Recent experimental (low-rate) dynamic contact angles for 14 solid surfaces are interpreted in terms of their solid surface tensions. Universality of these experimental contact angle patterns is illustrated; other reasons that can cause data to deviate from the patterns are discussed. It is found that surface tension component approaches do not reflect physical reality. Assuming solid surface tension is constant for one and the same solid surface, experimental contact angle patterns are employed to deduce a functional relationship to be used in conjunction with the Young equation to determine solid surface tensions. The explicit form of such a relation is obtained by modifying Berthelot’s rule together with experimental data; essentially constant solid surface tension values are obtained, independent of liquid surface tension and molecular structure. A new combining rule is also derived based on an expression similar to one used in molecular theory; such a combining rule should allow a better understanding of the molecular interactions between unlike solid–liquid pairs.

Nitrogen-containing plasmas are widely used to improve wettability, printability, bondability, and biocompatibility of polymer surfaces. Plasma-treatments fed with NH₃ give rise to N-functionalities, such as amino ( NH₂), imino ( CH NH), cyano ( C N) and others on polymers, plus oxygen-containing groups due to post-plasma atmospheric oxidation. This work deals with NH₃ plasma treatment of PA 6 foils and the evaluation of surface modification as a function of treatment time. The introduced functionalities were observed by streaming potential measurements (surface charge), X-ray photoelectron spectroscopy analysis (nature of introduced functionalities), atomic force microscopy (surface topography), and contact angle measurement (assessment of treatment effect). The results show that the introduction of N-containing groups is increasing with longer treatment time only to a certain extent where the negative effect of surface destruction prevails over the positive effect of introduction of functional groups. The treatment causes a shift of the isoelectric point (IEP) toward pH of 6.2 as compared to 4.2 found for the untreated foil. If the treatment time is longer than 1 min the IEP is shifted to lower pH, the number of amino groups on the surface is reduced and the contact angle is increased.

The contact angle of aqueous solutions of Triton X-100, Triton X-114, Triton X-165, sodium dodecylsulfate, sodium hexadecylsulfonate, cetyltrimethylammonium bromide, cetylpyridinium bromide, sodium N-lauryl sarcosinate, dodecyldimethyethylammonium bromide, tetradecyltrimethylammonium bromide and benzyldimethyldodecylammonium bromide on polytetrafluoroethylene, polymethyl methacrylate and nylon 6 was studied. The contact angle values were used in the Young equation for the polymer–solution interface tension calculation and for the determination of the critical surface tension of polymer wetting. The critical surface tension of polymer wetting was obtained on the basis of the relationship between the cosine of contact angle and/or the adhesion tension as a function of the surface tension of aqueous solution of studied surfactants and then was discussed in relation to the Lifshitz–van der Waals components and electron-acceptor and electron-donor parameters of polytetrafluoroethylene, polymethyl methacrylate and nylon 6 surface tension. The role of the parameter of interfacial interactions in the relationship between the critical surface tension of polymer wetting and the surface tension was also considered. This parameter was calculated by using the polymer–solution interface tension as well as the polymer and aqueous solutions of surfactant surface tension.

The Lifshitz–van der Waals/acid-base approach proposed by van Oss et al. is found to yield inconsistent solid surface tensions and components from contact angles, for fluorocarbon, polystyrene, and poly(methyl methacrylate) solid surfaces. It is also shown that the approach cannot predict the correct interfacial tensions of all liquid–liquid pairs in question: the predicted interfacial tensions range from 34% lower to 112% higher than the experimental values. Thus, the usefulness of the approach for surface tension components and interfacial tensions is open to question. The liquid surface tension components postulated since 1986 are also summarized.

The use of plasma treatment for the modification of polystyrene microtiter wells has been evaluated by contact angle measurements, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The contact angle data suggests that the effect of plasma treatment is first to clean the surface of volatile contamination, increasing the hydrophobicity, and then to introduce oxygen functionality into the surface, decreasing the hydrophobicity. The cleaning effect appears to occur in the first few seconds of treatment while the oxygenation effect increases with increased exposure to the plasma. The XPS and ToF-SIMS measurements show increasing surface oxygen concentration with plasma treatment time, with a concomitant reduction in aromaticity. Scanning tunnelling microscopy (STM) imaging reveals that plasma treatment significantly affects the adsorption of bovine serum albumin (BSA). Untreated surfaces exhibited areas where no BSA adsorption occurred. These regions ranged in size between 20 and 60 nm in diameter. Plasma treated surfaces, however, exhibited no such areas, with BSA adsorption appearing to be more uniform across the surface. The regions on the untreated surfaces where no BSA adsorption occurred are thought to be hydrocarbon (volatile) in nature possibly from the moulding process, which is removed in the first few seconds of plasma treatment.

The objective of this study was to determine the surface free energy components of aqueous-based cellulose ether films and compare these values with those of other cellulose polymers. The surface free energy parameters were calculated from the contact angles of sessile drops of apolar and polar liquids on cellulose ether films cast on glass slides using the Lifshitz–van der Waals/acid–base (LW/AB) approach according to the method of van Oss, Chaudhury and Good. The cellulose ethers studied were hydroxypropyl methylcellulose (HPMC), methylcellulose (MC), hydroxypropyl cellulose (HPC) and hydroxyethyl cellulose (HEC). The total surface free energy of these cellulose ethers ranged from 42 to 50 mJ m−². The contribution of the acid–base (AB) component of surface free energy to the total surface free energy of the polymers ranged from 4 to 12%, which was considerably lower than that of cellulose. The cellulose ethers demonstrated near monopolarity and had dominant electron donor (Lewis-base) character. The overall trend in the values of the thermodynamic terms derived from the surface free energy parameters as indicators of hydrophilicity and hydration were in good agreement with the relative bulk solubility and hydration behavior of the polymers. Independent estimates of the AB character of the polymers from work of adhesion terms calculated from the liquid wetting data agreed with those obtained from the surface free energy parameters. Calculation of the work of adhesion with substrates of varying surface free energy parameters indicated that acid–base interactions made a major contribution to the total work of adhesion between cellulose ethers and bipolar surfaces. Although no direct correlation could be established between the surface free energy parameters and the type of substitution on the cellulose backbone for the cellulose ethers, the values of the terms derived from the LW/AB approach were consistent with those of cellulose and ethylcellulose. The LW/AB approach provides a reasonably consistent method for estimating the surface properties of cellulose ethers and the resulting surface free energy parameters are shown to relate to the interfacial properties of the polymers.

The common measurement of the contact angle is performed in conditions not corresponding to true equilibrium states and gives non-equilibrium values, the advancing and receding contact angles. To solve this problem, a very simple experimental device, based on the Wilhelmy experiment, is proposed in the present paper. It is able to transfer mechanical energy to the three-phase system in a controlled way through a simple loudspeaker; the analysis of some common surfaces is made through this method showing as a new stable minimum of the surface free energy can be attained, independent on the initial conditions and corresponding to a value of the contact angle intermediate between the advancing and receding ones. A comparison is developed with literature results on heterogeneous and rough surfaces, some ‘first-order’-approximation equations proposed in the literature are examined and compared with the new results. A simple but useful theoretical treatment is also compared with the experimental results to allow a more detailed, although qualitative-level, analysis. An important consequence with respect to the calculations of solid surface free energies is indicated.

Effects of the exposure of ultraviolet (UV) excimer light on the physicochemical surface properties of polymer films were investigated by contact angle measurements and X-ray photoelectron spectroscopy (XPS). The UV light at wavelength of 172 nm was exposed to polyethylene (PE), polyimide (PI), and polytetrafluoroethylene (PTFE) films in ambient air. The advancing and receding contact angles of water on the unexposed and UV-exposed films were determined by the sessile drop and the Wilhelmy methods as a measure of the wettability. For the PE and PI films, remarkable decrease in the water contact angle was accomplished by the UV exposure of several or several 10 s. The XPS data showed that such increase in the wettability was attributed to the increased atomic oxygen concentration at the film surfaces. The wettability of the PTFE film did not change due to the UV exposure. When the UV-exposed PE and PI films were stored in ambient air, the increase in the water contact angle, i.e. the hydrophobic recovery, was observed over a time scale of several days. It was suggested that the gasification of the low-molecular weight oxidized materials as well as the reorientation and the migration of polymer chains in the oxidized surface layer was responsible for the hydrophobic recovery in air. The UV exposure was also attempted to the PI film being covered with a metal mesh to prepare the film having both non-exposed and UV-exposed surface regions. The differences in the advancing and receding contact angles between the both regions were observed on the continuous weight recording at constant interfacial moving velocity by the Wilhelmy method. The Wilhelmy method in combination with the UV lithography technique enabled the simultaneous evaluation of the wettabilities of the treated and untreated surfaces.

Semi-crystalline polyetheretherketone (PEEK) is of interest for providing hermetic sealing of implantable medical devices. Self bonding or autohesion is achieved by mild temperature and pressure treatment and is potentially useful to form joints in the encapsulation of active medical implants. The surfaces of PEEK films were treated in a radio-frequency (RF) plasma containing one of the gases Ar, N₂ and O₂, to achieve surface activation. The bond strength developed over the interface of PEEK films was evaluated by lap-shear testing. The effects of plasma conditions on the surface morphology, composition, and properties were determined using the profilometer, contact angle measurements, X-ray photoelectron spectrometry (XPS) and scanning electron microscopy (SEM). Results obtained show that plasma treatment of the PEEK films enhances their bonding strength, with the Ar treated films exhibiting the highest bond strength and nitrogen the lowest. Bond strength was shown to correlate positively with total oxygen content, with C–O group concentration and with the polar component of surface energy. Bond strength correlated negatively with CO group concentration.

The surface free energy is an essential paper property affecting liquid/ink interaction with the ink-jet paper surface. Different ways of calculating surface energy components for ink-jet papers is introduced. The results given by the very useful van Oss–Chaudhury–Good (vOCG) bi-bidentate model are compared with simpler mono-bidentate and mono-monodentate models. The unbalance in the acid–base (AB) values of the vOCG-model is compensated for, and occasional negative roots obtained are removed when applying the simpler mono-bidentate- and mono-monodentate models. The simple and elegant mono-monodentate model produces comparable values with the other models, and is thus recommended. The calculated percent work of adhesion between the probe liquids and substrates correlates well with surface energy component values. Also the percent work of adhesion between the inks and substrates correlates with surface energy values.

Contact angles can be of great value; however, making meaningful contact angle measurements and interpreting those measurements is complex. For years, researchers have addressed a wide variety of issues concerning contact angles. Some questions have been qualitatively answered; others remain open. In this paper, we focus on three issues which are particularly important for the measurement and use of contact angles: the appropriate definitions and use of macroscopic and microscopic contact angles, a brief survey of the length scales relevant to phenomena controlling contact angles, and the role of vibrations in determining contact angles. We emphasize contact angle issues relevant to heterogeneous surfaces, specifically, ambient surfaces prevalent in nature and industry.

Surface or interfacial phenomena, including wetting, adsorption, adhesion, and dissolution, are of significant interest for daily life as well as for industrial and engineering applications. Surface tension and the Hansen solubility parameter (HSP) both represent similar physical characteristics related to these phenomena. It is therefore interesting to study the relation between them, and in the present work, reported empirical relations between surface tension and HSP are critically investigated. There exists an approximately proportional relation between total surface tension and HSP, although the coefficient obtained in the present work is much smaller than the commonly reported ones. The result is supported by an estimation of the coefficient using a simple physical model. On the other hand, finding correlations between the partial components of surface tension and HSP appears to be difficult as they are measured differently. The uses of databases from which measurements are taken must also be taken into question. As an example, the surface tension components of diiodomethane are investigated, and the validity of the reported values are called into question.

We examined the possibility of the surface modification of hydrophobic polystyrene (PS) and poly(methyl methacrylate) (PMMA) by ozone aeration, UV irradiation and combination of ozone aeration and UV irradiation (ozone/UV) in distilled water. The surface states of treated films and particles were investigated by means of contact angle, atomic force microscopy (AFM) and FT-IR measurements. According to the contact angle measurements, the values of the contact angle of ultrapure water on treated PS films decreased with an increase in the elapsed time of the treatments. The most remarkable decrease was seen in ozone/UV treatment. On the other hand, the contact angle on treated PMMA films slightly increased to an equal extent after three types of treatments. The film and the particle surfaces of PS with aromatic rings were found to be well modified with hydroxyl (OH) and carbonyl (CO) groups and to give the most remarkable effects in ozone/UV treatments, whereas those of PMMA with no aromatic ring were hardly modified, merely resulting in a slight disorder in their surface roughness. The experimental results on surface modification of PS and PMMA revealed that the ozone/UV treatment in distilled water is usable as one of the useful techniques for the surface modification of polymers with aromatic rings.

The interaction between inks and substrates is critical during printing. Adhesion of the ink film is determined by the reciprocal interactions of polar and nonpolar (dispersive) components between polymer films and inks. The greater the similarity between the polar and dispersive components of inks, coating and substrates, the better the wetting and adhesion on the surface of printing substrate. Various liquid materials in printing such as inks, varnishes, lacquers, and adhesives contain high ratios of water. The highly polar nature of water makes the interaction of these materials unsuitable with predominantly disperse polymer surfaces. Some films with polyolefin structure, especially polypropylene, and polyethylene, are nonpolar and cannot form strong bonds with ink, varnish, or lacquer coatings due to their chemical structure. Increasing surface energy components overcomes the poor wetting and adhesion on polymer surfaces. In this review, the topics of water contact angle measurement and determination of surface energy, surface tension, and using sessile drop method for the wettability and ink adhesion of polymer films are surveyed. Information on structural and chemical processes was given that assists in obtaining wettable film surfaces. Recommendations were made for good adhesion and printability based on surface treatment methods and ink modification.

Ultra-high modulus polyethylene (UHMPE) fibres have been treated using a novel 'non-plasma' treatment allowing the incorporation of various chemical functional groups onto the polymer surface. The process comprises two steps: corona discharge treatment, followed by silanization of the polymer surface by a solution of an organo-functional silane. Corona discharge treatment incorporates oxygen-containing functionalities, e.g. reactive hydroxyl groups, onto the polymer surface. The presence of reactive -OH groups provides the possibility of covalent linkage of any organo-functional silane to the corona discharge-treated polymer in the form of a fibre, film, sheet, or powder. The effectiveness of the process was assessed by examining the interlaminar fracture energy and flexural modulus and by SEM analysis of the fracture surfaces of composites fabricated from the untreated, corona discharge-treated, ammonia plasma-treated, and the amine-grafted (using the novel process) UHMPE fabric. A significant improvement in interfacial adhesion was confirmed by increases in the interlaminar fracture energies and flexural moduli. The effectiveness of the process investigated is similar to the ammonia plasma treatment. SEM analysis of the fracture surfaces indicated a change in the fracture mode from purely adhesive for unmodified fibres, through to mixed failure mode for corona-treated material, to highly cohesive-in-fibre surface for amine-grafted UHMPE fibres. XPS analysis confirmed the incorporation of the amine groups onto the surface of polyethylene treated using the novel method.

Polypropylene sheets are treated with oxygen plasma for the interfacial control of GF/PP composites. The interfacial strength between glass fabric and PP resin is estimated by the T-peel test method. The evaluation of T-peel test data is done by both the T-peel strength method and the T-peel amplitude method. The T-peel strength value and T-peel amplitude value were respectively increased to about 50% and 120% compared with each value of non-treated specimens. The T-peel strength relates to the surface energy on the PP-sheet and the T-peel amplitude relates to the fracture pattern of the delamination surface. From SEM observations on the delamination surface, many voids in the space enclosed with fiber bundles are observed in the case of non-treated specimen and no void and fiber bridging are observed on the plasma treated specimens. It is found that interfacial properties between fiber and resin are improved by this plasma process.

The effects of oxygen-plasma treatment of ultra-high-modulus polyethylene (UHMPE) fiber on the transverse properties of the UHMPE fiber/vinylester composites have been investigated. The UHMPE fiber/vinylester unidirectional (UD) laminates were prepared with untreated and oxygen-plasma-treated UHMPE fiber. The oxygen-plasma treatment of the UHMPE fiber increases the transverse tensile strength and failure strain of UHMPE-fiber/vinylester composites and changes the failure initiation site from the interface to the interior of the UHMPE fiber. The oxygen-plasma treatment of the UHMPE fiber introduced micro-pits on the fiber surface; these micro-pits improve the interfacial adhesion in UHMPE fiber/vinylester composites through the mechanical interlocking between the micro-pits and the vinylester resin. Finite-element (FE) modeling was performed to investigate the effect of the micro-pits on stress transfer in the UHMPE-fiber/vinylester composite. The micro-pits are known to increase the stress transfer from the vinylester resin to the UHMPE fiber and this increased stress transfer is correlated with the improved transverse properties and the transition of the failure initiation site after oxygen plasma treatment.

The effects of oxygen plasma treatment on ultra-high-modulus polyethylene monofilaments have been investigated with particular reference to their adhesion to epoxy resins. The adhesion strength was monitored by pull-out tests, and the effects on the monofilaments was also studied by contact angle measurements, determination of gel content and scanning electron microscopy. The results of this investigation suggest that there are three contributions to the improvements in adhesion obtained from the plasma treatment. First, at comparatively short treatment times there is a general oxidation of the surface. Secondly, at intermediate treatment times cross-linking of the surface occurs, which increases the cohesive strength of the fibre surface. Finally, at long exposure times there is a pitting of the surface, which could give rise to a mechanical keying effect but may also reduce the fibre strength.

Improper or inadequate surface treatment is one of the commonest causes of failure in adhesive bonding, but the selection of a good surface treatment can bring marked improvements in the wet durability of adhesive bonds to metals and glasses, and can permit the bonding of otherwise unbondable materials such as polytetrafluoroethylene (ptfe) and the polyolefins. Untreated surfaces may be unsatisfactory for adhesive bonding because they may be contaminated, may lack polar chemical groups or the interface they make with an adhesive may be susceptible to hydrolysis.

Plasma-chemical modification is frequently used to improve the adaption of polymer surfaces to biological environments. In this regard amino functional groups play a key role. They provide an excellent basis for subsequent modifications with specific biomolecules. It would be of great value to get an amino functionalization independent of the specific material in use. The paper reports on an investigation concerning the feasibility of such an universal plasma functionalization procedure. Two different downstream microwave plasma sources were taken to apply a procedure, which was developed for high-grade modification of polystyrene (PS), to a number of other polymers including polyetheretherketone (PEEK), polyethyleneterephthalate (PET), polyethylenenaphthalate (PEN), polycarbonate (PC), polyethylene (PE), polymethylmethacrylate (PMMA) and fluorinated polymers. In many cases, very similar results were obtained. At maximum 5% of the surface were covered by nitrogen functional groups. In some cases, about 50% of total nitrogen functional groups were amino groups. The results suggest that a downstream ammonia plasma treatment indeed is a fairly universal method for high performance amino functionalization of polymeric biomaterials.