  We consider a model mixture of hard colloidal spheres and non-adsorbing
polymer chains in a theta solvent. The polymer component is modelled as a
polydisperse mixture of effective spheres, mutually noninteracting but excluded
from the colloids, with radii that are free to adjust to allow for
colloid-induced compression. We investigate the bulk fluid demixing behaviour
of this model system using a geometry-based density-functional theory that
includes the polymer size polydispersity and configurational free energy,
obtained from the exact radius-of-gyration distribution for an ideal
(random-walk) chain. Free energies are computed by minimizing the free energy
functional with respect to the polymer size distribution. With increasing
colloid concentration and polymer-to-colloid size ratio, colloidal confinement
is found to increasingly compress the polymers. Correspondingly, the demixing
fluid binodal shifts, compared to the incompressible-polymer binodal, to higher
polymer densities on the colloid-rich branch, stabilizing the mixed phase.
