Understanding molecular structures of buried polymer/metal interfaces is important for the design and development of polymer adhesives used in advanced microelectronic devices and polymer anticorrosion coatings for metals. The buried interfacial molecular structure between polystyrene (PS) and silver (Ag) was investigated using infrared-visible sum frequency generation (SFG) vibrational spectroscopy via a “sandwiched” sample geometry. SFG resonant signals from the phenyl C–H stretching vibrational modes were detected from the PS/Ag interface, suggesting that the PS phenyl groups at this buried polymer/metal interface are ordered. Spectral analysis indicated that the phenyl groups at the buried PS/Ag interface tilt toward the interface, pointing away from the Ag side.

Sum frequency generation (SFG) vibrational spectroscopy has been widely employed to investigate molecular structures of biological surfaces and interfaces including model cell membranes. A variety of lipid monolayers or bilayers serving as model cell membranes and their interactions with many different molecules have been extensively studied using SFG. Here, we conducted an in-depth investigation on polarization-dependent SFG signals collected from interfacial lipid monolayers using different experimental geometries, i.e., the prism geometry (total internal reflection) and the window geometry (external reflection). The different SFG spectral features of interfacial lipid monolayers detected using different experimental geometries are due to the interplay between the varied Fresnel coefficients and second-order nonlinear susceptibility tensor terms of different vibrational modes (i.e., ss and as modes of methyl groups), which were analyzed in detail in this study. Therefore, understanding the interplay between the interfacial Fresnel coefficients and χ(2) tensors is a prerequisite for correctly understanding the SFG spectral features with respect to different experimental geometries. More importantly, the derived information in this paper should not be limited to the methyl groups with a C3v symmetry; valid extension to interfacial functional groups with different molecular symmetries and even chiral interfaces could be expected.

Understanding the interfacial molecular structures of antifouling polymers in solutions is extremely important in research and applications related to chemistry, biology, and medicine. However, it is generally challenging to probe such buried solid/liquid interfaces in situ. We herein report a molecular-level study on detecting the interfacial molecular structures of an antifouling hydrogel material, poly(2-hydroxyethyl methacrylate) (PHEMA), in contact with water and bovine serum albumin (BSA) solution in situ using sum frequency generation (SFG) vibrational spectroscopy. To compare to and validate our in situ experiments, molecular-level structures of the substrate/PHEMA interface before and after water exposure were also detected. The detected strong O–H vibrational signals from water and hydroxyethyl and carbonyl vibrational signals from PHEMA prove that the PHEMA surface hydration was attributed to the interaction between water and PHEMA side hydrophilic groups. SFG experimental results verify that the adsorbed BSA molecules at the PHEMA/solution interface were disorderly arranged, supported by data from the laser scanning confocal microscopic (LSCM) experiment. This indicates the weak interaction between the BSA molecules and PHEMA surface. This direct detection of the surface hydrated structures of PHEMA sheds light on understanding the interfacial properties of antifouling materials in aqueous environments. The capability reported here to probe the PHEMA/solution interface and the hidden substrate/PHEMA interface after water exposure can be applied to investigate a broad range of interfaces of antifouling materials.

A new methodology was developed to probe glass transition temperatures (Tgs) of polymer thin films supported on gold (Au) substrates and confined between two solid (silica and silver) surfaces based on the surface plasmon polariton (SFPP) signals generated by sum frequency generation (SFG) spectroscopy. The measured Tgs for polymer (poly(methyl methacrylate), poly(benzyl methacrylate) and poly(ethyl methacrylate)) thin films supported on Au substrates showed similar thickness-dependent trend, that is, the Tg decreased as the thin film thickness decreased due to the free surface effect. However, the measured Tg of the (poly(methyl methacrylate)) thin films confined between two solid surfaces increased significantly with respect to the bulk value, indicating the strong interfacial effect when the free surface was replaced by a buried interface. This method to measure the Tg can be applied to study different polymer thin films supported on metal surfaces or confined between two solid surfaces with different surface chemistries. More importantly, SFG has the unique selectivity and sensitivity to study surfaces and interfaces, providing the feasibility to develop SFG into a powerful tool to detect surface, interfacial, and bulk Tgs of a polymer thin film simultaneously in the future.

•Gradiently varied packing/orientation states were discovered for polymer thin films on the gold substrates.•Polarization-dependent band splitting was correlated to evolution of the chain packing/orientation state.•A new experimental scheme using p-polarized light was developed to adjust the field vector direction for infrared absorption.Gradiently varied chain packing/orientation states of the polyacrylamide (PAL) thin films spin-coated on the gold (Au) substrates were found via the polarized reflection–absorption Fourier transform infrared spectroscopy (RA-FTIR). As the film thickness increases, the splitted amide I bands provide a direct evidence that the PAL thin films are of a gradiently varied bi-layered structure. In the bottom layer, most of the PAL molecules show random orientation which is induced by the non-favorable interaction from the adjacent Au surface. In the top layer, most of PAL molecules show parallel orientation to the Au surface which is induced by spin coating, evidenced by the enhanced low-frequency splitted amide I band (∼1658 cm−1) and N–H stretching modes of the amino groups when the light electric field vector is adjusted to be parallel to the Au surface. The observation reported in this study should be of universal significance for polymer thin films on the supported substrates, where the interfacial interaction as well as spin coating could vary the polymer packing/orientation states substantially.

Using ellipsometry, it is found the glass transition temperature of the spin-coated polyacrylamide (PAL) thin films on the supported silicon (Si) substrates with an oxide layer decreases with decreasing the film thickness. But Tgs of the as-prepared thin films are much higher than that of the bulk sample. Such observations can be attributed to the combined result of the “surface effect” and increased hydrogen bonding interaction between PAL chains due to spin coating/thin film confinement.

Sum frequency generation (SFG) vibrational spectroscopy was employed to study the interaction between memantine (a water-soluble drug for treating Alzheimer’s disease) and lipid bilayers (including zwitterionic PC and negatively charged PG lipid bilayers) at the molecular level in real time and in situ. SFG results revealed how the memantine affected these lipid bilayers in terms of the lipid dynamics, average tilt angle (θ), as well as angle distribution width (σ). It was found that memantine could adsorb onto the zwitterionic PC surface but did not affect the flip-flop rate of the PC bilayer even in the presence of 5.0 mM memantine, indicating the negligible interaction between memantine and the PC bilayer. However, for the negatively charged PG bilayer, it was found that the outer PG leaflet could be significantly destroyed by memantine at a relatively low memantine concentration (1.0 mM), while the inner PG leaflet remained intact. Besides, the θ and σ of CD3 groups in the outer PG lipid leaflet were calculated to be ∼82.0° and ∼19.5° after adding 5 mM memantine, respectively, indicating that these CD3 groups were prone to lie down at the membrane surface (versus the surface normal) with the addition of 5 mM memantine while nearly standing up without the addition of drug molecules. These monolayer- and molecular-level results could hardly be obtained by other techniques. To the best of our knowledge, this is the first experimental attempt to quantify the drug-induced orientational changes of lipid molecules within a lipid bilayer. The present work provided an in-depth understanding on the interaction between memantine and model cell membranes, which will potentially benefit the development of new drugs for neurodegenerative diseases involving drug–membrane interaction.

Understanding molecular structures of buried polymer/metal interfaces is important for the design and development of polymer adhesives used in advanced microelectronic devices and polymer anticorrosion coatings for metals. The buried interfacial molecular structure between polystyrene (PS) and silver (Ag) was investigated using infrared-visible sum frequency generation (SFG) vibrational spectroscopy via a “sandwiched” sample geometry. SFG resonant signals from the phenyl C–H stretching vibrational modes were detected from the PS/Ag interface, suggesting that the PS phenyl groups at this buried polymer/metal interface are ordered. Spectral analysis indicated that the phenyl groups at the buried PS/Ag interface tilt toward the interface, pointing away from the Ag side.

During the model membrane formation process from a lipid monolayer of 1,2-dipalmitoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (sodium salt) (DPPG) in air to a lipid bilayer of DPPG/deuterated-DPPG (dDPPG) on water, the intensity of sum frequency generation (SFG) vibrational signals from the DPPG molecules increased by ∼34 times. The increased signal intensity could be caused by inherently different molecular ordering of lipid molecules in the monolayer/bilayer or by optical effects induced by different contacting mediums (prism in air and prism contacting water) (see Figure 2). We resolve the two possibilities by analyzing tilt angles of the methyl groups at DPPG hydrophobic ends for the monolayer/bilayer which reflect the molecular ordering information and by evaluating the Fresnel coefficients which reflect the contacting medium-induced optical effect. DPPG molecules were more ordered after transformation from a lipid monolayer in air to a lipid bilayer on water, which induced a slight signal increase. The augmentation of the Fresnel coefficients was well-correlated to the enhancement of the SFG resonant signal. Therefore, this case study is applicable to similar interfacial systems probed by the SFG spectroscopy to deduce the relative contributions of interfacial molecular ordering and interfacial Fresnel coefficients to the detected SFG signal.