Antiferroelectric siloxane liquid crystal dimers with large molecular tilt under an applied electric field

The scope of the present study is the response of a series of antiferroelectric dimeric or bi-mesogenic siloxanes to an applied electric field with focus on their pretransitional behaviour and the field-induced antiferro-ferroelectric (AF-F) transition. Most of these compounds possess a molecular tilt close to 45° and spontaneous polarization in the field-induced ferroelectric (F) phase in the range of 250-300 nC cm^-2. In the dimers with a spacer length exceeding five carbons, a transformation from first to second order of the field-induced AF-F transition is found with temperature. Several different indications for this transformation are identified and their characteristics are discussed in the framework of the existing theoretical models. A large field-induced in-plane deviation of the sample optic axis was observed in the pretransitional region of several of the siloxane dimers and is likely due to the flexibility of the linking chains. The potential of the antiferroelectric bimesogenic siloxanes for displaying high contrast images and grey scale capability is shortly discussed. The large molecular tilt close to 45° in combination with the field-induced AF-F transition of second order seems to be the most attractive features of these materials.