Abstract
Nature elegantly utilizes the self-assembly of supramolecules to construct functional superstructures. The self-assembled superstructures are formed by cooperatively secondary forces such as amphiphilic effect, polar ability, hydrogen bonding, columbic interactions, van der Waals forces, metal coordination, ionic bonding, and the chirality. Among them, the chiral effect on the self-assembly is essential for the formation of helical morphology. A variety of helical morphology including helical conformation, hierarchical helical structures and helical crystalline morphology have been observed in the self-assembly. Recently, helical superstructures have been obtained from the self-assembly of amphiphilic block copolymers containing charged helical blocks in buffer solutions. The chiral effect on the self-assembly of block copolymers (i.e., coil-coil molecules) is essential for the formation of helical morphology. In contrast to coil-coil molecules, the self-assembly rod-coil molecules possess strong segregation strength for phase separation due to the characteristics of rod segment. As a result, they are able to self-assemble into periodic textures even for small oligomeric molecules so as to provide an excellent system for the examination of chiral effect on self-assembled superstructures. Amphiphilic molecules offer numerous opportunities for chemical variations, and thus provide a crucial direction for the controlled fabrication of superstructures. The self-assembly of chiral lipid molecules (i.e., chiral amphiphiles) has been extensively studied, and a variety of dynamically changing morphologies in liquid crystalline (fluid) state were observed. Carbohydrate sugars provide a rich library of chiral building blocks, which are also biocompatible that makes them attractive candidates for being used successfully in the design of self-assembly chemistry. The morphologies of sugar-based amphiphiles affected by the introduction of different hydrophilic or hydrophobic parts and of unsaturation in the lipophilic moiety have been thoughtfully studied. However, how chiral information transfers from primary molecular structure to quaternary aggregates for the self-assembly system is still a challenging course to scientists. Recently, our research group demonstrated a novel nanohelical structure that was obtained from the self-assembly of chiral block copolymers, poly(styrene)-block-poly(L-lactide) (PS-PLLA). Here, we simply introduce a sugar entity of (i.e., a chiral entity) to rod-coil molecules. The self-assembly of the sugar-based rod-coil amphiphiles gives rise to a variety of interesting morphology. The central theme of this study is to understand the self-assembly processes by exploring various secondary forces, in particular the effect of chirality. We attempt to examine the self-assembly mechanisms in different environments including solution and bulk states so as to understand the kinetic processes of assembly. The studies of sugar-based rod-coil amphiphiles have been carried out by differential scanning calorimetry (DSC), ultraviolet-visible spectrum (UV), circular dichroism (CD), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), and scanning probe microscopy (SPM) experiments. Sugar-based rod-coil amphiphiles exhibited both the lyotropic and thermotropic liquid crystalline behavior from our works. It is noted that complicate self-assembly process is involved in solution state. The sugar-based rod-coil amphiphiles appeared positive Cotton effect materials, and there is no odd-even effect with respect to the alkyl chain length of coil. Interestingly, we observed that the alkyl chain length of LC9 might be long enough to interrupt the arrangement of rod segments driven by liquid crystalline phase transformation. The self-assembly of LC9 and LC11 in solution or from melt appeared left-handed helical morphology as observed by TEM, FESEM and SPM. Surprisingly, banded spherulites were observed in the self-assembly of the chiral sugar-based rod-coil amphiphiles in bulk or thin film. The formation of the superstructure is driven by thermotropic liquid crystal behavior whilst the chiral effect of sugar-based entity induces the twisting of molecular aggregation. As a result, the transfer of chiral information from molecular level to quaternary superstructure can be identified.