Influence of plasma treatment on wettability and scratch resistance of Ag-coated polymer substrates

Development in science and technology is relevant with the production of innovative materials with their specific properties depending on application. Polymers with precisely defined properties play an increasingly important role in the development of modern technology. Especially, the growing interest in thin film coatings deposited on various polymers has been observed. Polymers are frequently used in different application fields, such as biomaterials, adhesive and protective coatings, microelectronic devices, thin-film technology and many others, due to their excellent physical and chemical properties [1–7]. Polymers are easy to manufacture and they are characterized by low density, flexibility and cost-effectiveness [8].

Desired surface properties of polymers such as hydrophilicity, roughness, crystallinity or conductivity are required for their successful application [2, 3]. On the other hand, they are often unsuitable to use due to their low surface energy leading to poor wettability, adhesion, print-ability and scratch-resistance [1, 8, 9]. For these reasons, surface of polymer should be somehow activated to change its properties [1–3, 8, 10]. According to Pelagade et al. [11] surface modification techniques of polymers can be divided into three categories such as: (1) cleaning or etching material from the surface, (2) surface reactions that create functional groups and cross linking and (3) deposition of thin films. The common methods of surface modification include mechanical, chemical or physical treatments. The most frequently used treatments are, e.g.: flame, corona discharge, plasma, photons, electrons, ion beam or X-rays [2, 3, 12, 13]. These methods are used to achieve changes of polymer such as formation of special functional groups at the surface, increase of surface energy, change of wettability, improvement of chemical inertness and dyeability, introduction of surface cross linking, removal of weak boundary layers or contaminants, modification of surface morphology or improvement of surface electrical conductivity [3, 8, 11, 13, 14].

According to Chan et al. [3] plasma treatment is probably the most versatile surface treatment technique. Plasma, sometimes called the fourth state of matter, has been known for a very long time. Plasma can be defined as a partly or fully ionized gas containing charged and neutral particles, e.g. electrons, positive and negative ions, molecules, radicals and atoms [3, 15–17]. In recent years, the use of plasma treatment of polymers for modification of their surface properties has been studied as an environmentally friendly alternative to the wet chemical techniques [1, 11, 15, 18]. This kind of surface modification has many advantages, including modification of just the outermost atomic layers of the substrate, reduction of thermal degradation and modification without affecting the bulk properties of the polymer [18–21]. Plasma surface modification is used to create new chemical groups on their surface, branching and cross linking of macromolecules and formation of low molecular weight oxidized structures [2, 11, 12, 15, 19, 22]. Appearance of polar groups on the polymer surface leads to a change in wetting behavior, thus, strengthening the hydrophilicity of treated materials [23]. However, the surface roughness is also affected by the plasma treatment that can also change its wettability [20, 24]. Surface properties of plasma treated polymers depend on, e.g., gases composition, exposure time, frequency and plasma power [8, 12, 13]. Polymers are often modified using noble gas (He, Ne, and Ar) or reactive gas (O₂) plasma [25, 26]. In general, reaction of plasma with a polymer surface can be divided into three groups: surface reactions, which lead to functional groups production, plasma polymerization, cleaning and etching [3]. Oxygen or nitrogen plasma treatment leads to implementation of polar functional groups on the polymer surface. The consequences of these processes are improvement of adhesion properties, bonding of polymers and hydrophilization of their surface [10, 15, 20, 27]. One of the most valid disadvantage of plasma treatment is the ageing effect. Functional polar groups formed on the treated surface are not stable in time, as the surface tends to recover to its previous state [10, 20, 22]. Surface ageing is characterized by two processes. The first one is the reorientation of the polar groups into the bulk polymer and the second is the mobility of small polymer chain segments into the matrix [10]. Thus, the results of these processes are a decrease in surface energy and a significant decrease of surface wettability [8, 10, 12, 13, 24].

Metallic films on polymer surface are important components in the microelectronic circuits, optical and mechanical devices, in the construction of diodes, CD’s; they also find application in medicine [13]. As mentioned, polymers are characterized by low surface energy and poor adhesive properties. Therefore, to ensure good adhesion between polymer and metal, the surface of polymer must be activated by, e.g., plasma treatment [10, 12, 13, 15]. Deposition of metallic thin film on polymer substrates can be performed using various physical and chemical processes [13]. Among these techniques we can distinguish electroless plating of polymers in solutions of metallic salts, electroforming, thermal spraying, magnetron sputtering and electron beam evaporation [13, 28].

In this paper, the properties of plasma treated polymers (polycarbonate and polytetrafluoroethy-lene) and metallic coatings deposited on their surface by electron beam evaporation have been described.

PTFE (from DuPont) and PC foils (from SABIC Innovative Plastics) were used as substrates for the investigations. PTFE and PC had thicknesses of 250 μm and 1 mm, respectively, and their size was equal to 100 mm x 100 mm. Plasma treatment was performed in a cylindrical discharge chamber with an argon gas. The working chamber was initially pumped to ca. 2 Pa. Next, argon was dosed and its flow was set in order to obtain ca. 17 Pa pressure in the chamber. Samples were treated for 60 s with different power regulated using pulse width modulation (PWM) method with the PWM fill factor equal to: 10 %, 30 % and 50 %. Maximum plasma power in the applied reactor with 100 % PWM coefficient was 500 W. The distance between electrodes was 12 mm.

After surface modification of polymers silver thin films were deposited by electron beam evaporation method. During the deposition, Ag pellets (99.999 %) were melted by electron beam and evaporated from a tungsten boat. During the process, the pressure was kept below 3 x 10^-3 Pa. Thin films were deposited with a rate of ca. 3.3 Å/s and their thickness was equal to 50 nm. During evaporation of silver thin films the power was equal to ca. 60 W. The voltage of the ion gun was 6 kV, while the current was 10 mA.

Immediately after plasma treatment the surface properties of polymers and thin films deposited on modified samples were analyzed by contact angle measurement. Surface wettability was assessed by a Theta Lite tensiometer (Attension). Liquid used for the contact angle determination was deionized water with a conductivity of 0.05 μS/cm. Contact angle measurements were performed according to the sessile drop method [29, 30].

Roughness of the modified polymers and thin films was determined with the aid of CCI Theta Lite optical profiler (Taylor Hobson). The surface diversification of polymers was examined by the value of root mean square roughness (Sq) and average roughness (Sa). Scratch resistance of Ag thin films deposited on unmodified and modified substrates was investigated using a Summers Optical’s Lens Coating Hardness Test Kit. For this purpose the abrasion test consisting in rubbing the surface of Ag coating with a cotton fabric pad for 50 cycles using applied load of 0.5 N was performed according to the well acknowledged standard [31, 32]. Thin film surface was examined for scratch resistance by optical microscope (Olympus BX51) while the change of roughness after scratch test of Ag thin films deposited on polymer substrates was evaluated by optical profiler (Taylor Hobson CCI Lite).

In this work the influence of argon plasma treatment on PTFE and PC surface properties and Ag coatings deposited by electron beam evaporation on unmodified and modified polymers were described. Analysis of contact angle measurements showed that surface wettability of PTFE and PC after argon plasma treatment with different powers was changed. The value of contact angle for deionized water decreased of about 40 % and 25 % for polytetrafluoroethylene and polycarbonate surface, respectively. The ageing effect was not observed. After 14 days the hydrophilic properties of polymer surfaces after argon plasma treatment were still observed. Moreover, Ag thin film deposited on the modified polymers exhibited good scratch resistance compared to the coatings on unmodified substrates. Plasma modification increased wettability and improved adhesion between the polymer surface and Ag coating and thus, it was possible to obtain good thin film scratch resistance of the prepared coatings. Such results show that metallic coatings deposited on plasma modified polymers are suitable for innovative application in, e.g., flexible electronics technology.