6 Cocoon Silk: From Mesoscopic Materials Design …
265
In terms of experimental results, it was revealed that the mechanical performance
of B. mori silk fibers can be improved by increasing the orientation function value.
For example, Liu et al. [18] carried out MD simulations to investigate the influence
of different orientation functions on the stability of corresponding crystal networks
and then compared the simulation results with the experimental results. Notably, the
simulated data fit well with the experimental data; that is, the breaking stress of B.
mori silk fibers increases with f [18].
Apart from the crystallite orientation function, the density of crystallites is another
important structural factor that determines the strength of silk fibers. Evidently, within
the framework of the crystal network structure, the fishnet-like topology of this crystal
network can further isolate the products of the breakage of individual crystallites by
bypassing the loading stress from the broken ones to the surrounding interconnected
crystallites [18]. Consequently, the more β-crystallites that participate in load sharing,
the stronger the network.
6.3.5 Level Five Structure of SF Materials
As discussed in Sect. 3.4, each individual nanofibril is indeed a molecular crystal
network. If a nanofibril is isolated, we can treat such nanofibrils as a single, individual
domain. If several nanofibrils are interconnected with each other and consequently
form a nanofibril network, they are treated as a multi-domain system [19, 32]. In
principle, the nanofibril network is defined as the level 5 structure of SF materials
in which the structural unit refers to the individual nanofibril, and the links between
neighboring nanofibrils are formed either by strong physical contact (for silk fibers)
or weak interactions (for non-fiber SF materials). It is apparent that the stability of
the nanofibrils themselves and the strength of inter-nanofibril interactions are the
two most important factors that are highly correlated to the macroscopic mechanical
properties [16, 19, 32]. In Sect. 6.3.4, the structural factors affecting the stability
of nanofibrils (crystal network) are intensively investigated. In this subsection, we
will comprehensively study the influence of the strength of interactions between
nanofibrils.
Figure 6.14 illustrates two typical nanofibril network architectures in SF materials: (a) systems in which the inter-nanofibril interaction strength is strong or infinite, and (b) systems in which the inter-nanofibril interaction strength is weak or
zero. AFM morphological results have shown that although both silk fibers and
SF hydrogels are composed of numerous nanofibrils, the inter-nanofibril interaction strength in SF hydrogels is very weak because the nanofibrils in SF hydrogels are patterned in a disorderly manner. However, SF fibers exhibit a system of
strong domain-domain interactions because SF fibers comprise a bundle of very
well-oriented twisted nanofibrils interlocked by adjacent nanofibrils to ensure that
the nanofibrils cannot move freely [16, 19, 32].
The above discussion reveals the importance of the helically twisted morphology
of SF nanofibrils. We have assumed that this characteristic morphology results from
265
In terms of experimental results, it was revealed that the mechanical performance
of B. mori silk fibers can be improved by increasing the orientation function value.
For example, Liu et al. [18] carried out MD simulations to investigate the influence
of different orientation functions on the stability of corresponding crystal networks
and then compared the simulation results with the experimental results. Notably, the
simulated data fit well with the experimental data; that is, the breaking stress of B.
mori silk fibers increases with f [18].
Apart from the crystallite orientation function, the density of crystallites is another
important structural factor that determines the strength of silk fibers. Evidently, within
the framework of the crystal network structure, the fishnet-like topology of this crystal
network can further isolate the products of the breakage of individual crystallites by
bypassing the loading stress from the broken ones to the surrounding interconnected
crystallites [18]. Consequently, the more β-crystallites that participate in load sharing,
the stronger the network.
6.3.5 Level Five Structure of SF Materials
As discussed in Sect. 3.4, each individual nanofibril is indeed a molecular crystal
network. If a nanofibril is isolated, we can treat such nanofibrils as a single, individual
domain. If several nanofibrils are interconnected with each other and consequently
form a nanofibril network, they are treated as a multi-domain system [19, 32]. In
principle, the nanofibril network is defined as the level 5 structure of SF materials
in which the structural unit refers to the individual nanofibril, and the links between
neighboring nanofibrils are formed either by strong physical contact (for silk fibers)
or weak interactions (for non-fiber SF materials). It is apparent that the stability of
the nanofibrils themselves and the strength of inter-nanofibril interactions are the
two most important factors that are highly correlated to the macroscopic mechanical
properties [16, 19, 32]. In Sect. 6.3.4, the structural factors affecting the stability
of nanofibrils (crystal network) are intensively investigated. In this subsection, we
will comprehensively study the influence of the strength of interactions between
nanofibrils.
Figure 6.14 illustrates two typical nanofibril network architectures in SF materials: (a) systems in which the inter-nanofibril interaction strength is strong or infinite, and (b) systems in which the inter-nanofibril interaction strength is weak or
zero. AFM morphological results have shown that although both silk fibers and
SF hydrogels are composed of numerous nanofibrils, the inter-nanofibril interaction strength in SF hydrogels is very weak because the nanofibrils in SF hydrogels are patterned in a disorderly manner. However, SF fibers exhibit a system of
strong domain-domain interactions because SF fibers comprise a bundle of very
well-oriented twisted nanofibrils interlocked by adjacent nanofibrils to ensure that
the nanofibrils cannot move freely [16, 19, 32].
The above discussion reveals the importance of the helically twisted morphology
of SF nanofibrils. We have assumed that this characteristic morphology results from