Organic compounds exhibiting the smectic C phase are made of rod-like molecules that have dipolar groups with lateral components. We argue that the off-axis character of the lateral dipolar groups can account for tilt in layered smectics (SmC, SmC*, SmI etc.). We develop a mean-field theory of the smectic C phase based on a single-particle potential of the form UC is proportional to sin(2theta) cos phi, consistent with the biaxial nature of the phase, where theta and phi are the polar and azimuthal angles, respectively. The hard-rod interactions that favour the smectic A phase with zero tilt angle are also included. The theoretical phase diagrams compare favourably with experimental trends. Our theory also leads to the following results: i) a first-order smectic C to smectic A transition above some value of the McMillan parameter alpha, leading to a tricritical point on the smectic C to smectic A transition line and ii) a first-order smectic C to smectic C transition over a very small range of values of the model parameters. We have also extended the theory to include the next higher-order term in the tilting potential and to include the effect of different tilt angles for the molecular core and the chain in the SmC phase.

A simple molecular theory of smectic C liquid crystals smectic C smectic A nematic isotropic n n q n q c O molecule X Y n q Z Chapter-6-162-Z-X plane, containing the director n . The long axis of any given molecule makes an angle with the positive Z axis, angle with n , and its projection in the X-Y plane makes the azimuthal angle with respect to the positive X axis. The relation between , and is given by cos = cos cos + sin sin cos 6.1)The molecular centres are randomly distributed within the layers in the X-Y plane as in the SmA liquid crystal.Since the nematic director is apolar, rotating the medium by 180 0 about the Y axis results in an identical arrangement i.e., there is a two fold axis of symmetry along the Y axis. Also, the Z-X plane which contains the director n is a mirror plane of symmetry. These two symmetries together represent a point of inversion about the origin O. Hence the medium can not sustain any net polarisation. Other smectic liquid crystals with molecular tiltIt has been recently found [4] that some compounds exhibit the smectic phase in which the direction of vector c of the successive layers is opposite i.e., it has anticlinic symmetry (SmC alt ). The suffix 'alt' stands for 'alternate' i.e., the inlayer projection of n is along the positive and the negative X directions in successive layers.The liquid crystal still can not sustain any net polarisation. A very interesting type of smectic liquid crystal (SmC*) results if the molecules are chiral. In the SmC* liquid crystals, each layer still has a two fold axis of symmetry along the Y axis, but there is no orthogonal mirror plane. The medium exhibits ferroelectric polarisation of the layers in the direction perpendicular to the tilt plane. However, due to chiral interactions, the direction of polarisation develops a gradual twist from layer to layer i.e., it becomes helical about the Z axis and the structure is heli-electrical [3]. Since the polarisation vector tends to align with an external electric field, the helix can be unwound by applying an electric field perpendicular to the Z axis. The molecular ordering in the unwound SmC* resembles that of the SmC, but the medium is ferroelectric. The helix can also be unwound by placing the SmC sample in a thin cell (thickness 2 m) whose surfaces are in the Z-X plane. Such a surface stabilised ferroelectric liquid crystal (SSFLC) exhibits a very useful electro optic effect when placed between appropriately oriented crossed polarisers. The field of view is dark Chapter-6 -163when the incident light is linearly polarised along n . If the direction of the applied electric field is reversed, the direction of the average tilt is also reversed, resulting in a change of 2 in the direction of n . Since the optic axis is parallel to the director, this results in a corresponding change in the direction of the optic axis of the medium and the field of view becomes bright. The dark field can be restored by rotating the cell by an angle 2 about the direction of electric field. This helps in the measuremen...

No abstract

No abstract

Experimental observations show that the nematic order parameter is signicantly enhanced as the thickness of the cell is reduced. Calculations of earlier theories do not agree well with the experimental data. We propose a simple extension of MaierSaupe theory to account for the enhancement. A molecule near the surface is assumed to feel the mean eld potential (the MaierSaupe type) and also the surface induced potential. This is included in the MaierSaupe theory. Our calculations are in better agreement with the experimental data.