The spontaneous polarisation and rotational viscosity in host-dopant mixtures

Abstract

In our search for systematic features we have studied the appearance of spontaneous polarisation in some simple cases of non-chiral smectic C mixtures with added chiral dopant. The electro-optic properties have been measured for each system and evaluated in the simplest possible dynamic model. In this way we determined the tilt angle, the polarisation and the rotational viscosity. Some methodological comments are made on the measurements, because their simplicity makes them well adapted for large scale screening of new compounds. If the host-dopant is a binary system we find that the coupling constant μ = ∂P/∂θ between polarisation and tilt behaves in a regular way. This is also true for the viscosity, which has an Arrhenius behavior with a dopant independent prefactor and an activation energy barrier increasing almost linearly with the dopant concentration. When the host is itself a binary mixture of fixed composition we find essentially the same behavior but with a lower value of pμ (i.e. the binary host gives a weaker rotational bias to the dopant molecules). If, on the other hand, we kept the dopant concentration constant while changing the relative composition of the host components, the situation is much more complex, with a viscosity that may exhibit maxima or minima, whereas the activation barrier remains constant. The coupling coefficient μ is still regular, being a linear function of the relative composition.

Keywords:

• Ferroelectric liquid crystals
• spontaneous polarisation
• rotational viscosity