Effective Viscosity of Unentangled Random Copolymer Films of Styrene and 4-Methoxystyrene with Different Copolymer Compositions

We show that the thickness, h0, dependence of the effective viscosity, ηeff(h0), of unentangled random copolymer films of styrene (St) and 4-methoxystyrene (MeOS) supported by silica (SiOx) can be easily tuned by tuning the MeOS concentration, XMeOS, in the copolymer. When XMeOS is increased from 0% to 100%, ηeff(h0) changes steadily from suppressed to enhanced with decreasing film thickness. At XMeOS = 10%, ηeff(h0) displays a nonmonotonic variation. Results obtained by fitting a three-layer model to the ηeff(h0) data show that ηeff(h0) of the XMeOS = 0% film was dominated by the mobility-enhancing effect of the free surface, but as XMeOS increases, the mobility-slowing effect of the substrate surface increases and dominates. The results also show that the propagation distance, lt, of the substrate effect increases with increasing XMeOS and is ∼2.6 to 6 times the radius of gyration of the polymer. We tentatively suggest the long propagation distances to be caused by the long-range van der Waals interactions. Analysis of the van der Waals interfacial potentials of the films supports this interpretation.