THE ISOTROPIC TO NEMATIC LIQUID-CRYSTAL TRANSITION FOR FLEXIBLE NONSPHERICAL MOLECULES

A torsionally flexible molecule is modeled as having a hard convex cor e and flexible caps in the polar regions. During an isomerization, the aspect ratio of the molecule changes from its minimum value in the ci s position to its maximum in the trans conformation. Intermolecular po tentials arise from hard core repulsion and repulsion from a square sh oulder potential that operates only when the flexible portions overlap . When this potential is used in an Onsager-density functional theory, extended to account for the torsional degree of freedom, both the sha pe of the molecule and the order parameter are density and temperature dependent. As the density increases in the isotropic phase, the molec ule becomes increasingly spherical. At the isotropic, nematic transiti on, the molecular shape abruptly elongates and this elongation continu es with further increases in density. Both the increased sphericalizat ion and the increased elongation are small effects (1%-2%) even for 5: 1 and 10:1 molecules undergoing a 20% length fluctuation. The primary order parameter [P2] and the relative density change at the isotropic, nematic transition are nearly independent of the amplitude of torsion al fluctuations. This study suggests that for estimates of the density changes at the I-N transition, hard body models should be sufficient.