The liquid crystalline phase behavior of dimerizing hard spherocylinders

Abstract

The phase behavior of dimerizing (associating) rigid particles is studied by both theory and computer simulation. The model molecule comprises a hard spherocylinder of length L and diameter D with a terminal square well bonding site embedded in one of the hemispherical caps. This model mimics the properties of simple hydrogen bonding mesogens; for example, mesogens with a carboxylic acid end group which are capable of forming dimers. A recently proposed theory of the isotropic (I)-nematic (N) phase transition for long hard spherocylinders with an attractive site at one end [R. P. Sear and G. Jackson, Mol. Phys. 82, 473 (1994)] is extended to shorter molecules. In the original theory the free energy is truncated at the level of the second virial coefficient. We now include the higher virial coefficients in an approximate manner with a Parsons type scaling. The accuracy of the theory is demonstrated by comparison with novel Monte Carlo simulation data for the same model. Excellent agreement is found for densities, pressures and degrees of association especially at the liquid crystalline phase transition. In comparing the results for the L/D=5 associating system with those for its nonassociating analogue, the nematic phase is seen to be stabilized relative to the isotropic phase, while the nematic (N)-smectic-A (SmA) transition occurs at approximately the same density. The I-N transition for the dimerizing system is clearly first order, while the N-SmA is essentially continuous. The smectic-A phase has a monolayer structure which is similar to that formed by the nonassociating system. Furthermore, a system of otherwise nonmesogenic molecules with L/D=3 has a stable liquid crystal phase when dimerization is made possible with the inclusion of the terminal bonding sites. Rather than being a nematic phase, this phase is surprisingly found to have the layered structure of a smectic-A phase. We discuss our results in terms of the increase in the ‘effective’ aspect ratio as a result of association.