The Onsager theory for the isotropic-nematic phase transition in a solution of rodlike particles is extented to the case of mixtures of such rods with different lengths. It is found that there is a significantly higher molefraction of long rods in the anisotropic phase than in the isotropic phase and a concomitant enhancement of the orientational order of these particles. This explains the fractionation that occurs between the isotropic and anisotropic phase of polydisperse solutions of rodlike polymers.
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Deblieck et al. (1980) studied this question.