Chemical reactions between model rubber systems and plasma polymerized acetylene films. Part A: XPS, AES and SIMS investigations

Chemical reactions between natural rubber (NR) and plasma-polymerized acetylene films were investigated using model systems in which natural rubber was replaced by the unsaturated compound squalene (C30H50) or its saturated analogue, squalane (C30H62). Analysis of plasma-polymerized acetylene films on steel substrates before and after reaction with model rubber systems was accomplished using X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and Auger electron spectroscopy (AES). The importance of unsaturation in the model rubber compound was demonstrated by comparing the results for the two model systems. For the squalene-based model rubber system, the results obtained were consistent with a model in which components of the curing system reacted with both squalene and the plasma-polymerized acetylene film to form a relatively large number of polysulfidic pendant groups. In the early stages of the reaction, pendant groups were probably terminated by cobalt ions since sulfides were detected by XPS and AES. Pendant groups were probably terminated by zinc ions in the later stages of the reaction. The pendant groups disproportionated to form a relatively large number of mono-, di-, and tri-sulfidic crosslinks between the model rubber compound and the plasma polymerized primer. In the squalane-based model rubber system, components of the curing system reacted with the plasma-polymerized acetylene film to a small extent to form a few relatively long polysulfidic pendant groups. However, there was little evidence for reaction of the curing system with squalane and for crosslinking between squalane and the plasma polymerized primer.