  Considering the density-density correlation function of a concentrated
polymer solution with multicomponent solvent we find a phase transition due to
the heterogeneity of excluded volume constant. This new phase transition
implies a strong enhancement of the scattered light intensity in the critical
region, which can explain a recent experiment showing strong light scattering
from a ternary polymer system consisting of polyethylene oxide (PEO) dissolved
in nitroethane and 3methyl-pentane.
