Revealing Buried Interfaces Through In-situ Observation of Phase Structure in Polymer Blends Using Sum-Frequency Generation Imaging

Adhesion technologies have been attracting much attention because polymer adhesives are lighter and have higher toughness compared to mechanical bonding. Aggregation states at solid interfaces are strongly related to their adhesion properties. Polymer chains directly contact the adherend surface, leading to both physical and chemical interactions. Thus, it is essential to understand the orientation of functional groups in polymer chains at the interface, and the resulting local conformations. In this study, we focused on sum-frequency generation (SFG) vibrational spectroscopy and imaging, which offer the best depth resolution at the sub-nanometer level among available techniques, to disclose the relationship between local orientational distribution of functional groups at the interface and adhesion strength. SFG imaging was applied to blend films of polystyrene and poly(methyl methacrylate) (PMMA) with different compositions on the quartz substrate, enabling the direct observation of wetting layers and phase-separated structures at the substrate interface. Furthermore, it was revealed that hydrogen bonding between side chain carbonyl groups of PMMA and hydroxy groups on quartz was promoted with increasing PMMA fraction. The interfacial adhesion strength concurrently increased with PMMA fraction due to effective hydrogen bonding at the interface. Therefore, it can be concluded that the interfacial aggregation states of polymer chains are crucial to control the adhesion properties.