Biomedical Engineering Reference
In-Depth Information
surface of growing fiber tips. The surfactant molecules interrupt the nucleation of
the gelators, leading to mismatch branching.
Enhanced side branching of fibers by surfactant molecules was also observed.
It was observed that the presence of a surfactant Span 20 could enhance the
side branching of GP-1 fibers formed in propylene glycol (PG), which converts
spherulitic fiber structures into comb-like and brush-like fibers when the surfactant
concentration is below 0.77 wt% (Figure 2.8c-e) [4a]. With increase of surfactant
concentration to above its critical micelle concentration (CMC) of 1.5 wt% in PG,
the fiber network was converted back into a spherulitic fiber network (Figure 2.8f).
However, the spherulitic fiber network formed in the presence of the surfactant is
denser, and the elasticity of the material is twice that in the absence of surfactant.
It is interesting to notice that two different surfactants (Tween 80 and Span
20) can enhance different patterns of fiber branching of the same gelator GP-1.
The presence of Tween 80 molecules enhances the tip branching of GP-1 fibers
formed in ISA, while Span 20 enhances the side branching of GP-1 fibers formed
in PG (Figure 2.8g,h). However, a clear understanding of the different behavior of
these two surfactants has not been obtained from the current data. Compared with
Span 20 (
0.0006 wt%) was used.
We cannot exclude the possibility of the surfactant Tween 80 enhancing the side
branching if the concentration of this surfactant is significantly increased, since
the side surface of fibers also provides an interface for the adsorption of surfactant
molecules. In addition, the different solvents used for the two systems also make it
difficult to draw a conclusion. Nevertheless, on the basis of the difference between
the molecular structures of these two surfactants, we guess that the following
factors can contribute to the different behavior observed for them:
1) The molecule of Tween 80 is much larger than that of Span 20. Therefore, the
molecule of Tween 80 is more rigid. It should be more capable of interrupting
the integration of GP-1 molecules on the fast-growing surface of fiber tips, on
the basis of energetic and entropic considerations.
2) GP-1 is much less soluble in the more polar solvent PG than in ISA, which
contributes to its high supersaturation in PG and the spherulitic growth of the
GP-1 fiber. The adsorption of Span 20 molecules on the side surface of the
fibers is kinetically favored due to the fast integration and nucleation of GP-1
molecules on the tip surface. Enhanced tip branching can be possible if the
gelation occurs under a sufficiently low supersaturation.
≥
0.05 wt%), only a tiny amount of Tween 80 (
≤
2.4.2
Engineering of Multi-Domain Fiber Networks
A multi-domain fiber network consists of a number of individual/single fiber
networks with clear boundary. The elasticity of a material with such a fiber network
is low due to the presence of the mechanically weak boundary area. Reducing the
boundary area between the single fiber networks and enhancing their interactions
are feasible ways to improve the elasticity of a material. On the basis of the
nucleation-growth mechanism, for a fixed mass of gelator, the boundary area can
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