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 CaF 2 before and after thermal annealing. PMMA on silica exhibits similar OCH 3 -dominated SFG spectra before and after annealing. In contrast, PMMA on CaF 2 shows a significant decrease in OCH 3 signals and enhancements of CH 2 and CH 3 signals upon annealing, revealing substantial molecular reorganization at the buried PMMA/CaF 2 interface. Quantitative analysis indicates that the OCH 3 groups adopt an average tilt angle of ~77° 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 CaF 2 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.