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Surface Characterization of C60-Containing Poly(ethyl methacrylate)/Poly(ethyl methacrylate) Blends H. L. Huang,† S. H. Goh,*,† J. W. Zheng,‡ D. M. Y. Lai,§ and C. H. A. Huan§ Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore, Institute of High Performance Computing, 1 Science Park Road, #01-01 The Capricorn, Singapore Science Park II, Singapore 117528, Singapore, and Institute of Materials Research and Engineering, National University of Singapore, 3 Research Link, Singapore 117602, Singapore Received December 27, 2002. In Final Form: April 18, 2003 The surface properties of C60-containing poly(ethyl methacrylate)s (PEMA-C60) and their blends with poly(ethyl methacrylate) (PEMA) were investigated by attenuated total reflectance Fourier transform infrared spectroscopy, scanning electron microscopy, time-of-flight secondary ion mass spectrometry, and contact-angle measurements. The polymer surface becomes more hydrophobic with increasing C60 content in the polymer. The PEMA-C60/PEMA blends show the surface enrichment of PEMA-C60. There is a good correlation between the surface hydrophobicity and the surface enrichment of PEMA-C60.
Introduction C60 and its derivatives possess interesting photonic, electronic, magnetic, and biomedical properties as a result of their unusual molecular symmetry.1,2 The combination of the intriguing properties of C60 and those of some specific polymers is of considerable interest, and these materials may find practical applications.3-8 We have recently studied the complexation behavior of poly(ethylene oxide) end-capped with C60.9-11 The incorporation of C60 induces a dramatic hydrophobic effect that enhances the yield of the complexes. The surface behavior of a polymer blend is very sensitive to the chemical structures and interpolymer interaction. For an immiscible two-phase blend, the surface enrichment of the low-surface-energy component is a universal phenomenon.12,13 Jones et al. studied the surface concentration profile of the deuterated polystyrene/hydrogenated polystyrene blend.14 The small difference in the polarizability of the C-H and C-D bonds produces a surface excess of the deuterated polystyrene component. C60 has a spherical three-dimensional structure that is very * Corresponding author. Telephone: +65-6874-2844. Fax: +656779-1691. E-mail address:
[email protected]. † Department of Chemistry, National University of Singapore. ‡ Institute of High Performance Computing. § Institute of Materials Research and Engineering, National University of Singapore. (1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (2) Kroto, H. W.; Fischer, J. E.; Cox, D. E. The Fullerenes; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1994. (3) Marcos, R. A.; Rispens, M. T.; Hummelen, J. C.; Janssen, R. A. J. Synth. Met. 2001, 119, 171. (4) Eklund, P. C.; Rao, A. M. Fullerene Polymers and Fullerene Polymer Composites; Springer: Berlin, 1999. (5) Geckeler, K. E.; Samal, S. Polym. Int. 1999, 48, 743. (6) Wudl, F. J. Mater. Chem. 2002, 12, 1959. (7) Dai, L. J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1999, C39, 273. (8) Prato, M. Top. Curr. Chem. 1999, 199, 173. (9) Huang, X. D.; Goh, S. H. Macromolecules 2000, 33, 8894. (10) Huang, X. D.; Goh, S. H. Macromol. Chem. Phys. 2000, 201, 281. (11) Song, T.; Goh, S. H.; Lee, S. Y. Macromolecules 2002, 35, 4133. (12) Jones, R. A. L.; Kramer, E. J. Polymer 1993, 34, 115. (13) Schmidt, I.; Binder, K. J. Phys. 1985, 46, 1631. (14) Jones, R. A. L.; Kramer, E. J.; Rafailovich, M. H.; Skolov, J.; Schwarz, S. A. Phys. Rev. Lett. 1989, 62, 280.
different from that of a polymer chain. It is likely that the incorporation of C60 may affect the surface properties of the parent polymer. Therefore, the surface enrichment may be observed when a C60-containing polymer is blended with its parent polymer. In the present paper, we report the surface properties of C60-containing poly(ethyl methacrylate) (PEMA-C60) and its blends with poly(ethyl methacrylate) (PEMA) based on the results from contactangle measurements, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), and time-of-flight secondary ion mass spectrometry (ToF-SIMS).
Experimental Section Materials. PEMA was synthesized by free-radical polymerization in tetrahydrofuran (THF) at 70 °C using 0.2 wt % 2,2′azobisisobutyronitrile as the initiator. PEMA-C60 was prepared following the synthetic route reported previously.15,16 Basically, poly(ethyl methacrylate-co-2-bromoethyl methacrylate) [poly(EMA-co-2-BEMA)] was first prepared by free-radical copolymerization. The bromine groups in the copolymer were converted to azide groups [poly(EMA-co-2-AEMA)] followed by a reaction with C60 to afford PEMA-C60. The C60 content was determined by thermogravimetry because C60 shows practically no weight loss up to 600 °C in a nitrogen atmosphere and PEMA decomposes completely by 500 °C.15,16 The two PEMA-C60 samples used in the present study contain 2.6 and 3.9 mol % of the fullerenated repeat unit and are designated as PEMA-C60-2.6 and PEMAC60-3.9, respectively. Preparation of the Blends. Appropriate amounts of polymers were separately dissolved in THF to form 2% (w/v) solutions. (15) Zheng, J. W.; Goh, S. H.; Lee, S. Y. Polym. Bull. 1997, 39, 79. (16) Zheng, J. W.; Goh, S. H.; Lee, S. Y. Fullerene Sci. Technol. 2001, 9, 487.
10.1021/la027077g CCC: $25.00 © 2003 American Chemical Society Published on Web 05/16/2003
Poly(ethyl methacrylate) Blends The polymer solutions were mixed and stirred for 24 h. To prepare the films for the contact-angle, SEM, and ToF-SIMS measurements, the solutions were directly spin-coated on 0.5 × 0.5 in. silicon wafers using a Laurell spin coater (WS-200-4T2/25P/HSP) at a speed of 1000 rpm. The silicon wafers were dipped in a dilute HF solution to remove the silicon oxide on the surface just before spin-coating. FTIR Measurements. Both the transmission Fourier transform infrared (TX-FTIR) and the ATR-FTIR measurements were performed on a Bio-Rad 165 FTIR spectrophotometer. The TXFTIR samples were prepared by adding polymer solutions onto KBr powder; the dried polymer-KBr mixtures were ground and pressed to form disks. For the ATR-FTIR measurements, solutions were cast on aluminum foils, and the films were allowed to air-dry, followed by further drying in a vacuum oven at 60 °C. For the collection of both the TX-FTIR and the ATR-FTIR spectra, 256 scans were signal-averaged at a resolution of 2 cm-1. A Ge prism with a 45° face cut was used as the internal reflectance element for all the ATR-FTIR collections. The depth of penetration (dp) in the polymer films in the ATR-FTIR experiments was estimated to be 0.8-1.0 µm, corresponding to a sampling depth (ds, which is defined as ds ) 3dp) of 2.4-3.0 µm from the surface for the IR absorption bands between 700 and 800 cm-1.17 SEM Measurements. The SEM micrographs were obtained using a JEOL JSM-5200 scanning electron microscope with an accelerating voltage of 20 kV. The film surfaces were first sputtercoated with gold before observation. Contact-Angle Measurements. The static contact angles of water on various surfaces were measured with an NRL-10000-230 optical goniometer (Rame`-Hart, Inc., U.S.A.) in an air atmosphere. All the samples were annealed in vacuo at 60 °C for 24 h after the spin-coating process, and the measurements were then made. The reported contact angles are an average of at least seven measurements in different locations on the film surface. The measured angles were within (2°. ToF-SIMS Measurements. The high-resolution mass spectra were obtained with a ToF-SIMS spectrometer, ION-ToF SIMS IV (ION-TOF, GmbH, Germany). A pulsed primary ion source (Ar+, 10 keV, ∼3 pA) was rastered over the sample area of 500 × 500 µm2. The mass spectra were acquired for 100 s with a fluency of 3000 at m/z > 15. To obtain information on the variation of the composition with the depth below the surfaces of these samples, depth profiles were conducted with a dual beam, low-energy sputter ion beam (Ar+, 3 keV, 10 nA, sputter area 500 × 500 µm2), and high-energy analysis ion beam (Ga+, 25 keV, 2pA, analysis area 200 × 200 µm2). Charge compensation with a low-energy electron flood gun was used during both the static and the dynamic measurements.
Results and Discussion FTIR Characterization. Figure 1 shows the ATRFTIR and TX-FTIR spectra of PEMA-C60-3.9. The FTIR spectrum of pure C60 consists of four absorbance bands at 527, 576, 1182, and 1429 cm-1.18 However, for PEMA-C60, only the 527 cm-1 band can be observed, and the other three bands are submerged beneath the strong PEMA bands. The ATR-FTIR and TX-FTIR spectra are very similar. However, the peak intensity of the 527 cm-1 band in the ATR-FTIR spectrum is stronger than that of the TX-FTIR spectrum. If the 527 cm-1 band area is normalized with the 750 cm-1 band [(CH2)n rocking vibration],19 the relative peak intensity of 527 cm-1 in the ATR-FTIR spectrum is about six times that of the TX-FTIR spectrum. Even though it is difficult to determine the surface composition accurately by FTIR, the surface enrichment of C60 to a depth of about 3 µm is clearly observed by comparing the ATR-FTIR and TX-FTIR spectra. (17) Chen, X.; Gardella, J. A., Jr.; Philip, L. K. Macromolecules 1992, 25, 6621. (18) Taylor, R. The Chemistry of Fullerenes; World Scientific: Singapore, 1995. (19) Socrates, G. Infrared Characteristic Group Frequencies; John Wiley & Sons: New York, 1994.
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Figure 1. TX-FTIR and ATR-FTIR spectra of PEMA-C60-3.9.
SEM Characterization. Figure 2a shows the SEM micrograph of the poly(EMA-co-2-AEMA)/PEMA (50/50) blend. Poly(EMA-co-2-AEMA) is miscible with PEMA, as is shown by the smooth surface topography. The incorporation of C60 to the PEMA chain dramatically changes the topography. For the PEMA-C60-2.6/PEMA (20/80) blend, the SEM micrograph shows the dispersion of the minor component as spherical domains of about 1.0 µm in diameter (Figure 2b). For the C60-containing polymers, the C60 moieties aggregate to form big clusters.20-22 Thus, the spherical domains are likely clusters of C60 moietes. As the PEMA-C60-2.6 content increases, the C60 clusters interconnect to form the matrix, leaving the non-C60containing component to form a doughnutlike structure (Figure 2c,d). Contact-Angle Characterization. The water contact angles are 63, 82, and 84° for PEMA, PEMA-C60-2.6, and PEMA-C60-3.9, respectively, indicating the enhancement of surface hydrophobicity upon the incorporation of C60. For the blends, the water contact angles do not increase linearly with the PEMA-C60 content (Figure 3). For example, the contact angles of the two PEMA-C60/PEMA (20/80) blends are about 12% larger than those based on the linear additivity rule. The contact-angle results show the surface enrichment of PEMA-C60 on the blend surface. ToF-SIMS Characterization. ToF-SIMS has been increasingly used to characterize the surfaces of polymer blends.23-26 The positive ToF-SIMS spectrum of the PEMA film is shown in Figure 4 (top). Cursory investigation of the spectrum reveals that the secondary ions emitted from the film surface are mainly the low-mass species (m/z ) 0-150), such as CH3+ (m/z ) 15), C2H5+ (m/z ) 29), and OCOC2H5+ (m/z ) 73). However, there are no characteristic ions in the high-mass region (m/z ) 500-800). For PEMAC60-2.6, there is no obvious difference in the low-mass region (m/z ) 0-150) compared to PEMA. However, in the high-mass region (m/z ) 500-800), PEMA-C60-2.6 shows the characteristic C60+ ion (m/z ) 720; Figure 4, (20) Wang, X. H.; Goh, S. H.; Lu, Z. H.; Lee, S. Y.; Wu, C. Macromolecules 1999, 32, 2786. (21) Song, T.; Goh, S. H.; Lee, S. Y. Polymer 2003, 44, 2563. (22) Song, T.; Dai, S.; Tam, K. C.; Lee, S. Y.; Goh, S. H. Polymer 2003, 44, 2529. (23) Chan, C. M.; Weng, L. T. Rev. Chem. Eng. 2000, 16, 341. (24) Li, L.; Chan, C. M.; Weng, L. T.; Xiang, M. L.; Jiang, M. Macromolecules 1998, 31, 7248. (25) Liu, S.; Chan, C. M.; Weng, L. T.; Li, L.; Jiang, M. Macromolecules 2002, 35, 5623. (26) Vanden Eynde, X.; Bertrand, P. Surf. Interface Anal. 1999, 27, 157.
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Figure 2. SEM micrographs of (a) poly(EMA-co-2-AEMA)/PEMA (50/50); (b) PEMA-C60-2.6/PEMA (20/80); (c) PEMA-C60-2.6/ PEMA (50/50); and (d) PEMA-C60-2.6/PEMA (80/20).
Figure 3. Water contact angles of (a) PEMA-C60-3.9/PEMA and (b) PEMA-C60-2.6/PEMA.
bottom). The positive spectrum shows that the C60 grafted on the PEMA chain fragmented to a series of characteristic ions with a mass difference of 24. This is consistent with the laser-induced fragmentation behavior of the C60+ ion, which dissociates to produce even-numbered fragments.27 As is shown in the negative spectra of the PEMA-C60 samples, C60 also fragmented to a series of characteristic fullerene ions. However, the ion intensities are much lower than those in the positive spectra. Therefore, our discussion focuses on the positive spectra. To verify whether the air-polymer interface is enriched with C60, SIMS depth profiling was conducted for PEMA(27) O’Brien, S. C.; Heath, J. R.; Curl, R. F.; Smalley, R. E. J. Chem. Phys. 1988, 88, 220.
C60. For the fluorinated polymers, the fluorinated ion intensities were normalized with that of C+.28 However, for the C60-containing polymer, it is not suitable to use C+ as the reference because C60 produces C+ when it is fragmented to C58 or other fullerene characteristic clusters. For pure PEMA, the -OCH2CH3 group has a low surface energy and the freedom to reorganize at the air-polymer interface, as is shown by the strongest ion intensity of C2H5+ in the static positive ToF-SIMS spectrum. Therefore, the C60+ ion intensity was normalized with that of C2H5+. Figure 5 shows the SIMS depth profile of C60 in the PEMA-C60-2.6 film. The C60 concentration decreases with an increasing depth. Because C60 shows the surface enrichment compared with C2H5+, the dynamic ToF-SIMS proves that C60 tends to enrich at the air-polymer interface. Figure 6 shows the positive ToF-SIMS spectrum of the PEMA-C60-2.6/PEMA (20/80) blend. Compared to those of PEMA-C60-2.6, the characteristic fullerene ion intensities decrease very slightly, and the intensities are not proportional to the PEMA-C60 bulk content. To make a valid quantitative analysis, the PEMA-C60 surface fraction was calculated by the following equation:
surface fraction )
(I500-800/I0-150)blend (I500-800/I0-150)PEMA-C60
where I500-800 is the integrated total ion intensity from m/z ) 500 to 800 and I0-150 is that from m/z ) 0 to 150. Figure 7 shows that the calculated surface fraction of PEMA-C60 is obviously larger than that of the bulk composition, indicating the surface enrichment of the (28) Huang, H. L.; Goh, S. H.; Lai, D. M. Y.; Huan, C. H. A. To be submitted for publication.
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Figure 6. Positive static ToF-SIMS spectra of PEMA-C60-2.6/ PEMA (20/80).
Figure 4. Positive static ToF-SIMS spectra of (top) PEMA and (bottom) PEMA-C60-2.6. Figure 7. Surface fraction of PEMA-C60 calculated from the positive static ToF-SIMS ion intensities. (a) PEMA-C60-3.9/ PEMA and (b) PEMA-C60-2.6/PEMA.
Conclusions
Figure 5. ToF-SIMS depth profile of C60 in the PEMA-C60-2.6 film.
PEMA-C60 component. Moreover, Figure 7 resembles Figure 3, showing that the surface hydrophobicity of the blends arises from the surface enrichment of the PEMAC60 component.
The incorporation of C60 to the polymer chain has a dramatic effect on the surface behavior of the parent polymer. The surface becomes more hydrophobic with an increasing C60 content in the polymer, as is shown by the increasing water contact-angle value. Both the ATR-FTIR and dynamic SIMS depth profile results show the surface enrichment of C60 on the film. A combination of the contactangle, SEM, and ToF-SIMS results show that the PEMAC60/PEMA blend surface is enriched with PEMA-C60. There is a good correlation between the contact-angle results and the ToF-SIMS results. Acknowledgment. The authors thank the National University of Singapore for financial support of this research. LA027077G
