ORDER PARAMETERS OF 2 PHASE-TRANSITIONS IN MIXTURES OF ELLIPSOIDAL AND SPHERICAL MOLECULES

The model is the ES mixture of ellipsoidal (E) and spherical (S) molec ules dispersed on a b.c.c. lattice with a ratio of concentrations diff erent from 1:1. Minimising numerically the effective thermodynamic pot ential, obtained in a ''modified'' mean-field approximation, the varia tion of the orientational and positional order parameters for differen t values of concentration, molecular parameter (of ellipsoidal molecul es), and interaction constants are studied. The coupling effects arisi ng from the interplay between orientational and positional degree of f reedom are also checked. Determining the critical temperatures for two kinds of transitions the phase diagram (the critical temperature-conc entration diagram) is obtained, where the transition lines define four regions corresponding to the completely disordered mixture, the only positionally ordered mixture, the only orientationally ordered mixture , and the positionally and orientationally ordered mixture. Thus, in l arge lines, the model of the ES mixture reproduces qualitatively some features of the behaviour of plastic and liquid crystals.