Role of side chains in phase equilibria of disklike mesogens

A theory of phase equlibria in solutions of discotic mesogens of a hard-cor e disklike center and attached side chains is developed in the Flory lattic e approximation. The role of the side chain stiffness and length in the pha se equilibria is studied numerically in detail for two cases of the hard-co re size, x=7 and 10. It is found that the chain stiffness has a profound ef fect on the molecular ordering, with stiff chains increasing and soft chain s decreasing the nematic order parameter S. The critical solute concentrati on for the first appearance of the stable nematic phase upsilon(x)* shifts towards lower values for stiff chains and to higher values for flexible one s. In addition, the soft chains suppress while the rigid ones enhance the c oexistence range of two nematic phases observed for x similar or equal to 9 .1 [M. Wnek and J. K. Moscicki, Phys. Rev. E 53, 1666 (1996)]. [S1063-651X( 98)04909-5].