Decay of interface correlation in thin polymer films

The long-range interface correlation in thin polymer films (polystyrene and fully brominated polystyrene) prepared by spin-coating is examined. Using diffuse X-ray scattering at small angles of incidence, the roughness correlation which dominates the surface morphology of the polymer film is probed from mesoscopic down to molecular in-plane distances. At the smallest replicable in-plane length scale, the crossover from a conformal to a statistically independent roughness spectrum is determined. The influences of molecular weight and film thickness are discussed. Compared to annealed samples, the as-prepared ones show a different scaling behavior, which is explained with simple models taking surface-bending rigidity into account. With a real-time annealing investigation, the decay of interface correlation after the onset of annealing has been followed. At annealing even below the glass-transition temperature, the roughness correlation is changing and disappears during sufficiently long annealing in the melt. Monitoring the changes with time probes the mobility of the polymer molecules at the polymer-vacuum interface. In thin films, the time constant is increased. The determined surface diffusion coefficient shows a slowing down as compared to the bulk behavior which may be attributed to the attractive, long-range substrate-interface interaction.