The effect of dipolar interactions on the liquid crystalline phase transitions of hard spherocylinders with central longitudinal dipoles

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Abstract
The results of isothermal—isobaric Monte Carlo (MC-NPT) simulations for N = 1020 hard spherocylinders of aspect ratio L/D = 5 with embedded central longitudinal point dipoles are presented. The effect of the dipolar interactions on the phase transitions to the mesophases is examined. For this aspect ratio, non-polar hard spherocylinders exhibit both nematic (N) and smectic-A (SmA) liquid crystalline phases. This study examines the effect of the dipoles on the densities and pressures of the transitions from the isotropic (I) fluid to these liquid crystalline phases. The long range of the dipole-dipole interaction is accounted for by using the reaction field approach with a self-consistent treatment of the dielectric of the boundary; this approach gives results indistinguishable from the full Ewald summation, which is very computationally costly. At moderate temperatures, the transition from the isotropic fluid to the nematic liquid crystalline phases appears to be postponed to higher densities by the inclusion of the dipole. This rather surprising result could stem from a reduction in effective aspect ratio of the aggregates as a result of anti-parallel side-by-side dipole pairing. As expected, the smectic-A phase is found to be stabilized with respect to the nematic phase as a consequence of the strong dipolar interactions that are possible within a layer. Indeed, at lower temperatures, the nematic phase vanishes altogether and an I—SmA transition is observed. Therefore the existence of an I-N—SmA triple point is predicted. The occurrence of a re-entrant nematic phase for this system cannot be ruled out. Unlike the non-polar system, the ordering transition exhibits appreciable hysteresis: at low temperatures glassy states are obtained by compressing the isotropic fluid, and a heating and annealing cycle is required to obtain stable smectic-A phases. This means that it is difficult to locate the precise positions of the phase transtions at lower temperatures, and care should be taken in interpreting the results. No evidence of ferroelectric behaviour is found in any of the states examined, even at low temperatures.