Plasma Polymer Films Prepared by Means of Decomposition of Polymeric Solids

Thermal evaporation of polyethylene (PE) and polypropylene (PP) is performed with or without an activation of volatile fragments in an rf glow discharge. Without plasma assistance evaporated films are a mixture of aliphatic hydrocarbons. These films are not stable in contact with water. The activation in plasma enhances cross-linking and the films become hydrocarbon plasma polymers. However, these plasma polymers resemble much more the original polymers. Introduction Evaporation of polymers has been attempted already in the 1960s [1,2]. The films were found composed of low molecular weight fragments and for better mechanical properties a post deposition UV light irradiation was recommended [2,3]. The evaporation of a large number of polymers has been treated by Hogarth and Iqbal [4]. We have decided to reinvestigate the above mentioned findings and compare a simple evaporation and “plasma assisted” evaporation [5] that may be seen as a specific plasma polymerization process. Experimental Evaporation of PP and PE took place in a cylindrical reactor pumped with rotary and diffusion pumps. After reaching the ultimate pressure of 2⋅10 Pa the pumping speed was throttled and argon (purity 99,99%) was fed in at a flow rate of 2.5 sccm and pressure 0.4 Pa. The evaporation was performed with or without simultaneous rf discharge (10 – 160 W) excited by a magnetron 78 mm in diameter equipped with a graphite target. Graphite known for its low sputtering yield was chosen to minimize the contribution of sputtering. The evaporation source was placed 5 cm above the magnetron, and substrates were placed 15 cm above the evaporation source (Fig.1). When usual molybdenum boat was used for the evaporation of the polymer a vigorous boiling led to bulk drops of polymer splashing out and reaching the substrates. A more sophisticated evaporation source was constructed. It was filled with a mixture of polymer and copper granules to improve thermal transfer to the polymer. Evaporation temperature was controlled by a thermocouple and set to 350 °C for both polymers. The low-density polyethylene in granular form was purchased from Aldrich, and the polypropylene in sheet form was obtained from Goodfellow. Silver coated glass for Fourier transform infrared spectroscopy (FTIR) and water contact angle