A simple molecular theory of smectic-C liquid crystals

Abstract

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...

Synthesis, mesomorphic properties and X-ray diffraction studies on 7-alkyloxy-3-(4-alkyloxyphenyl-4H-1-benzopyran-4-one: Crystal structure of 7-hexyloxy-3-(4-hexyloxyphenyl)-4H-1-benzopyran-4-one

“…7). The tilt angle in SmC phase could not be ascertained owing to a first order transition [21], a transition that involved a discontinuity in entropy [22]. Hence, the layer spacing only reflects the conformation of the molecules during phase transition.…”

Molecular model for de Vries type smectic-A–smectic-Cphase transition in liquid crystals

“…Molecular models for the SmC phase have been proposed by a number of authors [5,16,17,18,19,20]. Most of the theories assume the long molecular axis to be very highly oriented.…”

Comment on “Origin of tilted-phase generation in systems of ellipsoidal molecules with dipolar interactions’’

“…We have correlated the observations on smectic-C forming compounds with this picture, and we have extended the highly successful McMillan model of the smectic-A phase [10] to include an interaction arising from this process to develop a mean field theory of smectic-C liquid crystals. The theory [8,9] qualitatively describes most of the phase diagrams found in real compounds.…”

“…Returning to the dipolar origin of the tilting in the smectic-C phase, we [8,9] argued some time ago that the molecular structures of compounds exhibiting that phase are such that the transverse dipoles are not located on the molecular long axes, but slightly shifted away. In that case, the net dipolar interaction between neighboring freely rotating molecules in a smectic-A layer will be repulsive.…”