Polystyrene is a thermoplastic important for a variety of applications in high-technology areas like microelectronics, Labon-a-Chip devices, membranes, and cell culture [1,2]. Its properties of special interest are hardness, rigidity, ease of heat fabrication, thermal stability, low specific gravity and excellent thermal and electrical properties for insulating purpose [3,4]. However, low surface energy (SE), hydrophobicity, and insufficient reactivity of this polymer strongly affect functional qualities of many modern PS products. In order to reduce these drawbacks, i.e., to enhance the material adhesion properties, various methods like corona, glow and plasma treatment discharge as well as UV-lamp exposure are being applied to modify the surface layer (SL) of this polymer [5][6][7][8]. The increase in the surface energy induced by these methods can affect not only adhesion but also several tribological properties [9]. Lasers enable to perform precise changes in the properties of small fragments of surfaces of complex shapes [10,11]. The ultravioletlaser radiation can change the surface geometrical structure and initiate chemical reactions that cause formation of polar functional groups within the SL of the material to be modified [12][13][14]. Compared to UV exposure high energy nanosecond UV-laser irradiations will cause a significant higher density of radicals. Due to time-dependant diffusion process in the gas phase and surface layer, the radicals cannot be completely saturated by reaction with oxygen and therefore other recombination processes become more important [8,15]. The physicochemical phenomena associated with laser-induced alterations are still not fully understood and are continually a subject of intensive studies [16][17][18].
Recently, much attention has been paid to laser-assisted patterning and modification of PS with respect to micro-fluidics and cell culture applications [19,20]. In most works, the effects of KrF or Nd:YAG laser treatments on alterations in the polystyrene SL have been investigated [19,21,22]. Physicochemical changes in this layer, induced by the ArF excimer laser radiation, have also been reported [23]. However, importance of this issue was attached to the ablation regime.
Surface oxidation with no significant change in advancing contact angle and in surface roughness for below ablation threshold ArF laser-irradiated PS was reported elsewhere [8,24]. However in our examinations significant increase in surface roughness was noticed. Some differences in obtained results can arise from different experimental conditions like, non-and homogenized laser beams, different laser fluences, pulse repetition rates and sample preparation methods.
This work focuses on the effects of the ArF laser irradiation below the ablation threshold of polystyrene. The objective of the discussed investigation was to determine (i) the extent of oxidation and elemental composition of the SL, (ii) surface geometrical structure, and (iii) wettability and surface energy of the laser-irradiated PS samples.
Experimental
2.1. Materials
Polystyrene designated as Owispol 945 E (Dwory SA, Os ´wie ˛cim, Poland) was used in the investigation. Its melt flow rate (MFR)