Following the development of a methodology for determining the apolar components as well as the electron donor and the electron acceptor parameters of the surface tension of polar surfaces, surfaces of a number of quite common materials were found to manifest virtually only electron donor properties and no, or hardly, any electron acceptor properties. Such materials may be called monopolar; they can strongly interact with bipolar materials (e.g., with polar liquids such as water); but one single polar parameter of a monopolar material cannot contribute to its energy of cohesion. Monopolar materials manifesting only electron acceptor properties also may exist, but they do not appear to occur in as great an abundance. Among the electron donor monopolar materials are: polymethylmethacrylate, polyvinylalcohol, polyethyleneglycol, proteins, many polysaccharides, phospholipids, nonionic surfactants, cellulose esters, etc.

Strongly monopolar materials of the same sign repel each other when immersed or dissolved in water or other polar liquids. The interfacial tension between strongly monopolar surfaces and water has a negative value. This leads to a tendency for water to penetrate between facing surfaces of a monopolar substance and hence, to repulsion between the molecules or particles of such a monopolar material, when immersed in water, and thus to pronounced solubility or dispersibility. Monopolar repulsion energies can far outweigh Lifshitz-van der Waals attractions as well as electrostatic and “steric” repulsions. In aqueous systems the commonly observed stabilization effects, which usually are ascribed to “steric” stabilization, may in many instances be attributed to monopolar repulsion between nonionic stabilizing molecules. The repulsion between monopolar molecules of the same sign can also lead to phase separation in aqueous solutions (or suspensions), where not only two, but multiple phases are possible. Negative interfacial tensions between monopolar surfactants and the brine phase can be the driving force for the formation of microemulsions; such negative interfacial tensions ultimately decay and stabilize at a value very close to zero.

Strongly monopolar macromolecules or particles surrounded by oriented water molecules of hydration can still repel each other, albeit to an attenuated degree. This repulsion was earlier perceived as caused by “hydration pressure”.

A few of the relevant colloid and surface phenomena are reviewed and re-examined in the light of the influence of surface monopolarity on these phenomena.

This article will describe some of the recent progress in the area of plasma polymerization and plasma treatment. It is not intended to be an exhaustive overview of the field, but instead a summary of the highlights of research studies in this field selected by the authors according to the importance.

Comprehensive reviews on basic phenomena, theory, and reaction mechanisms of plasma-assisted processing and plasma polymerization will be covered in this review. Formation of diamond and amorphous carbon which has attracted considerable attention in the last few years will also be described.

Recent advances in plasma assisted deposition of composite metal/organic (polymeric or carbon) films will be discussed including deposition configurations and processes. Suggested applications particularly in optics and microelectronics will be emphasized.

Possible trends in future research and development of plasma deposition of organic films will also be outlined.

Corona discharge introduces polar groups into the polymeric surfaces and, as a consequence, improves the surface energy, wettability, and adhesion characteristics. The main chemical mechanism of corona treatment is oxidation. This article further discusses some special problems that are related to corona treatment of polyolefin films by reviewing the recent developments in this field, such as effect of corona treatment on adhesion, effect of resin additives on corona treatment, insufficient treatment and over-treatment of corona discharge, aging, and re-treatment. © 1999 John Wiley & Sons, Inc. Adv Polym Techn 18: 171–180, 1999
https://onlinelibrary.wiley.com/doi/abs/10.1002/(SICI)1098-2329(199922)18:2%3C171::AID-ADV6%3E3.0.CO;2-8

In this work, melt-spun polypropylene (PP) fibers were treated in an electric field of a corona discharge. The fibers were then characterized using the thermally stimulated current (TSC) spectroscopy. It has been shown that the electret state of corona-treated PP fibers is a result of the combination of Maxwell–Wagner polarization and charge trapping. Activation energies and relaxation times for these processes have been determined, and characteristics of trapping sites have been calculated. The electret state induced in PP fibers by the corona discharge treatment holds for a long time (several months). Our analysis of the effect of processing temperature and electric field intensity on the characteristics of the electret state in melt-spun PP fibers allows one to specify optimum technological regimes for industrial production of PP-based electret filter materials. © 2000 John Wiley & Sons, Inc. Adv Polym Techn 19: 312–316, 2000
https://onlinelibrary.wiley.com/doi/abs/10.1002/1098-2329%28200024%2919%3A4%3C312%3A%3AAID-ADV7%3E3.0.CO%3B2-X

Several commercial polymers (polyethylene, polyimide, polytetrafluoroethylene, polyvinylchloride and polycarbonate) have been treated by low temperature glow discharge plasmas in various gases, namely NH3, O2, Ar, and CF4. These surface modifications were performed in "pure" microwave (2.45 GHz, "single-mode") or in combined microwave/radio frequency (2.45 GHz/13.56 MHz, "dual-frequency") plasma. Important systematic changes of the surface composition, wettability, and adhesion of thin metal films were observed for different substrate bias values, and for the different gases. The modified surface-chemical structure is correlated with contact angle hysteresis of water drops; this helps to identify which surface characteristics are connected with the wettability heterogeneity and with adhesive bonding properties, and how they are influenced by plasma-surface interactions.

The Young's equation is the well-known relationship used to describe a sessile drop at equilibrium on top of a solid surface. This relationship has been discussed thermodynamically and microscopically for purely flat surfaces in the literature. To characterize the non-flatness of a surface, one may introduce the Wenzel's roughness r defined as the area of the wall surface devided by the area of its projection onto the horizontal plane. Obviously, r is equal to 1 once the surface is flat. For r>1, it is known that Young's equation has to be modified to take into account the increase of surface. The generalization of Young's relation is the so-called Wenzel's law. In this presentation, we will study this relation within microscopic models. We will in particular show that the roughness may enhance the wetting of the substrate even at the microscopic scale.

Radio frequency plasma treatment of polydimethylsiloxane (PDMS) is a useful way of increasing wettability to improve adhesion. Its main defect is the subsequent rapid recovery of hydrophobicity. The fluorosilicone polymethyltrifluoropropylsiloxane (PMTFPS) is another low surface energy silicone where improved wettability is often desired. We have directly compared the behavior of PMTFPS and PDMS using air, oxygen, helium and argon plasma gases. The effect of the plasma has been investigated by water contact angle changes and by x-ray photoelectron spectroscopy (XPS) surface characterization. Both an unfilled PMTFPS gum and a filled elastomer were examined. PMTFPS is affected in much the same way as PDMS, an oxidized silica-like surface region is produced. This is shown by the shift in the high resolution Si 2p spectra to higher binding energy which is most marked in the case of helium treatment. Significant improvements in water wettability occur with helium treatment having the greatest effect but the hydrophobic recovery is mostly complete within 24 hours in all cases. Multiple treatments followed by water storage are effective in maintaining wettability for at least a week.

A review of the author's investigations of the equilibrium contact angles of pure liquids on low- and high-energy solid surfaces, both bare and covered with a condensed monomolecular adsorbed film, includes the critical surface tension of wetting and the effect of homology on spreading by pure liquids, the causes of nonspreading on high-energy surfaces, and the existence and properties of autophobic liquids and oleophobic monolayers. Constitutive relationships are summarized in a table of critical surface tensions of wetting. The theory and application of the retraction method of preparing adsorbed monolayers from solution and the conditions for mixed films are presented. Studies of the wetting behavior of solutions of various surfactants and the resultant explanation of the function of a wetting agent are generalized to include nonaqueous systems. Following estimates of the reversible work of adhesion of liquids to solids, the part played by wetting in obtaining optimum adhesion by adhesives is outlined, and a fundamental explanation is given of constitutive effects in the development of strong adhesive joints. Future areas of research on wetting and adhesion are indicated.

The effect of roughness on the wettability of an idealized sinusoidal surface has been studied with a digital computer. The equations of Wenzel and of Cassie and Baxter are discussed in relation to the model. The heights of the energy barriers between metastable states of a drop are seen to be of utmost importance in determining the magnitude of contact angle hysteresis.

An experimental study of the wettability of rough surfaces over an extremely wide range of roughness is described. The theoretical wettability behavior of an idealized, rough surface agrees well with that of real surfaces. The theoretically predicted minimum in the curve of receding contact angle vs. roughness, for systems of high intrinsic contact angle, is experimentally verified.

Adhesion of polymers was determined as a function of temperature. The influence of the bonding times and temperatures indicates that the performance is established largely by the extent of wetting at the polymer-substrate interface. Considerations based on surface free energies show that most practical systems should exhibit complete wetting at equilibrium. The problem appears to involve establishing factors which retard or preclude wetting. Low substrate surface energy, high polymer viscosity, substrate topography, selective adsorption, and coacervation may be involved.

Certain aspects of the adsorption theory of adhesion are developed more fully than has been done previously. The consequences of nonreciprocity of spreading are pointed out, and are used to develop a more general practical point of view with respect to the adhesive bonding of materials of low-surface free energy. The system epoxy adhesive-(nonsurface-treated) polyethylene, normally considered nonadherent, is investigated experimentally in some detail. It is shown how this system, without material modification, can be made adherent. An area of study for possible adhesives for materials of lowsurface free energy is suggested.

A new type of radio frequency (rf) large area non equilibrium (‘cold’) plasma source operating at atmospheric gas pressures in an open reactor was presented. The source was based on a specially designed rf electrode with the gas flowing through an inner microstructure integrated in the electrode. A cylindrical source of 35 mm in diameter with approximately 900 hollow cathodes forming an integrated open structure and a rectangular 120×20 mm2 source of this type were tested. A typical rf power for the source operation is only several tens of watts. Experiments show that the performance of both sources at atmospheric pressure was substantially better in comparison with single cylindrical rf hollow cathodes of 400 μm in diameter. The argon and neon plasmas generated in the space between the main electrode and the substrate holder were uniform and very stable. The optical emission spectroscopy study, the rf current, voltage and impedance measurements, as well as the substrate temperature tests reveal three different power dependent regimes of these plasma sources. Effects of plasma treatment of surfaces were studied on both temperature sensitive samples (plastic webs) and metals (aluminum, steel). An effective cleaning of Al samples was observed after 5 min treatment in the neon plasma in an open reactor. After 1 s treatment of the Polyethylene web the surface tension increases from values<34 mN/m to values ⩾56 mN/m. Design of sources allows their direct scaling up and may bring a number of interesting applications in large area cold atmospheric plasma processing.

The surface of isotactic poly(propylene) foils was oxidized by corona discharge plasma in order to improve the adhesive characteristics. The dependence of the degree of surface oxidation on either the current density or the time of exposure was determined. Rapid increase of the free surface energy was observed at current densities ranging from 0.4 to 0.6 mA. A reduction of the exposure time of discharge at the foil surface has an effect similar to the reduction of current density. The change of free surface energy of extruded poly(propylene) was rapid, especially during the first 24 h, while for modified biaxially oriented poly(propylene) the decrease of free surface energy was substantially slower.

The influence of thermal treatment on the adhesion between isotactic poly(propylene) (iPP) and ethylene-propylene copolymer has been studied. The adhesive force between the polymer films was measured by performing peel tests. It was found that an interface layer has been formed. Its structure and thickness are dependent on the thermal history of the sample: the peel strength increases with annealing temperature and time, and the cooling rate, too, influences the peel strength. The method of preparing the iPP films has an effect on the adhesion of the sandwich sample as well.

Corona discharge treatment of isotactic polypropylene surfaces in N₂ and CO₂ was investigated by contact angle measurements and ESCA. The electrical characteristics of the discharges as well as the influence of indirect parameters (moment of air contact and ageing time) and direct parameters (applied charge, electrical field strength and film temperature) on the surface modification were determined. These investigations showed that electrons, emitted by photo effect are the dominant charge carriers and the main cause of surface activation. The active species in the surfaces (presumably radicals) can either perform crosslinking and H-abstraction or react with the discharge gas. In the N₂-discharge the polymer radicals can only react with atomic or excited nitrogen whereas in CO₂ they also react with ground state molecules. If the samples are brought into air contact after discharge leftover radicals are oxidized by atmospheric oxygen. In addition a UV-radiation causing activation in a surface layer was found. The bulk of the polymer is not influenced by corona discharge.

The surface modification of synthetic fiber fabrics via corona discharge treatment and subsequent graft polymerization was investigated. Polyethylene (PE) nonwoven fabric and polyamide-6 (PA-6) nonwoven fabric were used as base fabrics. Acrylic acid (AAc) was graft polymerized onto the fabrics via corona discharge pre-treatment. The grafted amounts of PAAc were dependent on the grafting time, that of PA-6 being higher than that of PE. It was confirmed that the surface of the fibers constructing the fabric was fully covered with PAAc after the 20 min reaction. The surface of the PAAc grafted fabrics was characterized by X-ray photoelectron spectroscopy. The leakage of electrostatic charge from the fabric was determined and the dyeability was studied with methylene blue. The period of time in which the charge potential attenuated to 1/2 of the initial potential decreased drastically by grafting with PAAc. The grafted amount was enough for dyeing the entire fabrics.