Demixing in isotropic binary mixtures of rodlike macromolecules

A binary mixture of long rigid rods of diameters D-i and lengths L-i (i = 1 , 2) may demix into two isotropic phases. and we give necessary conditions on the molecular size parameters for this transition to exist. These condit ions imply th;lt the two diameters must be sufficiently unequal, D-2/D-1 > (9/7+4/7 root 2)(2), or D-2/D-1 < (9/7-4/7 root 2)(2), while the length rat io is limited to an interval f_(D-2/D-1) < L-2/L-1 < f(+)(D-2/D-1). The fun ctions f(+/-) are given explicitly.