Interlayer interactions and the dependence of biaxiality of the chiral smectic-C* phase on electric field in the helical unwinding process.

We investigate the unwinding process of pure and racemized mixtures of (1-methylheptyloxycarbonyl)phenyl- 4'-carboctyloxy-biphenyl-4-carboxylate (MHPOOCBC) in homeotropic cells by studying (i) the optical texture under crossed polarizers and (ii) the tilted conoscopy as a function of the electric field for different temperatures. The tilted conoscopy yields biaxiality and the tilt angle for values of the latter up to 45 degrees . We find that the unwinding process depends on the optical purity of the sample and the temperature. A cell with pure MHPOOCBC surprisingly exhibits a large positive biaxiality in the initial stages of the unwinding process. The magnitude of the positive biaxiality decreases with a decrease in the enantiomeric excess (EE) of the compound and eventually it becomes negative for a material having a low EE value. The negative biaxiality observed in a mixture with a low EE value is normal and can be explained by a double sine-Gordon equation proposed by Meyer. Microscopic observations show that the special unwinding behavior (positive biaxiality) is related to the helical fractures, that is, to the existence of discretely coiled helical structures. We show that these helical fractures arise from the existence of a narrow energy well in the synclinic position. This implies that the elastic constant near the synclinic position is significantly larger than in other positions. By adopting a semiempirical equation for the interlayer interactions which replaces the conventional equation, we successfully simulate the various unwinding behaviors. We reasonably conclude that the special interlayer interaction is due to the steric interactions between the two adjacent surfaces formed by somewhat tilted terminal parts of the molecules.