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Soft Matter and Biological Physics
Eur. Phys. J. E 9, 107-126 (2002)
DOI: 10.1140/epje/i2002-10060-x

A molecular theory of smectic C liquid crystals made of rod-like molecules

A.S. Govind and N.V. Madhusudana

Raman Research Institute, C.V. Raman Avenue, Bangalore - 560 080, India

nvmadhu@rri.res.in

(Received 3 August 2002 / Published online: 3 December 2002)

Abstract
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 $U_{\rm C} \propto
\sin (2 \theta) \cos \phi$ , 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.

PACS
61.30.Cz - Molecular and microscopic models and theories of liquid crystal structure.
64.70.Md - Transitions in liquid crystals.
64.60.Cn - Order-disorder transformations; statistical mechanics of model systems.

© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2002