Absolute molecular structure of the polystyrene at the buried polystyrene/silica interface and its relationship to dewetting during annealing

Abstract

Dewetting is a common phenomenon observed from polymer thin films under stimuli, such as annealing, which directly affects the applications of polymer thin films. A key to understanding such dewetting behavior is to precisely probe the molecular structure evolution at the buried polymer/substrate interface during annealing. However, non-destructively probing the buried interface is very difficult at the molecular level and in situ. In this study, the specific surface- and phase-sensitive sum frequency generation (SFG) vibrational spectroscopy was used to investigate PS thin films on silica substrates during annealing. The results showed that the absolute orientation/conformation of PS at the film surface was significantly different from that of the flattened PS nanolayer formed near the substrate by dewetting. The dynamic molecular structure evolution of the PS nanolayer (the formation process of the flattened PS nanolayer) at the buried PS/silica interface was directly observed in situ. The dewetting occurred at the non-adsorbed (bulk, free) PS/the flattened PS nanolayer interface due to the autophobic effect. Furthermore, the effects of end groups and solvents on the PS dewetting rate were also elucidated. This study provides a molecular-level understanding of the PS structure at the buried PS/silica interface and its relationship to dewetting behavior.