THEORY OF PHASE-SEPARATION FOR A SOLUTION OF TRIDISPERSE ROD-LIKE PARTICLES

Within the Onsager theory, we study solutions of polydisperse thin har d rods. An approximate method by Odijk is used numerically to determin e the orientational distribution functions in the nematic phase in a s traightforward way. Next, the coexistence relations are used in determ ining phase transitions. In coexisting nematic phases equality of osmo tic pressure implies equal overall number density. A procedure is deve loped to locate equal chemical potentials between nematic phases of tr idisperse systems. For length ratios larger than 3, we find phase diag rams showing nematic three-phase regions, critical points, critical en d points and as a limit a tricritical point. The mechanism is not due to excluded-volume effects but to the interplay between orientational entropy and entropy of mixing. At higher concentrations the phase diag ram is independent of concentration. Below a certain concentration, co existence with an isotropic phase intervenes. (C) 1997 Elsevier Scienc e B.V.