Langmuir 1992,8, 1165-1169
1165
Mechanism of Ultraviolet and Electron Bonding of Perfluoropolyethers G. H. Vurens,* C. S. Gudeman, L. J. Lin, and J. S. Foster IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099 Received October 3, 1991. I n Final Form: January 16, 1992 It has recently been reported that thin films of perfluoropolyethers (PFPE’s)can be bonded to a variety of substrates under illumination of 185-nm ultraviolet (UV) light. In this paper we examine the bonding of thin perfluoropolyether films by low-energy electrons and UV light. It is shown that the bonding takes place through interaction of the perfluoropolyether molecule, with a low-energy photoelectron created by excitation of the substrate by the UV photons. The perfluoropolyether molecule subsequently undergoes a dissociative electron attachment, resulting in the formation of a radical and a F- ion. It is likely that radical propagation and termination steps then cross-linkthe polymer and attach it to the substrate. The similaritiesand differencesbetween electron-bondedand UV-bonded perfluoropolyether films are discussed. Introduction
sition of perfluoropolyethers by low energy (10-90 eV) It is shown electrons has been studied by D’Anna et Perfluoropolyethers (PFPE’s) are a group of polymers that there is an onset of 14 eV before positive ions can be that are widely used as l~bricanta,l-~ dielectric fluids: formed from a perfluoropolyether molecule during electron and mechanical and diffusion pump 0il.9,~because of their bombardment. high chemical and thermal stability. It was recently d e m o n ~ t r a t e dthat ~ ~thin ~ ~ ~films (up to 30 A) of perfluoropolyethers can be attached to a variety Perfluoropolyethers do show accelerated decomposition of substrates, such as amorphous carbon, silica, and gold, in contact with Lewis acid surfaces,61obut only at elevated by illumination of the substrate with 185-nm UV light. temperatures. Even at room temperature perfluoropoly~ - ~ ~ The attached perfluoropolyether lubricant does not rinse ethers are sensitive to high-energy e l e ~ t r o n s , ~ JXoff the substrate with Freon or other solvents (such as r a ~ s , ~ .~yJ -*r a y ~and , ~ ion ~ ~ beams.21 ~~ No evidence for a perfluorooctane) in which the polymer is soluble. Illuchemical change in the perfluoropolyether due to UV mination with 254-nm UV light does not result in the irradiation has been demonstrated until recently, although formation of a so-called bonded perfluoropolyether film perfluoropolyethers can be synthesized using UV light from on these substrates. The fact that the perfluoropolyether the photooxidation of perfluoroolefins.z2 Here only the film can be bonded by far-UV light is surprising if one peroxide linkages in the polymer chain have been shown considers the strong chemical and physical stability of to be susceptible to UV d e c o m p o s i t i ~ n .The ~ ~ decompoperfluoropolyethers. This work was performed so the mechanism of the bonding process and the nature of the (1) Sianesi, D.; Zamboni, V.; Fontanelli, R.; Buraghi, M. Wear 1971, attached perfluoropolyether film could be elucidated. 18, 85. (2) Miyamoto, T.; Sato, I.; Ando, Y. In Tribology and Mechanics of We show that if a thin film of perfluoropolyether above Magnetic Storage Systems; Bhushan, B., Ed.; Society of Tribologists a substrate is irradiated with UV light, photoelectrons and Lubrication Engineers, 1988; Vol. V, p 55. can be generated from the substrate. In this paper we (3) Suzuki, M.; Saotome, Y.; Yanagisawa, M. Thin Solid Films 1988, 160,453. also show how long-energy electrons (
