Probing Molecular Structures of Poly(dimethylsiloxane) at Buried Interfaces <i>in Situ</i>

Silicone materials such as poly­(dimethylsiloxane) (PDMS) are widely used in a variety of important applications such as polymer adhesives, packaging materials for microelectronics, polymer MEMS, microfluidics, biomedical implants, and marine antifouling coatings. In such applications, molecular structures of PDMS at buried interfaces will determine interfacial properties. Therefore, it is important to elucidate PDMS molecular structures at relevant buried interfaces. In this study, the interfacial structures of PDMS silicone elastomer in contact with silica and different polymer materials have been studied using sum frequency generation (SFG) vibrational spectroscopy. It was found that the PDMS methyl groups are ordered at the buried poly­(ethylene terephthalate) (PET)/PDMS and fused silica/PDMS interfaces. However, these methyl groups tend to adopt different orientations at different interfaces. Using the SFG spectral fitting results, the possible ranges of tilt angles and twist angles of PDMS methyl groups at the buried PET/PDMS and silica/PDMS interfaces were determined. At the PET/PDMS interface, the methyl groups tend to have large tilt angles (>70°) with small twist angles (<20°). At the silica/PDMS interface, methyl groups tend to adopt a broad distribution of tilt angles along with large twist angles. The absolute orientations of the PDMS methyl groups at the buried interfaces were determined from the interference pattern of the PDMS SFG signal with the nonresonant signal from a TiO₂ thin film. PDMS methyl groups tend to orient toward the PDMS bulk rather than the contacting substrates at both the PET/PDMS and silica/PDMS interfaces. However, at the polystyrene/PDMS and poly­(methyl methacrylate)/PDMS interfaces, PDMS methyl groups orient toward the hydrophobic polymer substrate surfaces. The different orientations of PDMS methyl groups at the investigated buried interfaces were correlated to interfacial polar interactions determined by substrate surface hydrophobicities