Formation Mechanism of High-Density, Flattened Polymer Nanolayers Adsorbed on Planar Solids

Thermal annealing is one of the most indispensable polymer fabrication processes and plays essential roles in controlling morphologies and properties of polymeric materials. We here report that thermal annealing also facilitates polymer adsorption from the melt on planar silicon (Si) substrates, resulting in the formation of a high-density polymer nanolayer with flattened chain confirmations. Three different homopolymers (polystyrene, poly(2-vinylpyridine), and poly(methyl methacrylate)), which have similar inherent stiffness and bulk glass transition temperature (T-g), but have different affinities with Si substrates, were chosen as models. Spin-cast films (similar to 50 nm in thickness) with the three polymers were prepared on cleaned Si substrates and then placed in a vacuum oven set at a temperature far above the bulk Tg. In order to monitor the polymer adsorption process at the solid-polymer melt interface during thermal annealing, we used the protocol that combines vitrification of the annealed films (via rapid quench to room temperature) and subsequent intensive solvent leaching (to remove nonadsorbed chains). The detailed structures of the residual films (i.e., flattened layers with 2-3 nm in thickness) were characterized by using X-ray reflectivity and atomic force microscopy. As a result, we found that the film thicknesses of the flattened layers for the three different polymers increase as a power-law of annealing time before reaching the "quasiequilibrium" state where the film growth is saturated. We have also revealed that the final thickness of the flattened layer at the quasiequilibrium state increases with increasing the solid-segment interaction, while the kinetics becomes more sluggish. The observed formation kinetics corresponds to a "zipping-down" process of the transient flattened chains on planar solids in order to further increase the number of solid/segment points, which is the driving force for flattening so as to overcome the conformational entropy loss in the total free energy.