Article pubs.acs.org/Langmuir
Molecular Ordering of Phenyl Groups at the Buried Polystyrene/ Metal Interface Xiaolin Lu,*,† John N. Myers,‡ and Zhan Chen*,‡ †
State Key Laboratory of Bioelectronics, School of Biological Science & Medical Engineering, Southeast University, Nanjing 210096, Jiangsu Province, P. R. China ‡ Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109 United States S Supporting Information *
ABSTRACT: Understanding molecular structures of buried polymer/metal interfaces is important for the design and development of polymer adhesives used in advanced microelectronic devices and polymer anticorrosion coatings for metals. The buried interfacial molecular structure between polystyrene (PS) and silver (Ag) was investigated using infrared-visible sum frequency generation (SFG) vibrational spectroscopy via a “sandwiched” sample geometry. SFG resonant signals from the phenyl C−H stretching vibrational modes were detected from the PS/Ag interface, suggesting that the PS phenyl groups at this buried polymer/metal interface are ordered. Spectral analysis indicated that the phenyl groups at the buried PS/Ag interface tilt toward the interface, pointing away from the Ag side.
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“sandwiched” sample geometry21 was adopted to measure the SFG resonant signals generated from the PS/Ag interface, which can disclose the molecular ordering information at the buried PS/Ag interface. Analysis of the SFG spectra revealed that the phenyl groups at the PS/Ag interface are ordered and tilt toward the buried interface.
INTRODUCTION Probing buried polymer/solid interfaces is desirable because the molecular structures of such buried interfaces impact the successful applications of many materials such as composites, adhesives, paints, and coatings.1−3 The interfacial molecular structures generally reflect the molecular interactions at the interfaces between two bulk phases, either physically or chemically or both. Rich information has been gathered to understand the molecular structures of buried polymer/solid interfaces concerning conformation of chain backbones,4,5 segregation of chain ends and side groups,6−8 and specific interactions between polymers and solid substrates.9−11 However, it is technically difficult to probe buried polymer/ solid interfaces, and few technologies have the capability to directly probe the buried interfaces in situ. Sum frequency generation (SFG) vibrational spectroscopy, which has been developed for more than two decades,12,13 has proved to be a powerful analytical tool to investigate buried polymer/solid interfaces.14−26 For example, the molecular structures at the interfaces of polystyrene (PS) and sapphire,14 PS and SOG,15,16 polyethylene (PE)−polypropylene (PP) blend and sapphire,8 polyacrylates and sapphire,17,18 styrenic surface-active block copolymer (SABC) and calcium fluoride (CaF2),19 polyacrylates and silver (Ag),20−22 polybutadiene-modified epoxy (PBME) and gold (Au),23 and organic field effect transistors24,25 have been successfully studied using the SFG spectroscopy. Nonetheless, only in recent years has SFG spectroscopy been employed to study the molecular structures of buried polymer/metal interfaces.20−23 In this article, we applied the SFG spectroscopy to study the molecular structure at the buried PS/Ag interface. A recently developed © 2014 American Chemical Society
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EXPERIMENTAL SECTION
When soft polymer chains meet a rigid wall, just like the case of the PS/Ag interface investigated in this article, entropy loss of the polymer chains between the polymer bulk and the rigid wall must render some incremental ordering of the polymer chains at the interface. Such incremental ordering originating from the self-adaption of the soft polymer chains at the interface should inherently exist and should be able to be observed experimentally. Here, to detect such molecular ordering at the buried interface, we sandwiched a PS (Scientific Polymer Products, Mw = 600 000) thin film between a transparent silica substrate and a smooth Ag substrate prepared by an electronbeam evaporator (Cooke Evaporator, Cooke Vacuum Products). Briefly speaking, a PS film on a transparent fused silica substrate was prepared by spin-coating. After annealing, a 500 nm thick Ag layer was electron-beam evaporated onto the PS surface. After annealing again, a sandwiched sample was ready for the SFG measurement. A commercial SFG system (EKSPLA, Lithuania) based on a Nd:YAG laser with an output pulse of 20 ps was used to probe such an interface. The infrared and visible beams penetrated through the transparent silica and overlapped at the PS/Ag interface. The reflected output SFG beam went through the transparent silica and was then collected by a detector, as shown in Figure 1. Such a “sandwiched” geometry has Received: May 26, 2014 Revised: July 10, 2014 Published: July 14, 2014 9418
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Lorentzian function over the infrared frequency range, as shown in eq 1.27,28 ISFG ∝ |χeff |2 = χNR eiα +
∑ q
Aq ωIR − ωq + i Γq
2
(1)
χeff is the effective second-order nonlinear optical susceptibility tensor at the surface or interface. The components of χeff can be probed using different polarization combinations of the input and output laser beams chosen in an SFG experiment. (In this study, ssp and ppp were used.) Aq, ωq, and Γq are the strength, resonant frequency, and damping coefficient of the vibrational mode q. α is the oscillating phase difference between χNR and the vibrational modes. Figure 3 shows the five normal
Figure 1. Schematics show the “sandwiched” sample geometry used for the SFG measurement (left) and the relation between the surface (or interface, XYZ) and molecular (abc) coordinate systems (right). Z axis is defined as the surface normal and XZ plane contains the input and output beams; c axis is the principle axis of the phenyl ring; ac plane contains the phenyl ring plane. The tilt angle is defined as the angle between Z axis and c axis. significant advantages for SFG measurement to probe the buried polymer/metal interface. First, the silica and the polymer have a similar refractive index, and as a result the intensities of the input light beams reaching the polymer/metal interface are almost independent of the polymer film thickness (because the film is thin, the IR absorption can be ignored); therefore, the intensity of the output SFG beam does not significantly change based on the thickness of the film. Second, upon excitation by the input beams, a strong surface plasmon resonance (SPR) signal generated from the metal surface can be observed,26 which serves as the nonresonant background in an SFG spectrum. With this strong nonresonant background, it is easier to collect signal from the interface and to determine whether the collected SFG spectrum is contributed from the polymer/metal interface.
Figure 3. Five normal vibrational modes of the C−H stretching vibrations for a phenyl group.
modes of the C−H stretching vibrations for a phenyl group.29−31 In the ssp spectrum shown in Figure 2, signals from the ν20b and ν2 modes appear as the two strong “negative” peaks located at 3020 and 3061 cm−1, respectively. A weak peak also appears at 2920 cm−1 which can be assigned to the methylene antisymmetric stretching (as) vibrational mode. In the ppp spectrum, signals from the ν7a, ν7b, and ν2 modes overlap as one broad “negative” peak. Spectral fitting can reveal the resonant frequency (ωq), strength (Aq), and damping coefficient (Γq) with respect to each vibrational mode, as shown in Table 1. The detection of the phenyl vibrational modes strongly suggests that the phenyl groups have certain orientational ordering at the buried PS/Ag interface, as we expected before. As a control experiment, we also collected SFG spectra of a PS thin film on a silica substrate, which reflect the molecular ordering information at the PS surface in air,32,33 as shown in Figure 4. The fitting results for the spectra in Figure 4 are also shown in Table 1. In the ssp spectrum shown in Figure 4, the dominant peak is from the ν2 mode and the two shoulder peaks are from the ν7a and ν7b modes. In the ppp spectrum, the two dominant peaks are from the ν7a and ν2 modes. To acquire more information related to the molecular orientation, certain ratios in Table 1 can be used to estimate the orientation angles of phenyl groups. When we prepared our samples, all PS samples were annealed at 80 °C (near the glass-transition temperature of PS, Tg) for more than 4 h. In this respect, the twist angles are considered to be isotropically distributed because the annealing temperature is high enough to relax the side phenyl groups. The remaining tilt angle of the phenyl groups is directly associated with the PS backbone orientation, which is strongly affected by the nearby Ag wall and thus needs to be evaluated. Because in all of the spectra the ν2 mode can be clearly differentiated, the experimentally measured susceptibility ratio of ssp over ppp polarization combinations for the ν2 mode was used to evaluate the tilt angles of the phenyl groups on the surface in air and at the buried polymer/metal interface.
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RESULTS AND DISCUSSION Figure 2 shows the collected SFG spectra with ssp (s-polarized sum frequency beam, s-polarized visible beam, and p-polarized
Figure 2. ssp and ppp spectra collected using the “sandwiched” sample geometry to probe the PS/Ag interface. The solid lines are the fitted results using eq 1
infrared beam) and ppp polarization combinations. These spectra are the averaged results from multiple spectra collected from each of the five different “sandwiched” samples. (See Figure S1 in the Supporting Information.) As we can see, over the scanned infrared frequency range, the nonresonant signals from the Ag surface (χNR) appear as a relatively smooth curve, and the molecular vibrational signals appear as the distinct peaks. The overall SFG intensity can be represented as a 9419
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Table 1. Fitting Parameters for PS Surface in Air, PS/Ag Interface, and SAM on Ag Surfacea Aq sample
ωq (cm−1)
ssp
2915 2965 3032 3053 3066 χNR α 2920 2945 3020 3035 3047 3061 3078 χNR α 2845 2880 2933 3065 χNR α
13
PS surface in air
PS/Ag interface
PETL SAM on Ag
a
ppp 9 21 0 19 −1.0 0
−12 16 36 0 0 23 −12 69
138 111 177 −24 154 −1.6
68 55 −1.6 −11 11 21 29 19 −1.6
Γq (cm−1) 8 8 7 6 6
assignment CH2 as CH3 as ν7a ν7b ν2
10 5 9 9 9 9 9
CH2 as unassigned ν20b ν7a ν7b ν2 ν20a
6 6 12 11
CH2 ss CH2 as CH2 as ν2
ss: symmetric stretching; as: antisymmetric stretching.
Figure 4. Collected ssp and ppp spectra from the PS film on the silica substrate, probing the PS surface in air. The solid lines are the fitted curves using eq 1. (2) χeff,ssp, ν2 (2) χeff,ppp, ν2
≅
Fssp, yyzχyyz , ν2 Fppp, xxzχxxz , ν2 + Fppp, zzzχzzz , ν2
Figure 5. Relation between the |χeff,ssp,ν2/χeff,ppp,ν2| and the tilt angle of the phenyl groups at the PS/Ag interface and the PS surface in air. The schematic in the bottom panel shows the phenyl group at the buried PS/Ag interface adopts a large tilt angle and the phenyl group at the PS surface in air adopts a small tilt angle with respect to the surface/ interface normal. The plane including X and Y axes defines the surface/interface plane, and the Z axis defines the surface/interface normal.
(2)
Figure 5 shows the calculated susceptibility ratios of ssp over ppp polarization combinations as a function of the averaged phenyl tilt angle (θ0) and angle distribution width (σ) assuming a Gaussian distribution28,34 for the PS surface in air and the buried PS/Ag interface. (See the Supporting Information.) From the intersection points between the calculated curves and the experimentally measured values, the phenyl orientations at the PS/Ag interface and the PS surface in air were found to be different. At the PS/Ag interface, the phenyl groups can have
either a single tilt angle of 67° (σ = 0°) or an averaged tilt angle of 90° with an angle distribution width of 15° (σ = 15°).35 At 9420
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contributed from the ν2 mode at 3065 cm−1 was clearly observed as a distinct “negative” peak. Fitting results indicate that the phase term (α) of this vibrational mode with respect to the nonresonant background is the same as that at the PS/Ag interface (−1.6 in radian for nonresonant signals in both cases and positive signal strength for ν2 signal). The phenyl groups at both PETL SAM surface and PS/Ag interface should be azimuthally isotropic. As we previously discussed, at such a surface or interface, only molecular dipoles along the surface or interface normal can be detected because the isotropic surface or interface does not allow any dipole component parallel to the surface or interface to be SFG active as a result of ensemble average. Therefore, the same phase term of the ν2 mode suggests the ensemble-averaged dipole directions for the phenyl groups at the SAM surface and the PS/Ag interface are the same, or both phenyl groups have the same averaged absolute orientation.36,37 On the SAM surface, the phenyl end group should point away from the metal surface because the thiol group at the other end should chemically react with the surface. Therefore, it is believed that the phenyl groups at the PS/Ag interface should also point away from the Ag side, as the phenyl groups at the SAM surface do. In fact, such a phenyl absolute orientation at the PS/Ag interface is favorable. Possibly in this way, the phenyl groups can avoid close contact with the rigid Ag wall, which can help the phenyl groups seek more free volume and maximize their own conformational entropy. We believe that if a large number of side phenyl groups contact the metal surface, then the entropy loss will be larger, and this is entropically unfavorable.
the PS surface in air, the phenyl groups can have either a single tilt angle of 26° (σ = 0°) or an averaged tilt angle of 11° with an angle distribution width of 15° (σ = 15°). In other words, the phenyl groups are more likely to tilt toward the interface while in contact with Ag and to “stand up” at the PS surface in air. It should be noted that similar orientation information for the phenyl groups at the PS surface in air has been reported,14 which is consistent with our SFG result. If we compare the SFG ssp spectrum collected from the PS/Ag interface with that from the PS surface in air, the signal from the ν20b mode was observed from the PS/Ag interface but was not or was too weak to be observed for the PS surface in air. Because both the PS/ Ag interface and the PS surface in air are azimuthally isotropic, only a molecular dipole component along the surface normal can be detected by SFG. The dipole transition direction of the ν20b mode is perpendicular to the phenyl ring main axis (principle axis in Figure 1, axis “c”). For this vibrational mode to be observed, the phenyl ring must be tilted to a certain extent so that a component of the dipole can lie along the surface/interface normal. Furthermore, the ν20b mode is a weak Raman mode, which means its Raman polarizability derivative is very small. Qualitatively speaking, this mode becomes SFGactive only when the phenyl ring at the surface or interface adopts a very large tilt angle versus the surface/interface normal. Correspondingly, when the phenyl ring at the surface or interface adopts a very small tilt angle or stands up, this mode should not be observed anymore, just like the phenyl groups on the PS surface in air previously investigated. Alternatively, the dipole transition moment of the ν2 mode is along the direction of the phenyl ring main axis, and this mode is highly Raman-active. In the ssp spectrum detected using the “sandwiched” geometry (Figure 2), both the ν20b and ν2 modes are present with a similar signal strength. If the phenyl ring tilts not much larger than 45° versus the interface normal, then the signal contributed from the ν2 mode should be much stronger than that from ν20b. The similar signal strength again suggests that the phenyl ring at the PS/Ag interface must have a large tilt angle versus the interface normal, tilting toward the interface. A comparison SFG experiment was performed for a selfassembled monolayer (SAM) of phenylethylthiol (PETL) on the Ag surface. The collected ssp spectrum is shown in Figure 6, and the fitted results are shown in Table 1. Besides the aliphatic C−H vibrational signals below 3000 cm−1, signal
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CONCLUSIONS In summary, for the first time, we have applied the SFG spectroscopy to examine the local molecular structure at the buried PS/Ag interface in situ. It was found that phenyl groups are ordered at the buried PS/Ag interface, tilting toward the interface. Furthermore, it was determined that phenyl groups at the PS/Ag interface point away from the metal surface. Such findings demonstrate our ability to reveal the detailed molecular structural information at the buried polymer/metal interfaces. As a natural extension of this work, future studies will be focused on revealing the chain conformation and chain dynamics at the buried polymer/metal interface.
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ASSOCIATED CONTENT
S Supporting Information *
SFG theory and phenyl orientation calculation. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*X.L.: E-mail:
[email protected] *Z.C.: E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (grant nos. 51173169 and 21004054), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and Human Resources, and Social Security Bureau of Zhejiang Province, China. X.L. is also
Figure 6. ssp spectrum collected from a PETL SAM layer on the Ag surface (dotted line) and the fitted curve using eq 1 (solid line). 9421
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grateful for the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, China (PAPD 1107037001). Z.C. and J.N.M. acknowledge the support from Semiconductor Research Corporation (SRC contract no. 2012-KJ-2282). Z.C. also thanks University of Michigan for supporting his sabbatical.
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