Effective Viscosity of Lightly UVO-Treated Polystyrene Films on Silicon with Different Molecular Weights

Recently, we found that a brief, 1 s exposure to ultraviolet ozone (UVO) can cause the effective viscosity, n _eff , of polystyrene films supported by oxide-coated silicon (PS-SiOx) to increase by more than 100 times. In this experiment, we study the phenomenon with different PS molecular weights, M _w , from 2.4 to 451 kg/mol. We found that n _eff was increased for all M _w 's when the film thickness, h ₀ , was decreased below an onset thickness comparable to the radius of gyration, R _g , of the polymer. For h ₀ greater than the onset thickness, the n _eff versus h ₀ dependence varies with M _w . Specifically, for M _w ≥ 60K g/mol n _eff was constant, equal to the bulk viscosity. For M _w < 60K g/mol, n _eff decreased with decreasing h ₀ in the same way as that of the pristine counterparts. X-ray photoelectron spectroscopy (XPS) shows that oxygenated groups are formed in the films after the UVO treatment. We propose that the oxygenated groups can interact with the OH groups on the SiOx surface to produce increases in n n _eff . Correspondingly, we found that the n _eff data could fit well to a three-layer model containing a dynamically dead layer on the substrate. Results of the fit are consistent with a surface layer with a thickness of ∼R _g and the following attributes. Below entanglement, the mobility of this layer is enhanced relative to the bulk polymer. Above entanglement, couplings between the surface chains and the chains in the inner, bulklike region force the surface chains to flow as the inner chains. As a result, enhanced mobility can only be found in short-range, local motions over depths of the order of the average distance between entanglements. This picture explains a variety of surface relaxation phenomena reported in the literature.