Diffusion of adsorbed theta-solvent polymers at a solid-liquid interface

We study how surface diffusion depends on temperature when this is varied below the theta temperature. In the polystyrene-cyclohexane system, we use FRAP (fluorescence recovery after photobleaching) to measure over times over 4 orders of magnitude, from 10 sec to 10$^{5}$ seconds. A fast component of motion is attributed to chains loosely bound to the surface. A slower component of motion is retained after rinsing; it is subdiffusive. At temperatures below the bulk coexistence temperature, the surface layer is thicker than a monolayer. We show that bulk phase separation of polymers in dilute solution produces a dense surface layer of emulsion and foamy near-surface structure.