pubs.acs.org/Langmuir © 2010 American Chemical Society
Characterization of Plasma Polymerized C, H, and O Containing Compounds by MALDI Mass Spectrometry Lijuan Peng and Gary R. Kinsel* Department of Chemistry and Biochemistry, Southern Illinois University at Carbondale, Carbondale, Illinois 62901, United States Received July 12, 2010. Revised Manuscript Received September 27, 2010 Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is used for the first time to characterize radio frequency plasma-deposited polymers and for investigation of the plasma polymerization process. The MALDI mass spectra of the plasma polymers of allyl alcohol, di(ethylene glycol) vinyl ether and ethylene glycol butyl vinyl ether are all reported using solvent-based MALDI sample preparation approaches. The MALDI mass spectra of each of the three plasma polymers contain distinctive polymer series ion signals having molecular weight distributions below 2000 Da. Unexpectedly, however, the ion signals from each of the three plasma polymers show a common polymer repeat unit of 44 Da, for which the chemical formula is most likely -(C2H4O)-, and no evidence of the expected radical chain polymerization polymer is detected. These results are discussed in terms of the likely involvement of gas-phase radical species having different stabilities in the radio frequency plasma environment.
Introduction Radio-frequency (rf) plasma polymerization has been widely used as a “clean technology” for the formation of materials in the last two decades.1-3 The process is solvent free and has the ability to create thin, pinhole-free, chemically and thermally stable polymer coatings having specific functional groups on most substrates (polymer, metal, glass, silica, and ceramic). More recently, rf plasma polymerization has played an increasingly important role in the field of biomedical engineering.2,4 For example, rf plasma polymer modification of stents and implants can provide a biocompatible interface between the body’s biological environment and the implanted synthetic medium for the purpose of reducing antagonistic reactions from a patient’s immune system.5,6 Although plasma polymerized compounds were first discovered as undesired byproduct of rf plasma research in the early 1930s,7 these compounds have only been extensively explored since the 1960s.8 A wide variety of plasma polymers having various chemical functionalities have been obtained through rf plasma polymerization of numerous monomers. For example, rf plasma polymerized acrylic acid, methacrylic acid or 2-aminobenzotrifluoride, subsequently treated with oxygen and ammonia plasma has been used to immobilize glucose sensors for monitoring blood sugar.9 Allylamine plasma polymer modified biochips are used for spatially controlled electro-stimulated DNA *To whom correspondence should be addressed. Phone: 618-453-6496. Fax: 618-453-6408. E-mail:
[email protected]. (1) Osada, Y. Vysokomol Soedin aþ 1988, 30, 1814. (2) Siow, K. S.; Britcher, L.; Kumar, S.; Griesser, H. J. Plasma Process Polym. 2006, 3, 392. (3) Forch, R.; Zhang, Z. H.; Knoll, W. Plasma Process Polym. 2005, 2, 351. (4) Cokeliler, D.; Caner, H.; Zemek, J.; Choukourov, A.; Biederman, H.; Mutlu, M. Biomed. Mater. 2007, 2, 39. (5) Hossainy, S. F. A.; Y., T.; Tung, A. C.; Pacetti, S. D. U.S. Patent 7247364, 2007. (6) Klee, D.; Hocker, H. Adv. Polym. Sci. 1999, 149, 1. (7) Linder, E. G.; Davis, A. P. J. Phys. Chem. 1931, 35, 3649. (8) Goodman, J. J. Polym. Sci. 1960, 44, 551. (9) Muguruma, H.; Karube, I. Trac.-Trend Anal. Chem. 1999, 18, 62. (10) Hook, A. L.; Thissen, H.; Hayes, J. P.; Voelcker, N. H. Biosens. Bioelectron. 2006, 21, 2137. (11) Szili, E.; Thissen, H.; Hayes, J. P.; Voelcker, N. Biosens. Bioelectron. 2004, 19, 1395.
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adsorption-desorption10 and cell transfection assays.11 Plasma polymer films of di(ethylene glycol) vinyl ether (EO2),12,13 triethylene glycol monoallyl ether,14 triglyme,15 tetraglyme,16,17 and 12crown-4-ether18 have been shown to resist protein adsorption and cellular adhesion. Low pressure plasma polymerization of Nisopropylacrylamide and N,N-diethylacrylamide allows thermoreversible swelling of thin hydrogel films.19 Varying duty cycle pulsed plasma polymerization of allyl alcohol yields extremely hydrophilic to super hydrophobic surface chemistries.20 Numerous other examples may be found in various literature reviews.1-3,21 Despite this broad exploration of various types of plasma polymers, the mechanism of plasma polymer formation remains under debate and the chemical structures of these plasma polymerized compounds have not been fully elucidated.21-24 Numerous analytical approaches have been used to characterize plasma polymers with a goal of gaining insight into the polymerization process. Mass spectrometry has been employed to analyze the gas composition of the plasma by collecting the plasma residues at the outlet of the plasma reactor.25,26 X-ray photoelectron spectroscopy (XPS), ToF-SIMS, and IR spectroscopy have also been (12) Wu, Y. L. J.; Timmons, R. B.; Jen, J. S.; Molock, F. E. Colloid Surf. B 2000, 18, 235. (13) Zhang, J.; Feng, X. F.; Xie, H. K.; Shi, Y. C.; Pu, T. S.; Guo, Y. Thin Solid Films 2003, 435, 108. (14) Beyer, D.; Knoll, W.; Ringsdorf, H.; Wang, J. H.; Timmons, R. B.; Sluka, P. J. Biomed. Mater. Res. 1997, 36, 181. (15) Hendricks, S. K.; Kwok, C.; Shen, M. C.; Horbett, T. A.; Ratner, B. D.; Bryers, J. D. J. Biomed. Mater. Res. 2000, 50, 160. (16) Murugesan, R.; Hanley, E.; Albrecht, R. M.; Oliver, J. A.; Heintz, J. A.; Lauer, J. L.; Shohet, J. L. J. Appl. Phys. 2008, 103. (17) Shen, M. C.; Wagner, M. S.; Castner, D. G.; Ratner, B. D.; Horbett, T. A. Langmuir 2003, 19, 1692. (18) Denes, A. R.; Somers, E. B.; Denes, F. S.; Wong, A. C. L. Biofilms 1998, 31. (19) Schmaljohann, D.; Beyerlein, D.; Nitschke, M.; Werner, C. Langmuir 2004, 20, 10107. (20) Rinsch, C. L.; Chen, X. L.; Panchalingam, V.; Eberhart, R. C.; Wang, J. H.; Timmons, R. B. Langmuir 1996, 12, 2995. (21) Inagaki, N. Plasma Surface Modification and Plasma Polymerization; Technomic Publishing: Basel, 1996. (22) Shen, M.; Bell, A. T. ACS Symp. Ser. 1979, 108. (23) Eliezer, S.; Eliezer, Y. The Fourth State of Matter: An Introduction to the Physics of Plasma; Taylor & Francis: London, 2001. (24) Smith, T.; Kaelble, D. H.; Hamermesh, C. L. Surf. Sci. 1978, 76, 203. (25) Smolinsky, G.; Vasile, M. J. Int. J. Mass Spectrom. Ion Phys. 1973, 12, 147.
Published on Web 10/20/2010
DOI: 10.1021/la1027866
17477
Article
Peng and Kinsel
Figure 1. Structures of the three monomers used in these studies.
routinely used to investigate the chemistry of the plasma polymer surface.3,27-30 Finally, despite the large sample requirements, solid state 13C NMR has also been shown to be a useful tool for investigation of plasma-polymerized compounds and can provide quantitative structural information that is not available from other analytical techniques.31,32 In this study we explore the use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for characterization of rf plasma-deposited bulk polymers and for investigation of the rf plasma polymerization process. MALDI MS has emerged in recent years as one of the most powerful tools for characterization of synthetic polymers because of the simplicity of sample preparation, a reasonable tolerance of impurities/salts, and the simplicity of the mass spectral interpretation which shows mainly singly charged quasi molecular ions.33,34
Experimental Section Chemicals and Materials. The monomer allyl alcohol (AA) was purchased from Acros, and the monomers di(ethylene glycol) vinyl ether (EO2) and ethylene glycol butyl vinyl ether (EGBVE) were purchased from Aldrich (Figure 1). All monomers were freeze-dried three times to remove excess adsorbed gases but were not purified further. All solvents, used as received, were of HPLCgrade purity and were purchased from Sigma-Aldrich and Fisher. The MALDI matrix 2,5-dihydroxybenzoic acid (DHB) was purchased from Sigma and used without further purification. Cationization reagents silver trifluoroacetate (AgTFA) and sodium trifluoroacetate (NaTFA) were purchase from Fluka, and cesium iodide was purchased from Johnson Matthey Chemicals. Glass slides (22 22 mm) were purchased from Electron Microscopy Sciences. Plasma Polymerization. Plasma polymerization reactions are carried out in a laboratory-built pulsed rf plasma reactor (Figure 2). The chamber consists of two round stainless steel electrodes, 28 cm in diameter and separated by 27 cm. Two inlets allow for the introduction of gases such as nitrogen, argon, oxygen, and the monomer vapors. A liquid nitrogen cold trap is employed to prevent excess reactant from reaching the rotary pump. The reactor pressure is maintained through adjustment of (26) OToole, L.; Mayhew, C. A.; Short, R. D. J. Chem. Soc. Faraday Trans. 1997, 93, 1961. (27) Groenewoud, L. M. H.; Engbers, G. H. M.; Terlingen, J. G. A.; Wormeester, H.; Feijen, J. Langmuir 2000, 16, 6278. (28) Oran, U.; Swaraj, S.; Friedrich, J. F.; Unger, W. E. S. Plasma Process. Polym. 2004, 1, 141. (29) Oran, U.; Swaraj, S.; Friedrich, J. F.; Unger, W. E. S. Plasma Process. Polym. 2005, 2, 563. (30) van Os, M. T.; Menges, B.; Foerch, R.; Vancso, G. J.; Knoll, W. Chem. Mater. 1999, 11, 3252. (31) Dilks, A.; Kaplan, S.; Vanlaeken, A. J. Polym. Sci. Polym. Chem. 1981, 19, 2987. (32) Kaplan, S.; Dilks, A. J. Polym. Sci. Polym. Chem. 1983, 21, 1819. (33) Cramer, R.; Burlingame, A. L. J. Mass Spectrom. 1999, 34, 1089. (34) Nielen, M. W. F. Mass Spectrom. Rev. 1999, 18, 309.
17478 DOI: 10.1021/la1027866
Figure 2. Schematic diagram of the rf plasma reactor. the flow rate of the gases. The rf signal at 13.65 MHz is provided by an ENI A-500 rf power amplifier. The system can be run in either continuous wave or pulsed mode. Gating of the rf signal in pulsed operation is controlled by a Tektronix AFG 3102 function generator and passed via power meter and matching network to the chamber electrode. Before each plasma run, the system is first cleaned by applying overnight a continuous oxygen plasma (400-500 mTorr, 25W). After system cleaning, the substrates are placed onto the grounded electrode and the reactor chamber is evacuated to base pressure (
