Engineering of Small-Molecule Gels Based on the Thermodynamics and Kinetics of Fiber Formation - Soft Fibrillar Materials: Fabrication and Applications - page 107

(cf. Figure 2.11a and c for GP-1/PG fiber networks). It was observed that with a
certain mass of gelator, the elasticity of the gel formed by GP-1 in PG decrease
linearly with the supersaturation of GP-1. This is contrary to the linear increase
observed in other gels (i.e., HSA/BB) with interpenetrating spherulites [36]. The
linear decrease of G with supersaturation means that a more elastic material can
be produced at a higher temperature for a fixed mass of gelator. The higher G at
a low supersaturation is due to the reduced nucleation rate, which contributes to
the formation of larger spherulites. This reduces the mechanically weak boundary
area between the spherulites and leads to the formation of more integrated
(interconnected) fiber networks (Figure 2.10a-c). It was observed that reducing
the nucleation rate by lowering the supersaturation or by using a suitable polymer
additive (PMMMA), the elasticity of the gel can be significantly improved [4c, 36].
For example, without PMMMA, the G of GP-1/PG gel at 40 ◦ Cismorethan
three times that obtained at 20 ◦ C. Increasing the concentration of PMMMA to
0.06 wt%, the G of the gels formed at the two temperatures is about the same,
namely about 75 000 N m -2 . This value is 5 times of that of the material obtained
at 20 ◦ C and 1.5 times of that of the material obtained at 40 ◦ C in the absence of the
polymer. The less significant increase in G at 40 ◦ C can be attributable to the lower
fiber mass of the material formed at this higher temperature. Compared with the
thermodynamic approach, the additive-mediated nucleation is superior in that the
gel can be formed at lower temperatures. This is important to protect the activity
of molecules, particularly biomolecules, to be incorporated into the networks. In
addition, the presence of long polymer molecules with multiple interacting points
with the gelator molecules can potentially strengthen the network by connecting
neighboring spherulites or neighboring fibers of a single spherulite. This will make
the network more resistant to external strains. For example, the critical strain

c
of the 3 wt% GP-1/PG gel (the minimal strain required to break down partially the
network structure of the soft material) formed at 20, 40, and 50 ◦ C in the absence of
PMMMA was found to be 0.7, 0.4, and 0.3%, respectively [36]. This indicates that
although the elasticity ( G ) of the material can be improved by forming the gel at a
higher temperature, the network becomes more brittle. Although the presence of
additive also enlarged the size of spherulites, the

γ

c values of the material obtained
at 20 ◦ C in the presence of 0.04 and 0.06% PMMMA are 1.5 and 2.0%, respectively
[36]. The results show that the presence of the polymer significantly reduced the
brittleness of the material.

γ

2.5.2
Improving the Elasticity of a Material by Enhancing Fiber Branching

It has been demonstrated that the elasticity of a given material can be improved
by enhancing the fiber branching. A power law function ( G = 1.07 × 10 6

ξ − 0.49 )
between the elasticity G and correlation length ξ of GP-1 fibers formed in ISA was
obtained [5]. Power law relations between G and pore size have been proposed
for polymer gels and networks. For example, for semi-flexible and flexible polymer
networks the exponents were found to be − 2and − 3, respectively [46]. According