The reasons for the appearance of the hysteresis of wetting are considered. The model is proposed according to which the hysteresis is the result of the orientations of molecules of wetting liquids which is preserved due to the action of surface forces even after the flow ceases.

Sessile-drop and captive-bubble techniques were used for contact angle measurements. The advancing and receding contact angles were measured for water and ethylene glycol at self-assembled monolayer surfaces of dodecanethiol, for water at methylated quartz surfaces, and for water at roughened polyethylene and polytetrafluoroethylene surfaces. It was found that for each technique used, sessile-drop and captive-bubble, different advancing contact angles and different receding contact angles were frequently obtained for nonideal systems with rough and heterogeneous solid surfaces. The disagreement between contact angles, as measured with the two different techniques, increased with increasing imperfection of the solid surface. Also, it was observed that solid surface roughness and heterogeneity affected a variation of the advancing and receding contact angles with drop (bubble) size. No contact angle change with respect to drop (bubble) size (in the range 1–7 mm base diameter) was observed when smooth and homogeneous solid surfaces were well prepared. It is possible that metastable states, which are responsible for the contact angle hysteresis, also affect the contact angle/drop (bubble) size relationship. These three-phase systems with sessile drop and captive bubble at heterogeneous and/or rough solid surfaces are complex because solid surface heterogeneity and roughness cause contortions in the shape of the three-phase contact line and the drop (bubble) surface in the vicinity of the three-phase contact line. These contortions may affect a variation of the internal free energy of the liquid drop (gas bubble). It is shown that a slight variation in the advancing contact angle value over a few millimeters change in drop (bubble) diameter does not guarantee a high-quality surface state. Measurements of the receding contact angles provide more information on the quality of the solid surface and they should always be included with the measurements of advancing contact angles.

Polyetheretherketone (PEEK) has been treated with oxygen-, air-, argon- and ammonia-plasmas, which greatly improve adhesion to an epoxide film adhesive. Treated surfaces can be stored under laboratory conditions for up to 90 days without significant loss of the improved adhesion properties. Contact angle measurements show that the surface energy of PEEK is much increased by plasma-treatment. X-ray photoelectron spectroscopy shows that the plasmas increase the amounts of oxygen and in some cases the amounts of nitrogen, and that new surface groups include -OH and -CO-. Wiping treated surfaces with acetone can reverse the effects of plasma-treatment.

Polymers have been applied successfully in fields such as adhesion, biomaterials, protective coatings, friction and wear, composites, microelectronic devices, and thin-film technology. In general, special surface properties with regard to chemical composition, hydrophilicity, roughness, crystallinity, conductivity, lubricity, and cross-linking density are required for the success of these applications. Polymers very often do not possess the surface properties needed for these applications. However, they have excellent bulk physical and chemical properties, are inexpensive, and are easy to process. For these reasons, surface modification techniques which can transform these inexpensive materials into highly valuable finished products have become an important part of the plastics and many other industries. In recent years, many advances have been made in developing surface treatments to alter the chemical and physical properties of polymer surfaces without affecting bulk properties. Common surface modification techniques include treatments by flame, corona, plasmas, photons, electron beams, ion beams, X-rays, and γ-rays.

Plasma treatment is probably the most versatile surface treatment technique. Different types of gases such as argon, oxygen, nitrogen, fluorine, carbon dioxide, and water can produce the unique surface properties required by various applications. For example, oxygen-plasma treatment can increase the surface energy of polymers, whereas fluorine-plasma treatment can decrease the surface energy and improve the chemical inertness. Cross-linking at a polymer surface can be introduced by an inert-gas plasma. Modification by plasma treatment is usually confined to the top several hundred ångströms and does not affect the bulk properties. The main disadvantage of this technique is that it requires a vacuum system, which increases the cost of operation.

Thin polymer films with unique chemical and physical properties are produced by plasma polymerization. This technology is still in its infancy, and the plasma chemical process is not fully understood. The films are prepared by vapor phase deposition and can be formed on practically any substrate with good adhesion between the film and the substrate. These films, which are usually highly cross-linked and pinhole-free, have very good barrier properties. Such films find great potential in biomaterial applications and in the microelectronics industry.

Very high-power microwave-driven mercury lamps are available, and they are used in UV-hardening of photoresist patterns for image stabilization at high temperatures. Other applications of UV irradiation include surface photo-oxidation, increase of hydrophilicity, and photocuring of paintings.

Pulsed UV-lasers are used in surface modification in many areas. Pulsed UV-laser irradiation can produce submicron periodic linear and dot patterns on polymer surfaces without photomask. These interference patterns can be used to increase surface roughness of inert polymers for improved adhesion. These images can also be transferred to silicon surfaces by reactive ion etching. Pulsed laser beams can be applied to inert polymer surfaces for increased hydrophilicity and wettability. Polymer surfaces treated by pulsed UV-laser irradiation can be positively or negatively charged to enhance chemical reactivity and processability. Pulsed UV-laser exposures with high fluence give rise to photoablation with a clean wall profile. There are many other practical applications of laser photoablation, including via-hole fabrication, and diamond-film deposition. The present review discusses all these current applications, especially in the biomedical and microelectronics areas.

Surface free energies have been calculated for solid fluorocarbon materials by employing a method that utilizes dielectric data and theoretical predictions of van der Waals (dispersion) interactions. Excellent agreement between the results of direct force measurements and those of the theory for retarded van der Waals interactions supports the methodology. Two relatively new fluorocarbon polymers have been identified as having the lowest known surface free energies of all bulk homogeneous polymeric solids. This study provides confirmation that estimates of solid surface free energies based on contact angle measurements with dispersive organic liquids depend on the dielectric properties of both the liquids and the solid.

The determination of solid and solid-liquid surface tensions is of importance in a wide range of problems in pure and applied science. There exist, at present, many indirect approaches for obtaining these values because it is not possible to measure directly surface tensions involving a solid phase. These various methods are often in considerable disagreement, both quantitatively and from a theoretical standpoint. The problem persists since most of these approaches have not been tested objectively through the prediction of physical phenomena which could be independently observed and thus used to validate the various theories.

The accurate measurement of contact angles is essential in many areas of applied surface thermodynamics. As was seen in Chapters 3 and 5, the contact angle provides a unique means of determining solid-vapor and solid-liquid surface tensions. The range of applications of this measurement is remarkable, both as a simple tool to assess, for example, the cleanliness of surfaces, and as a highly sensitive scientific measurement aimed at providing information on the solid surface tension and the physical state of the surface. When first encountered, the measurement of contact angles appears to be quite straightforward. This apparent simplicity is, however, very misleading, and experience has shown that the acquisition of thermodynamically significant contact angles requires painstaking effort. This chapter addresses the many practical issues pertaining to the measurement of contact angles and liquid surface tensions, including the preparation of suitable solid surfaces and measuring liquids.

In the vast majority of contact angle studies in the literature, the method used is direct measurement of sessile drops. Recent developments in image analysis and processing have increased the accuracy and reduced the subjectivity considerably (see Chapter 10). Nevertheless, there are certain limitations which leave room for other techniques. One of the limitations of the classical sessile-drop method is that the camera or imaging device will be focused on the largest meridian section, and hence reflect only the contact angles at the point in which the meridian plane intersects the three-phase line. Surface heterogeneity and/or roughness could well cause variations of the contact angle along the three-phase line. An alternate approach is to infer the contact angle from the drop contact diameter (see Chapter 10). But even on very good solid surfaces, on which such effects are absent, there is a systematic problem with this method: contact angles will change as a function of drop size for drops up to approximately 1 cm diameter. While this dependence can indeed be used to determine line tension (see Chapter 4), this and other similar effects interfere with the interpretation of contact angles in terms of surface energetics (see Chapter 3).

Numerous methodologies have been developed for the measurement of contact angles and surface tensions as outlined in Chapter 8 and Refs. 1-4. Liquid surface tension measurements commonly involve the determination of the height of a meniscus in a capillary, or on a fiber or a plate. Contact angles are most commonly measured by aligning a tangent with the profile of a sessile drop at the point of contact with the solid surface. Other notable methods are the Wilhelmy slide (Chapter 8) and the capillary rise technique (Chapter 9). An overview of such techniques reveals that in most instances a balance must be struck between the simplicity, the accuracy, and the flexibility of the methodology.

The interfacial energetics and wettability of small particles are of technological interest in many areas of applied science. Areas where such phenomena are important include the preparation of stable suspensions of particles (e.g., colour pigments in paints), the adhesion of particles to solid surfaces in various scenarios (e.g., lubrication), the dispersion of particles into a liquid or melt of a polymer, and the modification of particle surface properties through the adsorption of polymeric macromolecules or surfactants. The successful manipulation of the process being considered is largely determined by the physicochemical surface properties of the interacting surface components, and particularly the wettability and the surface (or interfacial) tension of the particles. The complexities of contact angle phenomena and surface tensions were discussed in Chapter 3.

Contact-angle measurements, the ASTM standard wetting test for polyolefin films, and X-ray photoelectron spectroscopy (XPS or ESCA) were used to characterize flame-treated polypropylene (PP) films. Two combustion models, STANJAN and PREMIX, were then used to determine the chemical and physical properties of the flames used to treat the PP films. Both the flame equivalence ratio and the position of the PP film in the flame are important variables in determining the extent of oxidation and improvement in wettability obtained by flame treating. The optimal equivalence ratio for the flame treatment of PP is 0.93, while the optimal luminous flame-to-film distance is 0-2 mm. Modeling of the combustion processes occurring in the flame provides evidence that the extent of treatment correlates closely with the concentrations of H, O, and OH radicals present in the flame. The extent of surface modification of the flame-treated PP does not appear to correlate with either the flame temperature or the concentraion of oxygen molecules. The mechanism of surface oxidation by flame treatment probably involves polymer-radical formation by O and OH, followed by rapid reaction of the polymer radicals with O, OH, and O₂.

The influence of film roughness on the wetting properties of vacuum-deposited polytetrafluorethylene (PTFE) thin films has been investigated using atomic force microscopy (AFM) and contact angle goniometry. Surface roughness has been characterized by atomic force microscopy in terms of RMS roughness (R_q) and fractal dimensions. A contact angle correlation with surface roughness, as determined by AFM, is evident from these results, which are discussed on the basis of wetting theory. The results also confirm that the high water contact angles (as high as 150°) recently observed at the surface of a new water repulsive coating material (mixture of PTFE and binder) are because of surface roughness. Such measurements clarify the effect of nanometer-size surface asperities on the wetting properties of hydrophobic coating.

The present invention is a polyester film base which has a surface approximately 5 nm thick. The surface of the film base has been altered to include about 6 to 15 atomic percent nitrogen in the form of imines, secondary amines and primary amines in the ratio of about 1:1:2. The invention also includes a film base whose surface includes oxygen in the form of hydroxyl, ether, epoxy, carbonyl or carboxyl groups wherein the oxygen is about 4 to 10 atomic percent above the original surface content of the base. The polyester film base can be either polyethylene terephthalate or polyethylene naphthalate.

Polypropylene (PP) film was treated with radio-frequency-induced oxygen plasma, followed by the vacuum deposition of aluminum (Al) thin film, and the peel strength of the Al deposited PP film (Al/PP) was examined. The peel strength of plasma-treated PP film varied widely in the range of 6.7 to 157 N/m depending upon the plasma treatment conditions, whereas that of the untreated PP was 5.2 N/m. The peel strength was minimized at oxygen pressure near 13.3 Pa (0.1 Torr), and decreased with increasing discharge power. The peel strength rapidly increased at the initial stage of plasma treatment (∼ several seconds), decreased at the second stage, and slightly increased again at the third stage. A good agreement was found between the peel strength of Al/PP and the amounts of oxygen introduced onto the PP surface at the initial stage. A short-time treatment was very effective to improve the adhesion of Al/PP. At the end of the second stage, a large amount of carbon was detected by XPS on the Al layer of the peeled interface of Al/PP, which gave a minimum peel strength. Cohesive failure of PP film might have occurred. SEM photograph showed that PP surface was etched by oxygen plasma at the thrid stage. These peel behaviors of Al/PP were explained by the chemical and physical changes of the PP surface caused by oxygen plasma treatment: (1) introduction of O-functional groups onto the PP surface at the initial stage, (2) formation of weak booundary layers resulting from the partial scission of PP molecules at the second stage, and (3) plasma etching of the PP surface at the third stage.

This study analyzes the combination of different tie layers with normal coextrusion and coating parameters: temperature, speed, and pretreatments. We wanted to determine the best combination possible and to issue recommendations for extrusion and coating. The typical structure was paper/tie layer/barrier layer/tie layer/LDPE (low-density polyethylene). When LDPE is coated on paper, pretreatments of the web are used (such as corona or flame). Sometimes ozone is sent onto the melt to increase natural oxidation. The same thinking could be applied to tie layers, but the trials showed that certain combinations lead to different conclusions. The effect of parameters such as corona or flame power, which were thought to depend only on the paper type, could also be influenced by the combination paper-tie layer. This could mean that web pretreatments are not always needed.

The treatment of surfaces by corona discharges is a well-established method to improve surface properties. The surface to be treated is moved continuously and is exposed to transient gas discharges, known as microdischarges, in air at atmospheric pressure between electrodes, where at least one electrode is covered with a dielectric barrier. Because of the short duration, only some 10 ns, the current through the microdischarges is predominantly carried by electrons. The ion temperature remains close to room temperature. Owing to these properties such discharges are qualified to treat surfaces which are sensitive to higher temperatures. For a large number of applications this treatment is adequate, but the adhesion of aqueous glues and inks to some plastic materials is insufficient if the surfaces are treated in this way. Furthermore, it is difficult to meet the requirements of surface properties of, for instance, polyolefine film (e.g. surface tension, adhesion). This material is not based on monomers containing chlorine or fluorine and is preferred for ecological reasons. This paper presents the results of experiments which demonstrate that in comparison to a common corona treatment significant improvements in surface properties of plastic materials can be achieved if repetitively generated pulse trains and reactive gases are used instead of air. If, for instance, the microdischarges are established in acetylene, thin films with a thickness of several namometres are formed on surfaces, which increase and stabilize the surface tension up to a level of 72 mN m−1. The state of the art of this new technology is discussed.

Argon plasma-pretreated polytetrafluoroethylene (PTFE) films were subjected to further surface modification by near-UV light-induced graft copolymerization with acrylic acid (AAc), sodium salt of styrenesulfonic acid (NaSS), and N,N-dimethylacrylamide (DMAA). The surface compositions and microstructures of the modified films were characterized by angle-resolved X-ray photoelectron spectroscopy (XPS). A stratified surface microstructure with a significantly higher substrate-to-graft chain ratio in the top surface layer than in the subsurface layer was always obtained for PTFE surface with a substantial amount of the hydrophilic graft. The stratified surface microstructure was consistent with the observed hysteresis in advancing and receding water contact angles. The graft yield increased with Ar plasma pretreatment time and monomer concentration. Covalent immobilization of trypsin on the AAc polymer-grafted PTFE films was facilitated by water-soluble carbodiimide (WSC). The effective enzyme activities increased initially with increasing surface concentration of the grafted AAc polymer but became saturated at a moderate AAc polymer concentration. The immobilized enzyme could still retain close to 30% of its original activity. Solution-coating of the polymeric acid-modified PTFE films with the emeraldine (EM) base of polyaniline readily resulted in an interfacial charge transfer interaction and a semiconductive PTFE surface.

Ion irradiation with various oxygen flow rates has been carried out to improve the wettability of polymethylmethacrylate (PMMA) to water and to enhance the adhesion between Al and the polymer. Ar+ ion and oxygen ion were irradiated on the polymer, and amounts of ions were changed from 5 × 1014 Ar+/cm2 to 5 × 1016 Ar+/cm2 by a broad ion beam source. Oxygen gas from 0 ml/min to 7 ml/min was flowed near the polymer surface during the ion irradiation, and the energy of ions was changed from 500 eV to 1500 eV. The wetting angle was reduced from 68° to 49° with the Ar+ ion irradiation only at 1 keV energy, to 43° with the oxygen ion irradiation, and dropped to 8° with Ar+ ion irradiation with flowing 4 ml/min oxygen gas near the polymer surface. Changes of wetting angle with oxygen gas and Ar+ ion irradiation were explained by a two-step chemical reaction among polymer matrix, energetic ions, and oxygen gas. The effects of Ar+ ion and oxygen ion irradiation were explained by considering formation of hydrophilic groups due to a reaction between irradiated polymer chain by energetic ion irradiation and blown oxygen gas, and enhanced adhesion between Al and PMMA was explained by the formation of electron acceptor groups in polymer and electron donors in metal, and by the chemical reaction in the interface between irradiated polymer surface and deposited metal.

The present invention is a polyester film base which has a surface approximately 5 nm thick. The surface of the film base has been altered to include about 6 to 15 atomic percent nitrogen in the form of imines, secondary amines and primary amines in the ratio of about 1:1:2. The invention also includes a film base whose surface includes oxygen in the form of hydroxyl, ether, epoxy, carbonyl or carboxyl groups wherein the oxygen is about 4 to 10 atomic percent above the original surface content of the base. The polyester film base can be either polyethylene terephthalate or polyethylene naphthalate.