Spatial and temporal dependence of diffusion in polystyrene thin films on silicon and carbon surfaces

The strong inhibition of chain diffusion in thin polystyrene (PS) films has been observed near an attractive silicon surface, and diffusion remains in hibited out to distances of several radii of gyration from the surface. The present study seeks to determine the time dependence of the diffusion coef ficient, and to examine the effect of a carbon surface on this diffusion. T he sputter-deposited carbon surface may serve as a model for carbon-black p articles employed in nanocomposites, which have recently been observed to r educe diffusion throughout a nanocomposite layer. The experiments employed a thin (similar to 15 nm) deuterated polystyrene (dPS) marker layer sandwic hed between two normal PS layers. Deuterium profiles were monitored in the annealed samples by secondary ion mass spectrometry. Strong segregation was observed at the silicon surface, but was inhibited at the carbon surface, allowing the diffusion behaviour to be studied in the latter case over long er annealing times. A finite-element computer program was developed to fit the observed diffusion profiles. The variation of the diffusion coefficient with depth is shown to be consistent with previous results, and diffusion is comparable at both the carbon and silicon surfaces. The diffusion coeffi cient decreases roughly in proportion to t(-1/2), and is discussed in the c ontext of reptation theory.