Bilayers in nanoparticle-doped polar mesogens.

Structures of the mesophases of five members of the 4-n-alkyl-4'-cyanobiphenyl homologous series (4-n-butyl-4'-cyanobiphenyl to 4-n-octyl-4'-cyanobiphenyl) doped with milled BaTiO_{3} nanoparticles were investigated by x-ray scattering. Clear solutions of each of the 4-n-alkyl-4'-cyanobiphenyls were first prepared in n-heptane and then doped with an n-heptane/nanoparticle dispersion, which led to gelation. The nanogels were found to be one-dimensional, multilayered, smectic nanostructures in each case. Surprisingly, a characteristic layer spacing of 4.5 nm was observed in all five homologues. Synchrotron x-ray scattering study of the multilayer structures of doped 4-n-pentyl-4'-cyanobiphenyl and 4-n-octyl-4'-cyanobiphenyl revealed nine orders of the primary Bragg reflection which were used to calculate the electron density profiles of the multilayers by Fourier analysis. The multilayers were found to consist of molecular bilayers wherein the mesogens were arranged in a head-to-head assembly of the polar head groups. The alkyl tails of the mesogenic molecules were freely movable and the tail-to-tail assembly was stabilized by heptane. The dissolved nanoparticles clearly induced a new self-assembled nanostructure in which the rigid aromatic part, and not the overall length, of the molecules defined the layer spacing.