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[Display omitted] • The specific surface-and phase-sensitive SFG spectroscopy revealed the absolute PS structure at the buried PS/silica interface. • The PS nanolayer near the substrate gradually formed during annealing. • The dewetting occurred at the non-adsorbed PS/the flattened PS nanolayer interface due to the autophobic effect. • As the solvent quality reduced, the dewetting rate of the PS film became slower. • This study provides a molecular-level understanding of the dewetting property of the polymer thin film. 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. [ABSTRACT FROM AUTHOR]

• SFG revealed the interfacial structural evolution of PS under confinement. • The penetration of the PS chain at the d 8 -PS/PMMA interface occurred at ∼423 K. • The molecular ordering/orientation evolved during the penetration. • The tilt angle of the phenyl group increased while the twist angle decreased. The geometrical confinement directly affects many crucial properties of polymer thin films by interfacial interactions. A key to understanding the role of these interfacial interactions is to specifically probe the confined interface at the molecular level and in situ. However, the direct and nondestructive detection of interfacial polymer structures under confinement is very difficult. In this study, specific interface-sensitive sum frequency generation (SFG) spectroscopy was applied to study the deuterated PS (d 8 -PS) film confined between hydrogenated poly (methyl methacrylate) (PMMA) films at the molecular level and in situ. The results showed that the ordering/orientation of the PS chain (backbone and phenyl group) evolved at the buried d 8 -PS/PMMA interface during annealing. The tilt angle of the PS phenyl group increased while the twist angle decreased with elevated annealing temperature. Both overall SFG ssp and ppp intensities decreased after annealing at 433 K for 24 h, indicating that PS chains entangled with or penetrated the PMMA chains at the confined interface. This study qualitatively and quantitatively revealed the interfacial structure and structural evolution of the PS chain confined between PMMA films in situ during annealing, which is beneficial to the molecular-level understanding of the interfacial chain conformational relaxation of polymers under confinement. [Display omitted] [ABSTRACT FROM AUTHOR]