Investigations of a homologous series of chiral siloxane based dimesogenic compounds

Abstract

A homologous series of siloxane based dimesogens was synthesized in racemic and enantiomeric form. The flexible part of the core is composed of a central hexamethyltrisiloxane unit and two polymethylene chains of variable length. Increasing the number of methylene units in the core promotes smectic phase behaviour whose polymorphism was studied by polarization optical microscopy. Transitions into metastable phases were detected and investigated by differential scanning calorimetry using different heating rates. Information about the crystallization tendency was obtained from annealing experiments by changing annealing temperature and duration. In contrast to short dimesogenic homologues which predominantly showed monomesogen-like behaviour, polymer-like behaviour was observed for the longer homologues. Small angle X-ray diffraction provided data on the temperature dependence of the smectic layer spacing of mesogens and their corresponding dimesogens linked by the siloxane spacer. No significant differences between the layer spacings of the two classes of compounds were detected which suggests chain folding in the dimesogenic compounds and a similar mesogenic orientation for the mesogens and dimesogens.