This work shows that phase separations in 2D polymer films can be tuned by employing entropically constraining grafting points. We present experimental results oil surface-grafted 2D polystyrene (PS)/polyvinylmethylether (PVME) blended Films using surface-anchored benzophenone derivates. In contrast with 2D films that have not been grafted, it was possible to raise the blended lower critical solution temperature (LCST) above room temperature by using low grafting point densities. Highly constrained films did not show polymer-polymer phase separation. In addition to the in situ structural analysis performed with surface probe microscopy (SPM), mu-beam-sized grazing incidence small-angle X-ray scattering (mu-GISAXS), and mu-X-ray reflectivity (mu-XRR), surface stress investigations performed using nano mechanical cantilever sensor (NCS) arrays gave detailed insight into the phase separation mechanism. Phase separations were shown to result in dominating attractive entropic spring mechanisms with opposing repulsive effects resulting from surface and interfacial energy changes.