Design of liquid crystalline block molecules with nonconventional mesophase morphologies: Calamitic bolaamphiphiles with lateral alkyl chains

Novel bolaamphiphiles, consisting of a biphenyl rigid core, polar 2,3-dihyd roxypropoxy groups at each terminal end, and an additional long alkyl chain in a lateral position have been synthesized. The structures of these terna ry block molecules were systematically changed by variation of the length a nd position of the alkyl chain, by introduction of additional spacer units between one of the 2,3-dihydroxypropyl groups and the rigid core, and by re placement of one of the 2,3-dihydroxypropoxy groups by a single hydroxy gro up. The influence of these structural variations on the liquid crystalline properties of these new materials was investigated by polarized-light micro scopy, differential scanning calorimetry, and X-ray diffraction. These inve stigation have shown that, by elongation of the lateral chain, a transition from a smectic monolayer structure (SmA(1)) via a strongly distorted layer structure (SmA(+)), a centered rectangular columnar phase (Col(r)/c2mm) an d a noncentered rectangular columnar phase (Col(r)/p2gg) to a hexagonal col umnar phase (Col(h)/p6mm) takes place. Elongation of the bolaamphiphilic co re leads to the loss of the columnar phases, which are replaced by smectic phases, whereas reduction of its length favors the hexagonal columnar phase . This phase sequence is explained as a result of the microsegregation of t he lateral alkyl chains from the rigid aromatic cores. The segregated alkyl chains are organized in columns that interrupt the smectic layers. The hyd rogen bonding keep the bolaamphiphilic cores fixed end to end, so that they form networks of cylinders around the lipophilic columns. The space requir ed by the alkyl chains with respect to the length of the bolaamphiphilic co res is restricted and largely determines the geometry of the cylinders, whi ch leads to the observed phase sequence. The obtained mesophase morphologie s, built up by three distinct sets of subspaces, are related to morphologie s of some ternary block copolymers. Furthermore, the investigated compounds represent a novel class of materials, capable of forming supramolecular co lumnar mesophases.