Molecular surface structure of comb-like polyoxyethylene with alkyl sulfonyl side-chains studied by sum-frequency generation vibrational spectroscopy

The orientation of alkyl sulfonyl side-chains on a series of polyoxyethylenes, CH₃–nSE (n?=?6, 8 or 10), and the effect of annealing and rubbing on the molecular orientation of the alkyl side-chains at the substrate surface, were investigated using sum-frequency generation (SFG) vibrational spectroscopy. Based on the SFG spectra and their quantitative interpretation, we deduced that the alkyl chains of CH₃–10SE are almost vertically oriented at the surface and that the terminal methyl groups of the alkyl chains are tilted from the surface normal as much as θ ≈ 40?±?5°, with a broad distribution of tilt angles. We also found that rubbing treatment induced the anisotropic orientation of the alkyl side-chains perpendicular to the rubbing direction, but their orientation was unchanged by annealing.     

Local structure at the air/liquid interface of room-temperature ionic liquids probed by infrared-visible sum frequency generation vibrational spectroscopy: 1-alkyl-3-methylimidazolium tetrafluoroborates

The air/liquid interface of 1-alkyl-3-methylimidazolium tetrafluoroborates with the general formula [C(n)mim]BF(4) (n = 4-11) was studied using infrared-visible sum frequency generation (SFG) vibrational spectroscopy. The probability of the gauche defect per CH2-CH2 bond in the alkyl chain decreases as the number of carbon atoms in the alkyl chain increases. This observation suggests that the interaction between the alkyl chains is enhanced as the alkyl chain length becomes longer. The frequencies of the C-H stretching vibrational modes observed in the SFG spectra are higher than those of the corresponding peak positions observed in the infrared spectra of the bulk liquids. This shift is consistent with a structure in which the alkyl chain protrudes from the bulk liquid into the air. A local structure, which originates from the intermolecular interaction between the ionic liquid molecules, is proposed to explain these observations.     

Adsorption of 1- and 2-butylimidazoles at the copper/air and steel/air interfaces studied by sum frequency generation vibrational spectroscopy

The structure of thin films of 1- and 2-butylimidazoles adsorbed on copper and steel surfaces under air was examined using sum frequency generation (SFG) vibrational spectroscopy in the ppp and ssp polarizations. Additionally, the SFG spectra of both isomers were recorded at 55 °C at the liquid imidazole/air interface for reference. Complementary bulk infrared, reflection-absorption infrared spectroscopy (RAIRS), and Raman spectra of both imidazoles were recorded for assignment purposes. The SFG spectra in the C-H stretching region at the liquid/air interface are dominated by resonances from the methyl end group of the butyl side chain of the imidazoles, indicating that they are aligned parallel or closely parallel to the surface normal. These are also the most prominent features in the SFG spectra on copper and steel. In addition, both the ppp and ssp spectra on copper show resonances from the C-H stretching modes of the imidazole ring for both isomers. The ring C-H resonances are completely absent from the spectra on steel and at the liquid/air interface. The relative intensities of the SFG spectra can be interpreted as showing that, on copper, under air, both butylimidazoles are adsorbed with their butyl side chains perpendicular to the interface and with the ring significantly inclined away from the surface plane and toward the surface normal. The SFG spectra of both imidazoles on steel indicate an orientation where the imidazole rings are parallel or nearly parallel to the surface. The weak C-H resonances from the ring at the liquid/air interface suggest that the tilt angle of the ring from the surface normal at this interface is significantly greater than it is on copper.     

Polar and azimuthal alignment of a nematic liquid crystal by alkylsilane self-assembled monolayers: effects of chain-length and mechanical rubbing

Alkylsilane self-assembled monolayers (SAMs) on oxide substrates are commonly used as liquid crystal (LC) alignment layers. We have studied the effects of alkyl chain length, photolytic degradation, and mechanical rubbing on polar and azimuthal LC anchoring. Both gradient surfaces (fabricated using photolytic degradation of C18 SAMs) and unirradiated SAMs composed of short alkyl chains show abrupt transitions from homeotropic to tilted alignment as a function of degradation or chain length. In both cases, the transition from homeotropic to tilted anchoring corresponds to increasing wettability of the SAM surfaces. However, there is an offset in the critical contact angle for the transition on gradient vs unirradiated SAMs, suggesting that layer thickness is more relevant than wettability for LC alignment. Mechanical rubbing can induce azimuthal alignment along the rubbing direction for alignment layers sufficiently near the homeotropic-to-planar transition. Notably, mechanical rubbing causes a small but significant shift in the homeotropic-to-tilted transition, e.g., unrubbed C5 SAMs induce homeotropic anchoring, but the same surface after rubbing induces LC pretilt.     

Entropically mediated polyolefin blend segregation at buried sapphire and air interfaces investigated by infrared-visible sum frequency generation vibrational spectroscopy

The segregation behavior of binary polymer blends at hydrophilic solid sapphire and air interfaces was investigated by infrared-visible sum frequency generation (SFG) vibrational spectroscopy. SFG spectra were collected from a bulk miscible blend consisting of identical molecular weight (approximately 54,000) and similar surface free energy (29-35 dyn/cm) components of atactic polypropylene (aPP) and aspecific poly(ethylene-co-propylene) rubber (aEPR). Characteristic CH resonances of the blend were contrasted with those of the individual components at both buried (sapphire/polymer) and free (air/polymer) interfaces. Preferential segregation of the aPP component was observed after annealing at both air/polymer and sapphire/polymer interfaces. SFG spectra revealed ordering of the polymer backbone segments with the methylene (CH2) groups perpendicular to the surface at the sapphire interface and the methyl (CH3) groups upright at the air interface. The SFG results indicate that the surface composition can be determined from the peak intensities that are characteristic of each component and that conformational entropy played a likely role in surface segregation. aPP occupied a smaller free volume at the surface because of a statistically smaller segment length (aPP is more flexible and has a shorter length). In addition, the high density of the ordered CH3 side branches enhanced the surface activity by allowing the long-chain backbone segments of aPP to order at the surface.     

Demonstrating the feasibility of monitoring the molecular-level structures of moving polymer/silane interfaces during silane diffusion using SFG

In this paper, the feasibility of monitoring molecular structures at a moving polymer/liquid interface by sum frequency generation (SFG) vibrational spectroscopy has been demonstrated. N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane (AATM, NH2(CH2)2NH(CH2)3Si(OCH3)3) has been brought into contact with a deuterated poly(methyl methacrylate) (d-PMMA) film, and the interfacial silane structure has been monitored using SFG. Upon initial contact, the SFG spectra can be detected, but as time progresses, the spectral intensity changes and finally disappears. Additional experiments indicate that these silane molecules can diffuse into the polymer film and the detected SFG signals are actually from the moving polymer/silane interface. Our results show that the molecular order of the polymer/silane interface exists during the entire diffusion process and is lost when the silane molecules traverse through the thickness of the d-PMMA film. The loss of the SFG signal is due to the formation of a new disordered substrate/silane interface, which contributes no detectable SFG signal. The kinetics of the diffusion of the silane into the polymer have been deduced from the time-dependent SFG signals detected from the AATM molecules as they diffuse through polymer films of different thickness.     

Probing polymer/polymer interfaces

Infrared-visible sum frequency generation spectroscopy (SFG) has been used to study the interface between poly(vinyl-N-octadecylcarbamate-co-vinyl acetate) (Comb) and deuterated or hydrogenated polystyrene (dPS or hPS) films. Strong methyl symmetric and Fermi resonance bands associated with the alkyl side chains of the Comb polymer are observed in the SFG spectra. In addition, for Comb/hPS spectra, symmetric and asymmetric vibration modes of phenyl groups are observed. The presence of asymmetric modes indicates the phenyl rings are tilted with respect to the interface normal.     

Vibrational Sum‐Frequency Generation Activity of a 2,4‐Dinitrophenyl Phospholipid Hybrid Bilayer: Retrieving Orientational Parameters from a DFT Analysis of Experimental Data

The vibrational nonlinear activity of films of 2,4‐dinitrophenyl phospholipid (DNP) at the solid interface is measured by sum‐frequency generation spectroscopy (SFG). Hybrid bilayers are formed by a Langmuir–Schaefer approach in which the lipid layer is physisorbed on top of a self‐assembled monolayer of dodecanethiol on Pt with the polar heads pointing out from the surface. The SFG response is investigated in two vibrational frequency domains, namely, 3050–2750 and 1375–1240 cm^?1. The first region probes the CH stretching modes of DNP films, and the latter explores the vibrational nonlinear activity of the 2,4‐dinitroaniline moiety of the polar head of the lipid. Analysis of the CH stretching vibrations suggests substantial conformational order of the aliphatic chains with only a few gauche defects. To reliably assign the detected SFG signals to specific molecular vibrations, DFT calculations of the IR and Raman activities of molecular models are performed and compared to experimental solid‐state spectra. This allows unambiguous assignment of the observed SFG vibrations to molecular modes localized on the 2,4‐dinitroaniline moiety of the polar head of DNP. Then, SFG spectra of DNP in the 1375–1240 cm^?1 frequency range are simulated and compared with experimental ones, and thus the 1,4‐axis of the 2,4‐dinitrophenyl head is estimated to have tilt and rotation angles of 45±5° and 0±30°, respectively.     

Ultrafast vibrational dynamics and spectroscopy of a siloxane self-assembled monolayer

Time and frequency domain sum-frequency generation (SFG) were combined to study the dynamics and structure of self-assembled monolayers (SAMs) on a fused silica surface. SFG-free induction decay (SFG-FID) of octadecylsilane SAM in the CH stretching region shows a relatively long time scale oscillation that reveals that six vibrational modes are involved in the response of the system. Five of the modes have commonly been used for the fitting of SFG spectra in the CH stretching region, namely the symmetric stretch and Fermi resonance of the methyl group, the antisymmetric stretch of the methyl, as well as the symmetric and antisymmetric stretches of the methylene group. The assignment of the sixth mode to the terminal CH(2) group was confirmed by performing a density function theory calculation. The SFG-FID measures the vibrational dephasing time (T(2)) of each of the modes, including a specific CH(2) group within the SAM, the terminal CH(2), which had never been measured before. The relatively long (～1.3 ps) dephasing of the terminal CH(2) suggests that alkyl monolayer structure is close to that of the liquid condensed phase of Langmuir Blodgett films.     

Photo-dimerization of a chalcone-based side chain polymer for the alignment of ferroelectric liquid crystals

The alignment and optical properties of ferroelectric liquid crystal cells, having alignment films of a chalcone-based side chain polymer treated by linearly polarized UV irradiation were investigated. The long absorption band of the UV/Vis spectra gradually decreased and the FTIR spectra shifted as the irradiation times increased, indicating that cyclo-addition and isomerization reactions of the chalcone-based side chains occurred. UV dichroism demonstrated anisotropic changes in the alignment films, with a maximum at low exposure energy (0.5 J cm^-2). Liquid crystal molecules were aligned perpendicular to the polarization direction of the linearly polarized UV radiation. The azimuthal anchoring energy of liquid crystal E7 on a chalcone-based side chain polymer surface increased with exposure energy. Well aligned defect-free cells and high contrast ratio were achieved with irradiation of longer than 5 min; the geometric conditions for a stable C2 structure may be satisfied at low temperature with slowly cooling.     

Molecular structure of an alkyl-side-chain polymer-water interface: origins of contact angle hysteresis

A new and direct approach to verify surface heterogeneity as the microscopic origin of contact-angle hysteresis is demonstrated. IR-visible sum-frequency-generation spectroscopy (SFG) was used to selectively probe the molecules at the interface of an alkyl-side-chain polymer [poly(vinyl n-octadecyl carbamate-co-vinyl acetate)] with water. The spectra indicate that in contact with water, the polymer surface is heterogeneous (having areas of differing surface energies). This evidence of surface heterogeneity supports the hysteresis observed in the advancing and receding contact angles of the polymer surface with water. The same measurements made for the chemically and structurally similar surface of an octadecyltrichlorosilane self-assembled monolayer indicates a homogeneous surface at the water interface. In this case, contact-angle hysteresis measurements implicate surface roughness as the cause of hysteresis. Atomic force microscopy measurements of roughness for these surfaces further support our conclusions. The polymer-water interface was probed using SFG at above-ambient temperatures, and an order-to-disorder transition (ODT) of alkyl side chains at the interface was observed, which closely follows the melting of crystalline side chains in the bulk. This transition explains the increased wettability of the polymer, by water, when the temperature is raised above the bulk melting temperature. Furthermore, the irreversibility of this ODT suggests that the disordered polymer-water interface is the thermodynamic equilibrium state, whereas the before-heating structure of this interface is a kinetically hindered metastable state.     

Novel cyanoterphenyl self-assembly monolayers on Au(111) studied by ellipsometry, x-ray photoelectron spectroscopy, and vibrational spectroscopies

The self-assembled monolayers (SAMs) of two asymmetric disulfides derivatives (namely, LC1 and LC2) were prepared on Au(111). The disulfides contain a pure alkyl chain and an alkyl chain terminated by a cyanoterphenyl group. LC1 and LC2 differ by the way the cyanoterphenyl group is attached onto the alkyl chain: it is expected to be aligned with the alkyl chain in the case of LC1 and perpendicular to it in the case of LC2 (T shape). The consequences in terms of surface coverage, chemical composition, and molecular conformation of the two SAMs are studied using ellipsometry, x-ray photoelectron spectroscopy (XPS), reflection absorption infrared spectroscopy (RAIRS), and broadband femtosecond sum-frequency generation (SFG). A model of coverage and tilt angle based on ellipsometry and XPS results shows that the SAM "manages" the large size of the terphenyl group by lowering the terphenyl containing chain coverage and by increasing the tilt. In the case of LC2, the disulfide breaks during molecular assembly, less terphenyl chains adsorb than pure alkyl chains, and the overall chain coverage is smaller than for LC1. RAIRS and SFG results show that these differences in surface coverage correspond to a drastically different orientation of the terphenyl axis, which lies nearly parallel to the surface for LC2, while it is tilted by approximately 28 degrees for LC1. This shows that the terphenyl group takes much more space on the surface in the case of LC2 and explains why the terphenyl coverage is found smaller for LC2. The anomalous SFG relative intensities observed in the region of CH stretch between CH2 and CH3 modes, and symmetric and antisymmetric modes, show that the chains are not in the fully stretched, all-trans conformation, LC2 being probably more distorted than LC1. These distorsions allow the molecules to occupy the space available below the large terphenyl group. The relative intensities of symmetric and antisymmetric modes are discussed qualitatively for some typical molecular conformations and orientations of the alkyl chain.     

Adsorption of sodium dodecyl sulfate at the hydrophobic solid/aqueous solution interface in the presence of poly(ethylene glycol): dependence upon polymer molecular weight

The polar orientation and degree of conformational order of sodium dodecyl sulfate (SDS) adsorbed at the hydrophobic octadecanethiol/aqueous solution interface in the presence of poly(ethylene glycol) (PEG) has been investigated as a function of the surfactant concentration and the molecular weight of the polymer. Sum frequency generation (SFG) vibrational spectroscopy was employed to obtain spectra of interfacial surfactant; weak SFG signals from interfacial polymer were also detected for polymer molecular weights of 900 and above. The phase of the SFG spectra indicated that both the surfactant and polymer had a net orientation of their CH2 and/or CH3 groups toward the hydrophobic surface. Spectra of SDS in the presence of mixed polymer/surfactant solutions showed increasing conformational order as the surfactant concentration was raised. At the lowest surfactant concentrations, the spectra of SDS were weaker in the presence of the polymer than in its absence. All PEG molecular weights investigated, with the exception of PEG 400, gave rise to significant inhibition of ordered surfactant adsorption below the critical micelle concentration. The greatest inhibitory effect was noted for PEG 900. Probing interfacial PEG specifically through the use of perdeuterated SDS revealed that the polymer spectral intensity decreased monotonically as the surfactant concentration was increased for all polymer molecular weights where a PEG spectrum was apparent. These findings are interpreted in terms of the displacement of preadsorbed polymer as the surfactant concentration increases. This result is compatible with observations of adsorption from SDS/PEG solutions at solid/solution and solution/air interfaces made using other techniques.     

Alkyl chain conformation and the electronic structure of octyl heavy chalcogenolate monolayers adsorbed on Au(111)

Adsorption states of dioctyl dichalcogenides (dioctyl disulfide, dioctyl diselenide, and dioctyl ditelluride) arranged on Au(111) have been studied by X-ray photoelectron spectroscopy (XPS), infrared-visible sum-frequency generation (SFG), and ultraviolet photoelectron spectroscopy (UPS). XPS measurements suggest that dioctyl dichalcogenides dissociatively adsorbed on Au(111) surfaces to form the corresponding monolayers having chalcogen-gold covalent bonds. The elemental compositions of octanechalcogenolates on Au(111) indicate that the saturation coverages of the octyl heavy chalcogenolate (OcSe, OcTe) monolayers are lower than that of the octanethiolate (OcS) self-assembled monolayers (SAMs). The SFG observations of the CH(2) vibrational bands for the heavy chalcogenolate monolayers strongly suggest that a discernible amount of gauche conformation exists in the monolayers, while OcS SAMs adopt highly ordered all-trans conformation. The intensity ratio of the symmetric and asymmetric CH(3) stretching vibration modes measured by SFG shows that the average tilt angle of the methyl group of the OcSe monolayers is greater than that of the OcS SAMs. The larger tilt angle of the methyl group and the existence of a discernible amount of gauche conformation in the OcSe monolayers are due to the lower surface coverage of the OcSe monolayers compared with the OcS SAMs. The smaller polarization dependence in the angle-resolved UPS (ARUPS) spectra of the OcSe monolayers than that of the OcS SAMs is caused by the more disordered structures of the alkyl chain in the former. XPS, SFG, and ARUPS measurements indicate a similar tendency for the OcTe monolayers. The density of states (DOS) observed by UPS at around 1.3 eV for OcS adsorbed on Au(111) is considered to be the antibonding state of the Au-sulfur bond. Similar DOS is also observed by UPS at around 1.0 eV for the OcSe monolayers and at approximately 1.6 eV for the OcTe monolayers on Au(111).     

Sum frequency generation study of the room-temperature ionic liquids/quartz interface

The purpose of this investigation is to study the ionic liquid/quartz interface with sum frequency generation vibrational spectroscopy (SFG). SFG spectroscopy was chosen for this study because of its unique ability to yield vibrational spectra of molecules at an interface. Different polarization combinations are used, which probe different susceptibilities, giving SFG the ability to determine molecular orientation at the interface. The ionic liquids used were 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF(4)], and 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF(6)]. To determine the influence of the molecular structure and charge on orientation at the interface, neutral, 1-methylimidazole, and 1-butylimidazole were also studied. Raman spectra and depolarization ratios were obtained for neat samples of 1-methylimidazole, 1-butylimidazole, and 1-butyl-3-methylimidazolium tetrafluoroborate recorded from 2700 to 3300 cm(-1). SFG spectra of the 1-methylimidazole/quartz interface showed both methyl and aromatic C-H vibrations. Orientation calculations determined that the ring of the molecule is tilted 45-68 degrees from normal, with the methyl group oriented 32-35 degrees from normal. The SFG spectra of 1-butylimidazole contain several resonances from the alkyl chain with only one weak resonance from the aromatic ring. Orientation calculations suggest that the ring is lying in the plane of the surface with the methyl group pointing 43-47 degrees from normal. The orientation of the [BMIM][PF(6)] ionic liquid was sensitive to trace amounts of water and had to be evacuated to <3 x 10(-5) Torr for the water to be removed. SFG spectra of both ionic liquids were similar, displaying resonances from the alkyl chain as well as the aromatic ring. Orientation analysis suggests the aromatic ring was tilted 45-90 degrees from normal for [BMIM][BF(4)], while the ring for [BMIM][PF(6)] was tilted 38-58 degrees from normal. This suggests the orientation of the molecule is influenced by the size of the anion.     

Sum frequency generation spectroscopic studies on phase transitions of phospholipid monolayers containing poly(ethylene oxide) lipids at the air-water interface

Vibrational sum frequency generation (SFG) spectroscopy was applied to study the phase transitions of the mixed monolayers of l-alpha-distearoyl phosphatidylethanolamine (DSPE) and DSPE covalently coupled with poly(ethylene oxide) at the amino head group (DSPE-EO(45), DSPE with 45 ethylene oxide monomers) at the air-water interface. The SFG spectra were measured for the mixed monolayers with the mole fractions of DSPE-EO(45) of 0, 1.3, 4.5, 9.0, 12.5, and 16.7% at the surface pressures of 5, 15, and 35 mN/m. The monolayer compression isotherms indicated that the mixed monolayers at 5, 15, are 35 mN/m are mainly in the so-called "pancake", "mushroom", and "brush" states, respectively. The SFG spectra in the OH stretching vibration region give rise to SFG bands near 3200 and 3400 cm(-1). The mean molecular amplitude of the former band due to the OH stretching of the "icelike" water molecules associated mainly with the hydrophilic poly(ethylene oxide) (PEO) chains, exhibits appreciable decrease on compression of the mixed monolayers from 5 to 15 mN/m. The result corroborates the model for the pancake-mushroom transition, which presumes the dissolution of the PEO chains from the air-water interface to the water subphase. Further compression of the mixed monolayers to 35 mN/m causes a slight decrease of the line amplitude, which can be explained by considering a squeezing out of water molecules from the hydrophilic groups of DSPE-EO(45) in the brush state, where the PEO chains strongly interact with each other to form a tight binding state of the hydrophilic groups. The relative intensities of the SFG bands due to the CH3 asymmetric and symmetric vibrations were used to estimate the tilt angles of the terminal methyl group of DSPE, indicating that the angle increases with increasing the mole fraction of DSPE-EO(45). The angles almost saturate at the mole fraction larger than 10%, the saturation angle being nearly 90 degrees at 5 mN/m, ca. 60 degrees at 15 mN/m, and ca. 47 degrees at 35 mN/ m. Then, the introduction of the hydrophilic PEO head group causes a large tilting of the alkyl groups of DEPE in the mixed monolayers.     

Preparation of alkanethiol monolayers on mild steel surfaces studied with sum frequency generation and electrochemistry

An n-alkanethiol, octadecanethiol (ODT), monolayer was successfully prepared onto an oxide-free mild steel (MS) surface under cathodic polarization in a 0.1 M LiCl/CH(3)OH solution containing 1 mM ODT. Cyclic voltammetry (CV) and electrochemical impedance (EIS) and sum frequency generation (SFG) spectroscopy were applied to study and characterize the adsorption of ODT at a MS surface. In 0.1 M LiCl/CH(3)OH solution containing 1 mM ODT, CV of the MS electrode shows a dramatic decrease in charging current and a positive shift in oxidation potential when compared to a solution without ODT. The interfacial capacitance was obtained as 2.52 microF/cm(2) from the impedance data. An average chain tilt angle of 48 degrees for the ODT molecules was deduced from the comparison of the interfacial capacitances of the ODT/MS and ODT/Au monolayers. X-ray photoelectron spectroscopy confirmed the formation of the ODT monolayer on mild steel. The ppp SFG spectrum of the ODT-modified MS features three strong methyl vibrational modes at 2877, 2943, and 2967 cm(-1), indicating the formation of the oriented and densely packed ODT monolayer. However, the appearance of the two weak CH(2) groups' vibrational modes at 2850 and 2914 cm(-1) implies the presence of defects in the ODT monolayer. ODT/Au films were prepared to compare with the ODT/MS films. Orientation analysis of the air/solid interface suggests that the methyl group of ODT/Au films has a tilt angle of 30 degrees , while the methyl group of ODT/MS films has a tilt angle of 23 degrees . Water was found to have an impact on the shape of the SFG spectra of ODT/MS. This suggests that the solution penetrated through the defects to reach the MS surface.     

Microscopic Investigation of Ethylene Carbonate Interface: A Molecular Dynamics and Vibrational Spectroscopic Study

Ethylene carbonate(EC) liquid and its vapor-liquid interface were investigated using a combination of molecular dynamics(MD)simulation and vibrational IR, Raman and sum frequency generation(SFG)spectroscopies. The MD simulation was performed with a flexible and polarizable model of the EC molecule newly developed for the computation of vibrational spectra. The internal vibration of the model was described on the basis of the harmonic couplings of vibrational modes, including the anharmonicity and Fermi resonance coupling of C=O stretching. The polarizable model was represented by the charge response kernel(CRK),which is based on ab initio molecular orbital calculations and can be readily applied to other systems. The flexible and polarizable model can also accurately reproduce the structural and thermodynamic properties of EC liquid. Meanwhile, a comprehensive set of vibrational spectra of EC liquid, including the IR and Raman spectra of the bulk liquid as well as the SFG spectra of the liquid interface, were experimentally measured and reported. The set of experimental vibrational spectra provided valuable information for validating the model, and the MD simulation using the model comprehensively elucidates the observed vibrational IR, Raman, and SFG spectra of EC liquid. Further MD analysis of the interface region revealed that EC molecules tend to orientate themselves with the C=O bond parallel to the interface. The MD simulation explains the positive Im[χ~((2))](ssp) band of the C=O stretching region in the SFG spectrum in terms of the preferential orientation of EC molecules at the interface. This work also elucidates the distinct lineshapes of the C=O stretching band in the IR, Raman, and SFG spectra. The lineshapes of the C=O band are split by the Fermi resonance of the C=O fundamental and the overtone of skeletal stretching. The Fermi resonance of C=O stretching was fully analyzed using the empirical potential parameter shift analysis(EPSA) method. The apparently different lineshapes of the C=O stretching band in the IR, Raman, and SFG spectra were attributed to the frequency shift of the C=O fundamental in different solvation environments in the bulk liquid and at the interface. This work proposes a systematic procedure for investigating the interface structure and SFG spectra, including general modeling procedure based on ab initio calculations, validation of the model using available experimental data, and simultaneous analysis of molecular orientation and SFG spectra through MD trajectories. The proposed procedure provides microscopic information on the EC interface in this study, and can be further applied to investigate other interface systems, such as liquid-liquid and solid-liquid interfaces.     

Molecular restructuring at poly(n-butyl methacrylate) and poly(methyl methacrylate) surfaces due to compression by a sapphire prism studied by infrared-visible sum frequency generation vibrational spectroscopy

Infrared-visible sum frequency generation (SFG) vibrational spectroscopy, performed in visible wavelength total internal reflection (TIR) geometry, was used to determine the molecular structures of poly(n-butyl methacrylate) (PBMA) and poly(methyl methacrylate) (PMMA) surfaces in air and in contact with a smooth sapphire surface with and without the application of pressure. C-H vibrational resonances were probed optically to nondestructively examine the buried polymer/sapphire interfaces and obtain information about the molecular orientation in situ. These findings are contrasted with those of the same polymers cast from a toluene solution directly on the sapphire prism surface and annealed. Compared to polymer surface conformation in air, the SFG spectra of the deformed (compressed) PBMA at the sapphire interface illustrate that the ester butyl side chain restructures and tilts away from the surface normal. However, the molecular conformation in the similarly deformed PMMA at the sapphire interface is identical to that obtained in air, which is dominated by the upright-oriented ester methyl side chains. For PBMA and PMMA spin cast on sapphire and annealed, the surface structure of the undeformed PBMA at the sapphire interface is identical to that of the deformed PBMA at the sapphire interface, while the PMMA conformation is different and shows alpha-methyl group ordering. Since the glass transition temperature of PBMA is below room temperature, the rubbery state of PBMA demonstrates a melt-like behavior, evidenced by the fact that PBMA is in conformation chemical equilibrium at the sapphire surface even under compression. Due to the high glass transition temperature of PMMA, compression freezes PMMA in a metastable state, revealed by the restructured molecular conformation when annealed against the sapphire surface. The results of this study demonstrate that structural changes at buried polymer surfaces due to the application of contact pressure can be detected in situ by TIR-SFG vibrational spectroscopy.