Molecular ordering of organic supramlecules at the ultra-thin film

Abstract

The molecular ordering of the synthesized supramolecules is strongly affected by the functionality, molecular shape and surface anchoring. The thermal analysis was performed by differential scanning calorimetry (DSC). Synchrotron X-ray diffraction (XRD), polarized optical microscopy (POM) results show that the asymmetric fan-shaped second generation dendrimer containing a $CO_2C_3H_7$ core group of the cylinder and dodecyl ($C_{12}H_{25}$) alkyl tails exhibits hexagonal columnar mesophase upon cooling from isotropic phase. Transmission electron microscopy (TEM) images show that the columns are strongly affected by the substrates and film thickness: The columnar aligned perpendicular to hydrophobic substrate, and it aligned parallel to the hydrophilic surface. Different morphologies are encountered depending on the thickness. Thick films of such a material exhibit highly ordered hexagonal structure where the columns are oriented either parallel or perpendicular depending on the substrate. However, at the ultrathin films state, low surface energy planes are identified through examination of the morphology of the fan-shaped dendrimer, which contain hexagonally shaped holes on a free surface. The film forms into pinholes (with sharp edges) that have orientations related by an approximate hexagonal symmetry. The predominant direction of the hole edges corresponds to the (10) molecular lattice direction. In order to understand self-assembling mechanism of the planar ordering, we investigate the surface texture of Langmuir-Blodgett monolayers and multilayers of the dendrimer at the air-water interface. Surface pressure-area ($\pi$-A) isotherm and Brewster angle microscopy (BAM) measurements show that the molecule forms a stable monolayer on a water surface with two phase transitions (gaseous phase → expanded phase → condensed phase). The electron diffraction (ED) patterns, transmission electron microscopy (TEM) and atomic force microscopy (AFM) images of the conden...