Tuning the Effective Viscosity of Random Copolymer films of Styrene and 4-Methoxystyrene by Varying the Copolymer Composition*

We show that thickness dependence of effective viscosity, η_eff(h₀), of random copolymer films of styrene (St) and 4-methoxystyrene (MeOS) supported by silica (SiOx) can be easily tuned by using different MeOS concentrations, X_MeOS. When X_MeOS is increased from 0% to 100%, η_eff of nanometer films changes steadily from suppressed to enhanced. At X_MeOS = 10%, η_eff(h₀) displays a non-monotonic variation. We explain our results by suggesting that MeOS interacts more strongly with SiOx than St does and the behavior of η_eff(h₀) is due to competitions between substrate effect and free surface effect. If substrate effect is stronger, η_eff of nanometer films is enhanced. If free surface effect is stronger, η_eff reduction occurs. When the two effects are comparable, our X_MeOS = 10% data suggests that substrate effect dominates in nanometer films but free surface effect dominates in thicker films, resulting in a non-monotonic η_eff(h₀) dependence.