A Sum Frequency Generation Vibrational Study of the Interference

ARTICLE pubs.acs.org/JPCC

A Sum Frequency Generation Vibrational Study of the Interference Effect in Poly(n-butyl methacrylate) Thin Films Sandwiched between Silica and Water Xiaolin Lu,*,†,‡,|| Matthew L. Clarke,‡,|| Dawei Li,§ Xinping Wang,† Gi Xue,§ and Zhan Chen‡,* †

Department of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Education Ministry, Zhejiang Sci-Tech University, Hangzhou, 310018, China ‡ Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States § Department of Polymer Science, Nanjing University, Nanjing, 210093, China

bS Supporting Information ABSTRACT: On the basis of the development in the literature, a thin film model was used to interpret the thickness-dependent sum frequency generation (SFG) spectra collected from the air/silica/poly(n-butyl methacrylate) (PBMA)/water system. By taking into account the Fresnel coefficients of the PBMA layers at the silica/PBMA and PBMA/water interfaces, the SFG spectra of the silica/PBMA and PBMA/water interfaces were obtained. The side chain methyl vibrational modes at the two interfaces were found to have opposite phases. This suggests that the side chain methyl groups at the two interfaces adopt different absolute orientations. It is believed that methyl groups at both interfaces point toward the PBMA bulk considering the unfavorable interactions between the hydrophobic methyl groups and hydrophilic silica surface and water. Orientation analysis indicates the side chain methyl groups at both interfaces tilt more toward the interfaces rather than be perpendicular to them. Applying the thin film model to the air/PBMA/ silica system, we found that the surface signals from PBMA in air dominate the SFG output on account of the relatively higher Fresnel coefficients at this surface over those of the buried PBMA/silica interface.

1. INTRODUCTION Sum frequency generation (SFG) vibrational spectroscopy has been developed into a powerful probe to study the surface and interfacial structures at the molecular level. 111 For example, it successfully elucidated molecular structures of various polymer surfaces and interfaces.5,1215 This technique intrinsically detects the second-order nonlinear susceptibility at polymer surfaces and interfaces in that the second-order nonlinear susceptibility of the polymer bulk vanishes under the electric dipole approximation. The generally used sample geometry in the SFG study was a polymer thin film on a substrate.2,1643 In this case, the polymer thin film was taken as a nonlinear polarization sheet which can generate the SFG signals, as shown in Figure 1. In many cases, the vibrational resonances in the collected SFG spectra were considered as the signals generated from one interface, i.e., the polymer surface in air. This is true only when the SFG contribution at one interface dominates over that at the other interface (such as the polymer/substrate interface). When the SFG signal generated from either interface cannot be neglected, the signals can interfere with each other and the assumption that the SFG signals are generated exclusively from one interface becomes invalid. Due to light beam reflection and refraction at both interfaces, the Fresnel coefficients responsible for the local field correction play a key role in determining the relative r 2011 American Chemical Society

SFG signal intensities at the two interfaces. Since silica is one of the most extensively used substrates, this paper discusses the SFG signal interference effect between the silica/poly(nbutyl methacrylate) (PBMA) and PBMA/water interfaces for a “face-down” geometry of the air/silica/PBMA/water system. A generalized method for analyzing the SFG signals generated from a polymer film on a substrate was developed by taking into account the contributions from the two interfaces. The SFG signals from the silica/PBMA and polymer/water interfaces were successfully deconvolved on account of the Fresnel coefficients at the two interfaces for ssp (s-polarized sum frequency output, s-polarized visible input, and p-polarized IR input) and sps polarization combinations. The orientations of the side chain methyl groups at both interfaces were analyzed. In view of the successes of our analysis, we extended the method to the analysis of the SFG spectra collected using the “face-up” geometry, i.e., SFG spectra from the air/PBMA/ silica system. It was found that in this case the SFG spectra are dominated by the surface signals from the air/PBMA interface. Received: March 14, 2011 Revised: May 7, 2011 Published: June 23, 2011 13759

dx.doi.org/10.1021/jp202416z | J. Phys. Chem. C 2011, 115, 13759–13767

The Journal of Physical Chemistry C

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Figure 1. The molecular structure of poly(n-butyl methacrylate) and a schematic showing the face-down geometry for the SFG experiment. In this study, Medium 3 was water: VI, visible beam; IR, infrared beam; SF, sum frequency signal beam.

2. EXPERIMENTAL SECTION 2.1. Materials. PBMA (Figure 1) was purchased from Scientific Polymer Products, Inc. (Mw = 180000, Dorval, QC, Canada). The fused silica windows of 1 in. diameter and 1/8 in. thickness were ordered from Esco Products, Inc. The fused silica substrates were treated sequentially by a sulfuric acid bath saturated with potassium dichromate, deionized water, and air plasma. The PBMA films with different thicknesses were prepared by spin coating the PBMA toluene solution onto the cleaned fused silica substrates. The film thickness was controlled by adjusting the spin speed and the polymer solution concentration (0.5% to 6.0% w/w). The films were then annealed overnight at 80 °C to ensure the complete solvent evaporation. The film thicknesses were measured by using a Dektak depth profilometer and found to vary between 20 and 350 nm. 2.2. SFG Experiment. SFG spectra were collected using a custom-designed EKSPLA system. The SFG setup has been reported in previous publications.2737 A “face-down” geometry was used in this research (Figure 1), in which the visible and IR beams passed through the back side of the substrate and through the polymer film and then overlapped at the polymer/water (Medium 3) interface. The SF beam was collected in the reflected direction. The input angles for the visible and infrared beams are 60° and 54°, respectively, versus the surface normal on top of the silica substrate. The beam diameters were approximately 500 μm. The pulse energies of the visible and IR beams were approximately 150 and 100 μJ, respectively. SFG spectra were collected using the ssp and sps polarization combinations. Both ssp and sps SFG spectra were detected in the frequency range between 2750 and 3050 cm1. The SFG spectra were finally normalized using the input laser powers.

3. RESULTS AND DISCUSSIONS 3.1. General Discussion. It has been discussed that the bulk contribution may affect the detected SFG resonant signals.4446 The comparison between the interface and bulk contributions can be done by comparing the thickness-dependent SFG spectra

Figure 2. The thickness-dependent ssp and sps spectra in the facedown geometry.

or comparing the reflection and transmission spectra.45,46 Figure 2 shows our thickness-dependent SFG spectra in ssp and sps polarization combinations using the “face-down” geometry, namely, air/silica/PBMA/water. From the spectra, it can be seen that the SFG spectra do change with the PBMA thickness. However, the resonant signals in these spectra vary periodically as a function of the PBMA thickness. If the resonant SFG signal is dominated by the contribution from the bulk PBMA, as the polymer film thickness increases, the intensity of this SFG resonant peak should initially increase (when the thickness is smaller than the coherent length) and then more or less stabilize (when the thickness is larger than the coherent length). We did not see this film thickness dependence of any peak intensity in Figure 2. As demonstrated in the previous publications,28,29 for such amorphous polymer thin films with the thickness on the order of 101 to 102 nm, the electric dipole approximation is still applicable and the observed resonant signals should come from the PBMA interfaces, instead of the polymer bulk. This judgment also agrees with the previous conclusion that an SFG spectrum in the reflection mode is dominated by the interfacial signals.45 3.2. Fresnel Coefficients of the Thin Film Model. Recent publications have shown that the bulk film thickness can have a substantial effect on the Fresnel coefficients responsible for the local field corrections of the input and output beams.42,4753 This indicates that the periodical variation of the SFG resonant signals may come from the change of Fresnel coefficients, supposing the susceptibilities at the interface(s) are constant. A thin film model was applied in this research as reported before.4855 Here, the PBMA thin film on the silica substrate is considered as a nonlinear 13760

dx.doi.org/10.1021/jp202416z |J. Phys. Chem. C 2011, 115, 13759–13767


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For the PBMA/air interface, we have

polarization sheet which can generate SFG signals. Since there are two interfaces, which are the silica/PBMA and the PBMA/ Medium 3 interfaces as shown in Figure 1, both interfaces can generate SFG resonant signals. The PBMA bulk is considered to have no contribution to the resonant signals as discussed in section 3.1. Now we can write down the following equations responsible for the SFG spectra in ssp and sps polarization combinations.56 ð2Þ

IðωÞ µ jχeff j2 ð2Þ χeff

¼F

χ

silica=PBMA silica=PBMA

+F

χ

+ χNR

ð2Þ Here it can be seen that for a polymer thin film the effective second-order nonlinear susceptibility consists of three parts, namely, the silica/PBMA part (Fsilica/PBMA χsilica/PBMA), the PBMA/Medium 3 part (FPBMA/Medium3 χPBMA/Medium3), and the nonresonant part (χNR). The silica/PBMA and PBMA/ Medium 3 parts are responsible for the observed resonant signals. Fsilica/PBMA and FPBMA/Medium3 are the Fresnel coefficients responsible for the local field corrections. With the defined input angles of visible and infrared beams, Fsilica/PBMA and FPBMA/ Medium3 are solely a function of the polymer film thickness (d). χsilica/PBMA and χPBMA/Medium3 are the interfacial second-order nonlinear susceptibilities. Since the film thicknesses in the current investigation (27322 nm) are much larger than the mean-square end-to-end distance of a PBMA chain (assuming an ideally flexible chain, Ree ≈ 8 nm), it is reasonable to assume that the film thickness has no substantial effect on the conformational order of the polymer chains at both interfaces. χsilica/PBMA and χPBMA/Medium3 are thus assumed constant which can be directly correlated to the polar orientation of the chemical groups at the corresponding interfaces, respectively. The periodical dependence of the SFG spectra on the film thickness should be a consequence of the periodical variation of the Fresnel coefficients in terms of the film thickness. As is known, Fsilica/PBMA and FPBMA/Medium3 are actually the overall Fresnel coefficients which can be decomposed into three L factors.56 These L factors are directly responsible for the local field corrections of the two input and one output beams. Since we are discussing the PBMA layers at the interfaces, the silica/PBMA and PBMA/Medium 3 interfaces indicate the PBMA layers at the silica/PBMA and PBMA/water interfaces in this paper. This definition of the interfaces is physically meaningful and provides a consistent treatment of the L factors for both the silica/PBMA and PBMA/Medium 3 interfaces, as the previous SHG study on C60 thin film.59 Referring to the previous publications related to the thin film model,48,49,52 we can write down the L factors as the following equations. For the silica/PBMA interface, we have ðωi Þ ¼ t p ð1 r23 e2iβ Þ Lsilica=PBMA xx

cosðθ2 Þ cosðθ1 Þ

s 2iβ Lsilica=PBMA ðωi Þ ¼ t s ð1 + r23 e Þ yy

p

Lsilica=PBMA ðωi Þ ¼ t p ð1 + r23 e2iβ Þ zz

n1 n2 n2silica=PBMA

ð3Þ

ð4Þ

cosðθ2 Þ cosðθ1 Þ

s LPBMA=Medium3 ðωi Þ ¼ eiΔ t s eiβ ð1 + r23 Þ yy p

ðωi Þ ¼ eiΔ t p eiβ ð1 + r23 Þ LPBMA=Medium3 zz

ð1Þ

PBMA=Medium3 PBMA=Medium3

p

p

ðωi Þ ¼ eiΔ t p eiβ ð1 r23 Þ LPBMA=Medium3 xx

n1 n2 n2PBMA=Medium3

ð6Þ ð7Þ ð8Þ

ωi is the beam frequency; tp and ts are the overall transmission coefficients at the silica/PBMA interface for the p- and s-polarized lights, respectively. rp23 and rs23 are the linear reflection coefficients at the PBMA/Medium 3 interface. β is the phase difference between a reflective beam and its secondary reflective beam after it propagates across the polymer thin film and reflects back.57 θ1 and θ2 are the beam incidence angles in the silica and PBMA polymer, respectively. n1 and n2 are refractive indices of the silica and PBMA polymer, respectively. nsilica/PBMA and nPBMA/Medium3 are the refractive indices of the PBMA interfacial layers at the silica/PBMA and PBMA/Medium 3 interfaces. The expressions for tp, ts, and β can be written as tp ¼

p t12 p p 2iβ 1 + r12 r23 e

ð9Þ

ts ¼

s t12 s s 2iβ 1 + r12 r23 e

ð10Þ

β¼

2π n2 d cos θ2 λ0

ð11Þ

tp12 and ts12 are the linear transmission coefficients at the silica/ PBMA interface. λ0 is the corresponding wavelength. d is the PBMA film thickness. The expressions for linear coefficients of rp23, rs23, tp12, and ts12 are r23 ¼

n3 cos θ2 n2 cos θ3 n3 cos θ2 + n2 cos θ3

ð12Þ

s ¼ r23

n2 cos θ2 n3 cos θ3 n2 cos θ2 + n3 cos θ3

ð13Þ

t12 ¼

2n1 cos θ1 n2 cos θ1 + n1 cos θ2

ð14Þ

s t12 ¼

2n1 cos θ1 n1 cos θ1 + n2 cos θ2

ð15Þ

p

p

In eqs 68, there exists a Δ, which is the phase difference required when considering the coherence during the addition of the two SFG output beams generated from the silica/PBMA and PBMA/Medium 3 interfaces.48,49,52 This phase difference has been explicitly explained previously.48,49 Since the two output SFG beams are generated by overlapping the input visible and infrared beams at the two interfaces (silica/PBMA and PBMA/ air), this propagation phase difference at the two interfaces for the output SFG, the input visible, and the input infrared beams should be separately expressed48,49,52

ð5Þ

ΔSF ¼ 13761

2πn2, SF d λSF cos θ2, SF

ð16Þ


The Journal of Physical Chemistry C ΔVI ¼

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2πn2, VI d 2πn1, VI d ðtan θ2, VI + tan θ2, SF Þ sin θ1, VI λVI cos θ2, VI λVI ð17Þ

ΔIR ¼

2πn2, IR d 2πn1, IR d ðtan θ2, IR + tan θ2, SF Þ sin θ1, IR λIR cos θ2, IR λIR ð18Þ

We can now write the Fresnel coefficients for the silica/ PBMA/air system. For example, the Fresnel coefficient of the silica/PBMA interface and the PBMA/air interface for the ssp polarization combination can be written as silica=PBMA Fssp ¼ Lsilica=PBMA ðωÞLsilica=PBMA ðω1 ÞLsilica=PBMA ðω2 Þ sin θ1, IR yy yy zz

ð19Þ PBMA=air PBMA=air PBMA=air Fssp ¼ LPBMA=air ðωÞLyy ðω1 ÞLzz ðω2 Þ sin θ1, IR yy

ð20Þ ω, ω1, and ω2 are the output sum, input visible, and input infrared beam frequencies, respectively. It should be noted that the overall Fresnel coefficients should include the transmission of the input visible, infrared, and output SFG beams at the top air/silica interface, which means that we need to take into account the linear Fresnel coefficients for the two input beams at the air/silica interface (01, t01) and one output beam at the same interface (10, t10). Finally, the overall Fresnel coefficients before the second-order nonlinear susceptibility tensor components for ssp polarization combination can be written as silica=PBMA Fssp 10 silica=PBMA 01 silica=PBMA 01 ¼ ts;SF Lyy ðωÞts;VI Lyy ðω1 Þtp;IR Lsilica=PBMA ðω2 Þ sin θ1, IR zz

ð21Þ

Figure 3. The calculated absolute Fresnel coefficients as a function of film thickness for ssp and sps polarization combinations in the “facedown” geometry.

Within the vibrational resonant frequency range, the secondorder nonlinear susceptibility tensor component can be expressed as a Lorentz function.

PBMA=Medium3 Fssp

¼

χ¼

10 PBMA=Medium3 01 PBMA=Medium3 01 ts;SF Lyy ðωÞts;VI Lyy ðω1 Þtp;IR LPBMA=Medium3 ðω2 Þ zz

sin θ1, IR

ð22Þ Similarly, the overall Fresnel coefficients for sps polarization combination can also be obtained: silica=PBMA Fsps 10 silica=PBMA 01 01 silica=PBMA ¼ ts;SF Lyy ðωÞtp;VI Lsilica=PBMA ðω1 Þts;IR Lyy ðω2 Þ sin θ1, VI zz

ð23Þ PBMA=Medium3 Fsps 10 PBMA=Medium3 01 01 PBMA=Medium3 ¼ ts;SF Lyy ðωÞtp;VI LPBMA=Medium3 ðω1 Þts;IR Lyy ðω2 Þ sin θ1, VI zz

ð24Þ The expressions for t10 and t01 are similar to eqs 1215. The output SFG spectral intensity can thus be expressed as silica=PBMA silica=PBMA PBMA=Medium3 PBMA=Medium3 χyyz + Fssp χssp Issp ðωÞ µ jFssp

+ χssp;NR eijssp j2

ð25Þ

silica=PBMA silica=PBMA PBMA=Medium3 PBMA=Medium3 χyzy + Fsps χyzy Isps ðωÞ µ jFsps

+ χsps;NR eijsps j2

ð26Þ

A

∑q ω2 ωqq + iΓq

ð27Þ

A q , ω q , and Γ q are the strength, resonant frequency, and damping coefficient of the vibrational mode q. Figure 3 shows , FPBMA/water , Fsilica/PBMA the calculated absolute values of Fsilica/PBMA ssp ssp sps and FPBMA/water (here Medium 3 is water). The phase difference sps and FPBMA/water , and the phase difference between Fsilica/PBMA ssp ssp silica/PBMA and FPBMA/water are shown in Figure 4. In the between Fsps sps following, we will fit the SFG spectra using eqs 2527. The only variables which will be used in fitting are Aq, ωq, and Γq for each peak from the two interfaces, the nonresonant background, and the phase differences mentioned above. 3.3. PBMA Spectral Analysis. The thickness-dependent spectra in Figure 2 demonstrate that the Fresnel coefficients of the two interfaces including the absolute value and the relative phase have a substantial effect on the detected SFG spectra, which can be related to their constructive or destructive interferences. We will first consider the individual contributions of the absolute Fresnel coefficients from each interface and the phase difference. Qualitatively, because the refractive indices of silica and PBMA are similar, reflection and refraction of the light beams at the silica/PBMA interface are not significant. Thus the absolute Fresnel coefficients of the PBMA/water interface do not change much as a function of the film thickness for both ssp and sps polarization combinations. Indeed, the calculated 13762



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Figure 4. The calculated and fitted phase differences between the Fresnel coefficients for the silica/PBMA and PBMA/water interfaces in terms of film thickness for ssp and sps polarization combinations in the “face-down” geometry.

absolute Fresnel coefficient is nearly invariable as a function of film thickness for both polarization geometries (Figure 3). Contrarily the absolute Fresnel coefficient for the silica/PBMA will exhibit more variation as changes in film thickness alter the propagation distance of the light beams between the silica/ PBMA and PBMA/water interfaces. The effect of multiple reflections must also be considered for this interface, and it is concluded that the change of the absolute values of the Fresnel coefficients at the silica/PBMA interface for ssp and sps polarization combinations over the film thickness is just a consequence of vectorial addition of the multiple reflected light fields. As seen in Figure 3, the absolute Fresnel coefficient for the silica/ PBMA exhibits substantial changes in magnitude as a function of thickness. The phase difference for both polarizations was also calculated and shown to vary linearly over the range of measured thicknesses (Figure 4). Our spectral fitting and spectral analysis were based on the calculated Fresnel coefficients including the absolute values and relative phases shown in Figure 3 and Figure 4. We used the absolute values of Fresnel coefficients and set the phase difference as one of the fitting parameters in order to get the best fitting results, as shown in Figure 5. The fitted phase differences between the Fresnel coefficients for the silica/PBMA and PBMA/water interfaces in terms of film thickness for ssp and sps polarization combinations are plotted in Figure 4 in comparison to the calculated values using the thin film model. The fitted parameters for the vibrational resonant peaks are listed in Table 1. From the fitted parameters in Table 1, we rebuilt the ssp and sps spectra of silica/PBMA and PBMA/water interfaces and plotted them in Figure 6. As shown in Figure 5, the fitted and

Figure 5. The experimental and fitted ssp and sps spectra viewed as a 3-D plot.

Table 1. Fitting Results for the ssp and sps Spectra Collected in Our Face-down Geometry (air/silica/PBMA/water)a silica/PBMA ωq

Aq

PBMA/water

Γq

Aq

Γq

assignment

ssp 2872

16

10

35

10

2895

13

10

32

10

unassigned (perhaps CH2)

2935 2960

14 65

10 12

25 57

10 12

side chain CH3 Fermi side chain CH3 as

side chain CH3 ss

sps 2910

21

10

18

10

CH2 as

2960

52

12

46

12

side chain CH3 as

a

Key: ss, symmetric stretching; as, antisymmetric stretching; Fermi, Fermi resonance.

experimental SFG spectra match up with each other, indicating that our fitting results are quantitatively or at least semiquantitatively reliable. From the fitting results, there are four vibrational resonant peaks for ssp spectra of the silica/PBMA and PBMA/water interfaces, which are located at 2872, 2895, 2935, and 2960 cm1, respectively. It should be noted that the peak positions were determined by spectral fitting. The two peaks at 2872 and 2935 cm1 are assigned to the symmetric stretching 13763



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Figure 7. The schematic shows the absolute orientation of ester methyl groups at the silica/PBMA and PBMA/water interfaces.

(ss) and Fermi resonance (Fermi) modes of the side chain methyl groups, respectively.28,29 The peak at 2960 cm1 is assigned to the antisymmetric stretching (as) mode of the side chain methyl groups.28,29 The 2895 cm1 peak might be contributed by the methylene groups. There are only two vibrational resonant peaks in the sps spectra of the silica/PBMA and PBMA/ water interfaces, which are located at 2910 and 2960 cm1. These two peaks are sequentially assigned to the “as” mode of the methylene groups and the “as” mode of the side chain methyl groups.28,29 Interestingly, for both the ssp or sps polarization combinations, the resonant peaks at the silica/PBMA and PBMA/water interfaces have opposite phases, as shown in Figure 6. In other words, the second-order nonlinear susceptibilities of the same vibrational modes at the two interfaces are of opposite signs, as shown in Table 1. Several papers have pointed out that the relative phase in SFG studies can help judge the absolute orientation of the chemical groups.51,5860 In other words, chemical groups which are “pointing toward” and “pointing away” basically generate two out-of-phase vibrations with respect to a standard (either a nonresonant background51,58 or a vibrational mode59,60). On the basis of this “principle”, the side chain methyl groups at the silica/PBMA and PBMA/water interfaces should have opposite absolute orientations. Since the silica surface and water are both hydrophilic, we believe that the hydrophobic side methyl groups at both the silica/PBMA and silica/water interfaces collectively point to the PBMA side, as shown in Figure 7. For both the silica/PBMA or PBMA/water interface, the side methyl “as” mode is the strongest mode in both the ssp and sps spectra. From the ratio of the fitted second-order nonlinear susceptibilities of ssp over sps, the orientation angles of the side methyl groups at both interfaces can be deduced. The side methyl groups can be taken as having C3v symmetry.61,62 The silica/PBMA and PBMA/water interfaces can be considered azimuthally isotropic. The second-order nonlinear susceptibilities collected in ssp and sps polarization combinations are related to the molecular hyperpolarizabilities and the tilt angles of the side methyl groups versus the surface or interface normal.56,63 χyyz, as ¼ Ns βcaa ðcos θ cos3 θÞ

ð28Þ

χyzy, as ¼ Ns βcaa cos3 θ

ð29Þ

1 χyyz, s ¼ Ns βccc ½cos θð1 + rÞ cos3 θð1 rÞ 2 Figure 6. The silica/PBMA and PBMA/water interfacial spectra rebuilt from the fitted results in Table 1. Thick lines indicate the overall spectra; thin lines indicate the individual resonant peaks. The negative peaks indicate out-of-phase resonances with respect to the positive peaks.

ð30Þ

The ratio of χyyz,as over χyzy,as is explicitly expressed as χyyz, as χyzy, as 13764

¼

cos θ cos3 θ cos3 θ

ð31Þ



Figure 8. The ratio of χyyz,as/χyzy,as as a function of the tilt angle θ0 and angle distribution σ for the methyl groups.

Figure 9. The calculated absolute Fresnel coefficients in terms of film thickness for using ssp, sps, and ppp polarization combinations in the face-up geometry.

Figure 8 shows the calculated χyyz,as/χyzy,as value as a function of tilt angle (θ0) for several different tilt angle distributions (0°, 10°, 20°, 30°, 34°, 40°) when a Gaussian distribution function is assumed. The ratios of χyyz,as/χyzy,as for the silica/PBMA and PBMA/water interfaces are plotted as straight lines in the same figure. Because the two ratios are similar, ∼1.2, it suggests that the possible orientation of the side methyl groups at both the silica/PBMA and PBMA/water interfaces lies between the two extremes of a tilt angle of 48° with a δ-distribution and a tilt angle of 90° with a distribution width of 35°, as shown in Figure 8.

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This deduced orientation agrees with the previous conclusion that the side methyl groups at the PBMA/water interface tilt more toward the PBMA surface in water rather than be perpendicular to it.29 3.4. Extension to the “face-up” Geometry. To detect the molecular structures of the polymer surfaces using SFG, the “face-up” geometry is generally used. On the basis of the same method described in section 3.2, we calculated the overall Fresnel coefficients for ssp, sps, ppp polarization combinations as a function of the film thickness for the face-up geometry. The ppp polarization combination is introduced here for completeness (which was not included in the study above because it is complicated and probes multiple terms of the χ(2) tensor). The air/PBMA/silica system was chosen as an example. The calculated curves are plotted in Figure 9. Comparing data in Figures 3 and 9, it can be seen that for the silica/PBMA interface the facedown geometry would provide a stronger spectrum than the faceup geometry. It is also very clear from Figure 9 that no matter what polarization combination is used, the absolute Fresnel coefficients of the PBMA/silica interface are much smaller than those of the air/PBMA interface. Furthermore, most of the published SFG studies on polymer surfaces dealt with hydrophobic groups, such as methyl, methylene and phenyl groups. Such chemical groups at the surface (in air) could be much more ordered than those at the buried interface, i.e., the polymer/silica interface. Therefore the second-order nonlinear optical susceptibility components of the polymer/air interface are much stronger. Along with the effect of the Fresnel coefficient discussed above, the contributions from the polymer surface usually dominate the SFG spectra. In this case, the contributions from the buried interfaces are many times weaker and can be ignored. However, sometimes in quantitative spectral analysis, especially orientation calculation, the contribution from the buried interface may not be neglected because it may still change the peak intensity or the spectral line shape. In this case, the method developed in this research can be used to analyze structural information of the polymer surfaces and buried polymer interfaces.

4. CONCLUSION While excellent work has been done to understand the role of interferences in SFG spectra when two interfaces are present,4750,52,53 in this paper we uniquely developed a methodology to investigate interfacial structures of polymer thin films at solid/solid and solid/liquid interfaces simultaneously using SFG. For the first time, this paper deduced SFG spectra of polymer films at the polymer/silica interface and the polymer/ water interface from the detected SFG spectra. Using these deduced spectra, we could study molecular structures of polymers at solid/solid and solid/water interfaces. The observed thickness-dependent SFG spectra in the facedown geometry of air/silica/PBMA/water system can be explained by the thin film model which includes the signal contributions from both the silica/PBMA and PBMA/water interfaces. By considering the relative phase of the second-order nonlinear susceptibilities of the side methyl vibrational modes at the two interfaces, we infer that the side methyl groups at the two interfaces were both pointing to the PBMA bulk with high tilt angles. This inference is consistent with the hydrophobic nature of the methyl groups and the hydrophilic nature of the water and silica surface. The extension of the thin film model to the face-up geometry suggests that for the face-up geometry, for example, 13765


The Journal of Physical Chemistry C air/PBMA/silica system, the surface signals dominate the SFG output.

’ ASSOCIATED CONTENT

bS

Supporting Information. A table of fitting results of the nonresonant background and figures of experimental and fitted spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] or [email protected]. )

Author Contributions

The first two authors contributed a similar amount of work toward this paper.

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 21004054 and 20973092), the Zhejiang Provincial Natural Science Foundation of China (Grant No. Y4100390), the start-up fund of Zhejiang Sci-Tech University (Grant No. 0913845-Y), the funds from the Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Education Ministry, Zhejiang Sci-Tech University (Grant No. 2009QN07), and the Department of Education of Zhejiang Province (Grant No. Y200909780). Z.C. is grateful for the support from the US Office of Naval Research (N00014-081-1211). ’ REFERENCES (1) Eisenthal, K. B. Chem. Rev. 1996, 96, 1343–1360. (2) Shen, Y. R. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 12104–12111. (3) Conboy, J. C.; Messmer, M. C.; Walker, R. A.; Richmond, G. L. Prog. Colloid Polym. Sci. 1997, 103, 10–20. (4) Buck, M.; Himmelhaus, M. J. Vac. Sci. Technol., A 2001, 19, 2717–2736. (5) Chen, Z.; Shen, Y. R.; Somorjai, G. A. Annu. Rev. Phys. Chem. 2002, 53, 437–465. (6) Williams, C. T.; Beattie, D. A. Surf. Sci. 2002, 500, 545–576. (7) Richmond, G. L. Chem. Rev. 2002, 102, 2693–2724. (8) Wang, H.-F.; Gan, W.; Lu, R.; Rao, Y.; Wu, B.-H. Int. Rev. Phys. Chem. 2005, 24, 191–256. (9) Allen, H. C.; Casillas-Ituarte, N. N.; Sierra-Hernandez, M. R.; Chen, X.; Tang, C. Y. Phys. Chem. Chem. Phys. 2009, 11, 5538–5549. (10) Geiger, F. M. Annu. Rev. Phys. Chem. 2009, 60, 61–83. (11) Guyot-Sionnest, P. Surf. Sci. 2005, 585, 1–2. (12) Rangwalla, H.; Dhinojwala, A. J. Adhes. 2004, 80, 37–59. (13) Loch, C. L.; Ahn, D.; Chen, C.; Chen, Z. J. Adhes. 2005, 81, 319–345. (14) Chen, Z. Prog. Polym. Sci. 2010, 35, 1376–1402. (15) Lu, X.; Chen, Z.; Xue, G.; Wang, X. Front. Chem. China 2010, 5, 435–444. (16) Zhang, D.; Ward, R. S.; Shen, Y. R.; Somorjai, G. A. J. Phys. Chem. B 1997, 101, 9060–9064. (17) Zhang, D.; Shen, Y. R.; Somorjai, G. A. Chem. Phys. Lett. 1997, 281, 394–400. (18) Gracias, D. H.; Zhang, D.; Lianos, L.; Ibach, W.; Shen, Y.-R.; Somorjai, G. A. Chem. Phys. 1999, 245, 277–284. (19) Chen, Z.; Ward, R.; Tian, Y.; Baldelli, S.; Opdahl, A.; Shen, Y.-R.; Somorjai, G. A. J. Am. Chem. Soc. 2000, 122, 10615–10620.

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A Sum Frequency Generation Vibrational Study of the Interference

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