The interfacial tension along the boundary formed between two immiscible polymer liquids has been measured by the pendant drop method. The polymers employed for the study are polyethylene, polydimethylsiloxane, poly(ethylene oxide), polytetrahydrofuran, poly(vinyl acetate) and an ethylene-vinyl acetate copolymer. Surface tensions of these polymers (against air) were also determined by the same technique. The values of interfacial tension between polyethylene and each of the five polar polymers, together with the surface tension data, were utilized to calculate the separate contributions to the surface tension by dispersion and dipole interaction forces, in accordance with the procedure proposed by Fowkes. The interfacial tension between two polar polymers was then analyzed in terms of these separate components of forces. An empirical relation has been shown to correlate the dipole interaction term in interfacial tension with the individual dipole force components of the two polar polymers involved.

A plot of the initial spreading pressures F sub ba or initial spreading coefficients S sub ba against the surface tensions of a homologous series of organic liquids b can be used to determine the critical surface tension for spreading on a second substrate liquid phase a. Straight-line relations are found for various homologous series. The intercept of that line with the axis of abscissas F sub ba 0, or S sub ba 0 defines a value of spreading for that series. This method is advantageous because it eliminates the need for measuring or calculating the contact angle of lens b floating on liquid a, it can be applied to any liquid substrate, and it is applicable even when spreading does not lie within the range of surface tensions of the members of the homologous series of liquids b. The value of spreading for the waterair interface was determined in this way using several homologous series of pure hydrocarbon liquids. The lowest value found was 21.7 dynescm at 20 deg C for the n-alkane series. Higher spreading values were obtained using olefins or aromatic hydrocarbons as the result of interaction between the unsaturated bond and the water surface. Since the results are analogous to those reported earlier for solid surfaces, it is concluded that the clean surface of water behaves as a low-energy surface with respect to low-polarity liquids. This result is to be expected if only dispersion forces are operative between each alkane liquid and water.

Low contact angle hysteresis is an important characteristic of superhydrophobic surfaces for nonstick applications involving the exposure of these surfaces to water or dust particles. In this article, a relationship is derived between the surface work of adhesion and the dynamic contact angle hysteresis, and the resulting predictions are compared with experimental data. Superhydrophobic surfaces with different contact angles and contact angle hysteresis were prepared by generating silicon pillars with varying pillar size and pitch. Surfaces were subsequently treated with fluoroalkyl silanes to modify further the hydrophobicity. The three-phase contact line established for such systems was related to the Laplace pressure needed to maintain a stable superhydrophobic state.

The surface chemistry and nanotopography of low-density polyethylene (LDPE) were modified by downstream, inductively coupled, radio frequency (rf) Ar plasma without inducing surface damage. The extent of surface modification was controlled by the applied ion energy fluence, determined from the plasma ion density measured with a Langmuir probe. The treated LDPE surfaces were characterized by atomic force microscope (AFM) imaging, contact angle measurements, and X-ray photoelectron spectroscopy (XPS). Analysis of AFM surface images confirmed that topography changes occurred at the nanoscale and that surface damage was insignificant. Contact angle measurements demonstrated an enhancement of the surface hydrophilicity with the increase of the plasma power. XPS results showed surface chemistry changes involving the development of different carbon-oxygen functionalities that increased the surface hydrophilicity. Physical and chemical surface modification was achieved under conditions conducive to high-density inductively coupled rf plasma.

The chemical modification of polystyrene surfaces by low-energy (10−100 eV) SF₅⁺, C₃F₅⁺, and SO₃⁺ ions was studied by X-ray photoelectron spectroscopy and two-laser ion trap mass spectrometry. The mechanism of fluorination was found to be dissimilar for SF₅⁺ and C₃F₅⁺ ions in this energy range at fluences of 10¹⁴−10¹⁶ ions/cm². SF₅⁺ was found to induce fluorination of the polymer surface by grafting reactive F atoms upon dissociation at impact. SF_n fragments were not found to be grafted or implanted into the polymer. Sulfur was detected on the polymer surface only at incident energies above 50 eV and was found to be sulfidic in nature. In contrast, C₃F₅⁺ ions induced grafting of both reactive F atoms and molecular C_mF_n fragments from the dissociation of the incident projectile. Larger proportions of highly fluorinated sites and thicker fluorocarbon layers were found for C₃F₅⁺ at all energies and fluences. A variety of aliphatic and aromatic fluorine bonding environments were detected on both SF₅⁺ and C₃F₅⁺ modified polystyrene surfaces.

The wetting and adhesion characteristics of 20 different surfaces have been studied systematically by both static water contact angle (θ) and dynamic contact angle measurement techniques: sliding angle (α) and advancing (θ_A) and receding (θ_R) contact angles. These surfaces cover surfaces of all traits, from smooth and flat to rough and artificially textured. Fourteen of the surfaces are flat, and they range from molded plastic sheets to solution coated polymer films to chemical vapor deposition polymerized polymer films and to self-assembled monolayers on Si wafers. The rest of the surfaces include 4 fluorosilane coated textured Si wafer surfaces and two natural surfaces derived from the front and back side of the rose petal. Static water contact angle data suggest that these surfaces vary from hydrophilic with θ at ∼71° to superhydrophobic with θ exceeding 150°. Plots of θ of these surfaces versus α, (cos θ_R – cos θ_A), and the contact angle hysteresis (θ_A – θ_R) all yield scattered plots, indicating that there is little correlation between θ and α, (cos θ_R – cos θ_A) and (θ_A – θ_R). Since the later three parameters have been mentioned to relate to adhesion semiempirically between a liquid droplet and the contacting surface, the present work demonstrates with generality that contact angle indeed does not relate to adhesion. This is consistent with a known but not well recognized fact in the literature. In this work, we study both the wetting and adhesion forces between water and these 20 surfaces on a microelectromechanical balance (tensiometer). When the water drop first touches the surface, the attractive force during this wetting step was measured as the “snap-in” force. The adhesion force between the water drop and the surface was measured as the “pull-off” force when the water drop separates (retracts) from the surface. The snap-in force is shown to decrease monotonously as θ_A decreases and becomes zero when θ_A is >150°. The very good correlation is not unexpected due to the similarity between the wetting and the “snap-in” process. The analysis of the pull-off force data is slightly more complicated, and we found that the quality of the water–surface separation depends on the surface “adhesion”. For surfaces that show strong adhesion with water, there is always a small drop of water left behind after the water droplet is pulled off from the surface. Despite this complication, we plot the pull-off force versus α, (cos θ_R – cos θ_A) and (θ_A – θ_R), and found very little correlation. On the other hand, the pull-off force is found to correlate well to the receding contact angle θ_R. Specifically, pull-off force decreases monotonically as θ_R increases, suggesting that θ_R is a good measure of surface adhesion. Very interestingly, we also observe a qualitative correlation between θ_R and the quality of the pull-off. The pull-off was found to be clean, free of water residue after pull-off, when θ_R is >∼90° and vice versa. The implications of this work toward surface contact angle measurements and print surface design are discussed.

Any desired surface wettability of a plastic surface can be produced by changing the concentration of the plasma gas, which here is a mixture of oxygen and a compound which includes fluorine. In the plasma treatment, the use of a third electrode consisting of a metal mesh for ion trapping can significantly decrease the etching effect. The plastic surface wettability, given by the contact angle of a water drop, does not have any direct relationship with the surface roughness due to etching in this experiment.

Despite the recognition of several sources of variation of contact angle, both between and within sessile drops on plane substrates, no comment has ever been made on the statistical treatment of observed angles, especially those around single drops. Circumperipheral observations are suggested as essential, and analysis using cos theta in an autocorrelation model is proposed as a general means of handling such data.

An atmospheric pressure non-equilibrium plasma (APNEP) developed in the UK by EA Technology Ltd is currently being investigated in collaboration with the University of Surrey. Of the many applications of surface modification that can be induced using plasmas, adhesion enhancement is one of the most commercially important. In this paper, we illustrate the use of an atmospheric plasma to enhance the adhesion characteristics of low-density polyethylene (LDPE) and poly(ethylene terephthalate) (PET). The polymers were treated in the remote afterglow region of an atmospheric pressure plasma to avoid the thermal effects that can cause degradation for thermally sensitive materials when placed in direct contact with the plasma. Reactive (oxygen containing) and inert (oxygen free) atmospheric plasmas rapidly impart adhesion enhancement by a factor of two to ten as measured by 180° peel tests. However, extended exposure to the atmospheric plasma does not impart additional adhesion enhancement as the surface is ablated revealing the underlying polymer with poor adhesive characteristics. In contrast, vacuum plasma treated LDPE and PET show increased adhesion with extended plasma treatment. An adhesion enhancement in excess of two to three orders of magnitude was found to be achievable for vacuum plasma treatment times greater than 10 min.

In recent work, we found that the stability of the negative corona charge in radio frequency plasma treated polytetrafluoroethylene (PTFE) films (18 µm thickness) strongly depends on the plasma sources, the exposure time and the condition of the film in the plasma, i.e. the film orientation on the holder and whether the film is one-sided metallized or non-metallized, as well as the film side for corona charged. Using Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy, we conclude that two factors affect the negative charge stability: oxide formed on the surface and positive charges trapped in the film. The oxides serve to retain the negative corona charges and the plasma-generated positive charges recombine with the negative corona charges and cause the corona charge discharge after heating.

Atmospheric pressure plasmas are commonly used to improve the wetting and adhesion properties of polymers. In spite of their use, the mechanisms for achieving these properties are unclear. In this regard, we report on a computational investigation of the gas phase and surface kinetics during humid-air corona treatment of polypropylene (PP) and the resulting modification of its surface properties while varying energy deposition, relative humidity (RH), web speed, and gas temperature. Using results from a global plasma chemistry model validated against experiments, we found that increasing energy deposition increased the densities of alcohol, carbonyl, acid, and peroxy radicals on the PP surface. In doing so, significant amounts of gas phase O₃ and N_xO_y are produced. Increasing the RH increased the production of peroxy and acid groups, while decreasing those of alcohol and carbonyl groups. Production of O₃ decreased while that of HNO₃ increased. Increasing the temperature decreased the concentrations of alcohol, carbonyl, and acid groups on PP while those of the peroxy radicals increased. For a given energy deposition, higher web speeds resulted in decreased concentrations of alcohols, peroxy radicals, carbonyl, and acid groups on PP.

A general framework for the physical description of partial discharge (PD) processes is presented that holds for different types of PD causing defects. A PD process is treated as a stochastic process consisting of short duration discharges (point-like in time) and charge carrier drift/recombination intervals between these discharges. It is determined by few basic physical parameters and, in a stochastic process framework, can be described in a closed form by a master equation. Since usually only the fast discharges can be measured as PD signals, a restricted possibility of observing a PD process results. The link between the stochastic process and observable quantities is derived.

A specific type of measurements is reported, the so-called phase-resolved partial discharge (PRPD) patterns. Here the total charge transferred during a discharge and the time or alternating current phase at which the discharge occurs are measured. Thus each discharge event is described by the two quantities, charge and phase angle. The modelling of the observation process is explicitly derived for this case. However, the used method can easily be generalized to other types of PD measurements.

The proposed approach yields new possibilities for the interpretation and analysis of PD patterns. Features of PD patterns can be derived analytically from the process parameters. Conversely, quantitative information about the discharge physics can be gained from measured patterns. Some limiting cases of model parameter values leading to typical pattern features are discussed explicitly.

Examples are presented that demonstrate the applicability of the model for three different discharge types (internal discharge in a gas-filled void, surface discharge in oil, corona in air).

A theory for the current and light pulses of positive glow corona from a point in air is presented; this phenomenon was first observed as an apparently continuous glow by Michael Faraday. Results are obtained, in concentric sphere geometry, for air at atmospheric pressure, by solving the continuity equations for electrons, positive ions, negative ions and metastable oxygen molecules, coupled with Poisson's equation. A series of ‘saw-toothed’ current pulses of period about is predicted with a DC current level. Accompanying the current peaks are discrete pulses of light 30 ns wide. Successive ‘shells’ of positive ions, from successive current pulses, carry 96% of the mean current. The mean current - voltage relationship has the classic square-law form. The seed electrons required for successive pulses are detached from negative ions by metastable oxygen molecules. Photo-ionization is crucial for the discharge at the anode and for the formation of negative ions throughout the gap. The pulse frequency varies with applied voltage and is found to be approximately proportional to the positive-ion mobility. The surface electric field at the central electrode remains close to Peek's onset field. The origin of onset streamers is explained and sub-microsecond voltage pulses are found to produce streamers. The results for concentric-cylinder electrodes are described briefly.

Positive and negative dc corona discharges in CO-air and -air mixtures were applied. Natural humid air was used. The step by step development with time of the formation of gas products after the action of the corona discharge was measured in situ. The discharge tube was situated in an IR gas cell. The IR absorption spectra were scanned from the area of the inter-electrode distance in successive time steps of the action of the discharge (about 1 min). Measurements were performed for three combinations of electrode materials, namely Mo-stainless steel, Mo-brass and Cu-brass. Reflection IR absorption spectra from the surfaces of the electrodes used were scanned after the action of the discharge. The influence of the electrode material on the development with time of the reaction products was observed. Polymer-metal complexes with possible catalytic activity are formed on the surfaces of electrodes. From measurements it resulted that the discharge processes consist of simultaneously acting volume processes of plasmochemical nature (probably initiated by electrons) and electrocatalytic surface processes on electrodes (probably initiated by photons).

This paper is aimed at calculating the electric field in the point-to-plane electrode system with the plate covered with a dielectric layer. With charge accumulation on the dielectric surface by corona discharge, the field in the dielectric is increased at the expense of a decrease in the gas gap. The charge accumulation on the dielectric surface proceeds to the maximum possible value when the normal component of the surface field vanishes. With the dielectric layer fully-charged, the percentage decrease of the field in the gas gap is maximum at the dielectric surface and declines along the gap axis to vanish at the point tip. The percentage decrease of the field becomes more pronounced with the increase of the diameter of the dielectric layer. The effect of inter-electrode spacing and the dielectric layer thickness on the field distribution is investigated. An accurate method of charge simulation was used for field calculation irrespective of the thickness of the dielectric layer and the gap geometry. With ion flow along the flux lines from the stressed point to the ground plane, the field enhancement factor increases and the volume charge density decreases along the flux lines. The voltages of the ion flow threshold and corona quenching are calculated and compared with previous measurements. The method of calculation is extended to calculate how high the surface potential of the charged dielectric needs to be to trigger a micro-spark in the electrostatic discharges from grounded point electrodes.

The relative importance of atmospheric negative ions in corona formation and breakdown in a radial electric field has been investigated, with special reference to effects occurring at an insulating surface of a cycloaliphatic polyurethane with dolomite filler. The inception times, corona charge, light emission (by UV photography) and sparkover voltages under lightning impulses have been recorded under natural atmospheric conditions and under the influence of negative ions introduced from an auxiliary corona. The presence of excess negative ions on the surface is shown to increase both the charge injected and the mean radius of the corona, attributable to an augmentation of streamer discharges. In similar experiments in air, the excess ions cause a transition to a `glow' discharge. Effects of negative ions on sparkover are not significant on the insulator surface, but they cause a small increase in air. Comparisons between the two cases lead to the conclusion that the most important effect of the ions on the surface is to provide `seed' electrons for streamer propagation, following ion detachment in the field of advancing streamer tips.

This paper describes theoretical and experimental investigations of the effect of an electrode coating on the onset voltage of a corona on negatively stressed electrodes. Dielectric-coated hemispherically-capped rod-to-plane gaps positioned in air are investigated. The onset voltage is calculated based on the self-recurring single electron avalanche developed in the investigated gap. Accurate calculation of the electric field in the vicinity of a coated rod and its correlation to the field values near a bare rod of the same radius are obtained using the charge simulation method. The calculated field values are utilized in evaluating the onset voltage of the corona. Also, laboratory measurements of the onset voltage on bare and coated electrodes are carried out. The effects of varying the field nonuniformity, the coating thickness and its permittivity on the onset voltage values are investigated. The results show that coating the electrodes with a dielectric material is effective in increasing the onset voltage of the corona on its surface. The calculated onset voltage values for coated and bare electrodes agree satisfactorily with those measured experimentally.

Porous polytetrafluoroethylene films were positively or negatively corona-charged at room or elevated temperatures and their charge-storage behaviour was investigated by means of isothermal surface-potential and thermally stimulated discharge-current measurements. In addition, electron micrographs of the sample morphology were taken and the influence of high humidities on the surface-charge decay was investigated. For comparison, nominally non-porous polytetrafluoroethylene films were studied in the same manner. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities if the relative humidity is not too high. Further investigations are under way in order to better understand this behaviour and to employ it for electret applications.

An 18 μm nonmetallized polytetrafluoroethylene (PTFE) film is treated in radio frequency (RF) plasma before a point-to-grid corona charged. The isothermal (170°C) surface potential measurement shows that the surface charge stability is significantly dependent on the plasma sources and treatment conditions. Oxygen (O₂), oxygen/helium (O₂/He) mixture gases and helium (He) plasma treatment enhance the film negative charge stability significantly but not hydrogen (H₂) plasma. Electron spectroscopy chemical analysis confirms that this superior negative charge retention for O₂ plasma treatment is a result of the high concentration of oxide groups on the subsurface during the plasma treatment.

Interest has grown over the past few years in applying atmospheric pressure plasmas to plasma processing for the benefits this can offer to existing and potential new processes, because they do not require expensive vacuum systems and batch processing. There have been considerable efforts to efficiently generate large volumes of homogeneous atmospheric pressure non-thermal plasmas to develop environmentally friendly alternatives for surface treatment, thin film coating, sterilization, decontamination, etc.

Many interesting questions have arisen that are related to both fundamental and applied research in this field. Many concern the generation of a large volume discharge which remains stable and uniform at atmospheric pressure. At this pressure, depending on the experimental conditions, either streamer or Townsend breakdown may occur. They respectively lead to micro-discharges or to one large radius discharge, Townsend or glow. However, the complexity arises from the formation of large radius streamers due to avalanche coupling and from the constriction of the glow discharge due to too low a current. Another difficulty is to visually distinguish many micro-discharges from one large radius discharge. Other questions relate to key chemical reactions in the plasma and at the surface. Experimental characterization and modelling also need to be developed to answer these questions.

This cluster collects up-to-date research results related to the understanding of different discharges working at atmospheric pressure and the application to polymer surface activation and thin film coating. It presents different solutions for generating and sustaining diffuse discharges at atmospheric pressure. DC, low-frequency and radio-frequency excitations are considered in noble gases, nitrogen or air. Two specific methods developed to understand the transition from Townsend to streamer breakdown are also presented. They are based on the cross-correlation spectroscopy and an electrical model.

An atmospheric pressure non-equilibrium plasma (APNEP) has been developed in the UK by EA Technology Ltd and is currently being investigated in collaboration with the University of Surrey. The main focus is the use of atmospheric pressure plasmas to modify the surfaces of commercially important polymers including polyolefins, poly(ethylene terephthalate) and poly(methyl methacrylate). These surface modifications include surface cleaning and degreasing, oxidation, reduction, grafting, cross-linking (carbonization), etching and deposition. When trying to achieve targeted surface engineering, it is vital to gain an understanding of the mechanisms that cause these effects, for example, surface functionalization, adhesion promotion or multi-layer deposition. Hence comparisons between vacuum plasma treated surfaces have also been sought with a view to using the extensive vacuum plasma literature to gain further insight. In this paper, we will introduce the APNEP and compare the key characteristics of the plasma with those of traditional vacuum plasma systems before highlighting some of the surface modifications that can be achieved by using atmospheric plasma. Data from the analysis of treated polymers (by spectroscopy, microscopy and surface energy studies) and from direct measurements of the plasma and afterglow will be presented. Finally, our current understanding of the processes involved will be given, particularly those that are important in downstream surface treatments which take place remote from the plasma source.

In this paper, we present data on the physics and phenomenology of plasma reactors based on the One Atmosphere Uniform Glow Discharge Plasma (OAUGDP^™) that are useful in optimizing the conditions for plasma formation, uniformity and surface treatment applications. It is shown that the real (as opposed to reactive) power delivered to a reactor is divided between dielectric heating of the insulating material and power delivered to the plasma available for ionization and active species production. A relationship is given for the dielectric heating power input as a function of the frequency and voltage at which the OAUGDP^™ discharge is operated.

The surface tension of a model, water-based, flexographic printing ink was measured at a range of surfactant concentrations along with the equilibrium contact angle formed with polymer substrates. The surface excess of surfactant at each concentration was calculated using the Gibbs adsorption isotherm and assumed equal to the concentration of surfactant at the interface. The change in the surface tension of the ink formulation was assumed to be determined entirely by the surface concentration of surfactant. This allowed the estimation of the surface tension at the solid–liquid and solid–vapour boundaries when in contact with substrate based on the values obtained for pendant drops. The associated polar and dispersive contributions to the surface tension were then calculated using the Young–Dupré equation. The values of the polar and dispersive surface tension components extracted in this manner were compared with those calculated using the approach of van Oss, Chaudhury and Good. The use of surface excess in estimating the contributions to surface tension was found to give far better agreement with experimental data than the van Oss approach which is intended for use with pure liquids.

Various carbon films which have been produced in plasma have been fluorinated so that their surface structures could be analysed. Every fluorinated carbon film shows a characteristic contact angle change which follows its surface structure. These samples can be classified into two groups: amorphous and crystal, from comparison of the ratios of the area due to fluorine in the C_1s peak to the total area of the C_1s peak in ESCA results.

For the authors' previous work see ibid., vol.10, p.487 (1977). Further measurements are reported on the decay of surface potential of negative corona charged polyethylene films and on the crossover effect reported earlier by Ieda and co-workers (1967). It is shown that when the duration of charging is short ( approximately 25 ms) the subsequent decay curves of surface potential are well-behaved and do not exhibit the crossover effect even though the initial surface fields are high. Experiments are also reported in which an air stream is directed along the surface of the films while corona charging. This also removes the crossover effect and is in agreement with results reported in which an air stream is directed along the surface of the films while corona charging. This also removes the crossover effect and is in agreement with results reported by Okumura (1976) for polystyrene-hexamethacrylate. It is concluded that excited molecules as well as photons produced in the corona discharge play an important role in inducing charge from surface states to enter the bulk of the polymer where they are much more mobile. This leads to rapid decay of surface potential at higher initial surface fields and the crossover effect is then observed.

There has been considerable interest in non-thermal atmospheric pressure discharges over the past decade due to the increased number of industrial applications. Diverse applications demand a solid physical and chemical understanding of the operational principals of such discharges. This paper focuses on the four most important and widely used varieties of non-thermal discharges: corona, dielectric barrier, gliding arc and spark discharge. The physics of these discharges is closely related to the breakdown phenomena. The main players in electrical breakdown of gases: avalanches and streamers are also discussed in this paper. Although non-thermal atmospheric pressure discharges have been intensively studied for the past century, a clear physical picture of these discharges is yet to be obtained.

Three different dielectric barrier-controlled discharge regimes in helium at atmospheric pressure under sinusoidal excitation have been obtained by varying the excitation frequency or the gas chemical composition: the filamentary discharge, which is the discharge that is usually obtained; the glow discharge, which is controlled by cathode secondary emission; and the homogeneous discharge, which is of a nature in between those of the filamentary and the glow discharges. All the characteristics that have been studied, such as the discharge current, the emission spectrum, the wettability and the chemical transformations of a polypropylene film, are related to the discharge-regime variation. The glow discharge is clearly more efficient than the others as a means of increasing the polypropylene-surface energy. Values as high as 62 mJm^-2 are obtained with this discharge whereas the maximum value after interaction with the filamentary one is 45 mJm^-2. This improvement in wettability is due to there being more O atoms implanted at the surface as well as to the addition of N atoms. The differences among in surface transformations have been correlated to the characteristics of these different discharges and more specifically to the localization of the electrical energy transfer into the gas and to the nature of the ions created during the discharge.

A study on oxygen-plasma treatment of ink-jet paper is presented. Paper was exposed to a weakly ionized, highly dissociated oxygen plasma with an electron temperature of 5 eV, a positive-ion density of 8 × 10¹⁵ m⁻³ and a density of neutral oxygen atoms of 5 × 10²¹ m⁻³. Optical emission spectroscopy (OES) was applied as a method for detection of the reaction products during the plasma treatment of the paper. OES spectra between 250 and 1000 nm were measured continuously during the plasma treatment. The wettability of the samples before and after the plasma treatment was determined by measuring the contact angle of a water drop. The appearance of the surface-functional groups was determined by using high-resolution x-ray photoelectron spectroscopy (XPS), while changes in the surface morphology were monitored with scanning electron microscopy (SEM). Already after 1 s of the plasma treatment the surface, which was originally hydrophobic, changed to hydrophilic, as indicated by a high absorption rate of a water drop into the paper. The OES showed a rapid increase of the CO and OH bands for the first few seconds of the plasma treatment, followed by a slow decrease during the next 40 s. The intensity of the O atom line showed reversed behaviour. The XPS analyses showed a gradual increase of oxygen-rich functional groups on the surface, while SEM analyses did not show significant modification of the morphology during the first 10 s of the plasma treatment. The results were explained by degradation of the alkyl ketene dimer sizing agent during the first few seconds of the oxygen-plasma treatment.

Plasma polymer surface modification is widely used in the biomedical field to tailor the surface properties of materials to improve their biocompatibility. Most of these treatments are performed using low pressure plasma systems but recently, filamentary dielectric barrier discharge (FDBD) and atmospheric pressure glow discharge (APGD) have appeared as interesting alternatives. The aim of this paper is to evaluate the potential of surface modifications realized with FDBD and APGD in different atmospheres (N₂+ H₂ and N₂+ NH₃ mixtures) on poly(tetrafluoroethylene) to determine the relative influence of both the discharge regime and the gas nature on the surface transformations. From XPS analysis, it is shown that the discharge regime can have a significant effect on the surface transformation; FDBDs operating in H₂/N₂ lead to a high concentration of amino-groups with high specificity but also important damaging on the surface. Glow discharges in both H₂/N₂ and NH₃/N₂ lead to lower concentrations of amino-groups with lower specificity but lower surface damaging. Therefore, this simple surface treatment seems to be an effective, low cost method for the production of uniform surface modification with amino-groups that can subsequently be used to graft various chemical functionalities used for biomaterial compatibility.

An atmospheric pressure glow discharge (APGD) was used for surface modification of polyethylene (PE) and polypropylene (PP). The discharge was generated between two planar metal electrodes, with the top electrode covered by a glass and the bottom electrode covered by the treated polymer sample. The discharge burned in pure nitrogen or in nitrogen-hydrogen or nitrogen-ammonia mixtures. The surface properties of both treated and untreated polymers were characterized by scanning electron microscopy, atomic force microscopy, surface free energy measurements and x-ray photoelectron spectroscopy. The influence of treatment time and power input to the discharge on the surface properties of the polymers was studied. The ageing of the treated samples was investigated as well. The surface of polymers treated in an APGD was homogeneous and it had less roughness in comparison with polymer surfaces treated in a filamentary discharge. The surface free energy of treated PE obtained under optimum conditions was 54 mJ m^-2 and the corresponding contact angle of water was 40° the surface free energy of treated PP obtained under optimum conditions was 53 mJ m^-2 and the contact angle of water 42°. The maximum decrease in the surface free energy during the ageing was about 10%.