Liquid crystal materials for use in display devices and light shutters are broadly of two types: nematic and smectic.
Nematic materials have long range orientational order, with molecules being free to move past one another in different directions while retaining a statistically parallel orientation of their long axes (‘the director’). The nematic phase is highly fluid and is used in display modes such as twisted nematic, supertwist, hybrid aligned mode, and bistable modes such as ZBD and PABN. Nematic polymer-dispersed liquid crystals (PDLC) are used in switchable glazing technology. However, they require constant power to maintain an ON-state and, moreover, exhibit unwanted haze at wide viewing angles.
Smectic liquid crystals have a layer structure and are more viscous than nematic materials. Molecules have orientational and positional order within a layer, but layers can move relative to each other. A smectic material may exist as one of a number of possible polymorphic modifications, depending on the arrangement of molecules within the layers. For example, molecules in a Smectic A phase have their long axes statistically perpendicular to the plane of the layers and the lateral distribution of the molecules within a layer is random. Molecules in a Smectic C phase also have a random lateral distribution but have their long axes tilted with respect to the plane of the layers.
Smectic A materials have been used in bistable displays (FIG. 1) which are written by inducing a highly scattering texture resulting from the motion of doped charged impurities under dc or low frequency ac electric fields. Erasure is by dielectric reorientation, at higher ac frequencies, to an optically clear state.
Smectic C materials have found use in fast-switching bistable displays based on the discovery that optically active Smectic C materials are ferroelectric, anitferroelectric or ferrielectric, and can be rapidly switched between two states if a suitably aligned thin (1-2 μm) layer is used.
A smectic material may exhibit different polymorphic modifications at different temperatures, and may reversibly transform to a nematic material at higher temperatures before becoming an isotropic liquid at a still-higher temperature (the clearing temperature).
Much work has been done by formulation chemists to produce mixtures of liquid crystal materials which are stable in the nematic phase over a wide temperature range, including temperatures both substantially above and substantially below room temperature, for both storage and operational purposes. A large number of wide temperature-range nematic materials have been made and are commercially available, typically with a number of components whose properties and proportions are carefully optimised to suit a particular display application. However, wide temperature-range smectic materials, in which the material exhibits a single polymorphic modification throughout the specified range, are not widely available and may not be optimised for different applications.
Aspects of the present invention are specified in the independent claims. Preferred features are specified in the dependent claims.
We have surprisingly found that a wide temperature-range smectic liquid crystal material may be made by taking a wide temperature-range nematic mixture and doping this with a mesogenic silicon-containing material. The host nematic mixtures may have been optimised in various ways (e.g. dielectric and optical anisotropies, switching speed etc.). Nematic mixtures are very complicated formulations, precisely formed in a balanced way; indeed it is known that some mixtures may require up to 20 individual components to achieve the necessary characteristics. Improper mixing of these components, or the presence of other additives, usually leads to unexpected and negative effects on the performance of the mixture. It is therefore a very surprising result that the addition of siloxane to such a complex mixture still generates a smectic phase of a practical temperature range. Additionally, these smectic mixtures are further useful since they retain, to a large extent, the other desirable characteristics of the host nematic material (e.g. optical anisotropy).
We have found that by forming specific mixtures of organosiloxane liquid crystals and non-siloxane liquid crystal materials, the overall material performance can be dramatically enhanced and tuned according to requirement. In addition, it is possible to use pseudo-LC organosiloxane materials as the additive. The pseudo-material contains an organosiloxane moiety and may induce smectic ordering within the mixture. These additives may contain chiral agents, chromophores, dichroic or fluorescent dyes, dielectric or refractive index enhancers or reducers, or others, for example.