Molecular Structure of Interphases Formed by Plasma Polymerized Acetylene Films and Steel Substrates
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Molecular Structure of Interphases Formed by Plasma Polymerized Acetylene Films and Steel Substrates P. I. Rosales, E. J. Krusling, F. J. Boerio, and R. G. Dillingham Department of Chemical and Materials Engineering University of Cincinnati Cincinnati, Ohio 45221-0012 ABSTRACT The goal of this research was to determine the molecular structure of interfaces between plasma polymerized acetylene films and steel substrates and to elucidate the mechanisms by which the films adhere to the substrates. Inductively-coupled, RF-powered plasma reactors interfaced to an FTIR spectrometer and to an XPS spectrometer were used to deposit films ranging in thickness from a few tens of nanometers to approximately one nanometer onto polished steel substrates. As the thickness of the films was decreased, features in the XPS and FTIR spectra that were characteristic of the bulk of the films decreased in intensity and features characteristic of the interface increased in intensity. A band was observed near 1706 cm-1 in FTIR spectra of the thickest films and attributed to carbonyl groups. Bands were observed near 1600 and 1045 cm-1 in spectra of the thinnest films and attributed to iron carboxylate and iron alkoxide groups that were responsible for adhesion of the films to the substrates. Oxygen required to form these groups was available from water or oxygen molecules adsorbed onto the walls of the reactor. INTRODUCTION Adhesion of natural rubber (NR) to metals is an important technology. Since NR does not adhere to most metals, some form of surface engineering is usually required before useful levels of adhesion can be obtained. Our research has shown that plasma polymerized acetylene films are excellent primers for bonding NR to steel [1]. Polished steel substrates were coated with ultra-thin (~75 nm) plasma polymerized acetylene films that were deposited in an inductively coupled RF reactor using argon as a carrier gas. The coated substrates were used to prepare miniature lap joints using NR as an "adhesive." The strength of as-prepared joints was about 31 MPa and failure was 100% cohesive in the rubber, indicating that initial adhesion at the NR/film and film/substrate interfaces was excellent. Bond durability was also excellent and no loss of strength was observed after aging joints in steam for 3 days. These results were remarkable since the substrates were polished before film deposition. The molecular structure of interfaces is an important issue in adhesion science. Therefore, we investigated reactions occurring at the NR/film interface using reflectionabsorption infrared spectroscopy (RAIR), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and Auger electron spectroscopy (AES). Because of the complexity of the reactions, a model rubber compound was used to simulate reactions at the NR/film interface [2-4]. The model compound consisted of a mixture of squalene, a low molecular weight, unsaturated analog of NR, zinc oxide, carbon black, sulfur, stearic acid, diaryl-p-di-phenyleneamine, cobal
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