Flattening process of polymer chains irreversibly adsorbed on a solid*

We report the structural relaxation process of irreversibly adsorbed polymer chains that lie flat on a solid (``flattened chains (FC)'') via thermal annealing. Amorphous polystyrene (PS) on quartz, which together constitute a weakly attractive system, was used as a model system and the local chain conformations of the FC were investigated by sum frequency generation spectroscopy (SFG). Two different film preparation processes (i.e., spin coating and dip coating) were used to create different initial chain conformations. PS films were annealed at T\textgreater \textgreater T$_{\mathrm{g}}$ to reach the ``quasiequilibrium'' state and subsequently rinsed with chloroform, a good solvent to uncover the buried FC. SFG results revealed that the backbone chains (constituted of CH and CH$_{\mathrm{2}}$ groups) of the FC preferentially orient to the weakly interactive substrate surface via thermal annealing regardless of the initial chain conformations, while the orientation of the phenyl rings becomes randomized. We postulate that increasing the number of surface-segmental contacts (i.e., enthalpic gain) is the driving force to overcome the conformational entropy loss in the total free energy.