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.

The retention of chemical structure and functional groups during plasma polymerisation was investigated. Usually plasma polymer layers, prepared by continuous wave radio-frequency plasma, are often chemically irregular in their structures and chemical compositions. To minimise these irregularities, low wattages and the pulsed plasma technique were applied to avoid fragmentations. The polymerisation of vinyl and acryl-type monomers was strongly enhanced in the dark phase (plasma-off) of a pulsed rf plasma caused by the reactivity of the vinyl or acryl-type double bonds. Bifunctional monomers with acryl or allyl double bonds and also polar groups such as OH, NH2, and COOH were used to produce plasma polymers with defined (regular) structures and a high density of a single type of functional groups. The maximum yields were 30 OH, 18 NH2, 24 COOH groups per 100 C atoms. To vary the density of functional groups a chemical copolymerisation with “chain-extending” comonomers such as butadiene and ethylene was initiated in the pulsed plasma. The composition of these copolymers was investigated by XPS and IR spectroscopy. Homopolymers and copolymer layers were deposited on polypropylene (PP) foils and then aluminium was thermally evaporated. The peel force increased considerably and showed a dependence on the density of functional groups. The plasma polymer deposition was also monitored in situ by the Self-Exciting Electron Resonance Spectroscopy (SEERS) to show correlations between plasma parameters and properties of the deposited plasma polymer layers measured “quasi-in situ” by coupling the plasma chamber with an XPS spectrometer.

The ageing of thin PET films in an oxygen plasma was investigated. After several hours exposure a large decrease in mechanical strength was observed. Plasma particle bombardment, chemical reactions and the plasma vacuum UV radiation cause extensive chemical and structural changes. The chemical reactions leading to the ageing process were identified.

Different types of plasma, irradiative and chemical activation were compared in terms of surface functionalization. Corona and spark jet plasmas are characterized by low gas temperatures and high rates in surface modification. UV irradiation in the presence of ozone does not involve any particle bombardment and acts only by enhanced photooxidative processes. Although ion implantation can be avoided, this method is not free of radiative damage in both the surface-near region and the bulk of polymers. Furthermore, its functionalization rate is low. In relation to low-pressure O₂ plasma modification, all treatments mentioned here have a low efficiency in adhesion promotion due to oxidative degradation of macromolecules and formation of molecular debris known as the “weak boundary layer”.

Single-type functionalizations with different types of functional groups at polypropylene (PP) and polytetrafluoroethylene (PTFE) surfaces were achieved using, instead of a simple plasma modification, either a combined plasmachemical–chemical process or the pulsed plasma-initiated homo-or copolymerization of monomers carrying functional groups. The combined process consists of O2 plasma pretreatment and wet-chemical reduction of O functional groups to OH groups using diborane, vitride (sodium bis (2-methoxyethoxy) aluminum hydride), or LiAlH4. The high degree of retained chemical structure and functional groups during the low-power pulsed plasma polymerization was found to be a prerequisite for producing well-defined, adhesion-promoting plasma polymer layers as model surfaces with high concentrations of exclusively or predominantly one type of functional group, such as OH, NH2, or COOH. The maximum concentrations of functional groups were found to be 31 OH, 21 NH2 or 25 COOH groups/100 C atoms using allyl alcohol, allylamine, or acrylic acid, respectively, as monomers in the plasma polymerization process and 14 OH groups/100 C atoms by applying the combined O2 plasma/diborane reduction process. To vary the density of functional groups, a so-called plasma-initiated gas-phase radical copolymerization with ethylene or styrene as a “chain-extending” comonomer, or butadiene as “chemical crosslinker” was employed. The peel strength of evaporated aluminum layers on unspecifically oxygen-plasma functionalized polypropylene (PP) and polyethylene (PE) shows in each case a maximum at 20 O per 100 C atoms. Initially the peel strength increased linearly with the concentration of functional groups when PP or polytetrafluoroethylene (PTFE) substrates were coated with plasma polymers or copolymers carrying a single type of adhesion-promoting functional groups. The ranking of the adhesion-promoting effect is CH2< NH2 (OH< COOH, and corresponds to the tendency to form chemical bonds between aluminum and the different functional groups.

Plasma treatment changes the solvent absorption and permeation as well as the swelling properties of polymers. Enchanced solvent absorption and swelling are effects of an improved solvent compatibility. The plasma introduces a large number of different groups at the polymer surface depending on the nature of the plasma. Fluorine-containing plasmas can replace hydrogen atoms of the polymer molecule with fluorine atoms. Moreover, fluorine-containing plasma polymer layers can be formed. All these processes reduce the resulting surface free energy, reduce the diffusion length of solvent molecules, and produce a barrier layer. We have studied the formation of solvent barriers by plasma fluorination and by crosslinking by ultraviolet (UV) radiation. Thin foils of polypropylene (PP) and polyethyleneterephthalate (PET) were used as substrates. CF₄, SF₆, and SOF₂ were applied as sources of fluorine atoms. Hexafluoropropene, tetrafluorethylene, and perfluorohexylethylene form plasma polymer layers on the polymer substrates. Test solvents were n-pentane, tetrachloroethylene, dimethylsulfoxide, and mixtures of n-pentane and methanol.Plasma treatment changes the solve The permeation rate of solvents through plasma-modified polymers was measured gravimetrically. Mass spectrometry was applied to analyze the permeating components of the solvent mixtures. Fluorination of surface layers by plasma-chemical (CF₄, SF₆) means considerably reduces the permeation rate of PP (95% barrier effect) and PET (100%). The preferred permeation of one component of the pentane/methanol mixture is influenced by the polarity of plasma-introduced groups at the polymer surface.

The peel strength of aluminium to polypropylene and poly(tetrafluoroethylene) was determined in dependence on the type and the concentration of functional groups on the polymer surface. For this purpose the polymer surface was equipped with monotype functional groups. The first method to produce monotype functionalized surfaces was an introduction of O functional groups using an oxygen plasma treatment and converting these groups to OH groups applying a wet chemical reduction. In result of this two-step treatment the hydroxyl group concentration at the polymer surface could be increased from 3–4 to 10–14 OH groups/100 C atoms. The second method consists in the deposition of a 150 nm adhesion-promoting layer of plasmapolymers or copolymers onto the polymer surface using the pulsed plasma technique. For that purpose functional groups carrying monomers as allyl alcohol, allylamine and acrylic acid were used. Applying the plasma-initiated copolymerization and using neutral “monomers” like ethylene or butadiene the concentration of the functional groups was varied.

A correlation of peel strength with the ability of forming chemical interactions between Al atoms and functional groups was found: COOH > OH >> NH₂ > H(CH₂–CH₂).

A new concept for molecular interface design in metal–polymer systems is presented. The main features of this concept are the replacement of weak physical interactions by strong covalent bonds, the flexibilization of the interface for compensating different thermal expansions of materials by using long-chain flexible and covalently bonded spacers between the metal and the polymer as well as its design as a moisture-repellent structure for hindering diffusion of water molecules into the interface and hydrolysis of chemical bonds. For this purpose, the main task was to develop plasmachemical and chemical techniques for equipping polymer surfaces with monotype functional groups of adjustable concentration. The establishing of monotype functional groups allows grafting the functional groups by spacer molecules by applying usual wet-chemical reactions. Four processes were favoured for production of monotype functional groups by highly selective reactions: the plasma bromination, the plasma deposition of plasma polymers, the post-plasma chemical reduction of O-functionalities to OH-groups, and the chemical replacement of bromine groups by NH₂-groups. The grafting of flexible organic molecules as spacers between the metal layer and polymer improved the peel strength of the metal. To obtain maximal peel strength of aluminium coatings to polypropylene films and occurrence of cohesive failure in the polypropylene substrate, about 27 OH groups per 100 C-atoms or 6 COOH groups per 100 C-atoms were needed. Introducing C_6–11-aliphatic spacers 1 OH or COOH group per 100 C-atoms contributed about 60% of the maximal peel strength of the Al–PP system, i.e. 2 or 3 spacer molecules per 100 C-atoms were sufficient for maximal peel strength.

The surfaces of polymers, namely polypropylene, copolymers and blends, were exposed to low pressure oxygen and atmospheric pressure air plasmas to improve their adhesion to polyurethane adhesives. A correlation is attempted between lap shear strengths of polypropylene-polyurethane composites and the relevant XPS, AFM and NEXAFS data. It was found that plasma functionalization improved the adhesion to maximum values even when the time of exposure was low: 1 to 10 seconds for low pressure plasmas, and 0.1 to 1 seconds in case of atmospheric plasma jet treatments. Thus, high lap shear strengths were obtained at relatively small oxygen contents. The improvement in shear strength at short time plasma exposures seems to be correlated to the complete smoothening of the supermolecular structure of stretched polypropylene foils as shown by AFM. Valence band XPS and derivatization techniques revealed more details of the oxygen functionalization on polypropylene. NEXAFS experiments confirmed a re-orientation of bonds and segments of the macromolecules by plasma exposure which are assumed to be responsible for adhesion improvement.

The chemical and morphological stabilities of polymer segments in the near-surface layer were investigated by spectroscopic methods such as X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. Model studies were undertaken with Langmuir–Blodgett films, self-assembled monolayers and oligomer films. For thin polymer layers (30 to 500 nm), the changes in molecular-weight distributions of some polymers were investigated systematically by size exclusion chromatography, matrix-assisted laser desorption/ionization–time-of-flight mass spectrometry and thermal-field flow fractionation for oxygen- and helium-plasma exposures. The polymer surfaces were found to be relatively stable at exposure to an oxygen low-pressure plasma up to ca. 2 s. This is important information to get maximum adhesion to metals in composites. In correlation to their redox potentials, potassium, aluminium and chromium react with oxygen functional groups at the polymer/metal interface. In a dedicated study, chromium was found to attack aromatic rings and form different reaction products.

Plasma treatment of silicone surfaces is a useful, environmentally-sound method of increasing wettability to improve adhesion. A thin, wettable silica-like layer is produced with various plasma gases such as argon, helium, oxygen and nitrogen. However, in each case the surfaces gradually recover their hydrophobicity. The silica-like layer is brittle and microcracking is evident at more severe levels of plasma treatment. The onset of cracking is a function of plasma gas, RF power, pressure and treatment time. Scanning electron microscopy has been used to characterize the cracks.

The hydrophobic recovery has been monitored by water contact angle changes. It occurs with both cracked and uncracked treated surfaces. There is an initial jump in hydrophobicity at the onset of cracking. Thereafter, the recovery of both cracked and uncracked surfaces broadly parallels each other with virtually complete recovery of original hydrophobicity within one week. These effects can be accounted for by rapid surface diffusion of low molecular weight material out of fresh cracks followed by slower bulk diffusion through the polymer matrix. Significant differences in recovery rates are also evident between different plasma gases.

Polymer surfaces typically have low surface tension and high chemical inertness and so they usually have poor wet-ting and adhesion properties. The surface properties can be altered by modifying the molecular structure using plasma immersion ion implantation (PIII). In this work, Nylon-6 was treated using oxygen/nitrogen PIII. The observed improvement in the wettability is due to the oxygenated and nitrogen (amine) functional groups created on the polymer surface by the plasma treatment. X-ray photoelectron spectroscopy (XPS) results show that nitrogen and oxygen plasma implantation result in C–C bond breaking to form the imine and amine groups as well as alcohol and/or car-bonyl groups on the surface. The water contact angle results reveal that the surface wetting properties depend on the functional groups, which can be adjusted by the ratio of oxygen–nitrogen mixtures.

Three nonpolar organic polymers containing small amounts of polar functionality were studied with regard to their surface characteristics. Two of the materials, potassium chlorate/sulfuric acid-oxidized polyethylene and poly(ethylene-co-acrylic acid) display variable surface polarities which can be reversibly accessed by heating films of the polymers in air or aqueous sodium hydroxide. Sodium-reduced Teflon–FEP did not exhibit this characteristic. A combination of contact angle, ESCA, and ATR IR data are used to display that the surface changes are caused by migration of functional groups within the outer few tens of angstroms of the surface.

Contact angle behavior of four relatively high surface tension ionic liquids (1,3-dimethylimidazolium methyl sulfate, 1-ethyl-3-methylimidazolium ethyl sulfate, 1-ethyl-3-methylimidazolium fluoroborate, and bis(hydroxyethyl)dimethylammonium methane sulfonate) was studied on seven hydrophobic surfaces and compared with water contact angle behavior. Smooth surfaces of various chemical compositions exhibit contact angles with ionic liquids that are lower than values obtained with water and that scale with liquid surface tension values. Contact angles of ionic liquids on rough perfluoroalkyl surfaces exhibit the highest contact angles reported for liquids other than water and are indistinguishable from those of water and not dependent on liquid surface tension. Superhydrophobic methylsilicone surfaces that exhibit high water contact angles and low hysteresis exhibit very low receding contact angles with ionic liquid probe fluids and high hysteresis. The potential for ionic liquids as probe fluids is argued because of their variable and controllable surface tension, interface charge density, interface dipole density, as well as their variable and controllable cation/anion structure and molecular volume.

Comments are made concerning the recent use of adjectives to describe solid surfaces that exhibit anomalously high water contact angle values. We suggest that the meaning of the word hydrophobic be resolved before it is modified, for example, to superhydrophobic and further modified, for example, to sticky superhydrophobic and before the definitions of these new words become issues of contention. The case is made that the first statement in the title is appropriate with experiments that demonstrate significant attractive interaction between liquid water and the surface of solid Teflon. Four types of experiments are described: the interaction of a silicon-supported covalently attached perfluoroalkyl monolayer (a model Teflon surface) with a sessile water drop (1) and with a thin film of water on a clean silicon wafer surface (2), the interaction of 1 and 12 microm diameter solid Teflon particles with a water droplet surface (3), and the interaction of a thin (<5 microm) Teflon film with a water droplet (4). The concepts of shear and tensile hydrophobicity are introduced, and the recommendation that two numbers, advancing and receding contact angle values, should be considered necessary data to characterize the wettability of a surface. That the words hydrophobic, hydrophilic, and their derivatives can and should only be considered qualitative or relative terms is emphasized.

We respond to a recent report in this journal that criticizes our experiments, which disproved the Wenzel and Cassie theories. The criticism is that we measured contact angles “with drops that were too small, ignoring the indications of existing theoretical understanding.” We take a step back to give an explanation of what we believe to be the reason that the “existing theoretical understanding” is wrong. We explain that the teaching of surface science has led generations of students and scientists to a misunderstanding of the wetting of solids by liquids. This continues as evidenced by this recent criticism and numerous recent papers. We describe several demonstrations that were designed to help teachers, students, and scientists overcome the widespread learning disability that is rooted in their faulty intuition and to help them regard wetting from the perspective of lines and not areas.

We review our 2006−2009 publications on wetting and superhydrophobicity in a manner designed to serve as a useful primer for those who would like to use the concepts of this field. We demonstrate that the 1D (three-phase, solid/liquid/vapor) contact line perspective is simpler, more intuitive, more useful, and more consistent with facts than the disproved but widely held-to-be-correct 2D view. We give an explanation of what we believe to be the reason that the existing theoretical understanding is wrong and argue that the teaching of surface science over the last century has led generations of students and scientists to a misunderstanding of the wetting of solids by liquids. We review our analyses of the phenomena of contact angle hysteresis, the lotus effect, and perfect hydrophobicity and suggest that needlessly complex theoretical understandings, incorrect models, and ill-defined terminology are not useful and can be destructive.

A view of contact angle hysteresis from the perspectives of the three-phase contact line and of the kinetics of contact line motion is given. Arguments are made that advancing and receding are discrete events that have different activation energies. That hysteresis can be quantified as an activation energy by the changes in interfacial area is argued. That this is an appropriate way of viewing hysteresis is demonstrated with examples.

The fiber/epoxy resin adhesion increases after plasma treatment on ultrahigh molecular weight polyethylene (UHMW-PE) fibers. The surface modification of UHMW-PE monofilaments was studied using a combination of techniques: contact-angle measurements, SEM, and pullout tests. The results may be summarized as follows: Infiuenced by different plasma parameters and draw ratios of the monofilaments, the adhesion increases by at least four times by plasma treatment. Failure in the pullout tests involve rupture within a treated monofilament and the skin of it was peeled off; the degree of peeling-off is affected by different plasma treatment conditions and draw ratios of the monofilaments. There is only a slight decrease in the surface energy of the treated monofilaments with aging time. Ways of combining plasma etching with other chemical treatments to further improve the fiber/resin adhesion have also been studied. © 1993 John Wiley & Sons, Inc.
https://onlinelibrary.wiley.com/doi/abs/10.1002/app.1993.070471116

Static secondary ion mass spectroscopy (fast atom bombardment mass spectroscopy), (SIMS (FABMS)) and Fourier transform infrared-photo-acoustic spectroscopy (FTIR-PAS) studies have been performed on samples of polypropylene subjected to different numbers of flame treatments. SIMS spectra allowed us to identify unambiguously the site of oxidation in the methyl pendant groups, because of the striking decrease in the intensity of the methyl fragment in positive-ion spectra. The behaviour of the surface concentrations of hydroxyl, formyl and carboxyl groups as a function of the number of flame treatments has also been observed, leading us to an hypothesis supporting the effectiveness of hydroxyl groups in promoting paint adhesion. FTIR-PAS spectra did not show evident changes on passing from untreated to flame-treated samples. This negative evidence is also important: it implies a limited depth of oxidation. In the light of previous XPS results and FTIR-PAS characteristics (thickness of the observed layer and sensitivity) we suggest a depth of oxidation of some 10 to 20 nm.

A study of the effects of flame treatments on a high-impact polypropylene has been performed. Both physico-chemical and mechanical properties have been investigated. The surface chemical composition has been determined by XPS, while the surface tension and the polarity were obtained through contact angle measurements. A remarkable agreement in the behaviour of chemical composition and polarity has been found, emphasizing the role of carbonyl and carboxyl groups. The adhesion of treated and untreated samples to paint coatings hua been mechanically tested. The force of adhesion remains quite constant after the first flame treatment. This suggests the importance of chemical interactions of the coating with the first layers of the polymer.

XPS and contact angle measurement have been used to study oxygen–plasma-treated polypropylene (PP) surfaces aged at variable temperatures. Surface rearrangement leading to low wettabillity has been observed, without alteration of the surface composition, as determined by XPS. Experimental results have been interpreted in terms of internal rearrangements of a modified layer, <5 nm thick, formed on top of the PP and immiscible with it.

We also modelled the composition of the surface layer and calculated the relative mobility of modified and non-modified polymer chains. On this basis, the experimentally observed behaviour can be interpreted in terms of surface rearrangement driven by a compromise between striving for lower surface tension and maximizing inter-and intramolecular interactions, mainly hydrogen bonds.

The surface composition observed after treatment with plasma, corona, flame or other for enhancing surface tension is then time dependent. For this reason, the procedure used for surface analysis, namely the time allowed for surface equilibration, should be specified in reports.

The deposition of plasma polymers on poly(acrylonitrile) (PAN) fibers has been investigated by X-ray photoelectron spectroscopy and dynamic contact angle measurements. Four polymerizable monomers were examined: tetrafluoromethane (TFM), perfluoropropene (PFP), tetramethyldisiloxane (TMS), and hexamethyldisiloxane (HMS). The deposition rate of TFM was undetectable and the treated fibers exhibited some fluorination and an increase of hydrophilicity, due to posttreatment oxidation after exposure to air. The deposition rate of PFP was quite slow and the formation of an incomplete fluorinated layer was observed, with a remarkable increase of the water advancing contact angles. TMS and more so HMS quickly formed continuous and reproducible polysiloxane layers having pronounced hydrophobic properties. The influence of the position of the fibers in the plasma reactor chamber was also investigated. A good uniformity of deposition was found when the fibers were placed at different points between the electrodes.

A simple but crude theoretical model involves summation of the pair potential function by integration and the use of dispersion, polar, and induction interactions for establishing the pair potential. For single-component systems the cohesive energy density, δ², the surface tension, γ, and the molar volume, V_m, are important. The theoretical model, as related to single-component systems, predicts a proportionality between δ² and $\text{γ/V}{}_{\mathrm{m}}{}^{\mathrm{<mtext>13</mtext>}}$ for molten metals and organic liquids, an increasing trend of γ with δ for polymers, and a maximum ideal tensile strength equal to about one-fourth of δ² for polymers and metals. Most of the experimental results are reasonably consistent with the theoretical predictions. For two-component interactions the model must be further modified. The δ_A or γ_A values are measures of the intensity of interactions within component A. For predicting the A–B interactions, the nature of the interactions within A and B must also be defined in terms of the fractional polarities p_A and p_B. The value of p_A can be determined either from the ionization potential, the polarizability, and the dipole moment of A or by interacting the polar material A with a nonpolar material B. The theory allows the prediction of the heat of mixing and of the ideal butt joint strength from δ_A, δ_B, p_A, and p_B and the prediction of interfacial tension from γ_A, γ_B, p_A, and p_B. While most of the available experimental data are poorly suited for exact quantitative testing of the theory, they are semiquantitatively consistent with it. The theory is useful for interpreting experimental data on polymer solubility, adhesive bond strength, wettability of polymers, and interfacial tension involving organic liquids and water or mercury. In particular, the interfacial tension between mercury and non-hydrogen-bonding organic liquids can be calculated quite accurately with the aid of the fractional polarities.