Engineering of Small-Molecule Gels Based on the Thermodynamics and Kinetics of Fiber Formation - Soft Fibrillar Materials: Fabrication and Applications - page 95

to its mismatch nucleation. The molecular structure of this polymer given below
satisfies all the criteria for a strong adsorptive additive.
The molecular structure of EVACP is as follows.

) ( ( CH 2 CH 2 CH 2 )

CH

y

x

O

C O

CH 3

Ethylene/vinyl acetate copolymer (EVACP)

The presence of a tiny amount of this polymer was found to significantly retard the
nucleation/induction time and growth rate of DHL fiber, and enhance themismatch
between the nucleation phase and substrate. On the basis of the nucleation-growth
model, this work develops a novel approach to the creation of self-supporting
materials from otherwise useless materials using a tailor-made polymer additive. It
was observed in later work that the presence of this polymer additive successfully
converted an interconnecting fibrillar network of GP-1 (Figure 2.7a) into spherulitic
networks (Figure 2.7b). Enhanced structural mismatch induced by the adsorption
of this additive significantly enhanced fiber branching. The enhanced structural
match was evidenced by the increase in the slope of the curve ln ( t g )( t g is
induction time) versus 1/[( /k T ) 2 T 3 ] (Figure 2.7c). However, the conversion

compromised the viscoelasticity of the gel, due to the mechanically weak nature of
spherulitic networks (Figure 2.7d). Nevertheless, it provides a simple approach to
the topological conversion from an interconnecting to a spherulitic fiber network.

μ

Surfactant-Enhanced Fiber Branching The interfacial adsorption of surfactant
molecules has long been well known. A surfactant molecule is amphiphilic,
with a hydrophilic head and one or two hydrophobic tails. This contributes to its
strong adsorption at the interface of two phases (i.e., water-air, two solvents) with
different polarity in order to reduce the interfacial tension (or surface free energy).
The large difference in the polarity of a gelator and a solvent (low solubility of the
gelator in the solvent) is essential to the phase separation (crystallization) and fiber
network formation in a gel system. Hence, the surfactant molecules present will
potentially affect the crystallization of gelators and hence the microstructure of the
fiber networks.
It was observed that the presence of a tiny amount of the surfactant Tween 80
can lead to the formation of highly branched fibers of GP-1 in ISA (Figure 2.8a,b)
[6a,b]. Without the surfactant, elongated fibers with a low degree of branching were
formed at low supersaturation of GP-1. The branching distance (correlation length)
of the GP-1 fibers was decreased from 2.5 μm to 400 nm when a tiny amount of
this surfactant was added. Similar effects of this surfactant on the branching of
lanosterol fibers formed in benzyl benzoate were also observed. This indicates that
the surfactant molecules at such a low concentrations preferentially adsorb on the