Surface Morphology and Molecular Chemical Structure of Poly(n-butyl

Chunyan Chen, Cheryl L. Loch, Jie Wang, and Zhan ChenZhan Chen ... Jie Wang, Mark A. Even, Xiaoyun Chen, Alvin H. Schmaier, J. Herbert Waite, and Zhan...
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Surface Morphology and Molecular Chemical Structure of Poly(n-butyl methacrylate)/Polystyrene Blend Studied by Atomic Force Microscopy (AFM) and Sum Frequency Generation (SFG) Vibrational Spectroscopy Chunyan Chen,† Jie Wang,‡ Sara E. Woodcock,‡ and Zhan Chen*,†,‡ Department of Chemistry and Department of Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109 Received May 29, 2001. In Final Form: November 2, 2001 Sum frequency generation (SFG) vibrational spectroscopy and atomic force microscopy (AFM) have been applied to study the poly(n-butyl methacrylate) (PBMA)/polystyrene (PS) blend surfaces. SFG showed that PBMA tended to segregate to the blend surfaces because of its lower surface tension. The phenyl groups on the pure PS surface orientated closely to the surface normal with a narrow angle distribution. Presence of PBMA dramatically affected the orientation angle of the phenyl groups on the blend surface. For example, on the surface of the blend with only 4 wt % PBMA in the bulk, the phenyl groups would tilt more toward the surface, but they did not completely lie down on the surface. Because of the larger orientation angle of the phenyl groups versus the surface normal, no ppp signal of PS could be detected on the PBMA/PS blend surfaces with our experimental geometry. AFM results showed that pure PS and PBMA surfaces were flat, but domain structures existed on the polymer blend surfaces. Selective solvent cyclohexane has been used to identify species on the PBMA/PS blend surface. Annealing and solvent effects on the blend surface morphology have also been investigated.

1. Introduction The surface properties of polymer materials, such as wettability, friction, adhesion, and biocompatibility, play crucial roles in many applications.1-5 Polymer blends are composed of two or more different polymer components and may exhibit surface properties that are different from those observed in the bulk. This is attributed to the difference between the surface structures and the bulk structures, which is induced by individual components’ characteristics. The proper design of polymer blends can provide materials with desired properties that cannot be provided by a single polymer material, allowing the polymer blends versatility in various applications.6 The study of the surface structures of polymer blends is very important for determining the surface properties, and an increased knowledge of the structures will greatly help people to tailor the desired surface properties of a polymer blend by mixing the appropriate polymers. Polymer blend surfaces have been studied by many different groups using theoretical calculations7,8 and various surface-sensitive techniques including X-ray pho* To whom all correspondence should be addressed. E-mail: [email protected]. Fax: 734-647-4865. † Department of Macromolecular Science and Engineering. ‡ Department of Chemistry. (1) Carbassi, F.; Morra, M.; Occhiellp, E. Polymer Surfaces: from Physics to Technology; John Wiley and Sons: Chichester, 1994. (2) Rataner, B. D.; Castner, D. G. Surface Modification of Polymeric Biomaterials; Plenum Press: New York, 1996. (3) Feast, W. J.; Munro, H. S. Polymer Surfaces and Interfaces; John Wiley and Sons: New York, 1987. (4) Feast, W. J.; Munro, H. S.; Richards, R. W. Polymer Surfaces and Interfaces II; John Wiley and Sons: New York, 1992. (5) Park, J. B.; Lakes, R. S. Biomaterials: an Introducion; Plenum Press: New York, 1992. (6) Polymer Blends and Composites in Multiphase Systems; Han, C. D., Ed.; Advances in Chemistry Series 106; American Chemical Society: Washington, DC, 1984. (7) Genzer, J.; Faldia, A.; Composto, R. J. Phys. Rev. E 1994, 50, 2373-2376. (8) Wu, D. T.; Fredrickson, G. H.; Carton, J. P. J. Chem. Phys. 1996, 104, 6387-6397.

toelectron spectroscopy (XPS),9-11 secondary ion mass spectroscopy (SIMS),11-13 atomic force microscopy (AFM),10,11,14-19 contact angle goniometry,20 and neutron reflectivity.21,22 For instance, the surfaces of immiscible polymer blend films, such as polystyrene (PS)/polystyrene partially substituted with bromine (PBrS),16,17 PS/poly(methyl methacrylate) (PMMA),10,18,19 and PMMA/poly(ethylene oxide) (PEO)11 have been widely studied. Contact angle goniometry has been used to study surface tension or surface energy.20,23 AFM is an efficient probe to detect surface morphology, friction, or adhesion properties through contact or tapping modes with excellent spatial resolution10,14-19 and can also be applied to image surfaces in water. XPS and SIMS have been used in determining the surface composition of polymer blends.9-13 The detec(9) Artyushkova, K.; Wall, B.; Koenig, J.; Fulghum, J. E. Appl. Spectrosc. 2000, 54, 1549-1558. (10) Ton-That, C.; Shard, A. G.; Daley, R.; Bradley, R. H. Macromolecules, 2000, 33, 8453-8459. (11) Affrossman, S.; Kiff, T.; O’Neil, S. A.; Pethrick, R. A.; Richards, R. W. Macromolecules 1999, 32, 2721-2730. (12) Feng, J. Y.; Weng, L. T.; Li, F.; Chan, C. M. Surf. Interface Anal. 2000, 29, 168-174. (13) Vanden Eynde X.; Bertrand, P. Appl. Surf. Sci. 1999, 141, 1-20. (14) Takahara, A.; Nakamura, K.; Tanaka, K.; Kajiyama, T. Macromol. Symp. 2000, 159, 89-96. (15) Grandy, D. B.; Hourston, D. J.; Price, D. M.; Reading, M.; Silva, G. G.; Song, M.; Sykes, P. A. Macromolecules 2000, 33, 9348-9359. (16) Affrossman, S.; Henn, G.; O’Neil, S. A.; Pethrick, R. A.; Stamm, M. Macromolecules 1996, 29, 5010-5016. (17) Slep, D.; Asselta, J.; Rafailovish, M. H.; Sokolov, J.; Winesett, D. A.; Smith, A. P.; Ade, H.; Strzhemechny, Y.; Schwarz, S. A.; Sauer, B. S. Langmuir 1998, 14, 4860-4864. (18) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 1996, 29, 3232-3239. (19) Dalnoki-Veress, K.; Forrest, J. A.; Dutcher, J. R. Phys. Rev. E 1998, 57, 5811-5817. (20) Wu, S. Polymer Interface and Adhesion; Marcel Dekker: New York, 1982. (21) Muller-Buschbaum, P.; Gutmann, J. S.; Stamm, M. Macromolecules 2000, 33, 4886-4895. (22) Grull, H.; Schreyer, A.; Berk, N. F.; Majkrzak, C. F.; Han, C. C. Europhys. Lett. 2000, 50, 107-112. (23) Wulf, M.; Grundke, K.; Kwok, D. Y.; Neumann, A. W. J. Appl. Polym. Sci. 2000, 77, 2493-2504.

10.1021/la0107805 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/26/2002

Poly(n-butyl methacrylate)/Polystyrene Blend

tion of the molecular level chemical structure of the surfaces has been studied by FTIR and FTIR-attenuated total reflection (ATR).24,25 Their surface sensitivity, however, is limited by the penetration depth of the infrared beam, which is in the same order of the wavelength. That is to say, FTIR-ATR detects signals from the surface layers of the material on the micrometer scale, which is too thick to correlate with the surface properties. As a powerful surface-specific spectroscopic tool with submonolayer sensitivity, sum frequency generation (SFG) vibrational spectroscopy is revolutionizing surface science.26-43 Recently, SFG has been successfully applied to study polymer surfaces.36,37,41,42,44-46 SFG collects surface vibrational spectrum of polymer material with submonolayer surface sensitivity,44,46 allowing various functional groups on the polymer surface to be identified. The orientation of these surface functional groups can also be deduced by using different polarization combinations of input and output laser beams in the SFG setup.29,47-49 SFG is different from high vacuum surface analytical tools such as XPS or SIMS in that it can be applied to study any surfaces or interfaces which are accessible by light, including surfaces in air or in water. In this paper, we have studied poly(n-butyl methacrylate) (PBMA)/polystyrene (PS) blends by SFG, supplemented with AFM. SFG results showed that PBMA component tended to segregate to the blend surface because of its lower surface tension. The AFM results showed that domain structures existed on various blend surfaces. We have also discussed solvent and annealing effects on the surface morphology of PBMA/PS blends. (24) Wang, D.; Ji, J.; Feng, L. X. Macromolecules 2000, 33, 84728478. (25) Cheng, S. S.; Chittur, K. K.; Sukenik, S. N.; Culp, L. A.; Lewandowska, K. J. Colloid Interface Sci. 1994, 162, 135-143. (26) Shen, Y. R. The Principles of Nonlinear Optics; Wiley: New York, 1984. (27) Shen Y. R. Annu. Rev. Phys. Chem. 1989, 40, 327-350. (28) Shen, Y. R. Nature 1989, 337, 519-525. (29) Guyot-Sionnest, P.; Hunt, J. H.; Shen, Y. R. Phys. Rev. Lett. 1987, 59, 1597-1600. (30) Miranda, P. B.; Shen, Y. R. J. Phys. Chem. B 1999, 103, 32923307. (31) Bain, C. D. J. Chem. Soc., Faraday Trans. 1995, 91, 1281-1296. (32) Eisenthal, K. B. Chem. Rev. 1996, 96, 1343-1360. (33) Gragson, D. E.; Richmond, G. L. J. Phys. Chem. B 1998, 102, 3847-3861. (34) Walker, R. A.; Gruetzmacher, J. A.; Richmond, G. L. J. Am. Chem. Soc. 1998, 120, 6991-7003. (35) Conboy, J. C.; Messmer, M. C.; Richmond, G. L. J. Phys. Chem. 1996, 100, 7617-7622. (36) Chen, Z.; Gracias, D. H.; Somorjai, G. A. Appl. Phys. B 1999, 68, 549-557. (37) Gracias, D. H.; Chen, Z.; Shen, Y. R.; Somorjai, G. A. Acc. Chem. Res. 1999, 320, 930-940. (38) Shultz, M. J.; Schnitzer, C.; Simonelli, D.; Baldelli, S. Int. Rev. Phys. Chem. 2000, 19, 123-153. (39) Pizzolatto, R. L.; Yang, Y. J.; Wolf, L. K.; Messmer, M. C. Anal. Chim. Acta 1999, 397, 81-92. (40) Kim, J.; Cremer, P. S. J. Am. Chem. Soc. 2000, 122, 1237112372. (41) Briggman, K. A.; Stephenson, J. C.; Wallace, W. E.; Richter, L. J. J. Phys. Chem. B 2001, 105, 2785-2791. (42) Gautam, K. S.; Schwab, A. D.; Dhinojwala, A.; Zhang, D.; Dougai, S. M.; Yeganeh, M. S. Phys. Rev. Lett. 2000, 85, 3854-3857. (43) Lo¨bau, J.; Wolfrum, K. J. Opt. Soc. Am. B 1997, 14, 2505-2512. (44) Chen, Z.; Ward, R.; Tian, Y.; Baldelli, S.; Opdahl, A.; Shen, Y. R.; Somorjai, G. A. J. Am. Chem. Soc. 2000, 122, 10615-10620. (45) Chen, Z.; Ward, R.; Tian, Y.; Eppler, A. S.; Shen, Y. R.; Somorjai, G. A. J. Phys. Chem. B 1999, 103, 2935-2942. (46) Wei, X.; Hong, S. C.; Lvovsky, A. I.; Held, H.; Shen, Y. R. J. Phys. Chem. B 2000, 104, 3349-3354. (47) Zhuang, X.; Miranda, P. B.; Kim, D.; Shen, Y. R. Phys. Rev. B 1999, 59, 12633-12640. (48) Hirose, C.; Yamamoto, H.; Akamatsu, N.; Domen, K. J. Phys. Chem. 1993, 97, 10064-10069. (49) Hirose, C.; Akamatsu, N.; Domen, K. Appl. Spectrosc. 1992, 46, 1051-1072.

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Figure 1. Molecular formulas of PS and PBMA. Table 1. Physical Properties of PS and PBMA molecular surface solubility weight Tg51 tension component (MW) (°C) (dyn/cm)20 toluene cyclohexane PS PBMA

190,000 181,000

100 20

40.7 31.2

good good

good poor

2. Experimental Section 2.1. Sample Preparation. The PS and PBMA polymers were purchased from Scientific Polymer Products, Inc. and were used as received. The physical properties of both polymers were compared and listed in Table 1, and the molecular structures are shown in Figure 1. Polymer films were prepared by spin coating 2 wt % pure polymer or polymer blend (with different ratios of two components) toluene solution onto fused silica (1-in. diameter, 1/8-in. thickness, from ESCO products, Inc.). The samples were spun at 3000 rpm for 30 s using a spin coater purchased from Specialty Coating System. All spin-cast samples were oven dried at 80 °C for 24 h before analysis. For the annealed samples, the annealing was carried out at 140 °C in the vacuum oven for 24 h. All samples were tested without annealing if not specially designated. 2.2. AFM. The topographic images were obtained by using a PicoSPM atomic force microscope purchased from Molecular Imaging, Inc. All measurements were performed in contact mode using contact silicon cantilevers from K-Tek International, Inc. The typical force constant was 0.12N‚m-1 and the scanning rate was set at 2.0 lines per second. 2.3. SFG. In an SFG setup, a pulsed visible laser beam (frequency ωvis) and a tunable pulsed IR beam (ωir) are overlapped on a surface. The light emitted by the nonlinear process at the sum frequency, ωsum ) ωvis + ωir, is detected by a photodetector. The intensity of the light at ωsum is proportional to the square of the sample’s second-order nonlinear susceptibility, which vanishes when a material has inversion symmetry under the electric-dipole approximation. The bulk materials usually possess inversion symmetry; therefore, they do not generate a sum frequency output, but surfaces, which lack inversion symmetry, do. Both theoretical calculations and experimental results show that SFG is submonolayer sensitive. A plot of SFG intensities versus the frequency of the IR laser produces the vibrational spectrum of the surface species. SFG is a polarized light experiment, and different polarization combinations of input and output beams can be used to deduce the orientation of surface molecules.29,47-49 In our experiments, sum frequency spectra were collected by overlapping a visible and a tunable IR beam on a polymer surface, at incident angles of 60° and 54°, respectively. The visible beam had a wavelength of 532 nm and was generated by frequencydoubling the fundamental output pulses of 20 ps pulse width from an EKSPLA Nd:YAG laser. The IR beam, tunable from 1000 to 4300 cm-1, was generated from an EKSPLA optical parametric generation/amplification and difference frequency system based on LBO and AgGaS2 crystals. Both beams were overlapped spatially and temporally on the sample. The diameters of both beams on the sample were about 0.5 mm. The sum frequency (SF) signal from the polymer surface was collected by a photomultiplier tube. A separate photomultiplier was used to collect the bulk SFG signal from a ZnSe plate as a reference channel. Two photodiodes monitored the input visible beam and IR beam powers by collecting the back reflections of these two beams from the focus lenses. SFG spectra from the sample’s surface can be normalized by the reference signal from ZnSe or by the powers of the input laser beams. In this work, SFG spectra with different polarization combinations, including ssp (s-

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polarized SF output, s-polarized visible input, and p-polarized infrared input), ppp, and sps were collected. As previously mentioned, SFG signal can only be generated from media that does not have inversion symmetry (under the electric-dipole approximation); thus, most bulk materials will not generate SFG signal since they possess inversion symmetry. The polymer blend system is special since it contains multiple components. Sometimes, the blend system is not uniform and may have phase segregations in the bulk. The boundaries between the different phases may not have inversion symmetry, resulting in SFG signals. Before discussing the details of our SFG spectra of PS/PBMA blends, we need to ensure that our SFG signal is primarily from the blend/air interfaces. The SFG spectra were first collected in air with the input laser beams traveling through the fused silica substrate to the blend film. Then, the polymer blend film was in contact with water so that the polymer/air interface was replaced by the polymer/water interface. The spectra collected were dramatically changed, and the signal intensities were decreased substantially. These changes indicated that the initial SFG spectra were primarily from the blend/air interface. If the substrate/polymer interface or bulk signal dominated the spectra, then there would be no significant change after the blend surface contacts the water. Upon exposing the blend surface to air again, its SFG spectra were recovered. From our AFM images, which we will show later, domain structures were detected on the surfaces of polymer blends. The height differences between domains were around 20 nm. Comparing to the wavelength of our input beams of the SFG set up, we believe that polymer blend surfaces are still optically flat. Domain sizes are in the order of hundreds of nanometers; therefore, 20-nm height differences in various domains will not affect the deduction of orientation of surface functional groups.50

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Figure 2. The ssp, sps, and ppp SFG spectra of PS.

3. Results and Discussion 3.1. Relation between Surface Morphology as well as Surface Chemical Structure and Bulk Concentration of Two Components in the Blend. SFG spectra of pure PS on different substrates including sapphire and oxidized Si have been studied in detail.41,42 At the PS/air interface, research shows that the phenyl groups of the PS thin film deposited on sapphire substrate are orientated to approximately parallel to the surface normal. On the oxidized Si substrate, phenyl groups at the PS/air interface are tilted away from the surface normal in an angle of 57°. There are five aromatic C-H vibrational stretching modes observed in the SFG spectra of PS, and they can be separated into two groups.42 The irreducible representation of modes ν2 (3060 cm-1), ν7a (3030 cm-1), and ν20a (3080 cm-1) is A1, and the ν7b (3055 cm-1) and ν20b (3020 cm-1) modes belong to the B1 representation. We have collected ssp, sps, and ppp SFG spectra of PS/ air interface of PS film on fused silica (Figure 2). All SFG spectra were dominated by the vibrational modes of the phenyl group. By detailed calculations (see later discussions and the Supporting Information), it was found that the phenyl groups on the pure PS surface orientated closely to the surface normal. Figure 3 shows the ssp, sps, and ppp SFG spectra of PBMA. The ssp spectrum was dominated by the symmetric stretch of the methyl group of the side chain at 2875 cm-1 and the Fermi resonance at 2940 cm-1. The methylene symmetric stretch at 2850 cm-1 was barely visible in this spectrum. Both the sps and ppp SFG spectra of PBMA were dominated by the antisymmetric stretch of the methyl group at 2960 cm-1. Detailed surface structures of PBMA have been deduced by SFG intensity ratio and absolute intensity measurements.52,53 (50) Simpson, G. J.; Rowlen, K. L. Chem. Phys. Lett. 2000, 317, 276281.

Figure 3. The ssp, sps, and ppp SFG spectra of PBMA.

The ssp, sps, and ppp SFG spectra of PS/PBMA blends are shown in Figures 4, 5, and 6, respectively. The SFG spectra of pure PS and PBMA are also shown as references. As mentioned, peaks between 3000 and 3100 cm-1 were assigned to the aromatic C-H stretches of PS, and their intensities could be used as indicators of surface coverage of PS. Peaks at 2875 and 2960 cm-1 came from the symmetric and asymmetric C-H stretches of methyl groups on PBMA molecules; their intensities could be used to deduce the surface coverage of PBMA. From the ssp, sps, and ppp SFG spectra, we could conclude that the PS/PBMA blend surfaces changed as a function of the bulk concentrations of two different components and that the PBMA likes to segregate to the surface. When the bulk concentration of PBMA component in the blend exceeded 60 wt %, PBMA covered almost the entire surface. There was barely any SFG signal from PS that could be observed in all these SFG spectra. When the concentration of PBMA was lower than 60 wt %, signals from the PS (51) Brandrup, J.; Immergut, E. H.; Grulke, E. A. Polymer handbook; John Wiley & Sons: New York, 1999. (52) Oh-e, M.; Lvovsky, A. L.; Wei, X.; Shen, Y. R. J. Chem. Phys. 2000, 113, 8827-8831. (53) Wang, J.; Paszti, P.; Even, M. A.; Chen, Z. submitted.

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Figure 6. The ppp spectra of PS/PBMA blends.

Figure 4. The ssp spectra of PS/PBMA blends.

Figure 7. The ssp, sps, and ppp SFG spectra of PS/PBMA blend with 4 wt % PBMA in the bulk. Figure 5. The sps spectra of PS/PBMA blends.

A1 vibrational mode, we have

could be detected, but the SFG spectra were still dominated by the PBMA. The surface structures of the blends with 20 wt % to 4 wt % PBMA were quite similar, evidenced by their similar SFG spectra. Even for the PBMA/PS blend with only 4 wt % PBMA in the bulk, PBMA was still easily detected on the surface (Figure 7). For the ppp spectra, there was no detection of any PS signal on all blend surfaces, even on the surfaces of blends with 80 or 96 wt % PS in the bulk (Figure 6). We will discuss this in detail below. We have shown detailed calculations of SFG spectra in the Supporting Information. As mentioned there, the SFG output intensity in the reflected direction can be readily written as47 2 I(ω) ∝ |χ(2) eff |

(1)

where χ(2) eff is the effective second-order susceptibility. Since the phenyl group is treated as C2v symmetry, for its

(2) |χeff,ppp,A1 |) | -Lxx(ω)Lxx(ω1)Lzz(ω2)cosβcosβ1sin β2χyyz,A1 + Lzz(ω)Lzz(ω1)Lzz(ω2)sin βsin β1sin β2χzzz,A1| (2)

The SFG signals corresponding to different vibrational modes in the ppp spectrum depend on the orientation of the functional groups. The signal may disappear at certain orientation angles. Figure 8 shows the calculated relative intensities of SFG signals from the combination of the ν2 (A1) and the ν7b (B1) modes as a function of the orientation angle of the surface phenyl groups versus the surface normal. If we assume the angle distribution is a δ function or the root-mean-square width of the Gaussian distribution of the orientation angles is 30°, we obtain the results shown in Figure 8a and Figure 8b, respectively. Since the intensity of the peak at 3060 cm-1 on the pure PS surface in the ppp SFG spectrum is stronger than that in the sps spectrum (Figure 2), we believe that the distribution used in Figure 8a is closer to the real situation. From further comparison of the peak intensities around 3060 cm-1 in the ssp, sps, and ppp SFG spectra of the pure PS surface

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Figure 8. Relations between the SFG signal intensity of the peak around 3060 cm-1 and the orientation angle of phenyl groups: (a) assuming that the angle distribution is a δ function and (b) assuming that the root-mean-square width (σ) of the Gaussian distribution of the orientation angles is 30°.

with the calculated results in Figure 8a, we could deduce that the surface phenyl groups orientated closely along the surface normal with a narrow angle distribution. From Figure 8a and b, we saw clearly that if phenyl groups had a larger orientation angle (e.g., >45°), the ppp signal became too weak to be detected. This was the case for the

Chen et al.

phenyl groups on the PS/PBMA blend surface (Figure 7). Comparison of the peak intensities around 3060 cm-1 in the ssp and sps SFG spectra of the PBMA/PS blend of 4 wt % PBMA in the bulk (Figure 7) with the calculated results in Figure 8a also confirms our conclusion that the orientation angle of the phenyl groups on this blend surface was larger than 45° versus the surface normal. Phenyl groups did not completely lie down on the surface, otherwise the SFG signals would disappear. This indicated that the phenyl groups would prefer to tilt more closely to the blend surface, compared to those on the pure PS surface, when PBMA appeared on the surface. Surface segregation on the polymer blend surface occurs because of the difference in surface energy between the two components. Since the surface energy of PBMA is lower than PS, it is reasonable for PBMA to segregate more on the surface. Surface structures of PBMA/deuterated PS blends have been studied by SIMS, XPS, and AFM.54 Deuteration of PS is to differentiate PS signals from PBMA signals in the SIMS experiment. For our SFG studies, no deuteration of PS is necessary, since the aromatic C-H stretching peaks of PS are at higher frequencies compared to normal C-H stretching peaks of PBMA. SIMS and XPS experiments show different surface segregation behaviors of PBMA on the blend surface because of different surface sensitivities of two techniques. SIMS results indicate that PBMA covers the entire surface for all blends, while XPS shows that PBMA saturates the blend surface until its bulk concentration reaches 65 wt %. Our SFG results are between the results of SIMS and XPS studies. Since SFG is a submonolayer surface

Figure 9. AFM topographic images of PS/PBMA blends: (a) pure PBMA, (b) 20 wt % PS/80 wt % PBMA, (c) 40 wt % PS/60 wt % PBMA, (d) 50 wt % PS/50 wt % PBMA, (e) 80 wt % PS/20 wt % PBMA, and (f) pure PS (the length scale and the height scale in all images are the same).

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Figure 10. AFM topographic images of PS/PBMA blend (with 20 wt % PS) thin film before and after being washed by cyclohexane.

sensitive technique, we believe that the different SIMS results might be due to the deuteration of PS. One important result deduced from SFG studies, which cannot be obtained from SIMS and XPS research, is that the orientation of phenyl groups of PS on the blend surface is different from the pure PS surface. As mentioned, XPS and SIMS need high vacuum to operate, while SFG can be applied to investigate polymer/air and polymer/water interfaces. Therefore, SFG studies of PS/PBMA blend surfaces in air can serve as references for SFG studies of surface restructurings of these surfaces in water, which is currently under investigation. We have repeated the AFM measurements of the PS/ PBMA surface published in ref 54 and have obtained compatible results (Figure 9). The topographic images illustrated that both pure PS and PBMA surfaces are quite flat. In contrast, domain structures have been observed on the blend surfaces, especially on the surfaces of blends with more than 50 wt % PBMA. Different surface domain structures such as islands, ribbon patterns, or valleys have been obtained, depending on the bulk concentrations of two components in the blend. PBMA and PS are generally immiscible; therefore, these different patterns result from surface phase segregation. In the PS/PBMA blend, the glass transition temperature (Tg) of PBMA (20 °C) is much lower than PS (100 °C).51 The low glass transition temperature of PBMA (even lower than the room temperature) indicates that PBMA molecules are quite mobile at room temperature, which is believed to play a crucial role in surface morphology. Because of the high mobility of PBMA molecules, they tend to “flow” around and are likely to enrich the lower area (dark area in AFM images). We have shown experimentally that islands (higher areas) on the PBMA/PS blend (with 20 wt % PS in the bulk) surface were PS by using a selective solventscyclo-

hexane.55 Cyclohexane is a good solvent to dissolve PS, but a poor solvent for PBMA. The original AFM image showed that there were many islands on this PBMA/PS blend surface. The diameter of these islands was around 400 nm and the height was about 20 nm. After being washed by cyclohexane, all the islands changed into holes (Figure 10). The diameter of these holes was the same as the islands before cyclohexane washing, and the depth of the holes was around 4 nm. This result showed that those islands are PS, which can be dissolved by cyclohexane. The materials in valley areas which cannot be dissolved by cyclohexane should be PBMA, supporting that the low glass transition temperature component PBMA tended to flow around and enriched the valley region. Questions arose when we compared our AFM results with our SFG results. The results from the AFM (Figure 10) indicated that islands on the surface of PBMS/PS with 20 wt % PS in the bulk were PS, and the SFG results (Figures 4-6) showed that for the surfaces of blend even with 40 wt % PS in the bulk, PS signals were hardly detected. Previous SIMS results also indicate that this blend surface is entirely covered by PBMA. Therefore, we believe that the PS islands are covered by a thin layer of PBMA (Figure 11). Because of the surface sensitivity, SFG and SIMS could detect the top PBMA layer but could not

(54) Affrossman, S.; Jerome, R.; O’Neil, S. A.; Schmitt, T.; Stamm, M. Colloid Polym. Sci. 2000, 278, 993-999.

(55) Walheim, S.; Ramstein, M.; Steiner, U. Langmuir 1999, 15, 4828-4836.

Figure 11. Scheme of phase segregation on PS/PBMA blend (with 20 wt % PS) surface.

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Figure 12. (a) AFM image of PS/PBMA blend (with 20 wt % PS) thin film surface with the film cast from chloroform solution onto the substrate; (b) AFM image of PS/PBMA blend (with 20 wt % PS) thin film surface with the film cast from toluene solution onto the substrate after annealing at 140 °C.

detect signals from PS islands under this thin PBMA layer. Cyclohexane was able to penetrate this thin layer and dissolve PS islands. Another question occurs if the PS islands are indeed covered by PBMA: why can no SFG signal of PS be detected from the “buried” interface between the top PBMA layer and the PS islands below? This issue is being currently addressed with SFG studies of polymer/polymer interfaces. The preliminary results showed that the SFG signals generated from the polymer/ polymer interface were much weaker than those from the polymer/air interface. Thus, SFG signals here were dominated by the interface between PBMA top layer and air. 3.2. Effect of Solvent on Film Morphology. We have studied the effects of solvents on the surface morphology of PBMA/PS blends. AFM images of PS/PBMA blend (with 20 wt % PS in the bulk) thin films spun cast from two different solvents, toluene and chloroform, are shown in Figures 10b and 12a. These films were cast from solutions with the same concentration (2 wt %) by using the same spinning rate (3000 rpm). Comparing these two images, we found that phase segregation occurred on both film surfaces. Both surfaces showed island structures, but the heights of those islands on two surfaces were quite different. The height of the islands indicates the degrees of the phase segregation; therefore, the phase segregations on PBMA/PS blend surfaces were affected by the different solvents. The average height of the islands on the surface of PS/PBMA blend spun cast from toluene solution was around 20 nm (Figure 10b); however, the average height of the islands on the film cast from chloroform solution was only about 5 nm (Figure 12a). As the solvent evaporated, during or shortly after spin coating, phase segregation occurred and islands formed on the blend surfaces. Many different factors such as the polymerpolymer interaction, polymer-solvent interaction, and the evaporation rate of solvents will affect the phase segregation and the islands formation. Since the boiling point of toluene is higher than chloroform, it will evaporate more slowly. If it is supposed that the different evaporation rates of toluene and chloroform are the determining factor, the phase segregation for the film cast from toluene solution should be more complete and the islands on the surface should be higher and more homogeneous. On the contrary, while using the chloroform as the solvent, the PS islands were not homogeneous because of chloroform’s much higher evaporation rate. This explains our observations in Figure 12a. 3.3. Effect of Annealing on Film Morphology. It has been reported19,56 that the formation of thin film is usually a nonequilibrium process and is subject to changes

Figure 13. Schematic representation of annealing effect on the surface morphology.

in surface morphology while being annealed to reach a new thermodynamic equilibrium state. Especially in our case, the glass transition temperature of PS is around 100 °C, higher than the sample preparation temperature (80 °C). As mentioned, for the PS/PBMA blend with 20 wt % PS in the bulk, AFM images showed that islands on the surface were PS. At the same time, SFG results showed that these PS islands were covered by a thin layer of PBMA. We have studied the effect of annealing on the morphology of PS/PBMS blend surfaces. AFM images (Figure 12b) showed that surface roughness of PS/PBMA with 20 wt % PS in the bulk became smaller after annealing. The possible effect of annealing on film morphology was shown schematically in Figure 13. Upon annealing at 140 °C, both components in the blend moved and local molecular rearrangement occurred, resulting in the decrease in the height of islands. As a low-surface energy component, PBMA molecules tended to move to the blend surface to minimize the free energy of the whole system at higher annealing temperature to reach to the equilibrium. The higher surface free energy component, PS, diffused into the bulk. As a result, the roughness of the surface decreased. SFG spectra of the annealed sample were similar to those of the sample without annealing, with PBMA dominating in all SFG spectra. It is interesting to compare our annealing effect for the PS/PBMA blend with some other blends with opposite annealing effect. Stamm and co-workers56 have studied deuterated polystyrene (PdS)/Poly (styrene-co-p-bromox(56) Affrossman, S.; O’Neil, S. C.; Stamm, M. Macromolecules 1998, 31, 6280-6288.

Poly(n-butyl methacrylate)/Polystyrene Blend

styrene) (PBrS, 0 e x e 1) blend thin film before and after annealing at 150 °C. The original AFM image of PdS/ PBrS with 30% PBrS in weight is similar to that of PS/ PBMA with 20 wt % PS. The surface roughness of a thin film composed of 30% PBrS in weight significantly increases after annealing. Before annealing, the islands that form on this PdS/PBrS surface are PBrS. Since the glass transition temperature of PBrS is 142 °C, which is only slightly lower than the annealing temperature 150 °C, they will not be very mobile. It is believed that the annealing at 150 °C had two effects: (a) diffusion of PS from pure PS blocks to PBrS blocks which causes the islands to grow and (b) a PdS-enriched region is formed on top of the PBrS islands. Therefore, surface roughness of this PdS/PBrS will become larger after annealing. This is different from our case. The annealing temperature in our experiment was much higher than the glass transition temperature of PS, thus PS diffused to the bulk. Also, PBMA had a significantly low Tg and “flowed”. The different annealing effects for PS/PBMA blend and PdS/ PBrS blend were reasonable. Conclusion The submonolayer surface sensitive vibrational technique, SFG, has been applied to study PS/PBMA blend surfaces. The SFG results of PS/PBMA blends demonstrated that the lower surface energy component, PBMA, tended to cover the surface. It was hard to detect any PS on the surface when its bulk weight concentration was lower than 40%. The presence of PBMA molecules directly affected the average orientation of phenyl groups on the surface. Through different polarization combinations of input and output beams of the SFG setup, it was found

Langmuir, Vol. 18, No. 4, 2002 1309

that the phenyl groups orientated closely to the surface normal on the pure PS surface. However, even for the blend with only 4 wt % bulk PBMA, the average orientation of the surface phenyl groups drastically changed. The result showed that the angle between the phenyl group and the surface normal increased, which indicated that the phenyl group tended to tilt more toward the surface because of the interactions with PBMA molecules. This also explained the absence of SFG signal of PS in ppp SFG spectra of all PBMA/PS blends. Various kinds of domain structures such as island or ribbon pattern, depending on the bulk concentrations of two components in the blend, have been observed on PS/ PBMA blend surfaces by AFM. This was the result of phase segregation. The selective solvent, cyclohexane, has been successfully used to identify the surface species. The effect of annealing on surface morphology was also investigated. The surface roughness of the blend thin film decreased after being annealed. This change was attributed to the local molecular rearrangement at high temperature, especially the strong mobility of PBMA molecules. The combination of SFG and AFM presents a more complete picture of PS/PBMA surfaces. Acknowledgment. This work was supported by the start-up fund from the University of Michigan. Supporting Information Available: The calculation of the orientation angles of the surface phenyl groups on the pure PS and the PBMA/PS blend surfaces is described in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. LA0107805