Low viscosity nematics : design of dopants

Abstract

The rotational viscosity, γ₁, of a nematic liquid crystal is of special importance because it determines the switching times for display devices.  However, the understanding of the molecular factors that control γ₁ is not well-developed.  This thesis details an experimental investigation into the relaxation dynamics of a wide range of nematogenic mixtures.  The dimeric materials used as dopants in the host nematic mixture, ZLI-4792, have been synthesised and their effect on the host studied.

There are a variety of techniques available for the determination of γ₁ and a direct method in which a sample with a uniformly aligned director is initially inclined at an angle to a magnetic field has been used.  Since the diamagnetic anisotropy is positive, the director in this orientation is unstable and it realigns parallel to the field; the relaxation time for this process is directly proportional to the rotational viscosity coefficient. To determine the relaxation time it is necessary to measure the time dependence of the angle between the director and the magnetic field.  ESR spectroscopy provides a powerful technique with which to do this.  A novel ESR spectroscopy experiment has been devised which allows the determination of the field-induced director relaxation time from a single ESR spectrum.  During the investigation into the dynamics of the doped nematic mixtures an unexpected gelling behaviour was observed for four of the dimeric dopants.  These gelled systems were then studied a range of techniques in order to develop an understanding of this novel behaviour.  Crystal structures of the pure dopants show that the molecules pack in a sheet array; this suggests the network, which traps the liquid crystal, has a sheet structure.  The form of the ESR spectrum obtained for the gelled systems shows that the magnetic field is unable to align the director, indicating that the director is pinned at the surface of the sheets.  Some Trimeric dopants were also synthesised and their gelling behaviour studied.

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