Polymer surface diffusion in the dilute limit

The molecular weight (M) dependence of the center-of-mass surface diffusion coefficient (D) is measured for polystyrene adsorbed from chloroform, a good solvent, in the dilute limit of surface coverage. On as-received smooth quartz surfaces, we reproduce D ∼ M^-3/2, the same power law observed earlier in aqueous systems for a much smaller range of M. This is consistent with computer simulations in the literature regarding 2D diffusion between obstacles. But on smoother surfaces, mica and thermally annealed quartz, we observe Rouse scaling, D ∼ M^-1, to our knowledge the first time this theoretically expected scaling has been observed for polymer diffusion at the solid-liquid interface. For polystyrene of the highest molecular weight diffusing on the rougher surfaces, in the range (1-6.7) × 10⁵ g mol^-1, the dependence on M softens to weaker than a power law of -3/2. This weaker dependence, consistent with D ∼ M^-1 over the limited range of available M, may suggest reversion toward Rouse behavior.