Semirigid macromolecules consisting of one or more rigid, rodlike sequences of units in combination with flexible (random coil) units are treated according to the adaptation of the lattice model to a system of rods. The lengths and locations of the rigid sequences within each macromolecule are considered to be fixed by its structure. They are separated by various specified numbers of random coiled units. Polydispersity is taken into account. General expressions are derived for the chemical potentials, and biphasic equilibria between nematic and isotropic phases are investigated. The presence of a very few random coiled units increases the disorder in the anisotropic phase, narrows the biphasic gap, and suppresses fractionation of species differing in length in polydisperse systems. The introduction of internal points of flexure, even in absence of intervening random units, produces similar effects to a degree that depends markedly on the locations of the points of flexure.
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