Order parameter theory of the pre-transitional behaviour in antiferroelectric liquid crystals

We propose an order parameter theory as a possible description of the pre-s witching behaviour of antiferroelectric liquid crystals (AFLCs). Assuming t hat the antiferroelectric state is preserved so that the directors in adjac ent layers ape on opposite sides of the smectic cone, we consider changes i n the molecular order about these directors. Our theory incorporates interl ayer and intralayer dipole-dipole interactions, the dipole-electric field i nteraction and the thermodynamic potential. The effective optic axis as a f unction of applied voltage can then be calculated and is found to be in qua litative agreement with experimental observations which show a superlinear dependence.