Changes of polymer material wettability by surface discharge Surface discharge and material treatment Surface treatment of materials in low temperature plasma belongs to the modern and very perspective plasmachemical processes. Especially surface discharges at atmospheric pressure are revealed as perspective because they do not require expensive vacuum devices. Moreover, energy density is very high in these discharges (up to 100 W.cm -3 ) and the discharge burns almost homogenously which leads to the relatively short treatment time. The reactor for plasma generation is shown in Fig. 1 left. Non-thermal plasma of the surface discharge is initiated by the pulsed voltage with the frequency of 7 khz. The electrode system itself is demonstrated in Fig. 1 right. The whole device could be pumped out and consequently filled by the defined gas mixture. Maximal size of treated sample is 10x10 cm. Contemporary configuration do not allow sample movement and thus the contact with plasma is done in small stripes, only, where the discharge burns. Of course, in practical industry application it is possible to use material movement through the discharge space and therefore the industrial treatment is more or less homogenous, contrary to our model experiment. And it is also possible to treat materials with the width over 1 m. Plasma treatment time itself could be varied in the range of few seconds to tens of minutes, in principle only few seconds are enough for the effect. Fig. 1: Photo of the discharge reactor (left, Universal plasma reactor UPS100W) and detail view on the electrode system for the surface discharge generation (right). Determination of surface energy Free surface energy surface phenomena Surface phenomena are complex processes taking place on the interface between two or three phases. The interface (i.e. surface as well) represents a region of definite thickness where composition and energy continuously change from one phase into the other. Intermolecular forces take a significant role in this region, too. These forces are cohesive forces of mutual attraction of molecules inside one phase and adhesive forces characterising molecule bonds from one phase on the surface of the other phase = adsorption. Molecules in the interface of interacting phases are in a different energy state than the same molecules
inside the appropriate phase. If we want to enlarge the interface (surface) area, it is necessary to transfer molecules from the inside of interface layer and thus to do work. Forces that prevent surface enlargement are called interface (surface) tension γ which is defined as a force perpendicularly affecting a length unit of liquid surface. Needed work is equal to the product of surface tension γ and surface increment da and it is called surface energy δ. Numerical value of surface tension γ in N/m simultaneously expresses work in J that is necessary to form new surface of 1 m 2. Values of surface tension γ expressed in J/m 2 are considered as values of specific G γ =, (1) A T, p, n where γ means surface (interface) energy, G is Gibbs free energy of the whole system, A is area of the interface, T is temperature, p is pressure, n is total amount of matter mols in the system. Free surface energy consists of disperse and polar part. London disperse forces contribute to the disperse part of free energy, electrostatic forces, hydrogen bonds and dipoledipole interactions contribute to the polar part. It is not possible to measure of solid matters by any direct method. Therefore this important parameter is mostly calculated from values of contact angle of various liquids with different polarity and different surface wetting. Drop shape represented by angle θ is determined by three interface tensions γ SV, γ SL, γ LV (Fig. 2). In balance, characterised by time stability of drop shape, an equilibrium contact angle on smooth, homogeneous, flat and solid surface can be described by Young equation γ LV cosθ = γ SV γ SL, (2) where γ LV means surface tension of liquid in equilibrium with its saturated vapour, γ SV is surface tension of solid matter in equilibrium with saturated liquid vapour and γ SL means interface tension between solid matter and liquid. Saturated vapour γ LV Liquid Solid matter θ γ SL γ SV Fig. 2: Equilibrium contact angle on smooth, homogenous, flat and solid surface. Liquid completely spreads on the surface of solid phase if θ = 0, liquid do not spread on the surface totally if θ = 180. Generally, surface is wetted by liquid when θ < 90 and nonwetted when θ > 90. Measurement of contact angle principles The most used method which is based on the liquid drop placed on flat solid surface (so called sessile drop methods ) is the tangential method. This method is based on the measurement of angle formed between drop profile tangent and solid pad in the point of contact of these two phases.
Measurement conditions Parameters that can influence the measurement are: quality of tested liquid drop size surface quality density drop surroundings surface preparation temperature drop volume partial surface solubility time evaporation adsorption and absorption gravitation For good measurement reproducibility, it is important to keep standard measurement conditions. The drop should be as small as possible because then it better forms spherical or elliptical shape. As the material surface is not generally homogeneous, it is necessary to carry out more measurements and calculate arithmetic mean value from the obtained data. Data evaluation surface energy calculation methods Surface energy of solid surfaces could be determined by various methods. Method selection depends on number of used liquids for contact angle measurements, on material of appropriate surface and on information that we would like to obtain from the measurement (see Table 1). Table 1 Method Information Min. number of liquids Application Examples Zisman critic surface tension 2 Non-polar surfaces PE, PTFE Fowkes Owens - Wendt - Rabel and Kaeble (OWRK) Extended Fowkes Wu (harmonic mean) Acid-base theory Equilibrium state theory disperse part of surface energy disperse, polar part and contribution of hydrogen to disperse and acid-base contribution to free surface energy 1 Non-polar systems PE, PTFE 2 universal polymers 3 specific questions about surface properties 2 (at least 1 polar) low-energy system 3 specific questions about surface properties plasma or corona treatment of polymers organic pigments, polymers biological systems 1 universal polymers Schultz 1 2 high-energy system glass, metals Schultz 2 2 high-energy system polymers
SEE SYSTEM Surface Energy Evaluation System Instrumentation: SEE system + SEE software (it is not possible to start up the program without the connected device) Principle: Deposited drop is recorded by colour CCD camera immediately after its placing on the substrate and it is displayed on the PC monitor. Further it is utilized using SEE software. Procedure: 1. Connect USB cable from the device to PC (if it is disconnected). 2. Start up the software in option Start/ Programy/ See system/ See software 5.4. 3. For records by CCD camera, bookmark CAMERA must be active. 4. Enter the file name (Picture Name) and file placement disk (in bottom-right). 5. Place the sample on metal table and focalize the camera on sample edge using a screw movement along axis y. 6. In the case that sample edge is not horizontal, unfasten the screw in down-left under the metal table and balance it. 7. By button depict the possibility of shot property adjustment. In part Auto Mode Control/ Exposure adjust the background (darken or raise, respectively). 8. Apply liquid on the sample using micropipette and focalize by the screw movement along axis y. 9. Save appropriate shot by clicking on button CAPTURE. All shots are depicted in the right column of the dialog window. 10. Analyze the drop and calculate surface energy using selected model. Bookmark Analysis 1. Select file for the analysis in the right dialog window. 2. Add liquid by clicking on button NEW LIQUID (in the case that we use more liquids, it is possible to change active liquid using shifts). 3. Analysis itself can be carried out by following ways: 3.1. 3-point analysis (Quick) - Activate by clicking button or F4 or in menu Analysis - Quick - Place 2 points on interface liquid-solid matter by clicking left button. - Place 1 point on drop outline. - Contact angle is determined by circular interpolation. 3.2. More-point analysis - Deactivate the 3-point analysis by button or F4 or in menu Analysis Quick. - Place 2 points on interface liquid-solid matter by clicking left button + Ctrl. - Place at least 4 points on drop outline by clicking the left button. - Carry out the measurements by button MEASURE. 4. Save measured data by button ADD. Contact angles + file name are saved on working pages for each tested liquid extra. 5. In the case that you want to remove some points, use button CLEAR or click on the appropriate point by the right button. In the case of more-point analysis, it is necessary to also hold button Ctrl at the interface liquid-solid matter.
Bookmark Calculation 1. Each new liquid has its own file. Click on the header and select the liquid from the menu. 2. Mark angles that you want to use for the calculation by the left button. 3. Select method for surface energy calculation in menu Models. 4. Calculation is depicted in the bookmark Reports it can be saved in txt format. After all measurements, do not disconnect USB cable from the device. Appendix to surface energy calculation Calculation, point 4 Select Acid-base model. Selection of three liquids (always diiodmethane and water + something) using <ctrl> + mouse left button Parameters in results: gtotal total surface energy glw disperse part of surface energy gab acid-base (polar) part of surface energy g+ acid part g- base part It is valid that: gtotal = glw + gab gab = 2 g + * g Tasks 1) Selected sample of polymer material treat by surface discharge at various time of interaction between plasma and polymer surface. Adjust different discharge conditions during the treatment (input power, gas flow).