  The interplay of nematic order and phase separation in solutions of
semiflexible polymers in solvents of variable quality is investigated by
density functional theory (DFT) and molecular dynamics (MD) simulations. We
studied coarse-grained models, with a bond-angle potential to control chain
stiffness, for chain lengths comparable to the persistence length of the
chains. We varied both the density of the monomeric units and the effective
temperature that controls the quality of the implicit solvent. For very stiff
chains only a single transition from an isotropic fluid to a nematic is found,
with a phase diagram of "swan-neck" topology. For less stiff chains, however,
also unmixing between isotropic fluids of different concentration, ending in a
critical point, occurs for temperatures above a triple point. The associated
critical behavior is examined in the MD simulations and found compatible with
Ising universality. Apart from this critical behavior, DFT calculations agree
qualitatively with the MD simulations.
