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Langmuir 2007, 23, 4346-4350
Plasma Surface Modification and Characterization of POSS-Based Nanocomposite Polymeric Thin Films Brian H. Augustine,*,† Wm. Christopher Hughes,*,‡ Kathryn J. Zimmermann,† Ashley J. Figueiredo,§ Xiaowen Guo,| Charles C. Chusuei,| and Jessica S. Maidment⊥ Department of Chemistry, MSC 4501 and Department of Physics, MSC 4502, James Madison UniVersity, Harrisonburg, Virginia 22807, Department of Chemistry, Sweet Briar College, Sweet Briar, Virginia 24595, Department of Chemistry, 142 Schrenk Hall, UniVersity of MissourisRolla, Rolla, Missouri 65409-0010, and Department of Chemistry, Randolph Macon Women’s College, Lynchburg, Virginia 24503 ReceiVed October 31, 2006. In Final Form: January 22, 2007 The effect of a remote oxygen plasma on nanocomposite hybrid polymer thin films of poly[(propylmethacrylheptaisobutyl-polyhedral oligomeric silsequioxane)-co-(methylmethacrylate)] (POSS-MA) has been examined by advancing contact angle, X-ray photoelectron spectroscopy (XPS), and variable-angle spectroscopic ellipsometry (VASE). Exposure to a 25 W remote oxygen-containing plasma was found to convert the surface of POSS-MA films from hydrophobic to hydrophilic within 20 s. The exposure time needed for this conversion to occur decreased as the O2/N2 ratio in the plasma environment increased, indicating a positive correlation between the hydrophilicity and the presence of oxygen in the plasma. Local bonding information inferred from high-resolution XPS data showed that the isobutyl bonding to the POSS moiety is replaced with oxygen as a result of plasma exposure. Finally, VASE data demonstrates that increasing the weight percent of POSS in the copolymer significantly impedes the oxygen plasma degradation of POSS-MA films. On the basis of these results, a model is presented in which the oxygen plasma removes isobutyl groups from the POSS cages and leaves a SiO2-like surface that is correspondingly more hydrophilic than the surface of the untreated samples and is more resistant to oxidation by the plasma. The ability to modify surfaces in this manner may impact the utility of this material for biomedical applications such as microfluidic devices in which the ability to control surface chemistry is critical.
I. Introduction For many applications, a combination of the properties of organic polymers with those of inorganic materials is desirable. Recently developed nanocomposite hybrid organic-inorganic materials based on polyhedral oligomeric silsequioxane (POSS) copolymers offer a possible avenue to exploiting the best properties of both organic and inorganic materials.1 Applications involving higher glass-transition temperatures,2,3 better flammability resistance,3,4 and resistance to degradation in low earth orbit5,6 have all been reported for POSS-based polymers. To date, no studies have been reported on the possibility of using these materials for microfluidic devices such as those needed for micrototal analysis (µ-TAS or “lab-on-a-chip”) systems. However, modifying the surface chemistry of polymer microfluidic devices has been discussed.7,8 In these devices, tailoring the surface wetting, biofouling, and electroosmotic flow (EOF) characteristics * Corresponding authors. E-mail:
[email protected],
[email protected]. † Department of Chemistry, James Madison University. ‡ Department of Physics, James Madison University. § Sweet Briar College. | University of MissourisRolla. ⊥ Randolph Macon Women’s College. (1) Li, G. Z.; Wang, L. C.; Ni, H. L.; Pittman, C. U. J. Inorg. Organomet. Polym. 2001, 11, 123-154. (2) Haddad, T. S.; Lichtenhan, J. D. Macromolecules 1996, 29, 7302-7304. (3) Zheng, L.; Kasi, R. M.; Farris, R. J.; Coughlin, E. B. J. Polym. Sci., Part A 2002, 40, 885-891. (4) Jeon, H. G.; Mather, P. T.; Haddad, T. S. Polym. Int. 2000, 49, 453-457. (5) Gilman, J. W.; Schlitzer, D. S.; Lichtenhan, J. D. J. Appl. Polym. Sci. 1996, 60, 591-596. (6) Gonzalez, R. I.; Phillips, S. H.; Hoflund, G. B. J. Appl. Polym. Sci. 2004, 92, 1977-1983. (7) Henry, A. C.; Tutt, T. J.; Galloway, M.; Davidson, Y. Y.; McWhorter, C. S.; Soper, S. A.; McCarley, R. L. Anal. Chem. 2000, 72, 5331-5337. (8) Lahann, J.; Balcells, M.; Lu, H.; Rodon, T.; Jensen, K. F.; Langer, R. Anal. Chem. 2003, 75, 2117-2122.
of the internal surfaces is of considerable interest for polymer microfluidics and has been demonstrated with other surfacemodification strategies. POSS-based polymers represent an attractive system that provides both the performance advantages of glass microfluidic devices9 and the fabrication advantages of polymeric devices.10,11 Poly(methyl methacrylate) (PMMA) microfluidic devices have a lower EOF than glass and are highly hydrophobic.12 Furthermore, the significantly lower glasstransition temperature and favorable optical properties such as UV transparency and low autofluorescence of PMMA lend themselves to uses in lab-on-a-chip devices.10 The primary focus of this article is to understand the changes in surface chemistry of oxygen plasma-treated POSS-MA thin films. However, the possibility of tuning the hydrophilicity and thus potentially the EOF of POSS-based PMMA surfaces using an oxygen plasma environment is shown in this article. II. Experimental Section Two types of substrates were prepared prior to the deposition of POSS thin films. Substrates prepared for X-ray photoelectron spectroscopy (XPS) analysis were on 4 in. Si(100) wafers in which 1500 Å of a refractory metal (Ta) was first deposited to minimize charging effects (henceforth called Si/Ta). The Ta films were deposited via magnetron sputter deposition at a deposition pressure of 2 × 10-3 Torr and a base pressure of at least 5 × 10-6 Torr. Si/Ta samples were stored in a N2 dry box in a fluoroware container, and polymer thin films were deposited on the Si/Ta substrates without (9) Landers, J. P. Anal. Chem. 2003, 75, 2919-2927. (10) Soper, S. A.; Ford, S. M.; Qi, S.; McCarley, R. L.; Kelly, K.; Murphy, M. C. Anal. Chem. 2000, 72, 642A-651A. (11) Becker, H.; Locascio, L. E. Talanta 2002, 56, 267-287. (12) McCormick, R. M.; Nelson, R. J.; Alonso-Amigo, M. G.; Benvegnu, J.; Hooper, H. H. Anal. Chem. 1997, 69, 2626-2630.
10.1021/la063180k CCC: $37.00 © 2007 American Chemical Society Published on Web 03/14/2007
POSS-Based Nanocomposite Polymeric Thin Films further cleaning within 10 days of deposition. Prior work has shown that SiO2 thin films in this size range deposited on refractory metal supports (such as Ta) serve to minimize charging while at the same time retain their bulk chemical properties.13-15 Substrates used for variable-angle spectroscopic ellipsometry (VASE) were 4 in. Si(100) wafers thermally oxidized at 1000 °C to a SiO2 thickness of ∼250 Å (henceforth called Si/SiO2). All samples were immediately cleaved into 1 × 1 cm2 pieces, cleaned with compressed N2, and stored in a N2 dry box prior to POSS thin film deposition and analysis. Compositions of 10, 20, and 45% by weight poly[(propylmethacrylheptaisobutyl-polyhedral oligomeric silsequioxane)-co-(methylmethacrylate)] (POSS-MA) copolymer were purchased and used as received (MW ≈ 50 000 amu for all compositions, Hybrid Plastics, Inc.). The copolymer was dissolved in CHCl3 solvent at a concentration of 2.0-2.5 mg/mL and spun-cast onto the two types of substrates at a spin speed of 1000 rpm to form a thin film ranging in thickness from 140 to 220 Å depending on the POSS composition in the polymer. Pure PMMA thin films were produced as a control by dissolving 200 µm PMMA beads (MW ) 75 000 amu, Polysciences, Inc.) in CHCl3 at a concentration of 2.0 mg/mL and spun-cast using the same conditions as for the POSS-MA films. Plasma surface modification was performed in a March PX-250 plasma chamber in a remote plasma configuration that consisted of a powered top electrode and perforated grounded electrode located 2.54 cm apart. Samples were placed on a floating electrode approximately 10 cm downstream from the plasma. All plasma treatments were performed at 25 W forward power using varying O2/N2 gas ratios with times between 0 and 2000 s. The base pressure in the system was
