Kevin Adlem

Improved Liquid Crystal Mixtures for STN Displays

Supertwisted nematic (STN) liquid crystal displays remain a leading technology for mobile communications applications, personal digital assistants (PDA) and personal convergence devices. Higher demands on display information content place ever increasing demands on the physical parameters of the liquid crystal mixture. To satisfy current and future display requirements, it is necessary to develop mixtures showing: ? Fast response times achieved by rotational viscosity reduction and birefringence increase. ? Small temperature dependence of the operating voltage (dV/dT), especially at low temperatures. ? Small frequency dependence of the operating voltage (dV/df), necessary for multi-level greyscale operation. ? Improved steepness (V90/V10) of the electro-optical curve to enable higher duty multiplexing of the display. In order to fulfil these requirements, liquid crystal mixtures with improved physical properties were developed. Examples of these mixtures are presented.

Parallel Synthesis of 4-alkyl-4'-cyanobiphenyl Liquid Crystals

In the search for novel nematic liquid crystal materials for display applications, it is common to investigate several different alkyl chain homologues of a particular core structure. It is sometimes possible to develop a synthetic pathway that allows homologous variation in the ultimate step, but the majority of published methods for liquid crystal synthesis begin from an alkyl chain containing intermediate. This results in a separate pathway for each homologue. A “one-pot” technique has been developed for the parallel synthesis of up to five homologues of a given core structure, exemplified by “the 4-alkyl-4′-cyanobiphenyl series, together with their isolation in gram quantities and high purity by preparative high performance liquid chromatography. The technique demonstrates that it is possible to identify and monitor each intermediate throughout the synthetic pathway, using gas chromatography/mass spectrometry. It is also shown that there are no substantive losses in yield for any homologue throughout the pathway. It is concluded that this technique is a viable method for the synthesis of novel liquid crystal materials.