Engineering of Small-Molecule Gels Based on the Thermodynamics and Kinetics of Fiber Formation - Soft Fibrillar Materials: Fabrication and Applications - page 94

of elongated fibers or unbranched fibers using tailor-made additives to enhance the
branching of fibers in order to acquire materials with desired micro/nanostructure
and rheological properties (Figure 2.6a,b). To achieve this, a suitable additive should
adsorb strongly on the surface of growing fibers to interrupt the structural match
between the nucleating phase and the surface of growing crystal fibers. Therefore,
the selection of additive is not arbitrary. On the basis of the results obtained from
theoretical calculations and experimental observations [4c, 6c, 32], we provide some
guidelines as follows:

1) Large molecule with a relatively rigid basic structure. The rigidity of additive
molecules can result from a variety of molecular features, such as the intra-
molecular bonding (i.e., hydrogen bonds, double, or triple covalent bonds) and
the presence of bulky functional groups in the backbone of the molecules.
Based on both energetic and entropic consideration, for different molecules
of similar types, larger molecules with somewhat rigid structures are easily
adsorbed at interfaces [32b,c].
2) Stronger interaction between additives and the substrate will lead to a stronger
adsorption at the surface [32]. Since the surface of crystals is highly ordered
and stiff, to obtain the maximal interactions by matching the structure of the
substrate, it is desirable to have short and relatively flexible functional groups
attached to the backbone of additive molecules so that they can adjust their
positions to obtain optimal interactions with the solid molecules at the surface
of crystals.
3) The adsorbed additives should interrupt the growth of crystal layers along
the substrate [32]. The repulsions can originate from steric, electrostatic,
polar/non-polar, or hydrophilic/hydrophobic forces, and can be achieved by
attaching some functional groups to the backbone of themolecules of additives.

Large molecules such as polymers with molecular structure that satisfy the above
criteria have proven to be effective. Beside polymers, surfactants are a class of
chemicals that have interfacial adsorbing property. Due to its amphiphilicity, a
surfactant molecule can adsorb at the interface of two phases with different polarity
to minimize the interfacial tension and free energy. Therefore, surfactants can also
be suitable additives.

Polymer-Enhanced Fiber Branching Suitable polymer additives have been used as
branching promoters in directing the formation of three-dimensionally intercon-
necting fiber networks from a non-gelling paste-like material. It was observed that
the presence of a tiny amount of an ethylene/vinyl acetate copolymer (EVACP) ad-
ditive can facilitate the formation of self-supporting materials with highly branched
fibers (Figure 2.6h) of L-DHL (lanosta-8,24-dien-3

-ol:24,25-dihydrolanosterol
56 : 44) in the solvent di-(2-ethyl-hexyl) phthalate (DIOP) [6c]. Without this ad-
ditive, L-DHL crystallized into unbranched short fibers (Figure 2.6g), which led to
the formation of a paste-like material. The branching promotion by this polymer
is due to its strong adsorption on the tip surface of L-DHL fibers, which leads

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