Engineering of Small-Molecule Gels Based on the Thermodynamics and Kinetics of Fiber Formation - Soft Fibrillar Materials: Fabrication and Applications - page 75

2
Engineering of Small-Molecule Gels Based on the
Thermodynamics and Kinetics of Fiber Formation
Jing-Liang Li and Xiang Yang Liu
2.1
Introduction
A small-molecule gel (SMG) is a typical class of supramolecular materials. An
SMG forms when a hot solution of a small-molecular gelator is cooled to a certain
temperature, namely gelation temperature T g . The gelator molecules, at a concen-
tration in excess of the equilibrium concentration at T g , self-organize into a three
dimensional (3D) fibrous network.
The first attempts to design molecular gelators can be dated backed to mid-1990
[1]. Although increasing understanding of the structural properties of molecules
required for self-assembly and fiber formation has been acquired since then [2],
the design of new gelators continues to be largely a trial-and-error process [2a].
Moreover, the strong solvent dependence of the gelling capacity of a gelator also
makes the design of new gelators a hard task [1c, 3]. If interconnecting 3D micro-
or nanofiber networks with the required organization can be constructed, ''new''
functional materials with the required functionalities can be produced [4]. This
design-and-production approach based on the reconstruction of the micro/nano fibril
structure of gels has become a robust and innovative route in producing new soft
functional materials. To develop an efficient approach to engineering the 3D fiber
network of an SMG, an understanding of the fiber network formationmechanism is
required. The self-assembly model explains how the fiber network is architected by
the self-assembly of small molecules through noncovalent interactions. However,
despite some success of this model in interpreting the experimental observation, it
is incapable of explaining some important phenomena in an SMG, for example,
the fiber branching. How does it occur, and is it possible to control the fiber
branching? In this context, the self-assembly model cannot provide a global view
on fiber network formation and engineering.
Interestingly, it was proven that the fiber network formation in SMGs is a
crystallization process, which consists of the nucleation and growth of fibers. On
the basis of this finding, the micro/nano structure of fiber networks in several
SMGs and their macroscopic properties were successfully manipulated by tuning