Langmuir 1992,8, 130-134
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Monolayer Chemical Lithography and Characterization of Fluoropolymer Films Terrence G. Vargo,t Patrick M. Thompson,$ Louis J. Gerenser,t Robert F. Valentini,§ Patrick Aebischer,§ Daniel J. Hook,? and Joseph A. Gardella, Jr.*?t Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214, Surface Science Section, Analytical Technology Division, Kodak Research Laboratories, Eastman Kodak Company, Rochester, New York 14650, and Section of Artificial Organs, Biomaterials and Cellular Technology, Brown University, Providence, Rhode Island 02912 Received May 6, 1991. I n Final Form: July 11, 1991 A means for surface chemical modification with micron level lithographicpatterning on fluoropolymers is described. The two-step process involves a novel glow discharge plasma modification. This process allows high densities of reactive linking functionalitiesto be synthesized at the surface of fluoropolymers without degradation of the morphology. This modification is accomplished with lithographicpatterns on poly(tetrafluoroethy1ene-co-hexafluoropropylene) (FEP). Further functionalization is then accomplished via reactionswith (aminopropy1)triethoxysilane(APTES) which results in controlled monolayer coverages of poly[ (aminopropyl)siloxane](APS). Imaging time of flight static secondary ion mass spectrometry provides maps of the resulting surfaces. The resulting surface promotes mouse neuroblastoma cell attachment and in some cases neurite outgrowth only at surface chemically modified regions. The bulk optical properties of the fluoropolymer are preserved.
Introduction The ability to control and construct the chemistry of surfaces and interfaces at the atomic and macromolecular level is one of the key reasons for the rapid development of new materials in the past decade.lt2 In our laboratories, a new fluoropolymer has been ~ h o w n which ~ - ~ exhibits interesting and novel properties (e.g., retention of inherently low surface energies with a concurrent high surface reactivity tospecific ~ h e m i s t r i e s ) .This ~ ~ ~materialis unlike other chemically modified/refunctionalizedsaturated fluoropolymers (i.e., poly(tetrafluoroethy1ene-co-hexafluoropropylene) (FEP) and poly(tetrafluoroethy1ene) (PTFE)'jv7in that the methods used in this process do not require an initial defluorinationlreduction step. Defluorination of fluoropolymers, whether through wet chemical6,7 or gas-phase methods (e.g., plasma treatment8t9), typically results in loss of low-energy characteristics as well as substantial damage to the surface morphology. Thus, the improvements via our modification method4t5 are related to the incorporation of highly reactive functionality onto a surface which exhibits fluoropolymer characteristics (e.g., low surface energy) with minimal damage to the surface morphology. The aim of this paper is to show the development of new architecturally constructed materials having various technological applications. As one example we introduce
* To whom correspondence should be addressed. of New York a t Buffalo. t Eastman Kodak Co. + State University
5 Brown University.
(1) Swalen, J. D.; et al. Langmuir 1981,3(6),932. (2)Laibanis,P. E.;Hickman, J. J.; Wrighton,M. S.; Whitesides, G. M. Science 1989,245,845. (3)Gardella, J. A.,Jr.; Vargo, T. G. US.Patent 4,946,903,1990. (4)Vargo,T. G.; Gardella, J. A., Jr.; Meyer, A. E.;Baier, R. E. J.Polym. Sci., Part A: Polym. Chem. 1991,29,555. (5) Hook,D. J.;Vargo, T. G.; Gardella, J. A., Jr.; Litwiler, K. S.; Bright, F. V. Langmuir 1991,7,142. (6)Costello, C. A.;McCarthy, T. J. Macromolecules 1987,20,2819. (7) Bening, R.C.; McCarthy, T. J. MacromolecuIes 1990,23,2648. (8)Clark, D.T.; Hutton, D. R. J. Polym. Sci., Part A: Polym. Chem. 1987,25,2643. (9)Morra, M.; Occhiello, E.; Garbassi, F. Langmuir 1989,5,876.
the capability to provide (within our reaction sequence) a lithographic patterning method for fluoropolymeric materials. A two-step approach is used where step one involvesa radio frequency glow discharge (RFGD) plasma treatment using a grid or mask having the desired dimensions (Figure 1). Following plasma modification, step two uses solution-phase chemistry to refunctionalize (at the monolayer level) the reactive sites on the fluoropolymer. Using this two-step approach, a lithographic application of a wide range of functionalities can then be covalently bonded to fluoropolymeric articles with no change in the micromorphology. Further, reactions at the monolayer coverage level enable efficient use of such functionalities and yield a material which retains bulk physical characteristics, including optical clarity. This latter property is important for a variety of technological applications which includes the field of cell biology and tissue culture research. While coating methods can provide similar outermost surface functionality, orientation of functionality and patterning are limited, optical characteristics are degraded, and adhesion of coatings to fluoropolymers is generally poor. The production of monolayer surface modifications, which are restricted to micron lateral dimensions on an insulator,2require the capabilities of modern microscopic methods. Fluorescent tagging and confocal fluorescent microscopy might achieve the necessary mapping of such functionalities; however, fluorescent tag specificity, efficiency, and potential chemical perturbation (among others) can limit the capability of this technique. In this work, imaging time of flight static secondary ion mass spectrometry (ToF-SIMS)1° is used because of (1)the ease in providing specific chemical information (without tagging or derivatization), (2) the sensitivity to monolayer surface concentrations, (3) the need for micron level spatial resolution, and (4) the application toward insulating surfaces. To demonstrate one utility of these modifications, we report results of mouse neuroblastoma cell attachment (IO)Benninghoven, A. 2.Phys. 1970,230,403.Benninghoven, A. J. Vac. Sci. Technol. 1985,A3 (3),451.
1992 American Chemical Society
Monolayer Chemical Lithography
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Langmuir, Vol. 8, No. 1, 1992 131
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Figure 1. Generic two-step method for refunctionalizing fluoropolymers (a). Step 1involves the exposure of ultrasonically cleaned (in hexane and methanol) FEP to hydrogen/methanol RFGD (Modified Harrick RFGD chamber: 13.56 MHz, 1 W/ cm3 power density, pressure 100mTorr, 20 s,llwith a nickel TEM grid (b) (Ladd Research Model 2-10031, 1000 mesh) placed in intimate contact with the FEP surface). After ultrasonically cleaning (in both hexane and methanol) the modified FEP is quickly dipped in and out of a ca. 1% distilled APTES in distilled dried hexane. The dipped FEP is then ultrasonically cleaned in hexane and methanol. The TEM grid illustrated in (b) has grid bars of ca. 7 pm which are spaced ca. 15 pm apart as determined from SEM analysis. The outer frame or ring of the grid (again measured via SEM) was ca. 150 pm.
and subsequent neurite outgrowth on poly(aminopropy1siloxane) (APS) patterned onto a fluoropolymer (fluorinated ethylene propylene (FEP))surface. This was done in order to evaluate this particular chemistry and its effect on neural cell attachment and growth as well as to evaluate the capability to confine cell attachment and growth within the patterned regions. Experimental Section FEP Surface Refunctionalization. Poly(tetrafluoroethylene-co-hexafluoropropylene)(FEP) was generously donated from Du Pont Electronics Division, Du Pont Co., Wilmington, DE, 19880-0019. FEP films were all ca. 50 pm thick as supplied, and all samples were ultrasonically cleaned in hexane and methanol before and after RFGD modification. Figure l a summarizes the generic steps utilized for the covalent attachment of siloxane polymer to the general class of fl~oropolymers.3-~ The extent of reaction a t monolayer coverages is governed by both the degree of initial plasma modification of the substratell and the subsequent exposure time to the coupling agent s ~ l u t i o n . ~ This was quantified by the combined use of electron spectroscopy for chemical analysis (ESCA) and fluorescence spectroscopy (using fluorescent tags that can be selectively and reversibly ~~~
(11)One observation of interest was that under RFGD conditions, in which no metallic grid was used, treatment times ranged from ca. 10 to
30 min3-6 depending on the extent of modification desired. With the metallic grids in place, we found that treatments resulting in the same concentration of functionality could be achieved in exposures of 1C-30 s. Apparently the low ionization potential of the nickel metal facilitates a localized plasma which is much higher in electron density than that normally associated with a plasma environment at the surface of an insulating material alone.
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Atomic Mass Units Figure 2. (a) ToF-SIMS positive ion mass spectrum of unmodified and cleaned (see Figure 1)FEP. (b) ToF-SIMS positive ion spectrum of APS as reacted onto the FEP substrate with no patterning. removed5). For this study, the attached APS to the FEP has a measured coverage of ca.
