6 Cocoon Silk: From Mesoscopic Materials Design …
283
the direction of molecular chains, nanofibrils elongate and give rise to interaction with
each other and consequently lead to the formation of nanofibril networks. In detail,
during stage 1, most SF molecules are in the random coil or α-helix conformations
and are therefore soluble. Gradually, owing to thermal fluctuations, such molecules
begin to move forward towards each other. Along with the breakage of the initial
intramolecular H-bonds and the formation of intermolecular H-bondings, the transition to β-sheets is triggered. The total free energy of the entire system is decreased
because of the formation of β-sheets, and the β-sheets have a more compact structure
than their precursors. In this regard, such a conformation transitional process can
be identified as a crystallization process. During stage 2, because more β-sheets are
present in the solution, inter-sheet interactions (such as hydrophobic interactions and
van der Waals interactions) further lead to the stacking of nearby β-sheets and result
in the crystallization of β-crystallites. β-crystallites are attributable to the closed
package and well-defined patterns of β-sheets and should be regarded as more stable
polymorphs of β-sheets. During stage 3, the shear force (such as the force in the silk
spinneret) orients the β-crystallites in a parallel direction. However, in the absence of
such shear forces (e.g., during the SF hydrogelation process), the orientation of crystallites is less orderly. In stage 4, the differences in nanofibril interaction strength and
the nanofibril network architecture result in the differing mechanical performance of
silk materials.
6.5.2 Nucleation Mechanism
The formation of the mesoscopic hierarchical crystalline network structure of SF
materials has been found to follow the nucleation mechanism [19]. The nucleation
kinetics of both the homogeneous nucleation and heterogeneous nucleation processes
can be thoroughly quantified by determining the nucleation rate, which is defined as
the number of mature nuclei created per unit volume time in the system. The nucleation rate is usually highly correlated to many factors, especially the concentration
of the nucleating phase and the surrounding chemical environments.
6.5.2.1 Homogeneous Nucleation
According to classical nucleation theory, nucleation occurs only if the nucleation
barriers can be overcome [62, 63]. In addition, the newly formed crystalline phases
are not thermodynamically stable until the dimensions are beyond the critical size.
Mathematically, the nucleation barrier G
∗ and critical size R c are represented by
Eq. 6.6.
G
∗
=
16πγ
3
3(ρ c μ)
2
and R c =
2γ
ρ c μ
(6.6)
283
the direction of molecular chains, nanofibrils elongate and give rise to interaction with
each other and consequently lead to the formation of nanofibril networks. In detail,
during stage 1, most SF molecules are in the random coil or α-helix conformations
and are therefore soluble. Gradually, owing to thermal fluctuations, such molecules
begin to move forward towards each other. Along with the breakage of the initial
intramolecular H-bonds and the formation of intermolecular H-bondings, the transition to β-sheets is triggered. The total free energy of the entire system is decreased
because of the formation of β-sheets, and the β-sheets have a more compact structure
than their precursors. In this regard, such a conformation transitional process can
be identified as a crystallization process. During stage 2, because more β-sheets are
present in the solution, inter-sheet interactions (such as hydrophobic interactions and
van der Waals interactions) further lead to the stacking of nearby β-sheets and result
in the crystallization of β-crystallites. β-crystallites are attributable to the closed
package and well-defined patterns of β-sheets and should be regarded as more stable
polymorphs of β-sheets. During stage 3, the shear force (such as the force in the silk
spinneret) orients the β-crystallites in a parallel direction. However, in the absence of
such shear forces (e.g., during the SF hydrogelation process), the orientation of crystallites is less orderly. In stage 4, the differences in nanofibril interaction strength and
the nanofibril network architecture result in the differing mechanical performance of
silk materials.
6.5.2 Nucleation Mechanism
The formation of the mesoscopic hierarchical crystalline network structure of SF
materials has been found to follow the nucleation mechanism [19]. The nucleation
kinetics of both the homogeneous nucleation and heterogeneous nucleation processes
can be thoroughly quantified by determining the nucleation rate, which is defined as
the number of mature nuclei created per unit volume time in the system. The nucleation rate is usually highly correlated to many factors, especially the concentration
of the nucleating phase and the surrounding chemical environments.
6.5.2.1 Homogeneous Nucleation
According to classical nucleation theory, nucleation occurs only if the nucleation
barriers can be overcome [62, 63]. In addition, the newly formed crystalline phases
are not thermodynamically stable until the dimensions are beyond the critical size.
Mathematically, the nucleation barrier G
∗ and critical size R c are represented by
Eq. 6.6.
G
∗
=
16πγ
3
3(ρ c μ)
2
and R c =
2γ
ρ c μ
(6.6)