ARTICLE pubs.acs.org/JPCC
Absolute Orientation of Ester Side Chains on the PMMA Surface Kailash C. Jena, Paul A. Covert, Shaun A. Hall, and Dennis K. Hore* Department of Chemistry, University of Victoria, Victoria, British Columbia, V8W 3V6, Canada ABSTRACT: Phase-resolved sum-frequency generation spectroscopy has been used to measure the complex, nonlinear susceptibility spectra of the ester methyl groups at the poly(methyl methacrylate)air interface. The response of the ester methyl symmetric stretch is found to be in phase with that of the aliphatic methyl symmetric stretch in an ordered octadecylsilane monolayer. However, because the local bonding environments of these two methyl groups differ significantly, the signs of the molecular hyperpolarizability tensor elements are not necessarily the same. Therefore, the ester methyl orientation cannot be inferred from the phase measurements alone. To address this ambiguity, the relevant tensor elements were calculated for both types of methyl groups. Results of these calculations, combined with spectroscopic evidence, indicate that the ester methyl groups at the polymer surface are oriented with their hydrogen atoms directed toward the air. This finding may be used to better understand the hydrophobicity and reactivity of the polymer surface.
’ INTRODUCTION The surface structure of polymers is critical to their function in a wide variety of technological, industrial, laboratory, and biomedical applications. For example, biosensors often utilize enzymes immobilized on polymer supports. The activity of the enzyme may be adversely affected by minor changes in its secondary and tertiary structure, and such changes may be imparted by the hydrophobicity of an adjacent solid support. It has already been established that knowledge of the bulk polymer structure alone is not sufficient to predict the surface composition, as various components of the polymer chain may selectively segregate to the surface.1 The surface structure of poly(methyl methacrylate) (PMMA) in particular has received much attention.210 One of the challenges inherent in such studies is finding a tool that is sufficiently specific to the polymer surface, without being overwhelmed by the composition of the bulk phase. Over the past decade, vibrationally resonant sum-frequency generation (SFG) spectroscopy, a nonlinear optical three-wave mixing technique, has proven itself to be especially well-suited to the selective investigation of polymer surfaces.1,1019 This specificity stems from the fact that, under the electric dipole approximation, key elements of the second-order nonlinear susceptibility χ(2) are zero in bulk environments. However, if the material is ordered in a polar manner as expected at the surface, χ(2) 6¼ 0 and a signal proportional to |χ(2)|2 is measured. Furthermore, if the energy of the infrared pump beam is tuned over vibrational resonances, this then provides surface-specific information for each chemical moiety present in the interfacial region. Shortly after the first demonstration of this technique,20,21 effort has been directed toward quantifying surface structures based on the magnitude of the observed |χ(2)|2 signal, often utilizing various combinations of input and output polarization states to aid in the analysis.2226 However, despite this rich quantitative information, a key characteristic of the surfaces has in many cases remained elusive. This has been the direction of the functional groups in an absolute r 2011 American Chemical Society
sense for example, whether methyl groups have their hydrogen atoms directed toward or away from the bulk polymer. For this reason, when one reports a CH3 mean tilt angle of θ with respect to the surface normal, the ambiguity between θ and (180 θ) remains to be resolved. In some cases, a simple physical or chemical argument suffices for example in the case of a covalently bound species immobilized on the surface, or an amphipathic surfactant at the airwater interface. In other cases, such as that of a long, tangled polymer chain, the result is not obvious and must be determined experimentally. A quantitative analysis of polarized SFG spectra obtained from the PMMAair interface has previously revealed that ester methyl groups are oriented approximately perpendicular to the surface.18 Resolving the absolute orientation of these functional groups would provide a fundamental chemical insight to rationalize the hydrophilicity of the PMMA surface and its subsequent reactivity. Because SFG spectroscopy relies on polar orientation to observe χ(2) 6¼ 0, it follows that it must be possible to resolve this polarity, that is to know the absolute orientation of the bonds in resonance with the infrared pump beam. The challenge is that the sense of the bond manifests itself only in the phase of χ(2). We therefore require a phase-resolved SFG experiment to tease out this information, as has been experimentally demonstrated by a variety of interferometric schemes.2739 The underlying principle in such measurements is a comparison of interference fringes obtained from the sample with those obtained from a reference material. However, in moving from the experimental result to conclusions based on the measured phase, knowledge of the molecular hyperpolarizability of all materials is critical. In this article, we report the absolute orientation of the PMMA ester methyl groups to be directed toward the air. We demonstrate that arriving at this conclusion requires a careful discussion of the Received: June 9, 2011 Published: July 19, 2011 15570
dx.doi.org/10.1021/jp205712c | J. Phys. Chem. C 2011, 115, 15570–15574
The Journal of Physical Chemistry C
ARTICLE
Figure 2. Phase determination of (a) PMMA with respect to (b) z-cut quartz. The points are the measured data at each position of the phase shifting unit. Lines represent a fit to the model used to extract the phase shift between each sample and the local oscillator. The final (relative) phase of PMMA is the difference between the two phase shifts. For clarity, fringe intensities have been scaled from zero to unity; offsets from zero are not displayed. Figure 1. (a) SFG ssp intensity spectrum across the PMMA ester methyl group symmetric stretch, proportional to the magnitude squared of the second-order susceptibility. (b) Amplitude spectrum, proportional to the second-order susceptibility, obtained from the square root of the intensity spectrum. (c) Two possibilities for the phase spectrum in the vicinity of the vibrational resonance, obtained from an interferometry measurement.
phase reference, and consideration of the molecular nonlinear properties of the sample and reference methyl groups.
’ EXPERIMENTAL METHODS PMMA (Scientific Polymer Products NY, molecular weight 35 000 g/mol) films were spin coated at 2000 rpm from a 2 wt %/wt solution in chloroform onto 8 mm thick glass substrates. We have recently demonstrated our experimental setup for acquiring phase-resolved SFG spectra.26 Briefly, a local oscillator derived from y-cut quartz is overlapped with the sample SFG signal in a collinear manner. All beams are incident to the airPMMA interface at 70, using approximately 200 μJ of IR from 2900 3000 cm1, and 100 μJ of visible fixed at 18 797 cm1 (532 nm). All spectra are acquired in the ssp polarization scheme, averaging 100 pulses per data point, detecting s-polarized SFG with the input visible beam s-polarized and infrared beam p-polarized. SFG interferograms are obtained by varying the phase between the local oscillator- and sample-originating SFG field by means of a (45 tilting fused silica plate. ’ RESULTS AND DISCUSSION Experiments. With the phase shifting unit and source of local oscillator removed (conventional SFG setup), we obtain the ssp intensity spectrum shown in part a of Figure 1 (points), proportional to |χ(2)|2. The square root of each point then provides the amplitude |χ(2)| spectrum shown in part b of Figure 1 (points). The band near 2955 cm1 has been demonstrated to be the symmetric stretch of the ester methyl group (shown in the inset of part a of Figure 1).1113 With the phase-shifting unit and local
oscillator source in place, we obtain interferograms at eight IR beam energies between 29482960 cm1. The same IR energies are then used to obtain interferograms with a thick piece of z-cut quartz replacing PMMA in the sample position. Interferograms obtained at ωIR = 2950 cm1 are shown in Figure 2 (points). One notices that the amplitude of the fringes varies considerably as a function of the phase-shifting unit tilt angle, and the bottom of the fringes occurs at a lower intensity as the tilt angle increases. This is a result of the amplitude correction in the form of the Fresnel transmission coefficients when the transmission of all three beams is considered through the fused silica plate.26 This amplitude correction is taken into account in the interference line shape according to the procedure described in ref 26. By fitting to this line shape, we are able to determine the phase of χ(2) at each IR energy with respect to that of our z-cut quartz sample. An example of the fit is shown in part b of Figure 2 (lines). However, if the polarity of the quartz reference is not known (i.e., the direction of its positive x axis has not been identified), then there is a 180 ambiguity in the resulting PMMA phase. This is displayed in part c of Figure 1 (points) in a manner that highlights the phase ambiguity. Because there are no other modes of significant amplitude nearby, we can approximate the IR energy-dependence of the second-order susceptibility of the ester CH3 symmetric stretch with a Lorentzian χð2Þ ðωIR Þ ¼
A ω0 ωIR iΓ
ð1Þ
Here, A is the mode amplitude, governed by the ensemble average of molecular hyperpolarizabilities for the individual contributing ester methyl groups, ω0 is the mean resonance frequency, Γ is the line width, and i = (1)1/2. In consideration of the data presented in Figure 1, we rewrite eq 1 in the form χ(2) = |χ(2)|eij. Now, the frequency dependence of the amplitude may be expressed explicitly as jAj jχð2Þ jðωIR Þ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðω0 ωIR Þ2 þ Γ2 15571
ð2Þ
dx.doi.org/10.1021/jp205712c |J. Phys. Chem. C 2011, 115, 15570–15574
The Journal of Physical Chemistry C
ARTICLE
Figure 3. Comparison of the measured phase of PMMA and OTS. The frequency is displayed relative to ω0 = 2956 cm1 for PMMA (blue), and relative to ω0 = 2873 cm1 for OTS (red). One can see that the sign of the phase on resonance is the same for both molecules.
Similarly, the phase of χ(2) may be written as AΓ jðωIR Þ ¼ arctan Aðω0 ωIR Þ
ð3Þ
Using these definitions, we perform a simultaneous fit of eq 2 to the data in part b of Figure 1, and eq 3 to the data in part c of Figure 1. The fit resulted in |A| = 17.89, ω0 = 2956 cm1, and Γ = 5.105 cm1 and is indicated by the solid lines in Figure 1. Note that the fits in parts a and b of Figure 1 would be identical for mode amplitudes (A, as indicated by the factor A appearing in the numerator and denominator of the arctangent argument in eq 3. The distinction in the two possibilities for the sign are seen in part c of Figure 1. If the sign of the amplitude were known, this information may be obtained from quadrant preservation in the eq 3 arctangent evaluation. In particular, when A > 0, j = π/2 on resonance (ωIR = ω0); when A < 0, j = π/2 on resonance. The first step in resolving this phase ambiguity is to compare PMMA to a material of known structure. For this, we have prepared an octadecylsilane (OTS) monolayer on glass following a published protocol.40 The terminal CH3 group on OTS, known to be directed away from the glass, has a symmetric stretch on resonance with the IR energy at 2873 cm1. Interferograms obtained for IR energies in the neighborhood of this mode are compared with interferograms obtained from our z-cut quartz reference in the same orientation as was used in the PMMA experiment. Figure 3 plots the phase of PMMA (blue) and OTS (red) against the relative frequency Δω = ωIR ω0 of their respective vibrational modes. We observe that both curves display the same phase characteristics about the resonance frequencies, that is they are not shifted by 180 with respect to each other. Because it has been established that methyl groups ordered as in OTS have j = π/2 on resonance,27,38,41,42 the ester methyl group of PMMA therefore also has j = π/2 on resonance. Both samples are studied in external reflection from a dielectricair interface, so the local field corrections are all real and positive. However, it is important to note that this determination alone does not necessarily indicate that the direction of the methyl group is the same for both materials. It has already been established that different electronic environments may result in variation of the sign of the molecular hyperpolarizability tensor elements.29 The OTS methyl symmetric stretch (ω0 = 2873 cm1) at the end of a long alkyl chain is in a significantly different environment from the PMMA ester CH3 symmetric stretch (ω0 = 2956 cm1), as evidenced by the ∼80 cm1 difference in resonance frequencies. The relationship between the hyperpolarizability tensor elements for these two methyl
Figure 4. (a) Methyl hexanoate, illustrating the local coordinate systems defined for the ester CH3 (blue axes) and aliphatic CH3 (red axes). (be) Elements of the methyl hexanoate polarizability tensor R(1) lm and dipole moment vector μn in the local CH3 frame as a function of the normal mode coordinate, Q. The a, b, and c axes are defined according to the convention illustrated in part a. Data plotted with points are obtained from our calculation; solid lines indicate fits to a second-order polynomial; dashed lines are the derivatives about Q = 0. Data in red correspond to the methyl group at the end of the alkyl chain; data in blue are for the ester methyl group. Polarizability derivative results are shown (1) (1) for (b) ∂R(1) aa /∂Q, (c) ∂Rbb /∂Q, and (d)∂Rcc /∂Q. Dipole moment derivative results are shown for (e) ∂μc/∂Q.
groups therefore remains to be established; we address this with electronic structure calculations. Calculations. In general, we must consider the ensemble average of all participating hyperpolarizability elements in χ(2) NÆR(2)æ, where N is the number of methyl groups in a noncentrosymmetric environment and R(2) is their hyperpolarizability tensor. If we consider that the methyl groups have no azimuthal orientation (isotropy in the plane of the surface) and no preferred twist angle (rotation about the methyl C3 axis), then we are concerned with only the tilt angle, θ, with respect to the surface normal. With our experimental choice of beam polarizations (ssp), this results in the expression ð2Þ
ð2Þ χð2Þ Nð2Rð2Þ ccc þ Raac þ Rbbc ÞÆcos θæ ð2Þ
ð2Þ 3 Nð2Rð2Þ ccc Raac Rbbc ÞÆcos θæ
ð4Þ
where the angular brackets represent an ensemble average of cos θ and cos3θ over all the methyl group tilt angles. We now have (2) (2) the task of comparing R(2) aac , Rbbc , and Rccc for the two different methyl group environments. eq 4 also serves to highlight the importance of the sign of the R(2) components in that they manifest themselves in the χ(2) response as prefactors to either cos θ or cos3θ terms. Because these are odd powers of the cosine, the contribution of the sign of the R(2) elements will be influenced by the absolute orientation of the bond: cos θ = cos (180 θ) and cos3θ = cos3 (180 θ). This is in contrast to spectroscopic 15572
dx.doi.org/10.1021/jp205712c |J. Phys. Chem. C 2011, 115, 15570–15574
The Journal of Physical Chemistry C
ARTICLE
and that the ester methyl groups are close to the surface normal, tilted in the range θ = 030 depending on the width of their orientation distribution.18 (We reiterate that, until now, this was equivalent to θ = 150180 prior to the resolution of the absolute orientation.) Because the ester methyls were determined to be nearly upright at the surface (parallel or antiparallel to the surface normal), resolving their direction is especially critical, as this has implications for the hydrophilicity of the PMMA surface, and its reactivity toward oxygen plasma and other surface modifications. Our finding that these groups direct their hydrogen atoms toward the air may account for the observed water contact angle of 6070,43,44 hydrophilic, yet suggestive that the ester sp3 oxygen atom is not exposed at the surface. Figure 5. Proposed structure of the PMMAair surface with ester methyl groups (highlighted in yellow) oriented with their hydrogen atoms directed toward the air. For contrast, alpha methyl groups are highlighted in blue.
techniques such as IR absorption and Raman scattering that depend on even (second and fourth) powers of the cosine term where cos2θ = cos2 (180 θ) and so it is not possible to resolve the directionality of chemical bonds. This additionally draws attention to the necessity of determining the signs of all participating R(2) elements, not just their magnitudes. For this calculation, we have employed methyl hexanoate since it contains a methyl ester similar to PMMA, and a methyl group at the end of an alkyl chain similar to OTS. For each CH3 group, part a of Figure 4 illustrates that we define the local c axis as pointing from the methyl carbon toward the methyl hydrogen atoms; the a axis is eclipsed with one of the methyl CH bonds; b is perpendicular to both a and c to form a right-handed coordinate system. Our scheme will be to evaluate the harmonic approximation ! ð1Þ ∂Rlm 1 ∂μn ð2Þ ð5Þ Rlmn ωIR ω0 iΓ ∂Q ∂Q where R(1) is the linear polarizability, μ is the dipole moment, Q is the normal mode coordinate for the CH3 symmetric stretch, and l, m, n represent any of the molecular frame Cartesian coordinates a, b, c. We perform a geometry optimization and subsequent Hessian calculation at the B3LYP/6-31G(d,p) level with a polarizable continuum solvent model. We then determine (1) (1) R(1) aa , Rbb , Rcc , and μc for 7 displacements about Q for each methyl group. The results are shown in parts be of Figure 4 for the alkyl methyl group (red) and the ester methyl group (blue). Q = 0 represents the equilibrium geometry; the 3 points with Q < 0 are compressions along the normal mode coordinate; the 3 points with Q > 0 are extensions. The dashed lines are the slopes evaluated at Q = 0, representative of the sought derivatives in eq 5. We observe that, in all cases, the signs of the derivatives are identical for the alkyl methyl group and ester methyl group, and their values are also very similar. We can now conclude that, because the measured phase of the PMMA ester methyl symmetric stretch is the same as that of the OTS methyl symmetric stretch (Figure 3) and their hyperpolarizability tensor elements have the same sign, they have the same absolute orientation. Our proposed conformation is illustrated in Figure 5, with the ester methyl groups highlighted in yellow. A previous study has determined that the alpha methyl groups are tilted close to the plane of the PMMA surface (as highlighted in blue in Figure 5),
’ CONCLUSIONS We have determined that, at the surface of PMMA, the polymer side chains are oriented such that their ester methyl groups are directed toward the air. We have additionally illustrated that resolving the absolute orientation of surface chemical functional groups requires a careful consideration of the molecular response of all materials, including the ones chosen as a standard for comparison. To this end, we have used a nonlinear optical technique together with electronic structure calculations to resolve the ambiguity in the direction of the surface side chains. Efforts to alter the hydrophobicity of the PMMA surface rely on an understanding and subsequent control of the surface composition and morphology. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for support of this science with a Discovery Grant. Equipment was purchased with assistance from the Canadian Foundation for Innovation (CFI) Leaders Opportunity Fund, the British Columbia Knowledge Development Fund (BCKDF), and the University of Victoria. S.A.H. is grateful to NSERC for an Alexander Graham Bell graduate scholarship. We thank Prof. Matthew Moffitt for stimulating discussions. ’ REFERENCES (1) Chen, Z.; Shen, Y. R.; Somorjai, G. A. Annu. Rev. Phys. Chem. 2002, 53, 437–465. (2) Soga, I.; Granick, S. Colloids Surf., A 2000, 170, 113–117. (3) Watts, J. F.; Leadley, S. R.; Castle, J. E.; Blomfield, C. J. Langmuir 2000, 16, 2292–2300. (4) Carriere, P.; Grohens, Y.; Spevacek, J.; Schultz, J. Langmuir 2000, 16, 5051–5053. (5) Pireaux, J. J.; Gregoire, C.; Caudano, R.; Vilar, M. R.; Brinkhuis, R.; Schouten, A. J. Langmuir 1991, 7, 2433–2437. (6) Erber, E.; Tress, M.; Mapesa, E. U.; Serghei, A.; Eichhorn, K.-J.; Voit, B.; Kremer, F. Macromolecules 2010, 43, 7729–7733. (7) Deimel, M.; Rulle, H.; Liebing, V.; Benninghoven, A. Appl. Surf. Sci. 1998, 134, 271–274. (8) Durning, C. J.; O’Shaughnessy, B.; Sawhney, U.; Nguyen, D.; Majewski, J.; Smith, G. S. Macromolecules 1999, 32, 6772–6781. (9) Shaffer, J. S.; Chakraborty, A. K. J. Chem. Phys. 1991, 95, 8616–8630. 15573
dx.doi.org/10.1021/jp205712c |J. Phys. Chem. C 2011, 115, 15570–15574
The Journal of Physical Chemistry C
ARTICLE
(10) Wang, J.; Woodcock, S. E.; Buck, S. M.; Chen, C.; Chen, Z. J. Am. Chem. Soc. 2001, 123, 9470–9471. (11) Tateishi, Y.; Kai, N.; Noguchi, H.; Uosaki, K.; Nagamura, T.; Tanaka, K. Polym. Chem. 2010, 1, 303–311. (12) Rao, A.; Rangwalla, H.; Varshney, V.; Dhinojwala, A. Langmuir 2004, 20, 7183–7188. (13) Li, Q.; Hua, R.; Cheah, I. J.; Chou, K. C. J. Phys. Chem. B 2008, 112, 694–697. (14) Clarke, M. L.; Chen, C.; Wang, J.; Chen, Z. Langmuir 2006, 22, 8800–8806. (15) Lu, X.; Shephard, N.; Han, J.; Xue, G.; Chen, Z. Macromolecules 2008, 41, 8770–8777. (16) Kweskin, S. J.; Komvopoulos, K.; Somorjai, G. A. Langmuir 2005, 21, 3647–3652. (17) Liu, Y.; Messmer, M. J. Am. Chem. Soc. 2002, 124, 9714–9715. (18) Wang, J.; Chen, C.; Buck, S. M.; Chen, Z. J. Phys. Chem. B 2001, 105, 12118–12125. (19) Miyamae, T.; Nozoye, H. Surf. Sci. 2003, 532535, 1045–1050. (20) Shen, Y. R. The Principles of Nonlinear Optics; John Wiley & Sons: New York, 1984. (21) Shen, Y. R. Nature 1989, 337, 519–525. (22) Wei, X.; Hong, S.-C.; Zhuang, X.; Goto, T.; Shen, Y. R. Phys. Rev. E 2000, 62, 5160–5172. (23) Gan, W.; Wu, D.; Zhang, Z.; Feng, R.-R.; Wang, H.-F. J. Chem. Phys. 2006, 124, 114705. (24) Lu, R.; Gan, W.; Wu, B.; Chen, H.; Wang, H. J. Phys. Chem. B 2004, 108, 7297–7306. (25) Hall, S. A.; Hickey, A. D.; Hore, D. K. J. Phys. Chem. C 2010, 114, 9748–9757. (26) Jena, K. C.; Covert, P. A.; Hore, D. K. J. Chem. Phys. 2011, 134, 044712. (27) Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. J. Chem. Phys. 2009, 130, 204704. (28) Superfine, R.; Huang, J. Y.; Shen, Y. R. Opt. Lett. 1990, 15, 1276–1278. (29) Superfine, R.; Huang, J. Y.; Shen, Y. R. Chem. Phys. Lett. 1990, 172, 303–306. (30) Ji, N.; Ostroverkhov, V.; Chen, C.; Shen, Y. R. J. Am. Chem. Soc. 2007, 129, 10056–10057. (31) Zhang, L.; Tian, C.; Waychunas, G.; Shen, Y. R. J. Am. Chem. Soc. 2008, 130, 7686–7694. (32) Tian, C.; Ji, N.; Waychunas, G. A.; Shen, Y. R. J. Am. Chem. Soc. 2008, 130, 13033–13039. (33) Tian, C.; Shen, Y. R. J. Am. Chem. Soc. 2009, 131, 2790–2791. (34) Ji, N.; Ostroverkhov, V.; Tian, C. S.; Shen, Y. R. Phys. Rev. Lett. 2008, 100, 096102. (35) Stiopkin, I. V.; Jayathilake, H. D.; Bordenyuk, A. N.; Benderskii, A. V. J. Am. Chem. Soc. 2008, 130, 2271–2275. (36) Yamaguchi, S.; Tahara, T. J. Chem. Phys. 2008, 129, 101102. (37) Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. J. Am. Chem. Soc. 2010, 132, 6867–6869. (38) Mondal, J.; Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. J. Am. Chem. Soc. 2010, 132, 10656–10657. (39) Chen, X.; Hau, W.; Huang, Z.; Allen, H. J. Am. Chem. Soc. 2010, 132, 11336–11342. (40) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92–98. (41) Sovago, M.; Vartiainen, E.; Bonn, M. J. Phys. Chem. C 2009, 113, 6100–6106. (42) Sovago, M.; Vartiainen, E.; Bonn, M. J. Chem. Phys. 2010, 133, 229901. (43) Ma, Y.; Cao, X.; Feng, X.; Ma, Y.; Zou, H. Polymer 2007, 48, 7455–7460. (44) Jung, Y. C.; Bhushan, B. Nanotechnol. 2006, 17, 4970–4980.
15574
dx.doi.org/10.1021/jp205712c |J. Phys. Chem. C 2011, 115, 15570–15574
