INSTABILITY OF A NEMATIC PHASE DESCRIBED BY THE MAIER-SAUPE MODEL

The stability of a nematic phase in binary mixtures of Liquid crystals (LC) with non-nematogenic solvents, was analyzed using the combined F lory-Huggins and Maier-Saupe models. Stability was described using gen eral criteria developed by Dorgan [Liq. Crys. 10, 347 (1991)], applied to the g(phi(1),s) function describing the free energy density of the system (phi(1)=volume fraction of LC, s=orientational order parameter ). Equivalent criteria were obtained by replacing s by the particular s(phi(1)) function that minimizes g and making d(2)g[phi(1),s(phi(1))] /d phi(1)(2)>0, to define stability. Recent results reported by Shen a nd Kyu [J. Chem. Phys. 102, 556 (1995)] were reanalyzed using general stability criteria. It was found that the spinodal curve for the nemat ic phase extended beyond the limit fixed by these authors. In particul ar, the spinodal enters the region where the isotropic phase is the st able one, in agreement with other calculations reported by Dorgan for a nematic phase described by a modified Flory-Ronca model. A pure LC m ay exist in a metastable state in a small temperature range located ab ove T-NI (equilibrium temperature of the nematic to isotropic transiti on of the pure LC). (C) 1998 American Institute of Physics.