Structural studies of liquid crystals and Langmuir monolayers of dendritic molecules.

摘要

Dendritic molecules are powerful building blocks in forming well structured crystals and liquid crystals not observed in either the more traditional rod-like mesogens or the newer discotic molecules. The ability of self assembly into disc- or sphere-like macro-molecular assemblies, or dendrimers, from monodendrons allows the formation of self-organized columnar or cubic structures of various sizes and symmetries. We have used small angle x-ray scattering to study a series of first-generation monodendrons in their mesophases. We found that the lattice parameters of the same mesomorphic structures increase as the sizes of contituent molecules are made larger. In certain conditions, a small change in monodendron architectures while maintaining their overall backbone geometry, can result in completely different liquid crystalline structures.; To explore the possible structures formed by these novel molecules in lower dimensions, we have used grazing incidence x-ray diffraction (GID) to probe the structures of monolayers of monodendrons at the air-water interface. Unlike the two-chain phospho-lipids of biological importance, we observed that our two-chain dendntic amphiphiles form highly ordered centered rectangular lattice with an uniform tilt of peripheral alkyl chains toward their nearest neighbors in the compressed solid phase. In a similar study of Langmuir film of three-chain monodendrons, the in-plane structure, however, is an oblique lattice with a tilt of alkyl chains toward a less symmetrical direction.; Complementary to the information obtained by GID, we applied x-ray reflectivity (XR) measurements to monolayers of a wide range of higher-generation monodendrons to investigate charge density distribution along surface normal. We found that monodendrons, in the compressed state, are oriented such that the hydrophilic cores are bound to water surface, the aromatic benzene rings form lower density sublayer just above water surface, and the hydrophobic alkyl chains form a high density sublayer at the top. We also observed, despite the difference among various monodendrons, a uniform increase in layer thickness in all monolayers as we further compressed the films to higher surface pressures while keeping them in the same solid phase. This qualitatively confirms that in the Langmuir monolayers of monodendrons there exist tilts in the alkyl chains, as quantitatively observed in our GID scans, and the tilt amplitude depends on the surface pressure.