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Orientational Analysis of Interfacial Molecular Groups of 2-Methoxyethyl Methacrylate Monomer Using Femtosecond Sum Frequency Generation Spectroscopy Stephanie Chong Chan, Joon Hee Jang, and Katherine Leslee Asetre Cimatu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11124 • Publication Date (Web): 09 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016
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Orientational Analysis of Interfacial Molecular Groups of 2-Methoxyethyl Methacrylate Monomer Using Femtosecond Sum Frequency Generation Spectroscopy Stephanie C. Chan1, Joon Hee Jang2, Katherine A. Cimatu*,1 1
Department of Chemistry and Biochemistry, Ohio University, 100 University Terrace, 136
Clippinger Laboratories, Athens, Ohio 45701-2979, United States 2
Department of Nanomedicine, Houston Methodist Research Institute, 6670 Bertner Avenue,
Houston, Texas 77030, United States Corresponding Author * Katherine A. Cimatu, Ohio University, Department of Chemistry and Biochemistry, 295 Clippinger Laboratories, Athens, Ohio 45701; telephone (740) 593-2308; fax (740) 593-0148; email
[email protected]
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Abstract This study employs femtosecond sum frequency generation (FS-SFG) spectroscopy to identify the functional groups at the air-liquid interface of 2-methoxyethyl methacrylate(MEMA) monomer. Based on the collected spectra, the methoxy group (–OCH3), the methylene (-CH2) group from the ethyl side chain of MEMA, the alpha-methyl group (α-CH3), and the alkenemethylene (=CH2) groups are present at the interface. Tilt angle was determined by calculating the intensity ratio values from the fitting results, and then the ratios are compared to the simulated SFG curves. The results derived from the conventional polarization combination (SSP, PPP, and PSS) approach shows average tilt angles of 60° for -OCH3 symmetric stretches vibration dipole. Using the polarization mapping method, the tilt angle for the symmetric stretch of the –OCH3 group was 48 ° with an intensity ratio (SSP/PPP) of 17. The orientation distribution of the functional groups was also obtained from the amplitude ratios using the two methods. In conclusion, the MEMA monomer is partially ordered at the air-liquid interface as a result of influences from its different functional groups. These results suggest that the α-CH3 and -OCH3 symmetric stretches are tilted further away from the surface normal, thus deviating from the presumed model of well-ordered interfacial molecules. Introduction The prevention of biofilm formation and other interface-related phenomena occurring at air, liquid, and solid interfaces are important issues in fundamental research and technological development. Specifically, fouling phenomena such as corrosion and biofilm formation can be prevented by changing bulk properties or by modifying surfaces with functionalized selfassembled monolayers or polymer coatings.1-7 Currently, polymers based on polyesters, acrylics, polyurethanes, vinyl acrylics, and organosilanes are used as coatings. Understanding the
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interfacial structures of these polymeric materials is essential in designing polymers with targeted functions for industrial applications. Thus, both the physical and chemical properties of these materials are important. The identification of the interfacial molecules is important, but understanding the average orientation and the distribution of tilt angles adopted by the existing interfacial functional groups at air-solid and liquid-solid interfaces is also of importance.8-13 It is significant to understand the interfacial structures of polymers and polymer thin films, which can be correlated to their functional, mechanical, chemical, and physical properties by also understanding the interfacial properties of their respective monomers through specific functionalization Studying the monomers will allow us to characterize the specific effects of different functional groups without restriction in finding their most favorable conformation at the air interface by satisfying the minimum free energy requirement. The interfacial molecules are expected to reorganize and adopt the conformation that minimizes free energy at the surface.9, 14-15 In this study, 2-methoxyethyl methacrylate (MEMA) is chosen as a representative of the methacrylate-based functional monomers Methacrylate and acrylate monomers, as well as their associated derivatives, are used extensively in the polymer plastic industry. Due to their doublebond structure, the alkene-CH2 groups are reactive and easily polymerize. For example, the polymeric material poly(2-methoxyethyl acrylate) (pMEA) has been used as a coating material for artificial organs because it is bioinert and compatible with blood.11, 16-17 The 2-methoxyethyl methacrylate functional monomer is an essential building block of poly(2-methoxyethyl methacrylate). Thus, the effect of functional groups on the ethyl side of the methacrylate monomer become of great interest.
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Femtosecond vibrational sum frequency generation (SFG) spectroscopy is employed to observe the surface orientation of the alpha-methyl (α-CH3), methylene (CH2), methoxy (– OCH3) and alkene-methylene (=CH2) groups. These functional groups of the MEMA monomer can then be probed within the CH spectral region. By investigating their contribution in this way, the orientation of the interfacial molecules of the MEMA monomer can also be correlated to the final structures of the polymer itself. The final structures of these polymers at the interface can be attributed to the chemical structure of the monomers and how the orientation of the molecules are affected by varying the end-group position of the functional groups at the ethyl side of MEMA. Previous publications focusing on different kinds of monomers as small molecules have been reported which included discussion on the orientation of the monomers at the interface using SFG.18-21 A good example would be the orientational analysis of three diols which mainly focused on the CH stretching modes of methylene groups as a model for the methylene-only molecular system. It has been stated that the orientation and conformational analysis of the ethylene glycol , as an example of a monomer, was based on the polarization analysis and using the polarization selection rule which was used to obtain the vibrational mode assignment for methylene group at the vapor-liquid interface. They have concluded that tilt angle was around 37° ± 8° for the methylene group.18 Our study would provide average orientation information on MEMA monomers, including the contributions of each of the functional groups to the orientation, by fitting the SFG spectra obtained from different polarization measurements. As mentioned, SFG is a suitable vibrational technique for studying different combinations of interfaces between two bulk phases. SFG spectroscopy has existed for more than three decades and has been continuously improved and developed in conjunction with
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material science developments; it has found application in the study of energy materials, soft condensed substances, nanoparticles, biomaterials, and even atmospheric chemistry.22-33 In general, this second-order nonlinear optical technique is interface-specific with monolayer sensitivity. The most significant aspect of this technique is its ability to probe the interfaces between two bulk phases at the gas, liquid, and solid interfaces, which differentiates it from other vibrational techniques. SFG is an advanced surface technique, and interpretation of the SFG spectra is quite challenging. The smaller and simpler molecules are expected to organize well at the surface, which makes the interpretation of the SFG spectrum less complicated because the individual peaks are more resolved in the spectrum. However, if the samples of interest or molecules become more complex and multifunctional, overlapping peaks and an increase in the degree of disorder result to a more difficult peak assignment and fitting. Additionally, SFG intensity is proportional to three molecular parameters: 1) molecular hyperpolarizability, 2) the concentration and number density of the functional groups at the interface, and 3) the orientation of the functional groups (both the average orientation and the width of the orientation distribution). Thus, monomeric and polymeric surfaces with a more challenging structure will introduce difficulty in the direct interpretation of their spectra.34 Studies performed in our group are also characterizing the polymer thin films of the homopolymers of these methacrylate-based monomers using SFG spectroscopy. But the scope of this paper only focuses on the analysis of the interfacial molecules of MEMA monomers, including the vibrational mode assignment, orientation and the distribution of the tilt angles of the molecular groups at the air interface. As mentioned, the functional groups at both ends of the MEMA monomer can be probed within the CH region using sum frequency generation (SFG), as shown in Figure 1. While it can provide a lot of information in short range of vibrational
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frequency, the SFG spectra can be complex with overlapped peaks. To resolve this, two methods were chosen to obtain the orientation of the interfacial molecular groups. The conventional polarization measurement requires collecting the spectra with different polarization combinations. Various angles of the SFG polarizer will be utilized through the polarization mapping method (PMM). The PMM shows peaks that transition during an S-to-P polarization of the resultant SFG beam. The SFG spectra from both the S and P polarizations are fitted simultaneously. This approach is applicable for complicated and partially ordered molecules.3537
This method has been used by Chen and Baldelli groups which indicated that mapping of the
spectral polarization significantly improves the result in SFG analysis.35-39 The results of these two approaches are compared and discussed with spectra fitted for 9 peaks. We also used PMM to support and confirm the number of peaks, the peak positions, and the orientation of the chemical groups. Sum Frequency Generation Background Sum frequency generation spectroscopy is a second-order nonlinear process technique that probes various interfaces and obtains the vibrational spectra of the interfacial molecules. An SFG signal only arises when two pulsed laser beams spatially and temporally overlap at the interface. Thus, a specific geometry is chosen to set both the fixed frequency visible beam, , and the mid-infrared beam, , at a certain angle to overlap and generate the SFG beam, . The resulting emitted SFG light is the sum of both frequencies of light and is generated at an angle that can be calculated using the phase-matching condition. Consequently, the emitted SFG light intensity is resonantly enhanced when the mid-IR frequency matches any of the vibrational modes of the interfacial molecules. The SFG vibrational spectrum can be presented as intensity plotted against the IR frequency.40-43
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Compared with other second-order nonlinear techniques, there are more specific selection rules for SFG. First, and most importantly, the SFG photons are only generated in a noncentrosymmetric environment, which exists at interfaces and surfaces where the inversion symmetry is broken. Thus, the SFG process is interface specific and differentiates from the bulk phase, i.e., isotropic distribution. Second, SFG is dependent on the net polar orientation of the interfacial molecules. Thus, less SFG photons and intensity are acquired and observed when equal numbers of molecules are arranged oppositely, thus canceling their induced dipole moments. Third, an increase in disordered surface structure due to the preferred interaction of molecules can also affect the SFG and the quality of the vibrational spectrum. In addition, SFG is a coherent process. Because the emitted SFG light and the two incident beams have magnitude, direction, and phase, this technique has been used to determine the orientation of functional groups at the interface. This information is obtained by acquiring SFG spectra using different polarization combinations. These polarization combinations are then related to their nonlinear susceptibility tensor elements. For example, the calculated intensity ratio between two polarization combinations can be used to determine the average tilt angles of the interfacial molecules from the surface normal.44 The details of SFG theory have been discussed thoroughly elsewhere.32,
44-48
A short
summary is provided in Supporting Information to present some of the important equations that describe SFG theory. Orientational Analysis The orientational analysis is the determination of the tilt angle distribution of various functional groups from the surface normal. By recording the SFG spectra at different polarizations—more specifically, SSP, PPP, PSS, and SPS combinations—the fitted values can
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be used to estimate amplitude or intensity ratios, which can then be compared to the simulated curves. Theoretically, this approach makes the estimation of the nonlinear susceptibility tensor elements possible. The mathematical relationship between the effective susceptibility of each polarization combination and the molecular hyperpolarizability is well documented and is also shown in Supporting Information, where the simulated curves were derived.44-45, 49 A simplified SFG equation related to its intensity is shown in Equation 1:
∝ +
(1)
where the subscripts and are the resonant and non-resonant susceptibilities. is independent of frequency over the range of interest, whereas is the sum of the Lorentzian line shapes related to the vibrational spectra of the molecules at the interface within the range that the peaks are observed.50 To analyze the data obtained with each distinct polarization combination, the Lorentzian line shape is adapted to also account for the line broadening as shown in Equation 2: ∝ | | ∝
! "#
+ $ % (2)
where N is the molecular number density of the species responsible for the vibrational transitions, and & and '& are the resonance frequency and the damping constant of the qth vibrational mode, respectively. is the incident IR frequency While ( is the orientational averaged hyperpolarizability that can be further defined as the product of IR diple moment and Raman polarizability tensor. ) is the phase of the non-resonant response. Instead of making the non-resonant contribution negligible, we opted to add it to the fitting equation to account for any
contributions of the monomer bulk material to because the material can still be considered a dielectric medium with some permittivity. Equation 3 is a simplified version of the fitting
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equation that accounts for the broadband width of the IR beam profile by adding a Gaussian * with a spectral width of +* to the SFG fitting equation:50 function centered at
+ ∝ $,- −
/! 0 1 / 0
× ∑&
4
! "5
+ 6 $ % (3)
The full width-half maximum is approximately 200-250 cm-1 for one center.32 The amplitude factors, 6& and 6 , are proportional to the product of the orientation-averaged transition dipole moment and polarizability or the molecular hyperpolarizabilities as shown in Equation 2.51 Polarization Mapping Orientation analysis using polarization combinations is the conventional method used to obtain molecular orientation information at an interface. However, interpreting the resulting spectra is difficult, especially when there are excessive unresolved peaks. Overlapping peaks in the SFG spectra can obscure proper peak assignment and create ambiguity in the orientation analysis. The most effective way to eliminate or reduce the spectral overlap is to maximize spectral resolution. Although the SFG setup can achieve a fine line shape at a particular wavenumber, most of the SFG spectra suffer from peak overlap.32 Alternatively, measuring spectral intensity changes at various polarization angles can simplify the assignment of overlapping peaks by making other peaks as part of the convoluted spectra more prominent at each specific polarization angle.35-37 As noted from previous works performed by Z. Chen, H.F. Wang, and S. Baldelli groups, mapping improves SFG analysis which includes approaches such as polarization null angle (PNA) analysis and polarization mapping method (PMM). For PNA analysis, this method involves setting the polarization of the incident beams similar to polarization mapping method, set the IR wavenumber fixed at a resonant frequency for a specific vibrational mode, and then, collect the spectrum as a function of SFG polarization angle. This method is well-suited for well-separated multiple peaks without
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interference to obtain the surface orientation of molecules.52-54 For example, to cite its advantages the accuracy and sensitivity of the interfacial molecules were reported studying methanol at vapor/liquid interface. They reported that PNA method is more accurate compared to polarization intensity ratio by an order of magnitude and this method is dependent on the set-up configuration and also affected by the changes in the polarization angle of the visible beam compared to the IR beam.54 On the other hand, according to Z. Chen et. al. and Baldelli et.al., the PMM analysis can provide a better description of the surface at a molecular level through chemical identification and orientation and also effective for convoluted spectra.36, 39They have described this method to present a more reliable spectral information by means of characterizing the surface using seven different polarization. As performed in this study, our simultaneous fitting according to them will reduce the bias in fitting. As will be shown in the results and discussion, a 2D contour plot of the polarization angle, σ, as a function of the IR wavenumber helps in the visualization of different vibrational modes attain maximum intensity.39 Another work of Baldelli et.al. utilized three methods to perform quantitative orientation analysis of acetonitrile on a rutile titanium dioxide surface. These methods include distinct polarization combination, polarization mapping, and null angle method. The study concluded that the distinct polarization combination results have agreed with the PMM results and reported that the acetonitrile has a tilt angle of 30 ̊. 37 To use this approach in our experiment, the polarizations of visible, mid-infrared (IR), and SFG beams are set to 45° from p-polarization, p-polarization, and in 15° increments from 0° to 90°. The complete expression of the effective susceptibility at a given SFG polarization angle, σ, is as follows:
= sin ; sin ; cos ; >
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+ sin ; cos ; cos ; >>
+ cos ; sin ; cos ; >>
+cos ; cos ; cos ; >>>
(4)
= sin ; sin 45° cos 0° >
+ sin ; cos 45° cos 0° >>
+ cos ; sin 45° cos 0° >>
+ cos ; cos 45° cos 0° >>>
(5)
where σ represents the SFG polarizing angle from the p-polarization adjusted every 15°. >>
and >> are the symmetrically disallowed components of the generated SFG signals. Thus, the second order nonlinear susceptibility can be simplified as shown in Equation 6:37 = F=
1
√2
F sin ; + cos ; >>>
0
GHHI
(6)
0
GIII
By substituting Equation 6 into Equation 3, the final SFG intensity is described as follows:
4KKK L MNO P" QRM P
∝ J J ∝
! "#
+ F STU ; + VWS; XYYY $ %
(7)
where 6--- and X--- are resonant and non-resonant amplitude, respectively. The normalization factor, is omitted and absorbed to amplitude in Equation 7. F and F respectively correspond to resonant and non-resonant part of the ratio of SSP to PPP amplitude. After fitting SFG experimental data, the resonant amplitude ratio, is used to deduce orientation of the monomer molecules of interest.
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For simultaneous fitting of the spectra derived from polarization mapping, Equation 8 is considered. This equation accounts for any effect from the non-resonant contribution. A Gaussian function is used to account for the width of the mid-IR beam. + ∝ $,- −
/! 0 1 / 0
× ∑&
4KKK L MNO P" QRM P ! "5
+ F STU ; + VWS; XYYY $ %
(8) Experimentally, the MEMA monomers are characterized using femtosecond sum frequency generation spectroscopy. In SFG, the air-liquid interface of pure MEMA monomers is exposed to the pulsed incident beams, which generate the SFG spectrum and thereby satisfy the selection rule. In this study, the SFG spectra were acquired using two methods. To perform orientational analysis, the polarizations of the SFG, a fixed 795-nm beam, and the mid-infrared beam were adjusted to obtain SSP, PPP, PSS, and SPS polarization combinations; S is perpendicular, whereas P is in a parallel position relative to the plane of incidence. The SFG spectra were fitted using Equation 3 in MATLAB R2014b to obtain the estimated average tilt angles for the –OCH3, -CH3, =CH2, and -CH2 functional groups from the SFG simulation curves derived using Wolfram Mathematica 9. Conversely, the polarization mapping method (PMM) was performed on the SFG spectra collected by setting the mid-IR beam at p-polarization, the fixed 795-nm beam 45 degrees from p-polarization, and the SFG polarizer at 0° - 90° in 15° increments. This method was used to resolve convoluted peaks present in the SFG spectrum and to obtain the distribution of each vibrational mode as a function of the angle set in the SFG polarizer. Detailed SFG theory on the generation of the simulated curves used for estimation of the average tilt angles and the orientation distribution from the SFG intensity used in this paper are provided in the Supporting Information.
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Experimental Methods A.
Sample Preparation. Eicosanoic acid and the 2-methoxy methacrylate monomer were
purchased from Sigma-Aldrich and used without further purification. Two milligrams of ≥ 99.0% purity eicosanoic acid was dissolved in 1 mL of chloroform. This solution was used to form a monolayer on water in a clean petri dish. This measurement is essential for air-liquid interface alignment using the SSP polarization combination. After SFG signal optimization, the eicosanoic acid on water was replaced with the MEMA liquid monomer (min 85% purity from PolySciences, Inc. and 99% purity from Sigma-Aldrich). The alignment of the beam for the pure MEMA liquid monomer was estimated using an SFG reference beam and an estimated volume of the monomer in a clean petri dish without changing the height of the sample stage. The average humidity and room temperature were 25% and 24 °C, respectively. B.
Femtosecond SFG Spectroscopy (FSSFG) The detailed set-up was described previously.55 Briefly, the femtosecond sum frequency
generation spectroscopy (FSSFG) used a temporally and spatially overlapped fixed 795-nm beam and IR beam at the surface plane with incident angles set to 50° and 60°, respectively, which generated the third beam (the SFG beam) 52° from the surface normal. The setup utilized Solstice from SpectraPhysics to generate the fundamental beam at 795 nm, which was then sent to a 50:50 beam splitter. The first 50% of the 795-nm beam was sent to a delay line composed of dielectric mirrors, an attenuator, a Fabry-etalon, collimating lenses, a half-waveplate, and a focusing lens before the sample stage. Simultaneously, the other 50% of the 795-nm beam was sent to an optical parametric amplifier to generate a collinear signal and idler beams. Next, both the signal and the idler beams arrived at the same time and space at the difference frequency (DFG) crystal to generate the mid-infrared beam. The mid-IR beam path was composed of
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mirrors, a periscope assembly, a half-waveplate, and a focusing IR lens. After overlapping, the SFG beam was collected through a series of optics including a set of collimating lenses, mirrors, and a polarizer. The SFG signal was acquired and saved using a combination of a spectrograph and a charge-coupled device camera. For using the conventional polarization combination approach, the spectra were collected by utilizing the polarization configurations of SSP, PPP, and PSS; each letter in the combinations represents the polarization of the SFG, 795 nm, and mid-IR beams, respectively. Initially, the system was optimized for gold using the overlap and timing of the PPP polarization, and the xaxis was calibrated using the IR spectrum of a polystyrene film. Next, the gold spectra were collected with a 3-sec acquisition time at 2800 cm-1, 2900 cm-1, 3000 cm-1, and 3100 cm-1 including their respective background spectra. All spectra were background corrected, and the four center wavenumbers were summed. The final gold spectrum was fitted using a Gaussian function, and the full-width half-maximum (FWHM) was obtained. The FWHM value (~404 cm1
for the four centers) was considered as a factor in the Lorentzian line shapes present in the
vibrational spectra and in the correction of the mid-IR beam profile in fitting the SFG spectra. Three trials of the SFG spectra of the MEMA monomer were collected for 9 minutes of data acquisition from the 4 mid-IR center wavenumbers of 2800 cm-1, 2900 cm-1, 3000 cm-1, and 3100 cm-1 including 9 minutes of background data at each center. Each SFG spectrum was background corrected, smoothed using 10 points, and averaged to obtain the standard deviation from the three trials of one center wavenumber. The background-corrected SFG spectra from all four centers were then summed and stitched together. An example of the SFG spectra is shown in Supporting Information Figure S2 for the SSP polarization of the MEMA monomer. After data collection and processing, the SFG spectra are fitted according to their polarization using
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MATLAB 2014b. The resulting amplitude fitting values of the SSP, PPP, and PSS polarizations were compared by taking the ratios of SSP to PPP and SSP to PSS to estimate the average tilt angle from the surface normal of the functional groups of the MEMA. An example of the simulated plot of the estimated SFG intensity shows the curves for both methyl symmetric and asymmetric stretches later in the article. The ratio values were then compared to the formulated simulation curves from Mathematica 9.0 to estimate the average tilt angles and the distribution of the tilt angles or angular distribution. Details on the orientational analysis are available in Supporting Information. For the polarization mapping method, as described earlier in the text, the fixed 795-nm beam was set to 45° from p-polarization, the mid-IR polarization was set at p-polarization, and the SFG polarizer was changed in increments of 15° from 0° - 90°.36-37, 39 The SFG spectra were collected every 15° for 9 min of data acquisition. Three trials and background spectra were performed for each SFG polarization angle. All spectra were acquired at the four different centers of 2800 cm-1, 2900 cm-1, 3000 cm-1, and 3100 cm-1. The same method of data processing used for the conventional polarization combination method was applied for the spectral results of the polarization mapping method. An example of one SFG polarization angle including the spectral data obtained from different centers and its total spectrum are available in Supporting Information Figure S3. Next, all the spectra taken from 0° to 90° are simultaneously fitted using Mathematica 9.0. The values were extracted as .csv files, which were then transferred to Origin 9.1 to obtain the individual fitted data for each spectrum of the PMM results, and the colored final polarization two-dimensional map was obtained using Mathematica 9.0, where the x, y, and z-axes were set to wavenumbers (cm-1), SFG polarization angles (°), and SFG intensity (arbitrary units), respectively. From the fitting results, gamma values (FY/YYY ) were obtained for each
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vibrational mode present in the SFG spectrum. The gamma values are the amplitude ratios of SSP and PPP as shown in Equation 8. If the square of the amplitude ratios or the square of gamma values were calculated and used as intensity ratio values, the estimated values were compared to the orientation curve, from which the average were obtained. Notably, because the bulk of the monomers are isotropic, the rotation angle was not considered in the orientation curve. Results and Discussion The 2-methoxyethyl methacrylate monomer was chosen not only for its applications in polymer science but also because the methoxy (–OCH3) group substitution at the ethyl side chain can be probed in the CH region. The vibrational peaks from the –OCH3 functional group can be distinguished from the vibrational modes of other functional groups present at the interface. Suggested peak assignments for the 2-methoxyethyl methacrylate monomer (MEMA) Figure 1. A ChemDraw 3D representation of a 2-methoxyethyl methacrylate monomer where 1, 2, and 3 are assigned to the alpha-methyl (α-CH3), alkene-methylene (alkene-CH2), and methoxy (–OCH3) groups, respectively.
As shown in Figure 1, the functional groups in the MEMA monomer structure that are probed in the CH region are the alpha-methyl (α-CH3), alkene-methylene (alkene-CH2), alkane-methylene (-CH2), and methoxy (–OCH3) groups.
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1
9
2 3 4 5 6 78
10000 9000
SFG Intensity (a.u.)
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8000 7000 6000 5000
SSP
4000 3000
Figure 2. SFG spectra of 2-methoxyethyl methacrylate monomer at the air-liquid interface. The SSP, PPP, and PSS SFG spectra with error bars are shown in black with a red fit line, blue with a magenta fit line, and green with a navy blue fit line, respectively. The line traces for the four polarizations are displaced for clarity. The horizontal lines labelled as 1, 2, 3, 4, 5, 6, 7, 8, and 9 from the fittings are positioned at ~2748 cm-1,~2816 cm-1, ~2864 cm-1, ~2902 cm-1, ~2937 cm-1, ~2974 cm-1, ~2998 cm-1, ~3010 cm-1, and ~3122 cm-1.
All spectra were collected at 4 centers: 2800
2000
PPP PSS
1000 0
2600 2700 2800 2900 3000 3100 3200
Wavenumber (cm-1)
cm-1, 2900 cm-1, 3000 cm-1, and 3100 cm-1. Then, the spectra from each center of the
broadband mid-IR beam were added for each designated polarization combination. The summation and individual SSP spectra from each center are shown, as an example, in Supporting Information Figure S2. With the SSP polarization combination, the number of peaks is initially assigned to five (~2730 cm-1, 2810 cm-1, 2885 cm-1, 2930 cm-1, and 3000 cm-1), but for more consistency between the three chosen polarization combinations and the polarization mapping method results, we decided to fit the data with nine peaks. The 9-peak fitting results for SSP, PPP, and PSS polarization combinations are available in Supporting Information Tables S1-S3. Figure S4 also shows the SSP, PPP, and PSS spectra plotted and presented without reducing the number of data points, and it includes the error bar or the scatter for each data point. The shoulder positioned at ~2748 cm-1 is a vibrational peak that is assigned as an overtone of a CH bending mode56; however, for our purposes, we will leave it unassigned. As shown in Figure 2, the peaks occur at ~2816 cm-1
16-17, 57-58
, ~2860 cm-1, ~2902 cm-1
59-60
, 2937 cm-1, 2974 cm-1,
2998 cm-1, 3010 cm-1, and ~3122 cm-1. The peaks were assigned as the methoxy symmetric CH stretch (–OCH3 SS)11, 16-17, 57, 61, fermi related methylene group vibrational mode34, 62, α-methyl symmetric CH stretch (\-CH3 SS)11,
16-17, 59
, α-methyl symmetric stretch split by Fermi
Resonance (\-CH3 FR)63-64, α-methyl asymmetric stretch (α-CH3 AS)11,
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, methoxy
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asymmetric stretch (–OCH3 AS)9, 11-13, 34, 59-60, alkene-methylene CH stretch (=CH2 SS),65 and a broad peak that may be due to the presence of hydroxyl peaks from the adsorption of water13, 6669
, respectively. The last broad peak is more pronounced in the PPP and PSS polarizations. A
summary of the vibrational peak assignment for the three polarization combination is listed in Table 1. Again, the 1H NMR and IR spectra of the MEMA monomer were collected and are presented in Supporting Information Figure S5 to confirm the purity and the number peaks present in the IR spectrum and showed a very small peak close to the ~2816 cm-1 peak at approximately 2738 cm-1. The NMR and IR data were taken from the same sample source. To verify the presence of the ~2738 cm-1 peak, the SFG experiments were performed using a MEMA monomer from two different sources with different purities (PolySciences, ≥85% purity; and Sigma Aldrich, 99% purity) at the SSP polarization combination. The shoulder peak at ~2738 cm-1 was observed in both samples. The –OCH3 peak dominated the SFG spectrum, implying that the –OCH3 methoxy group is likely present at the interface and is partially ordered or partially aligned. Contributions from the α-CH3, -CH2, and alkene-CH2 functional groups remain in the SSP spectrum, but the relative contribution and distribution of each functional group at the interface is unknown. For the PPP polarization, two broad peaks and one narrow peak were observed in the SFG spectrum. Again, the PPP spectrum was fitted with 9 peaks, and the suggested peak assignments are the same as with the SSP polarization. The peaks positioned at ~2758 cm-1, ~2831 cm-1, ~2865 cm-1 17, 60, ~2895 cm-1, 2956 cm-1, ~2978 cm-1 59-60, ~2992 cm-1, 2999 cm-1, and ~3100 cm-1 are assigned to an unassigned peak, methoxy symmetric CH stretch, fermi related methylene group vibrational mode34, 62, α-methyl symmetric stretch, α-methyl symmetric stretch split by Fermi Resonance (\-CH3 FR)63-64, α -methyl asymmetric stretch59-60, methoxy
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asymmetric stretch (–OCH3 AS)9, 12, 34, 60, 70, and a broad peak due to the presence of hydroxyl peaks from water adsorption13, 66-69, respectively. The two broad peaks are likely a combination of overlapping peaks for multifunctional partially ordered small molecules. From a previous publication55, the peak at ~3000 cm-1 was assigned to the alkene-CH2 CH stretch at SSP for the 2-hydroxyethylmethacrylate (HEMA) monomer with an –OH functional end group. Interestingly, for the MEMA monomer, we still observed the peak at ~ 3010 cm-1 with the SSP polarization and assigned the peak as the alkene-CH2 CH stretch. On the other hand, a peak positioned at 2996 cm-1 is present in the PPP spectrum which is about 14 cm-1 away to the position of the =CH2 vibrational mode. Thus the peak at 2996 cm-1 can be assigned to –OCH3 asymmetric stretch.
9, 11-13, 34, 59-60
There is almost no contribution from the alkene-CH2 CH
stretch in the PPP SFG spectrum, as shown in Figure 2. Table 1. List of the spectral peak positions(fitted) and suggested vibrational peak assignments of 2methoxylethyl methacrylate(MEMA) monomer for SSP, PPP, and PSS polarization combinations.
Vibrational Assignment SSP (cm-1) 2748(±5) 2816(±1)
Polarization PPP (cm-1) 2754(±3) 2831(±1)
PSS (cm-1) 2752(±10) 2811(±6)
Fermi related methylene group vibrational mode34, 62 \-Methyl (-CH3) symmetric stretch 59-60 \ − Methyl (-CH3) Fermi Resonance (FR) 63-64 \-Methyl (-CH3) asymmetric stretch11, 59-
2860(±10)
2865(±3)
2853(±3)
2903(±2) 2937(±1)
2895(±4) 2956(±2)
2895(±2) 2934(±4)
2975(±1)
2979(±2)
2964(±4)
Methoxy (-OCH3) asymmetric stretch9, 11-
2998(±4)
2992(±2)
2991(±2
Unassigned56 Methoxy (-OCH3) symmetric strech11, 1617, 57,58, 61, 69, 71
60
13, 34, 59-60
Alkene-Methylene (=CH2) CH stretch65 3009(±3) 2999(±4) 3002(±5) the presence of hydroxyl peaks of 3134(±20) 3100(±7) 3100(±6) adsorbed water molecules13, 66-68 PSS spectra were also collected from 4 different mid-IR wavenumber centers and summed; the final PSS spectrum is also shown in Figure 2. The peak assignments of the
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spectrum including its fitting results are available in Supporting Information Table S3. Because the peaks were identified, estimating the average orientation and orientation distribution of specific functional groups at the interface would be very interesting. Two approaches were applied in this project: a conventional polarization combination and polarization mapping methods. Orientation Analysis of SSP, PPP, and PSS polarization combinations For this approach, the spectra were collected at different polarizations, and Equation 3 was used to fit each spectrum individually to estimate the relative amplitude values for each contributing peak. As shown in Supporting Information, we only used the results of the 9-peak fitted spectra in Tables S1-S3. The description for the derivation of these simulated curves is given in Supporting Information including Table S4, which contains the values of parameters used for obtaining the simulated SFG curves.18, 44, 72-73 The R-values used for the simulation were 3.4 and 1.721,
74
for alpha-methyl and methoxy groups, respectively, chosen from previous
publications where R = 1.66 – 4.0.21,
44-45, 47, 73-76
and the refractive indices of the MEMA
monomer used for SFG(nSFG), visible (nvis), and IR(nIR) were 1.4884, 1.4848, and 1.4810, respectively.77 The refractive index of the interface between air and the MEMA monomer was set to 1.2424.18,
73
The refractive indices were obtained from a dispersion formula used for
poly(methyl methacrylate) with the assumption that the refractive index values are similar to their monomers with the same structural backbone.77 Because we have a co-propagating experimental geometry, the molecular hyperpolarizabilities will have a weak dependence on the refractive indices of the IR beam.18 Figure 3 presents the estimated SFG intensity of the CH3 symmetric and asymmetric stretches with C3V point group symmetry for SSP, PPP, and PSS polarization combinations.
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2
Abs[C3VssPPP] 2 Abs[C3VssSSP] 2 Abs[C3VssPSS]
0.6
Simulated SFG Intensity, A.U.
0.6
Simulated SFG Intensity, A.U.
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2
Abs[C3VasPPP] 2 Abs[C3VasSSP] 2 Abs[C3VasPSS]
0.4
0.4
0.2
0.2
0.0
0
20
40 60 Tilt Angle, θ
80
0.0
0
A
20
40 60 Tilt Angle, θ
80
B
Figure 3. Estimated SFG intensity of the CH3 (A) symmetric and (B) asymmetric stretches at the SSP, PPP, and PSS polarization combinations.
As shown in Figure 3A, taking the ratio between SSP and PPP amplitude values of the CH3 asymmetric stretch will result in a similar tilt angle since the SFG intensity values of both SSP and PPP does not vary as a function of tilt angle. SSP and PPP simulated SFG intensity curves have the same profile especially from 0° - 45°. Thus, it is reasonable to take the ratio between SSP and PSS to obtain the average orientation (tilt angle from the surface normal) and orientation distribution (angular distribution of each possible tilt angles) since their profiles are different. Then, in Figure 3B, the SSP and PPP polarization simulated SFG intensity profiles as a function of tilt angle are similar in a tilt angle range of 5° - 85°. With PSS polarization intensity profile varying from 0°- 60°, it is reasonable to use the ratio of SSP and PSS and obtain reasonable values for the orientation.
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Simulated SFG Amplitude Ratio
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10
129o
61o
5
0
20
40
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4 o θ=0 o θ=10 o θ=20 o θ=30 o θ=40 o θ=50 o θ=60 o θ=70 o θ=80 o θ=90
3
2 0
80 100 120 140 160 180
20
40
80
B
0.0001
175o
0.0000
Simulated SFG Amplitude Ratio
A
5o
60
Distribution Angle, σ
Tilt Angle, θ
Simulated SFG Intensity Ratio
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θ=0
0.2
θ=20 θ=30 θ=40
0.0
θ=50 θ=60 θ=70 θ=80
-0.2
0
20
40
60
80 100 120 140 160 180
Tilt Angle, θ
o
θ=10
θ=90
0
20
40
60
o o o o o o o o o
80
Distribution Angle, σ
D
C
Figure 4. A and C. Simulated SFG intensity ratio curves are presented between the symmetric and asymmetric stretches of the SSP and PSS polarizations for the methyl group. The curves are also marked with the corresponding fitting results to obtain the average tilt angles. B and D. Simulated SFG amplitude ratios between the SSP and PSS as a function of the distribution angle. As indicated, there are several possible distribution angles for several specific tilt angles. The horizontal solid line and vertical bars indicate the calculated SFG intensity ratio and uncertainty error with a confidence level of 95% obtained from the fitting analysis.
We used Mathematica 9.0 to generate the simulation curves for the delta (+) function orientation ratio that generates the simulated SFG intensity curves as a function of tilt angle (theta, ϴ) and distribution orientation curves. The simulated SFG intensity curves are derived from plotting the absolute value of the squared ratio between C3V SS SSP and C3V SS PSS ]^_>
(Abs[]^_>]2 ) versus ϴ, which represents the tilt angle from the surface normal. Most of the time, these interfacial molecular groups do not have the same tilt angles, making our assumption
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using the delta function orientation less accurate. Thus, we also opted to obtain the simulated curve for the SFG symmetric stretching amplitude ratio versus the distribution (in theta, ϴ) of the tilt angles, for which the distribution of the tilt angles obeys a Gaussian distribution.12,
44
We
used the following equations: `abSSTaU cTSdeTfbdTWU =
g
Phij√k
$
lmln 0 0o0
(9)
where the radians are changed to degree and p, pq , ;, and ; can be represented as the tilt angle, average or mean tilt angle, standard deviation, and the variance, respectively. ; dictates the deviation of a set of numbers distributed about the average value of the tilt angle. In plotting the curves, ;hij is used as the x-axis and as the distribution angle for varying average tilt angles. Then, the amplitude distribution was expressed as Equation 10: u
n
6r-sTdbc$ cTSdeTfbdTWU = 6fS t n
vwGxyy MNOz{|∗ ~~w hwh{
n
un
MNOz{|∗ ~~w hw h{
(10)
where i can also be represented by the chosen polarization combination, such as SSP or PSS, deduced from a C3v point group and for a symmetric stretch. Equation 11 was utilized to obtain the simulated normalized amplitude ratio of the simulated curve as a function of distribution angle: 4Yhi w ]^_>
XWerasT$c 6r-sTdbc$ adTW cTSdeTfbdTWU = 4Yhi w ]^_> (11) These equations are applicable to the fitting results obtained from the conventional polarization combination and polarization mapping methods.
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The coding of the simulation calculations was based on the reported SFG theory. After generating the curves for the SSP and PSS polarizations and fitting the spectra, the estimated amplitude values were compared with the curves. In Figure 4A and 4C, the simulated SFG intensity curves are shown for symmetric and asymmetric stretches of the methyl functional group in SSP and PSS polarizations. Notably, uncertainty errors in the main text and all the error bars in the graphs were calculated with a 95% confidence level. As labeled, Figure 4A corresponds to the α-CH3 symmetric stretch. The SSP and PSS intensity ratio for the CH3 group were 9±4. As shown in Figure 4A and listed in Table 2, the intensity ratio value of 9 represents two average orientation tilt angles: 61°± 13° and 119° ± 26°. However, the average tilt angle orientations of the methyl group indicates that the conformation of the MEMA monomer at the air-liquid interface is partially ordered with an angle of 61° from the surface normal, especially when compared to ordered molecules at the same interface or with a self-assembled monolayer that has a 10° - 20° average tilt angle.78 Whereas Figure 4B shows the distribution angle of the orientation for α-CH3 SS vibrational mode. The calculated amplitude ratio from fitting values is 3±0.8 α-CH3 SS. Table 2 summarizes the average orientation and distribution orientation including the uncertainty error values of the methyl symmetric and asymmetric stretches at a 95% confidence level (CL). Thus with an amplitude ratio of 3 for ‑CH3 SS from the α-CH3 group, the distribution angles (Table 2) represent a relatively narrow distribution of the varying tilt angles especially for a tilt angle of 60 ° that has a distribution of 8°. As the tilt angle from the surface normal increases, the distribution angle increases. The results from using the intensity and amplitude ratios are comparable because not only that the tilt angle was determined but the distribution of this specific angle is also obtained that can be relevant in relation to the surface coverage for a specific vibrational mode.
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In Figure 4C, the intensity ratio obtained from SSP and PSS the -CH3 asymmetric stretch (AS) is 7E-5± 3E-5, and when correlated to the simulation curve, the average tilt angle orientations of -CH3 AS stretch are 5± 2° and 175± 81° from the surface normal. To note, the high uncertainty error values at a 95% CL can be from the fitting accuracy of the convoluted spectra and the difference in the signal-to-noise ratio of both the SSP and PSS SFG spectra. Then, with the distribution curve shown in Figure 4D, the –CH3 AS stretch has an amplitude ratio value of 9 ± 20 x 10-3, from which a very narrow distribution angle of 4± 9° is obtained for a tilt angle of 0°. Since the calculated values obtained from the ratio of SSP and PSS asymmetric stretches are small, the fitted result barely satisfies the assumption of a single orientation in the simulated curve. Thus, finding the average orientation and distribution orientation with good accuracy is difficult, as shown in Figures 4C and 4D. Table 2. List of possible average tilt angles of the methyl(-CH3) functional group presents at the interface. These values are estimated from the simulation curves generated by Mathematica. The reported uncertainty error was calculated with a 95% confidence level from the fitting analysis.
Methyl Group (CH3 group)
Peak Amplitude (Fitting result)
Intensity Ratio (SSP/PSS)
Symmetric Stretch
Asymmetric Stretch
Average Tilt Angle
Amplitude Ratio (SSP/PSS)
61°± 13° 822± 49(SSP) 271± 43(PSS)
9 ± 4
119°± 26°
3.03± 0.66
1.9± 4.1(SSP) 225± 40(PSS)
7E-5± 3E-5
5°± 2°
0.009± 0.020
Orientation Distribution
8°± 3° (60° tilt angle) 17°± 8° (70° tilt angle) 20°± 9° (80° tilt angle) 21°± 9° (90° tilt angle) 4°± 8° (0° tilt angle)
175°± 81°
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118o
60o
35 30 25 20 15 0
20
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60
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8 θ=0
o
θ=10
7
θ=20 θ=30 θ=40
6
θ=50 θ=60 θ=70
5
θ=80 θ=90
o o o o o o o o o
4 0
80 100 120 140 160 180
10
20
30
40
50
60
70
80
90
Distribution Angle, σ
Tilt Angle, θ
B
0.8
Simulated SFG Amplitude Ratio
A Simulated SFG Intensity Ratio
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1
0.6 0.4
31o
0.2
149o
0.0
-0.2
o θ=0 o θ=10 o θ=20 o θ=30 o θ=40 o θ=50 o θ=60 o θ=70 o θ=80 o θ=90
0
-0.4 -0.6 0
20
40
60
80 100 120 140 160 180
Tilt Angle, θ
0
20
40
60
80
Distribution Angle, σ
D
C Figure 5. A and C. Simulated SFG intensity curves are presented between the symmetric and asymmetric stretches of the SSP and PSS polarizations for the methoxy group. and the curves are also marked with corresponding fitting results to obtain the average tilt angles. B and D. Simulated SFG amplitude ratios between the SSP and PSS polarizations as a function of distribution angle. As indicated, there are several possible distribution angles for several specific tilt angles. The horizontal solid line and vertical bars indicate the calculated SFG intensity ratio and uncertainty error with a confidence level of 95% obtained from the fitting analysis.
Next, Figure 5 shows the average orientation and orientation distribution of the methoxy (-OCH3) group. In Figure 5A, this is an intensity ratio curve of the methoxy symmetric stretch (OCH3 SS) as a function of tilt angle from the surface normal. The tilt angles obtained from using an intensity ratio value of 36± 14 are 60° and 118° (Table 3). Whereas for the orientation distribution of this vibrational mode in Figure 5B, the amplitude ratio of 6 ± 1 obtained three distribution angles. For example, a distribution angle of 14°± 3° for a tilt angle of 70° from the surface normal (Table 3). The value represents a relatively narrow distribution of the varying tilt
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angles. The tilt angle is similar to what was obtained from using the intensity ratio with a 10° difference. This observation suggests that most of the –OCH3 groups are tilted further from the surface normal because of the relative distribution obtained for different tilt angles at the airliquid interface with larger tilt angle values. On the other hand, the tilt angle and orientation distribution for the methoxy asymmetric stretch (-OCH3 AS) are presented in Figure 5C and 5D. The tilt angle is determined by using the intensity ratio between the amplitudes of two polarizations. Thus, with the intensity ratio of 0.2± 0.5, the tilt angle is obtained with an angle 31° from the surface normal. While the amplitude ratio value of 0.4± 0.7 obtained four distribution angles for a tilt angle range of 0°31°, as shown in Table 3. Again, the results are comparable from using both the intensity and amplitude ratios because one estimate derived from the simulated curve has distribution angle of 6° for the tilt angle of 30° (Table 3). The –OCH3 AS stretch has a narrow distribution width with tilt angles from 0° - 30° from the surface normal, which indicates that most of this vibrational stretch population exists within this specific distribution of angles and that they are aligned at the interface. If we compare the width of the distribution of angles, the α–CH3 groups have a narrower distribution of angles compared to –OCH3 groups for both symmetric and asymmetric stretches. Whereas, the values of the tilt angles from the surface normal indicate that the -CH3 and –OCH3 symmetric stretches are tilted ~60° away from the surface normal by using the intensity ratio, as summarized in Tables 2 and 3. The result of this orientation is partly good. Concluding to have a poor orientation says otherwise where no SFG signal is observed in the CH region. Since sufficient signal is still obtained in our experiment, an SFG spectrum can still be plotted as a function of IR wavenumber. On the other hand, the tilt angle of the asymmetric stretches has a
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26° difference from 5°(-CH3 AS) to 31° (-OCH3 AS). The discrepancy in the tilt angle could be due to the low intensity of the vibrational stretch and peak overlapping which can affect the fitting accuracy and the estimated tilt angle values. Overall, the α-CH3 functional groups still have similar molecular conformation with – OCH3 functional groups at the interface especially the symmetric stretches. The values observed using the intensity and amplitude ratios are consistent, and the trend between the two groups is similar; i.e., the –OCH3 SS functional group is tilted at 60°compared to the tilt angle of α-CH3 SS functional group at 61°from the surface normal. The orientational analysis using the SSP/PPP ratio is not presented in this paper because as mentioned earlier, the intensity profiles are similar for both the symmetric and asymmetric stretches of the methyl group as a function of tilt angle. Table 3. List of possible average tilt angles of the methoxy (-OCH3) functional group present at the interface using the conventional polarization combination approach.These values are estimated from the simulation curves generated by Mathematica. The reported uncertainty error was calculated with a 95% confidence level from the fitting analysis.
Methoxy Group (-OCH3 group) Symmetric Stretch
Asymmetric Stretch
Peak Amplitude (Fitting result)
Intensity Ratio (SSP/PSS)
Average Tilt Angle
Amplitude Ratio (SSP/PSS)
60°± 24° 1463± 29(SSP) 243± 43(PSS)
36± 14
118°± 46°
6 ± 1
29± 34(SSP)
0.2± 0.5
31°± 98°
0.4± 0.7
69± 26(PSS)
149°± 464°
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Orientation Distribution 14°± 3° (70° tilt angle) 18°± 3° (80° tilt angle) 19°± 4° (90° tilt angle) 28°± 43° (0° tilt angle) 23°± 36° (10° tilt angle) 18°± 27° (20° tilt angle) 6.0°± 10° (30° tilt angle)
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Figures 4 and 5 are the simulated curves obtained for a 9-peak fitting. Thus, a simultaneous 9-peak fitting was also used for the polarization mapping method to have consistency between the orientation calculations for the two approaches. To check the difference in the fitting individual spectrum of SSP, PPP, and PSS, a global fitting is performed using Mathematica.
The fitted SFG spectra of SSP, PPP, and PSS polarization combinations,
including the fitting parameters, are presented in the Supporting Information as Figure S6 and Table S5. The average tilt angles and the orientation distributions were also estimated based on the fitting values provided by the global fitting, as shown in Supporting Information Figures S7 and S8. The results have marginal differences with individual fitting using MATLAB 2014b program; the intensity and amplitude ratios resulted to varied average tilt angles and distribution angles for the respective tilt angles from the simulated curves. The difference in values could be due to the accuracy of fitting the SFG spectra using different algorithms, the method of fitting, and the parameters of the equation. However, the overall estimated orientation of MEMA monomer at the air-liquid interface are comparable. Table S6 summarizes the average tilt angles and the orientation distribution of the functional groups of interest. Polarization Mapping Method To compare with the conventional polarization method, the SFG spectra were acquired every 15° from 0° to 90° by adjusting the SFG polarizer position in front of the spectrograph and detector, as stated in the experimental section. For consistency, the assignment of the peaks was similar to the suggested peak assignments used for the conventional polarization method. As shown in Figure 3, the asymmetric stretching transition dipole usually exhibits strong intensity in PPP polarization, which corresponds to 0 degree SFG polarization angle. Thus, it is clearer in PMM to identify overlapped peaks. For instance, 6th and 7th peaks are assigned as –CH3 AS and
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–OCH3 AS which show expected maximum intensities at ~0 degree and ~15 degrees SFG polarization. While the =CH2 group’s CH stretch shows a maximum intensity at ~75 degrees SFG polarization. After data processing, the spectra from 0° to 90° were individually imported and then simultaneously fitted using Mathematica 9.0 and Equation 8. The fitting results are provided in Supporting Information Table S7. The spectra in Figure 6A present the simultaneous fitting of the individual SFG spectra of the MEMA monomer from 0° to 90°. The fitting R-square value of 0.9671 was obtained after global or simultaneous fitting. After fitting, the γ-values for each vibrational mode were obtained; the gamma value is the relative amplitude ratio of the SSP and PPP polarizations. A smoothed and fitted 2-D polarization map of the SFG spectra of the MEMA monomer is shown in Figure 6B, where the x-, y-, and z- axes are represented by the IR wavenumber, SFG polarizer angle, and
fitted SFG spectra intensity obtained from the simultaneous fitting,
respectively. The 2-D map enhances the visualization of the vibrational modes from 0° - 90°. The transition of the spectra from 90° to ~60° indicates that the contributions from the -CH2 symmetric stretch (~2863 cm-1) and the –CH3 and –OCH3 asymmetric stretch (2980 and 2995 cm-1) vibrational modes are almost negligible. However, interestingly, at an SFG polarization angle of 30°, the presence of 9 peaks is observed. This trend is the same as the SFG polarization angles move from 45° to 15°. From 15°- 0°, there is almost no contribution from the alkene-CH2 CH symmetric stretch, and the broad peak between 2950 cm-1 to 3000 cm-1 is an overlap of the α-CH3 and –OCH3 asymmetric stretches positioned at 2980 cm-1 and 2995 cm-1, respectively.
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1
2
3
4
5 6 78
9
6000
SFG Intensity (a.u.)
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5000 4000
90 deg(s)
3000
75 deg
2000
60 deg 45 deg 30 deg
1000
15 deg 0 deg(p)
0 2600
2700
2800
2900
3000
3100
3200
Wavenumber (cm-1)
A
B Figure 6. A. Individual SFG spectra obtained from 0°-90° were globally fitted using Mathematica 9.0. B. A twodimensional representation of the polarization mapping of the SFG spectrum taken at 0° - 90° SFG polarization for every 15° as a function of the wavenumber.
The global fitting results of the polarization mapping method can be treated and applied using the same simulation curves to determine the average orientation and orientation distribution of the interfacial functional groups. In this case, because all of the amplitude ratios are obtained from the fitting results of the polarization mapping method, the average orientation and the distribution of some selected peaks are estimated. Figure 7A and B present the SFG intensity ratio as a function of tilt angle which has an intensity (γ2) value of 3± 0.4 and distribution angle for the -CH3 symmetric stretch, respectively. As a reminder, in polarization
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mapping method theory, γ is equal to SSP/χPPP or the amplitude ratio between SSP and PPP
Simulated SFG Intensity Ratio
6
4
20o
160o
2
0 0
20
40
60
Simulated SFG Amplitude Ratio
polarizations. Thus estimated tilt angles of the –CH3 SS are 20± 3° and 160± 22°.
o θ=0 o θ=10 o θ=20 o θ=30 o θ=40 o θ=50 o θ=60 o θ=70 o θ=80 o θ=90
2
1 0
80 100 120 140 160 180
20
40
18 16
132o
48o
10 8 6 4 2 0 0
20
40
60
80 100 120 140 160 180
Tilt Angle, θ
Simulated SFG Amplitude Ratio
20
12
80
B
A
14
60
Distribution Angle, σ
Tilt Angle, θ
Simulated SFG Intensity Ratio
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5 o θ=0 o θ=10 o θ=20 o θ=30 o θ=40 o θ=50 o θ=60 o θ=70 o θ=80 o θ=90
4
3
2 0
20
40
60
80
Distribution Angle, σ
D
C
Figure 7. A. The simulated curve for the intensity ratio of SSP and PPP, which is used to estimate the average tilt angles of -CH3 SS stretch. B. The simulated curves for the amplitude ratio that determines the estimated distribution per tilt angle of the -CH3 SS stretch. C. The estimated tilt angle of the –OCH3 SS while D. presents the distribution of tilt angle for the –OCH3 SS. The horizontal solid line and vertical bars indicate the calculated SFG intensity ratio and uncertainty error with a confidence level of 95% obtained from the fitting analysis.
Additionally, the estimated tilt angles from the surface normal for the –CH3 SS of the αCH3 group are 61± 13° and 119± 26° (Table 2) using the SSP/PSS intensity ratios from the conventional method while tilt angles of 20± 3° and 160± 22° are obtained with the PM method. These results have a difference of ~41° between the two methods. Some part of the
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difference could have been due to experimental differences such as humidity, room temperature, data acquisition, and beam alignment. However, the major part of the difference would originate from the insensitivity of the orientation curve in SSP/PPP intensity. Focusing only on the first estimated value between 61° and 20° indicates that the α–CH3 functional group is organized within a certain degree of order at the interface. Although tilt values vary between the conventional polarization combination and PM methods, the results suggest that reasonable peak assignment and the interfacial molecules’ orientation
are detectable by the two SFG
spectroscopy methods. Figure 7C and D show simulated SFG intensity ratio curve as a function of the orientation tilt and distribution the orientation distribution of the symmetric stretches of -OCH3 group, respectively. An intensity ratio of 17± 3 is obtained from the –OCH3 symmetric stretch positioned at 2816 cm-1 with an estimated gamma value, γ–OCH3, of 4± 0.3. The average tilt angle orientations for the –OCH3 group are 48± 7° and 132.0± 20°, summarized in Table 4. The tilt angles of the – OCH3 symmetric stretch derived by the two approaches indicate a varying organization of these functional groups at the interface with a 12° difference. The –OCH3 symmetric stretch, with a γ–OCH3 of 4± 0.3°, resulted to narrow distribution that ranges from 17° - 35° (Table 4). The smallest tilt angle value that has a distribution of 17° has a tilt angle of 50° to which it only has 2° difference from what was obtained using the intensity ratio. The distribution for all possible tilt angles for the –OCH3 symmetric stretch is relatively similar for each angle but broader compared to the values obtained using the conventional polarization combination orientational analysis. At the same time, the tilt angle values from the simulated distribution curves from the conventional polarization combination
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approach (70°- 90°) in Figure 5D are similar to the tilt angle values derived from polarization mapping from 50°- 90°, indicating that the –OCH3 symmetric stretch is tilted farther away from the surface normal. The results of asymmetric stretches of α-CH3 and –OCH3 groups are available in the Supporting Information (Figure S9). Figure S9 describes the average tilt angle orientation and distribution orientation for the asymmetric stretches of α-CH3 and –OCH3 groups. The (γ-α-CH3)2 and (γ–OCH3)2 for the asymmetric stretches values are and 3 x 10-13 ± 2 x 10-5 and 1± 1, respectively, in Table 4. When the values are compared to the simulated curve determining the average orientation tilt angle, there are no possible average tilt angles obtained from the surface normal. This finding indicates that the square of the gamma values is not within the approximation of one orientation based on the derivation of the simulated curves. Again, the orientational distribution angles are obtained by inserting the γ-values from the simultaneous fitting for the two vibrational modes to the simulated curve. The distribution angles for both – OCH3 and α-CH3 are not obtained because the gamma values are again outside the boundaries of the simulated curves. This result indicates that the estimated values do not fit in the assumed orientation for the simulated curves. Overall, the orientation of the asymmetric stretches for these two vibrational modes is not estimated because the gamma values obtained from polarization mapping do not conform to the simulated curves.
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Table 4. List of possible average tilt angles of selected functional groups presents at the interface using the polarization mapping method results. These values are estimated from the simulation curves generated by Mathematica. The reported uncertainty error was calculated with a 95% confidence level from the fitting analysis.
Polarization Mapping Results
Methoxy Group (-OCH3 group)
Symmetric Stretch
Asymmetric Stretch Methyl Group (α-CH3 group)
Intensity Ratio (SSP/PPP)
Amplitude Ratio (SSP/PPP)
48°± 7° 17± 3
132°± 20°
4 ± 0.3
1± 1
Not within the boundary of the curve 20°± 3°
2± 2
3± 0.4
160°± 22°
2 ± 0.1
3E-13 ± 2E5
Not within the boundary of the curve
6E-7 ± 0.04
Symmetric Stretch
Asymmetric Stretch
Average Tilt Angle
Orientation Distribution 17°± 1° (50° tilt angle) 29°± 2° (60° tilt angle) 33°± 2° (70° tilt angle) 34°±3° (80° tilt angle) 35°±3° (90° tilt angle) Not within the boundary of the curve 15°± 1° (0° tilt angle) 11°± 0.7° (10° tilt angle) Not within the boundary of the curve
Organization of the Interfacial Functional Groups The tilt angles from the surface normal denote the relative orientation of these functional groups at the interface, which is related to the organization of the molecules in terms of each functional group at the surface. A highly ordered and defect-free monolayer with an efficient packing density results in tilt angles closer to the surface normal for one specific functional group. However, our results on monomers have partially ordered conformation, allowing for contributions from several functional groups at the interface. With the presence of varying functional groups at the interface, intermolecular forces and interactions can form between the
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groups. Again, the degree of order of these functional groups at the surface is also dependent on finding the lowest surface free energy leading to their final molecular conformation. The purposes of this paper are to determine a) vibrational mode peak assignment of the MEMA and b) compare the polarization combination approach with the polarization mapping method to estimate the orientation of the \-CH3 and –OCH3 groups. More specifically, usage of the intensity and amplitude ratios to obtain the average tilt angles and orientation distribution of the interfacial functional groups of the MEMA monomer at the interface, respectively. In addition to the identification of functional groups present at the interface, we used the conventional polarization combination approach using different polarization settings and polarization mapping method using varying polarization of SFG angles in 15° increments to obtain the experimental amplitude and intensity ratios. After taking the SFG spectra at different polarization combination, the presence of α-CH3, -OCH3, -CH2CH2-, and =CH2 groups in the spectrum can indicate a partially-ordered surface. With the orientational analysis, we were able to estimate the possible tilt angles responsible for this degree of order. As mentioned, the amplitude and intensity ratios were obtained from fitting the SFG spectra. In our case, we opted to assign and fit all the spectra with nine peaks by comparing the spectral peak intensity from conventional and polarization mapping methods with simulated SFG intensity profile as well as previously reported assignment. The amplitude ratio from fitting were then compared to the simulated intensity curve as a function of tilt angle. We also represented the simulated amplitude ratio as a function of distribution angle for a specific tilt angle. The distribution angle or orientation distribution pertains to the narrow or broad distribution of these functional groups at a particular tilt angle. The two approaches resulted with similar values for the average tilt angles and orientation distribution and by also taking into consideration the ratios are of different
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nature. Both methods have a different level of difficulty and interpretation; for example, the tilt angle values obtained between SSP/PSS for a conventional polarization combination approach had a tilt angle of 60° for the –OCH3 symmetric stretch. On the other hand, using PMM, resulted to a tilt angle of 48°. This 12° difference can be a result of the low SFG signal of PSS which has affected the accuracy of fitting and estimation of the amplitude ratios and increased the uncertainty errors at the 95% confidence level. Thus, the average tilt angles and orientation distribution values were also influenced by the fitting accuracy and uncertainty errors. In the case of PMM, the simulated curves for C2v point group symmetry were obtained but the simulated SSP/PPP ratio curves and the ratios do not coincide. An example of the simulated curves for the C2v point group symmetry is available in Supporting Information Figure S10. The intensity and amplitude ratios cannot be compared to the simulated curves because the generated curve is a straight line for both graphs. This result led to no estimation of the orientation for both the -CH2 and =CH2 functional groups. Thus, we only focused on the –OCH3 and α-CH3 functional groups for estimation of tilt angles and orientation distribution. We found that the polarization mapping method is also applicable to study the MEMA monomer at the air-liquid interface. The difficulty in fitting was due to a low SFG signal at 0° (ppolarized). The signal is already half of the expected intensity from a regular PPP polarization SFG spectrum. Thus the collection of SFG spectra with low signal-to-noise ratio raises challenges which lead to difficulty in fitting. The same argument can be made that due to a low signal-to-noise ratio, small inaccuracies are present in the calculated ratios and in comparison to simulated curves. The benefit of using the PMM is that simultaneous fitting can be performed where all the peaks are in the same position for the seven SFG spectra from 0°-90°, and the gamma (F HHI ) values are also obtained for all of the functional groups. We only focused on III
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functional groups that have C3v point group to determine the average tilt angles and orientation distribution. Conclusion The 2-methoxyethyl methacrylate (MEMA) monomer was characterized by SFG spectroscopy at the air-liquid interface to identify the contributing functional groups at the interface and to determine the average tilt angle. The MEMA monomer was chosen as a model system to determine orientation because we were able to probe the molecules within the CH region. The SFG spectra obtained using different polarization combinations and SFG polarization angles were acquired at 4 different mid-IR center wavelengths with 200-250 cm-1 FWHM. To cover the broad CH region, combining the 4 centers such that the IR intensity was relatively similar within the probed wavelength range was necessary. Two acquisition approaches were applied to further understand the behavior of these functional groups at the interface by determining their orientation at the air-liquid interface. The resulting average tilt angles and the orientation distributions from these methods suggested that the MEMA monomers are partially ordered at the air-liquid interface. The significance of this study is that we revealed that specific functional groups can have preferred orientations. For example, the –OCH3 SS obtained from the intensity ratio of SSP/PSS has a tilt angle of 60° from the surface normal using the conventional polarization combination method. In conclusion, we are able to identify the functional groups of the MEMA monomer, especially the –OCH3 and =CH2 groups, that were well separated from the convoluted peaks of the α-CH3 and -CH2 groups. The –OCH3 groups are difficult to identify in MEMA polymeric form; thus, we studied the MEMA monomer. These studies will help us understand whether this specific functional group has an effect on MEMA polymer organization. Additionally, the =CH2 groups were not present when the monomer is polymerized. Learning
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how these functional groups are affected at the air-liquid interface with its different chemical environment will be beneficial. ACKNOWLEDGMENTS We acknowledge with sincere gratitude Professor Michael Jensen and Caitlin Anderson for allowing us to perform IR measurements and Professor Jixin Chen for his helpful assistance and discussion with the MATLAB fitting. We are also grateful to Professor Steven Baldelli for his helpful discussions. The current work has been supported by the start-up fund provided by the Department of Chemistry and Biochemistry, the College of Arts and Sciences and the Vice President for Research at Ohio University. Additionally, the authors would like to thank Nanoscale and Quantum Phenomena Institute (NQPI) and Condensed Matter and Surface Science (CMSS) for their additional financial support. SUPPORTING INFORMATION AVAILABLE [extended SFG background, derivation of simulation SFG curve, MATLAB fitting results and parameters for SFG SSP, PPP, and PSS polarization combination, NMR and IR spectra of MEMA monomer, Mathematica Global Fitting spectrum and results for SSP, PPP, and PSS combination, Mathematica Global Fitting results for polarization mapping method, C2v point group simulated curves] REFERENCES 1. Groenzin, H.; Li, I.; Shultz, M. J., Sum-Frequency Generation: Polarization Surface Spectroscopy Analysis of the Vibrational Surface Modes on the Basal Face of Ice I(H). J Chem Phys 2008, 128. 2. Frau, A. F.; Pernites, R. B.; Advincula, R. C., A Conjugated Polymer Network Approach to Anticorrosion Coatings: Poly(Vinylcarbazole) Electrodeposition. Ind Eng Chem Res 2010, 49, 9789-9797. 3. Fu, Y.; Cai, M.; Zhang, E.; Cao, S.; Ji, P., A Novel Hybrid Polymer Network for Efficient Anticorrosive and Antibacterial Coatings. Ind Eng Chem Res 2016, 55, 4482-4489. 4. Johansson, K.; Bergman, T.; Johansson, M., Hyperbranched Aliphatic Polyesters and Reactive Diluents in Thermally Cured Coil Coatings. Acs Appl Mater Inter 2009, 1, 211-217.
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