The line energy associated with the triple phase contact line is a function of local surface defects (chemical and topographical); however, it can still be calculated from the advancing and receding contact angles to which those defects give rise. In this study an expression for the line energy associated with the triple phase contact line is developed. The expression relates the line energy to the drop volume, the interfacial energies, and the actual contact angle (be it advancing, receding, or in between). From the expression we can back calculate the equilibrium Young contact angle, θ ₀, as a function of the maximal advancing, θ _A, and minimal receding, θ _R, contact angles. To keep a certain maximal hysteresis between advancing and receding angles, different line energies are required depending on the three interfacial energies and the drop's volume V. We learn from the obtained expressions that the hysteresis is determined by some dimensionless parameter, script K sign, which is some normalized line energy. The value of script K sign required to keep a constant hysteresis (θ _A - θ _R) rises to infinity as we get closer to θ ₀ = 90°.

The control of the surface wettability of poly (methyl methacrylate) (PMMA) substrates has been successfully demonstrated using an Ar2* excimer lamp radiating 126 nm vacuum ultraviolet (VUV) light. Each of the samples was exposed to 126 nm VUV light in air over the pressure range of 2×10−4-105 Pa. Although at the process pressures of 10, 103, and 105 Pa, the PMMA surfaces became relatively hydrophilic, the degree of hydrophilicity depended markedly on the pressure. The minimum water contact angles of the samples treated at 10, 103, and 105 Pa were about 50°, 33°, and 64°, respectively. These values were larger than those of PMMA substrates hydrophilized through 172 nm VUV irradiation conducted under the same conditions. On the other hand, after 126 nm VUV irradiation conducted under the high vacuum condition of 2×10−4 Pa, the PMMA substrate surface became carbon-rich, probably due to preferential cross-linking reactions, as evidenced by x-ray photoelectron spectroscopy. This surface was hydrophobic, showing a water contact angle of about 101°. Although the 126 nm VUV-irradiated surfaces appeared relatively smooth when observed by atomic force microscope, very small particles with diameters of 30-60 nm, which probably originated from the readhesion of photodecomposed products, existed on all of the sample surfaces.

A new method for the measurement of apparent contact angles at the global energy minimum on real surfaces has been developed. The method consists of vibrating the surface, taking top-view pictures of the drop, monitoring the drop roundness, and calculating the contact angle from the drop diameter and weight. The use of the new method has been demonstrated for various rough surfaces, all having the same surface chemistry. In order to establish the optimal vibration conditions, the proper ranges for the system parameters (i.e., drop volume, vibration time, frequency of vibration, and amplitude of vibration) were determined. The reliability of the method has been demonstrated by the fact that the ideal contact angles of all surfaces, as calculated from the Wenzel equation using the measured apparent contact angles, came out to be practically identical. This ideal contact angle has been compared with three methods of calculation from values of advancing and receding contact angles.

Hydrophobic solid surfaces with controlled roughness were prepared by coating glass slides with an amorphous fluoropolymer (Teflon^® AF1600, DuPont) containing varying amounts of silica spheres (diameter 48 μm). Quasi-static advancing, θ_A, and receding, θ_R, contact angles were measured with the Wilhelmy technique. The contact angle hysteresis was significant but could be eliminated by subjecting the system to acoustic vibrations. Surface roughness affects all contact angles, but only the vibrated ones, θ_V, agree with the Wenzel equation. The contact angle obtained by averaging the cosines of θ_A and θ_R is a good approximation for θ_V, provided that roughness is not too large or the angles too small. Zisman's approach was employed to obtain the critical surface tension of wetting (CST) of the solid surfaces. The CST increases with roughness in accordance with Wenzel equation. Advancing, receding, and vibrated angles yield different results. The θ_A is known to be characteristic of the main hydrophobic component (the fluoropolymer). The θ_V is a better representation of the average wettability of the surface (including the presence of defects).

Many experiments have been performed globally to investigate ways of improving adhesion to polymers. This paper discusses current atmospheric surface activation systems, appropriate measurements of wettability and adhesion, over-treatment effects and surface analysis techniques relative to optimizing the adhesion of inks, paints, coatings and adhesives to polymer surfaces. Recommendations for improved activation by substrate and application are discussed.

One of the most important claims for the plasma technique as a surface treatment is that it modifies only a few atomic layers of materials. However, with polymers, this assumption must be carefully verified to keep the bulk mechanical properties constant. Besides the oxidation of the film, with specific plasma conditions such as high power and duration, the polypropylene film structure is also modified in the bulk through vacuum ultraviolet absorption and thermal relaxation. This change is associated with smectic- and amorphous-phase transformation into an α-monoclinic phase, with a rapid rate for the smectic transformation and a slower rate for the amorphous transformation. At the same time, the crystallite size increases, and the polypropylene film texture is planar and moderated (1.7 mrd at the maximum of the distribution, with a discharge power of 100 W and a treatment duration of 10 min). © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2007–2013, 2004
https://onlinelibrary.wiley.com/doi/abs/10.1002/polb.20071

To many people, plasma is a laboratory curiosity or limited in scale. Few know that plasma is a commercial process used daily in the treatment of fabrics, non-woven webs and film. This paper reviews applications and processes used to modify materials up to 60 in. in width in a roll-to-roll plasma system. The applications are quite varied. Sometimes, the process is simply to change the surface energy, while at other times, far more sophisticated processes, such as plasma-enhanced chemical vapor deposition (PECVD) processes, are employed to provide a chemical barrier or alter the tribological properties. As will be seen in this review presentation, plasma is extremely versatile and applicable to high-volume web applications.

This work deals with the study of the durability, in a hot/wet environment, of structural adhesively bonded polyetherimide (PEI) assemblies used in power electronics packaging technology. An overall approach is proposed, for which the epoxy joint-PEI substrates assembly on the one hand, and the adhesive system components (substrate surface and bulk adhesive) on the other hand, are studied separately with different analytical techniques. The first part of this work was devoted to the substrate surface state and to its modification using a cold plasma treatment of the PEI surface. Then for chosen parameters (power, duration) contact angle measurements indicated an increased surface tension resulting from surface decontamination (removal of release agent and carbon contaminants) and from the creation of polar species, such as esters or carboxylic acid groups, on the PEI surface (XPS analyses). The second part of this study concerned the bulk adhesive ageing in an ethylene glycol-water solution at 70°C. Mass uptake measurements versus time showed the liquid diffusion in the bulk adhesive associated with a microscopic damage of the epoxy system. An overall plasticizing of the adhesive with a considerable decay of the α-transition temperature of one of the two adhesive epoxy-amine networks (TGDDM-BAPP) was also highlighted using rheometry. However, in these ageing conditions, the adhesive glassy modulus decreases slighty because of the thermomechanical stability of the other epoxy network. In the third part, the asymmetric wedge test showed the beneficial effect of the cold plasma treatment on the epoxy/PEI interface durability in the aggressive medium.

The polymers PET, PA6, PVDF, HD-PE, and PP are activated by a commercially available plasma jet system at atmospheric pressure to improve adhesive bondability. The adhesion properties of the activated surfaces are evaluated by lap shear tests. The results are correlated with the surface properties that are investigated by XPS, AFM, and contact angle measurements. In addition the influence of operational parameters of the plasma treatment is studied. The activated samples exhibit a substantially increased bonding strength. The improvement can be related to an increase of oxygen concentration, and to changes of the topology of the substrate surface induced by the thermal component of the plasma. The most influential parameters in the plasma treatment are the distance between substrate and nozzle exit and the treatment time.

This study concerns the surface and adhesive properties of isotactic polypropylene (iPP) modified by an electric discharge plasma and affected by long-term hydrophobic recovery of the polymer surface after modification. The investigations were focused on the change in polarity of the modified polymer expressed by the polar fraction as well as on the decrease in the surface free energy, its polar component and mechanical work of adhesion (A _m) to polyvinyl acetate. A _m of modified iPP to polyvinyl acetate as a function of polar fraction can be described by a mathematical formula. It has been confirmed that the most intensive decrease in the surface and adhesive properties investigated is produced by the long-term hydrophobic recovery of the polymer appears in the course of the first 30 days after its modification. During subsequent aging the process of polymer hydrophobic recovery proceeds more slowly. It has been found that the value of surface and adhesive properties of iPP as well as the dynamics of the decrease in these properties during hydrophobic recovery of the surface after modification is, in the main, dependent on the iPP crystallinity.

This paper follows the “defense” of the Good-van Oss-Chaudhury (GvOC) acid-base approach made in Part I and carries out a detailed analysis of the Zisman critical surface energy and, mainly, of the Neumann equation-of-state (EQS) theory. The analysis is made on both a “practical” and a theoretical basis, trying to highlight the acceptable fitting results of axisymmetric drop shape analysis (ADSA) methods and their independence of the assumed thermodynamic foundations of EQS. Some new and original criticisms of the EQS approach are raised and it is shown that other purely semiempirical models, represented by different fitting equations with the same number of parameters, can represent the data measured by ADSA method with the same goodness as EQS. The equation of state appears as one of many semiempirical approaches for the evaluation of surface free energy of solids. Independent of the previous analysis, the criteria used in ADSA measurements are evaluated and some comments made on them.

The acid-base approach to the calculation of solid surface free energy and liquid-liquid interfacial tensions is a practical example of application of correlation analysis, and thus it is an approximate approach. In these limits, and provided that wide and well-obtained sets of contact angles or interfacial tension data are used for their computation, surface tension components can be considered as material properties. Although their numerical value depends on the characteristics of the chosen reference material, their chemical meaning is independent on the selected scale. Contact angles contain accessible information about intermolecular forces; using surface tension component (STC) acid-base theory, one can extract this information only making very careful use of the mathematical apparatus of correlation analysis. The specific mathematical methods used to obtain these results are illustrated by using as an example a base of data obtained by the supporters of the equation-of-state theory (EQS). The achievements are appreciably good and the agreement between STC and EQS is discussed.

The purpose of this study was to develop a standard methodology for measuring the surface free energy (SFE), and its component parts, of dental biomaterials. The contact angle of each of four samples of two materials--low density polyethylene and poly(methyl methacrylate)--was measured three times in each of six liquids (1-bromonaphthalene, diiodomethane, ethylene glycol, formamide, glycerol and distilled water). Critical surface tension estimates were obtained from Zisman plots. Data were then analyzed by the least-squares method to estimate the components of SFE. Estimates were also made for each of 12 liquid triplets, and by maximum likelihood and Bayesian analyses. The use of liquid triplets could yield misleading estimates of the components of SFE. A testing protocol is suggested in which multiple test liquids are used, and multiple methods of statistical analyses employed. SFE is important, in that high SFE is desirable when adhesion is required, but undesirable if plaque resistance is needed. Methodology that avoids some of the limitations of existing studies has been proposed.

The surface characteristics of ethylene-vinyl acetate (EVA) were modified by argon, air, and oxygen plasma at atmospheric pressure. The surface energies of the EVA were evaluated by contact angles according to a sessile-drop method and adhesion energy (G(IC)) was estimated by a 180 degrees peel test with polyurethane (PU). After the plasma treatments, the surface free energies (or specific polar component) of the EVA increased about five times compared to that of virgin EVA. The adhesion between the EVA and the PU is significantly improved by the plasma treatment. Especially, Ar/air/O(2) plasma treatment increases G(IC) of EVA/PU up to about 600% compared to that of the sample using virgin EVA.

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

In this paper, we report and discuss the results of the surface treatment, using an atmospheric pressure dielectric barrier discharge (DBD), of selected polymer films which have no bonded oxygen in their intrinsic structures. Contact angle, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) data are presented with respect to post-treatment characterisation and the dependence of these outcomes on the salient processing variables: energy dissipated, exposure duration and inter-electrode gap. Under the treatment conditions used, remarkably uniform treatment and markedly stable modified surface properties result from the test surfaces exposed to the discharge, even at transit speeds simulating those associated with continuous on-line processing. The DBD system thus described, provides chemically mild and mechanically non-destructive means of altering surface properties, targeting improved surface characteristics and potentially better application performance.

To prevent the loss of fiber strength, ultrahigh-molecular-weight polyethylene (UHMWPE) fibers were treated with an ultraviolet radiation technique combined with a corona-discharge treatment. The physical and chemical changes in the fiber surface were examined with scanning electron microscopy and Fourier transform infrared/attenuated total reflectance. The gel contents of the fibers were measured by a standard device. The mechanical properties of the treated fibers and the interfacial adhesion properties of UHMWPE-fiber-reinforced vinyl ester resin composites were investigated with tensile testing. After 20 min or so of ultraviolet radiation based on 6-kW corona treatment, the T-peel strength of the treated UHMWPE-fiber composite was one to two times greater than that of the as-received UHMWPE-fiber composite, whereas the tensile strength of the treated UHMWPE fibers was still up to 3.5 GPa. The integrated mechanical properties of the treated UHMWPE fibers were also optimum. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 463–472, 2004
https://onlinelibrary.wiley.com/doi/10.1002/polb.10727

Commercial polymers in thin film form were used for modification by atmospheric RF plasma. The influence of the plasma treatments using Ar and Ar+O₂ on surface energy, morphology and chemical structure of the films was investigated. It was revealed that both modifications caused surface activation of the polymer film, but they obeyed different mechanisms enhancing polymer wettability. First, surface graphitization due to argon sputtering caused hydrogen to free the surface and then reacts with oxygen in the air. Second, surface oxidation is connected with the functional group formation. The reactions of Ti with the polymer led to the simultaneous formation of TiCl₂, TiC, Ti-oxide and they contributed to film adhesion. In comparison with Ar, the mixed Ar+O₂ RF plasma treatment was a more timesaving process and had more influences on surface activation and film adhesion.

In this paper, we report and discuss the results of the surface treatment, using an atmospheric pressure dielectric barrier discharge (DBD), of selected polymer films which have no bonded oxygen in their intrinsic structures. Contact angle, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) data are presented with respect to post-treatment characterisation and the dependence of these outcomes on the salient processing variables: energy dissipated, exposure duration and inter-electrode gap. Under the treatment conditions used, remarkably uniform treatment and markedly stable modified surface properties result from the test surfaces exposed to the discharge, even at transit speeds simulating those associated with continuous on-line processing. The DBD system thus described, provides chemically mild and mechanically non-destructive means of altering surface properties, targeting improved surface characteristics and potentially better application performance.

The surface energy parameters of polycaprolactone (PCL) were determined at 160 and 180°C from its interfacial tensions with probe polymers. The probe polymers were polystyrene (PS) and poly(methyl methacrylate) (PMMA). This method is based on the well-known relationship between blend interfacial tension and polymer surface energy parameters, and requires the use of at least two probe polymers, whose surface energy parameters at the temperature of interest have been independently determined. It also requires direct measurement of blend interfacial tension at the high temperatures of interest. The interfacial tensions were obtained from direct measurements by the imbedded fiber retraction method. The following results were obtained: (a) γ^P (polar component) values for PCL was within the range reported using other methods, (b) γ^D (dispersion component) values for PCL decreased with increasing temperature, consistent with expectations and (c) γ^D values for PCL were on the high end, but still within the rather broad range of reported values.

In the present paper, the following topics are reviewed in detail: (a) the available adhesives, as well as their recent advances, (b) thermodynamic factors affecting the surface pretreatments including adhesion theories, wettability, surface energy, (c) bonding mechanisms in the adhesive joints, (d) surface pretreatment methods for the adhesively bonded joints, and as well as their recent advances, and (e) combined effects of surface pretreatments and environmental conditions on the joint durability and performance. Surface pretreatment is, perhaps, the most important process step governing the quality of an adhesively bonded joint. An adhesive is defined as a polymeric substance with viscoelastic behavior, capable of holding adherends together by surface attachment to produce a joint with a high shear strength. Adhesive bonding is the most suitable method of joining both for metallic and non-metallic structures where strength, stiffness and fatigue life must be maximized at a minimum weight. Polymeric adhesives may be used to join a large variety of materials combinations including metal-metal, metal-plastic, metal-composite, composite-composite, plastic-plastic, metal-ceramic systems. Wetting and adhesion are also studied in some detail in the present paper since the successful surface pretreatments of the adherends for the short- and long-term durability and performance of the adhesive joints mostly depend on these factors. Wetting of the adherends by the adhesive is critical to the formation of secondary bonds in the adsorption theory. It has been theoretically verified that for complete wetting (i.e., for a contact angle equal to zero), the surface energy of the adhesive must be lower than the surface energy of the adherend. Therefore, the primary objective of a surface pretreatment is to increase the surface energy of the adherend as much as possible. The influence of surface pretreatment and aging conditions on the short- and long-term strength of adhesive bonds should be taken into account for durability design. Some form of substrate pretreatment is always necessary to achieve a satisfactory level of long-term bond strength. In order to improve the performance of adhesive bonds, the adherends surfaces (i.e., metallic or non-metallic) are generally pretretead using the (a) physical, (b) mechanical, (c) chemical, (d) photochemical, (e) thermal, or (e) plasma method. Almost all pretreatment methods do bring some degree of change in surface roughness but mechanical surface pretreatment such as grit-blasting is usually considered as one of the most effective methods to control the desired level of surface roughness and joint strength. Moreover, the overall effect of mechanical surface treatment is not limited to the removal of contamination or to an increase in surface area. This also relates to changes in the surface chemistry of adherends and to inherent drawbacks of surface roughness, such as void formations and reduced wetting. Suitable surface pretreatment increases the bond strength by altering the substrate surface in a number of ways including (a) increasing surface tension by producing a surface free from contaminants (i.e., surface contamination may cause insufficient wetting by the adhesive in the liquid state for the creating of a durable bond) or removal of the weak cohesion layer or of the pollution present at the surface, (b) increasing surface roughness on changing surface chemistry and producing of a macro/microscopically rough surface, (c) production of a fresh stable oxide layer, and (d) introducing suitable chemical composition of the oxide, and (e) introduction of new or an increased number of chemical functions. All these parameters can contribute to an improvement of the wettability and/or of the adhesive properties of the surface.

Neumann−Good's parallel strip model (J. Colloid Interface Sci. 1972, 38, 341) was used to analyze the contact angle hysteresis for a liquid on a heterogeneous surface composed of alternatively aligned horizontal apolar (θ = 70°) and polar (θ = 0°) strips. The critical size of the strip width, below which the contact angle hysteresis disappears, was determined on the basis of the analysis of the activation energy for wetting to be from 6 to 12 nm. This calculated value of the critical strip size is 1 order of magnitude smaller than that of 0.1 μm, which has been commonly considered as the limit of heterogeneity size causing the appearance of the contact angle hysteresis.

Polyolefins such as polyethylene, polypropylene and their copolymers have excellent bulk physical/chemical properties, are inexpensive and easy to process. Yet they have not gained considerable importance as speciality materials due to their inert surface. Polyethylene in particular holds a unique status due to its excellent manufacturer- and user-friendly properties. Thus, special surface properties, which polyethylene does not possess, such as printability, hydrophilicity, roughness, lubricity, selective permeability and adhesion of micro-organisms, underscore the need for tailoring the surface of this valuable commodity polymer. The present article reviews some of the existing and emerging techniques of surface modification and characterisation of polyethylene. Surface modification of polymers, polyethylene in particular, has been extensively studied for decades using conventional tools. Although some of these techniques are still in use, they suffer from distinct shortcomings. During the last two decades, different means of surface modification have been thoroughly explored. The increasing expectancy for smart materials in daily life has, of late, sharply influenced research in the area of surface modification. Technologies that involve surface engineering to convert inexpensive materials into valuable finished goods have become even more important in the present scenario. In this review article we have attempted to broadly address almost all conventional and modern techniques for the surface modification of different physical forms and chemical compositions of polyethylene. This article will hopefully stimulate further research in this area and result in the development of polyolefins with multi-functional and responsive surfaces, which would ultimately lead to the commodities of polyolefins with smart surfaces.