Engineering of Small-Molecule Gels Based on the Thermodynamics and Kinetics of Fiber Formation - Soft Fibrillar Materials: Fabrication and Applications - page 92

monomers into the crystal lattice in an imperfect manner. The change in area of the
1700 and 3200 cm − 1 peaks, corresponding respectively to the dimerization of the
carboxylic acid groups and hydroxyl non-covalent interactions during crystallization
was monitored. The rate constant for hydroxyl interactions linearly increased as a
function of cooling rate while a plateau was observed for the rate of dimerization
at cooling rates between 5 and 7 ◦ Cmin − 1 . At cooling rates greater than 5-7 ◦ C
min − 1 , HSA monomers did not effectively dimerize before being incorporated into
the crystal lattice causing crystal imperfections, impeding linear epitaxial crystal
growth and producing branched fibers.
2.4
Strategies for Engineering the Micro/Nano Structure of Fiber Networks
On the basis of the crystallization mechanism, the structure of a fiber network can
be manipulated by controlling the primary nucleation rate of the gelator and growth
(branching) of fibers. The primary nucleation rate determines the total number of
individual (single) fiber networks in a certain gel volume. The branching density
determines the correlation length and pore size of the fiber network, as well as the
compactness of the individual fiber network. As has been discussed, if the individual
fiber network is not compact, neighboring fiber networks can penetrate into each
other, forming an interconnecting (single) fiber network. On the other hand,
compact fiber networks contribute to the formation of multi-domain networks. For
a single fiber network, the branching density is the most important parameter to
determine its macroscopic properties, while for a multi-domain network the size
of the individual fiber network is more important. In the following sections, we
will present the strategies developed so far to control fiber branching in single fiber
networks and the size of the individual fiber networks in multi-domain networks.
2.4.1
Engineering of ''Single'' Fiber Networks
As discussed in previous sections, the branching distance is important in deter-
mining the elasticity of a single fiber network. The fiber branching is essentially
controlled by the mismatch nucleation during fiber growth. For a given system,
supersaturation and the presence of impurities that affects the structural match can
affect the mismatch nucleation and thus fiber branching. In the following sections,
we will present the control of fiber branching through the thermodynamic and
additive-mediated approaches. The schematic description of the strategy is given
in Figure 2.6a,b.
2.4.1.1 Effects of Supersaturation/Super Cooling on Fiber Branching
As discussed in Section 2.3.3, supersaturation is an important factor that controls
the fiber branching. Supersaturation, the thermal dynamic driving force, not only
affects the nucleation rate, but also affects fiber branching by decreasing (at higher