Mechanism of Density Evolution of Polystyrene Adsorbed Layers on the Substrate

The density evolution of polystyrene (PS) adsorbed layers on phenyl-modified SiO₂-Si substrates was investigated. The thickness and density of flattened layer on substrates with above 75% phenyl content increased over annealing time and could approach 4.7 nm and 1.37 g/cm³ at equilibrium, respectively, which were much higher than those on SiO₂-Si. The annealing time for flattened chains to reach equilibrium increased with an increasing phenyl content on the substrate. The interface sensitive sum frequency generation vibrational spectroscopy (SFG) technique revealed that both the amount and the strength of the interfacial π-π interaction between the phenyl groups of substrates and in PS chains increased with annealing time. This resulted in more stretched chains perpendicularly, leading to a denser and thicker adsorbed layer with a closest-packing structure, driven by favorable enthalpy processes. Our work provides important insight into the densification mechanism of adsorbed flattened layers.