Enhanced Thermal Stability of Polystyrene by Interfacial Noncovalent Interactions

The local conformation of polystyrene (PS) and deuterated PS at the interface with quartz substrates, which were covered with phenyl groups using phenyltrimethoxysilane (PTS) under various conditions, was examined by sum frequency generation spectroscopy. As evidenced by the red-shift of the wavenumber of the v(2) vibration mode for phenyl groups, it was claimed that PTS phenyl groups standing vertically from the quartz surface induced the perpendicular orientation of PS phenyl rings by energetically favorable parallel-displaced pi-pi interactions at the interface. The local conformation of PS chains strongly anchored onto the substrate by the pi-pi interactions remained almost unchanged for 48 h even at a temperature 60 K higher than the bulk glass transition temperature. That is, the interfacial pi-pi interactions facilitated the adsorption of PS chains on the substrate to attain a large enthalpic gain, resulting in significantly slower dynamics of PS chains at the interface. Our results illustrate the importance of the interfacial noncovalent interactions in controlling the structure and dynamics of macromolecular chains at the interface as well as in the thin film geometry.