Polyacrylonitrile fibers were treated with a nitrogen glow-discharge plasma. The surfaces of untreated and treated fibers were examined with contact angle measurements, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Surface energy calculations of the fibers were carried out from contact angle measurements using the relationships developed by Fowkes. It is found that plasma treatment causes a reduction in water contact angle on the fiber surfaces. The dispersion component of surface energy changes slightly, while the polar component is increased significantly from 14.6 mN/m to 58.7 mN/m and the total surface energy increase is 139%. The increase of surface energy is mainly caused by the introduction of hydrophilic groups on the fiber surfaces after plasma treatment.

Influence of the unit energy (E_u) of corona discharge used for modification of pure polylactide (PLA) and polylactide nanocomposite (PLAC) containing 5 wt% of an aluminosilicate nanofiller (Cloisite 30B) on water (Θ_W) and diiodomethane (Θ_D) contact angles as well as on surface free energy (γ_s) of these polymers was studied. Θ_W and Θ_D as advancing contact angles were measured with use of a goniometer while γ_s was calculated by the Owens–Wendt method. It was found that Θ_W increased with the rising E_u while Θ_D remained approximately constant. Assuming E_u = const, it could be stated that the increase in γ_s was much more evident for PLA than for PLAC. This increase resulted practically from the change in the polar component of γ_s because the dispersive component for the two materials only slightly decreased with increase in E_u.

In this paper, a dielectric barrier discharge (DBD) operated at (sub)atmospheric pressure in a 95/5% He/CF₄ mixture is employed to increase the hydrophobicity of a poly(ethylene terephthalate) (PET) film. This paper studies the influence of different operating parameters on the hydrophobic properties of the PET film using contact angle measurements. Results clearly show that the hydrophobicity of the PET film is only enhanced when using large gas flows. Moreover, this work demonstrates that operating pressure and discharge power have a significant influence on the rate of plasma modification as well as on the uniformity of the plasma treatment. Also important to mention is that no ageing effect is observed. As a result, one can conclude that the utilized DBD is an efficient tool to create stable, hydrophobic PET surfaces.

Adhesion of epoxy resins on copper foils for printed circuit board (PCB) applications is improved by nearly a factor of 5, using surface cleaning and deposition of a 15-nm-thick film in a low-pressure remote plasma-enhanced chemical vapor deposition process. The cleaning pretreatment, using an N₂–O₂ oxidizing gas mixture with moderate heating (343 K), gives the best results. This pretreatment removes the carbonaceous contaminants present on the topmost surface of the sample and slightly oxidizes the copper into CuO. This oxide is then reduced during the deposition treatment, presumably by reaction with the aminopropyltrimethoxysilane (APTMS) precursor. The surface roughness is unchanged after treatment, thereby showing that the improvement of the copper/epoxy adhesion is only due to the chemistry of the plasma coating. Applying these results to dielectric barrier discharges allows us to achieve the same level of adhesion, which, therefore, does not depend on the process.

Polytetrafluoroethylene (PTFE) surfaces were treated by oxygen and nitrogen species generated either in a remote (filtered) RF plasma or in an ion gun. In the first case, the majority of the species reaching the surface are neutral molecules, whereas in the second case, ions are the reactive agent. In this paper, we show that ions alone do not lead to a significant grafting of new functions on the PTFE surface. The XPS analysis of the treated surface show identical behaviour with oxygen and nitrogen ion treatment, and the evolution of the C1s peak shape suggest a progressive sputtering, leading to defluorination of the surface. The nitrogen plasma treatment lead to a subsequent grafting that is attributed mostly to the “excited neutrals”, but we suggest here that the ions could play a significant role in the activation process of the surface. The exposure of PTFE to an oxygen plasma lead to chemical etching of the surface, different from the physical sputtering induced by the ion treatment, that lead to a super-hydrophobic behavior of the surface attributed to an increase in the surface roughness.

Plasma, considered as the fourth state of matter, is playing a key role as a modern discipline. Plasma processing is drawing attention from various technology sectors such as microelectronics, automotive, and surface modifications of polymers. Some examples of additional new applications include functional coatings for architectural glass, mercury-free lamps, plasma-treated packaging for food, beverage and pharmaceutical industries, as well as nanomaterials. With the emergence of all new technological applications from basic research in academic, industrial, or government laboratories, plasma is set to have a brilliant future.

Low-pressure plasma generated in a typical parallel plate reactor and atmospheric pressure plasma produced by a plasma needle were utilized to modify the surface of poly(styrene–butadiene–styrene) (SBS) elastomers. An RF discharge (13.56 MHz) in helium was used in the both cases. The SBS surfaces were investigated by T-peel tests, contact-angle measurements, and IRS–FTIR spectroscopy. It has been found that such plasma treatments drastically improve the strength of adhesive-bonded joints between the SBS surfaces and polyurethane adhesives, however, the plasma needle operation has turned out to be more effective. The molecular processes proceeding on the SBS surfaces have been briefly discussed.

An environmentally friendly plasma amination process for the activation of polymers prior to electroless metallization using dielectric barrier discharges (DBD) at atmospheric pressure was investigated. One focus of the work was on the correlation between plasma parameters and palladium coverage on the polymer on the one hand and the palladium coverage and adhesion of a galvanic copper metallization on the other hand. Using XPS spectroscopy it was found that a DBD treatment of polyimide (PI) films with mixtures of N₂ and H₂ leads to considerably higher Pd surface concentrations than on untreated reference samples or foils treated in air-DBD. The Pd coverages achieved result in peel strengths of a copper metallization of up to 1.4 N · mm⁻¹.

Polyethylene (PE) samples were activated by an atmospheric pressure plasma jet. The improvement in adhesive bond strength is attributed to the incorporation of oxygen-containing functional groups into the PE surface. Optical emission spectroscopy in combination with XPS analysis shows differences in the surface reactions for a plasma jet operated with air or pure nitrogen. The results indicate that the surface modifications take place in two different environments with respect to location and time: (a) reactions while the substrate is hit by the plasma jet, and (b) reactions outside the plasma jet after the treatment.

The polyethylene (PE) coatings could be very promising for various branches of industry due to their chemical stability and impact resistance. Plasma modification of powder has recently attracted much interest because of new prospects to control the interfacial properties. Plasma modification also significantly enhanced the adhesion of the polymer to the substrate. Powders find wide application in various branches of industry like paintings, biotechnology, filling for composite materials etc., but the plasma modification of powder surface has not found such application as plasma modification of flat solid materials. This is due to problems connected with the three dimensional geometry, necessity of solid mixing (due to the aggregation phenomenon) and the large surface area of powders which should be treated. We investigated plasma modification of PE powder, its adhesion properties on steel surface and mechanism influencing this adhesion. PE powder was modified using various working gases and chemicals. It was found that adhesion properties were strongly influenced by concentration of oxygen containing groups and also by PE crosslinking after modification. The value of crosslinking depends on used working gas and chemicals. The ternary mixture of O₂/H₂O/methanol was found to be an appropriate working gas for plasma treatment of PE for adhesion purposes. The treated PE had good wettability, low crosslinking and very high adhesion to the steel substrate.

After the impact of the great review of M. Strobel and C. S. Lyons on contact angle measurements, we discuss some claims of the authors. The Wilhelmy method is not generally “the best technique for measuring the contact angle hysteresis” as the authors claimed. Otherwise, we think that, even though equilibrium contact angle is an “unattainable” angle, the most-stable contact angle obtained from the system relaxation is experimentally accessible. The most-stable contact angle is energetically significant for evaluating quantitatively the surface energy value of rough, chemically homogeneous surfaces from the Wenzel equation, and the average surface energy of smooth, chemically heterogeneous surfaces from the Cassie equation. The most-stable contact angle, the advancing contact angle and the receding contact angles enable the thermodynamic description of the range of contact angle hysteresis and the distribution of metastable system configurations.

The potential of contact angle measurements (CAM) as an analytical tool to characterize surface treatments or modifications is often not fully exploited. Agreeing with Strobel and Lyons, comparing contact angles is often much more reasonable than comparing deduced data like surface energies, because the latter are based on models, in turn involving the influence and knowledge of intermolecular forces at the respective interfaces. For a comprehensive picture, the measurement of contact angles itself has to be considered together with the appropriate model and the available techniques to carry out CAM. An appropriate measurement procedure will be given and a brief discussion of some models to derive free surface energy from CAM.

In this commentary we discuss the assay by M. Strobel and C. S. Lyons on contact angle measurements, critical and popular topic in surface/plasma science community. We agree with stressing the importance of dynamic contact angle measurements (i.e. the evaluation of both advancing and receding). However, we make some remarks about the meaning of angle hysteresis with particular regard to the concepts of roughness and chemical heterogeneity, on the basis of our experience in hydrophobic and super-hydrophobic surfaces. Further, we describe our different point of view in the dispute between Wilhelmy balance and sessile drop methods.

Contact angles are used to solve research and manufacturing problems in an industrial environment. Contact angle measurements are scientific, readily acquired using relatively low-cost instruments and simple procedures, are agreeable for use in environments from academic research laboratories to industrial manufacturing facilities, and are an extremely powerful method for characterizing surfaces. The measurement of dynamic contact angles is rate-dependent at high capillary numbers. Water is a preferred probe liquid for contact angle measurements not only because of the importance of aqueous systems in science and industry, but also because water has the highest surface tension of any commonly available probe liquid and therefore has measurable contact angles on most polymeric materials. Most theories of solid surface energy have a basis in Young's equation, which employs the equilibrium contact angle. If surface energy or surface energy component calculations are made, both the advancing and the receding contact angle data should be used in those calculations.

An atmospheric pressure oxygen and helium plasma was used to activate the surface of poly(methyl methacrylate) (PMMA). The plasma physics and chemistry was investigated by numerical modeling. It was shown that as the electron density of the plasma increased from 3 × 10¹⁰ to 1 × 10¹² cm⁻³, the concentration of O atoms and metastable oxygen molecules (¹Δ_g) in the afterglow increased from 6 × 10¹⁵ to 1 × 10¹⁷ cm⁻³. Exposing PMMA to the afterglow for times between 0 and 30 s led to a 35° ± 3° decrease in water contact angle, and a ten-fold increase in bond strength to several adhesives. X-ray photoelectron spectroscopy of the polymer revealed that after treatment, the surface carbon attributable to the methyl pendant groups decreased 5%, while that due to carboxyl acid groups increased 7%. The numerical modeling of the afterglow and experimental results indicate that oxygen atoms generated in the plasma oxidize the polymer chains.

In this work, an experimental investigation of fluorine gas (F₂) plasma treatment of polypropylene (PP) film reveals the evolution of PP fluorination. Surface analysis of fluorinated PP surfaces describes a surface modification process that is initially quite rapid but slows sharply as the fluorination progresses. The fluorination reaction occurs more rapidly at the PP film surface and evidence of a treatment gradient is seen in the ESCA sampling depth of 10 nm. The increasingly fluorinated surface becomes less reactive to the plasma chemistry and develops a fully fluorinated, cross-linked surface layer that eventually extends the full ESCA sampling depth.

The surface modification of polyethylene powder using a plasma reactor based on a dielectric barrier discharge in air at atmospheric pressure and ambient temperature is investigated. The process is inexpensive, and the necessity of any vacuum equipment and technical gases is alleviated. The efficiency of the modification process was successfully demonstrated by ESCA measurements that proved formation of new functional groups at the modified powder surface. The modification effect was also evaluated by means of dynamic capillarity rising measurements. Powder capillarity tests proved significant powder capillarity changes. The reduction of the modification effect was also limited (max. reduction of about 20% during 1 100 d after the modification date).

Underwater plasma and glow discharge electrolysis are interesting new methods for polymer surface functionalization. The achievable content of O-containing functional groups exceeds that of oxygen glow discharge gas plasmas by a factor of two (up to ca. 56 O/100 C). The percentage of OH groups among all O-containing groups can reach 25 to 40%, whereas it is about 10% in the gas plasmas. Addition of hydrogen peroxide increases the fraction of OH groups to at most 70% (27 OH/100 C). The liquid plasma systems are also able to polymerize acrylic acid and deposit the polymer as very thin film on substrate surfaces or membranes, thereby retaining about 80% of all COOH functional groups (27 COOH/100 C).

Non-thermal atmospheric pressure plasma jets/plumes are playing an increasingly important role in various plasma processing applications. This is because of their practical capability to provide plasmas that are not spatially bound or confined by electrodes. This capability is very desirable in many situations such as in biomedical applications. Various types of ‘cold’ plasma jets have, therefore, been developed to better suit specific uses. In this paper a review of the different cold plasma jets developed to date is presented. The jets are classified according to their power sources, which cover a wide frequency spectrum from DC to microwaves. Each jet is characterized by providing its operational parameters such as its electrodes system, plasma temperature, jet/plume geometrical size (length, radius), power consumption, and gas mixtures used. Applications of each jet are also briefly covered.

Combined surface treatments using plasma fluorination and surface roughening were applied to investigate whether they could increase the peel property of PET beyond the value needed for use as a release coating of pressure-sensitive adhesive tapes. The peel strength of PET treated with CF₄/He APG plasmas decreased to approximately 100 N · m⁻¹, but not quite to the ideal value of PTFE, 20 N · m⁻¹. We also prepared PET with a rough surface (matte PET) to examine the effect of surface roughening. The matte PET peel strengths were decreased by plasma fluorination; the roughest matte PET showed even lower peel strength than PTFE. We conclude that the combined treatments could be effective in the formation of a surface with high peel property on PET.

PTFE surface modifications have been realized using low-pressure RFGD, DBD and APGD in different atmospheres. Compared to the RFGD NH₃ plasma, the DBDs operating in H₂/N₂ lead to similar surface concentrations of amino groups and similar surface damage, but with a much higher specificity. Both APGDs in H₂/N₂ and NH₃/N₂ lead to lower concentrations of amino groups, but with similar specificity, and with lower surface damage than the RFGD treatment. A method is proposed to evaluate the efficiency of the different discharges for amine surface functionalization of PTFE, and it is concluded that the NH₃/N₂ APGD discharge is the one that give the best results for an effective surface treatment.

Polyolefin surfaces, namely isotactic poly(propylene) (iPP) and low-density polyethylene (LDPE), were modified by corona discharge plasma. The chemical changes on the modified surfaces were observed, deeply affecting the surface and the adhesive properties of the studied materials. The hydrophobic recovery in the case of iPP is considerably dependent on the polymer crystallinity. The presence of the processing agents in the LDPE has a significant influence on the surface hydrophobization dynamics.

Surface modification induced on poly(ethylene terephthalate) (PET) fibers by cold SF6 plasma treatment has been investigated systematically as a function of plasma device parameters. The observed wettability modifications of fibers plasma-treated under different operating conditions were correlated to their dyeability modifications and to the changes in surface chemical composition, determined by X-ray Photoelectron Spectroscopy (XPS), and topography, investigated by atomic force microscopy (AFM). Optical emission spectra from the SF₆ plasma at different pressures gave information on its content of fluorine atoms. A striking transition was observed between the increased hydrophilicity and high dyeability, imparted by plasma treatment at low pressure (<0.2 mbar), mainly as a consequence of surface etching and surface activation, and the increased hydrophobicity, imparted by plasma treatment in the higher pressure regime (0.2–0.4 mbar), consequent to extended surface fluorination.

Atomic force microscopy (AFM), contact-angle measurements, and X-ray photoelectron spectroscopy (XPS or ESCA) were used to characterize biaxially oriented poly(propylene) (PP) films modified by exposure to a corona discharge. Surface analysis was performed on PP films modified at various corona energies to explore the changes in surface topography, wettability, and oxidation state resulting from the corona treatment. Even at low corona energies, water-soluble low-molecular-weight oxidized materials (LMWOM) are formed. These LMWOM products agglomerate into small topographical mounds that are visible in the AFM images. For the detection of LMWOM on corona-treated surfaces, AFM appears to be at least as sensitive as contact-angle measurements or ESCA. A major advantage of AFM relative to the other surface analytical techniques used to confirm the presence of the LMWOM is that no washing of the surface with water is required in conjunction with the AFM analysis.

Three polyethylene (PE) types with different branching structures were subjected to air, water and ethanol aerosol-assisted dielectric barrier discharges (DBD) for surface modification. Using the air DBD the incorporated oxygen concentration was found to be independent on the branching of PE in contrast to the introduction of OH groups, which was PE-2 > PE-1 > PE-3. For water-aerosol DBD the succession of OH concentration was in the order of PE-1 > PE-2 > PE-3. Ethanol aerosol-assisted DBD produced the lowest concentration of OH groups also independent on the branching of PE. The chemical nature of introduced oxygen functional groups was inspected by X-ray photoelectron spectroscopy (XPS) and assigned as CO, >CO/CHO/OCO and OCO.

The effect of different nebulized liquids directly introduced into the dielectric barrier discharge (DBD) was compared with simple air DBD treatment of polyethylene foils. Water, alcohols and aqueous solutions of different organic substances (environmentally compatible) and water soluble polymers were applied as aerosols and injected into the DBD zone. The DBD residence time (number of treatment cycles) and the power were varied. The durability of the surface modification effect was studied after removing of Low-Molecular Weight Oxidized Material (LMWOM) by washing the samples with water and ethanol. The modified foils were characterized by XPS and contact angle measurements as a function of the applied plasma conditions. The concentration of functional groups at modified surfaces was estimated by derivatization and subsequent XPS measurement.

In this study, the surfaces of ultrahigh molecular weight polyethylene (UHMWPE), poly(ethylene terephthalate) (PET), and polytetrafluoroethylene (PTFE) films were treated with a helium-water vapor plasma at atmospheric pressure and room temperature. Surface changes related to hydrophilicity, chemical funtionalization, surface energy, and adhesive strength after plasma treatment were investigated using water contact angle (WCA) measurements, X-ray photoelectron spectroscopy (XPS), and mechanical T-peel tests. Results indicate increased surface energy accompanied with enhanced hydrophilicity. WCA decreased by 36, 50, and 16% for UHMWPE, PET, and PTFE, respectively, after only 0.4 s treatment. For UHMWPE, it is shown that the surface functionalization can be tailored depending on the plasma exposure time. Aging studies performed for these three polymers show the stability of the surface groups as indicated by a small increase in WCA values of plasma treated samples which can be attributed to cross-linking of surface and subsurface polymer chains. XPS analysis of the surfaces show increased oxygen content via the formation of polar, hydroxyl-based functional groups. Furthermore, major changes in the polymer structure of PET are observed, possibly due to the opening of the aromatic rings caused by the plasma energetic species. T-peel test results show an 8, 7.5, and 400-fold increase in peel strength for UHMWPE, PET, and PTFE, respectively. Most importantly, it is shown that water-vapor based plasmas can be a promising, “green,” inexpensive route to promote the surface activation of polymers.

Film samples of low-density polyethylene (LDPE) and biaxially oriented poly(propylene) (BOPP) were surface modified by vacuum ultraviolet (VUV) irradiation using a Kr resonant lamp at λ = 123.6 nm in low-pressure ammonia gas, and were then stored in air. The time-dependence of the surface properties was monitored using several complementary surface-sensitive techniques such as contact angle goniometry (CAG), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectroscopy (ToF-SIMS), which allows one to determine the surface energy, and chemical composition at different depths. The relative importance of four possible mechanisms involved in surface hydrophobic recovery is discussed, and we show that in our particular case the main mechanism is rotational and/or translational motion of polymer chains and chain segments. This restructuring determines the observed “loss” of functional groups, which occurs within the first few monolayers of the surface (∼1 nm), as shown by the ToF-SIMS results, and which leads to the observed decrease in the surface energy. In the deeper surface regions (∼10 nm) long-lived radicals react with oxygen and water vapor upon exposure to the atmosphere, leading to an increase in the concentration of bound oxygen, as observed by XPS. Finally, CAG measurements show that the hydrophobic recovery is reversible and can be significantly reduced by cross-linking near the surface, as illustrated by depth sensing nano-indentation measurements on BOPP surfaces.

Recently, there has been increased interest in using atmospheric pressure plasmas for materials processing, since these plasmas do not require expensive vacuum systems. However, APGDs face instabilities. Therefore, special plasma sources have been developed to overcome this obstacle, which make use of DC, pulsed DC and AC ranging from mains frequency to RF. Recently, the APPJ was introduced, which features an α-mode of an RF discharge between two bare metallic electrodes. Basically, three different geometric configurations have been developed. A characterization of the APPJs and their applications is presented.

Non-thermal plasmas under atmospheric pressure are of great interest in polymer surface processing because of their convenience, effectiveness and low cost. In this paper, the treatment of Polyethylene terephthalate (PET) film surface for improving hydrophilicity using the non-thermal plasma generated by atmospheric pressure glow discharge (APGD) in air is conducted. The discharge characteristics of APGD are shown by measurement of their electrical discharge parameters and observation of light-emission phenomena, and the surface properties of PET before and after the APGD treatment are studied using contact angle measurement, x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEh4). It is found that the APGD is homogeneous and stable in the whole gas gap, which differs from the commonly filamentary dielectric barrier discharge (DBD). -4 short time (several seconds) APGD treatment can modify the surface characteristics of PET film markedly and uniformly. After 10 s APGD treatment, the surface oxygen content of PET surface increases to 39%, and the water contact angle decreases to 19°, respectively.

The experimental work reported here is devoted to the electrical study of two atmospheric pressure dielectric barrier discharge (DBD) reactors operating at high gas flow, conceived for surface treatment applications in spatial afterglow conditions. Both reactors are of coaxial geometry with the dielectric covering the active electrode, and are driven by a power generator delivering quasi-sinusoidal voltage waveforms in the 100–160 kHz range. The influence of the gas flow value and of the input power on the electrical operation of these systems is investigated. The comparative study performed here, by means of electrical measurements, reveals the influence of parameters such as geometrical dimensions and dielectric material used on the operation of the DBD. Power factor measurements are used to quantify the reactors' electrical performance. Optical diagnostics and kinetic modelling reveal a high chemical activity of the systems appropriate for the treatment of surfaces at atmospheric pressure.

In this paper, continuing previous work, we report on the installation and the testing of an experimental dielectric barrier discharge (DBD) reactor run in a controlled atmospheric pressure gaseous environment other than air. Here, the effects of a N₂-DBD treatment on the surface of a test polymer material (UHMW polyethylene) are examined, reported, discussed and compared to results obtained previously following air-DBD treatment. Surface analysis and characterization were performed using x-ray photoelectron spectroscopy, contact angle measurement and scanning electron microscopy before and following the DBD processing described. The discharge parameters used were correlated with the changes in the surface characteristics found following DBD treatments of various durations in a nitrogen atmosphere. The work focuses on the control of the gaseous environment supporting the discharge and on the possibility of overcoming the potentially dominant effect of reactive oxygen-related species, derived from any residual air present. The results obtained underline the very high reactivity of such species in the discharge, but are encouraging in respect of the possibility of the implantation or generation of functional groups other than oxygen-related ones at the surface of interest. The processing conditions concerned simulate 'real' continuous high speed processing, allowing the planning of further experiments, where various gaseous mixtures of the type X + N₂ will be used for controlled surface functionalization.