Identification of Gas-Phase Reactive Species and Chemical Mechanisms Occurring at Plasma-Polymer Surface Interfaces Michelle L. Steen, Carmen I. Butoi, and Ellen R. Fisher* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872 Received May 4, 2001. In Final Form: August 3, 2001 In the present work, we explore the underlying chemistry occurring on the molecular level during plasma processing of a variety of polymeric membranes, continuing our studies on plasma modification of polymeric membranes for permanent hydrophilicity. Characterization of gas-phase species as well as molecules generated at the membrane surface during plasma modification was performed using optical emission spectroscopy and mass spectrometry. Plasma-surface interactions of OH radicals were assessed with our direct, nonintrusive radical-imaging experiments based on laser-induced fluorescence. Results show that OH has a moderate reactivity of ∼50% on all membranes studied, regardless of applied plasma power. Alternate reactor designs were employed to examine formation of reactive species and the effects of ions on the surface modification. Collectively, these experiments provide information on the chemical mechanisms responsible for surface modification and, ultimately, afford better process control for plasma treatments of polymers. I. Introduction Plasma modification of polymers occurs when gas-phase species react at a surface to form stable products with physical and/or chemical properties that differ from those of the bulk polymer. 1-3 Reactive neutrals, ions, electrons, and photons generated in the plasma can all interact simultaneously with the polymer to alter surface chemical composition and wettability. 4-6 As a result, the nature and energy distributions of the species impinging on the polymer surface ultimately determine the observed change in the surface properties. 7-11 Electron impact processes influence the density of these species; however, direct energy transfer arises from ions and metastables, which abstract atoms from the surface and break polymer chains. Radical species can combine at free radical sites created in the polymer and implant new functional groups, thereby altering the surface composition of the polymer. 2 We recently reported plasma modification of polysulfone (PSf), polyethersulfone (PES), and polyethylene (PE) membranes for permanent wettability. 12,13 The polymer surfaces are completely wettable immediately after plasma treatment, and this wettability remains more than a year after treatment for the PSf and PES. The permanence of the hydrophilic modification is a direct result of covalently bound O-H, C-O, and C-O x (e.g., CdO, O-CdO) groups introduced by H 2 O plasma treatment. Furthermore, complete penetration of the plasma through the membrane is achieved with our experimental design. Accordingly, environmental scanning electron micrographs (ESEMs) confirm complete wetting of the entire porous structure in situ. To further explore the molecular-level chemistry, we determined the gas-phase composition of our H 2 O plasma under the same conditions used to process PSf membranes (25 W, 50 mTorr, 2 min). Excited-state OH and H atoms as well as H 2 were identified in the optical emission spectrum. 12 At the low plasma powers used to treat our membranes, no emission from excited O atoms was observed. Clearly, all of the species generated in the plasma, including ions and UV photons, 4 may contribute to plasma modification. Direct evidence of the nature of the plasma-surface interactions, however, is needed to ultimately define the contribution of specific plasma species to the observed changes in the composition and wettability. Hence, our first step toward this end was to identify excited-state species generated only during plasma modification. For example, optical emission spec- troscopy (OES) confirmed the presence of CO* during treatment of PSf, not observed without a membrane in the reactor. Thus, we were able to detect reactive plasma species as well as products of the surface modification of PSf. Polymer modification processes can further benefit from a thorough description of the actual chemical reactions occurring at the plasma-polymer interface. 14 One such study was performed by Badyal and co-workers, who identified and monitored the time-dependent concentra- tions of plasma-polymer interfacial reaction products and intermediates. 15 These experiments entailed mass spectral detection of gas-phase species permeating across the plasma-polymer interface during O 2 plasma surface * To whom correspondence should be addressed. E-mail: erfisher@lamar.colostate.edu. (1) Hollahan, J. R.; Bell, A. T. Techniques and Applications of Plasma Chemistry; Wiley: New York, 1974. (2) Inagaki, N. Plasma Surface Modification and Plasma Polymer- ization; Technomic Publishing: Lancaster, PA, 1996. (3) d’Agostino, R. Plasma Deposition, Treatment, and Etching of Polymers; Academic Press: San Diego, CA, 1990. (4) Clark, D. T.; Dilks, A. J. Polym. Sci., Polym. Chem. Ed. 1977, 15, 15. (5) Gerenser, L. J. J. Adhes. Sci. Technol. 1987, 1, 303. (6) Egitto, F. D.; Matienzo, L. J. IBM J. Res. Dev. 1994, 38, 423. (7) Seebock, R. J.; Kohler, W. E.; Romheld, M. Contrib. Plasma Phys. 1992, 32, 613. (8) Janes, J.; Huth, C. J. Vac. Sci. Technol., A 1992, 10, 3522. (9) Hopwood, J. Appl. Phys. Lett. 1993, 62, 940. (10) Winters, H. F. Top. Curr. Chem. 1980, 94, 69. (11) Godyak, V. A.; Piejak, R. B. Phys. Rev. Lett. 1990, 65, 996. (12) Steen, M. L.; Hymas, L.; Havey, E. D.; Capps, N. E.; Fisher, E. R. J. Membr. Sci. 2001, 188, 97. (13) Steen, M. L.; Jordan, A. C.; Hymas, L.; Fisher, E. R. J. Membr. Sci., submitted for publication. (14) Hopkins, J.; Badyal, J. P. S. J. Phys. Chem. 1995, 99, 4261. (15) Wheale, S. H.; Barker, C. P.; Badyal, J. P. S. Langmuir 1998, 14, 6699. 8156 Langmuir 2001, 17, 8156-8166 10.1021/la0106642 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/22/2001