The use of vibrationally resonant sum-frequency generation (VR-SFG) spectroscopy to investigate the structure of surfaces and interfaces generally relies on the assumption that only the surface/interface is probed and that the vibrational mode assignments are known and correct. To make vibrational mode assignments, two fundamental aspects of the technique must be dealt with. First, not all vibrational modes observed in linear spectroscopic techniques, such as IR and Raman, are necessarily present in the VR-SFG spectrum. Second, while it is generally assumed that VR-SFG only probes the surface, this technique in actuality is sensitive to molecules in any environment with broken symmetry. Previously published assignments for the aromatic CH stretching modes of polystyrene surfaces are contradictory, and one purpose of this work is to revisit those assignments. In addition to thin films of polystyrene, we have collected VR-SFG spectra of dimethylphenyl silane, polystyrene covered with a layer of poly(methyl methacrylate) (PMMA), and plasma-treated polystyrene to aid our mode assignments. Density functional theory calculations were also performed on styrene oligomers. Based on these experimental and theoretical results, we have determined that not all the expected vibrational modes are observed in the VR-SFG spectrum of polystyrene. We have also found that one particular mode, the ν2 symmetric stretch, accounts for two of the observed peaks, one from the exposed surface and a second feature from a subsurface layer within the polymer thin film. These two features appear at separate frequencies (11 cm−1 separation) because this mode is very sensitive to the local density, which is higher in the bulk than at the surface. With these experimentally validated mode assignments, VR-SFG spectra in the aromatic CH stretching region can be interpreted more reliably. More importantly, these results demonstrate that great care must be taken in assigning VR-SFG spectra. These results also show that VR-SFG can be used to interrogate more than just free surfaces and buried interfaces; any area of broken symmetry is probed with this technique.

Plasma treatment of polymer materials introduces chemical functionalities and modifies the material to make the native hydrophobic surface more hydrophilic. It is generally assumed that this process only affects the surface of the material. We used vibrationally resonant sum-frequency generation spectroscopy to observe changes in the orientation of phenyl groups in polystyrene (PS) thin films on various substrates before and after plasma treatment. VR-SFG selectively probes regions of broken symmetry, such as surfaces, but can also detect the emergence of anisotropy. On dielectric substrates, such as fused silica, the spectroscopic peak corresponding to the symmetric stretching (ν2) mode of the phenyl rings was undetectable after plasma treatment, showing that surface phenyl rings were altered. This peak also diminished on conducting substrates, but the intensity of another peak corresponding to the same mode in a bulklike environment increased significantly, suggesting that plasma treatment induces partial ordering of the bulk polymer. This ordering is seen on conducting substrates even when the polymer is not directly exposed to the plasma. Annealing reverses these effects on the polystyrene bulk; however, the surface phenyl rings do not return to the orientation observed for untreated films. These results call into question the assumption that the effects of plasma treatment are limited to the free surface and opens up other possibilities for material modification with low-temperature plasmas.Keywords: bulk polymer modification; field-responsive polymers; molecular orientation; plasma treatment; sum frequency generation; surface modification;

In vibrationally resonant sum-frequency generation (VR-SFG) spectra, the resonant signal contains information about the molecular structure of the interface, whereas the nonresonant signal is commonly treated as a background and has been assumed to be negligible on transparent substrates. The work presented here on model chromatographic stationary phases contradicts this assumption. Model stationary phases, consisting of functionalized fused-silica windows, were investigated with VR-SFG spectroscopy, both with and without experimental suppression of the nonresonant response. When samples are moved from CD3OD to D2O, the VR-SFG spectrum was found to change over time when the nonresonant signal was present but not when the nonresonant signal was suppressed. No effect was seen when the solvent was changed and pressurized to 900 psi. These results suggest that the response to the new solvent manifests primarily in the nonresonant response, not the resonant response. Any structural changes caused by the new solvent environment appear to be minor. The nonresonant signal is significant and must be properly isolated from the resonant signal to ensure a correct interpretation of the spectral data. Curve-fitting procedures alone are not sufficient to guarantee a proper interpretation of the experimental results.

Previous assignments of the CH stretching modes for surface-bound alkylsilanes in vibrationally resonant sum-frequency generation (VR-SFG) spectra have differed, leading to uncertainty in how to interpret vibrational spectra of these systems. In particular, the assignment of Fermi resonances, including which modes are coupled, has been unclear. To aid in these assignments, partially deuterated alkylsilanes were synthesized and characterized by FT-IR and VR-SFG. Density functional theory (DFT) calculations complement the spectroscopic investigation. Based on these results, we identify multiple contributions to the VR-SFG feature at ∼2950 cm−1; this is primarily a Fermi resonance between the symmetric methyl stretch and symmetric methyl bending modes at 2945 cm−1 with a high-frequency shoulder at ∼2960 cm−1, assigned to the methyl antisymmetric stretch, and a small contribution from the antisymmetric stretch of the ω CH2 group at ∼2930 cm−1. The feature at 2880 cm−1 is assigned as the symmetric methyl stretch. Improved mode assignments will aid the interpretation of vibrational spectra with an aim toward developing a better understanding of the molecular basis of retention in liquid chromatography.

Chemical separations in reversed-phase liquid chromatography (RPLC) are made possible by the detailed molecular-level interactions that take place between analyte molecules and the interface between the stationary and mobile phases. Changes in operational conditions lead to different retention times, suggesting that the structures of the stationary phase and the stationary/mobile phase interface are altered. However, the details of such alterations, and their relationship to separation performance, are not well understood. In this study, the conformations of model RPLC stationary phases were investigated with all-atom molecular dynamics simulations to elucidate the detailed structural response of the stationary phase to different conditions. The model system consists of a quartz surface functionalized with dimethyloctadecylsilane immersed in H2O. Pressurization of the mobile phase did not affect the structure of the stationary phase, in agreement with previous studies. The choice of grafting positions on the surface, however, had a significant impact on the stationary phase structure. Specifically, randomization of the grafting positions led to increased local density, steric crowding, and stationary phase swelling compared with uniform chain placement with the same surface coverage. This finding, which has not been previously established, demonstrates the importance of characterizing the system-specific, chain-placement effects on the behavior of the RPLC stationary phase when attempting to study the molecular mechanisms controlling chromatographic retention.

The interpretation of vibrationally resonant sum-frequency generation (VR-SFG) spectra is often complicated by two factors: spectral congestion and the presence of a nonresonant signal. With broadband VR-SFG systems, the spectra are further distorted because of incomplete sampling, or apodization, of the resonant free induction decay (FID) in the time domain. An experimental method is presented that takes advantage of these time-domain effects to obtain more accurate parameters than can be achieved by fitting a single VR-SFG spectrum. VR-SFG spectra are acquired at multiple delay times of the visible pulse with full suppression of the nonresonant signal and then simultaneously fit to determine a single set of spectral parameters. The multiple, independent spectra serve to constrain the results of the fitting. This variable time-delay approach allows for improved determination of the parameters of the resonant spectrum, including the proper number of peaks in the spectrum, and is demonstrated for thin films of polystyrene and surface-bound octadecylsilane. An additional advantage of the technique is that it can be implemented with minimal modification to an operational broadband VR-SFG system.

Sum frequency generation (SFG) spectroscopy is a powerful tool for probing the orientations of molecules at surfaces and interfaces, but oversimplification in the treatment of the nonresonant (NR) contribution has obscured some fundamental limitations in the analysis of SFG spectra. These difficulties are demonstrated for the case of polystyrene thin films. The NR signal invariably distorts the spectrum and can cause changes in the spectra even in the absence of actual structural changes. The NR signal originates not only from the substrate but from all materials in the system and should not be modeled as having a frequency-independent amplitude. Because of its complicated nature, NR signal must be isolated experimentally in order to obtain meaningful results. Suppression of NR signal, however, causes a different type of distortion, due to apodization of the resonant signal in the time-domain. Experimental methods need to be refined to take these limitations into account and to obtain unique spectral parameters to be used for orientation determination.

The nonresonant (NR) portion of sum-frequency generation (SFG) spectra of polystyrene (PS) thin films is shown to contain physical information and affect the analysis of the surface structure. When the NR signal is suppressed, PS thin films on three different substrates produce the same spectrum, suggesting the same structure at the free surface. Annealing or aging of PS films on silicon causes the NR signal to increase significantly compared with a fresh sample, indicating that the substrate is not the sole source of NR signal. A method is proposed for improved analysis of SFG spectra. First, spectra obtained with NR suppression are used to determine the resonant parameters. After these are constrained, the NR amplitude and phase parameters are determined more uniquely from unsuppressed spectra. NR-SFG must be properly handled for the analysis of SFG spectra to be mathematically unique and physically meaningful.Keywords (keywords): coherent spectroscopy; multiparameter curve fitting; nonlinear vibrational spectroscopy; polymer surface structure; surface spectroscopy;