  The radius of gyration of a polymer chain immersed in a low molecular weight
solvent is known to vary monotonically with the solvent quality. Here, we
consider the behavior of a chain immersed in a high molecular weight solvent
(polymer melt). Unsurprisingly, we find that, as the incompatibility between
the chain and the polymer melt is increased, the two limiting conformations of
the chain are the ideal random coil (R = aN^1/2) and the dense globule (R =
aN^1/3). We show, however, that between these two limits the radius of gyration
of the chain presents a hysteretic behavior .
