Plasma polymerization of allyl alcohol on polyolefin-based materials: Characterization and adhesion properties

Hydroxyl-rich surfaces have been obtained through the plasma polymerization of allyl alcohol on various polyolefin-based substrates. The influence of plasma parameters (deposition time and power), the type of substrate, and water washing on the chemical structure of the formed plasma polymers was investigated. The characterization involved contact angle measurements, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) analyses. To increase the sensitivity of XPS towards hydroxyl groups, derivatization with trifluoroacetic anhydride was performed. In addition, the adhesion between plasma-polymerized layers and a two-component polyurethane lacquer was evaluated by a 90 degrees peel test. It was shown that power had a pronounced influence on both water wettability and hydroxyl group retention, while deposition time mainly influenced the film thickness. A relationship between the extent of hydroxyl functionality and the loss of material during the washing procedure was also found. Furthermore, it was shown that the chemical structure of the plasma polymers was not seriously affected by the choice of substrate. On the contrary, the adhesion properties of the substrates with plasma-polymerized layers were found to be highly dependent on the substrate used. For one of the substrates, the adhesion tests revealed an excellent lacquer adhesion when appropriate process parameters during the plasma deposition were used. The other substrate showed considerably lower peel forces. The difference in adhesion properties between the substrates may be explained as an effect of vacuum-ultraviolet (VUV) emission present during the plasma deposition. The VUV emission is able to create radicals in the nearsurface region of the substrate and thereby cause chain scission reactions, lowering the cohesive strength of the substrate. Finally, this study showed that reflection absorption spectroscopy (RAS) is a suitable FTIR technique for obtaining structural information about plasma-polymerized layers thinner than 50 nm.