogen bonds and consequently
lead to a significant enhancement in the mechanical strength of β-crystallites [45].
From another perspective, compared with smaller crystallites, larger crystallites should contain more defects/mismatches, which also deteriorate their stability.
According to crystallization theory, the formation of large crystallites normally relies
on the package having a certain degree of ordering or symmetry [16]. During the
normal crystal growth process, crystallites grow together via self-epitaxial nucleation, which consequently results in an ordered assembly. Nevertheless, under
specific conditions, e.g., when the supersaturation is so high that the nucleation barrier
for mismatched epitaxial nucleation decreases rapidly, some mismatches may occur
between the parent crystals and nucleating layers of crystals. In this regard, the newly
deposited layers of growing crystals would deviate from the optimal structural match
position, and mismatched/misaligned structural packing would be observed [16].
W. Qiu and X.-Y. Liu
predicted stress–strain profiles of silkworm silk fibers are in agreement with the
results of experimental measurements. According to their reports, during the silk
fiber stretching process, the extension linearly increases with the responsive force in
the linear region, until the force reaches a threshold, after which the β-crystallites start
to split. The splitting of β-sheets (within β-crystallites) results from the breakage of
H-bondings, and assuming that the occurrence of H-bondings (along the β-strands) is
uniform everywhere, a larger amount of force is required to extract longer molecules
from β-crystallites because more H-bondings must be broken [44]. In other words, if
the fibrous axis (the c direction) of a β-crystallite is larger, such crystallites become
more robust. However, the beneficial effect attributable to crystallite size in the c
direction (i.e., L c ) on the mechanical stability is limited. In contrast, Xu et al. [32]
have shown that if the crystallite size L c is larger than 6 nm, the influence of size on
the splitting force becomes negligible. In natural silkworm silk fibers, the measured
L c values were more than 6 nm (Table 6.1). Thus, the splitting force of β-crystallites
in silk materials should not be correlated to the crystallite size along the c direction
[32]. However, Keten et al. [45] performed a series of large-scale molecular dynamics
(MD) simulations to investigate the effects of crystallite size (in the b direction) and
reported that small β-crystallites with a size of only a few nanometers can give rise
to greater strength, stiffness, and toughness compared to large β-crystallites [45].
According to the results of their simulation study, in the crystal network, if the
length of β-crystallites is 2–4 nm along the interchain direction (L b ), the mechanical
properties are optimal [45]. This experimental finding is in accordance with the
reported results; the strength and toughness of silk fibers are enhanced when the βcrystallite size is reduced from 10 nm to 6.5 nm and further reduced to 3 nm. This can
be attributed to the difference in geometry and stress distribution pathways of small
and large β-crystallites. For large crystals, the hydrogen bonds are directly stretched
because of tension (i.e., the β-crystallites are pulled in a direction parallel to that of the
hydrogen bonds). In contrast, for small crystals that can be deformed when exposed
to shear forces, hydrogen bonds are pulled orthogonal to the bonding direction. This
shear deformation pathway can optimally utilize hydrogen bonds and consequently
lead to a significant enhancement in the mechanical strength of β-crystallites [45].
From another perspective, compared with smaller crystallites, larger crystallites should contain more defects/mismatches, which also deteriorate their stability.
According to crystallization theory, the formation of large crystallites normally relies
on the package having a certain degree of ordering or symmetry [16]. During the
normal crystal growth process, crystallites grow together via self-epitaxial nucleation, which consequently results in an ordered assembly. Nevertheless, under
specific conditions, e.g., when the supersaturation is so high that the nucleation barrier
for mismatched epitaxial nucleation decreases rapidly, some mismatches may occur
between the parent crystals and nucleating layers of crystals. In this regard, the newly
deposited layers of growing crystals would deviate from the optimal structural match
position, and mismatched/misaligned structural packing would be observed [16].