Multilayered crystalline structures and liquid crystalline phases in a mesogen with siloxane tails

Abstract

N,N′-Terephthalylidine-bis-4-(2 pentamethyldisiloxyethyl)aniline, TBS2A, which contains a mesogenic group with two di-siloxane tails, and TBS3A with two tri-siloxane tails were synthesized. Liquid crystalline behaviour in these compounds were studied by DSC and polarized light microscopy. Temperature-induced structural transitions in TBS2A were studied using variable temperature X-ray diffraction. At least three structural phase transitions were observed: crystal-crystal transformation (from Cr2 to Cr3) at 125°C, crystalline phase (Cr3) to a smectic liquid crystalline phase (S_b) at 140°C, and the melting of the S_b phase at 152°C. A room temperature crystalline phase (Cr1) with a layer repeat of 29.5 Å is also observed. The low temperature Cr2 phase is characterized by a layer repeat of 26.3 Å and exhibits a long spacing of 105 Å corresponding to a repeat unit of four layers. The high temperature Cr3 phase has a layer repeat of 22.2 Å and exhibits a long spacing of 44 Å from a repeat unit with two layers. The mesophase has a single layer with a repeat of 22.2 Å. The di-siloxane tails in the Cr3 and the mesophase are fully interdigitated. The isotropic melt shows a diffuse halo at 24 Å corresponding to the separation of adjacent layers, and a second halo at 6.5 Å attributed to the separation of the mesogens/chains within the layers. These results are interpreted with structural models.