Real Space Imaging and Molecular Packing of Dendronized Polymer−Lipid Supramolecular Complexes

Abstract

The present work describes the molecular structure and the topology in real space of self-assembled comblike liquid crystalline polymers based on supramolecular ionic complexation of dendronized polymers and lipids. The molecular organization was studied as a function of the generation of the dendronized polymers and the length of the lipid tails, both parameters being known to strongly influence the final liquid crystalline (LC) structures stable at thermodynamic equilibrium (Macromolecules 2007, 40, 2822-2830). Transmission electron microscopy (TEM) on selectively stained samples allowed imaging of the structures in real space, the identification of individual domains in the various LC phases, and direct comparison of lattice periods with values obtained by small-angle X-ray scattering (SAXS). TEM and SAXS were found in perfect agreement, and surprisingly, the alkyl tails were shown to form the discrete domains in all LC columnar phases. On the basis of these results, various possible molecular packing motifs in the LC phases were considered. First, the combination of density measurements of individual dendronized polymers and lipids with the ratio of lipid-dendronized polymer complexation, measured by elemental analysis, allowed establishing the exact volume fraction of the two supramolecular components and thus, the radii of cylinders and lamellae widths in the various columnar and lamellar LC phases observed. Then, by comparing these topological values with the contour length of the lipids, it was possible to conclude that, in all columnar phases, the lipids adopt a fully stretched configuration with homeotropic arrangement, while in the lamellar phase, lipids tails are essentially interdigitated. The control of lattice periods of LC structures in the range of 2-5 nm and the possibility to confine the lipids in the cylindrical domains make these materials as possible templates for nanoporous materials and ultra densely packed nanochannels.