Substrate-dependent interfacial structures of ultrathin poly(methyl methacrylate) films upon annealing revealed by sum frequency generation vibrational spectroscopy

Abstract

Understanding interfacial molecular structures at the polymer/adjacent materials interface is essential for optimizing the performance of energy-related devices. However, it remains insufficiently explored due to limited interface-specific techniques. Here, we employed sum frequency generation (SFG) vibrational spectroscopy to investigate the substrate-dependent interfacial structures of spin-coated ultrathin poly(methyl methacrylate) (PMMA) films (∼10 nm) on silica and CaF₂ before and after thermal annealing. PMMA on silica exhibits similar OCH₃-dominated SFG spectra before and after annealing. In contrast, PMMA on CaF₂ shows a significant decrease in OCH₃ signals and enhancements of CH₂ and CH₃ signals upon annealing, revealing substantial molecular reorganization at the buried PMMA/CaF₂ interface. Quantitative analysis indicates that the OCH₃ groups adopt a tilt angle of ∼77° (assuming a δ-distribution) after annealing, suggesting a more lying-down or disordered orientation. These substrate-dependent differences arise from weaker interfacial interactions and the hydrophobic nature of the CaF₂ surface, which permits greater chain relaxation compared with the hydrogen-bond-constrained PMMA/silica interface. This study provides molecular-level and in situ insights into substrate-dependent structural evolution in polymer thin films and offers guidance for the interface engineering in photoelectric, photovoltaic, and energy-storage devices.