Anal. Chem. 2000, 72, 4908-4913
Sulfonation of Poly(N-vinylcarbazole) Studied by Combined Time-of-Flight Secondary Ion Mass Spectrometry and X-ray Photoelectron Spectroscopy Lu-Tao Weng,* Philip C. L. Wong, and Kachun Ho
Materials Characterization and Preparation Facility, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong Suhua Wang, Zhaohua Zeng, and Shihe Yang
Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong
As an effective hole-transport semiconductor, poly(N-vinylcarbazole) (PVK) has been extensively studied as a potential electronic and optical material.1-3 Most previous research work has been devoted to doped PVK, blends of PVK and semiconductor nanoparticles; little work has been done on chemical modification of PVK.4,5 Applications of PVK in optoelectronic devices depend strongly on its compatibility with other components, and chemical functionalization of PVK is one way to improve compatibility. We have developed a synthetic approach to functionalize PVK using acetyl sulfate as the sulfonating agent, based on the fact that acetyl sulfate has been widely used to sulfonate polystyrene and other aromatic and unsaturated polymers. 6,7
Ascertaining the success of sulfonation of PVK requires a detailed characterization using appropriate analytical techniques. In particular, one is interested to know where the sulfonation takes place (carbazole groups or the backbone of PVK) and the level of sulfonation. This paper shows how time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) can be jointly used to provide both qualitative and quantitative analysis of the PVK sulfonation process. XPS has been widely used to study the modifications of polymers after chemical treatments such as plasma, corona, and wet chemistry.8 In particular, XPS quantitative elemental analysis has been used to determine the level of chemical modification, and the detailed peak analysis (curve fitting) of core-level electrons (C 1s, O 1s, N 1s, etc.) gives some insight into the chemical changes of polymer surfaces. XPS curve fitting is not always straightforward, however, because of the small chemical shifts of different functionalities and complicated structural changes due to chemical treatments. A complementary technique is desirable. TOF-SIMS (and static SIMS in general) has gained importance during the past decade in the surface characterization of polymers because of its high molecular specificity and surface sensitivity.8,9 This technique can provide detailed information on surface molecular structures including polymer tacticity,10 sequence distribution,11 end groups,12 and branching and cross-linking.13 The main drawback of TOF-SIMS is difficulty in quantification, which can be complemented by a quantitative technique such as XPS. Therefore, XPS and TOF-SIMS are a good combination for the surface characterization of polymers, and indeed, the combined
* Corresponding author: (e-mail)
[email protected]; (fax) (852) 2358 2451. (1) Chan, H. S. O.; Ng, S. C.; Sim, W. S.; Tan, K. L.; Tan, B. T. G. Macromolecules 1992, 25, 6029. (2) Cao, Y.; Qiu, J.; Simth, P. Synth. Met. 1995, 69, 187. (3) Touihri, S.; Safoula, G.; Bernede, J. C. Polym. Degrad. Stab. 1998, 60, 481. (4) See, for example: Zamora, F.; Gonzalez, M. C. J. Macromol. Sci., Phys. B 1998, 37, 601. (5) Cai, R. F.; Bai, X.; Chen. Y.; Huang, Z. E. Eur. Polym. J. 1998, 34, 7. (6) Makowski, H. S.; Lundberg, R. D.; Singhal, G. U.S. Patent No. 3,870,841, to Exxon Research and Engineering Co., March 11, 1975. (7) Feng, K.; Zeng, Z. H.; Ou-Yang, W.; Li, Z. M. J. Appl. Polym. Sci. 1997, 64, 1651.
(8) Briggs, D. Surface analysis of polymers by XPS and static SIMS; Cambridge University Press: New York, 1998. (b) Chan, C. M. Polymer Surface Modification and Characterization; Hanser: New York, 1994. (9) Bertrand, P.; Weng, L. T. In Surface characterization: a user’s sourcebook; Brune, D., Hellborg, R., Whitlow, H. J., Hunderi, O., Eds.; Wiley-VCH: New York, 1997; p 334. (10) Vanden Eynde, X.; Weng, L. T.; Bertrand, P. Surf. Interface Anal. 1997, 25, 41. (11) Galuska, A. A. Surf. Interface Anal. 1997, 25, 1. (12) Affrossman, S.; Bertrand, P.; Hartshorne, M.; Kiff, T.; Leonard, D.; Pethrick, R. A.; Richards, R. W. Macromolecules 1996, 29, 5432. (13) Lianos, L.; Quet, C.; Duc, T. M. Surf. Interface Anal. 1994, 21, 14.
A series of sulfonated poly(N-vinylcarbazole) (PVK) samples have been systematically studied by time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS). Negative TOF-SIMS results provided unambiguous evidence that sulfonate groups are chemically attached to the carbazole moiety of PVK. The positive SIMS spectrum of PVK was, however, little affected by the sulfonation reaction. The degree of sulfonation was quantitatively determined by XPS. Therefore, the combination of TOF-SIMS and XPS is useful to follow the sulfonation reaction, both qualitatively and quantitatively. The SIMS intensities of some characteristic fragments are linearly related to the degree of sulfonation, suggesting that quantitative analysis is possible from TOFSIMS data.
4908 Analytical Chemistry, Vol. 72, No. 20, October 15, 2000
10.1021/ac000152c CCC: $19.00
© 2000 American Chemical Society Published on Web 09/21/2000
Chart 1. Repeat Unit of PVK
use of these two techniques has been shown to be very fruitful in surface studies of copolymers,14 polymer blends,15 and even carbon fiber surface functionalization.16 In this paper, XPS and TOF-SIMS were used to characterize the PVK before and after sulfonation, seeking a detailed characterization of the sulfonation process. A second objective of this paper was to test the quantitative aspect of TOF-SIMS data obtained from this functionalized polymer system. As indicated above, it is normally difficult to extract quantitative information from TOF-SIMS data. However, recent work on copolymers and polymer blends has shown that TOF-SIMS quantitative analysis is possible when the most characteristic peaks are selected to represent polymer constituents.17-19 It is desirable that this conclusion be tested with more polymer systems; PVK samples with controlled sulfonation may be good candidates for this purpose. EXPERIMENTAL SECTION Sample Preparation. PVK (repeat unit in Chart 1) was obtained from Aldrich. The detailed description of the PVK sulfonation reaction was published previously.20 Briefly, PVK was dissolved in tetrahydrofuran (THF) and mixed with a solution of sulfonating agent (acetyl sulfate), followed by heating to ∼75 °C and refluxing for 5 h, and then adding ethanol to terminate the sulfonation reaction. The sulfonation products were precipitated by adding cyclohexane; the precipitate was vacuum filtered and washed. The degree of sulfonation was controlled by changing the molar ratio of sulfonating agent to PVK. The samples used in this study were PVK, PVK-30, PVK-50, and PVK-60, which were obtained by using a molar ratio of sulfonating agent to PVK of 0, 0.39, 0.65 and 0.78, respectively. The samples for XPS and TOF-SIMS are polymer thin films, spin coated on silicon wafers. PVK and sulfonated PVK samples were dissolved in THF to a concentration of ∼4% (w/v). One or two drops of polymer solution were placed onto silicon disk, which was then spun at ∼3000 rpm for 30 s. Spectroscopic Methods. XPS measurements were performed with a PHI 5600 multitechnique system using a monochromatized (14) Zhao, J.; Rojstaczer, S. R.; Gardella, J. A., Jr. J. Vac. Sci. Technol. A, 1998, 3046. (15) Lhoest, J.-B.; Bertrand, P.; Weng, L. T.; Dewez, J.-L. Macromolecules 1995, 28, 4631. (16) Weng, L. T.; Poleunis, C.; Bertrand, P.; Carlier, V.; Sclavons, M.; Franquinet, P.; Legras, R. J. Adhesion Sci. Technol., 1995, 9, 859. (17) Weng, L. T.; Bertrand, P.; Lauer, W.; Zimmer, R.; Busetti, S. Surf. Interface. Anal. 1995, 23, 879. (18) Briggs, D.; Davies, M. C. Surf. Interface. Anal. 1997, 25, 725. (19) Vanden Eynde, X.; Bertrand, P. Surf. Interface Anal. 1999, 27, 157. (20) Wang, S. H.; Zeng, Z. H.; Yang, S. H.; Weng, L. T.; Wong, P. C. L.; Ho, K. Macromolecules, 2000, 33, 3232.
Figure 1. XPS core-level spectra C 1s, N 1s, and S 2p for PVK and PVK-50.
aluminum anode (14 kV, 25 mA). The spectrometer was operated in a fixed analyzer transmission (FAT) mode. A survey scan was recorded for each sample, followed by the recording of C 1s, N 1s, O 1s, and S 2p peaks where appropriate. The pass energy was 186 eV for survey scans and 23.4 eV for detailed scans. An electron flood gun was used for charge compensation. Binding energies were referenced to the neutral aliphatic carbon of PVK, set at 285 eV. XPS intensities were converted to atomic concentrations using experimental sensitivity factors (C 1s, 0.314; N 1s, 0.499; O1s, 0.733; S 2p, 0.717) provided by the manufacturer. TOF-SIMS measurements were carried out on a Physical Electronics PHI 7200 TOF-SIMS spectrometer equipped with two ion guns (Cs+ for high mass resolution spectroscopy and 69Ga+ for spatially resolved imaging) and a reflectron TOF analyzer. The detailed descriptions and performances of this instrument have been reported previously.21,22 In this work, high-resolution mass spectra were obtained using an 8-kV Cs source. The scanned area was 200 µm × 200 µm and the total ion dose for each spectrum acquisition was
