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Department of Energy for financial assistance (K.R.). We also wish to thank the ILL, Grenoble, for the use of neutron-scattering facilities, particularly Albert Wright for this help in collecting the data. We are also grateful to Tom Lee for his help in the use of the COSMIC molecular
modeling routine, which was donated by Smith, Kline and French Research, Welwyn. Registry No. (Polyethylene glycol monomethyl ether)(methacrylate)(styrene)(graft copolymer), 115115-55-4.
Synchrotron Radiation Studies of Poly(tetrafluoroethy1ene) Photochemistry R. R. Rye* and N. D. Shinn Sandia National Laboratories, Division 11 14, Albuquerque, New Mexico 87185 Received March 31, 1989. In Final Form: June 20, 1989 Irradiation of poly(tetrafluoroethy1ene) (PTFE) with monochromatic photons in the energy range 25 eV < hu < 1000 eV protects the surface against alkali etching used to prepare the surface for adhesion. Previous studies have shown that this protection against etching is associated with radiation-induced cross-linking and/or branching. The absence of a true threshold energy behavior (i.e., no energy at which the effect turns on or off) at the C(1s) and F(1s) core levels demonstrates that radiation-induced cross-linking results from low-energy valence excitations. Photon-stimulated desorption (PSD) measurements of neutral fluorocarbon fragments evolved during irradiation have been correlated with the cross-linking kinetics and subsequent etch resistance. Both an increase in the fluorocarbon desorption rate and a decrease in the radiation dose required to achieve a given etch resistance are found for photon energies just above the F( 1s) binding energy. These changes result from the increase in the photon absorption coefficient at the core level, suggesting that both experimental observations result from the increased cross-linking in the near-surface region. Simple kinetic models are used to explain both the initial prompt onset and subsequent linear rise in the fluorocarbon PSD profiles. Mechanistic aspects affecting the efficiency of cross-linking and the use of PSD to monitor this near-surface photochemistry are discussed.
Introduction Poly(tetrafluoroethy1ene) (PTFE) presents a special problem for adhesion applications because of its extreme chemical and physical inertness. For successful adhesion, methods must be used which modify the lowenergy surface of PTFE. The most widely used such method involves chemical etching of the surface with sodium,' either in liquid ammonia or as a 1:l complex of sodium and naphthalene (Tetra-Etch, W. L. Gore and Associates, Newark, DE). This etching step results in a highly porous surface with the chemical attack reported to extend to a depth of 10 000 A.' A mechanical effect resulting from interlocking with this highly porous surface has been suggested to be a major factor in adhesion to PTFE.2 It is clear that good adhesion to PTFE is not simply a result of defluorination. If it were, then the adhesion strength-after chemical etching-could be correlated to some measure of the near-surface fluorine concentration. No such correlation was found for samples which were exposed to ionizing radiation, photon^,^-^ or elect r o d prior to etching. Preirradiation of the surface leads to major differences, after etching, between irradiated and nonirradiated regions of the surface in color (1) Benderly, A. A. J . Appl. Polym. Sci. 1962,20, 221. (2) Rye, R. R.; Martinez, R. J. J . Appl. Polym. Sci. 1989, 37, 2529. (3) Rye, R. R. J.Polym. Sci., Polym. Phys. 1988, 26, 2133. (4) Rye, R. R.; Martinez, R. J. Mat. Res. SOC.S y m p . Proc. 1988, 119, 265. (5) Rye, R. R. Langmuir, in press.
0743-7463/90/2406-0142$02.50/0
contrast3 and the resulting adhesion strength2v4v5but not in the near-surface carbon to fluorine ratio.3 The visual appearance of the sample after etching is a better qualitative measure of subsequent adhesion strengths than the near-surface F concentration. After etching, the irradiated regions of the surface have the near-white appearance of unetched PTFE while the irradiated regions are dark and discolored. The adhesive strength, determined with epoxy resins, of the irradiated regions is only 3% of the strength of the nonirradiated surface. Thus, both the adhesion strength and the visual appearance of etched PTFE are strongly dependent on irradiation of the surface. In contrast, the F concentration from Xray photoelectron spectroscopy (XPS) decreases by approximately a factor of 40 for the etched surface relative to virgin PTFE and nearly a factor of 20 for a surface irradiated and then etched. Thus, if we accept the decrease in F by XPS as a measure of chemical etching, the extent of etching differs by only a factor of 2 for the irradiated and nonirradiated samples. The problem is that the F concentration, as measured by XPS samples to a depth of only -30 A, may be irrelevant if the chemical modification required for adhesion extends to depths of the order of 1 0 000 A.2 The radiation-induced modification of the etching behavior of PTFE can be used as a photolithographic technique for controlled adhesion. The mechanism for this process has been proposed to be radiation-induced crosslinking and/or branching in the near-surface which produces a more rigid6 surface resistant to the deep b 1990 American Chemical Societv
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